The effects of amphetamine on neuromuscular transmission in the lobster.

In vitro electrophysiological techniques were used on an excitatory neuromuscular junction of a walking-limb stretcher muscle of the lobster in order to define the pharmacology of amphetamine on a glutamatergic synapse. A single electrical stimulus in the presence of the drug produced a train of action potentials in the axon innervating the neuromuscular junction, resulting in a staircase-like enhancement of the evoked excitatory junction potential; both effects were antagonized by relatively small doses of haloperidol. In contrast, amphetamine reduced the amplitude of the spontaneous miniature junctional potential, suggesting a postsynaptic site of action; this depressant response to the drug was not antagonized by haloperidol. The data demonstrate that amphetamine concomitantly produces both excitation and depression at a single synapse.