A pharmacological analysis of the burst events induced in midbrain dopaminergic neurons by electrical stimulation of the prefrontal cortex in the rat

Electrical stimulation of the prefrontal cortex produces an inhibition-excitation (IE) activity pattern in the majority of responsive midbrain dopaminergic neurons. The excitatory phase often contains events, time-locked to the stimulation, which resemble natural bursts. The present study investigated the relationship between the inhibition and time-locked bursts by reducing the impact of the inhibition through membrane hyperpolarisation with the dopamine agonist apomorphine (i.v.) or antagonism with the GABA(A) antagonist picrotoxin (i.v. and iontophoretic). Apomorphine abolished or reduced time-locked bursting in all IE cells. Picrotoxin reduced the initial inhibition in the majority of IE cells, and abolished or reduced time-locked bursting at the highest intravenous dose. However, reductions in the initial inhibition were not systematically related to reductions in time-locked bursting. Hence, the phenomena do not appear to be causally related. Instead, time-locked bursts appear to be based on a straightforward excitation, which makes them closely analogous to natural bursts.