Antidromic latency variations of nigral compacta neurones

In-vivo deep brain recordings of intranigral grafted cells in a mouse model of Parkinson's disease

We recently showed that intranigral transplantation of embryonic neurons in a mouse model of Parkinson's disease led to anatomical and functional recovery of the nigrostriatal pathway. Here we report, in-vivo electrophysiological characteristics of these grafted neurons 2 months after transplantation. Extracellular activity was mapped within the transplant using microarray electrodes and exploration was done with antidromic and orthodromic striatal stimulation. Grafted neurons expressed spontaneous electrophysiological activity with dopaminergic-like characteristics, and antidromic and orthodromic responses suggest a functional recovery of the nigrostriatal loop.

Neocortical efferent neurons with very slowly conducting axons: strategies for reliable antidromic identification

Although simple in concept, reliable antidromic identification of efferent populations poses numerous technical challenges and is subject to a host of sampling biases, most of which select against the detection of the neurons with slowly conducting axons. This problem is particularly acute in studies of the neocortex. Many neocortical efferent systems have large sub-populations with very slowly conducting, nonmyelinated axons and these elements have been relatively neglected in antidromic studies of neocortical neurons. The present review attempts to redress this problem by analyzing the steps that must necessarily precede antidromic identification and the sampling biases associated with each of these steps. These steps include (1) initial recognition that the microelectrode is near a neuron; (2) activation of the efferent axon via the stimulating electrode; (3) conduction of the antidromic impulse from stimulation site to soma; (4) detection of the antidromic spike in the extracellular record and (5) discriminating antidromic from synaptic activation. Experimental strategies are suggested for minimizing the sampling biases associated with each of these steps; most of which can be reduced or eliminated by appropriate experimental procedures. Careful attention to such procedures will make it possible to better understand the nature and function of the information flow along the very slowly conducting axonal systems of the neocortex.

Burst-firing activity of presumed 5-HT neurones of the rat dorsal raphe nucleus: electrophysiological analysis by antidromic stimulation

We recently reported raphe neurones which frequently fired spikes in short bursts. However, the action potentials were broad and the neurones fired in a slow and regular pattern, suggesting they were an unusual type of 5-hydroxytryptamine (5-HT) neurone. In the present study, we investigated whether these putative burst-firing 5-HT neurones project to the forebrain and whether all spikes fired in bursts propagate along the axon. In anaesthetised rats, electrical stimulation of the medial forebrain bundle evoked antidromic spikes in both burst-firing neurones and in single-spiking, classical 5-HT neurones recorded in the dorsal raphe nucleus. Although the antidromic spike latency of the single-spiking and burst-firing neurones showed a clear overlap, burst-firing neurones had a significantly shorter latency than single-spiking neurones. For both burst-firing neurones and classical 5-HT neurones, antidromic spikes made collisions with spontaneously occurring spikes. Furthermore, in all burst-firing neurones tested, first, second and third order spikes in a burst could be made to collide with antidromic spike. Interestingly, in a small number of burst-firing neurones, antidromic stimulation evoked spike doublets, similar to those recorded spontaneously. From these data we conclude that burst-firing neurones in the dorsal raphe nucleus project to the forebrain, and each spike generated by the burst propagates along the axon and could thereby release transmitter (5-HT).

Postnatal development of the electrical activity of rat nigrostriatal dopaminergic neurons

