Acetylcholine-sensitivity of thalamic neurones: its relationship to synaptic transmission

Time related effects on functional brain connectivity after serotonergic and cholinergic neuromodulation

Psychopharmacological research, if properly designed, may offer insight into both timing and area of effect, increasing our understanding of the brain's neurotransmitter systems. For that purpose, the acute influence of the selective serotonin reuptake inhibitor citalopram (30 mg) and the acetylcholinesterase inhibitor galantamine (8 mg) was repeatedly measured in 12 healthy young volunteers with resting state functional magnetic resonance imaging (RS‐fMRI). Eighteen RS‐fMRI scans were acquired per subject during this randomized, double blind, placebo‐controlled, crossover study. Within‐group comparisons of voxelwise functional connectivity with 10 functional networks were examined (P < 0.05, FWE‐corrected) using a non‐parametric multivariate approach with cerebrospinal fluid, white matter, heart rate, and baseline measurements as covariates. Although both compounds did not change cognitive performance on several tests, significant effects were found on connectivity with multiple resting state networks. Serotonergic stimulation primarily reduced connectivity with the sensorimotor network and structures that are related to self‐referential mechanisms, whereas galantamine affected networks and regions that are more involved in learning, memory, and visual perception and processing. These results are consistent with the serotonergic and cholinergic trajectories and their functional relevance. In addition, this study demonstrates the power of using repeated measures after drug administration, which offers the chance to explore both combined and time specific effects. Hum Brain Mapp 38:308–325, 2017. © 2016 Wiley Periodicals, Inc.

Cholinergic mechanisms in spinal locomotion-potential target for rehabilitation approaches

Previous experiments implicate cholinergic brainstem and spinal systems in the control of locomotion. Our results demonstrate that the endogenous cholinergic propriospinal system, acting via M₂ and M₃ muscarinic receptors, is capable of consistently producing well-coordinated locomotor activity in the in vitro neonatal preparation, placing it in a position to contribute to normal locomotion and to provide a basis for recovery of locomotor capability in the absence of descending pathways. Tests of these suggestions, however, reveal that the spinal cholinergic system plays little if any role in the induction of locomotion, because MLR-evoked locomotion in decerebrate cats is not prevented by cholinergic antagonists. Furthermore, it is not required for the development of stepping movements after spinal cord injury, because cholinergic agonists do not facilitate the appearance of locomotion after spinal cord injury, unlike the dramatic locomotion-promoting effects of clonidine, a noradrenergic α-2 agonist. Furthermore, cholinergic antagonists actually improve locomotor activity after spinal cord injury, suggesting that plastic changes in the spinal cholinergic system interfere with locomotion rather than facilitating it. Changes that have been observed in the cholinergic innervation of motoneurons after spinal cord injury do not decrease motoneuron excitability, as expected. Instead, the development of a “hyper-cholinergic” state after spinal cord injury appears to enhance motoneuron output and suppress locomotion. A cholinergic suppression of afferent input from the limb after spinal cord injury is also evident from our data, and this may contribute to the ability of cholinergic antagonists to improve locomotion. Not only is a role for the spinal cholinergic system in suppressing locomotion after SCI suggested by our results, but an obligatory contribution of a brainstem cholinergic relay to reticulospinal locomotor command systems is not confirmed by our experiments.

Acetylcholine

This article traces the development of knowledge about the physiology and pharmacology of acetylcholine and its receptors between 1930 and 2005, with emphasis on contributions by members of the British Pharmacological Society, and by other British pharmacologists and physiologists.

A population of nicotinic receptors is associated with thalamocortical afferents in the adult rat: laminal and areal analysis

