Tremorogenesis by LON-954 [N-carbamoyl-2-(2,6-dichlorophenyl) acetamidine hydrochloride]: evidence for the involvement of 5-hydroxytryptamine

Evidence for the involvement of central serotonergic mechanisms in cholinergic tremor induced by tacrine in Balb/c mice

Tacrine is a potent and reversible inhibitor of acetylcholinesterase (AChE) in the brain. It produces tremor in animals, which is believed to be due to an increase in the brain acetylcholine level following AChE inhibition. The present study was undertaken to investigate the involvement, if any, of biogenic amines in the genesis of this motor dysfunction. Administration of tacrine (10-20 mg/kg, i.p.) produced dose- and time-dependent tremor in Balb/c mice. While in vivo inhibition of striatal AChE activity was observed only for the highest dose of tacrine, a dose-dependent increase in striatal choline acetyltransferase activity was obtained. Serotonin (5-HT) levels, as assayed following a sensitive HPLC-electrochemical procedure, were significantly increased in nucleus caudatus putamen, nucleus accumbens, substantia nigra, nucleus raphe dorsalis, olivary nucleus and the cerebellum. However, dopamine or norepinephrine levels remained unaltered in these areas of the brain. In animals treated with p-chlorophenylalanine, a specific tryptophan hydroxylase inhibitor and 5-HT depletor, tacrine failed to elevate the levels of 5-HT in the brain regions, and significantly attenuated tremor response to the drug. Tacrine-induced tremor was also significantly (83%) attenuated by 5-HT(2A/2C) receptor antagonist mianserin (5 mg/kg, i.p.), but methysergide (5 mg/kg, i.v.) could block tacrine-induced tremor only by 20%. Atropine (5 mg/kg, i.p.) antagonized tacrine-induced tremor by about 53%, but a combination of atropine and mianserin completely blocked the tremor response. These results indicate that the cholinergic tremor produced by tacrine in Balb/c mice is mediated via central serotonergic mechanisms, and stimulation of 5-HT(2A/2C) receptors plays a pivotal role in this motor dysfunction.

Simple pharmacological test battery to assess efficacy and side effect profile of centrally acting muscle relaxant drugs

INTRODUCTION

Centrally muscle relaxants (CMRs) are used mainly for treating muscle spasticities of neurological origin, and painful muscle spasms due to rheumatologic conditions. Their use is frequently associated with dose-limiting adverse effects. New drugs with improved side-effect characteristics are badly needed. However, there is no general agreement in the pharmacological literature on what methods are adequate to assess CMR effect and side effects in behaving rodents, which may hinder the development of new drugs. Here we report on the establishment of a simple pharmacological test battery, which was used to compare efficacies and side effect profiles of 11 compounds with central muscle relaxant action, in mice (intraperitoneal application).

METHODS

For measuring muscle relaxant activity, (1) a new tremor model (GYKI 20039-induced tremor) and (2) the morphine-induced Straub-tail assay were used. The former, newly developed method has advantages over harmaline- or LON-954-induced tremor. For detecting side effect liability (ataxia, sedation, impairment of voluntary motor functions), (1) the rota-rod test, (2) measurement of spontaneous motility, (3) the weight-lifting test and (4) the thiopental sleep test were used.

RESULTS

Among the 11 muscle relaxant compounds tested (tolperisone, eperisone, silperisone, diazepam, baclofen, tizanidine, afloqualon, mephenesin, zoxazolamine, memantine and carisoprodol), the calculated safety ratios (i.e. ID50 for side effect/ID50 for muscle relaxant effect) varied in a wide range. Silperisone seems to have the most advantageous profile (safety ratios range between 1.7 and 3.3 in the different pairs of assays) compared to the other tested drugs with lower (one or more ratios below 1.5, and often far below 1) and more varying ratios.

DISCUSSION

Therapeutic indices calculated from the results of these in vivo experiments for the clinically used muscle relaxants are in agreement with their adverse effect profiles in humans. Thus the present test battery seems to be suitable for predicting the possible clinical utility of newly synthesized compounds.

Effects of p-chlorophenylalanine on striatal acetylcholinesterase activity and on biogenic amine levels in nuclei raphe and caudate-putamen during physostigmine-induced tremor in rats

The biochemical mechanism underlying tremor is unknown. We investigated the effects of p-chlorophenylalanine (pCPA), a serotonin (5-HT) depletor, on the neurochemical processes in nuclei raphe dorsalis (NRD) and caudatus putamen (NCP) paralleling physostigmine-induced tremor in rats. Peak of physostigmine tremor correlated with increase in 5-HT in NRD and NCP, and a decrease in striatal dopamine (DA), as assayed employing high pressure liquid chromotography-electrochemistry. Administration of pCPA caused significant decrease in DA, norepinephrine (NE) and 5-HT levels in both the nuclei, without affecting striatal NE content and acetylcholinesterase activity. pCPA pretreatment significantly inhibited physostigmine-induced tremor and blocked corresponding increase in the levels of 5-HT in NRD and NCP. These results indicate involvement of central 5-HT, but not DA or NE, in the genesis of physostigmine tremor.