Extra- and intracellular recordings were obtained in vivo from dopaminergic nigrostriatal neurons in rat pups ranging in age from postnatal day (PD) 1 to PD28, and in adult rats. Neurons from PD1-3 rats were active at very low rates in a random pattern, rarely showed bursting activity, and often exhibited long periods of up to several minutes of silence. Spontaneous spikes were of relatively low amplitude and long duration. The mean firing rate increased and became more regular over time, and short bursts consisting of only 2 spikes were observed. By the second postnatal week, the initial segment component of the spontaneous spike resembled that seen in adults, but the somadendritic component was still relatively small, and there was often a very marked temporal delay between the two. Near the end of the second postnatal week, neurons exhibited a transient phase of pacemaker-like activity. Mean firing rates continued to increase with time, as did the incidence and complexity of bursting activity. The spontaneous firing rate, pattern and spike morphology approached adult values by the fourth postnatal week. Antidromic responses from neostriatum were obtained as early as PD1, and consisted of a significantly greater proportion of full initial segment-soma dendritic spikes compared to nigrostriatal neurons from adult rats. There was usually a long delay between the initial segment and somadendritic components of the spike. Mean antidromic latency and mean antidromic threshold did not vary significantly from PD1-3 to adults. Axonal conduction velocity reached maximal adult values by PD16-21. Neostriatal-evoked orthodromic responses consisted principally of a poststimulus inhibition whose duration decreased from PD1 through adulthood. Pure excitatory responses were very rarely observed at any age. Intracellular recordings from PD2, PD3 and PD5 rats revealed striatal-evoked inhibitory postsynaptic potentials in non-dopaminergic nigral neurons with a mean onset latency (9.8 +/- 3.8 ms) which did not differ from that previously reported for adult rats.

Electrophysiological properties of nigrothalamic neurons after 6-hydroxydopamine lesions in the rat

Extracellular recordings were made from electrophysiologically identified nigrothalamic cells in the substantia nigra pars reticulata of anaesthetized rats. The firing rate, firing pattern and responses to striatal stimulation were investigated in normal animals and in animals in which dopamine concentration in the ipsilateral striatum was reduced by more than 90%. At relatively short times after the lesion (less than 10 days) the mean firing rate of the spontaneously active cells in the population was significantly reduced and there was an increase in the occurrence of bursting activity. There was also a significant increase in the number of silent cells, located by antidromic stimulation from the thalamus. In spite of this reduction in mean firing rate the responses of neurons to stimulation of either the ipsilateral striatum or ventromedial thalamus was much larger in cells from lesioned animals. At longer times after the lesion (more than six months) the average firing rate of the neurons had returned to normal but there was still a prevalence of bursting activity and a consequent reduction in mean inter-spike intervals. There was little evidence of the previous hyper-responsiveness to thalamic stimulation but the responsiveness to striatal stimulation was still significantly elevated.

Do antidromic latency jumps indicate axonal branching in nigrostriatal and hypothalamo-neurohypophysial neurons?

Neurons in many brain regions exhibit discontinuous decreases in antidromic latency with small increases in stimulating current. We used an electrophysiological test requiring a single stimulating electrode to determine whether these 'latency jumps' are due to shifts in the site of spike initiation to the same or different axon branches. Latency jumps in response to stimulation of the striatal terminal fields of substantia nigra, pars compacta neurons represent spike initiation on different branches while those seen in paraventricular neurons with pituitary stalk stimulation usually reflect a change in site on a single branch.

The mode of projections of single locus coeruleus neurons to the cerebral cortex in rats

Axonal distributions of single locus coeruleus neurons within the cerebral cortex were examined with antidromic stimulation technique combined with cortical lesions (frontal lobotomy and lobectomy). In urethan-anesthetized rats, stimulating electrodes were implanted in 10 points extending over nearly the entire cerebral cortex, and antidromic responses of single locus coeruleus neurons to stimulation of these stimulus sites were analysed. Fifty percent of locus coeruleus neurons examined were activated antidromically from at least one cortical point in the cerebral cortex. The pattern and extent of axonal distributions of single locus coeruleus neurons in the cortex appeared to vary from cell to cell. From the results obtained in rats with the cortical lesions, it is concluded that in addition to locus coeruleus neurons with intracortical axons running from rostral to caudal, there are the neurons projecting to the occipital cortex without innervating the frontal cortex and those projecting simultaneously to the frontal and occipital cortex with two axonal branches. There was no topographic order between the recording sites within the locus coeruleus and the projection sites in the cortex.

