Excitatory and inhibitory effects of dopamine on neuronal activity of the caudate nucleus neurons in vitro

Abstract

Effects of dopamine on the rat caudate nucleus neurons were examined in a slice preparation using an intracellular recording technique. Perfusion of the bath with a low concentration (1 μM) of dopamine produced a depolarization concomitant with an increase in the spontaneous firing and the number of action potentials evoked by a depolarizing pulse applied into the cells. In contrast, higher concentrations (100–500 μM) of dopamine inhibited the spontaneous and current-induced firings without apparent effects on the resting membrane potential. In addition, during application of a high concentration (100 μM) of dopamine there was a marked elevation of the threshold potential of the action potential elicited by a higher depolarizing current. Simultaneous application of haloperidol (0.5–5 μM) antagonized both excitatory and inhibitory effects induced by the low and high concentrations of dopamine, respectively. In addition, the excitatory effect induced by a low concentration (1 μM) of dopamine was antagonized by domperidone (0.5 μM), a selective D 2 receptor antagonist, while the inhibitory effect by a high concentration (100 μM) was blocked by SCH 23390, a selective D 1 receptor antagonist. These results strongly suggest that the postsynaptic sites of caudate nucleus neurons have at least two subtypes of dopamine receptors (D 1 and D 2 receptors) that mediate inhibitory and excitatory responses of the neuron to dopamine, respectively.