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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 microM) 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 microM) 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 microM) 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 microM) 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 microM) of dopamine was antagonized by domperidone
(0.5 microM), a selective D2 receptor antagonist, while the inhibitory effect by a
high concentration (100 microM) was blocked by SCH 23390, a selective D1 receptor
antagonist. These results strongly suggest that the postsynaptic sites of caudate
nucleus neurons have at least two subtypes of dopamine receptors (D1 and D2 receptors)
that mediate inhibitory and excitatory responses of the neuron to dopamine, respectively.