In the adult rat brain, a prominent population of nicotinic cholinoceptors binds 3H-nicotine with nanomolar affinity. These receptors are abundant in most thalamic nuclei and in neocortical layers 3/4, which receive a major thalamic input. To test whether cortical nicotinic receptors are associated with thalamocortical afferents, unilateral excitotoxic (N-methyl-D-aspartate) lesions were made in one of four thalamic nuclear groups (anterior, ventral, medial geniculate, or dorsal lateral geniculate) or in temporal cortex. After 1 or 4 weeks of survival, cortical 3H-nicotine binding was quantified via autoradiography. Thalamic lesions resulted in a partial loss of 3H-nicotine binding in ipsilateral cerebral cortex. In each thalamic lesion group, the greatest decrease (35-45%) occurred within the cortical layers and area (i.e., cingulate, parietal, temporal, or occipital cortex) receiving the densest thalamocortical innervation. Binding of 3H-nicotine was also reduced within the thalamus local to the lesion, particularly at the longer survival time. Saturation analysis, performed in frontoparietal cortical tissue homogenates following ventral thalamic lesions, revealed a significant (34%) reduction in receptor density but not affinity. Direct excitotoxic lesions of the neocortex (temporal cortex) tended to preserve 3H-nicotine binding in layers 3/4, despite local neuronal loss. These results, taken with other published findings, suggest that some nicotinic cholinoceptors in adult rat cerebral cortex are located on thalamocortical terminals. This organizing principle appears to apply not only to sensory and motor relay projections but also to association nuclei that project to allocortical areas. These receptors may provide a local mechanism for nicotinic cholinergic modulation of thalamocortical input.

Does barbiturate anesthesia modify the neuronal properties of the somatosensory thalamus? A single-unit study related to nociception in the awake-pentobarbital-treated rat

By means of extracellular recordings, we studied thalamic ventrobasal complex neurons of rats tested first awake, and then anesthetized with pentobarbital. In both conditions, we found two groups of units in both states. The first group, displaying a spontaneous bursting activity, was not obviously responding to peripheral stimuli. Another group, displaying a single-spike activity, was almost exclusively activated by innocuous and/or noxious and innocuous mechanical stimuli. Still in this group, units specifically driven by noxious stimuli were only found under pentobarbital. These data, different from classical findings, emphasize the interest of the awake preparation in order to study nociceptive cellular mechanisms at the thalamic level.

Cholinergic responsiveness of neurons in the ventroposterior thalamus of the anesthetized rat

Acetylcholine has been implicated as an important neurotransmitter in the mechanisms of thalamic activation. Cholinergic mechanisms are thought to directly underlie the high level of excitability observed in thalamic relay neurons during waking and rapid eye movement sleep. We sought to determine if the cholinergic responsiveness of neurons in the ventroposterior nuclei of the thalamus in rat is consistent with this view. Neurons in the chloral hydrate-anesthetized rat were studied with extracellular recording and microiontophoretic application of cholinergic agents. In most cases (63% of 63 cells), the ejection of the agonist, carbachol, had no observable effect on spontaneous activity. Facilitation (25%), inhibition (8%) and inhibition followed by facilitation (3%) were also observed. Carbachol ejections that by themselves were ineffective in altering spontaneous activity proved capable, in 93% of 28 cells, of antagonizing the uniformly facilitatory responses produced by glutamate ejection. The putative M1-selective, cholinergic agonist, McN-A-343, was also ineffective alone in altering spontaneous activity in the majority of cases (74% of 27 cells) and produced only inhibitory responses in the remaining seven neurons studied. Interacting applications of McN-A-343 and glutamate resulted, in all cases, in antagonism of glutamate facilitation (N = 12). The various responses to applied cholinergic agonists were all capable of being antagonized by muscarinic receptor-blocking agents. Both the high proportion of inhibitory responses and the antagonism of glutamate facilitatory responses suggest that ventroposterior neurons in the rat differ from other thalamocortical relay neurons in the rat and cat with regard to cholinergic responsiveness. Additionally, the lack of predominantly facilitatory responding renders it unlikely that cholinergic mechanisms directly underlie increases in excitability of ventroposterior neurons observed during waking and rapid eye movement sleep.

Midlatency auditory evoked responses: differential effects of a cholinergic agonist and antagonist

The effects of a cholinergic antagonist (scopolamine) and agonist (physostigmine) on the auditory middle latency evoked responses (MLRs) were studied in 7 normal male volunteers. Scalp recordings were made from a central (Cz) electrode referenced to linked ear lobes on one channel and to a non-cephalic, sternovertebral reference on a second channel. Three components were statistically analyzed for changes in latency and amplitude: Pa, with peak positivity in the 25-40 msec latency range, Nb, with peak negativity 40-50 msec, and P1, with peak positivity 50-65 msec. Control recordings included responses to click rates of 1, 5, 8 and 10/sec; as has been previously reported, P1 showed a marked decrease and disappeared at the faster rates of stimulation whereas Pa showed no change in amplitude. Intravenous injections of scopolamine resulted in a rapid and complete disappearance of P1 and a slight increase in Pa; concurrently, the subjects reported feeling drowsy but were awake with eyes open through the recordings. Subsequent injections of physostigmine resulted in a rapid reversal of the scopolamine effects so that the subjects became alert, Pa decreased, and P1 reappeared and increased to control amplitudes. Rapid click rates caused P1 to diminish, as in the control period, indicating a common P1 recovery cycle in both the control and physostigmine conditions. These data are discussed in terms of the hypothesis that the P1 generator system is comprised of a cholinergic brain-stem-thalamic component of the ascending reticular activating system.

