The responsiveness of M2-muscarinic receptors in the posterior hypothalamus and brain stem of vasopressin hypertensive rats.

The response of an endogenous inhibitor of cAMP-dependent protein kinase (type I inhibitor) to tremorine was used as an index of sensitivity of control muscarinic M2-receptors. Tremorine induced a dose-dependent increase in type I inhibitor activity in the posterior hypothalamus and brain stem. The action of the compound was blocked by pretreatment with aminophylline and atropine. Prolonged, 28 days treatment with lysine vasopressin (1 U/kg/day ip) induced hypertension and modified the dose-response curve for tremorine. Five times higher doses of tremorine than in normotensive rats were necessary to induce statistically significant increase in type I inhibitor activity in the posterior hypothalamus and brain stem suggesting subsensitivity of M2-muscarinic receptors in the brain areas responsible for the regulation of blood pressure.

Pharmacokinetics of muscarinic cholinergic drugs as determined by ex vivo (3H)-oxotremorine-M binding.

The pharmacokinetic parameters of muscarinic cholinergic drugs after intravenous (IV) and oral administration to mice was determined with ex vivo (3H)-oxotremorine-M (3H-Oxt) binding to the brain. Oxotremorine had a long duration of action, and arecoline had a short one. There was a significant correlation between the ex vivo ED50 and the in vitro inhibition constants (Ki). Tremorine, a prodrug, inhibited ex vivo binding, but was relatively inactive in in vitro binding. The quaternary amines, methylscopolamine and oxotremorine-M, and the hydrophilic compound, pirenzepine, were relatively weak in inhibiting ex vivo binding because of their poor penetration of the blood-brain barrier. Oxotremorine and BM-5 were similarly bioavailable to the brain by the IV and the oral route. These results indicate that the pharmacokinetic profile of muscarinic cholinergic drugs can be determined with ex vivo (3H)-Oxt binding.

GABAergic, dopaminergic and cholinergic interaction in tremorine-induced tremors in mice.

Antitremor effects of systemically administered GABA agonists (GABA and sodium valproate) and GABA antagonists (bicuculline and picrotoxin) were studied in mice against tremorine-induced tremors. None of the GABA agonists were found to possess any antitremor effect, whereas GABA antagonists were found to possess protective effect against tremorine-induced tremors. Simultaneous administration of GABA agonists with sub-effective doses of anticholinergic agent (scopolamine) did not potentiate antitremor effect of scopolamine, whereas GABA antagonists and effective doses of scopolamine when administered simultaneously resulted in antagonism of protective effect of scopolamine. Similarly, when GABA agonists and sub-effective doses of dopaminergic agent, bromocriptine, were administered simultaneously, the protective effect of bromocriptine was potentiated. When GABA antagonists and effective doses of bromocriptine were administered simultaneously, the protective effect of bromocriptine was antagonized. The modification of the protective effect of anticholinergic and dopaminergic agents by GABAergic agents has been explained on the basis of neurotransmitter interaction.

Modification of drug-induced tremor by systemic administration of kainic acid and quisqualic acid in mice.

The effects of excitatory amino acids, kainic acid and quisqualic acid, on the tremorine- and harmaline-induced tremor were quantitatively examined in mice using the power spectral analyzing method. The severity of the tremor was determined quantitatively in terms of the cumulative sum of the mean square value of the data. Kainic acid enhanced the tremor induced by tremorine but depressed the tremor induced by harmaline. Quisqualic acid depressed the tremor induced by both tremorine and harmaline in a dose-dependent manner. Kainic acid shifted the frequency of each component of the tremor induced by tremorine to the high frequency side, but quisqualic acid did not affect the frequency of tremor of the tremor induced by tremorine. The frequency of tremor of the tremor induced by harmaline was shifted by both excitatory amino acids to the low frequency side, and another component of tremor in the power spectral densities developed, of which the mean square values were very small. The present results suggest that, at least in part, the glutamatergic system can take a role on the modification of drug-induced tremor.

The role of adrenergic mechanism in tremorine-induced tremors in rats: antitremor effect of beta-adrenoceptor antagonists.

Tranylcypromine (TCP) pretreatment was found to accelerate the tremorogenic activity of tremorine in rats. Conversely, reserpinization delayed the onset of induction of tremors, and a significant diminution in their intensity was observed in these rats. A comparative study of the antitremor activity of beta-adrenoceptor antagonists against this tremor-model showed that butoxamine (beta 2-antagonist) and propranolol (nonselective antagonist) were able to afford a rapid and powerful protection, whereas a weaker and delayed effect was observed in rats treated with the beta 1-antagonist, acebutolol. Furthermore, the antitremor activity of butoxamine and propranolol but not that of acebutolol was found to be potentiated and diminished in rats pretreated with reserpine and TCP, respectively. It was inferred that beta 2-receptor modulated the tremorogenic activity of tremorine, and that inhibition by propranolol or butoxamine of this subtype beta-adrenoceptor resulted in rapid and powerful suppression of tremors, and that the antiadrenergic activity of acebutolol was unlikely to have a role in its antitremor effect.

