Hemorrhage activates catecholaminergic neurons sensitive to GABA in the nucleus of the solitary tract with ascending projections to the subfornical organ in rats

The activity of neurons in the region of the nucleus of the solitary tract (NTS) that were antidromically identified by electrical stimulation of the rat subfornical organ (SFO) was tested for a response to microiontophoretic application of gamma-aminobutyric acid (GABA), hemorrhage (10 ml/kg b.w.t.), or local administration of the chemical neurotoxin, 6-hydroxydopamine (6-OHDA), into the SFO stimulation site. Microiontophoretically (MIPh) applied GABA caused a decrease excitability in 22 out of 24 neurons tested, and the inhibition was blocked by MIPh-applied bicuculline, a GABAA antagonist, but not by phaclofen, a GABAB antagonist. Of these neurons that responded to GABA, 17 displayed an increase in neural firing in response to hemorrhage, while 5 were unresponsive. The occurrence of both antidromic spikes and post-stimulus inhibition of 9 out of 13 neurons tested was completely abolished by the injection of 6-OHDA into the SFO. These results suggest that neurons in the region of the NTS, which carry peripheral baroreceptor information to the SFO, receive GABAergic inhibitory inputs via a GABAA receptor mechanism, and imply that part of these neurons are catecholaminergic.     

Study of the H-reflex of human hand muscles using the post-stimulus histogram method

The method of post-stimulus histograms of the single motor unit potentials permits the under-threshold facilitation to be revealed and the antidromic blocking of H-reflex to be avoided up to sub-maximal for M-response stimuli. The H-reflex was revealed only under abnormal conditions and was not observed in the noraml subjects. In the experiments the motoneurons responded to impulses from above, therefore the inhibition of the motoneurons itself cannot be a mechanism of the H-reflex inhibition. The alternative explanation is the presynaptic inhibition.     

Intrinsic regulation of locus coeruleus neurons: electrophysiological evidence indicating a predominant role for autoinhibition

Locus coeruleus neurons were antidromically activated and the resulting post-stimulation inhibition was compared to the interspike interval and examined for its dependency on antidromic invasion and stimulus intensity. The post-stimulation inhibition seen in these cells following antidromic activation approximated the interspike interval, was critically dependent on the antidromic invasion of the cell inder study and was only weakly dependent on stimulus intensity. These results suggest that the post-stimulation inhibition following antidromic activation in the locus coeruleus is mediated principally by autoinhibition and not by hypothesized local inhibitory interactions between locus coeruleus neurons.     

Neurophysiological characterization of commissurally projecting dentate neurons in the rat

Commissural neurons in the dentate hilus and in the deep dentate granule cell layer were recorded intracellularly in vivo, in conjunction with combined injection of the antrograde tracer Phaseolus vulgaris leucoagglutinin (PHA-L) at sites of electrical stimulation. Two hilar neurons responded with short latency antidromic spikes to stimulation of the contralateral dentate infrapyramidal molecular layer, but did not show any synaptic potentials, suggesting that these neurons do not receive commissural hilar input, either directly or indirectly, from the stimulating sites. On the other hand, 3 dentate-hilar border neurons responded to the contralateral hilar stimulation with antidromic spikes, excitatory postsynaptic potentials (EPSPs), orthodromic spikes, and inhibitory PSPs (IPSPs), suggesting a rich synaptic interaction both commissurally and locally in this region. No direct commissural inhibition was observed in any of the cells. PHA-L injection at the stimulation site indicated that commissural hilar axon terminals project to a limited region of the contralateral molecular layer in a lamellar fashion, and have only a sparse distribution in the contralateral hilus. The results indicate that rapidly conducting commissural neurons in the dentate gyrus are themselves inhibited in an indirect manner by commissural fibers.     

Tuberoinfundibular neurons in the basomedial hypothalamus of the rat: electrophysiological evidence for axon collaterals to hypothalamic and extrahypothalamic areas.