Cholinergic neurons of the feline pontomesencephalon. II. Ascending anatomical projections

Immunoreactivity for choline acetyltransferase (ChAT) was analyzed in unoperated cats and in cats in which stereotaxic lesions were made in the pedunculopontine and laterodorsal tegmental nuclei. The fine reaction product revealed moderate to dense ChAT-immunoreactive fiber plexuses throughout the telencephalon, diencephalon, and midbrain. A pontomesencephalic origin of cholinergic innervation to virtually every nucleus of the diencephalon, as well as to various midbrain and basal telencephalic sites was indicated in the cats with lesions, in which the optical density of ChAT-immunoreactivity was significantly decreased as compared to controls. Pontomesencephalic lesions produced no changes, however, in the density of ChAT staining in the cerebral cortex, basolateral amygdala, or caudate nucleus. In addition to ChAT-positive terminal fiber arborizations which were widely distributed, cholinergic fibers-of-passage were traced in the unoperated and operated feline brains. The general course of ChAT fibers cut in cross-section was followed in successive transverse levels, and although pathways originating from the pedunculopontine nucleus demonstrated orientations in every direction, many demonstrated a rostral course. A particularly dense aggregate of ascending ChAT-positive fibers was localized in the dorsolateral sector of the pedunculopontine area which could be followed at more rostral levels into the central tegmental fields and the compact part of the substantia nigra. From the central tegmental fields, numerous ChAT-immunopositive fibers cut in cross-section continued to course rostrally in the intralaminar, reticular and lateroposterior nuclei of the thalamus, and a distinct bundle of ChAT fibers coursing dorsolaterally was observed medial to the optic tract ascending to the lateral geniculate. ChAT fibers with dorsolateral orientations were additionally observed in the zona incerta, ventral anterior thalamus, and ansa lenticularis on route to the reticular thalamus, the globus pallidus, and the substantia innominata. Pathways consisting of fibers traced from ChAT-containing cells in the laterodorsal tegmental nucleus could be traced to medial structures such as the periaqueductal gray, ventral tegmental area and dorsal raphe. Medially placed ChAT fibers were additionally followed through the ventral tegmental area, the midline thalamus, and the hypothalamus, up to the medial and lateral septal nuclei. The trajectories of the ascending cholinergic pathways from the pontomesencephalon are discussed in relation to locally generated electrophysiological responses in the cat.

Neurotoxic lesions of the dorsolateral pontomesencephalic tegmentum-cholinergic cell area in the cat. II. Effects upon sleep-waking states

Kainic acid was injected bilaterally (4.8 micrograms in 1.2 microliters each side) into the dorsolateral pontomesencephalic tegmentum of cats in order to destroy the cholinergic neurons located in that region and thus to study the effects of their destruction upon sleep-waking states. The kainic acid produced a large area of nerve cell loss and/or gliosis centered in the dorsolateral tegmentum-cholinergic cell area, that includes the pedunculopontine tegmental (PPT) and laterodorsal tegmental (LDT) nuclei rostrally (A1-P2), and the parabrachial (PB) and locus coeruleus (LC) nuclei caudally (P3-P5). The mean loss of choline acetyltransferase (ChAT)-immunoreactive neurons within this area was 60% with a range from 25% to 85% across 11 cats. The mean loss of tyrosine hydroxylase (TH)-immunoreactive neurons, differentially distributed through the same region, was 35% with a range of 0-50%. Whereas the kainic acid lesions appeared to have only slight effects upon wakefulness and slow-wave sleep, they had marked effects upon paradoxical sleep (PS), which varied in degree across animals. In cats with the most extensive destruction of cholinergic neurons, PS was eliminated in the first few weeks following the lesion and then reappeared as isolated episodes characterized by sparse, low amplitude PGO spikes in association with few eye movements and an activated cortex, though in absence of neck muscle atonia. Although these PS-like episodes varied in amount, they were significantly less than baseline PS in percent and in duration for the group of 11 animals over one month recording. The PGO spike rate was significantly reduced; the EMG amplitude was significantly increased, marking a loss of neck muscle atonia. The percent of PS-like epochs, the rate of PGO spiking and the EMG amplitude on postlesion day 28 were found to be significantly correlated with the volume of the lesion within the dorsolateral pontine tegmentum-cholinergic cell area. The percent PS-like episodes and PGO spike rate were significantly correlated with the number of remaining ChAT-immunoreactive neurons, but not with the number of remaining TH-immunoreactive neurons within this region. These results suggest that cholinergic pontomesencephalic neurons may be critically involved in the generation of paradoxical sleep and its phasic events.

