Inhibition and Excitation Mediated Respectively by Dopamine D-1 and D-2 Receptors on the Caudate Nucleus Neurons Receiving Dopaminergic Input from the Substantia Nigra: Microiontophoretic and in vitro Studies

Abstract

There are conflicting results regarding whether dopamine acts as an excitatory or inhibitory neurotransmitter on the caudate nucleus (CN) neurons; for instance, stimulation of the pars compacta of the substantia nigra (SN) produces EPSP and action potentials in the CN neuron, and these potentials are blocked by dopamine receptor antagonists, while iontophoretically applied dopamine depolarizes the membrane concomitant with an inhibition of spontaneous firing. In addition, iontophoretically applied dopamine usually inhibits spontaneous and glutamate-induced firing, although an increase in the firing is sometimes observed. These discrepant results are considered to be due to the action of dopamine on different receptors, which are adenylate cyclase-linked D-1 and unrelated D-2 receptors, since both receptors are known to be present in the CN. Therefore, electrophysiological studies using an iontophoretic technique in vivo and intracellular recording methods in vitro were performed to elucidate whether or not the D-1 receptor mediates the inhibitory effects and D-2 the excitatory actions, and to determine whether both D-1 and D-2 receptors coexist in the same CN neuron.