Substantia nigra stimulation evoked antidromic responses in rat neostriatum

Electrical stimulation of the substantia nigra of rats elicits a burst of small amplitude waves with a latency of 4-6 ms that may last for 10-15 ms throughout much of the neostriatum. Frontal cortex stimulation also elicits a burst response, which can occlude the substantia nigra response. The substantia nigra evoked burst response was still present after chronic neocortical ablation or thalamic transection or both treatments combined. The response corresponds to the first sharp negative wave of the substantia nigra evoked neostriatal field potential. Single substantia nigra evoked action potentials were recorded in neostriatum with a mean latency of 9.8 ms, ranging from 4-22 ms. These action potentials were considered to be antidromic because they were occluded during appropriate collision intervals by orthodromic action potentials elicited by frontal cortex stimulation. Subthreshold frontal cortex conditioning stimulation did not alter the threshold for activation from substantia nigra. The refractory period for the axon was at least as long as that for the soma and ranged between 0.8-2.0 ms. The antidromic responses failed to follow low frequency stimulation (less than 40 Hz for 3000 ms). This failure occurred in the axon between substantia nigra and globus pallidus. The burst response and first sharp negative wave of the field potential probably represent the antidromic activation of the ubiquitous and densely packed medium spiny neostriatal projection neurons. These responses occur at the same latency, respond in the same manner to twin pulse and repetitive stimulation and are occluded by frontal cortex stimulation in the same manner as antidromic action potentials.

Antidromic activation of dorsal raphe neurons from neostriatum: physiological characterization and effects of terminal autoreceptor activation

Three types of neurons, distinguished on the basis of their spontaneous firing rates and patterns, extracellularly recorded waveforms and responses to neostriatal stimulation, were observed in the dorsal raphe nucleus in urethane-anesthetized rats. Type 1 neurons (presumed to be serotonergic) fired spontaneously from 0.1 to 3 spikes/s in a regular pattern, with initial positive-going bi- or triphasic action potentials. Type 1 cells exhibited long-latency antidromic responses to neostriatal stimulation (mean +/- S.E.M. 24.9 +/- 0.3 ms) that sometimes occurred at discrete multiple latencies, and supernormal periods persisting up to 100 ms following spontaneous spikes. Type 2 cells fired spontaneously in an irregular, somewhat bursty pattern from 0 to 2 spikes/s with initial negative-going biphasic spikes, and were antidromically activated from neostriatal stimulation at shorter latencies than Type 1 cells (21.8 +/- 0.9 ms). Type 3 cells were characterized by initial positive-going biphasic waveforms and displayed a higher discharge rate (5-30 spikes/s) than Type 1 or Type 2 cells. Type 3 cells could not be antidromically activated from neostriatal stimulation. The relatively long conduction time to neostriatum of the Type 1 presumed serotonergic neuron is discussed with respect to previous interpretations of the synaptic action of serotonin in the neostriatum. In conjunction with these antidromic activation studies, the neurophysiological consequences of serotonergic terminal autoreceptor activation were examined by measuring changes in the excitability of serotonergic terminal fields in the neostriatum following administration of the serotonin autoreceptor agonist, 5-methoxy-N,N-dimethyltryptamine (5-MeODMT). The excitability of serotonergic terminal fields was decreased by intravenous injection of 40 micrograms/kg 5-MeODMT, and by infusion of 10-50 microM 5-MeODMT directly into the neostriatum. These results are interpreted from the perspective of mechanisms underlying autoreceptor-mediated regulation of serotonin release.

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.

Nucleus basalis neurons exhibit axonal branching with decreased impulse conduction velocity in rat cerebrocortex

