Involvement of both cholinergic and catecholaminergic pathways in the central action of methylphenidate: a study utilizing lead-exposed rats.

The effects of methylphenidate (MPH) and the cholinergic agonists nicotine and oxotremorine were tested on the spontaneous multiple unit activity in the mesencephalic reticular formation of two groups of rats. In control rats i.v. MPH (1 mg/kg), nicotine (0.125 mg/kg), and oxotremorine (0.5 mg/kg) all attenuated the unit activity with latencies of less than 10 min. In another group of rats, exposed to lead acetate since birth, the extent of attenuation of unit activity induced by MPH and nicotine was reduced and the latency of effect was delayed by 45--50 min. The latency of the oxotremorine effect was not changed but the attenuation of unit activity was more pronounced in the lead-treated group. Pretreatment with spiroperidol, to inhibit the aminergic receptors, diminished the inhibitory effect of MPH in the control group but not in the lead-treated group, whereas the attenuating effect of oxotremorine was not affected in either group. These data support our previous evidence that MPH exerts its action in the central nervous system by a cholinergic pathway in addition to published catecholaminergic pathways. Furthermore, the present findings indicate that chronic lead-exposure in rats results in cholinergic hypofunction and supersensitivity at central cholinergic receptor sites. This alteration of central cholinergic function may be partially attributed to the malnutrition observed in the lead-exposed animals.