THE EXCITATION OF THALAMIC NEURONES BY ACETYLCHOLINE

Enhanced cortical responsiveness during natural sleep in freely behaving mice

Cortical networks exhibit large shifts in spontaneous dynamics depending on the vigilance state. Waking and rapid eye movement (REM) sleep are characterized by ongoing irregular activity of cortical neurons while during slow wave sleep (SWS) these neurons show synchronous alterations between silent (OFF) and active (ON) periods. The network dynamics underlying these phenomena are not fully understood. Additional information about the state of cortical networks can be obtained by evaluating evoked cortical responses during the sleep-wake cycle. We measured local field potentials (LFP) and multi-unit activity (MUA) in the cortex in response to repeated brief optogenetic stimulation of thalamocortical afferents. Both LFP and MUA responses were considerably increased in sleep compared to waking, with larger responses during SWS than during REM sleep. The strongly increased cortical response in SWS is discussed within the context of SWS-associated neuro-modulatory tone that may reduce feedforward inhibition. Responses to stimuli were larger during SWS-OFF periods than during SWS-ON periods. SWS responses showed clear daily fluctuation correlated to light-dark cycle, but no reaction to increased sleep need following sleep deprivation. Potential homeostatic synaptic plasticity was either absent or masked by large vigilance-state effects.

The inhibitory action of noradrenaline and other monoamines on spinal neurones

1. L-Noradrenaline (NA), 5-hydroxytryptamine (5-HT) and acetylcholine (ACh) were administered micro-electrophoretically to feline lumbar neurones while recording their spike potentials extracellularly.2. There was no evidence to suggest that NA acts as an excitatory transmitter in the spinal cord.3. NA had potent inhibitory effects on some interneurones as revealed by a depression of spontaneous and synaptic firing and on the firing to a local application of an excitant amino acid. The effects on Renshaw cells and motoneurones were less marked.4. The depressant actions of 5-HT were less marked than those of NA. ACh and carbamylcholine had depressant effects on some NA-sensitive interneurones but were invariably far less potent and on other NA-sensitive cells were completely inactive.5. NA had no detectable effect on the normal spike amplitude but when the action potentials were reduced by excessive depolarization then both NA and synaptic inhibition increased the spike amplitude; this effect could be due to a hyperpolarization of the cell membrane.6. There was a correlation between the distribution of NA-sensitive cells and the relative densities of NA-containing terminals in various layers of the grey matter.7. It was postulated that NA acts as an inhibitory transmitter released from the terminals of descending pathways in the spinal cord. Other possible mechanisms were discussed but lacked experimental support.

Molecular biology of glutamate receptors in the central nervous system and their role in excitotoxicity, oxidative stress and aging

Forty years of research into the function of L-glutamic acid as a neurotransmitter in the vertebrate central nervous system (CNS) have uncovered a tremendous complexity in the actions of this excitatory neurotransmitter and an equally great complexity in the molecular structures of the receptors activated by L-glutamate. L-Glutamate is the most widespread excitatory transmitter system in the vertebrate CNS and in addition to its actions as a synaptic transmitter it produces long-lasting changes in neuronal excitability, synaptic structure and function, neuronal migration during development, and neuronal viability. These effects are produced through the activation of two general classes of receptors, those that form ion channels or "ionotropic" and those that are linked to G-proteins or "metabotropic". The pharmacological and physiological characterization of these various forms over the past two decades has led to the definition of three forms of ionotropic receptors, the kainate (KA), AMPA, and NMDA receptors, and three groups of metabotropic receptors. Twenty-seven genes are now identified for specific subunits of these receptors and another five proteins are likely to function as receptor subunits or receptor associated proteins. The regulation of expression of these protein subunits, their localization in neuronal and glial membranes, and their role in determining the physiological properties of glutamate receptors is a fertile field of current investigations into the cell and molecular biology of these receptors. Both ionotropic and metabotropic receptors are linked to multiple intracellular messengers, such as Ca2+, cyclic AMP, reactive oxygen species, and initiate multiple signaling cascades that determine neuronal growth, differentiation and survival. These cascades of complex molecular events are presented in this review.

