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PubMed1998

GABAA receptor-mediated currents in interneurons and pyramidal cells of rat visual cortex.

1. We compared gamma-aminobutyric acid (GABA)-mediated responses of identified pyramidal cells and fast spiking interneurons in layer V of visual cortical slices from young rats (P11-14). 2. The frequency of spontaneous inhibitory postsynaptic currents (sIPSCs) was similar in pyramidal cells and interneurons (1.7 vs. 1.9 Hz). For events with 10-90% rise times less than 0.9 ms, no significant differences were found in mean amplitude (61 vs. 65 pA), mean rise time (0.58 vs. 0.61 ms), or the first time constant of decay (tau 1, 6.4 vs. 6.5 ms) between pyramidal cells and interneurons. The second decay time constant (tau 2) was significantly longer in interneurons than in pyramidal cells (49 vs. 22 ms). The difference in sIPSC decay kinetics between two cell types also existed in adult rats (P36-42), suggesting the kinetic difference in not due to differential development of GABAA receptors in these cell types. 3. The decay kinetics of monosynaptic evoked IPSCs were also longer in interneurons. As in the case of sIPSCs, the difference was accounted for by the second decay time constant. tau 1 and tau 2 were, respectively, 13 and 64 ms for interneurons and 12 and 47 ms for pyramidal cells. 4. Cell-attached patch recordings revealed that the mean open time for single Cl- channels in response to 2 microM GABA was significantly longer in interneurons than pyramidal cells (5.0 vs. 2.8 ms). The chord conductance of these channels in interneurons (12 pS) was significantly smaller than in pyramidal cells (15 pS). Single channel currents reversed polarity when the pipette potential was approximately -10 mV for both cell types. 5. These results show that there is a functional diversity of GABAA receptors in electrophysiologically and morphologically identified cortical pyramidal cells and interneurons. This diversity might derive from the different molecular composition of the receptors in these two cell types.

Action Potentials↗
PubMed1976

Convergence on interneurones mediating the reciprocal Ia inhibition of motoneurones. II. Effects from segmental flexor reflex pathways.

Interneurones identified as mediating the disynaptic reciprocal Ia inhibition of motoneurones (referred to as "Ia inhibitory interneurones") were recorded in the lumbar spinal cord of the cat. Volleys in ipsilateral and contralateral high threshold muscle afferents, cutaneous and high threshold joint afferents evoked a mixture of polysynaptic excitation and inhibition. These effects were ascribed to pathways activated by flexor reflex afferents (FRA) and in addition a specific ipsilateral low threshold cutaneous pathway. Ia inhibitory interneurones excited monosynaptically from flexor nerves received stronger net excitation by volleys in ipsilateral FRA than did extensor coupled interneurones, while the opposite pattern was seen from the contralateral FRA. These patterns are similar to those found in flexor and extensor motoneurones respectivey. The FRA inhibition in Ia inhibitory interneurones was partly mediated by "opposite" Ia inhibitory interneurones, i.e. those which are mediating the Ia inhibition of Ia inhibitory interneurones. The extent to which the FRA inhibition is transmitted by Ia inhibitory interneurones was roughly estimated by its susceptibility to recurrent depression by antidromic ventral root stimulation. The main conclusion is that most segmental pathways seem to evoke their effects in parallel to motoneurones and Ia inhibitory interneurones which are monosynaptically linked to the same muscle. The functional importance of this conclusion is discussed in a following report.

Animals↗
PubMed1976

Convergence on interneurones mediating the reciprocal Ia inhibition of motoneurones. III. Effects from supraspinal pathways.

Supraspinal effects were investigated in interneurones identified as mediating the disynaptic reciprocal Ia inhibition of motoneurones (referred to as Ia inhibitory interneurones). It was revealed that volleys in the vestibulospinal tract may evoke mono- and disynaptic EPSPs in interneurones monosynaptically excited from extensor muscles, i.e. extensor coupled Ia inhibitory interneurones. Flexor coupled interneurones instead received disynaptic inhibition. Volleys in the rubrospinal tract evoked a dominating polysynaptic excitation, usually mixed with inhibition, in flexor as well as extensor coupled interneurones. Disynaptic rubrospinal EPSPs and IPSPs were also revealed. The pyramidal tract also gives rise to a dominating polysynaptic excitation, usually mixed with inhibition, in flexor as well as extensor coupled Ia inhibitory interneurones. Rubrospinal and pyramidal volleys were shown to facilitate transmission in various segmental reflex pathways to the Ia inhibitory interneurones. A detailed comparison reveals a striking parallelism of segmental and supraspinal effects on alpha-motoneurones and Ia inhibitory interneurones connected to the same muscles. This considerably strengthens the hypothesis of an "alpha-gamma-linkage in the reciprocal inhibition".

