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PubMed2006

Serotonin-immunoreactive CPT interneurons in Hermissenda: identification of sensory input and motor projections.

Serotonin immunoreactive (5-HT-IR) neurons identified as cerebropleural ganglion triplets (CPTs) in Hermissenda may be homologues of 5-HT-IR neurons identified in other opisthobranch molluscs. In studies of isolated nervous systems and semi-intact preparations we used a combination of immunohistochemical techniques and fluorescent labeling with Lucifer yellow to identify 5-HT-IR CPT neurons after investigating sensory inputs and motor neuron projections. Here we show that identified 5-HT-IR CPT interneurons receive sensory input from mechanoreceptors and photoreceptors. In semi-intact preparations with intact pedal nerves P1 and P2, cutaneous stimulation of the middle or tail regions of the foot with calibrated von Frey hairs elicited spikes recorded from identified CPT interneurons. Illumination of the eyes evoked a small complex excitatory postsynaptic potential (EPSP) and resulted in a modest increase in the spike discharge of CPT interneurons. Immunostaining of Lucifer yellow-labeled neurons revealed that CPT interneurons projected an axonal process to the contralateral pedal ganglion. Depolarization of CPT interneurons with extrinsic current evoked EPSPs and spikes recorded from identified VP2 pedal neurons, motor neurons previously shown to elicit movement of the anterior foot. Extrinsic current stimulation of CPT interneurons in semi-intact preparations evoked movement of the anterior foot but did not facilitate ciliary activity or evoke PSPs recorded in identified VP1 ciliary motor neurons. Our results show that CPT neurons are polysensory interneurons that contribute to reflexive foot contractions in Hermissenda.

Action Potentials↗
PubMed2005

Physiology and morphology indicate that individual spinal interneurons contribute to diverse limb movements.

Overlapping neuronal networks have been shown to generate a variety of behaviors or motor patterns in invertebrates, but the evidence for this is more circumstantial in vertebrates. The turtle spinal cord can produce multiple forms of hindlimb scratching movements as well as hindlimb withdrawal, but it is still uncertain whether individual spinal cord interneurons contribute to the motor output for more than one type of limb motor pattern. In this study, individual spinal cord interneurons were recorded intracellularly in vivo in spinal immobilized turtles, and, after characterization, were filled with Neurobiotin. Interneurons that were rhythmically activated during multiple forms of ipsilateral fictive hindlimb scratching often had axon-terminal arborizations in the ventral horn of the spinal cord hindlimb enlargement. This provides some of the strongest evidence to date that interneurons involved in multiple forms of scratching contribute directly to hindlimb motor output. Moreover, most of these interneurons were also active during contralateral fictive scratching and during ipsilateral fictive hindlimb withdrawal, suggesting that they contribute to motor output for these additional behaviors as well. Such interneurons may provide the cellular basis for the contralateral contributions to ipsilateral scratching that have been demonstrated previously. Taken together, these findings suggest that diverse vertebrate limb movements are produced by spinal cord interneuronal networks that include some shared components.

Action Potentials↗
PubMed2002

Mechanisms of dopamine activation of fast-spiking interneurons that exert inhibition in rat prefrontal cortex.

Prefrontal cortical dopamine (DA) modulates pyramidal cell excitability directly and indirectly by way of its actions on local circuit GABAergic interneurons. DA modulation of interneuronal functions is implicated in the computational properties of prefrontal networks during cognitive processes and in schizophrenia. Morphologically and electrophysiologically distinct classes of putative GABAergic interneurons are found in layers II-V of rat prefrontal cortex. Our whole cell patch-clamp study shows that DA induced a direct, TTX-insensitive, reversible membrane depolarization, and increased the excitability of fast-spiking (FS) interneurons. The DA-induced membrane depolarization was reduced significantly by D1/D5 receptor antagonist SCH 23390, but not by the D2 receptor antagonist (-)sulpiride, D4 receptor antagonists U101958 or L-745870, alpha1-adrenoreceptor antagonist prazosin, or serotoninergic receptor antagonist mianserin. The D1/5 agonists SKF81297 or dihydrexidine, but not D2 agonist quinpirole, also induced a prolonged membrane depolarization. Voltage-clamp analyses of the voltage-dependence of DA-sensitive currents, and the effects of changing [K(+)](O) on reversal potentials of DA responses, revealed that DA suppressed a Cs(+)-sensitive inward rectifier K(+) current and a resting leak K(+) current. D1/D5, but not D2 agonists mimicked the suppressive effects of DA on the leak current, but the DA effects on the inward rectifier K(+) current were not mimicked by either agonist. In a subgroup of FS interneurons, the slowly inactivating membrane outward rectification evoked by depolarizing voltage steps was also attenuated by DA. Collectively, these data showed that DA depolarizes FS interneurons by suppressing a voltage-independent 'leak' K(+) current (via D1/D5 receptor mechanism) and an inwardly rectifying K(+) current (via unknown DA mechanisms). Additional suppression of a slowly inactivating K(+) current led to increase in repetitive firing in response to depolarizing inputs. This D1-induced increase in interneuron excitability enhances GABAergic transmission to PFC pyramidal neurons and could represent a mechanism via which DA suppresses persistent firing of pyramidal neurons in vivo.

