Antagonism of methylphenidate-induced behavior by neostigmine or hemicholinium-3

Evidence for monoaminergic involvement in triadimefon-induced hyperactivity

Triadimefon is a triazole fungicide that produces hyperactivity in both mice and rats similar to that seen following administration of compounds with catecholaminergic activity (e.g., d-amphetamine). To determine whether the triadimefon-induced hyperactivity is due to an action on CNS catecholaminergic systems, we evaluated the effects of combined treatment of triadimefon with either the tyrosine hydroxylase inhibitor d,l-alpha-methyl-p-tyrosine methyl ester HCl (alpha MPT) or the amine depletor reserpine. Adult male Long-Evans hooded rats, approximately 70 days of age were used. Dosage-effect functions were determined for alpha MPT (0-200 mg/kg IP), reserpine (0-2.5 mg/kg IP), d-amphetamine (0-3 mg/kg IP), and methylphenidate (0-40 mg/kg IP). Motor activity was measured as photocell interruptions in figure-eight mazes. The interaction between triadimefon and alpha MPT was determined with the following groups: 1) vehicle control; 2) 200 mg/kg triadimefon PO; 3) 100 mg/kg alpha MPT; and 4) both alpha MPT and triadimefon. A similar design was used to determine the interaction between triadimefon and reserpine (0.62 mg/kg), alpha MPT and d-amphetamine (1.5 mg/kg), and reserpine and methylphenidate (5.0 mg/kg). In the first experiment alpha MPT did not block the increased motor activity produced by triadimefon (i.e., both triadimefon alone and alpha MPT in combination with triadimefon produced significant increases in motor activity). alpha MPT did, however, block d-amphetamine-induced hyperactivity. Since alpha MPT did not antagonize the effect of triadimefon, these data suggest that increased motor activity produced by triadimefon is not mediated through release of newly synthesized catecholamines.(ABSTRACT TRUNCATED AT 250 WORDS)

Independent GABAergic and cholinergic modulation of apomorphine-induced stereotyped rearing in the rat

The injection of GABA into the caudate nucleus inhibited the stereotyped rearing induced by apomorphine in a dose-related manner. Muscimol, a potent GABAergic agonist shared this effect. The inhibitory effect of GABA was easily counteracted by bicuculline but not by pretreatment with atropine. Injection of carbachol into the caudate nucleus inhibited the stereotyped rearing induced by systemically-applied apomorphine in a dose-related manner. This inhibitory effect was easily abolished by atropine but not bicuculline. Thus, the stereotyped rearing induced by apomorphine, an effect due to an increased excitatory state of the dopaminergic system in the caudate nucleus, could be modified (inhibited) by augmentation of either the GABAergic or of the cholinergic state excitation. The two modulatory systems did not appear to be interlinked; most probably, they influence the dopaminergic effect independently of one another.

Cholinergic modulation of stimulant-induced behavior

Stimulant-induced stereotypy, presumably mediated by nigrostriatal dopaminergic neurons, can be altered by the administration of cholinergic agonists or antagonists. Cholinergic, striatal interneurons have been postulated as the site of these effects, although the specific site of interaction between cholinergic and dopaminergic systems is unknown. The study reported here examined the effects of the cholinesterase inhibitor physostigmine, and several other cholinergic and anticholinergic drugs, on stimulant-induced behavior. Stereotypy was inhibited by physostigmine to the same degree whether induced by direct (apomorphine) or by indirect acting (amphetamine and methylphenidate) stimulants. The results are interpreted as indicating that the site of cholinergic modulation of stimulant-induced stereotypy is postsynaptic to the dopaminergic neurons which mediate stereotypy.