Animals↗
PubMed1978

Graded synaptic transmission between local interneurones and motor neurones in the metathoracic ganglion of the locust.

1. In the metathoracic ganglion of the locust some neurones can effect changes in the membrane potential of identified post-synaptic motor neurones without themselves spiking. 2. These 'non-spiking' neurones have processes only within the metathoracic ganglion, and therefore are local intraganglionic interneurones. 3. The absence of spikes in the interneurones reflects their normal physiological state and is not due to the experimental conditions. 4. When the interneurones are depolarized by the injection of current pulses lasting several hundred milliseconds, post-synaptic motor neurones are either depolarized, or hyperpolarized, for the duration of the pulse. 5. The magnitude of the change in post-synaptic voltage is graded according to the amount of presynaptic current. 6. A number of physiological tests indicate that the graded effects upon motor neurones are mediated by chemical synaptic transmission. For example, an evoked hyperpolarization of a motor neurone can be reversed in polarity by simultaneously hyperpolarizing the motor neurone with injected current. 7. At their resting potential some interneurones tonically release sufficient transmitter to have a measurable post-synaptic effect. The injection of depolarizing and hyperpolarizing currents into these interneurones effects opposite changes in post-synaptic potential. 8. Other interneurones must be depolarized from resting potential before a post-synaptic effect is observed, and hyperpolarizing currents have no post-synaptic effect. In these interneurones it is estimated that a depolarization of only 2 mV is sufficient to effect the release of transmitter. 9. The membrane potentials of non-spiking interneurones can fluctuate by as much as 15 mV during active movements of the hind legs and individual p.s.p.s as large as 5 mV can be recorded. Therefore, summed p.s.p.s or even single ones are expected to be the electrophysiological signals effecting transmitter release from these interneurones.

Action Potentials↗
PubMed1980

The control of sets of motoneurones by local interneurones in the locust.

1. A motoneurone innervating a muscle in a hind leg of a locust is controlled in a graded manner by many non-spiking, local interneurones. There is overlap and fractionation of control between these interneurones. Some interneurones depolarize the motoneurone over part of its range, others hyperpolarize it, whilst some do both. 2. The interneurones organize the small number of motoneurones that innervate one muscle into overlapping sets of various sizes. A motoneurone can therefore be activated individually or in particular combinations with its fellow motoneurones. 3. The motoneurones innervating two muscles of a joint are also organized into overlapping sets by many local interneurones. This permits the motoneurones to the two muscles to be activated reciprocally, together, or independently. 4. One interneurone can elicit a co-ordinated movement of one, two or even three joints in a hind leg that are components of the normal behaviour of the locust. 5. A single interneurone acting alone does not usually elicit the maximum output from one motoneurone, nor a complete piece of behaviour. A stronger contraction of a muscle and a more complete movement results from the action of groups of interneurones. 6. It is suggested that local interneurones, exerting graded control over motoneurones are a major element in the organization of motor patterns in the locust.

Animals↗
PubMed1985

Sources of input to interneurones mediating group I non-reciprocal inhibition of motoneurones in the cat.

