Cholinergic mechanisms in the caudate nucleus

Effects of intrastriatal injections of atropine and methacholine on the apomorphine-induced gnawing in the rabbit

To find out the anatomical location of the target point of cholinergic-dopaminergic equilibrium, atropine (40 microng) or methacholine (10 microng) were injected through previously implanted cannulas into various places of caudate nucleus and putamen of the rabbit, and the effect of the injections on stereotype gnawing induced by subcutaneously or intravenously administered apomorphine (1--2 mg/kg) was assessed. The intensity of gnawing was measured using a special apparatus, counting each bite. Atropine inhibited the stereotype, while metacholine potentiated it. The effects were evident with the method used, but difficult to reveal with the classical method of assessing the intensity of stereotyped behavior, based on visual observation. The results suggest that the striatum is not a target point for the cholinergic component of the cholinergic-dopaminergic equilibrium in the central nervous system.

Interactions of nigrostriate synaptic transmission, iontophoretic O-methylated phenethylamines, dopamine, apomorphine and acetylcholine

Recordings were made from, and drugs applied to, neurons in the caudate nucleus of unanesthetized cats, using multibarrel micropipette electrodes. The substantia nigra was stimulated by sterotactically placed electrodes. Three O-methylated derivatives of dopamine, meta-methoxyphenethylamine (m-MPEA), para-methosy-phenethylamine (p-MPEA) and 3,4-demethoxyphenethylamine (DIMPEA), inhibited most, excited a few, and had no detectable effect on a substantial number of the cells upon which they were tested. A statistically significant correlation was found between the effects of dopamine (DA) and the three O-methylated derivatives on the same populations of cells. Iontophoretic release of the O-methylated derivatives could not prevent the actions of DA, nor could it block synaptically mediated effects of the nigrostriate pathway. It is concluded that the three O-methylated products are partial agonists of DA. The findings are difficult to reconcile with the suggestion that the experimental parkinsonian-like symptoms caused by O-methylated phenethylamines are the consequence of blockade of dopaminergic synapses. No correlation, negative or positive, was found between the effects of DA and of acetylcholine (ACh). The findings do not support the theory that balanced sets of antagonistic synapses, one dopaminergic, the other cholinervic, operate upon individual neurons in the caudate nucleus. Apomorphine and dopamine were shown to have similar effects on a substantial number of neurons, even though the onset and offset of the effect of apomorphine were slower than those of DA. This observation agrees with the suggestion that some of the central effects of apomorphine are due to an action at dopaminoceptive receptor sites.

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.

Frontal-striatal control of behavioral inhibition in the rat

The direct application of crystalline dopamine, D-amphetamine or scopolamine in microgram quantities to the ventral anterior region of the corpus striatum (VAS) of rats increased their responding for food on a modified DRL-30 sec schedule of reinforcement. Similar applications of norepinephrine were less effective than dopamine, while the anticholinesterase eserine depressed responding. Electrolytic lesions of the ventrolateral, but not the dorsomedial, prefrontal cortex of rats also increased their response rates. These results were interpreted as being consistent with the idea of a dopamine-acetylcholine antagonism in the VAS whose net output modulate behavioral inhibition. This striatal mechanism may be influenced by the ventrolateral prefrontal cortex.

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.

Effects of dopaminergic receptor agonists and antagonists on the activity of the neo-striatal cholinergic system

The effects of various neuroleptics and of apomorphine on the metabolism of ACh were examined in the neostriatum of the rat. For this purpose, a specific radio-enzymatic assay for brain ACh was used. This method is based on the preliminary purification of the choline esters by liquid cation exchange, separation of choline and ACh on thin layer chromatography plates, hydrolysis of ACh then reactylation of the choline moiety with a purified and stabilized rat brain choline acetyltransferase. The rat neostriatal ACh levels were decreased by neuroleptics of the phenothiazine and butyrophenone type and increased by apomorphine. An "in vivo" estimation of the rate of utilization of ACh was obtained by measuring the decline in neostriatal ACh content following the local microinjection of hemicholinium-3. This compound blocked almost totally the synthesis of ACh in these conditions. Chlorpromazine (15 mg/kg) enhanced neo-striatal ACh utilization and apomorphine (10 mg/kg) antagonized this effect. Neuroleptics did not effect ACh levels in the parietal cerebral cortex and the hippocampal formation. The modifications of the activity of neostriatal cholinergic neurons by chlorpromazine and apomorphine were still observed following the degeneration of the nigro-neostriatal dopaminergic fibers induced by the injection of 6-hydroxydopamine into the substantia nigra. The results strongly suggest that dopaminergic receptors as defined by their pharmacological interaction with neuroleptics and apomorphine are localized on neostriatal ACh neurons.