The origins of cholinergic and other subcortical afferents to the thalamus in the rat

The origins of the cholinergic and other afferents of several thalamic nuclei were investigated in the rat by using the retrograde transport of wheat germ agglutinin conjugated-horseradish peroxidase in combination with the immunohistochemical localization of choline acetyltransferase immunoreactivity. Small injections placed into the reticular, ventral, laterodorsal, lateroposterior, posterior, mediodorsal, geniculate, and intralaminar nuclei resulted in several distinct patterns of retrograde labelling. As expected, the appropriate specific sensory and motor-related subcortical structures were retrogradely labelled after injections into the principal thalamic nuclei. In addition, other basal forebrain and brainstem structures were also labelled, with their distribution dependent on the site of injection. A large percentage of these latter projections was cholinergic. In the brainstem, the cholinergic pedunculopontine tegmental nucleus was retrogradely labelled after all thalamic injections, suggesting that it provides a widespread innervation to the thalamus. Neurons of the cholinergic laterodorsal tegmental nucleus were retrogradely labelled after injections into the anterior, laterodorsal, central medial, and mediodorsal nuclei, suggesting that it provides a projection to limbic components of the thalamus. Significant basal forebrain labelling occurred only with injections into the reticular and mediodorsal nuclei. Only injections into the reticular nucleus resulted in retrograde labelling of the cholinergic neurons in the nucleus basalis of Meynert. The results provide evidence for an organized system of thalamic afferents arising from cholinergic and noncholinergic structures in the brainstem and basal forebrain. The brainstem structures, especially the cholinergic pedunculopontine tegmental nucleus, appear to project directly to principal thalamic nuclei, thereby providing a possible anatomical substrate for mediating the well-known facilitory effects of brainstem stimulation upon thalamocortical transmission.

Choline acetyltransferase immunoreactivity in the rat thalamus

The distribution of choline acetyltransferase immunoreactivity in the rat thalamus was investigated by using a specific monoclonal antibody and was compared with the pattern of acetylcholinesterase staining. The only choline acetyltransferase-immunoreactive cell bodies in the thalamus were in the medial habenula. A wide range of densities of immunoreactive fibers and varicosities was found. The highest densities of stained varicosities were in the anteroventral, reticular, lateral mediodorsal, and intralaminar nuclei. At the other extreme, the anterodorsal, ventroposteromedial, and paraventricular nuclei were almost devoid of immunoreactive varicosities. A light density of fibers was observed in several medial nuclei, including parataenial, reuniens, and gelatinosus. Most other nuclei contained moderately dense regions of varicose fibers that were often heterogeneous or patchy. The pattern of choline acetyltransferase immunoreactivity in the thalamus was in general similar to that of acetylcholinesterase. A marked discrepancy, however, was found in the anterodorsal nucleus, which was intensely stained for acetylcholinesterase but contained no apparent choline acetyltransferase immunoreactivity. Numerous physiologic studies have demonstrated striking effects of acetylcholine on thalamic activity. The present study provides a description of choline acetyltransferase-immunoreactive structures in the thalamic nuclei, providing a first step toward elucidating the anatomical basis for the physiologic and functional importance of cholinergic transmission in the thalamus.