Intracellular recordings have been made from laminae V-VI interneurones interposed in pathways of non-reciprocal inhibition of motoneurones from group I afferents of triceps surae and/or plantaris. A comparison of actions of brief stretches of triceps surae and plantaris with actions of electrical stimulation of nerves of these muscles revealed that I a afferents influenced about a half of the interneurones excited by I b afferents. Electrical stimulation of seven muscles nerves, three cutaneous nerves, posterior knee joint and interosseous nerves, the red nucleus and the pyramidal tract, evoked excitatory post-synaptic potentials (e.p.s.p.s) in various proportions of interneurones. Most of the interneurones were excited monosynaptically, or both monosynaptically and disynaptically by group I afferents. This, together with a very small proportion of interneurones in which e.p.s.p.s were evoked only disynaptically, indicates that the same last-order interneurones may subserve di- and trisynaptically evoked inhibition of motoneurones. Other fibre systems excited these interneurones monosynaptically (interosseal, rubro- and corticospinal), disynaptically (cutaneous, rubro- and corticospinal) and polysynaptically. The coupling of the earliest e.p.s.p.s from group II and joint afferents could not be established, but was consistent with their mediation by only one or two interposed interneurones. Inhibition was evoked from all fibre systems with excitatory input to the same or to other interneurones of the sample, except from group II afferents, the effects of which were found in a much smaller number of cells and may not be fully representative.

Action Potentials↗
PubMed2003

NMDA and AMPA receptors contribute to the nicotinic cholinergic excitation of CA1 interneurons in the rat hippocampus.

In the hippocampus, glutamatergic inputs to pyramidal neurons and interneurons are modulated by alpha7* and alpha3beta4* nicotinic acetylcholine receptors (nAChRs), respectively, present in glutamatergic neurons. This study examines how nicotinic AMPA, and NMDA receptor nAChR activities are integrated to regulate the excitability of CA1 stratum radiatum (SR) interneurons in rat hippocampal slices. At resting membrane potentials and in the presence of extracellular Mg2+ (1 mM), nicotinic agonists triggered in SR interneurons excitatory postsynaptic currents (EPSCs) that had two components: one mediated by AMPA receptors, and the other by NMDA receptors. As previously shown, nicotinic agonist-triggered EPSCs resulted from glutamate released by activation of alpha3beta4* nAChRs in glutamatergic neurons/fibers synapsing directly onto the neurons under study. The finding that CNQX caused more inhibition of nicotinic agonist-triggered EPSCs than expected from the blockade of postsynaptic AMPA receptors indicated that this nicotinic response also depended on the AMPA receptor activity in the glutamatergic neurons synapsing onto the interneuron under study. Nicotinic agonists always triggered action potentials in CA1 SR interneurons. In most interneurons, these action potentials resulted from activation of somatodendritic AMPA receptors and alpha7* nAChRs. In interneurons expressing somatodendritic alpha4beta2* nAChRs, activation of these receptors caused sufficient membrane depolarization to remove the Mg2+-induced block of somatodendritic NMDA receptors; in these neurons, nicotinic agonist-triggered action potentials were partially dependent on NMDA receptor activation. Removing extracellular Mg2+ or clamping the neuron at positive membrane potentials revealed the existence of a tonic NMDA current in SR interneurons that was unaffected by nAChR activation or inhibition. Thus integration of the activities of nAChRs, NMDA, and AMPA receptors in different compartments of CA1 neurons contributes to the excitability of CA1 SR interneurons.

Animals↗
PubMed2002

alpha7 nicotinic acetylcholine receptors on GABAergic interneurons evoke dendritic and somatic inhibition of hippocampal neurons.

GABAergic interneurons in the hippocampus express high levels of alpha7 nicotinic acetylcholine receptors, but because of the diverse roles played by hippocampal interneurons, the impact of activation of these receptors on hippocampal output neurons (i.e., CA1 pyramidal cells) is unclear. Activation of hippocampal interneurons could directly inhibit pyramidal neuron activity but could also produce inhibition of other GABAergic cells leading to disinhibition of pyramidal cells. To characterize the inhibitory circuits activated by these receptors, exogenous acetylcholine was applied directly to CA1 interneurons in hippocampal slices, and the resulting postsynaptic responses were recorded from pyramidal neurons or interneurons. Inhibitory currents mediated by GABA(A) receptors were observed in 27/131 interneuron/pyramidal cell pairs, but no instances of disinhibition of spontaneous inhibitory events or GABA(B) receptor-mediated responses were observed. Two populations of bicuculline-sensitive GABA(A) receptor-mediated currents could be distinguished based on their kinetics and amplitude. Anatomical reconstructions of the interneurons in a subset of connected pairs support the hypothesis that these two populations correspond to inhibitory synapses located either on the somata or dendrites of pyramidal cells. In 11 interneuron/interneuron cell pairs, one presynaptic neuron was observed that produced strong inhibitory currents in several nearby interneurons, suggesting that disinhibition of pyramidal neurons may also occur. All three types of inhibitory responses (somatic-pyramidal, dendritic-pyramidal, and interneuronal) were blocked by the alpha7 receptor-selective antagonist methyllycaconitine. These data suggest activation of these functionally distinct circuits by alpha7 receptors results in significant inhibition of both hippocampal pyramidal neurons as well as interneurons.