Action Potentials↗
PubMed2002

A non-alpha7 nicotinic acetylcholine receptor modulates excitatory input to hippocampal CA1 interneurons.

The nicotinic acetylcholine receptor (nAChR), particularly the alpha7 subtype, has received profound attention for its role in modifying excitatory postsynaptic currents (EPSCs) in hippocampal pyramidal neurons as well as in neurons from other brain regions. Here, we tested the possibility that an nAChR could affect EPSCs in the interneurons of rat hippocampal slices. Using whole-cell patch-clamp technique on CA1 stratum radiatum interneurons and U-tube application of agents, we show that nicotinic agonists enhance EPSC frequency in interneurons. Among the agents tested, cytisine and mecamylamine were the most effective agonist and antagonist, respectively, suggesting a role for alpha3beta4-containing nAChRs in the modulation of interneuron EPSCs. Ligands selective for the alpha7 nAChR had very little or no effect on interneuron EPSCs. Low concentrations of nicotine also enhanced EPSC frequency, implicating the involvement of non-alpha7 nAChRs in controlling interneuron excitability in smokers. We conclude that nAChR-dependent EPSC modulation in the hippocampus is both subtype- and neuron-specific and that a non-alpha7 nAChR, presumably alpha3beta4, controls glutamate transmission to CA1 interneurons.

Acetylcholine↗
PubMed2003

Interneuronal projections to identified cilia-activating pedal neurons in Hermissenda.

Neural networks have been shown to support the generation of more than one behavioral motor act. In the nudibranch mollusk Hermissenda, Pavlovian conditioning results in light, the conditioned stimulus (CS), evoking both inhibition of locomotion and foot contraction. The synaptic organization of the eyes and optic ganglion is well documented; however, the characterization of the neural network mediating visually modulated behaviors is incomplete. We have now characterized synaptic connections between identified photoreceptors and a newly identified interneuron (II(b)), identified synaptic projections from type I and type II interneurons to an inhibitory interneuron (III(i)) and to two newly identified pedal neurons, VP1 and VP2. Here we show that VP1 activates ciliary movement on the anterior foot and VP2 innervates the anterior foot and ventral tentacle. Stimulation of the photoreceptors with light produced two effects on the activity of VP1 and VP2. First, light inhibits type I(i) and II(i) interneurons and disinhibits VP1 and VP2. Depolarization of type II(e) interneurons also disinhibits VP1 and VP2. Second, the light-elicited depolarization and increased tonic activity of VP1 and VP2 is produced by excitatory synaptic input from ipsilateral and contralateral type II(b) interneurons. Pedal neurons VP1 and VP2 receive similar synaptic input from type I, II, and III(i) interneurons; this is in agreement with previous research showing that the visual pathway influences both ciliary locomotion and foot movement. The organization of the visual system in Hermissenda provides for the expression of cellular and synaptic plasticity supporting learning without altering the networks ability to carry out the requirements for normal visual processing.

Animals↗
PubMed2006

Electrophysiological classification of somatostatin-positive interneurons in mouse sensorimotor cortex.