Muscarinic action of acetylcholine in the rat ventromedial thalamic nucleus

Several lines of evidence suggest a role for ACh in the mediation of cerebello-thalamic transmission. The physiological, pharmacological and biochemical experiments described were designed to test this hypothesis for the rat cerebello-thalamic pathway. Unilateral electrolytic lesions of the superior cerebellar peduncle resulted in modest falls of CAT from both ventromedial thalamic nuclei (contralateral 35%, ipsilateral 15%). Iontophoretic application of ACh to relay cells evokes three types of response (i) excitation (ii) inhibition (iii) polyphasic combinations of (i) and (ii). The type of response evoked was directly related to the firing pattern of the cell. Thus, for example, excitatory responses were never recorded during high-frequency bursting but were easily evoked following a switch to tonic, single-spike activity. All responses to ACh and synaptic responses to cerebellar stimulation were sensitive to muscarinic but not to nicotinic cholinergic antagonists. The nicotinic antagonist mecamylamine was a potent blocker of excitant amino acid responses but had no effect on cerebellar-evoked synaptic responses. Cholinergic and anticholinergic agents had a profound action on relay cell firing pattern. ACh promoted single-spike activity whereas atropine promoted high-frequency bursting. The actions of ACh are discussed with reference to recently discovered voltage-sensitive ionic conductances. Because of the modulatory action of ACh on relay cell firing pattern and excitability no firm conclusion can be reached concerning the hypothesis under test here. We tentatively suggest a dual role for ACh as both neurotransmitter and neuromodulator.

A pharmacological investigation of the electrically evoked convulsive activity induced by administration of catechol in the anaesthetized rat

1 The response evoked by electrical stimulation at the wrist has been recorded from muscles of the forelimb of anaesthetized rats induced to convulse by administration of catechol.2 This response can be divided into three temporally distinct components, the characteristics of which have been described.3 The probability of occurrence of the two early components of the response has been measured before and after administration of various drugs. The results show that the first component is not affected by cholinoceptor or adrenoceptor blocking drugs or anticholinesterase agents. The probability of occurrence of the second component is significantly reduced by cholinoceptor blocking drugs and increased by physostigmine.4 The implications of these results in explaining the convulsant actions of catechol are discussed.

Localization of calcium channels in Paramecium caudatum

1. Electrical recordings from Paramecium caudatum were made after removal of the cilia with chloral hydrate and during ciliary regrowth to study the electrical properties of that portion of the surface membrane enclosing the ciliary axoneme. 2. Removal of the somatic cilia (a 50% reduction in membrane surface area) results in an almost complete elimination of the regenerative Ca response, all-or-none Ba2+ spike, and delayed rectification. 3. A twofold increase in input resistance resulted from the 50% reduction in membrane surface area. 4. The electrical properties remained unchanged, despite prolonged exposure to the chloral hydrate, until the cilia were mechanically removed. 5. Restoration of the Ca response accompanied ciliary regrowth, so that complete excitability returns when the cilia regain their original lengths. 6. It is concluded that the voltage-sensitive Ca channels are localized to that portion of surface membrane surrounding the cilia. 7. Measurements of membrane constants before and after deciliation and estimations of the cable constants of a single cilium suggest that the cilia of Paramecium may be fully isopotential along their length and with the major cell compartment.

Brain stem stimulation and the acetylcholine-evoked inhibition of neurones in the feline nucleus reticularis thalami

1. In cats anaesthetized with halothane and nitrous oxide, the responses to iontophoretically applied acetylcholine (ACh) and to high-frequency stimulation of the mid-brain reticular formation (MRF) were tested on spontaneously active neurones in the nucleus reticularis thalami and underlying ventrobasal complex.2. The initial response to MRF stimulation of 90% of the ACh-inhibited neurones found in the region of the dorsolateral nucleus reticularis was an inhibition. Conversely, the initial response of 82% of the ACh-excited neurones in the ventrobasal complex was an excitation. Neurones in the rostral pole of the nucleus reticularis were inhibited by both ACh and RMF stimulation.3. The mean latency (and s.e. of mean) for the MRF-evoked inhibition was 13.7 +/- 3.2 ms (n = 42) and that for the MRF-evoked excitation, 44.1 +/- 4.2 ms (n = 35).4. The ACh-evoked inhibitions were blocked by iontophoretic atropine, in doses that did not block amino acid-evoked inhibition. In twenty-four ACh-inhibited neurones the effect of iontophoretic atropine was tested on MRF-evoked inhibition. In all twenty-four neurones atropine had no effect on the early phase of MRF-evoked inhibition but weakly antagonized the late phase of inhibition in nine of fourteen neurones.5. Interspike-interval histograms showed that the firing pattern of neurones in the nucleus reticularis was characterized by periods of prolonged, high-frequency bursting. Both the ACh-evoked inhibitions and the late phase of MRF-evoked inhibitions were accompanied by an increased burst activity. In contrast, iontophoretic atropine tended to suppress burst activity.6. The possibility is discussed that electrical stimulation of the MRF activates an inhibitory cholinergic projection to the nucleus reticularis. Since neurones of the nucleus reticularis have been shown to inhibit thalamic relay cells, activation of this inhibitory pathway may play a role in MRF-evoked facilitation of thalamo-cortical relay transmission and the associated electrocortical desynchronization.