Acetylcholine↗
PubMed2006

Same spinal interneurons mediate reflex actions of group Ib and group II afferents and crossed reticulospinal actions.

The aim of the study was to analyze interactions between neuronal networks mediating centrally initiated movements and reflex reactions evoked by peripheral afferents; specifically whether interneurons in pathways from group Ib afferents and from group II muscle afferents mediate actions of reticulospinal neurons on spinal motoneurons by contralaterally located commissural interneurons. To this end reticulospinal tract fibers were stimulated in the contralateral medial longitudinal fascicle (MLF) in chloralose-anesthetized cats in which the ipsilateral half of the spinal cord was transected rostral to the lumbosacral enlargement. In the majority of interneurons mediating reflex actions of group Ib and group II afferents, MLF stimuli evoked either excitatory or inhibitory postsynaptic potentials (EPSPs and IPSPs, respectively) or both EPSPs and IPSPs attributable to disynaptic actions by commissural interneurons. In addition, in some interneurons EPSPs were evoked at latencies compatible with monosynaptic actions of crossed axon collaterals of MLF fibers. Intracellular records from motoneurons demonstrated that both excitation and inhibition from group Ib and group II afferents are modulated by contralaterally descending reticulospinal neurons. The results lead to the conclusion that commissural interneurons activated by reticulospinal neurons affect motoneurons not only directly, but also by enhancing or weakening activation of premotor interneurons in pathways from group Ib and group II afferents. The results also show that both excitatory and inhibitory premotor interneurons are affected in this way and that commissural interneurons may assist in the selection of reflex actions of group Ib and group II afferents during centrally initiated movements.

Afferent Pathways↗
PubMed1985

The processing of mechanosensory information by spiking local interneurons in the locust.

The responses and receptive fields of a group of spiking local interneurons in the metathoracic ganglion of the locust were defined by making intracellular recordings from them while moving joints of a hindleg and stimulating external mechanoreceptors. Some interneurons respond both to inputs from internal mechanoreceptors (proprioceptors) at particular joints and to inputs from an array of external mechanoreceptors. The effects of both types of receptor can be excitatory or inhibitory. Other interneurons respond to proprioceptive input alone. There is a spectrum of responses. At one extreme are interneurons that respond tonically, the frequency of their spikes being determined by the angle of a particular joint. At the other extreme are interneurons that respond phasically to imposed movements of a joint in any direction. Inbetween are interneurons that respond with either a rapidly or a more slowly adapting change in the frequency of their spikes to the displacement of a joint in only one direction. Each movement of a particular joint excites or inhibits several interneurons with a range of different response characteristics. An interneuron typically receives inputs from only one joint, though some are excited by both femoral and tibial receptors. The interneurons spike during active movements of a leg elicited by direct stimulation of individual motor neurons, and during movements elicited by tactile stimulation of other parts of the body.

Animals↗
PubMed1986

Local nonspiking interneurons involved in gating of the descending motor pathway in crayfish.