Spectral analysis of tremorine and cold tremor electromyograms in animal species of different size.

In 14 mice (36.2 +/- 5.4 g), 17 rats (425 +/- 46.4 g), and 11 rabbits (3,200 +/- 340 g) a comparative electromyographic analysis of tremorine tremor and cold tremor was performed in gastrocnemius and tibialis anterior muscles using spectral analysis. The mean frequency of cold tremor decreased with increasing body weight (mice: 40.2 +/- 4.5 Hz; rats: 31.3 +/- 4.9 Hz; rabbits: 16.4 +/- 3.2 Hz). With tremorine tremor no such allometric correlation was found for tremor frequency and body weight (mice: 17.7 +/- 3.6 Hz; rats: 19.6 +/- 5.1 Hz; rabbits: 15.9 +/- 2.1 Hz). Cross spectral analysis revealed that during cold tremor the flexor muscle (tibialis anterior) and the extensor muscle (gastrocnemius) of rabbits are activated alternately. The mean phase shift between the activation of flexor and extensor muscle was -155.5 degrees. Stronger activation was observed in the flexor muscle. Tremorine tremor was characterized by synchronous activation of flexor and extensor muscles with a mean phase angle of 3.0 degrees and a predominance of the extensor muscle. The results suggest that the nervous mechanisms for the generation of tremorine tremor and cold tremor are different.

Effects of striatal and pallidal lesions and intrapallidal GABA and opiate drugs on tremorine-induced rigidity in the rat.

Tremorine-induced hindlimb rigidity was prevented by the neurotoxin kainic acid in one neostriatum. A unilateral globus pallidus (GP) electrolesion reduced the tone of the contralateral leg (CL) and differentially modified tremorine rigidity. The ipsilateral leg rigidity was reduced and the CL rigidity was increased. Resting tone and tremorine-induced rigidity were measured in CL after injection in one GP of drugs that modify pallidal GABA function. The GABA drugs tested in GP had no consistent effects on resting limb tone. By contrast, baclofen and muscimol, agonists for GABA receptors, both prevented tremorine rigidity, whereas the antagonists bicuculline and picrotoxin increased rigidity. Opiate receptor agonists in GP did not effect tone of rigidity but naltrexone prevented rigidity in CL leg. The findings suggest a close involvement of GP in mediating cholinergic limb rigidity.

Neuropharmacological evaluation of RX 77368--a stabilised analogue of thyrotropin-releasing hormone (TRH).

L-Pyroglutamyl-L-histidyl-L-2,3-dimethylprolineamide (Pyr-His-Dmp . NH2; RX 77368) a stabilised analogue of thyrotropin-releasing hormone (TRH) has been examined for neuropharmacological effects in animal tests. The compound was more potent than either TRH or clinically established drugs in four animal tests of antidepressant potential (reserpine reversal, clonidine antagonism, tremorine reversal and learned immobility). RX 77368 also antagonised barbiturate sleeping time. Given by itself to rats the peptide produced arousal as characterised by EEG and EMG measurements and delayed the onset of sleep. The arousal induced was not accompanied by increases in locomotor activity. The profile of pharmacological activity for RX 77368 did not correspond to the profiles of tricyclic antidepressants, psychic-stimulants or analeptic drugs. The possible clinical uses for such a molecule are discussed.

The effect of intraventricular phenylethylamine and octopamine on the central effects of tremorine and pilocarpine and the acetylcholine level in the rat brain.

Phenylethylamine (PEA, 50 and 100 microgram ivc) and octopamine (OCT, 50 and 250 microgram ivc) potentiated the tremorine (10 mg/kg ip) induced hypothermia in the rat. This effect was partially antagonized by atropine (10 mg/kg ip). PEA and OCT significantly prolonged the duration of pilocarpine (100 mg/kg iv) induced catalepsy in rats. PEA (100 microgram ivc) and OCT (250 microgram ivc) depressed the acetylcholine (ACh) level in the cerebral cortex and striatum but did not affect it in the hippocampus. In addition, these amines enhanced the synthesis of ACh in the cerebral cortex, and PEA also in the rat striatum.

Oxotremorine-tolerance: muscarinic blockade or homeostatic adaptation?

Tremorine pretreatment of mice induces tolerance to some effects of oxotremorine. In the state of tolerance even the highest doses of oxotremorine did not cause antinociception, this blockade being insurmountable. Oxotremorine decreased motility and amphetamine hypermotility, and both effects were diminished by tremorine pretreatment. Amphetamine hypermotility increased in the tolerance state. The increase of cerebral acetylcholine level due to oxotremorine was diminished by tremorine pretreatment. It is suggested that a special blockade of cerebral muscarinic receptors might play a role in the tolerance phenomenon, moreover it is possible that some excitation develops in the CNS. A homeostatic adaptation may be involved a role in this kind of tolerance.