General anaesthetics and the acetylcholine-sensitivity of cortical neurons

1The effects of general anaesthetics on neuronal responses to iontophoretically-applied acetylcholine have been examined in slices of guinea-pig olfactory cortex maintained in vitro. 2 Acetylcholine excited 61% of the prepiriform neurones tested. The excitation was blocked by atropine, but not by dihydro-beta-erythroidine or gallamine. 3 Alphaxalone reversibly depressed the acetylcholine-sensitivity of prepiriform neurones. Pentobarbitone did not consistently depress the acetylcholine sensitivity of these cells. 4 Ether, methoxyflurane, trichloroethylene and halothane caused a dose-related augmentation of acetylcholine-induced firing. 5 These results show that general anaesthetics do not necessarily depress the sensitivity of nerve cells to all excitatory substances and that different anaesthetics may affect a particular excitatory process in various ways.

Inhibitory effects of acetylcholine on neurones in the feline nucleus reticularis thalami

1. Short iontophoretic pulses of acetylcholine (ACh) inhibited almost every spontaneously active cell encountered in the nucleus reticularis thalami of cats anaesthetized with a mixture of halothane, nitrous oxide and oxygen. On 200 cells the mean current needed to eject an effective inhibitory dose of ACh was 67 +/- 2 nA. When the ACh-evoked inhibition was mimicked by gamma-aminobutyric acid (GABA) or glycine on the same cell, the current required to release ACh was found to be approximately twice as great as that required to release an equally effective dose of GABA or glycine. 2. ACh inhibitions developed with a latency which was very much shorter than that for ACh excitation in cells of the ventrobasal complex. The latency of the ACh-evoked inhibition was as rapid as the onset and offset of the excitation of the same cells glutamate and their inhibition by GABA or glycine. 3. The firing pattern of ACh-inhibited neurones in the nucleus reticularis was characterized by periods of prolonged, high frequency bursts, and their mean firing frequency was 22 Hz. Raster dot displays and interspike interval histograms showed that whereas ACh suppressed the spikes that occurred between bursts much more readily than those that occurred during bursts, all spikes were equally sensitive to the depressant action of GABA and glycine. Large doses of ACh provoked or exaggerated burst activity. 4. ACh-evoked inhibition was extremely sensitive to blockade by short iontophoretic applications of atropine, which had no effect on the inhibitions evoked on the same cell equipotent doses of GABA or glycine. The ACh-evoked inhibitions were also antagonized by dihydro-beta-erythroidine released with slightly larger currents. When tested on the same cell, small iontophoretic applications of picrotoxin and bicuculline methoiodide blocked the inhibition evoked by GABA but had no effect on that evoked by ACh. Iontophoretic strychnine only rarely affected the inhibition evoked by ACh, while readily blocking the inhibition evoked on the same cell by an equipotent dose of glycine. In two cats, intravenous strychnine (1-2 mg/kg) had no effect on the ACh-evoked inhibition, while greatly reducing the sensitivity of the cell under study to glycine. 5. Only four out of forty-eight ACh-inhibted cells tested were inhibited by iontophoretic applications of either guanosine or adenosine 3':5'-phosphate. 6. Cells of the nucleus reticularis have been shown to have an inhibitory action on the thalamic relay cells, which are excited by ACh. It is suggested that the presence of both ACh excited and inhibited cells in different nuclei of the thalamus could be of considerable functional significance in gating sensory transmission through the thalamus.