The effects of anti-Parkinson drugs on cortical neurones

1. The effects of a potential anti-Parkinson drug, benapryzine, have been compared with those of benzhexol, atropine and procaine on the excitatory responses induced by acetylcholine and L-glutamate on feline cortical neurones using the microiontophoretic technique.2. All the drugs tested reduced the excitatory responses evoked by acetylcholine and L-glutamate. However, benapryzine, benzhexol and procaine more effectively reduced the excitatory responses to L-glutamate than those to acetylcholine whereas atropine was more effective against acetylcholine-induced excitation.3. In the presence of procaine the amplitude of the extracellular spikes was decreased. This effect was also observed during applications of benapryzine and benzhexol.4. Tests on the isolated frog sciatic nerve indicated that benapryzine and benzhexol had local anaesthetic actions respectively greater than and equivalent to those of procaine.5. It was concluded that the effects of benapryzine and benzhexol on cortical neurones were probably related to their local anaesthetic properties. The possibility that a local anaesthetic action may account for the effects of these drugs and of many other commonly used anti-Parkinson drugs in Parkinson's disease is discussed.

Effects of morphone on choline acetyltransferase levels in the caudate nucleus of the rat

1. Choline acetyltransferase (choline-o-acetyltransferase 2.3.1.6.) concentrations were determined in the caudate nucleus, thalamus, and cortex of control and morphine treated rats. The enzyme was assayed using a modified radiochemical method on a number of selected days, one hour after the last injection of 30 mg/kg of morphine and also during the subsequent phase of abstinence from morphine.2. Significant lowering of choline acetyltransferase activity in the caudate nucleus area was found in two cases, one hour after the first dose of morphine and upon subsequent abstinence from morphine.3. The enzyme activity in the two other parts of the brain remained at the normal levels.4. The presence of endogenous inhibitors formed during morphine administration was excluded.5. The relationship of a possible effect of morphine on the tissue binding of the enzyme and the subsequent lowering of its activity was tested by homogenization of the caudate nucleus area in different media. The decrease in enzyme activity occurred in all extraction media one hour after morphine administration.6. Inhibitory effects of in vitro addition of morphine to caudate nucleus homogenate, obtained from normal and morphine treated rats, were found to occur only at very high concentrations of the drug, negating the possibility of direct inhibitory effects of morphine.7. These experiments suggest two possible causes of the observed effects, which can be responsible for the lowering of enzyme activity, and can be operative simultaneously: (1) a negative feedback mechanism of accumulated acetylcholine, occurring after the first dose of morphine, and (2) the possible changes in enzyme configuration produced by morphine treatment.

Responses evoked in the cerebellar cortex by stimulation of the caudate nucleus in the cat

1. Responses evoked in the cerebellar cortex following stimulation of caudate nucleus are described.2. The evoked responses recorded from the surface of the cerebellar cortex were found to be of two types, one with a short (4-6 msec) latency and one with a longer (12-17 msec) latency.3. The short latency response was maximal in the lobulus simplex, the longer latency response was maximal in paramedian lobule.4. Following lesions in the inferior olive the longer latency response was absent.5. Recordings from within the cerebellar cortex showed that the short latency response was uniformly distributed throughout the grey matter, the longer latency response was maximal in the region of the Purkinje cell bodies.6. It was concluded that the short latency response was due to activation via the mossy fibres and the longer latency response to activation via the climbing fibres.7. It was found that responses could be evoked in the cerebellum following stimulation of only the latero-ventral part of the caudate nucleus; stimulation of the rest of the nucleus caused no response in the cerebellum. This division of the caudate nucleus into two parts is similar to the subdivision of the caudate nucleus made by other workers using different criteria.

The action of dopamine on neurones of the caudate nucleus

1. Dopamine applied iontophoretically to neurones of the caudate nucleus of cats caused excitation of some (9% of those encountered) and depression of others (60%). Some cells have been found affected both by dopamine and by acetylcholine.2. The effects of dopamine could be prevented by the previous iontophoretic administration of phenoxybenzamine, but not by dichloroisopropylnoradrenaline.3. Responses evoked in caudate neurones by electrical stimulation of substantia nigra were depressed by dopamine. No evidence for enhancement of the effects of nigral stimulation through the application of dopamine were detected.4. Stimulation of nucleus centromedianus thalami depressed firing of caudate neurones.5. The hypothesis that dopamine may act as an inhibitory synaptic transmitter within the caudate is put forward.