Uropod motor neurons in the terminal abdominal ganglion of crayfish are continuously excited during the abdominal posture movement so that subthreshold excitatory postsynaptic potentials from the descending statocyst pathway can elicit spike activity in the motor neurons only while the abdominal posture system is in operation. Local nonspiking interneurons in the terminal ganglion were also found to show sustained membrane potential change during the fictive abdominal posture movement. Artificial membrane potential change of these interneurons by intracellular current injection in the same direction as that actually observed during the abdominal movement caused similar excitation of uropod motor neurons. Artificial cancellation of the membrane potential change of these interneurons during the abdominal movement also caused cancellation of the excitation of uropod motor neurons. We concluded that the continuous excitation of uropod motor neurons during the fictive abdominal movement was mediated, at least partly, by the local nonspiking interneurons. Fourteen (36%) out of 39 examined nonspiking interneurons were judged to be involved in the excitation of uropod motor neurons during the fictive abdominal movement. Another 25 interneurons (64%) were found not to be involved in the excitation of motor neurons, although most of them had a strong effect on the uropod motor neuron activity when their membrane potential was changed artificially. The interneurons that were involved in the excitation of motor neurons during the abdominal movement included both of the two major structural types of nonspiking interneurons in the terminal ganglion, i.e., those in the anterolateral portion and those in the posterolateral portion. No strict correlation was found between the structure of nonspiking interneurons and their function in the control of uropod motor neuron activity.

Abdomen↗
PubMed1995

Mechanisms of selective long-term potentiation of excitatory synapses in stratum oriens/alveus interneurons of rat hippocampal slices.

1. We investigated long-term potentiation (LTP) of synaptic transmission in different populations of interneurons in the CA1 region of rat hippocampal slices using whole cell recordings. We elicited excitatory postsynaptic currents (EPSCs) in interneurons located in stratum oriens near the alveus (O/A) or in stratum lacunosum-moleculare near the stratum radiatum border (L-M) by electrical stimulation of nearby axons in stratum oriens and radiatum, respectively. 2. High-frequency stimulation (100 Hz, 1 s) of axons in conjunction with postsynaptic depolarization (to -20 mV) increased the peak amplitude of test EPSCs elicited at -80 mV in O/A interneurons. The mean peak amplitude of EPSCs was significantly potentiated relative to the control period at 10 min (39 +/- 7% increase, mean +/- SE; n = 11 cells) and 30 min (30 +/- 1% increase; n = 5 cells) after tetanization. Similar stimulation did not produce potentiation of EPSCs in L-M interneurons (n = 7 cells). 3. This selective LTP in O/A interneurons was reversibly blocked by the N-methyl-D-aspartate receptor antagonist (+/-)2-amino-5-phosphonopentanoic acid (AP-5). Tetanization in the presence of 25 microM AP-5 did not increase the amplitude of EPSCs (8 cells). After washout of AP-5 (4 cells), a second tetanization resulted in long-term potentiation of EPSCs. 4. LTP was dependent on the activation of metabotropic glutamate receptors. The peak amplitude of EPSCs was not increased 5-10 or 15-20 min after tetanization during bath application of the metabotropic glutamate receptor antagonist (RS)-alpha-methyl-4-carboxyphenylglycine (500 microM) (n = 5 cells). 5. Inclusion of the Ca2+ chelator 1,2-bis(2-aminophenoxy)-ethane-N,N,N',N'-tetraacetic acid (BAPTA; 25 mM) in the patch pipette blocked LTP in O/A interneurons. In five cells recorded with BAPTA-containing electrodes, the mean peak amplitude was not significantly increased after tetanization. Thus a rise in postsynaptic intracellular Ca2+ appeared necessary for the induction of LTP in these interneurons. 6. Incubation of slices with the inhibitor of nitric oxide synthase N omega-nitro-L-arginine methyl ester (100 microM) before and throughout the recording session also blocked the increase in EPSC amplitude at 5-10 min (5 cells) and 15-20 min (3 cells) after tetanization. NO synthesis may therefore be necessary for LTP in O/A interneurons. 7. These results suggest that LTP of excitatory synapses is selectively produced in O/A but not L-M interneurons, and that this LTP shares similar characteristics with LTP in hippocampal CA1 pyramidal cells.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1995

Electrophysiological evidence that dentate hilar mossy cells are excitatory and innervate both granule cells and interneurons.