Effects of serotoninergic drugs on tremor induced by physostigmine in rats

We investigated the effects of various serotoninergic drugs and serotonin (5-HT) depletion on physostigmine-induced visible tremor in rats. Physostigmine (0.25-1.5 mg/kg) caused dose-dependent tremor, initiated at 3-5 min (latency decreases as dose increases) and lasted for 30-35 min. Serotonin agonists, 8-hydroxy-2-(di-n-propylamino) tetralin (2.5 mg/kg) and buspirone (5 mg/kg) augmented the tremor response caused by physostigmine. The 5-HT(1)/5-HT(2) receptor antagonist, metergoline (1 mg/kg), and 5-HT(2) blocker, cyproheptadine (10 mg/kg) significantly decreased the duration (40%) as well as intensity (45-50%) of physostigmine-tremor. The 5-HT(2a)/5-HT(2c) antagonist ritanserin (5 mg/kg) significantly reduced the duration (60%) without affecting the intensity of the tremor. In 5-HT depleted rats (p-chlorophenylalanine; 300 mg/kg, for 3 days), physostigmine failed to produce tremor. Interestingly, in these animals, administration of a non-specific 5-HT agonist, 5-methoxy-N,N-dimethyl tryptamine, caused high intensity tremor. These results suggest that presence of 5-HT at the pre-synaptic terminals is needed for the tremor response by physostigmine and the response is greatly mediated via post-synaptic 5-HT receptors. The overall data indicated a direct involvement of central 5-HT system in the cholinergic tremor induced by physostigmine.

Behavioral and neurochemical alterations following intracerebroventricular administration of anti-serotonin antibodies in adult Balb/c mice

The effects of intracerebroventricular injections of serotonin (5-HT) antibodies were studied for changes in 5-HT, dopamine (DA), their metabolites and norepinephrine (NE) as well as 5-HT mediated behavior in adult mice. While nociceptive thresholds (tail-flick latency) were inhibited in antibody treated animals, tremor response to 5-methoxy-N,N-dimethyl tryptamine administration was increased. 5-HT and DA in the nucleus raphe dorsalis (NRD), substantia nigra (SN), nucleus caudatus putamen (NCP) and in the substantia grisea centralis, and NE in the former two nuclei were significantly decreased in these animals. 5-Hydroxyindoleacetic acid was unaffected in all nuclei except NRD, where it was inhibited. Homovanillic acid and 3,4-dihydroxyphenylacetic acid were inhibited in all nuclei except in NCP. The brunt of insult was more evident in NRD and SN where all neurotransmitters were inhibited for a longer period. 5-HT turnover was increased in all the nuclei, however only SN showed increased DA turnover. It may be assumed that the observed neurochemical and behavioral changes were the consequence of the antibodies binding to 5-HT, which in turn influenced the anatomically and functionally connected neurotransmitters. While the study contributes to the existing understanding of central neurotransmitter control on behavior, it fails to delineate the underlying mechanism. The possibility of developing a useful, drug-free 5-HT deficient animal model for studying clinical disorders, as well as for solving some of the basic questions related to the physiological functions of 5-HT in adult animals are envisaged from the study.

Tremorogenesis by physostigmine is unrelated to acetylcholinesterase inhibition: evidence for serotoninergic involvement

Studies were performed to bring out a serotoninergic involvement in physostigmine tremor, hitherto known to be working via the cholinergic system. 5-Hydroxytryptamine (5-HT) was estimated fluorimetrically after isolation on Sephadex G-10 and acetylcholinesterase (AChE) was assayed spectrophotometrically. The dose-dependent tremor was quantified by a double-blind study. No correlation (r = 0.01) existed between tremor and AChE inhibition since the non-tremoring dose of physostigmine caused the same degree of enzyme inhibition. An increase of 5-HT was found to be correlated (r = 0.59) with the duration and intensity of tremor. Cholinergic antagonists atropine (2 and 5 mg/kg, i.p.), scopolamine (0.5, 1.0, 2.0 mg/kg, i.p.) and mecamylamine (1 mg/kg, i.p.) failed to block the tremor while the 5-HT antagonists methysergide (5 mg/kg, i.v.) and cyproheptadine (10 and 30 mg/kg, s.c.) could afford more than 60% protection. These results suggest a serotoninergic rather than a cholinergic component in the genesis of physostigmine tremor.