Dopamine (DA) D1 receptors are distributed in the nucleus accumbens and the amygdala, two regions of the mesocorticolimbic DA system known to be activated by aversive environmental stimuli. The objective of the present study was to determine the contribution of D1 receptors in these brain regions to the expression of a fear-motivated behavior, notably, potentiated startle in rats. Injection of the DA D1 receptor antagonist SCH 23390 into the amygdala blocked the ability of a conditioned light stimulus previously paired with footshock to enhance acoustic startle amplitudes. Bilateral intracerebral administration of SCH 23390 into the nucleus accumbens had no effect on fear-potentiated startle. The observed opposing effects of amygdaloid DA D1 receptor antagonism on fear expression, along with earlier research demonstrating the involvement of ventral tegmental area (VTA) DA neurons on fear-potentiated startle, suggest a role for mesoamygdaloid activity in conditioned excitatory fear reactions.
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