In pentobarbital or urethane anesthetized rats, the activity of 889 mediobasal hypothalamic neurons was studied for evidence of a response to median eminence stimulation. Evidence of antidromic invasion, which indicated a projection to the median eminence, identified 134 cells (15%) as 'tuberoinfundibular' neurons. Antidromic spike latencies ranged from 0.5 to 14.0 msec (4.3 +/- 2.9 S.D.); conduction velocities were under 1.0 m/sec and were generally slower for tuberoinfundibular neurons located closest to the ventral surface of the hypothalamus. Certain tuberoinfundibular neurons followed paired median eminence shocks at frequencies up to 500 Hz; an increase in both the threshold and the latency for the second antidromic spike was observed with interstimulus intervals under 4 msec. Only 38% of tuberoinfundibular neurons were spontaneously active; 24 of 29 spontaneously active neurons displayed evidence of recurrent inhibition with durations up to 150 msec and at latencies which approximated that of the antidromic spike but which did not depend upon antidromic invasion. Similar responses were observed from 33 spontaneously active non-tuberoinfundibular neurons. Evidence of orthodromic excitation in response to median eminence shocks was observed from 22 other medial hypothalamic neurons. Latencies for excitation ranged from 1.5 to 9.0 msec (mean 4.5 +/- 2.1 S.D.). Simultaneous antidromic invasion from other hypothalamic and extrahypothalamic sites was observed from 8 tuberoinfundibular neurons. These sites included the anterior hypothalamic area (2 cells), the preoptic area (3 cells) and the thalamic nucleus medialis dorsalis (3 cells). These results indicate the presence of axon collaterals within the tuberoinfundibular system; some appear to terminate locally within the hypothalamus, while others extend rostrally and dorsally into extrahypothalamic areas. These connections may provide pathways for extrahypothalamic distribution of peptides which regulate adenohypophyseal secretion, and suggest that these peptides may subserve alternate regulatory roles within the central nervous system.     

Physiological properties of the output neurons in the deep layers of the superior colliculus of the rabbit

Using antidromic and orthodromic stimulation techniques, we studied physiological properties of the output neurons in the deep layers of the superior colliculus (SC) of 34 New Zealand rabbits. SC cells antidromicaly activated from the contralateral predorsal bundle (PDB) could also be activated by stimulation of the contralateral SC and ipsilateral central lateral nucleus of the thalamus (CL). The majority of these output neurons responded predominantly to the stimulation of the optic nerve, and only a small proportion of the output neurons were responsive to the stimulation of somatosensory and auditory (and/or vestibular) nerves. These results suggest that the orienting reflex might be elicited mainly by visual afferents in the rabbit The output SC neurons were subject to a 70 ms inhibition after antidromic stimulation of the PDB and a 40 ms inhibition after transsynaptic (orthodromic) stimulation of the optic chiasm (OX), indicating that the output neurons in the deep layers of the SC might be subject to at least two inhibitory circuits. These results are discussed in the context of a putative saccadic suppression circuitry model.     

The characteristics of the antidromic discharge of cat dorsal raphe neurons during repetitive activation

Slowly discharging neurons in the cat dorsal raphe could be classified into 3 types according to the behavior of antidromic spike discharges during repetitive stimulation of the medial forebrain bundle at 10 Hz. In the types 1 and 2, the latency of antidromic discharge was gradually prolonged to reach an asymptote, whereas no marked change occurred in the type 3. The type 2 neurons, which had a slower conduction velocity, showed a greater prolongation than the type 1 neurons. The maximum length of this prolongation was not significantly correlated with the initial latency. During 10 Hz stimulation some neurons showed repeatedly a conduction block after a sequence of initial decrease and later increase in latency. The spontaneous discharge was strongly suppressed during 10 Hz stimulation. During 1 Hz stimulation just after the cessation of 10 Hz stimulation, the prolonged antidromic latency was gradually restored in parallel with the recovery of the spontaneous discharge. Circumstantial evidences seem to be in favor of the idea that hyperpolarization of the axonal and somatic membranes is mainly responsible for the observed behavior of antidromic spikes of type 1 and 2 neurons.     

