Unit activity alterations induced in the mesencephalic periaqueductal gray by local electrical stimulation

In the framework of a series of investigations concerning the neural substrate of aversion, electrophysiological methods were used in order to specify, within the rat's periaqueductal gray (PAG), functional properties, viz. conduction velocity and refractory period, of PAG neurons already assessed in previous studies by means of behavioral methods, and to gather data on their local synaptic relationships. Unit activities were recorded from the periaqueductal gray with the aim of analyzing those alterations that would be induced by locally applying an electrical stimulation with parameters shown to elicit escape behavior. An implanted row of electrodes allowed the application of a stimulation to several sites aligned along a mediolateral or a rostrocaudal axis through the periaqueductal gray. The results indicate that an electrical stimulation applied to the periaqueductal gray may induce its effects through the activation of a number of dendrites and many slow conducting fibers running in a great variety of directions and branching within the periaqueductal gray. Their refractory period was surprisingly low (0.6 ms) for slow conducting fibers (below 1 m/s). The local circuitry appears to include many inhibitory connections. Their organization is assumed to be partly recurrent. Stimulation-induced inhibition becomes predominant when the stimulation is moved away from the recorded neuron along the mediolateral axis, but not along the rostrocaudal axis.