THE EXCITATION AND DEPRESSION OF MAMMALIAN CORTICAL NEURONES BY AMINO ACIDS

Amino acids related to L-glutamic and gamma-amino-n-butyric acid have been administered electrophoretically, and by pressure ejection, into the extraneuronal environment of single neurones in the pericruciate cortex of cats anaesthetized with allobarbitone or allobarbitone-urethane. Acidic amino acids related to glutamic acid, particularly N-methyl-D-aspartic acid, excited cortical neurones. Neutral amino acids related to gamma-amino-n-butyric acid, particularly 3-amino-1-propanesulphonic acid, depressed cortical neurones. Some of the depressants blocked the antidromic invasion of Betz cells by pyramidal volleys. There are no essential differences between the sensitivities of cortical and spinal neurones towards locally administered amino acids. A transmitter function of such amino acids within the mammalian central nervous system is considered unlikely.

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.

Effects of oxotremorine on neuronal RNA and chromatin in thalamic cholinoceptive sites

Neuronal nucleic acid responses were examined within the rat thalamic ventrobasal nuclear complex (VBC) and nucleus reticularis (NR) following single intraperitoneal injections of the central muscarinic-cholinergic (M2) receptor agonist oxotremorine (0.1, 0.7, or 1.0 mg/kg). After stoichiometric azure B and Feulgen staining of brain sections, scanning-integrating cytophotometry was used to quantify azure B-ribonucleic acid (RNA) content, Feulgen-DNA levels, and changes in the susceptibility of chromatin to Feulgen acid hydrolysis (F-DNA yield) of neurons on an individual basis. Changes in neuronal nucleolar volume were also determined histometrically. Within the VBC, oxotremorine produced marked dose-dependent elevations in neuronal RNA content and nucleolar volume with increased F-DNA yield (chromatin activation) in a proportion of VBC neurons. In contrast, within the NR, oxotremorine elicited reductions in RNA levels, F-DNA yield and nucleolar volume. The data demonstrate that oxotremorine-induced central muscarinic receptor stimulation is associated with metabolic correlates of thalamic VBC neuroexcitation and NR neuron depression. The overall study lends further credence to the hypothesis that muscarinic-cholinergic mechanisms are operative within the mammalian thalamus.

Localization of glutamate in trigeminothalamic projection neurons: a combined retrograde transport-immunohistochemical study

Trigeminothalamic projection neurons are important components of the pathways for conscious perception of pain, temperature, and tactile sensation from the orofacial region. The neurotransmitters utilized by trigeminal neurons projecting to the thalamus are unknown. By use of a monoclonal antibody specific for fixative-modified glutamate and a polyclonal antiserum against glutaminase, we recently identified neurons in the trigeminal sensory complex that contain glutamate-like immunoreactivity (Glu-LI) and glutaminase-like immunoreactivity. In the present study, we utilized combined retrograde transport-immunohistochemical techniques to localize putative glutamatergic trigeminothalamic neurons. Following injection of the retrograde tracer, wheatgerm agglutinin conjugated to horseradish peroxidase (WGA:HRP), into the ventroposterior medial thalamus (VPM), the number of neuronal profiles that were double-labeled with WGA:HRP and Glu-LI was greatest in principal sensory nucleus (Pr5), followed by subnuclei interpolaris (Sp5I) and caudalis (Sp5C). The average percentages of projection neurons double-labeled with Glu-LI were approximately 60-70% in Pr5 and Sp5I and 40% in Sp5C. The majority of double-labeled profiles in Sp5C were located in the magnocellular layer, as opposed to the marginal and substantia gelatinosa layers. A large injection site that spread into the intralaminar thalamic nuclei and nucleus submedius--areas implicated in the processing of nociceptive information--resulted in an increase in the ratio of single-labeled to double-labeled projection profiles in Sp5C. These results suggest that glutamate may be the neurotransmitter for a majority of trigeminothalamic projection neurons located in Sp5I and Pr5. However, on the basis of anatomical association, glutamate does not appear to be the major transmitter for neurons in Sp5C that forward nociceptive information to the thalamus.