Protective cap over CA1 synapses: extrasynaptic glutamate does not reach the postsynaptic density

Numerous data indicate that nonsynaptic release of glutamate occurs both in normal and pathophysiological conditions. When reaching receptors in the postsynaptic density (PSD), glutamate (Glu) could affect the synaptic transmission. We have tested this possibility in the hippocampal CA1 synapses of rats, either by applying exogenous Glu to the CA1 neurons or by disruption of Glu transporter activity. L-Glu (400 microM) was directly applied to the hippocampal slices acutely isolated from the rats. It produced a strong inhibition of both ortho- and antidromically elicited action potentials fired by CA1 neurons while the excitatory postsynaptic current (EPSC) measured in these neurons remained totally unaffected. The optical isomer D-Glu which is not transported by the systems of Glu uptake inhibited not only orthodromic and antidromic spikes, but also EPSC. Non-specific glutamate transporter inhibitor DL-threo-beta-hydroxyaspartic acid (THA, 400 microM) mimicked the effects of exogenous Glu and produced strong inhibition of both orthodromic and antidromic spikes, without any influence on the amplitude of EPSCs. Dihydrokainate (DHK, 300 microM), selective inhibitor of GLT-1 subtype of glutamate transporter, exerted a significant inhibitory action on the orthodromically evoked spikes and also on the EPSC. Our results indicate that extrasynaptic and PSD membranes of CA1 neurons form separate compartments differing in the mechanisms and efficiency of external Glu processing: the protection of PSD markedly prevails.     

Excitation and inhibition processes in the neurons of the thalamic motor nuclei of normal cats and following an N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-induced lesion of the nigrostriatal dopaminergic system

Peculiarities of excitation and inhibition evoked in motor thalamic nuclei (VA-VL) neurons by electrical stimulation of red nucleus were studied on intact cats and after injection of N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP; 5 mg/kg i.m., p.d. during five days). Two days after the last injection as much as 48% of nigral neurons were destroyed and the content of dopamine in the caudate nucleus fell to 30% as compared to intact animals. Before acute experiments all cats were anaesthetized with ketalar and immobilized with myorelaxine. It was found that MPTP injections caused a decrease of the inhibition duration and effectiveness in relay and nonrelay VA-VL neurons. The inhibition deficiency was accompanied by shortening of latencies of orthodromic responses evoked by red nucleus stimulation and facilitation of antidromic spikes invasion into somata of relay neurons after motor cortex stimulation. It was suggested that the reduction of GABAergic nigro-thalamic influences modulated by dopamine underlay the developing deficiency of inhibition.     

Neuronal activity of the substantia nigra (pars compacta) after injection of kainic acid into the caudate nucleus

In rats the caudate nucleus (Cd) of one side was injected with 2.5 μg kainic acid (KA) in 1 μl saline and that of the other side with saline of the same volume. Three to sixty days later, neuronal activities of the substantia nigra were examined by stimulating the caudate nucleus on each side. Two major types of activity were distinguished: pure inhibition and antidromic excitation followed by inhibition. With 65 substantia nigra units sampled from the side of saline injection, the frequency of incidence for the pure inhibition was 58.5% and that for the antidromic excitation followed by inhibition was 21.5%. In five units (7.7%), caudate stimulation remained ineffective. A total of 97 substantia nigra units was recorded from the side of KA injection and characterized as follows: (i) 42.3% had no effect of caudate stimulation, this being much higher than the control. Pure inhibition was encountered at a reduced frequency (29.9%) and its duration was markedly shortened. These findings are interpreted to mean that GABAergic strionigral inhibitory neurons were killed or damaged by locally injected KA. (ii) Antidromic excitation was seen as frequently as in the control side (19.6%). Inhibition following it was not reduced in duration. The antidromic inhibition of substantia nigra neurons from the caudate nuclei was suggested to be due to dopaminergic self-inhibition as proposed by Groves and his co-workers.     