The effect of baclofen and aminoxyacetic acid on catalepsy in the rat.

Gabaergic compounds, baclofen and aminoxyacetic acid (AOAA) potentiate the catalepsy induced by neuroleptics. This effect indicates their functional antagonism towards the central dopaminergic system. Both compounds exert a central antiserotonin effect. Baclofen, but not AOAA, also shows weak cholinolytic properties. However, the potentiation of cataleptogenic action of neuroleptics by baclofen and AOAA is not related to their effect on the central serotonergic or cholinergic systems.

Genetical differences in sensitivity to tremorine and oxotremorine in mice.

Male mice from 14 strains were injected i.p. with tremorine (3.0 mg/kg) or oxotremorine (0.15 or 0.1 mg/kg). Large inter-strain differences in the degree and duration of the subsequent hypothermia were noted. 2 strains, BALB/c and Simpson, were particularly sensitive to the hypothermic effect of oxotremorine. The offspring from a cross between BALB/c and Simpson were less sensitive than the parental strains, suggesting genetic complementation. A set of 7 recombinant inbred (RI) lines derived from strains C57BL and BALB/c were tested with oxotremorine. 5 RI lines resembled strain C57BL in their response and 2 RI lines resembled strain BALB/c. It was concluded that strains C57BL and BALB/c differ at a gene which has a major effect on the response to oxotremorine.

Direct or indirect action of histamine on dopamine metabolism in the rat striatum?

Intraventricular administration of 500 microgram of 2-pyridylethylamine, an agonist of the histamine (Hi) H1-receptor, produced a 20% increase of striatal HVA in the rat while the Hi H2-receptor agonist 4-methylhistamine had no influence on HVA and DOPAC levels. L-Histidine (1.5 g/kg) or amodiaquine (60 mg/kg) given i.p. increased HVA and DOPAC levels to the same extent as did pyridylethylamine. Histidine combined with tremorine had an additive effect with respect to the increase of DA acidic metabolites while mepyramine slightly attenuated the tremorine-induced rise of HVA.

Possible involvement of 5-hydroxytryptamine and cyclic-AMP in tolerance to tremorine analgesia in mice.

The phenomenon of tolerance to the analgesic action of tremorine in mice was studied by the hot-plate and tail-clip methods. Reduction in 5-HT levels in brain by parachlorophenylalamine pretreatment decreased the ED50 of tremorine analgesia in tremorine tolerant mice. 5-Hydroxyptophan, L-Dopa or alpha-methyl-para-tyrosine did not influence the analgesic response to tremorine in tremorine tolerant animals. However, theophylline was found to enhance the tolerance to tremorine analgesia. Brain 5-HT and cAMP are probably involved in tremorine tolerance, whereas neither noradrenaline nor dopamine is involved in the phenomenon.

Some aspects of neurophysiological basis of insecticide action.

When the insecticide parathion was administered to awake, unrestrained rats with chronically implanted brain electrodes, it was observed that the latency of the averaged flash-evoked potential in the visual cortex and superior colliculus was increased and the amplitude was decreased 2 to 4 hours later with responses returning to pretreatment levels about 8 hours after administration. Similarly, after administration of several dose levels of parathion in the rat, durations of phases of the maximal electroshock seizure (MES) pattern were altered to the greatest extent 4 hours later, but effects disappeared at 24 hours. These effects of parathion on the MES and evoked potentials coincided with a fall in blood and brain acetylcholinesterase (AChe) activities but disappeared after AChe inhibition had reached its peak and stabilized. Brain AChe activities required 2 to 4 weeks for recovery whereas blood AChe activity recovered in 1 week following inhibition by parathion (at least 2 mg/kg body weight). Studies in the monkey demonstrated similar results. Because these measurements of central nervous system function returned to normal despite continued inhibition of AChe activity, the results are interpreted to mean either that adaptation of evoked potentials or MES responses to prolonged AChe inhibition can occur in the rat and monkey after parathion administration or that some of the effects of parathion do not depend on AChe inhibition. Administration of DDT (100 mg/kg by mouth) to awake, unrestrained rats markedly increased the amplitude of spontaneous electrical activity in the cerebellum, whereas there was much less effect on electrical activity recorded simultaneously in the occipital cortex, reticular formation, and medial geniculate body. Similarly, DDT administration had marked effects on the averaged, sound evoked potential recorded in the cerebellum; DDT caused the appearance and increased the amplitude of an early component of this response not usually present during control recordings. Sound-evoked potentials recorded simultaneously from the frontal and occipital cortex and reticular formation were affected less or were decreased in amplitude by administration of DDT.