Observations on the pharmacology of cholinoceptive neurones in the rat brain stem

The pharmacology of spontaneously active cholinoceptive neurones in the brain stem of rats anaesthetized with urethane has been investigated using microiontophoresis to administer muscarinic and nicotinic agonists and antagonists. 2. Acetylcholine (ACh) excited most cells but occasionally depressed their activity. Muscarine, and the muscarinic agonists methacholine and bethanechol produced prolonged excitation or inhibition of cells whereas nicotine produced prolonged excitations but no inhibitions. 3 Atropine selectively antagonized ACh excitations and both excitation and inhibition of neuronal activity produced by muscarine and muscarinic agonists, but not the excitations produced by nicotine, glutamate or DL-homocysteic acid. 4 Dihydro-beta-erythroidine (DHBE) and tubocurarine antagonized both ACh and nicotine excitations but not those induced by glutamate or DL-homocysteic acid. Inhibitions by ACh or muscarine were not affected. 5 It is concluded that excitations of cholinoceptive neurones in the rat brain stem may be mediated by activation of both muscarinic and nicotinic receptors whereas inhibitions are mediated by activation of a muscarinic receptor.

An analysis of the representation of the forelimb in the ventrobasal thalamic complex of the albino rat

1. Glass micro-electrodes have been used to record from a total of 998 units situated in the ventrobasal thalamic complex in the deeply anaesthetized albino rat. 2. Of these units 889 responded to electrical stimulation of the contralateral forelimb and fifty-one to the contralateral hind limb. The remaining units consisted of those with receptive fields on the trunk, head and those which responded to stimulation of more than one limb. Only the latter group of units showed any spontaneous activity in the absence of intentional stimulation. 2. Of the units which responded to electrical stimulation of the contralateral forelimb the receptive fields, modality and latencies of response were accurately determined for 505 units. The mean latency to supramaximal stimulation at the wrist was 4.49 (+/- 0.04 S.E. of mean) msec; and to mechanical stimulation (for 146 of these units) at the centre of the receptive field 6.58 (+/- 0.12) msec. The modalities were distributed as follows: light pressure, 391; heavy pressure, 47; hair movement, 40; claw sensitive, 15 and joint movement, 12 units. 4. The forelimb representation within the ventrobasal thalamic complex was somatotopically organized, the over-all appearance being that of an incompletely closed fist, palmar surface uppermost, thumb media, with the wrist caudal and the digital tips rostral and dorsal. 5. The central projection was distorted, some parts showing expanded representation, notably the tips of digits II and III and the medial wrist pad. Other parts were contracted, e.g. the wrist, forearm and shoulder. 6. Units with receptive fields consisting of the whole of a walking pad had shorter mean latencies, to tactile stimulation, than those whose field was a single spot on a pad. 7. Units were found to show an abolute unresponsive time to the second of a pair of identical supramaximal electrical stimuli of up to 50 msec, and a relative unresponsive time which could last up to 500 msec. The absolute unresponsive and relative unresponsive times to the second of a pair of tactile stimuli was shorter being 30 and 150 msec respectively. 8. The effect of decortication was to increase the excitability of thalamic units to peripheral stimulation both in the initial and later discharges.

Micro-electrophoretic studies in the cat pulvinar region: effect of acetylcholine

1. In the posterior half of the pulvinar of cats anaesthetized with halothane and nitrous oxide, the majority of neurons were fired by ACh released with small electrophoretic currents. In the anterior part of that nucleus, ACh had more variable effects: excitation, depression or none. 2. In comparison with L-glutamate, DL-homocysteic acid and DL-aspartic acid, ACh appeared to be the most potent excitant. 3. ACh-induced discharges were easily and reversibly blocked by low doses of atropine. In most cases, ACh effects could not be blocked selectively by mecamylamine or dihydro-beta-erythroidine. 4. Nicotine failed to mimic ACh, whereas carbachol was a potent excitant and was readily blocked by low doses of atropine. 5. The histochemical reaction to acetylcholinesterase was moderate in the pulvinar. 6. These observations support the view that pulvinar cells differ from other thalamic cells.

