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

Distribution of choline acetyltransferase immunoreactivity in the vestibular nuclei of normal and unilateral vestibular neurectomized cats

Post-lesion recovery of vestibular functions is a suitable model for studying adult central nervous system plasticity. The vestibular nuclei complex (VN) plays a major role in the recovery process and neurochemical reorganizations have been described at this brainstem level. The cholinergic system should be involved because administration of cholinergic agonists and antagonists modify the recovery time course. This study was aimed at analysing the postlesion changes in choline acetyltransferase immunoreactivity (ChAT-Ir) in the VN of cats killed 1 week, 3 weeks or 1 year following unilateral vestibular neurectomy. ChAT-positive neurons and varicosities were immunohistochemically labelled and quantified (cell count and surface measurement, respectively) by means of an image analysing system. The spatial distribution of ChAT-Ir within the VN of control cats showed darkly stained neurons and varicosities mainly located in the caudal parts of the medial (MVN) and inferior (IVN) VN, the nucleus prepositus hypoglossi (PH) and, to a lesser extent, in the medial part of the superior vestibular nucleus (SVN). Lesion-induced changes consisted in a significant increase in both the number of ChAT-positive neurons (IVN, SVN) and the surface of ChAT-positive varicosities (IVN, SVN, PH). They were observed bilaterally in the acute (1 year and 3 weeks) and compensated (1 year) cats for the SVN and PH, while they persisted only in the IVN on the lesioned side in the compensated cats. These findings demonstrate vestibular lesion-induced reorganization of the cholinergic system in the IVN, SVN and PH which could contribute to postural and oculomotor function recovery.

In vivo pharmacological effects of dihydro-beta-erythroidine, a nicotinic antagonist, in mice

The comparative in vivo pharmacology of mecamylamine and dihydro-beta-erythroidine (DH beta E) in mice was studied. Modulation of the behavioral effects (antinociception, hypomotility, motor impairment and hypothermia) of nicotine in mice by DH beta E and mecamylamine were carried out. After SC administration, DH beta E and mecamylamine were nearly equipotent in blocking nicotine's effects except for antinociception, in which mecamylamine was clearly more potent. Intrathecal injection of DH beta E was also effective in blocking the antinociceptive effect of nicotine. In vivo interaction of DH beta E with calcium and calcium channels, involved in the central actions of nicotine, showed that intrathecal administration of DH beta E failed to reduce the antinociception induced by diverse drugs which increase intracellular calcium such as thapsigargin, (+/-)-BAYK 8644 and calcium, indicating that this antagonist does not affect calcium-dependent mechanisms involved in antinociception. On the other hand, mecamylamine blocked the antinociceptive effect of the calcium modulatory drugs, suggesting that it may be acting on calcium-dependent mechanisms involved in the intracellular signaling process. We conclude that DH beta E, a nicotinic neuromuscular antagonist, is able to block some of the central actions of nicotine after systemic and intrathecal administration. The mechanism of blockade is different from that of mecamylamine, a classical ganglionic antagonist, and may involve a direct action of DH beta E on nicotine receptor.

Response of neurons of the rat anterior hypothalamic-preoptic area to carbachol

Behavioural effects of carbachol given into the hypothalamic/preoptic area have been demonstrated but there is a paucity of information about the response of single neurons to carbachol. The aim of the present study was to determine the response of spontaneously firing neurons in the rat hypothalamic/preoptic area to application of carbachol by iontophoresis or by pressure injection in a dose and volume comparable with that used in behavioural studies. Extracellular single unit recordings showed a significant decrease in mean firing rate in 82% of neurons responding to iontophoretic carbachol and in 75.5% of neurons responding to carbachol injected about 600 microns away. An increase in firing rate occurred in only 15 and 17.6% of neurons, respectively. Application of saline did not alter the mean firing rate while application of glutamate into the same areas or ejection into the vicinity of the same neurons caused an increase in mean firing rate in 94% of responding neurons. The results indicate that a decrease in mean firing rate is the predominant neuronal response to carbachol in the anteromedial hypothalamic/preoptic area of the rat and we suggest that this decrease may be associated with behavioural responses to carbachol.