Classification of inhibitory interneurons is critical in determining their role in normal information processing and pathophysiological conditions such as epilepsy. Classification schemes have relied on morphological, physiological, biochemical, and molecular criteria; and clear correlations have been demonstrated between firing patterns and cellular markers such as neuropeptides and calcium-binding proteins. This molecular diversity has allowed generation of transgenic mouse strains in which GFP expression is linked to the expression of one of these markers and presumably a single subtype of neuron. In the GIN mouse (EGFP-expressing Inhibitory Neurons), a subpopulation of somatostatin-containing interneurons in the hippocampus and neocortex is labeled with enhanced green fluorescent protein (EGFP). To optimize the use of the GIN mouse, it is critical to know whether the population of somatostatin-EGFP-expressing interneurons is homogeneous. We performed unsupervised cluster analysis on 46 EGFP-expressing interneurons, based on data obtained from whole cell patch-clamp recordings. Cells were classified according to a number of electrophysiological variables related to spontaneous excitatory postsynaptic currents (sEPSCs), firing behavior, and intrinsic membrane properties. EGFP-expressing interneurons were heterogeneous and at least four subgroups could be distinguished. In addition, multiple discriminant analysis was applied to data collected during whole cell recordings to develop an algorithm for predicting the group membership of newly encountered EGFP-expressing interneurons. Our data are consistent with a heterogeneous population of neurons based on electrophysiological properties and indicate that EGFP expression in the GIN mouse is not restricted to a single class of somatostatin-positive interneuron.

Algorithms↗
PubMed1976

Integration of directional mechanosensory input by crayfish interneurons.

1. Interneurons activated by mechanosensory hairs on the crayfish telson respond selectively to directional displacements of the medium; the directions of maximum sensitivity lie 180 degrees apart in approximately the rostrocaudal plane, corresponding to the directional sensitivities of the two populations of primary afferent neurons. We have examined the basis for this selectivity by intracellular recording in the interneurons, correlating subthreshold potentials with activity evoked in identified afferents by bending single hairs or by producing nearfield displacements of the medium. 2. Interneurons can usually be caused to discharge by a brief train of impulses in single sensory axons. Unitary EPEPs are associated with arriving affterent spikes in the fourth (sensory) root; each primary interneuron receives convergence from several sensory axons, all sensitive to the same direction of movement. Since each afferent axon is drawn from a pair innervating a single sensory structure, this remarkable specificity of connection is unlikely to depend on an anatomical mode of address. 3. Higher order interneurons receive from directionally sensitive lower order interneurons of the same class, as well as from primary afferents of that class. The responses of such cells may show much more decrement during a train of displacement stimuli than do those of lower order cells. Directionality does not appear to be enhanced. 4. During the "null phase" some interneurons appear to be actively inhibited: bending of single hairs 180 degrees away from the effective direction may produce membrane hyperpolarization and slow spontaneous discharges, and shocks to afferent roots produce mixtures of monosynaptic EPSPs and polysynaptic IPSPs.

Animals↗
PubMed1982

Activities of identified interneurons, motoneurons, and muscle fibers during fictive swimming in the lamprey and effects of reticulospinal and dorsal cell stimulation.

1. Application of D-glutamate to the isolated spinal cord of the lamprey produces phasic activity in ventral roots, which is similar to that of the muscles of the intact swimming animal (5,18). Therefore, the isolated spinal cord may be used as a convenient model for the investigation of the generation of locomotor rhythms in a vertebrate. 2. Almost all slow muscle fibers exhibited excitatory junctional potentials (EJPs) during swimming activity. The number of EJPs per cycle increased with the intensity of ventral root (VR) bursting. Few twitch fibers were active, and these fired action potentials only during high intensities of VR bursts. 3. As was found by Russell and Wallén (25), myotomal motoneurons had oscillating membrane potentials during fictive swimming which, on the average, reached a peak depolarization in the middle of the VR burst (phi = 0.21 +/- 0.05; phi = 0 is defined as the onset of the VR burst, and the duration of the cycle is set equal to 1). Membrane potential oscillations in fin motoneurons were antiphasic to those of nearby myotomal motoneurons (peak depolarization phi = 0.68 +/- 0.05). 4. Lateral interneurons had oscillating membrane potentials in synchrony with those of myotomal motoneurons (peak depolarization phi = 0.21 +/- 0.10). Interneurons with axons projecting contralaterally and caudally (CC interneurons) had oscillating membrane potentials that peaked significantly earlier in the cycle (peak depolarization phi = 0.06 +/- 0.12). 5. Edge cells were only weakly modulated during fictive swimming. Their peak depolarizations occurred near the end of the VR burst (phi = 0.33 +/- 0.10). Most giant interneurons were not phasically modulated during fictive swimming. 6. Repetitive intracellular stimulation of Müller cells during fictive swimming generally evoked an increased burst intensity in ipsilateral VRs and a decreased burst intensity in contralateral VRs. The cells M3, B1, and B2 also produced increases or decreases in the frequency of VR bursts. Repetitive intracellular stimulation of sensory dorsal cells could also change the intensities and timing of VR bursts. 7. This study is an initial survey of lamprey spinal interneurons that participate in swimming activity. Lateral interneurons and CC interneurons are active during fictive swimming and probably help coordinate the undulations of the body, but their roles in pattern generation are not known. The central pattern generator is subject to modification by descending and sensory inputs.