Cholinergic and non-cholinergic transmission in the medial geniculate nucleus of the cat

1. Studies involving the electrophoretic administration of antagonists of ACh (atropine, DHbetaE) and cholinesterase inhibitors (neostigmine, physostigmine) to MGN neurones indicate that ACh is an excitatory transmitter in the feline MGN, most probably released from fibres which originate in or traverse the mesencephalon.2. Auditory afferents to the MGN, cortico-geniculate fibres and the excitatory fibres which mediate ;spontaneous' firing of MGN neurones are unlikely to be cholinergic.3. Almost all geniculo-cortical relay cells are excited by ACh, this excitation being mediated by receptors which have both muscarinic and nicotinic properties. The excitation of relay cells by ACh is sometimes preceded or followed by a depression of firing which is resistant to atropine and DHbetaE, but the significance of this depression is unknown.4. The firing of many unidentified MGN neurones is depressed by ACh in the absence of any excitation, and this depression is blocked by both atropine and DHbetaE, and potentiated by anticholinesterases. This type of depression by ACh may be related to cholinergic inhibition, but this possibility has yet to be investigated.

Short-latency excitation of brain stem neurones in the rat by acetylcholine

A fast excitatory response to acetylcholine (ACh) which has not previously been reported, has been found in the rat brain stem. Micro-iontophoretic applications of ACh to single brain stem neurones in unanaesthetized rats excited 81% and inhibited 3% of neurones studied. Two types of excitatory response were distinguished by their time course. Type I ACh excitation of neurones was of long latency resembling that previously reported in various parts of the brain. Type II excitation was of short latency, similar to that of micro-iontophoretically applied glutamate ions and to ACh excitation of Renshaw cells.

Studies on cholinergic transmission in the medial geniculate nucleus

1. Studies were made on the effects of iontophoretically and intravenously administered cholinergic antagonists on the synaptic responses of medial geniculate (MG) neurones evoked by stimulation of the auditory cortex, inferior colliculus and mesencephalic reticular formation.2. Atropine specifically blocked a proportion of the excitatory responses evoked by stimulating the auditory cortex, inferior colliculus and reticular formation, although it was without effect on some of them.3. Neostigmine and eserine facilitated some excitatory synaptic responses evoked by inferior collicular stimulation.4. It is suggested that the feline MG nucleus receives excitatory cholinergic, as well as non-cholinergic, pathways from the auditory cortex, inferior colliculus and lower brain stem. The cholinergic pathways from the auditory cortex may be either corticofugal fibres or recurrent axon collaterals of afferent projections from the MG nucleus to the cortex. Those from the lower brain stem are possibly the cholinesterase-containing fibres described by Shute & Lewis (1967).

Properties of cholinoceptive neurones in the medial geniculate nucleus

1. Acetylcholine (ACh), other cholinomimetics, cholinesterase inhibitors and cholinergic antagonists were administered iontophoretically to medial geniculate (MG) neurones and their effects on chemically or neurally evoked responses recorded extracellularly.2. Acetylcholine had excitant actions on 45% of the neurones tested. Most of these were of a slow time course. Desensitization to the excitant effects was frequently observed.3. Acetylcholine excited 91% of neurones activated antidromically by stimulation of the auditory cortex, 71% of neurones activated synaptically from the auditory cortex, 74% of neurones activated from the inferior colliculus and 100% of geniculo-cortical relay neurones.4. Acetylcholine had depressant effects, which were generally of a rapid time course, on 29% of MG neurones. No desensitization to the depressant effects was observed.5. On 4% of neurones, ACh had both excitant and depressant effects. Such "dual" effects were manifested either as an initial excitation followed by a depression, or as a depression followed by an excitation.6. Eserine, neostigmine and edrophonium potentiated both excitant and depressant actions of ACh on many cells. Neostigmine and edrophonium occasionally antagonized the effects of ACh.7. Atropine, hyoscine, dihydro-beta-erythroidine, hexamethonium and (+)-tubocurarine antagonized both excitant and depressant effects of ACh. The muscarinic blocking agents were usually more effective than the nicotinic agents.8. Carbamylcholine, acetyl-beta-methylcholine, nicotine, butyrylcholine, arecoline and pilocarpine had excitant, depressant or no effects on MG neurones. Generally, carbamylcholine was more potent than acetyl-beta-methylcholine and ACh, which were more potent than nicotine. Butyrylcholine, arecoline and pilocarpine were even less potent, often having no effect.9. The cholinomimetics generally had similar effects to those of ACh on the same neurones, but sometimes were quite different. Carbamylcholine, acetyl-beta-methylcholine and nicotine antagonized the effects of ACh on some neurones.10. The results suggest that cholinoceptive receptors on MG neurones are not homogeneous. Although there are possibly some purely muscarinic and purely nicotinic receptors, the majority appear to be of intermediate muscarinic-nicotinic type. These mediate either excitation or inhibition.