Single neurons in the basal forebrain (nucleus basalis area) were antidromically activated from the frontal or parietal cortex in anesthetized rats. Wide ranges of antidromic latencies were observed overall, with frontal and parietal stimulation yielding values ranging from 1.0 to 26.0 ms and 1.6-24.0 ms, respectively. Individual neurons often exhibited multiple antidromic latencies, such that deeper sites of stimulation or greater stimulation amplitudes generally yielded discretely different, shorter latencies than more superficial sites or lower amplitudes of stimulation. Single neurons were also often driven from neighboring sites (1-2 mm apart) within the frontal cortex, but no cell was coactivated from both frontal and parietal cortices. Finally, patterns and rates of spontaneous activity varied markedly among these cortically projecting neurons, with some cells being non-spontaneous and others exhibiting tonic rates of 30-40 Hz. Impulse waveforms also differed among driven cells, from relatively low-amplitude, negative spikes to large-amplitude, entirely positive spikes in unfiltered signals. These results indicate that cortically projecting, putatively cholinergic neurons in the basal fore-brain form a physiologically heterogeneous population in terms of impulse conduction velocity, spontaneous discharge, and spike waveforms. Our finding of multiple antidromic latencies and driving from neighboring sites indicate that these fibers may be highly branched in local terminal fields, but that individual cells may project exclusively to a single cortical area. Faster conduction velocities for deep compared to superficial cortical stimulation sites imply that these fibers may become non-myelinated upon entering cortical terminal fields, or that they may become markedly thinner as they travel within the cortex. This system of cholinergic cortical afferents differs in many physiologic aspects from the other non-thalamic cortical input systems of catecholamine or indoleamine neurons.

Autoreceptor-mediated changes in dopaminergic terminal excitability: effects of increases in impulse flow

The effect of spontaneous and stimulation-induced alterations in impulse flow on the antidromic excitability of nigrostriatal dopaminergic neurons were investigated in urethane-anesthetized rats. Terminal excitability was found to be inversely related to the rate of spontaneous activity of nigral neurons. Conditioning stimulation applied to dopaminergic axons in the medial forebrain bundle was found to decrease terminal excitability, but axonal conditioning stimulation was without effect on antidromic responses evoked from the medial forebrain bundle. Decreases in terminal excitability induced by medial forebrain bundle stimulation could be blocked by local infusions of haloperidol into the region of the terminal fields, suggesting that the effect was receptor-mediated. These results are consistent with the proposal that nigrostriatal dopaminergic neurons may modulate the impulse-dependent release of dopamine from striatal nerve terminals as a function of firing rate by autoreceptor-mediated alterations in the electrical properties of the terminal membrane.

Regulation of cerebello-cortical transmission in the rat ventromedial thalamic nucleus

On the basis of antidromic stimulation we have identified two distinct neuronal populations in the rat ventromedial thalamic nucleus. The largest population (96%) are thalamo-cortical relay cells which project via the internal capsule to the cerebral cortex. The smaller population of cells (4%) project caudally to the reticular formation and superior colliculus. These two cell types could be distinguished further on the basis of their patterns of spontaneous discharge. Relay cells fluctuate between two activity patterns (i) a rhythmic pattern characterized by periods of high-frequency bursting, and (ii) a more tonic discharge pattern of single spikes. The caudally projecting cells had a characteristic fast, regular type of spontaneous firing. Brachium conjunctivum stimulation evokes two distinct responses in thalamic relay cells. (i) a short-latency single spike, (ii) a longer latency, rhythmic response of 2-3 spikes. Both excitatory responses are followed by a period of cell quiescence. The type of response is dependent upon the cell's firing pattern. The short-latency response occurs during tonic, single-spike activity whilst the longer latency response occurs during high-frequency bursting activity. The short-latency response can be altered to the long latency response by increasing the level of anaesthesia or by applying a conditioning shock to known inhibitory pathways. Conversely the long latency response can be altered to the short-latency response by decreasing anaesthesia or by stimulation of the reticular formation. It is argued that both response types are evoked monosynaptically by activation of the same cerebello-thalamic fibres but that different ionic conductances which are active at different levels of membrane polarization are responsible for the two response patterns. Efficient time-locked cerebello-thalamo-cortical transmission occurs only during tonic single-spike activity, when cerebellar stimulation evokes a short-latency response. Such transmission is allowed or disallowed by the fine balance between converging excitatory and inhibitory afferents. In addition to a monosynaptic excitatory input from the cerebellar nuclei, relay cells received converging synaptic inputs from the substantia nigra, cerebral cortex, reticular formation and superior colliculus. Due to the anatomical arrangement in the rat it proved impossible to assess the role of the pallidum. The population of caudally projecting cells also received several converging synaptic inputs, but unlike those influencing relay cells, these inputs were all excitatory.(ABSTRACT TRUNCATED AT 400 WORDS)