Actions of acetylcholine and nicotine on rat locus coeruleus neurons in vitro

Intracellular recordings were made from neurons of the rat locus coeruleus in a slice of tissue cut from the pons and superfused in vitro at 37 degrees C. Both acetylcholine, in the presence of a muscarinic antagonist, and nicotine depolarized locus coeruleus neurons and increased the rate of action potential discharge. These effects persisted in a superfusing solution that contained no calcium ions and a raised magnesium ion concentration; because such solutions block synaptic potentials in the locus coeruleus, it was concluded that the acetylcholine and nicotine acted directly on the cell from which the recording was made. Acetylcholine-evoked depolarizations were blocked by hexamethonium or dihydro-beta-erythroidine, but not by alpha-bungarotoxin. Recordings of membrane current showed that acetylcholine and nicotine caused a net inward current, the amplitude of which increased linearly as the membrane potential was changed from -40 to -90 mV; the extrapolated reversal potential was about -20 mV (extracellular potassium concentration was 2.5 mmol/l). Depolarizations caused by nicotine declined during the presence of the agonist when the period of superfusion was continued for more than 2 min; repeated applications of nicotine evoked reproducible depolarizations only when the interval between them was at least 1 h. It is concluded that neurons of the rat locus coeruleus have nicotinic receptors with properties similar to those in peripheral ganglia with respect to reversal potential for the ion channel, insensitivity to alpha-bungarotoxin and propensity to desensitize.

Excitatory and differential disinhibitory actions of acetylcholine in the lateral geniculate nucleus of the cat

Single neurones were recorded in the dorsal lateral geniculate nucleus (d.l.g.n.) of adult cats anaesthetized with a mixture of halothane, nitrous oxide and oxygen. The multibarrel-glass micro-electrodes were filled with sodium acetate, L-glutamate, acetylcholine (ACh), gamma-aminobutyric acid (GABA) and bicuculline. In normally innervated, spontaneously active d.l.g.n. cells, ACh and L-glutamate elicited increased firing rates. After elimination of the excitatory input from the retina by retinal photocoagulation, the effects of ACh and L-glutamate were similar. This proves that both drugs have direct excitatory effects on d.l.g.n. cells and that disinhibition is not the most prominent influence of ACh in the d.l.g.n. The excitatory action of ACh on relay cells in the d.l.g.n. was strongly influenced by barbiturates. Sub-narcotic levels of sodium pentobarbitone completely abolished the excitation by ACh while the response to L-glutamate remained unchanged. Excitation, centre-surround antagonism and periphery effects were elicited by spots of light and by large field phase-reversing gratings with and without central sparing of the receptive field area. Binocular inhibition was elicited with the phase-reversing grating presented to the non-dominant eye. After localized destruction of the retinal receptive field area, retinogeniculate excitation ceased and an isolated lateral inhibition was observed in the acutely deafferented d.l.g.n. cells. The time course and strength of this inhibition was disclosed by raising the background discharge with microiontophoretically applied L-glutamate. With increasing size of retinal lesions the strength of isolated lateral inhibition decreased exponentially. A maximal intrageniculate range of more than 1000 microns was derived from computations of the lateral extent of deafferentation in the d.l.g.n. The inhibition acted beyond the classic surround inhibition of d.l.g.n. cells and thus was named long-range lateral inhibition. Microiontophoretically applied GABA elicits a strong inhibitory effect at the d.l.g.n. cells which is antagonized by bicuculline. Centre-surround antagonism, binocular inhibition and long-range inhibition were blocked by bicuculline and thus proven to be GABAergic. Each class of inhibition was differentially influenced by microiontophoretically applied ACh. Long-range inhibition was disinhibited, centre-surround antagonism was enhanced, and binocular inhibition was not significantly changed. In contrast to ACh excitation, the disinhibitory action of ACh was not suppressed by pentobarbitone.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of HI-6 and pralidoxime on neuronal RNA in thalamic cholinergic sites