The activation of phosphatidylinositol turnover is not directly involved in the modulation of neurotransmitter release mediated by presynaptic muscarinic receptors

In the rat cerebral cortex, the comparative effects of various muscarinic agonists on the release of [3H]dopamine ([3H]DA), [3H]acetylcholine ([3H]ACh), and [3H]5-hydroxytryptamine ([3H]5-HT) from superfused nerve endings and on phosphatidylinositol (PI) turnover were studied. Acetylcholine (ACh) was found to be the most potent among the agonists tested on all three release systems examined, and also on the activation of PI turnover. Oxotremorine and bethanechol were very weak agonists when tested as stimulators of PI turnover. However, oxotremorine was very effective as a release modulator, while bethanechol was completely ineffective. Our data suggest that the activation of PI turnover is not directly involved in the modulation of neurotransmitter release mediated by presynaptic muscarinic receptors.

Cholinoceptive properties of respiratory neurones in the rat medulla

The effects of iontophoretically applied acetylcholine, the acetylcholine agonists nicotine and muscarine, and the antagonists atropine, dihydro-beta-erythroidine (DH beta E) and mecamylamine, together with the excitatory amino acids, glutamate and D,L-homocysteic acid (DLH) were examined on the activity of respiratory-related neurones in the rat medulla and were compared with effects on non-respiratory brain stem neurones. Most neurones were excited by acetylcholine and no inhibitory responses were seen. Glutamate and DLH also excited but there was a trend for the phasic activity of respiratory neurones to be converted to a tonic discharge. Nicotine also excited most neurones to which it was applied and these responses were blocked by DH beta E but not by atropine. Muscarine also caused excitation and these responses were blocked by atropine but not by DH beta E. Both antagonists blocked acetylcholine-induced excitation but had no effect on responses to glutamate or DLH. Mecamylamine was without effect. It is concluded that the proportion of cholinoceptive respiratory neurones in the rat brain stem is similar to that for non-respiratory neurones. It seems likely that both nicotinic and muscarinic receptors are present on the majority of respiratory neurones and that both contribute to the response produced by iontophoretically-applied acetylcholine.

Asymmetric distribution of acetylcholine receptors and M channels on prepyriform neurons

The responses of pyramidal neurons of rat prepyriform cortex to ionophoretic application of acetylcholine (ACh) were studied in a submerged, perfused brain slice. ACh excited some neurons but only if applied to an area near to the cut surface of the slice. This area contained the basal dendrites of the pyramidal cells and some cell bodies. No excitation was seen if ACh was applied at depths of 250 microns or more from the cut surface, an area which contained only apical dendrites, although the apical dendrites were very sensitive to excitatory amino acids such as aspartate (Asp) and glutamate (Glu). On all neurons which did not discharge to ionophoretic application of ACh, ACh potentiated the response to Glu and Asp. No potentiation of amino acid responses was obtained on apical dendrites. The potentiation had a time course similar to that of the discharge of neurons which fired to ACh. This observation suggests that pyramidal neurons have receptors for ACh on basal but not apical dendrites. The ACh response in the basal dendrite-soma region was elicted by pilocarpine and blocked by atropine but not curare. This was true whether the response studied was direct excitation or potentiation of the response to an amino acid. The ACh response was associated with a voltage-dependent increase in membrane resistance which had a slow time course and appeared to be due to a turning off of an M current, as described by Brown and Adams (1980) in sympathetic ganglion cells. The effects of ACh were minimal at the resting potential but increased with depolartization. ACh had no effect on the current-voltage relation of the cell, except at depolarized potentials of less than -60 mV. Ionophoretic application of Ba2+ to the basal dendritic region resulted in potentiation of the amino acid responses and sometimes induced a discharge similar to that of ACh. Since Ba2+ mimics the ACh response, presumably by a direct blockade of the M channel, the effects of Ba2+ on apical dendrites were tested to determine whether these dendrites contain M channels associated with a transmitter receptor other than ACh. However, Ba2+ did not induce potentiation in apical dendrites, suggesting that M channels are also restricted to the basal dendrites or cell bodies.(ABSTRACT TRUNCATED AT 400 WORDS)

Inhibition of apomorphine-induced climbing in mice by cholinergic drugs and neuroleptics

Apomorphine (ap) was administered subcutaneously to mice kept in individual cages. Ap elicited an abnormal vertical climbing behaviour. The muscarinomimetics physostigmine and oxotremorine as well as the neuroleptics clozapine and haloperidol inhibited the climbing produced by ap 1 mg/kg. A small inactive dose of physostigmine potentiated the effect of clozapine but not that of haloperidol. The anticholinergic atropine antagonized the effect of physostigmine, oxotremozine, clozapine and haloperidol. The climbing behaviour produced by ap is presumably due to stimulation of dopamine receptors and this effect can be antagonized either by blockade of dopamine receptors or by activation of muscarinic receptors. Some lines of evidence suggest that the ap-inhibitory effect of clozapine may be partly due to a muscarinomimetic effect.