An electrophysiological study of the accessory olfactory bulb in the rabbit--II. Input-output relations as assessed from analysis of intra- and extracellular unit recordings

The input-output relations of the rabbit accessory olfactory bulb were studied by intra- and extracellular single unit recordings following electrical stimulation of the vomeronasal nerves, the lateral olfactory tract and the corticomedial amygdala. Cellular activity of accessory bulb mitral cells evoked by stimulation of the vomeronasal nerves consisted of a brief excitation with a latency of 16 ms. This initial response was followed by a period of reduced firing probability which was due to an inhibitory postsynaptic potential. In many cases this secondary response was followed by a second excitatory postsynaptic potential on which action potentials were generated at higher stimulus intensities. Deeper cells in the granule cell layer responded with a long latency, long duration, excitation, often consisting of bursts of 2-3 spikes. The majority of mitral cells were antidromically invaded by amygdala stimulation. The latencies of the antidromic spikes showed a wide range of variation (12-80 ms). Due to this great variation in antidromic latency the inhibitory postsynaptic potential following the antidromic action potential was rather modest but prolonged in duration. In many cases the onset of the inhibitory postsynaptic potential preceded the antidromic response. The majority of cells did not respond to lateral olfactory tract stimulation. Only 10% of the tested cells were invaded antidromically by stimulation at this site. These neurons were also driven antidromically by amygdala stimulation. We conclude that, although the physiological characteristics of mitral cells of the main and accessory olfactory bulb are very similar, there are important differences. The efferent fibres of the accessory bulb conduct at very slow and variable rates and project directly to the corticomedial amygdala.

Raphe - cerebellum interactions. I. Effects of cerebellar stimulation and harmaline administration on single unit activity of midbrain raphe neurons in the rat

The firing patterns of single raphe units at the posterior midbrain level were examined in chloralosed rats to assess the effects of cerebellar stimulation and/or harmaline administration. Raphe cells were grouped according to their spontaneous firing rate and other characteristics into two categories. From a total sample of 160 cells, 106 (66%) presenting a slow regular discharge pattern were classified as serotonergic (5-HT cells), whereas 35 (22%), having a faster firing rate, were considered non serotonergic (NS cells). Moreover, 19 (12%) raphe units were non categorized. Cerebellar juxtafastigial (JF) stimulation modified the discharge pattern of 56 (35%) raphe units. The remaining 65% were unaffected by the stimulation. Of the 41 5-HT cells affected by JF stimulation, 28 neurons (68%) showed a systematic increase of their firing rate, whereas of the 12 NS cells affected 8 neurons (66%) were inhibited. It thus appears that cerebellar stimulation has an opposite effect on raphe units according to the cell types. Harmaline administration suppressed the activity of 5-HT cells and increased the discharge rate of NS cells. Moreover, we noticed in the latter units a phase modulation of the firing pattern by pauses occurring with a fixed periodicity of 2.5 to 10 s. Considered in the context of previous studies, these results strongly suggest an inhibitory influence of the raphe system on the olivo-cerebellar circuitry.

Dopaminergic terminal excitability following arrival of the nerve impulse: the influence of amphetamine and haloperidol