Action Potentials↗
PubMed1985

Effects of groups of propriospinal interneurons on fictive swimming in the isolated spinal cord of the lamprey.

Fictive swimming activity was induced in isolated spinal cords of adult lampreys Ichthyomyzon unicuspis and Petromyzon marinus by addition of D-glutamate or N-methyl-D,L-aspartate (NMA) to the bathing fluid. Propriospinal interneurons are defined as nerve cells within the spinal cord with projections longer than 1 segment. The hypothesis that propriospinal interneurons contribute to intersegmental coordination during fictive swimming was tested using electrical stimulation, extracellular recording, and separated compartments. Stimulation of the split caudal end of the spinal cord indirectly excited ascending propriospinal interneurons, which enhanced and entrained bursts in rostral contralateral ventral roots. Indirect electrical stimulation of descending propriospinal interneurons could delay and diminish bursts in caudal contralateral ventral roots. Extracellular recordings from the rostral and caudal split ends of the spinal cord sometimes showed spike activities in phase with contralateral or ipsilateral ventral roots. Inhibition of 1-3 segments by spot applications of glycine or gamma-aminobutyric acid (GABA) did not interrupt normal coordination or rostrocaudal phase lag. When a middle region of spinal cord was inhibited in a compartment with GABA or glycine, the caudal spinal cord could entrain the bursts in rostral ventral roots. In a few preparations the caudal region induced antiphasic bursts in previously silent rostral roots through the inhibited region. The maximum separation for caudal-upon-rostral antiphasic entrainment was approximately 20 segments in Ichthyomyzon and 36 segments in Petromyzon. Increased concentrations of an excitatory amino acid in a rostral compartment could produce descending entrainment of bursts in an adjacent caudal compartment at a higher frequency with rostrocaudal phase lag. The rostral-upon-caudal entrainment could still occur through spot applications of GABA or glycine but not through long inhibited regions. Two hypothetical groups of propriospinal interneurons are proposed for the coordination of swimming activities in the isolated spinal cords of adult lampreys. 1) Crossed, ascending interneurons may be excited in phase with nearby motoneurons and may excite and entrain rostral pattern generators on the opposite side. 2) Short, commissural interneurons may be excited in phase with nearby motoneurons and may inhibit contralateral generators.

Animals↗
PubMed1990

Pharmacologic analysis of inhibition produced by last-order intermediate nucleus interneurons mediating nonreciprocal inhibition of motoneurons in cat spinal cord.

1. The aim of this study was to investigate the effects of drugs blocking glycinergic and GABAergic transmission on the postsynaptic inhibition of hindlimb motoneurons produced by activation of last-order laminae V-VI interneurons, which are coexcited by muscle and cutaneous afferents and have axonal branches projecting to the Clarke's column. 2. In anesthetized cats with right spinal cord hemisected and both dorsal columns cut between L4 and L5 segments, stimulation of the Clarke's column (CC) at L3-L4 level produced a short-latency, presumably monosynaptic, inhibitory potential that could be recorded either from L7 or S1 ventral rootlets by means of the sucrose-gap technique (iVRP) or intracellularly from hindlimb motoneurons (IPSP). These potentials have been attributed to antidromic activation of a population of last-order interneurons mediating nonreciprocal inhibition of motoneurons. 3. The early iVRP and IPSP produced by CC stimulation was practically abolished 10-20 s after the intravenous injection of strychnine (0.1 mg/kg) and replaced by an excitatory synaptic potential followed by delayed, slow, strychnine-resistant inhibitory potential. 4. Monosynaptic reflexes (MSR) elicited by stimulation of group I gastrocnemius (GS) afferents were inhibited during the occurrence of the CC-iVRP. This inhibition was significantly reduced after intravenous strychnine. On the other hand, the inhibition of the GS-MSR, produced by conditioning stimulation of the posterior biceps and semitendinosus (PBSt) nerve with trains of pulses applied 25-35 ms before the test stimulus, was practically unchanged after the intravenous injection of strychnine. 5. The CC-iVRP and the associated inhibition of GS-MSRs were not significantly affected after the intravenous injection of 0.1 mg/kg of picrotoxin, which clearly reduced the dorsal root potentials (DRP), the late component of the iVRP, and the inhibition of MSRs produced by PBSt volleys. 6. The effect of strychnine and picrotoxin was tested on the monosynaptic iVRP elicited by single intermediate nucleus interneurons that were antidromically activated from the CC and responded both to low-threshold cutaneous fibers and to group I or group II afferents. In three experiments where the interneuronal activity could be kept after the drug injection, it was possible to show that strychnine abolished the interneuronally elicited iVRP, which was replaced by an excitatory synaptic potential with onset preceding the interneuronal activity. In another experiment, it was possible to show that the interneuronally elicited iVRP was not affected by an intra-aortic injection of picrotoxin (0.5 mg/kg) that reduced to one-half the DRP and the iVRP produced by group I PBSt volleys.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1995

