The role of dopamine released from distal and proximal dendrites of nigrostriatal dopaminergic neurons in the control of GABA transmission in the thalamic nucleus ventralis medialis in the cat

Halothane-anaesthetized cats implanted with push-pull cannulae were used in this study. Amphetamine was applied in the pars reticulata or pars compacta of the substantia nigra in order to determine the role of dopamine released from distal or proximal dendrites of dopaminergic cells in the control of GABAergic transmission in the nucleus ventralis medialis of the thalamus. When applied for 30 min in either the pars reticulata or the pars compacta, amphetamine (10(-6) M) enhanced to a similar extent the local release of 3H]dopamine synthesized from 3H]tyrosine, these effects being seen mainly during the drug application. The amphetamine-evoked release of dopamine in the pars reticulata produced a long lasting reduction in the release of 3H]GABA synthesized from 3H]glutamine in the nucleus ventralis medialis as well as in the paralamellar zone of the nucleus ventralis lateralis. Opposite effects were observed when amphetamine (10(-6) M) was applied in the pars compacta. In complementary experiments, single unit recordings were made in the intermediate part of the pars reticulata, some of the cells being identified by antidromic activation from the nucleus ventralis medialis. Whether applied in the pars reticulata or pars compacta, amphetamine (10(-6) M, 10 min) evoked a reversible decrease in the firing rate of most recorded cells whether or not they were identified as projecting to the nucleus ventralis medialis. Therefore, the decreased release of 3H]GABA in the nucleus ventralis medialis seen following application of amphetamine in the pars reticulata of the substantia nigra could result from an inhibition of nigrothalamic GABAergic neurons. Since the nucleus ventralis medialis is also innervated by GABAergic neurons originating in the entopeduncular nucleus, single unit recordings were made from cells in this nucleus during the application of amphetamine (10(-6) M, 10 min) into the pars compacta of the substantia nigra, some of which were identified antidromically as projecting to the nucleus ventralis medialis. Most cells identified or not were found to be activated during this treatment. These results suggested that the increased release of 3H]GABA seen in the nucleus ventralis medialis following application of amphetamine in the pars compacta of the substantia nigra might be linked to the enhanced firing rate of entopeduncular-thalamic GABAergic neurons.