Electrophysiological properties of neurons in the caudal ventrolateral medulla projecting to the paraventricular nucleus of the hypothalamus in rats

Extracellular recordings were made from neurons in the caudal ventrolateral medulla in urethane-chloralose-anesthetized rats. Stimulation of the paraventricular nucleus (PVN) in the hypothalamus evoked antidromic action potentials in 71 neurons. On the basis of antidromic spike latencies, these neurons could be divided into fast- (24 neurons) and slow-conducting cell groups (47 neurons). Slow-conducting cells showed irregular and slow spontaneous discharges, while a majority of the fast-conducting cells did not show spontaneous discharges. The spontaneous activity of slow-conducting cells was suppressed by i.v. clonidine administration. The effects of clonidine could be consistently reversed by administration of the alpha 2-adrenergic antagonist, yohimbine. The responses by clonidine and yohimbine remained unimpaired in baroreceptor-denervated rats. Vagus nerve stimulation produced an excitation in 80% of slow-conducting cells tested. Baroreceptor activation induced by i.v. administration of phenylephrine inhibited about half of slow-conducting cells tested. Similar elevation of blood pressure in baroreceptor-denervated rats did not show any effect. These physiological and pharmacological properties of slow-conducting cells were similar to those previously reported for catecholaminergic cells in other parts of the brain. The results show the existence of two different populations among neurons in the caudal ventrolateral medulla which project directly to the PVN, and suggest that the presumed A1 catecholaminergic cells are involved in the afferent pathway from cardiovascular baroreceptors and the vagus nerve to the PVN.     

Baclofen increases the potassium conductance of rat locus coeruleus neurons recorded in brain slices

Baclofen causes a concentration-dependent inhibition of spontaneous firing, hyperpolarization and resistance decrease in locus coeruleus (LC) neurons recorded intracellularly in a brain slice preparation. The (-) isomer is active while the (+) isomer has little or no activity which indicates that the baclofen effect is stereoselective. Baclofen action on LC neurons is a direct postsynaptic effect since it remains in low Ca2+, high Mg2+ media. Baclofen actions on LC neurons are resistant to the GABAA antagonist bicuculline. The baclofen-induced hyperpolarization reverses at the K+ equilibrium potential, as estimated by the reversal potential of the post-stimulus hyperpolarization which follows an evoked train of action potentials. When the K+ concentration in the superfusion media is increased, the reversal potential for the baclofen-induced hyperpolarization shifts linearly with a slope of 61 mV per 10-fold change as predicted by the Nernst equation for a pure K+ conductance. The baclofen-induced K+ conductance increase is prevented by addition of the K+-channel blocker Ba2+ to the external media. Taken together, these data suggest that baclofen directly hyperpolarizes LC neurons by activation of GABAB receptors which leads to an increase in K+ conductance.     

Evidence for self- and neighbor-mediated postactivation inhibition of locus coeruleus neurons

Activation of locus coeruleus (LC) neurons is typically followed by inhibition of impulse activity lasting hundreds of ms. Previous studies have implicated two possible mechanisms for this postactivation inhibition: collateral synaptic interactions among LC neurons; and spike-induced, calcium-activated potassium conductance in the soma-dendritic membrane of LC cells. In the present study, antidromic or sensory stimuli were presented at near-threshold intensities for activation of LC neurons. A special computer program accumulated activity for trials yielding driven responses separately from that for trials of identical stimuli during the same train that failed to evoke activity. We found significant inhibition of LC impulse activity for antidromic or sensory stimuli that failed to excite the recorded cell as well as for stimuli that activated the recorded cell. The former result precludes an essential role of intrinsic inhibitory membrane currents (e.g. calcium-activated potassium conductance) in generating postactivation inhibition. Administration of the alpha antagonist piperoxane reduced the magnitude of inhibition on both driven and non-driven trials. Our findings indicate that inhibition on non-driven trials appears to be a synaptically mediated phenomenon, perhaps reflecting norepinephrine released from neighboring LC neurons that are activated. Furthermore, our data support the presence of a spike-dependent mechanism that also contributes substantially to postactivation inhibition in these cells. Thus, the overall results indicate the presence of two intracoerulear mechanisms that mediate postactivation inhibition characteristic of noradrenergic LC neurons.     