Regional characteristics of the membrane response of an identified crayfish nonspiking interneuron to intracellularly injected current.

1. We investigated the membrane response of the local directionally selective (LDS) interneuron, a nonspiking cell identified in the terminal abdominal ganglion of crayfish, to intracellularly injected current in different regions within the cell by single-electrode, discrete current-clamp experiments. The site of electrode impalement into the cell was visualized in situ together with the cell structure under a dissecting microscope. 2. The LDS interneuron has dendritic branches on both hemiganglia connected by a thick segment crossing the midline. Irrespective of the site of electrode impalement, the interneuron showed outward rectification upon depolarization from the resting potential level. When hyperpolarizing current was injected, a linear relationship was observed between the voltage response of the interneuron and the amount of injected current. Upon large hyperpolarization, however, the interneuron showed inward rectification. 3. The input resistance of the interneuron measured within the linear range of the membrane response was significantly lower in the transverse segment than in lateral dendrites (0.001 < P < 0.01). The time constant of the transient voltage response to step current injection was also significantly shorter in the transverse segment than in the lateral dendrites (0.001 < P < 0.01). 4. Although the regional difference in the input resistance could be accounted for, at least partly, by different geometric conditions of each dendritic branch into which current was injected, the regional difference in the time constant of the membrane response cannot be accounted for by structural differences because the time constant is independent of the membrane area. It is thus suggested that the passive properties of the interneuron membrane that are related to its response time constant show regional variability within the cell.

Animals↗
PubMed1996

Central pattern generator interneurons are targets for the modulatory serotonergic cerebral giant cells in the feeding system of Lymnaea.

1. The objective of the experiments was to explore the modulatory functions of the serotonergic cerebral giant cells (CGCs) of the Lymnaea feeding system by examining their synaptic connections with the central pattern generator (CPG) interneurons and the modulatory slow oscillator (SO) interneuron. 2. One type of modulatory function, "gating," requires that the CGCs fire tonically at a minimum of 7 spikes/min. Above this minimum level the CGCs control the frequency of CPG interneuron oscillation-- "frequency control," a second type of modulation. In an SO-driven fictive feeding rhythm, an increase in the frequency of the rhythm, with increased CGC firing rate, resulted from a reduction in the duration of the N1 (protraction) and N2 (rasp) phases of the feeding cycle with little effect on the N3 (swallow) phase. 3. The CGCs excited the N1 phase interneurons SO and N1M (N1 medial) cells but had no consistent effects on the N1 lateral cells. The CGC-->SO postsynaptic response was probably monosynaptic (< or = 200 ms in duration) with unitary 1:1 excitatory postsynaptic potentials (EPSPs) following each CGC spike. The CGC-->N1M excitatory response was slow and nonunitary, and a burst of CGC spikes evoked a depolarization of the N1M cells that lasted up to 10 s and triggered N1M cell bursts. Both CGC-->SO and CGC-->N1M excitatory responses could be mimicked by the focal application of serotonin (5-HT). 4. Both CGC-->SO and CGC-->N1M excitatory connections systematically increased the N1M cell firing rate within the CGCs' physiological firing range (0-40 spikes/min). This was due to both the direct (CGC-->N1M) and indirect (CGC-->SO-->N1M) excitatory synaptic pathways. The CGC-induced increase in N1M cell firing rate probably accounted for the reduced duration of the N1M cell feeding burst by causing a more rapid reversal of the feeding cycle from the N1 phase to the N2 phase. This phase reversal was due to the previously described recurrent inhibitory pathway (N1-->N2 excitation followed by N2-->N1 inhibition). 5. The CGCs' ability to provide a depolarizing drive to the N1M cells meant that this excitatory connection was also likely to be important for gating. 6. Activity in the CGCs produced nonunitary, long-lasting, excitatory postsynaptic responses on the N2 ventral (N2v) CPG interneurons, and these were likely to be involved in both the gating and the frequency control by the CGCs on the N2 phase of the feeding rhythm. Suppressing CGC tonic firing initially increased the duration of the N2v plateau (which determines the duration of the N2 phase of the feeding cycle, frequency function) but eventually led to a loss of N2v plateauing (gating function). 7. Nonunitary, weakly inhibitory CGC-->N2 dorsal responses were recorded that could be mimicked by the application of 5-HT. 8. Spikes in the CGCs evoked 1:1 monosynaptic EPSPs in the N3 tonic (N3t) CPG interneurons. This excitatory effect could be mimicked by the application of 5-HT. Within the physiological range of CGC firing, this excitation did not appear to influence the firing rate of the N3t cells. 9. N3 phasic (N3p) CPG interneurons showed biphasic (hyperpolarizing followed by depolarizing) unitary responses to spikes evoked in the CGCs. The inhibitory synaptic response was maintained in a high-Ca2+/high-Mg2+ (Hi-Di) saline and was mimicked by the focal application of 5-HT, indicating that it was probably monosynaptic. The excitatory component was, however, reduced in a Hi-Di saline, indicating that it was probably polysynaptic. Suppressing the CGCs during an SO-driven feeding rhythm caused the N3p cells to fire less, suggesting that the removal of the excitatory component of the response might be significant. 10. We conclude that the general depolarizing effects of the CGCs on a number of the CP

