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Abstract
Extracellular single unit recordings were obtained from the nucleus accumbens of urethane
anesthetized rats. It was found that electrical stimulation of the basal lateral and
basal medial nuclei of the amygdala produced strong excitatory responses in neurons
of the nucleus accumbens, in particular the medial region. Latencies of activation
were relatively short with a mean of 10.7 ms. Dopamine applied iontophoretically had
a marked attenuating effect on the excitatory response of nucleus accumbens neurons
to amygdala stimulation. The spontaneous activity of all neurons recorded from the
nucleus accumbens was also suppressed by dopamine, but the excitatory response was
more sensitive to dopamine inhibition than the spontaneous activity. Neurons in the
nucleus accumbens showed a variety of responses to single-pulse electrical stimulation
of the ventral tegmental area (VTA). Some units in the nucleus accumbens received
convergent inputs from both the amygdala and the VTA. Stimulation of the VTA also
attenuated the response of nucleus accumbens neurons to excitatory inputs from the
amygdala. A train of 10 pulses (0.15 ms, 200--600 microA) at 10 Hz delivered to the
VTA at 100 ms before stimulation of the amygdala caused attenuation of the original
excitatory response. The attenuating effect could be observed irrespective of whether
individual single-pulse stimulation of the VTA elicited a response in that particular
accumbens neuron or not. 6-Hydroxydopamine injected into the VTA 2 days prior to the
recording experiment, or haloperidol injected intraperitoneally 1 h before the recording
session, abolished this attenuating effect. However, responses to single-pulse stimulations
of the VTA were not abolished. The results suggest that the attenuation of the excitatory
response to amygdala stimulation was due to the release of dopamine from mesolimbic
dopaminergic neurons. Responses to single-pulse stimulations of the VTA were probably
due to activation of non-dopaminergic neurons projecting from the same area. It is
suggested as a working hypothesis that this inhibitory effect of dopamine may be an
important function of the mesolimbic dopamine pathway in modulating the extent to
which limbic structures can exert an influence on the motor system through the accumbens.