Modulation of recurrent inhibition in cat association cortex by reticulocortical arousal

Lesion-induced degeneration and single unit techniques were used to study the effects of reticular arousal and antidromic callosal stimuli on middle suprasylvian gyrus neurons in the cat. In acute experiments done 2 weeks–6 mo after ablation of the contralateral homotypical cortex, most neurons showed inhibitory responses to 400 Hz callosal volleys. Excitatory reticular stimulation 300 msec prior to the callosal stimulus increased the duration of the antidromic callosal inhibition. An interpretation of the data is made in terms of recurrent inhibitory circuits in association cortex.     

Inhibitory action of the ventral noradrenergic bundle on the lateral hypothalamic neurons through alpha-noradrenergic mechanisms in the rat

Electrical stimulation of the ventral noradrenergic bundle (V-NA bundle) produced 3 types of responses in lateral hypothalamic neurons: IPSPs, a polysynaptic EPSP-IPSP sequence and antidromic spikes. The IPSPs were considered to be monosynaptic due to the fixed latencies seen at stimulus intensities. Iontophoretic application of an alpha-NA antagonist blocked only the presumed monosynaptic inhibition. Most of the glucose-sensitive neurons were inhibited by stimulation of the V-NA bundle. These results may account for the hyperphagia and obesity produced by selective lesions of the V-NA bundle.     

Eye movement related neurons in the cat pontine reticular formation: projection to the flocculus

This study examines projection to the cerebellar flocculus of eye movement-related neurons in the median and paramedian part of the cat pontine tegmentum between the trochlear and the abducens nucleus. They were identified by rhythmic activity related to horizontal vestibular nystagmus induced by sinusoidal rotation. These neurons were classified into several groups by their discharge patterns during nystagmus, using criteria of earlier studies on saccadic eye movements and vestibular nystagmus in the monkey. Electrical stimulation of the ipsilateral flocculus elicited antidromic spike responses in a number of burst-tonic neurons and long-lead and medium-lead burst neurons. These neurons were located in and around the medial longitudinal fasciculus, the nucleus raphe pontis and the nucleus reticularis tegmenti pontis. A few neurons tested were also activated antidromically by stimulation of the contralateral flocculus. In contrast, no pauser neurons were activated from the ipsi-lateral flocculus. It is concluded that eye movement-related neurons in the medial pontine tegmentum, except for pauser neurons, directly project to the flocculus and may convey information about eye movements of visual and vestibular origins to the flocculus.     

Extra- and intracellular recordings from dorsal column postsynaptic spinomedullary neurons in the cat

Dorsal column postsynaptic (DCPS) spinomedullary neurons in the dorsal horn of spinal segments L6-S1 of adult cats anesthetized with sodium pentobarbital were identified by antidromic stimulation of cervical dorsal columns that were dissected free of, and electrically isolated from, the rest of the spinal cord. The neurons were categorized with respect to natural stimulation of their cutaneous receptive fields. An equal number of low-threshold mechanoreceptive and wide-dynamic-range neurons were found. No DCPS neurons could be classified as nociceptive-specific. All neurons received input from low-threshold mechanoreceptors with myelinated axons. There was no evidence that any neurons received monosynaptic input from unmyelinated, primary afferent fibers. The average conduction velocity of the antidromic responses was 45.7 m/s. Nearly half of the DCPS cells showed an antidromic spike followed by synaptically driven responses that were probably evoked by antidromic invasion into the intraspinal collaterals of A-beta primary afferent fibers that ascended the dorsal columns. Intracellularly recorded synaptic responses of DCPS neurons to dorsal column and receptive field stimulation usually consisted of an EPSP with overriding spike potentials followed by a prolonged IPSP whose amplitude decreased markedly as the stimulus frequency was increased in the range of 5 to 30 Hz. The results indicate that DCPS neurons constitute a projection system capable of signaling innocuous and tissue-damaging mechanical stimuli. The DCPS projection may play a role in the modulation of touch and pain perception.     