Animals↗
PubMed1996

Information theoretic analysis of dynamical encoding by four identified primary sensory interneurons in the cricket cercal system.

1. The stimulus/response properties of four identified primary sensory interneurons in the cricket cercal sensory system were studied using electrophysiological techniques. These four cells are thought to represent a functionally discrete subunit of the cercal system: they are the only cells that encode information about stimulus direction to higher centers for low intensity stimuli. Previous studies characterized the quantity of information encoded by these cells about the direction of air currents in the horizontal plane. In the experiments reported here, we characterized the quantity and quality of information encoded in the cells' elicited responses about the dynamics of air current waveforms presented at their optimal stimulus directions. The total sample set included 22 cells. 2. This characterization was achieved by determining the cells' frequency sensitivities and encoding accuracy using the methods of stochastic systems analysis and information theory. The specific approach used for the analysis was the "stimulus reconstruction" technique in which a functional expansion was derived to transform the observed spike train responses into the optimal estimate (i.e., "reconstruction") of the actual stimulus. A novel derivation of the crucial equations is presented. The reverse approach is compared with the more traditional forward analysis, in which an expansion is derived that transforms the stimulus to a prediction of the spike train response. Important aspects of the application of these analytical approaches are considered. 3. All four interneurons were found to have identical frequency tuning, as assessed by the accuracy with which different frequency components of stimulus waveforms could be reconstructed with a linear expansion. The interneurons encoded significant information about stimulus frequencies between 5 and 80 Hz, which peak sensitivities at approximately 15 Hz. 4. All four interneurons were found to have identical stimulus/response latencies. The mean latency between a stimulus component and the corresponding elicited spike was 17 ms. All four interneurons also had identical integration times. The integration time, measured by the duration of stimulus, which could affect the probability of spiking, was approximately 50 ms. 5. The accuracy of the encoding can be expressed as a signal-to-noise ratio, where the noise is a scaled difference between the original signal and the best estimate of the signal. Peak signal-to-noise ratios of approximately 1 were obtained for the cells across all stimulus power levels, using only the linear expansion term. Analysis of the data indicated that the consideration of second-order nonlinear transformations of the stimulus would not have increased the calculated encoding accuracy. 6. The encoding accuracy also can be expressed in the information theoretic units of bits/second, which characterizes the information transmission rate of the cell. Bits/second values varied between 10 and 80 for the 22 different cells in our experimental set. The information rate values were highly correlated with the mean spike rates of the interneurons, but were not correlated with the stimulus power levels. However, normalizing the absolute information rates by the mean spike rate in each case yielded a measure of bits/spike that was remarkably invariant across all experiments. The measured bits/spike rate was approximately 1 for all experiments. This result is discussed in the context of recent theoretical studies on optimal encoding. 7. Although the dynamic sensitivities of the four interneurons were identical, their directional sensitivities are known to be orthogonal. Thus the cells are complementary to one another from a functional standpoint: whereas a particular cell will be insensitive to air currents from some directions, one or more of the other three cells will be sensitive to stimuli from those directions...

