CHOLINERGIC MECHANISMS IN THE CEREBELLUM

From cAMP to adenosine: an illuminating shift of focus

In a remarkable career, straddling five decades, John Phillis pursued with fierce determination and exceptional energy the main goal of his scientific life, to throw light on the chemical agents that control brain function. Starting in Australia, he settled in North America, first in Canada, then in the USA, where his long tenure at Wayne State brought his career to its culmination.

Muscarinic receptors in the central nervous system of the rat. II. Distribution of binding of [3H]propylbenzilylcholine mustard in the midbrain and hindbrain

The distribution of muscarinic receptors has been studied in the rat midbrain and hindbrain by counting silver grains in light microscope autoradiographs of the specific (atropine-sensitive) binding of [3H]propylbenzilylcholine mustard in cryostat sections. Of the 78 areas studied 6 had grain counts between 6 and 9 times the nonspecific level ("high"), and a further 15 had counts 4-6 times non-specific ("intermediate"). The basilar pontine nuclei and the ventral nuclei of the lateral lemniscus had high counts. Among the cranial nerve motor nuclei the facial and hypoglossal nuclei had high counts and the motor trigeminal nucleus and nucleus ambiguus had medium counts. The interpeduncular nucleus as a whole had low counts but there were two bands of intense staining on each side around the entry zone of the bundles of afferent cholinergic fibres from the habenula. Intermediate levels of binding occurred over the inferior colliculus and the superficial and intermediate grey layers of the superior colliculus. The molecular layer of the vestibulocerebellar vermis was distinctly labelled.

Muscarinic receptofs in the central nervous system of the rat. III. Postnatal development of binding of [3H]propylbenzilylcholine mustard

The postnatal development of muscarinic receptors has been studied in 7 selected areas from the brains of 1-17-day-old rats by counting silver grains in light microscope autoradiographs of the specific (atropine-sensitive) binding of [3H]propylbenzilylcholine mustard in cryostat sections. A major part of the adult receptor density is present at 1 day of age, a time when only a small fraction of the adult number of synapses has yet been formed. Of the areas studied the hypoglossal nucleus is the most precocious in muscarinic receptor development, and the dentate gyrus the latest (associated with the late development of the dentate granule cells). The pattern of receptor distribution changes with development. The caudate-putamen first develops receptor in patches, beginning at the lateral (ventricular) surface. The pontine nuclei develop receptor in a medial to lateral sequence. The maturation of the adult laminar pattern of the olfactory bulb depends on the alignment of cells (especially the mitral cells). The neocortex initially has uniform labelling throughout its depth, and later the labelling in layer 4 becomes relatively less dense (probably associated with the ingrowth of afferent fibres). The hippocampal formation first develops receptor evenly over the pyramidal cell dendrites; later receptor appears over the newly formed dentate stratum moleculare and becomes much reduced over the hippocampal stratum lucidum and stratum lacunosum-moleculare (probably associated with the ingrowth of afferent fibres from the dentate gyrus and entorhinal area). In the cerebellum muscarinic receptor is found only in the lobules which receive the primary vestibular afferents. In the neonate it is present in the granular layer, but this later disappears and is replaced by the adult pattern of labelling in the molecular layer.

Development of neurons containing acetylcholinesterase and cholinacetyltransferase in dispersed cell culture of rat cerebellum

Cells from one-day-old cerebellum were grown for up to 30 days in dispersed cell culture. The characteristic neurons (deep cerebellar, Golgi and Purkinje cells) maintained their properties. It was found histochemically that some of the large cells display strong AChE activities in the perikaryon and in some processes, while biochemically the specific activities of the marker enzymes of the acetylcholine system, AChE (EC 3.1.1.7) and ChAc (EC 2.3.1.6), were increased and unchanged, respectively. During cultivation, the number of AChE-positive neurons increased. It can be inferred from these studies that, besides the AChE-positive (cholinoceptive) cells, ChAc-active (cholinergic) neurons (possibly Golgi II. type cells and some neurons in the deep cerebellar nuclei) are present in the cerebellum of the rat.

