PHYSIOLOGICAL-PROPERTIES OF RAT VENTRAL PALLIDAL NEURONS RECORDED INTRACELLULARLY IN-VIVO

1. The physiology of ventral pallidal (VP) cells was investigated usin g in vivo intracellular recording and staining techniques in adult rat s. Based on electrophysiological criteria, three different types of ce lls were found: type A cells, which fired phasic spikes that did not e xhibit a substantial afterhyperpolarization (AHP), type B cells, which exhibited a slow ramplike depolarization that preceded the short-dura tion action potential; the spike was followed by a prominent AHP, and type C cells, which were the only cells that fired spikes in couplets or bursts, with the spikes in a burst exhibiting a progressive increas e in duration and a decrease in amplitude. These cells also exhibited a rebound low threshold spikelike event. Furthermore, 18% of the VP ce lls recorded exhibited a slow subthreshold oscillation of the membrane potential (<1 Hz). 2. The response of VP cells to stimulation of fibe rs arising from the prefrontal cortex, nucleus accumbens, and mediodor sal thalamic nucleus (MD) was examined. In contrast to our initial pre dictions, all cells responded to nucleus accumbens stimulation with ex citation. Type A and B cells responded to nucleus accumbens stimulatio n with excitation and to MD stimulation with anti dromic-like response s, orthodromic excitation, or evoked inhibitory postsynaptic potential s. Only type A cells responded to prefrontal cortical stimulation. Typ e C cells only responded to stimulation of the nucleus accumbens, whic h resulted in evoked excitatory postsynaptic potentials. 3. The cells in the VP therefore can be segregated into three physiologically defin ed groups according to action potential discharge patterns and their r esponse to afferent fiber stimulation.