Quantitative azure B-RNA cytophotometry was employed to compare effects of the oximes HI-6 and pralidoxime (2-PAM) to those of atropine sulfate (AS) on neuronal RNA metabolism in the thalamic ventrobasal nuclear complex (VBC) and nucleus reticularis (NR). The ability of these compounds to mitigate soman (pinacolyl methylphosphonofluoridate)-induced neuronal RNA alterations (i.e., VBC-RNA depletion/NR-RNA elevation) in these muscarinic cholinergic sites was also determined. Generally, HI-6 (125 mg/kg, i.p.) and 2-PAM (43.2 mg/kg, i.m.) elicited similar patterns of neuronal RNA changes, i.e., diminution of VBC-RNA and NR-RNA with oximes alone; partial amelioration of soman (1.5 LD50, s.c.)-induced VBC-RNA loss; and slight or no effect on soman induced NR-RNA accumulation. HI-6 produced more severe RNA reduction than 2-PAM in both brain regions of non-poisoned rats, whereas 2-PAM was more effective in reversing the effects of soman in these two regions. The muscarinic antagonist, AS, also produced VBC-RNA depletion and partially counteracted the VBC-RNA loss in soman intoxicated rats. Unlike the oximes, however, AS resulted in NR-RNA accumulation and it also antagonized soman induced NR-RNA elevation. Neither oxime reactivated soman inhibited brain acetylcholinesterase but HI-6 did reactivate appreciable plasma cholinesterase. The overall data suggest that HI-6 and 2-PAM do exert pharmacologic actions on cholinergic neurons in the rat CNS. However, the greater effectiveness of HI-6 over 2-PAM in countering lethal actions of soman does not appear to be correlated with oxime mediated restoration of neuronal RNA levels in these two cholinergic regions.(ABSTRACT TRUNCATED AT 250 WORDS)

Cytophotometric analyses of thalamic neuronal RNA in soman intoxicated rats

Quantitative azure B-RNA cytophotometry was used to monitor metabolic responses of individual neurons within the ventrobasal nuclear complex (VBC) and nucleus reticularis (NR) of the rat thalamus following administration of soman (0.5, 0.9 or 1.5 LD50, sc). A dose-dependent depression in brain acetylcholinesterase (AChE) was evidenced. With respect to thalamic RNA responses, a complex pattern of RNA alterations was evidenced, with these two regions generally exhibiting opposite patterns of dose-related RNA changes. With sub-lethal dosages of soman, RNA accumulation was evidenced in the acetylcholine (ACh) mediated excitatory VBC region and RNA depletion in the ACh mediated inhibitory NR neurons. With a lethal dose, an opposite RNA response pattern was observed in both thalamic regions. It is postulated that the observed RNA response pattern with sub-lethal dosages of soman is what one would anticipate with cholinergic brainstem reticular formation activation. The absence of such a response with lethal doses strongly suggests some disruption of functional excitatory cholinergic activity and perhaps also an impairment of inhibitory cholinergic synaptic activity.

The effects of naloxone, morphine and methionine enkephalinamide on Ia afferent terminations in the cat spinal cord

Naloxone, morphine, Met5-enkephalinamide (MENKA) and procaine were administered microelectrophoretically near extracellularly stimulated extensor muscle group Ia afferent fibres and terminations in the lumbar spinal cord of cats anaesthetized with pentobarbitone sodium. Observations were made of effects on the electrical threshold, on the depolarizing action of GABA or piperidine-4-sulphonate (P4S), and on bicuculline-sensitive primary afferent depolarization (PAD) generated by tetanic stimulation of flexor muscle low threshold afferents. All 4 agents reversibly elevated the threshold of Ia fibres in the dorsal column and Ia terminations in the ventral horn. The depolarizations of terminations by GABA or P4S were also reduced, an effect, which for all except MENKA, probably accounted for a concomitant reduction in PAD. In the absence of a consistent effect on either threshold or depolarization by GABAmimetics, MENKA reversibly diminished PAD, an action blocked by naloxone. Intravenously administered naloxone, in doses known to enhance spinal monosynaptic excitation in the cat, had no effect on GABAergic PAD and little or no effect on Ia termination threshold. The results are discussed in relation to a naloxone-sensitive effect of MENKA which reduces transmitter release from GABAergic axo-axonic synapses on Ia terminals, but which does not account for the enhancement of spinal reflexes by naloxone.

