Electrophysiology and pharmacology of neurons of the medial zona incerta: an in vitro slice study.

The electrical properties and chemical sensitivities of neurons located in the medial zona incerta (ZI), a region containing a clustering of dopaminergic cell bodies known as the A13 cell group, were studied in Sprague-Dawley female rats in the in vitro slice preparation. Extracellular single cell recordings were made from 473 neurons located in the medial ZI. The neurons generally displayed a slow and regular firing pattern with a mean firing rate of 4.7 impulses/s. Some neurons exhibited an orthodromic response to electrical stimulation of the ventromedial nucleus of the hypothalamus (VMH). The response consisted of a short-term, variable-latency increase in activity, indicating a predominantly excitatory input from the VMH to the medial ZI. The chemical sensitivity of medial ZI neurons was tested by micropressure application of various neurotransmitters and peptides. Dopamine and serotonin were found to inhibit the spontaneous firing in many of the neurons tested, whereas none of the peptides tested consistently produced a short-term change in firing rate. Over half (51%) of the cells that were orthodromically excited by VMH stimulation were also inhibited by the exogenous application of dopamine. Since many of the cells in the medial ZI are dopaminergic and are believed to be autoregulated, the present findings indicate that a portion of the VMH neurons that project to the medial ZI may be synapsing on dopaminergic neurons.