Action Potentials↗
PubMed1996

Selective expression of transient outward currents in different types of acutely isolated hippocampal interneurons.

1. Voltage-dependent outward currents in CA1 interneurons were studied with the use of whole cell voltage-clamp techniques. Tissue slices containing strata lacunosum-moleculare and radiatum (L-M-R regions) of the hippocampal CA1 region were prepared. Neurons were then isolated from these tissue slices with the use of an acute dissociation procedure. The morphologies of the isolated neurons were distinct from those of pyramidal cells and correlated with those of interneurons identified in the L-M-R regions after immunohistochemical stainings. 2. Total outward currents were elicited from the isolated cells by depolarization steps applied after a 300-ms hyperpolarization prepulse to 100 mV from a holding potential of 50 mV. Delayed outward currents were obtained by intercalating a 120-ms step at 55 mV between the hyperpolarizing prepulse and the depolarization. The intercalating step served to inactivate transient outward currents. Transient outward current were isolated by subtracting the delayed outward currents from the total outward currents. 3. Interneurons were subgrouped on the basis of their ability to produce transient outward current in response to the above protocol. 4. The two groups of interneurons possessed distinct morphological features. Cells producing transient outward currents had polygonal-shaped somata with thick primary processes that gave rise to smaller secondary processes at a short distance from the soma. Interneurons without activatable transient outward currents had somata that were not polygonal and they had more slender primary dendritic processes. 5. These results suggest that interneurons in the L-M-R regions can be divided into two groups on the basis of the presence or absence of voltage-dependent transient outward currents. The two groups of cells differentiated on this basis also have distinguishable morphological traits. The difference in the properties of the outward current may be a factor contributing to the variation in the firing pattern of recorded interneurons reported in previous studies.

Animals↗
PubMed1996

Tachykinin-mediated modulation of sensory neurons, interneurons, and synaptic transmission in the lamprey spinal cord.

1. Tachykinin-like immunoreactivity is found in the dorsal roots, dorsal horn, and dorsal column of the lamprey. The effect of tachykinins on sensory processing was examined by recording intracellularly from primary sensory dorsal cells and second-order spinobulbar giant interneurons. Modulation of synaptic transmission was examined by making paired recordings from dorsal cells and giant interneurons, or by eliciting compound depolarizations in the giant interneurons by stimulating the dorsal root or dorsal column. 2. Bath application of tachykinins depolarized the dorsal cells. This effect was mimicked by stimulation of the dorsal root, suggesting that dorsal root afferents may be a source of endogenous tachykinin input to the spinal cord. The depolarization was reduced by removal of sodium or calcium from the Ringer, or when potassium conductances were blocked, and was not associated with a measurable change in input resistance. Dorsal root stimulation also caused a depolarization in the dorsal cells, and this effect and that of bath-applied substance P, was blocked by the tachykinin antagonist spantide. 3. The tachykinin substance P could reduce inward and outward rectification in the dorsal cells, the effect on outward rectification only being seen when potassium conductances were blocked by tetraethylammonium (TEA). 4. Substance P increased the excitability of the dorsal cells and giant interneurons, shown by the increased spiking in response to depolarizing current pulses. The increased excitability was blocked by the tachykinin antagonist spantide. 5. Substance P modulated the dorsal cell action potential, by increasing the spike duration and reducing the amplitude of the afterhyperpolarization. The spike amplitude was not consistently affected. 6. Stimulation of the dorsal column resulted in either depolarizing or hyperpolarizing potentials in the giant interneurons. The amplitude of the depolarization was increased by substance P, whereas the amplitude of the hyperpolarization was reduced. These effects occurred independently of a measurable change in postsynaptic input resistance, suggesting that the modulation occurred presynaptically. Paired recordings from dorsal cells and giant interneurons failed to reveal an effect of substance P on dorsal cell-evoked excitatory postsynaptic potentials (EPSPs), suggesting that the potentiation of the dorsal column-evoked depolarization was due to an effect on other axons in the dorsal column. Dorsal root-evoked potentials could also be increased in the presence of substance P, although this effect was less consistent than the effect on dorsal column stimulation. 7. These results suggest that tachykinins modulate sensory input to the lamprey spinal cord by increasing the excitability of primary afferents and second-order giant interneurons, and also by modulating synaptic transmission. Tachykinins may result in potentiation of local spinal reflexes and also modulation of descending reticulospinal inputs to the spinal locomotor network as a result of potentiation of spinobulbar inputs.

