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PubMed2002

Functional connectivity between neurons generating resting discharge in renal sympathetic neurons in the rabbit.

In anaesthetised rabbits, we analysed the cross-correlations of resting discharges in pairs of simultaneously recorded renal neurons. The study of 428 significant cross-correlations showed three pure types of neuronal co-ordination and two combined effects. Pure shared input was observed in 202 pairs of neurons (47.2%). The width of cross-correlogram peak was 222.9 +/- 7.2 ms. The peak height of shared input was 4.8 +/- 0.2 spikes s(-1) and was significantly related with correlogram width. Pure excitatory connection was found in 24 neuronal pairs (5.6%). Its peak width was 7.9 +/- 1.9 ms and it was shifted from time zero by 13.3 +/- 3.4 ms. The amplitude of the peak was 19.6 +/- 4.6 spikes s(-1). In 165 pairs of neurons (38.6%), excitatory connection was combined with shared input component. Pure reciprocal activation was seen in only 5 pairs of neurons (1.1%) while in 32 pairs (7.5%) it was accompanied by shared input co-ordination. The distance between narrow peaks measured in the combined type of co-ordination was 32.5 +/- 3.5 ms and the mean peak height was 4.1 +/- 0.7 spikes s(-1). Shared input in pure form and that accompanying both excitatory connection and reciprocal activation were significantly related to the frequency of discharge of neurons contributing to the cross-correlogram. The incidence and basic properties of the encountered types of neural coordination may indicate a pattern of interconnections between cells generating vasomotor tone in renal sympathetic neurons.

Action Potentials↗
PubMed1999

The lateralized readiness potential: relationship between human data and response activation in a connectionist model.

Psychophysiological measures such as the lateralized readiness potential (LRP) have been used to study information processing in the Eriksen flankers task. The data provided by these measures are consistent with a continuous flow theory, which proposes that the output of stimulus evaluation is continuously available to the response channels. Cohen et al. (1992) realized this theory in a connectionist model and showed that its behavior corresponded to that of human subjects in the flankers task. We report here a modification of the model and an analysis of the degree to which simulated LRPs (based on the activation functions of the response units of the model) resemble the actual LRPs of human subjects in the same task. Across a variety of different experimental conditions and outcomes, there was a marked correspondence between the simulated and actual LRPs. These observations strengthen the propriety of the connectionist model and of the continuous flow theory on which it is based.

Attention↗
PubMed1976

Transient responses of rabbit retinal ganglion cells to photic and electrical stimuli.

The relationships between the center and the surround of the receptive field of the rabbit retinal ganglion cell were investigated. This was done by coupling localized light spots and electrical activation of the retina and by analyzing the time of the excitatory and inhibitory periods. The responsiveness to the electrical transretinal pulse revealed a) that ON stimulation in OFF-center cells and OFF stimulation in ON-center cells, elicited a primary period of inhibition with a short latency; b) the long latency response of surround stimulation was not preceded by an inhibitory period unless the center was simultaneously stimulated in the same direction; c) a transient response to a stationary spot of light is followed by a period of inhibition. These results are discussed in relation to the known cellular retinal networks.

Animals↗
PubMedDate not supplied

Activation of glutamate transporters in rods inhibits presynaptic calcium currents.

We found that L-glutamate (L-Glu) inhibits L-type Ca2+ currents (ICa) in rod photoreceptors. This inhibition was studied in isolated rods or rods in retinal slices from tiger salamander using perforated patch whole cell recordings and Cl(-)-imaging techniques. Application of L-Glu inhibited ICa by approximately 20% at 0.1 mM and approximately 35% at 1 mM. L-Glu also produced an inward current that reversed around ECl. The metabotropic glutamate receptor (mGluR) agonists t-ADA (Group I), DCG-IV (Group II), and L-AP4 (Group III) had no effect on ICa. However, the glutamate transport inhibitor, TBOA (0.1 mM), prevented L-Glu from inhibiting ICa. D-aspartate (D-Asp), a glutamate transporter substrate, also inhibited ICa with significantly more inhibition at 1 mM than 0.1 mM. Using Cl imaging, L-Glu (0.1-1 mM) and D-Asp (0.1-1 mM) were found to stimulate a Cl- efflux from terminals of isolated rods whereas the ionotropic glutamate receptor agonists NMDA, AMPA, and kainate and the mGluR agonist, 1S,3R-ACPD, did not. Glutamate-evoked Cl- effluxes were blocked by the glutamate transport inhibitors TBOA and DHKA. Cl- efflux inhibits Ca2+ channel activity in rod terminals (Thoreson et al. (2000), Visual Neuroscience 17, 197). Consistent with the possibility that glutamate-evoked Cl- efflux may play a role in the inhibition, reducing intraterminal Cl- prevented L-Glu from inhibiting ICa. In summary, the results indicate that activation of glutamate transporters inhibits ICa in rods possibly as a consequence of Cl- efflux. The neurotransmitter L-Glu released from rod terminals might thus provide a negative feedback signal to inhibit further L-Glu release.