Dopaminergic neurons in the rat ventral tegmental area. III. Effects of -and -amphetamine

By use of various histochemical techniques, it was shown that both DA and non-DA cells in the VTA project to the NAc. Of these VTA-NAc output cells, the great majority were DA-containing cells. A small number of non-DA cells were encountered most frequently in the lateral part of the VTA. Correspondingly, two distinct groups of neurons, types I and II, could be identified by antidromic stimulation of the NAc. Several lines of evidence suggest that type I cells are DA-containing neurons. The evidence may be summarized as follows:

1. (1) type I cells had a slow-bursting or regular firing pattern, slow discharge rate and wide spike duration which appears to be identical to the characteristics of DA neurons originally described by Bunney et al.¹⁶;

2. (2) the great majority of these cells could be activated antidromically by stimulation of the NAc;

3. (3) the conduction velocity and absolute refractory period of type I cells are consistent with unmyelinated fine DA fibers;

4. (4) injection of 6-OHDA, but not 5,7-DHT directly in the MFB blocked antidromic responses of these cells;

5. (5) they were extremely sensitive to intravenously administered DA agonist apomorphine (ID₅₀ = 7 μg/kg); and

6. (6) direct fluorescence histochemical examination of serial sections from brains of animals in which type I cells have been identified by antidromic stimulation of the NAc showed that type I cells are most likely catecholamine-containi ng neurons. By contrast, type II cells possessed an entirely different spectrum of physiological characteristics; in addition, they showed no consistent response to apomorphine and their antidromic responses to stimulation of the NAc were not affected by 6-OHDA. It is concluded that (1) VTA output neurons consist of both DA and nonDA neurons, and (2) identified types I and II neurons in the VTA by antidromic stimulation of the NAc are DA and non-DA cells, respectively.

Bladder contractility-related neurons in Barrington's nucleus: axonal projections to the spinal cord in the cat

Barrington's nucleus projects directly to the sacral parasympathetic nucleus. The purpose of this study was to clarify whether neurons in Barrington's nucleus that increase their firing during bladder contractions project to the spinal cord and, if so, to which level(s) the axon reaches. Single units were recorded in Barrington's nucleus of cat with glass microelectrodes, and the termination level of descending axons was determined by antidromic stimulation of the spinal cord. Thirty-nine neurons projecting to the spinal cord were located in rostral parts of the dorsolateral pontine tegmentum, medial and ventral to the mesencephalic trigeminal tract. This finding is consistent with previous neuronal tracing studies. All neurons increased their firing rates during contraction associated with micturition. In 19 examined neurons, the most caudal level of the descending axon distributed between the L7 and the S3 level. Stimulation of the axon at this most caudal level resulted in antidromic spike latencies ranging between 19.5 msec and 45.0 msec. These antidromic latencies were much smaller than previously reported orthodromic conduction times between neurons in Barrington's nucleus and sacral preganglionic neurons innervating the bladder. The mean conduction velocity of the descending axon from the cell body to the border between Th13 and the L1 ranged between 7.2 m/sec and 27.7 m/sec. The decrease of the mean conduction velocity was observed at the lumbar as well as at the sacral segments, suggesting that axons issue collaterals to the lumbar level as well as to the sacral level.     

Reciprocal connections between the lateral hypothalamus and the frontal complex in the rat: electrophysiological and anatomical observations

Inputs to rat lateral hypothalamus (LHA) from prefrontal cortex (FC), and vice versa, were studied by intracellular recording, and by retrograde horseradish peroxidase (HRP) method. Stimulation of the FC evoked 3 types of responses: a polysynaptic EPSP-IPSP sequence, IPSPs alone, or antidromic response in LHA neurons. Forty-five per cent of IPSPs were considered to be monosynaptic since the latencies were constant when stimulus intensity was changed. The neurons labeled in the FC following electrophoretic injections of HRP into LHA were located in the medial and sulcal FC. In these cortical areas, not only pyramidal neurons in layer V, but also non-pyramidal neurons in layer VI were labeled. Stimulation of the LHA evoked an EPSP-IPSP sequence, or antidromic response in FC neurons. Some of the fast EPSPs were considered to be monosynaptic. The neurons labeled in the LHA following HRP injection into the FC were either relatively large spherical neurons or small ovoid-shaped neurons. These were distributed diffusely throughout the LHA.