The depolarization of feline ventral horn group Ia spinal afferent terminations by GABA

The unmyelinated terminal regions of extensor muscle Ia afferent fibres were stimulated electrically near lumbar motoneurones in anaesthetised cats using 300 microseconds pulses of less than 1 microA passed through the central NaCl barrel of seven barrel micropipettes. Such terminations were identified by anodal blocking factors of less than four and the latency of the antidromic impulse recorded in the appropriate peripheral muscle nerve. Although the effects of microelectrophoretically administered GABA were occasionally complex, the most consistent finding was a reduction in termination threshold followed by an increase. Both this reduction in threshold by GABA, and that produced by tetanic stimulation of low threshold flexor afferents (PAD) were diminished by microelectrophoretic bicuculline methochloride. This GABA antagonist alone elevated the threshold of some terminations but did not reduce the depolarizing action of either potassium or L-glutamate. Furthermore, since reductions in threshold by GABA, but not by either potassium or L-glutamate, were associated with a decrease in PAD, GABA appears to increase terminal membrane conductance. Since neither GABA nor bicuculline methochloride influenced the threshold or afferent depolarization of non-terminal regions of Ia fibres, there results are consistent with the function of GABA as a depolarizing transmitter at gabergic axoaxonic synapses upon the terminals of Ia afferent fibres synapsing with motoneurones.

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.

Localization of cholinergic muscarinic receptors in rat brain by light microscopic radioautography

Injection of a potent, cholinergic muscarinic antagonist, [3H]3-quinuclidinyl benzilate ([3H]QNB), results in a localization of the drug to muscarinic receptors in rat brain. The distribution of these drug receptors was examined in various brain regions previously thought to contain cholinergic neurons. They were localized to dendritic regions in the hippocampus, corpus striatum, nucleus accumbens and cerebral cortex. Particularly in the hippocampus, the receptor distribution may correspond to that for cholinergic nerve terminals.

The sensitivity of paramedian reticular neurones to acetylcholine

1. Of paramedian reticular neurones a significantly higher proportion of those antidromically activated from the cerebellum than of those orthodromically activated from this source were excited by acetylcholine. 2. Receptors for acetylcholine were of the muscarinic type. 3. No differences were found in the proportions of cholinoceptive and non-cholinoceptive cells responding to stimulation of cranial and limb nerves or to changes in blood pressure either spontaneous or induced. 4. Either a cholinergic pathway to paramedian reticular cells projecting to the cerebellum was not activated in these experiments or the receptors for acetylcholine are not located at synapses. An association between muscarinic receptors and acetylcholinesterase may be present with cells of this area.

Muscarinic excitation: a microelectrophoretic study on cultured smooth muscle cells

1 Acetylcholine was applied iontophoretically to smooth muscle cells cultured from taeniae coli of new-born guinea-pigs. Responses were recorded with intracellular microelectrodes.2 Acetylcholine induced depolarization, spike generation and contraction. Large conductance increases could be measured during the action of acetylcholine.3 Injection of depolarizing currents through the recording electrode reversed the sign of potential responses. The reversal potential was -5 to -25 mV.4 Minimum latencies of responses to acetylcholine were 120-500 ms. These values were not attributable to diffusion time.5 Attention is drawn to the long latencies of a variety of muscarinic responses, and the suggestion made that muscarinic mechanisms as a class may be characterized by a long activation time.

Spontaneous quantal transmitter release: a statistical analysis and some implications

1. Miniature end-plate potentials (m.e.p.p.s) were intra- and extracellularly recorded from neuromuscular junctions in rat phrenic nerve-diaphragm preparations in vitro.2. Statistical analysis of the intervals between m.e.p.p.s showed that when the mean number of events in time t was plotted as a function of the variance of the events in time t there was a significant deviation from the straight line relationship expected for a Poisson process. Computer simulation showed that this deviation is explicable if release was generated by the random phasing of the activity of a number of releasing sites.3. There was no indication that release of one quantum influences the probability of release of remaining quanta (drag, clustering). It is suggested that m.e.p.p.s whose amplitude is larger than the mode result from the release of the contents of vesicles whose volume is also supramodal.4. The effects of depolarization of nerve terminals upon the variance-mean curve suggest an increase in the activity of sites rather than an increase in their number.5. Statistical analysis indicated at least 200 +/- 100 (mean +/- 1 S.E.) releasing sites. This number is of the same order as the number of sites of vesicle aggregation and presynaptic membrane density seen in electron micrographs of nerve terminals of this preparation.