Amino Acid Transport System X-AG↗
PubMedDate not supplied

Do neocortical pyramidal neurons display stochastic resonance?

Neocortical pyramidal neurons in vivo are subject to an intense synaptic background activity that has a significant impact on various electrophysiological properties and dendritic integration. Using detailed biophysical models of a morphologically reconstructed neocortical pyramidal neuron, in which synaptic background activity was simulated according to recent measurements in cat parietal cortex in vivo, we show that the responsiveness of the cell to additional periodic subthreshold stimuli can be significantly enhanced through mechanisms similar to stochastic resonance. We compare several paradigms leading to stochastic resonance-like behavior, such as varying the strength or the correlation in the background activity. A new type of resonance-like behavior was obtained when the correlation was varied, in which case the responsiveness is sensitive to the statistics rather than the strength of the noise. We suggest that this type of resonance may be relevant to information processing in the cerebral cortex.

Action Potentials↗
PubMedDate not supplied

Turning on and off with excitation: the role of spike-timing asynchrony and synchrony in sustained neural activity.

Delay-related sustained activity in the prefrontal cortex of primates, a neurological analogue of working memory, has been proposed to arise from synaptic interactions in local cortical circuits. The implication is that memories are coded by spatially localized foci of sustained activity. We investigate the mechanisms by which sustained foci are initiated, maintained, and extinguished by excitation in networks of Hodgkin-Huxley neurons coupled with biophysical spatially structured synaptic connections. For networks with a balance between excitation and inhibition, a localized transient stimulus robustly initiates a localized focus of activity. The activity is then maintained by recurrent excitatory AMPA-like synapses. We find that to maintain the focus, the firing must be asynchronous. Consequently, inducing transient synchrony through an excitatory stimulus extinguishes the sustained activity. Such a monosynaptic excitatory turn-off mechanism is compatible with the working memory being wiped clean by an efferent copy of the motor command. The activity that codes working memories may be structured so that the motor command is both the read-out and a direct clearing signal. We show examples of data that is compatible with our theory.

Action Potentials↗
PubMed2001

Axons of sacral preganglionic neurons in the cat: I. Origin, initial segment, and myelination.

Parasympathetic preganglionic neurons in the cat sacral spinal cord innervate intraspinal neurons and pelvic target organs. Retrograde tracing studies have revealed little of the morphology of their axons including their origin, initial segments, or their myelin, due to methodological limitations. Intracellular labeling of single neurons with neurobiotin or HRP has overcome these problems. Axons were studied in 24 preganglionic neurons. In 21 neurons the axon originated as a branch of a dendrite, without a detectable axon hillock, at distances from the soma ranging from 10 to 110 microm (average 34.1 microm ). In 3 neurons the axon was derived from the soma. Initial segments, present in all cells, ranged from 15 to 40 microm (average 26.8 microm). Nearly all axons followed the initial segment with unmyelinated segments that varied between 59 to 630 microm, followed by myelin and nodes of Ranvier. Internodal distances were variable and relatively short (average 93 microm). Axonal diameters measured over the intraspinal course in 18 axons averaged 1.3 microm (range 0.6-2.4 microm) and were relatively constant compared with other neurons. Spine-like protrusions were observed on the initial segments of 12 cells. Axon collaterals originated from unmyelinated sections and nodes of Ranvier. Antidromic action potentials showing initial segment, soma-dendritic inflections, did not differentiate between soma-derived and dendrite-derived axons. The data suggest that axons originating from a dendrite are the normal structure of preganglionic neurons in the lateral sacral parasympathetic nucleus. It is proposed that the particular structure of these axons may be part of a timing mechanism that coordinates preganglionic neurons with other spinal neurons involved in target organ reflexes.

Animals↗
PubMed2004

Antagonism by local anesthetics of sodium channel activators in the presence of scorpion toxins: two mechanisms for competitive inhibition.

1. The interaction of veratridine (VTD), a Na+ channel activator, scorpion alpha-toxin (LQ), an open state Na+ channel stabilizer, and the local anesthetic, lidocaine (LID), a channel inhibitor, at the neuronal sodium channel was assessed by measuring VTD-dependent slow depolarizations of frog sciatic nerve using the sucrose-gap method. 2. The slow depolarizing action of veratridine was potentiated more than 10-fold by the peptide LQ toxin, whereas its competitive inhibition by lidocaine was unchanged by LQ. 3. We conclude that the antagonism between VTD and a LID molecule during slow depolarization is allosteric, involving a trapping of the Na+ channel by LID in the inactivated state that has a very low affinity for VTD. 4. The binding of VTD to the open state of the channel, which is stabilized by LQ, may be inhibited by orthosteric competition at overlapping sites since both LID and VTD bind avidly and rapidly to open channels.

