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 activity in the rostral ventromedial medulla (RVM) was recorded simultaneously in rats that were either awake or anesthetized with pentobarbital or ketamine. Autocorrelograms and cross-correlograms were calculated to illustrate the distribution of interspike intervals under each of these conditions. The spontaneous activity in both anesthetized groups, but not the awake group, displayed a low incidence for short intervals (about 1-36 ms) between spikes. However, this low incidence of short interspike intervals was found in some awake rats following the administration of subanesthetic dosages of ketamine, pentobarbital, or morphine. The occurrences, or lack of occurrences of these short interspike intervals may be a marker for the anesthetized state of the rat. A basal spiking pattern, as illustrated by three different types of correlograms, was observed, and these basal patterns did not usually change 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 sometimes be inhibitory, while at other times they might be excitatory; however, cells of the same class mainly shared excitatory connections. In 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 activity of other neurons in the local network.