Dopamine evoked inhibition of single cells of the feline putamen and basolateral amygdala.

Abstract

1. In cats under pentobarbitone or halothane anaesthesia, neurones of the putamen and basolateral amygdala were inhibited with a similar time course by iontophoretic applications of dopamine and gamma-aminobutyric acid (GABA), ejected with relatively short (20 sec) low intensity (less than 40 nA) pulses of positive current from five and seven barrelled extracellular micropipettes. The use of a stereotaxically positioned guide tube, sealed to the skull with dental cement, made it possible to obtain stable recording conditions and to correlate the stereotaxic position of the cells with the position of the micro-electrode tracks determined histologically by the post-mortem reconstruction of serial sections. 2. Since in cats anaesthetized with pentobarbitone none of the cells were found to be spontaneously active, the relative potency of dopamine and GABA were compared on glutamate excited cells. Approximately 2-5 times more current was required to release sufficient dopamine to cause just submaximal inhibition, equal in magnitude and duration to that evoked by GABA. 3. In nitrous oxide/halothane anaesthetized cats, approximately one quarter of the cells were spontaneously active. Relative potency studies showed that for dopamine, currents 2-0 and 1-6 times larger than those used for GABA were required to inhibit glutamate excited and spontaneously active cells respectively. 4. When the depth distribution of the cells was compared with the sensitivity of the cells to dopamine and GABA, the most sensitive cells were found to lie within the putamen and the basolateral amygdala. 5. On more than one third of the cells tested, iontophoretic application of the neuroleptic, alpha-flupenthixol of more than 3 or 4 min in duration, greatly reduced or abolished the inhibition of the cells by dopamine without impairing their sensitivity to GABA. 6. In four cats, large I.V. injections of alpha-flupenthixol (10 mg/kg) and the more potent neuroleptic pimozide (1 mg/kg) had no significant effect on the dopamine or GABA sensitivity of seventy cells in the putamen and basolateral amygdala. 7. Our results are in keeping with the view that dopamine has a predominantly inhibitory action in the mammalian forebrain. However the failure of I.V. neuroleptics to modify the sensitivity of the cells to dopamine suggests that the dramatic effects of neuroleptics on animal behaviour may not be explicable simply in terms of a generalized blockade of dopamine receptors at post-synaptic sites.