A CORRELOGRAM ANALYSIS OF THE ACTIVITY IN THE ROSTRAL VENTROMEDIAL MEDULLA OF AWAKE RATS AND IN RATS ANESTHETIZED WITH KETAMINE OR PENTOBARBITAL FOLLOWING THE ADMINISTRATION OF MORPHINE

The spiking activity of single units and their corresponding mass acti vity in the rostral ventromedial medulla (RVM) was recorded simultaneo usly in rats that were either awake or anesthetized with pentobarbital or ketal-nine. Autocorrelograms and cross-correlograms were calculate d to illustrate the distribution of interspike intervals under each of these conditions. The spontaneous activity in both anesthetized group s, but not the awake group, displayed a low incidence for short interv als (about 1-36 ms) between spikes. However, this low incidence of sho rt interspike intervals was found in some awake rats following the adm inistration of subanesthetic dosages of ketamine, pentobarbital, or mo rphine. The occurrences, or lack of occurrences of these short intersp ike intervals may be a marker for the anesthetized state of the rat. A basal spiking pattern, as illustrated by three different types of cor relograms, was observed, and these basal patterns did not usually chan ge after the administration of morphine to awake or anesthetized rats, even though the cells became excited or inhibited. Cross-correlograms revealed that the synapses between ''on'' and ''off' cells might some times be inhibitory, while at other times they might be excitatory; ho wever, cells of the same class mainly shared excitatory connections. I n summary, ketamine and pentobarbital do alter the firing patterns of individual neurons in the RVM, but, no matter what state the rat is in , the activity of individual RVM neurons is strongly related to the ac tivity of other neurons in the local network.