Variations in the excitability of the axons and terminal fields of dopaminergic neurons of the substantia nigra were examined as a function of time following the nerve impulse in urethane-anesthetized, immobilized rats. Excitability was measured by determining the threshold, defined as the minimum current required to consistently activate dopaminergic neurons antidromically. Threshold currents were maximal immediately following the action potential and declined exponentially to a plateau. The interval during which threshold current declined, the phasic period, was significantly longer for stimulation of neostriatal terminal fields as contrasted to stimulation of axons along the medial forebrain bundle. A positive correlation was observed between antidromic response latency and the duration of this phasic period for both sites of stimulation, an observation consistent with the view that the site of initiation of the antidromic action potential is of smaller diameter in the neostriatum than in the medial forebrain bundle Amphetamine, which promotes dopamine release and/or re-uptake blockade, reduced dopaminergic terminal excitability in the neostriatum at all intervals examined. This effect increased at successively shorter intervals during the phasic portion of the excitability curve. Haloperidol, a dopamine antagonist, increased the excitability of dopaminergic terminal fields, an effect which was also more marked earlier in the phasic interval. Neither amphetamine nor haloperidol had a significant effect on the excitability of dopaminergic axons in the medial forebrain bundle. Variations in dopaminergic terminal excitability after impulse arrival, and the effects of amphetamine and haloperidol on this phenomenon suggest that terminal excitability is determined by events related to arrival of the nerve impulse including activation of presynaptic 'autoreceptors' by dopamine released from the synaptic ending.

Reversible effects of tetanus toxin on striatal-evoked responses and [3H]-gamma-aminobutyric acid release in the rat substantia nigra

1 The effects of sublethal doses of tetanus toxin on gamma-aminobutyric acid (GABA)-mediated synaptic transmission and [3H]-GABA release were studied in the rat substantia nigra. 2 Intranigral injections of tetanus toxin at 1-5 times the mouse LD50 dose produced ipsiversive circling behaviour which was maximal after 1 week and lasted 2-3 weeks. Rats then displayed normal behavior suggesting that the effects of the toxin were fully reversible. 3 In the treated nigra of circling rats there was a reduction in the striatal-evoked inhibition of compacta and reticulata neurones, but no change in their spontaneous firing rates. Some forms of striatal-evoked excitation were also reduced. Once rats had recovered from circling no alterations in the synaptic responses were detected. 4 In circling rats there were no differences in the sensitivities of neurones in the treated and untreated nigra to GABA or to other inhibitory neurotransmitters. 5 The Ca2+-dependent, K+-evoked release of [3H]-GABA from slices prepared from the treated nigra of circling rats was less than that from the untreated nigra of circling rats. No differences in nigral [3H]-GABA release were observed once rats had recovered from the circling behaviour. 6 The results demonstrate that doses of tetanus toxin which produce reversible behavioural effects can interfere reversibly with GABA-mediated synaptic transmission by a presynaptic mechanism which probably involves a reduction in transmitter release.

The influence of striatal stimulation and putative neurotransmitters on identified neurones in the rat substantia nigra

The response of two populations of neurones in the substantia nigra (nigro-striatal compacta neurones and reticulata neurones) to microelectrophoretically administered putative neurotransmitters and stimulation of the ipsilateral striatum has been investigated in anaesthetized rats. There were marked differences between compacta and reticulata neurones in respect to their action potential configurations, spontaneous firing rates and their responses to striatal stimulation. However, both compacta and reticulata neurones were excited and/or inhibited by striatal stimulation, although inhibition was usually the predominant response in both neuronal populations. Compacta neurones were strongly inhibited by noradrenaline (NA) and dopamine (DA) but were unaffected by acetylcholine (ACh) and 5-hydroxytryptamine (5-HT). Reticulata neurones were excited by ACh and showed mixed responses to 5-HT, DA and NA. Excitant amino acids overdepolarized compacta neurones preventing them from firing rapidly, but induced large increases in reticulata neurone firing rate; effects that were readily antagonized by D-alpha-aminoadipate. Compacta neurones were less sensitive than reticulata neurones to GABA and glycine. The action of these inhibitory amino acids were selectively and reversibly antagonized by bicuculline methochloride and strychnine, respectively. The striatal-evoked inhibition of both compacta and reticulata neurones was reversibly reduced by bicuculline methochloride and irreversibly reduced by tetanus toxin, but was unaffected by strychnine. These results demonstrate that nigrostriatal-compacta neurones and reticulata neurones are physiologically and pharmacologically distinct neuronal populations and both receive inhibitory GABAergic and excitatory striatal inputs.