Periaqueductal gray neurons' activity in a mesencephalic slice preparation.

The electrophysiological characteristics of periaqueductal gray (PAG) neurons were studied using intracellular techniques in guinea pig brainstem slices maintained 'in vitro'. Input resistance and time constant ranged between 60-110 M omega and 11-20 ms respectively. Direct activation elicited action potentials generated by a Na+-conductance and characterized by a remarkable Ca2+-dependent plateau in the falling phase and a long-lasting afterhyperpolarization which is probably due to a Ca2+-dependent K+-conductance. These cells also showed a slow return to the resting membrane potential after hyperpolarizing pulses which was Ca2+-dependent. All PAG neurons had a low resting membrane potential and displayed a tonic spontaneous firing at a frequency around 7 impulses/s.