Characteristics of tail-tremor induced by nicotine in rats

Rodent models of tremor

This review focuses on rodent models of tremor, particularly those induced by pharmacological agents. Harmaline is one of the most frequently used tremor-generating drugs and harmaline-induced tremor is regarded as a model of essential tremor. Harmaline acts on inferior olive neurons, causing enhanced neuronal synchrony and rhythmicity in the olivocerebellar system. In addition, it selectively induces cerebellar Purkinje cell death, speculatively because of excessive glutamate release from nerve terminals of the olivocerebellar system onto Purkinje cells. Systemic administration of cholinomimetics can also produce generalized tremor, and muscarinic receptors on striatal neurons are thought to be the best candidate for the tremor-generating mechanism. On the other hand, dopaminergic neurotoxins, which are used in models of parkinsonism, have yet to be used for experimental analysis of tremor, because tremors induced by dopamine depletion in rodents are less remarkable than those induced by harmaline or cholinomimetics. Recently developed gamma-aminobutyric acid (GABA)(A) receptor alpha-1 subunit knockout mice exhibit postural and kinetic tremors, and clearly reproduce the features of essential tremors. Although from a phenomenological point of view, rodent models of tremor cannot entirely mimic human tremor disorders, they have useful advantages in the analysis of pathophysiological mechanisms underlying tremor. Development of convenient and reproducible methods for evaluating rodent tremor is therefore recommended.

[Assessment of anti-tremorogenic drugs using nicotine-induced tail-tremor model and elucidation of the mechanism]

Repeated administration of nicotine causes a tremor only in the tail (tail-tremor) of rats. The tremor is accompanied with locomotor hyperactivity without rigidity and immobility of the whole body, suggesting the involvement of the mechanism associated with the movement. The tail-tremor induced by nicotine was suppressed by nicotinic acethylcholine (nACh) receptor antagonists, but not by muscarinic acethylcholine (mACh) receptor antagonists. Moreover, the tail-tremor was suppressed by beta-adrenoceptor antagonists and benzodizepines. The tremor at rest is observed only in Parkinson's disease, which is improved by the use of mACh receptor antagonists. An essential tremor is one of the typical tremor connected with the movement (postural tremor) and improved with beta-adrenoceptor antagonists. These findings and results suggest that the nicotine-induced tail-tremor is useful for the study of the essential tremor as an animal model. On the other hand, daily administration of nicotine resulted in an augmentation of the tail-tremor. The development of the tail-tremor was suppressed by nACh receptor antagonists, N-methyl-D-aspartate (NMDA) receptor antagonists and nitric oxide (NO) synthase inhibitors. These results suggest that central nACh receptors are essential for the onset and further development of the tail-tremor induced by repeated administration of nicotine, and that NO formation mediated by NMDA receptors is involved in the developmental mechanisms.

Involvement of 5-hydroxytryptamine(1A) receptors in nicotine-induced tail tremor in rats

Involvement of the serotonergic system in tail tremor induced by repeated administration of nicotine was investigated in rats. Tail tremor induced by nicotine (0.5 mg/kg, s.c.) was suppressed by a 5-HT(1A) receptor antagonist, N-¿2-[4-(2-methoxyphenyl)-1-piperazinyl-]ethyl¿-N-(2-pyridinyl)cycloh exanecarboxamide trihydrochloride (WAY-100635; 0.3-3 mg/kg, i.p.), but not by a 5-HT(2) receptor antagonist, ketanserin (0.1-0.3 mg/kg, i.p). The 5-HT(1A) receptor agonists, buspirone (1-20 mg/kg, i.p.), gepirone (1-10 mg/kg, i.p.), tandospirone (1-10 mg/kg, i.p.) and (+/-)-8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT; 0.01-0.1 mg/kg, s.c.), enhanced the tail tremor. The enhancement of tail tremor by buspirone (10 mg/kg, i.p.) was blocked by WAY-100635 (0.3-3 mg/kg, i.p.). These findings suggest that nicotine-induced tail tremor is mediated by 5-HT(1A) receptors and that 5-HT(1A) receptor antagonists are effective in the treatment of tremor.

Involvement of nitric oxide in development of tail-tremor induced by repeated nicotine administration in rats

Daily administration of nicotine (0.5 mg/kg per day s.c.) to rats caused a tremor that appeared only in the tail (tail-tremor) and which became more marked over 8 days. Nitric oxide (NO) synthase inhibitors, Nw-nitro-L-arginine (10 mg/kg per day i.p.) or Nw-nitro-L-arginine methyl ester (20 and 40 mg/kg per day i.p.), administered each day before nicotine attenuated the development of the tail-tremor. However, neither Nw-nitro-L-arginine (2-10 mg/kg i.p.) nor Nw-nitro-L-arginine methyl ester (10-40 mg/kg i.p.) affected the tail-tremor that developed after 14 days of repeated nicotine administration. The noncompetitive N-methyl-D-aspartate (NMDA) receptor antagonist, MK-801 ((+)-5-methyl-10,11,-dihydro-5H-dibenzo[a,b]cyclohepten-5,10-imine hydrogen maleate) at 0.2 mg/kg per day (i.p.), or competitive antagonist, CPP (3-[(+/-)-2-carboxypiperazin-4-yl] propyl-1-phosphonic acid) at 2 mg/kg per day (i.p.), administered each day before nicotine attenuated the development of the tail-tremor. MK-801 (0.01-0.2 mg/kg i.p.) but not CPP (0.5-4 mg/kg i.p.) suppressed the tail-tremor that developed after 14 days of repeated nicotine administration. These results suggest that NO formation mediated by NMDA receptors is involved in the mechanisms underlying the tail-tremor induced by the repeated administration of nicotine.

The new muscarinic M1-receptor agonist YM796 evokes yawning and increases oxytocin secretion from the posterior pituitary gland in rats

The present experiments were performed to examine the effects of a new muscarinic M1-receptor agonist, (-)-YM796 ((-)-S-2,8-dimethyl-3-methylene-1-oxa-8-azaspiro[4.5]decane L-tartrate monohydrate), on yawning and oxytocin secretion from the posterior pituitary gland in rats YM796, at doses of 2.5-50 mg/kg (SC), elicited yawning. The yawning response was markedly increased by pretreatment with a beta-adrenoceptor antagonist, pindolol (20 mg/kg, IP), which per se did not elicit yawning. The yawning induced by YM796 (10 mg/kg, SC) in combination with pindolol (20 mg/kg, IP) was inhibited by scopolamine (0.5 mg/kg, SC), a muscarinic receptor antagonist, and pirenzcpine (300 micrograms/ rat, ICV) and EEDQ (N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline) (5 mg/kg, IP), muscarinic M1-receptor antagonists, but not by spiperone (0.5 mg/kg, SC), a dopamine D2-receptor antagonist, 4-DAMP (4-diphenylacetoxy-N-methylpiperidine methiodide) (100 micrograms/rat, ICV), a muscarinic M3-receptor antagonist, and [d(CH2)5, Tyr(Mc)2, Orn8]-vasotocin (100 ng/rat, ICV), an oxytocin receptor antagonist. YM796 at 2.5-50 mg/kg (SC) did not exert an action on prolactin levels but increased oxytocin secretion from the posterior pituitary gland in rats. This augmentation of oxytocin secretion by YM796 was inhibited by scopolamine (0.5 mg/kg, SC) and pirenzepine (3 mg/kg, SC), but not by mecamylamine (1 mg/kg, IP), a nicotinic receptor antagonist. The present findings obtained with YM796 suggest that the muscarinic M2-receptor stimulation participates in causing yawning behavior and oxytocin secretion in rats.