Anesthetics, Local↗
PubMed2000

ACTH activates rapid eye movement-related phasic inhibition during REM sleep in patients with infantile spasms.

OBJECTIVES: Phasic inhibition index (PII) is the rate of the simultaneous occurrence of phasic chin muscle activity (PCMA) and rapid eye movement bursts during rapid-eye-movement sleep (REMS). In naive patients with infantile spasms (IS), the PII value was found to reflect their prognosis. We studied the effects of adrenocorticotropic hormone (ACTH) on REMS components including PII in IS. METHODS: REMS parameters were examined in 18 IS patients before and after ACTH treatment. The effects of corticosteroids (CSs) were examined in 3 patients with congenital adrenal hyperplasia (CAH) and 3 with nephrotic syndrome (NS). RESULTS: ACTH decreased PII and PCMA in IS patients. In CAH patients, physiological doses of CSs corrected the increased intrinsic ACTH level and increased PII. In NS patients, therapeutic doses of CSs suppressed PCMA without affecting PII. CONCLUSION: ACTH suppressed PCMA through CSs, and reduced PII directly. ACTH was hypothesized to eliminate IS through these dual modes of action.

Adolescent↗
PubMed1997

Facilitation and inhibition of the acoustic startle reflex in the rat after a momentary increase in background noise level.

Small increments in background noise were shown to increase the amplitude of a subsequently elicited acoustic startle reflex (ASR) in rats by as much as 100% under optimal conditions. Increment lead time (5-160 ms) and level (1.5-15 dB), initial noise level (30-70 dB), startle level (95-125 dB), number of test days (1-5), and drug condition (diazepam or saline ip) were varied in 6 experiments. Prepulse facilitation (PPF), measured by difference scores, was greatest for intermediate increments (3 dB) and lead times (20-40 ms) and was replaced by prepulse inhibition (PPI) for higher values, especially in the later test days. Diazepam reduced baseline ASR and diminished PPI, but it did not affect PPF. These data argue against hypotheses that attribute PPF of this sort to either temporal integration within the ASR pathways or to the elicitation of a nonspecific arousal reaction by the prepulse.

Acoustic Stimulation↗
PubMed1998

Taste responses in the greater superficial petrosal nerve: substantial sodium salt and amiloride sensitivities demonstrated in two rat strains.

A great quantity of research has focused on neural responses of the chorda tympani nerve (CT) to taste stimuli. This report examined salt and sugar sensitivity of the greater superficial petrosal nerve (GSP) and the effect of amiloride on these neural responses. In addition to Sprague-Dawley (SD) rats that have CT responses typical of most rat strains, we included Fischer 344 (F344) rats whose CT responses to sodium chloride (NaCl) are higher than those of other strains. After a stimulation series in which water served as the rinse, a series of stimuli was presented in 100 microM amiloride. The GSP was highly responsive to NaCl, sodium acetate (NaAc), ammonium chloride, and sucrose; NaCl and NaAc responses were strongly suppressed by amiloride. Relative responses to NaCl were significantly higher in F344 than in SD rats. In summary, the GSP is highly sensitive to salt and sugar stimulation, and palatal taste receptors have a considerable degree of amiloride sensitivity.

Afferent Pathways↗
PubMedDate not supplied

Development of X- and Y-cell retinogeniculate terminations in kittens.

The cat retinogeniculocortical pathways are organized chiefly into two parallel independent neuronal streams, one involving X-cells of the retina and lateral geniculate nucleus, and the other, Y-cells. Development of the Y-cell pathway is more seriously affected by visual deprivation than is the X-cell pathway and we reasoned that some insight into the underlying mechanisms of these effects could be gained from studies of normal development. We therefore injected horseradish peroxidase into physiologically identified X- and Y-cell retinogeniculate axons to examine the postnatal development of their terminations in kittens. As we report here, at 3-4 weeks of age, most optic tract axons can be identified physiologically as members of the X- or Y-cell class. X-cell terminal fields in lamina A or A1 are wider at 3-4 weeks than they are in adults, while Y-cell terminal fields are narrower than in adults. During the second and third postnatal months, X-cell terminal arbors progressively contract while Y-cell arbors expand so that, by 12 weeks of age, the adult pattern is seen. These data, and the results of our earlier study of the effects of monocular lid suture on these terminal arbors, suggest that enlargement of Y-cell terminations in geniculate lamina A or A1 during development may be accompanied by competitive pruning of X-cell terminations within these same laminae.

Animals↗