Modification of the responses of brain stem neurones to transmitter substances by anaesthetic agents

1. The effects of microiontophoretic applications of acetylcholine (ACh), (-)-noradrenaline ((-)-NA) and 5-hydroxytryptamine (5-HT) have been investigated on spontaneously active brain stem neurones in decerebrate unanaesthetized rats and in rats anaesthetized with either tribromoethanol, urethane or pentobarbitone.2. Four types of responses to both (-)-NA and 5-HT were seen. These were: simple excitation; excitation preceded by a short-lasting inhibition; short-lasting inhibition and prolonged inhibition. Three types of responses to ACh were seen: an excitation with long latency of onset; excitation with short latency of onset, resembling the response to an excitant amino acid, and a short-lasting inhibitory response.3. The types of responses to microiontophoretically applied ACh, (-)-NA or 5-HT in anaesthetized and unanaesthetized animals were similar.4. The number of ACh excitatory responses with short latency of onset were significantly reduced in the pentobarbitone-anaesthetized group and a small but significant increase in the number of 5-HT inhibitory effects were observed in each anaesthetized group of animals.5. A significantly greater proportion of slower firing neurones (less than 10 spikes/s) were found in the pentobarbitone-anaesthetized animals.6. The effects of microiontophoretically applied and i.v. administered pentobarbitone were studied on spontaneously active neurones which responded consistently to ACh and a control agonist.7. Pentobarbitone administered by either route reduced the firing rate of most neurones studied and was shown to antagonize specifically the excitation of neurones by exogenously applied ACh.8. It is suggested that postsynaptic antagonism of endogenously released ACh may be a contributing factor in the mechanism of action of pentobarbitone.

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.

The interaction of presynaptic polarization with calcium and magnesium in modifying spontaneous transmitter release from mammalian motor nerve terminals

1. The relationship between motor terminal polarization and miniature end-plate potential (m.e.p.p.) frequency was examined in the presence of various Ca, Mg and K concentrations ([Ca], [Mg] and [K]) and also at modified bathing osmolarity levels. The polarization changes were obtained with ;electrotonic' and ;focal' polarizing currents and with rapid changes in bathing [K].2. M.e.p.p. frequency increased exponentially with electrotonic depolarizing currents, but failed to decrease similarly with hyperpolarizing currents. An increase in bathing [K] to 15 mM increased the sensitivity of the terminals to presynaptic hyperpolarization.3. The slope, on semilogarithmic coordinates, of the function relating m.e.p.p. frequency to electrotonic polarizing currents (the release-current function) was unchanged when bathing [Ca] was raised from 2 to 8 mM. When [Ca] was reduced to 0.5 mM the slope of this function was reduced initially but eventually approached the same slope as in control [Ca]. A similar effect was also found in the presence of 15 mM-KCl.4. The relationship between m.e.p.p. frequency and log [K], at various [Ca], resembled the relationships between m.e.p.p. frequency and presynaptic polarizing currents.5. An increase in bathing [Mg] or osmolarity had a similar effect to a reduction of [Ca].6. Tetrodotoxin (TTX) at a concentration of 10(-6) g/ml. was found to reduce m.e.p.p. frequency, at various [K], by a constant fraction of about 30%.7. In some of the junctions ;anodic break-down' was observed. An examination of this phenomenon with focal polarizing currents disclosed an unusual type of ;anodic break-down', with rapid ;on' and ;off' responses. This phenomenon may indicate that release depends on the influx of positively charged particles into the nerve terminals.8. It is concluded that nerve terminal depolarization accelerates exponentially the activity of a membrane component bearing three Ca molecules, the rate of acceleration being independent of bathing [Ca].