Blockade of central beta-adrenoceptors attenuates tremor induced by 5-hydroxytryptamine (5-HT)-receptor activation in rats

Animal models of tremor

Animal models of tremor have been widely used in experimental neurology, because they are an indispensable requirement for understanding the pathophysiology of human tremor disorders and the development of new therapeutic agents. This review focuses on three approaches to produce tremor in animals (application of tremorgenic drugs, experimental central nervous system lesions, study of genetic mutants) and their use in simulating tremor syndromes of humans. Whereas harmaline induces a postural/kinetic tremor in animals that shares some features with human essential tremor/enhanced physiological tremor, MPTP tremor is the best model available for rest tremor in people. The tremor following experimental lesion of the ventromedial tegmentum in primates closely resembles Holmes tremor in humans, whereas cerebellar intention tremor is mimicked by cooling of the lateral cerebellar nuclei. The "campus syndrome," discovered in a breed of Pietrain pigs, might be a useful model of human orthostatic tremor. However, no animal model has yet been generated that exactly recreates all features of any of the known tremor disorders in humans. Problems encountered when comparing tremor in animals and humans include differing tremor frequencies and the uncertainty, if specific transmitter abnormalities/central nervous system lesions seen in animal tremor models are characteristic for their human counterparts. The search for adequate tremor models continues.

Changes in neurotransmitter sensitivity in the mouse neocortical slice following propranolol and theophylline administration

1. The mouse neocortical slice has been used to examine the sensitivity of neurones to isoprenaline, 5-hydroxytryptamine (5-HT) and adenosine acutely and following chronic treatment of animals with propranolol or theophylline. 2. While having little effect alone, all three agonists enhanced the d.c. depolarizing potential produced by N-methyl-D-aspartate (NMDA). The effect of (-)-isoprenaline (0.2 microM) was shared by (+)-isoprenaline at the much higher concentration of 10 microM. 3. Superfusion of slices with theophylline or 8-phenyltheophylline blocked responses to adenosine with evidence of selectivity. A single injection of theophylline 24 h before slice preparation did not alter agonist sensitivity, but when administered daily at 100 mg kg-1 for 14 days, the xanthine caused a loss of sensitivity to adenosine and (-)-isoprenaline but not 5-HT. The lower dose of theophylline, 10 mg kg-1 daily, also led to a loss of adenosine responses but no change of sensitivity to the amines. 4. Following the 14 day treatment with theophylline at 100 mg kg-1 daily in two groups of mice, responses to adenosine recovered to control levels after 20 days. 5. Propranolol superfusion blocked responses to both isomers of isoprenaline and 5-HT but did not affect sensitivity to adenosine. 6. Chronic treatment with propranolol at 25 mg kg-1 daily for 14 days induced a loss of sensitivity to (-)-isoprenaline and 5-HT but not adenosine. A lower dose of 5 mg kg-1 daily caused no change in responses to adenosine or 5-HT, but yielded an increased sensitivity to (-)-isoprenaline. 7. The results are discussed with respect to reports of receptor up-regulation in binding studies; caution is clearly required in extrapolating from such work to receptor activity in a functional system, especially in the case of theophylline and adenosine.

Effect of beta-adrenergic receptor antagonists on nicotine-induced tail-tremor in rats

The effects of various beta-adrenergic receptor antagonists on nicotine-induced tail-tremor were investigated in rats. Atenolol (5 and 10 mg/kg, IP), arotinolol (5 and 10 mg/kg, IP), and carteolol (5 and 10 mg/kg, IP), hydrophilic beta-adrenergic receptor antagonists, did not affect the tail-tremor induced by nicotine given at a dose of 0.5 mg/kg SC. However, propranolol (5-20 mg/kg, IP) and pindolol (5-20 mg/kg, IP), nonselective and lipophilic beta-adrenergic receptor antagonists, did suppress the tail-tremor dose dependently. In contrast, metoprolol (5-20 mg/kg, IP), lipophilic and beta 1-selective adrenergic receptor antagonists, did not show such an effect. These results suggest that nicotine-induced tail-tremors may be mediated through central beta 2-adrenergic receptors as an appearance and developmental mechanism.

The effect of theophylline on essential tremor: the possible role of GABA

The chronic administration of theophylline was studied in twenty patients with essential tremor in a double-blind cross-over trial. The tremor was improved significantly after four weeks of treatment. In mice the chronic administration of theophylline was compared with propranolol on the modulation by adenosine, 5-HT, (-)isoprenaline or GABA of NMDA-induced depolarisation of neocortical slices. Adenosine depolarisation was abolished by two-weeks treatment with theophylline but not propranolol. Potentiation by (-)isoprenaline of NMDA responses was reduced by theophylline (100 mg/kg/day) and propranolol treatment (25 mg/kg/day), but a lower dose of propranolol further increased it. The enhancement by 5-HT of NMDA-induced depolarisation was unaffected by the pretreatment with theophylline, while the higher dose of propranolol blocked it. GABA caused no significant change of NMDA depolarisation in control slices, but after theophylline treatment (100 mg/kg/day) and propranolol administration at both doses it significantly potentiated NMDA depolarisation. The enhancement of GABA sensitivity might be an important common factor in decreasing the essential tremor after propranolol and theophylline treatment.