1. The hypothesis that dentate hilar "mossy" cells are excitatory was tested by simultaneous intracellular recording in rat hippocampal slices. Mossy cells were recorded simultaneously with their potential targets, granule cells and interneurons. The gamma-amino-butyric acid-A (GABAA) receptor antagonist bicuculline was used in most experiments to block the normally strong inhibitory inputs to granule cells that could mask excitatory effects of mossy cells. Some cells were recorded with electrodes containing the marker Neurobiotin so that their identity could be confirmed morphologically. 2. A mossy cell action potential was immediately followed by a brief depolarization in a granule cell in 20 of 1,316 pairs (1.5%) that were recorded in the presence of bicuculline. The mean amplitude of depolarizations was 1.99 +/- 0.24 (SE) mV when the postsynaptic membrane potential was -55 to -65 mV. Depolarizations could trigger an action potential if the granule cell was depolarized from its resting potential so that its membrane potential was -50 to -60 mV. These data suggest that mossy cells excite granule cells monosynaptically. 3. Monosynaptic excitation of an interneuron by a mossy cell was recorded in 4 of 47 (8.5%) simultaneously recorded mossy cells and interneurons, also in the presence of bicuculline. The mean interneuron depolarization was 1.64 +/- 0.29 mV when the interneuron membrane potential was approximately -60 mV. When an interneuron was at its resting potential (-52 to -63 mV), action potentials were often triggered by the depolarizations. 4. Without bicuculline present, mossy cells had no apparent monosynaptic effects on granule cells, as has been previously reported. However, effects that appeared to be polysynaptic were observed in 5 of 92 pairs (5.4%). Specifically, a small, brief hyperpolarization occurred in granule cells 2.5-7.3 ms after the peak of a mossy cell action potential. Given the results indicating that mossy cells excite interneurons, and the long latency to onset of the hyperpolarization, one possible explanation for the hyperpolarization is that mossy cells excited interneurons that inhibited granule cells. 5. The results suggest that mossy cells are excitatory neurons. In addition, mossy cells appear to innervate both granule cells and interneurons that are located within several hundred micrometers of the mossy cell soma. The only detectable effect on granule cells in this area under normal conditions appears to be disynaptic and inhibitory. However, when GABAA-receptor-mediated inhibition is blocked, monosynaptic excitation of granule cells by mossy cells can be detected.

2-Amino-5-phosphonovalerate↗
PubMed1996

Plasticity in the multifunctional buccal central pattern generator of Helisoma illuminated by the identification of phase 3 interneurons.

1. The mechanism for generating diverse patterns of buccal motor neuron activity was explored in the multifunctional central pattern generator (CPG) of Helisoma. The standard pattern of motor neuron activity, which results in typical feeding behavior, consists of three distinct phases of buccal motor neuron activity. We have previously identified CPG interneurons that control the motor neuron activity during phases 1 and 2 of the standard pattern. Here we identify a pair of interneurons responsible for buccal motor neuron activity during phase 3, and examine the variability in the interactions between this third subunit and other subunits of the CPG. 2. During the production of the standard pattern, phase 3 excitation in many buccal motor neurons follows a prominent phase 2 inhibitory postsynaptic potential. Therefore phase 3 excitation was previously attributed to postinhibitory rebound (PIR) in these motor neurons. Two classes of observations indicated that PIR was insufficient to account for phase 3 activity, necessitating phase 3 interneurons. 1) A subset of identified buccal neurons is inhibited during phase 3 by discrete synaptic input. 2) Other identified buccal neurons display discrete excitation during both phases 2 and 3. 3. A bilaterally symmetrical pair of CPG interneurons, named N3a, was identified and characterized as the source of phase 3 postsynaptic potentials in motor neurons. During phase 3 of the standard motor pattern, interneuron N3a generated bursts of action potentials. Stimulation of N3a, in quiescent preparations, evoked a depolarization in motor neurons that are excited during phase 3 and a hyperpolarization in motor neurons that are inhibited during phase 3. Hyperpolarization of N3a during patterned motor activity eliminated both phase 3 excitation and inhibition. Physiological and morphological characterization of interneuron N3a is provided to invite comparisons with possible homologues in other gastropod feeding CPGs. 4. These data support a model proposed for the organization of the tripartite buccal CPG. According to the model, each of the three phases of buccal motor neuron activity is controlled by discrete subsets of pattern-generating interneurons called subunit 1 (S1), subunit 2 (S2), and subunit 3 (S3). The standard pattern of buccal motor neuron activity underlying feeding is mediated by an S1-S2-S3 sequence of CPG subunit activity. However, a number of "nonstandard" patterns of buccal motor activity were observed. In particular, S2 and S3 activity can occur independently or be linked sequentially in rhythmic patterns other than the standard feeding pattern. Simultaneous recordings of S3 interneuron N3a with effector neurons indicated that N3a can account for phase-3-like postsynaptic potentials (PSPs) in nonstandard patterns. The variety of patterns of buccal motor neuron activity indicates that each CPG subunit can be active in the absence of, or in concert with, activity in any other subunit. 5. To explore how CPG activity may be regulated to generate a particular motor pattern from the CPG's full repertoire, we applied the neuromodulator serotonin. Serotonin initiated and sustained the production of an S2-S3 pattern of activity, in part by enhancing PIR in S3 interneuron N3a after the termination of phase 2 inhibition.