Histochemical localization of acetylcholinesterase in the cerebellum and optic tectum of four freshwater teleosts

A histochemical study has been carried out on the localization of acetylcholinesterase (AChE) in the cerebellum and optic tectum of four species of freshwater teleosts. AChE distribution in the cerebellar cortex of teleosts shows differences among the species examined and, in the trout, also differences between different cerebellar areas. This uneven kind of enzyme distribution corresponds to a similar variety of AChE patterns noticed in other vertebrates, especially mammals. AChE distribution in the optic tectum shows a prevalent pattern characterized by precise laminar distribution of enzymatic activity which is alternatively strong, weak or absent in the different tectal layers. The results suggest that most of sensitive imput and many systems of stimuli propagation may be mediated by cholinergic mechanisms in the optic tectum of telecosts.

Effect of iontophoretic application of cholinergic agonists and their antagonists to guinea-pig pelvic ganglia

1. The electrical activity of guinea-pig pelvic ganglion cells following iontophoretically applied cholinergic drugs, alone and during orthodromic nerve stimulation via the hypogastric nerve, has been recorded intracellularly.2. Iontophoretic application of nicotine (Nic) and acetylcholine (ACh) reduced membrane resistance and caused a depolarization which in approximately 80% of cells led to the firing of action potentials. In the remainder, depolarization was unaccompanied by firing.3. Iontophoretic application of Nic and ACh reduced or abolished the amplitude of successively evoked orthodromic responses.4. ACh-induced depolarization, unlike that caused by tetanic stimulation, was not followed by a subsequent increase in the frequency of synaptic potentials.5. Di-hydrobetaerythroidine (DHbetaE) and atropine (Atr) inhibited the response to both orthodromic stimulation and iontophoretic application of Nic and ACh.6. There was no evidence for the existence of muscarinic receptors in guinea-pig pelvic ganglia. Iontophoretic application of muscarinic agonists alone and after tetanic stimulation of the hypogastric nerve produced no significant depolarization of the ganglion cell membrane.

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

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Acetylcholine sensitivity of cerebellar neurones in the cat

1. Cholinomimetics, acetylcholine antagonists and some other compounds of pharmacological interest were administered electrophoretically near neurones within the vermal cerebellar cortex of anaesthetized (pentobarbitone) and unanaesthetized (cerveau isolé) cats.2. The neurones were identified by position within the cortex, spontaneous activity, and the responses to afferent and antidromic stimulation.3. Purkinje cells, but neither granule nor basket cells, were excited by cholinomimetics, and the acetylcholine receptors had muscarinic properties. Excitation was often preceded by depression of the spontaneous firing.4. Intravenously administered atropine and dihydro-beta-erythroidine did not depress the synaptic excitation of cerebellar neurones evoked by impulses in mossy, climbing or parallel fibres.5. Acetylcholine is thus unlikely to be an excitatory transmitter within the feline cerebellum, particularly at mossy fibre-granule cell synapses, despite the presence of relatively high levels of acetylcholinesterase within mossy fibre terminals.

Pharmacological properties of cholinoceptive neurones in the medulla and pons of the cat

1. Iontophoretic application of acetylcholine to neurones in the medulla and pons of the unanaesthetized decerebrate cat excited 35.5% and inhibited 22%. This was not a representative sample of lower brain stem neurones, since most penetrations were made medially. Some cholinoceptive neurones were in the medial reticular formation.2. The time course of the response to different doses has been described.3. Nicotine and nicotinic agents 1,1-dimethyl-4-phenylpiperazinium iodide (DMPP) and choline phenyl ether (TM1) excited neurones excited by acetylcholine but, with one exception, did not inhibit neurones inhibited by acetylcholine.4. Muscarinic agents mimicked both the excitatory and inhibitory responses to acetylcholine. (+/-)-muscarine and bethanechol (beta-methylcholine carbamate) had a prolonged and powerful inhibitory action.5. Oxotremorine [1-(2-oxopyrrolidino)-4-pyrrolidino-butyne-2] had a long-lasting excitatory action on neurones excited by acetylcholine.6. Eserine and neostigmine potentiated the action of acetylcholine but in addition they had a strong excitatory effect on many brain stem neurones, independently of the action of acetylcholine.7. Excitatory and inhibitory responses to acetylcholine could be antagonized by gallamine, hexamethonium and atropine. Dihydro-beta-erythroidine (DHbetaE) antagonized only excitatory responses. (+)-Tubocurarine was a weak antagonist.8. It is concluded that the excitatory responses have both nicotinic and muscarinic properties but the inhibitory responses are only muscarinic. Since the pharmacology of these two types of response is different, it is concluded that the inhibitory action is not due to an indirect effect from excitation of a neighbouring inhibitory neurone.