Action Potentials↗
PubMed1999

Processing of gustatory information by spiking local interneurons in the locust.

Despite the importance of gustation, little is known of the central pathways responsible for the processing and coding of different chemical stimuli. Here I have analyzed the responses of a population of spiking local interneurons, with somata at the ventral midline of the metathoracic ganglion, during stimulation of chemo- and mechanoreceptors on the legs of locusts. Volatile acidic stimuli were used to selectively activate the chemosensory neurons. Different members of the population of local interneurons received depolarizing or hyperpolarizing inputs during chemosensory stimulation. Many of the same interneurons that received chemosensory input also received mechanosensory inputs from tactile hairs on the leg, but others received exclusively mechanosensory inputs. Chemosensory inputs occurred with a short and constant latency, typical of monosynaptic connections. The chemosensory receptive fields of the spiking local interneurons mapped the surface of a hind leg so that spatial information relating to the location of a taste receptor was preserved. The amplitude of potentials in interneurons during chemosensory stimulation varied in a graded manner along the long axis of the leg, thus creating gradients in the chemosensory receptive fields of interneurons. Some interneurons were depolarized to a greater extent by chemical stimuli applied to basiconic sensilla on distal parts of the leg, whereas others were depolarized more by chemical stimulation of more proximal sensilla.

Animals↗
PubMed2000

Inhibition and disinhibition of pyramidal neurons by activation of nicotinic receptors on hippocampal interneurons.

Nicotinic acetylcholine receptors (nAChRs) are expressed in the hippocampus, and their functional roles are beginning to be delineated. The effect of nAChR activation on the activity of both interneurons and pyramidal neurons in the CA1 region was studied in rat hippocampal slices. In CA1 stratum radiatum with muscarinic receptors inhibited, local pressure application of acetylcholine (ACh) elicited a nicotinic current in 82% of the neurons. The majority of the ACh-induced currents were sensitive to methyllycaconitine, which is a specific inhibitor of alpha7-containing nAChRs. Methyllycaconitine-insensitive nicotinic currents also were present as detected by a nonspecific nAChR inhibitor. The ACh-sensitive neurons in the s. radiatum were identified as GABAergic interneurons by their electrophysiological properties. Pressure application of ACh induced firing of action potentials in approximately 70% of the interneurons. The ACh-induced excitation of interneurons could induce either inhibition or disinhibition of pyramidal neurons. The inhibition was recorded from the pyramidal neuron as a burst of GABAergic synaptic activity. That synaptic activity was sensitive to bicuculline, indicating that GABA(A) receptors mediated the ACh-induced synaptic currents. The disinhibition was recorded from the pyramidal neuron as a reduction of spontaneous GABAergic synaptic activity when ACh was delivered onto an interneuron. Both the inhibition and disinhibition were sensitive to either methyllycaconitine or mecamylamine, indicating that activation of nicotinic receptors on interneurons was necessary for the effects. These results show that nAChRs are capable of regulating hippocampal circuits by exciting interneurons and, subsequently, inhibiting or disinhibiting pyramidal neurons.

Acetylcholine↗
PubMed2001

Spinal interneurons that receive input from muscle afferents are differentially modulated by dorsolateral descending systems.

The possibility that descending systems have differential actions on the spinal interneurons that receive input from muscle afferents was investigated. Prolonged, physiological inputs were generated by stretch of the triceps surae muscles. The resulting firing patterns of 25 lumbosacral interneurons were recorded before and during a reversible cold block of the dorsolateral white matter at the thoracic level in nonparalyzed, decerebrate preparations. The strength of group I muscle afferent input was assessed from the response to sinusoidal tendon vibration, which activated muscle spindle Ia afferents directly and tendon organ Ib afferents via the resulting reflex force. The stretch-evoked responses of interneurons with strong responses to vibration were markedly suppressed by dorsal cold block, whereas the stretch-evoked responses of interneurons with weak vibration input were enhanced. The cells most strongly activated by vibration received their primary input from Ia afferents and all of these cells were inhibited by the cold block. These results suggest that a disruption of the descending system, such as occurs in spinal cord injury, will lead to a suppression of the interneuronal pathways with group Ia input while enhancing excitability within interneuronal pathways transmitting actions from higher threshold afferents. One possible consequence of this suppression would be a decreased activity among the Ia inhibitory interneurons that mediate reciprocal inhibition, resulting in abnormal reciprocal relations between antagonists and promoting anomalous muscle cocontraction.

Animals↗