Animals↗
PubMed1997

Properties of unitary IPSCs in hippocampal pyramidal cells originating from different types of interneurons in young rats.

Whole cell recordings were used in hippocampal slices of young rats to examine unitary inhibitory postsynaptic currents (uIPSCs) evoked in CA1 pyramidal cells at room temperature. Loose cell-attached stimulation was applied to activate single interneurons of different subtypes located in stratum oriens (OR), near stratum pyramidale (PYR), and at the border of stratum radiatum and lacunosum-moleculare (LM). uIPSCs evoked by stimulation of PYR and OR interneurons had similar onset latency, rise time, peak amplitude, and decay. In contrast, uIPSCs elicited by activation of LM interneurons were significantly smaller in amplitude and had a slower time course. The mean reversal potential of uIPSCs was -53.1 +/- 2.1 (SE) mV during recordings with intracellular solution containing potassium gluconate. With the use of recording solution containing the potassium channel blocker cesium, the reversal potential of uIPSCs was not significantly different (-58.5 +/- 2.6 mV), suggesting that these synaptic currents were not mediated by potassium conductances. Bath application of the gamma-aminobutyric acid-A (GABA(A)) receptor antagonist bicuculline (25 microM) reversibly blocked uIPSCs evoked by stimulation of all interneuron subtypes. In bicuculline, the mean peak amplitude of uIPSCs recorded with potassium gluconate was reduced to 3.5 +/- 4.4% of control (n = 7). Similarly, with cesium methanesulfonate, the mean amplitude in bicuculline was 2.9 +/- 3.1% of control (n = 13). Application of the GABA(B) receptor antagonist CGP 55845A (5 microM) resulted in a significant and reversible increase in the mean amplitude of uIPSCs recorded with cesium-containing intracellular solution. Thus uIPSCs from all cell types appeared under tonic presynaptic inhibition by GABA(B) receptors. Paired stimulation of individual interneurons at 100- to 200-ms intervals did not result in paired pulse depression of uIPSCs. For individual responses, a significant negative correlation was observed between the amplitude of the first and second uIPSCs. A significant paired pulse facilitation (154.0 +/- 8.0%) was observed when the first uIPSC was smaller than the mean of all first uIPSCs. A small, but not significant, paired pulse depression (90.8 +/- 4.0%) was found when the first uIPSC was larger than the mean of all first uIPSCs. Our results indicate that these different subtypes of hippocampal interneurons generate Cl(-)-mediated GABA(A) uIPSCs. uIPSCs originating from different types of interneurons may have heterogeneous properties and may be subject to tonic presynaptic inhibition via heterosynaptic GABA(B) receptors. These results suggest a specialization of function for inhibitory interneurons and point to complex presynaptic modulation of interneuron function.

Animals↗
PubMed2000

Selective depolarization of interneurons in the early posttraumatic dentate gyrus: involvement of the Na(+)/K(+)-ATPase.