Behavioral and biochemical studies in rats following prenatal treatment with beta-adrenoceptor antagonists

Increased motor activity and poor performance in the active avoidance test were observed in the offspring of rats treated with dl-propranolol or sotalol during pregnancy, but not with atenolol and d-propranolol. All substances were administered in drinking water from days 8-22 of gestation. A significant increase in the density of muscarinic acetylcholine receptors in the hippocampus was found for dl-propranolol and sotalol, at 35 and 20 days of age, respectively. Twenty-day-old pups born to dl-propranolol-treated rats exhibited a non-significant decrease in the number of beta-adrenoceptors in the frontal cortex. Assuming that all the beta-adrenoceptor antagonists tested had access to the developing fetal brain, the effect of dl-propranolol and sotalol on behavior could stem from central beta 2-adrenoceptor blockade. In view of the lack of behavioral changes after atenolol, a beta 1-selective adrenoceptor antagonist, it is suggested that the clinical use of beta 1-selective adrenoceptor antagonists during pregnancy might be safer for the fetus than beta 2-adrenoceptor antagonists.

Spermidine/spermine-N1-acetyltransferase activity in isoprenaline-stimulated rat salivary glands

In parotid, sublingual and submaxillary glands stimulated by continuous intravenous infusion of isoprenaline at various doses for 3 h, the concentrations of the polyamines putrescine, spermidine, spermine and N1-acetylspermidine as well as the activities of ornithine decarboxylase and spermidine/spermine-N1-acetyltransferase were determined. Ornithine decarboxylase is the enzyme that catalyses the formation of putrescine from ornithine. The most striking effect was observed in parotid glands. Here, the beta-adrenergic agonist increased the activity of spermidine/spermine-N1-acetyltransferase markedly, but only slightly (if at all) that of ornithine decarboxylase. It also increased the concentrations of N1-acetylspermidine and putrescine markedly. The result obtained in this gland indicates interconversion of higher polyamines to putrescine.

Modulation of oxotremorine-induced tremor by central beta-adrenoceptors

The muscarinic agonist oxotremorine was used to induce tremor in rats pretreated with methylatropine. An objective assessment of tremor intensity was accomplished by means of an accelerometer-based recording system. The non-selective, lipophilic beta-adrenoceptor antagonist propranolol dose-dependently suppressed tremor intensity, whereas the R-isomer of propranolol was without effect, verifying beta-adrenoceptor involvement. Since the hydrophilic, non-selective beta-antagonist nadolol was ineffective, the effect appears to be located inside the blood-brain barrier. The beta 2-selective antagonist ICI 118, 551 dose-dependently reduced tremor intensity, whereas selective blockade of beta 1-adrenoceptors with metoprolol had no effect, indicating the participation of a beta 2-adrenoceptor. On the other hand, the lipophilic beta 2-agonist clenbuterol dose-dependently enhanced tremor induced by oxotremorine. Determination of circulating plasma catecholamine concentrations revealed that the effect of beta-antagonists on tremor was not secondary to an effect on the oxotremorine-induced rise in catecholamine levels. Thus, the results suggest that beta 2-adrenoceptors located inside the blood-brain barrier are able to modulate oxotremorine-induced tremor in rats.

Blockade of central beta-adrenoceptors attenuates tremor induced by 5-hydroxytryptamine (5-HT)-receptor activation in rats

The effect of beta-adrenoceptor antagonists, varying in lipophilicity and receptor selectivity, were studied on tremor elicited by L-5-hydroxytryptophan (L-5-HTP) in rats pretreated with a peripherally acting decarboxylase inhibitor and a monoamine oxidase inhibitor, or by the directly acting 5-HT agonist 5-methoxy-N,N-dimethyltryptamine (5-MeODMT). Plasma levels of the beta-adrenoceptor antagonists were determined simultaneously. The non-selective lipophilic adrenoceptor antagonist propranolol was found to dose-dependently reduce tremor intensity, whereas the non-selective hydrophilic adrenoceptor antagonist sotalol had no effect, indicating a central site of action. Furthermore, beta 1-selective blockade with the adrenoceptor antagonist metoprolol had no effect on tremor intensity, whereas the beta 2-selective antagonist ICI 118,551 dose-dependently suppressed tremor intensity, suggesting that the beta-adrenoceptor subtype involved is of the beta 2-type. These results suggest that blockade of centrally located beta 2-adrenoceptors are able to attenuate the tremor response following 5-hydroxytryptamine receptor activation.