Interneurons innervating dentate granule cells are potent regulators of the entorhino-hippocampal interplay. Traumatic brain injury, a leading cause of death and disability among young adults, is frequently associated with rapid neuropathological changes, seizures, and short-term memory deficits both in humans and experimental animals, indicating significant posttraumatic perturbations of hippocampal circuits. To determine the pathophysiological alterations that affect the posttraumatic functions of dentate neuronal networks within the important early (hours to days) posttraumatic period, whole cell patch-clamp recordings were performed from granule cells and interneurons situated in the granule cell layer of the dentate gyrus of head-injured and age-matched, sham-operated control rats. The data show that a single pressure wave-transient delivered to the neocortex of rats (mimicking moderate concussive head trauma) resulted in a characteristic ( approximately 10 mV), transient (<4 days), selective depolarizing shift in the resting membrane potential of dentate interneurons, but not in neighboring granule cells. The depolarization was not associated with significant changes in action potential characteristics or input resistance, and persisted in the presence of antagonists of ionotropic and metabotropic glutamate, and GABA(A) and muscarinic receptors, as well as blockers of voltage-dependent sodium channels and of the h-current. The differential action of the cardiac glycosides oubain and stophanthidin on interneurons from control versus head-injured rats indicated that the depolarization of interneurons was related to the trauma-induced decrease in the activity of the electrogenic Na(+)/K(+)-ATPase. In contrast, the Na(+)/K(+)-ATPase activity in granule cells did not change. Intracellular injection of Na(+), Ca(2+)-chelator and ATP, as well as ATP alone, abolished the difference between the resting membrane potentials of control and injured interneurons. The selective posttraumatic depolarization increased spontaneous firing in interneurons, enhanced the frequency and amplitude of spontaneous inhibitory postsynaptic currents (IPSCs) in granule cells, and augmented the efficacy of depolarizing inputs to discharge interneurons. These results demonstrate that mechanical neurotrauma delivered to a remote site has highly selective effects on different cell types even within the same cell layer, and that the electrogenic Na(+)-pump plays a role in setting the excitability of hippocampal interneuronal networks after injury.

2-Amino-5-phosphonovalerate↗
PubMed1996

Lamprey spinal interneurons and their roles in swimming activity.

An isolated lamprey spinal cord generates rhythmic ventral root bursting that closely resembles swimming activity: ventral roots on opposite sides of the spinal cord burst in alternation, and rostral ventral roots lead caudal ventral roots. This rhythmic activity is induced by superfusion of the spinal cord with an excitatory amino acid and is called 'fictive' swimming. Three main types of spinal interneurons that are active during fictive swimming have been characterized: small excitatory interneurons with ipsilateral axons, large inhibitory interneurons with ipsilateral descending axons (lateral interneurons), and inhibitory commissural interneurons. The synaptic connectivities of these inter-neurons can account for the pattern of motoneuron excitation and inhibition occurring during fictive swimming, and it has been proposed that the synaptic interactions of these spinal neurons form the unit segmental oscillators of the swim network. Computer modeling has confirmed that this network can generate rhythmic activity resembling fictive swimming. The core of the model is the reciprocal inhibition between commissural interneurons on opposite sides of the cord. Evidence that the commissural interneurons are essential to rhythm generation comes from lesion studies in which the spinal cord was split down the midline and also from photo-ablation studies in which commissural interneurons were inactivated by illumination after retrograde labeling with a photo-toxic tracer. In both types of experiments, rhythmic activity of fictive swimming can be abolished, supporting the view that the commissural interneurons are necessary for rhythmogenesis.

Action Potentials↗
PubMed1986

Activity of crayfish abdominal-positioning interneurones during spontaneous and sensory-evoked movements.

The premotor interneurones that produce coordinated abdominal movements in crayfish (Procambarus) when stimulated directly, are also 'sensorimotor'. Sets of these interneurones respond in predictable ways to touching the body surface. One set of interneurones (type I) is activated to spiking by touch, while another (type II) receives only subthreshold influences. Several of these interneurones have overlapping receptive fields on the body surface. Touching areas of overlap activates groups of interneurones which discharge at low to moderate frequencies, rather than producing a high-frequency discharge of a single cell. No single positioning interneurone has been identified which is solely responsible for a "voluntary' (spontaneous) motor programme. When active, the positioning interneurones contribute to the production of the behaviour as a member of a constellation of such cells. The results show that this motor system comprises interneurones with sensory as well as motor properties. Although single cells can produce coordinated movements when stimulated at high frequencies, these positioning interneurones appear to function as 'command elements' within a large 'command system' and not as individual units.

Abdomen↗