Putative Nociceptive Modulatory Neurons in the Rostral Ventromedial Medulla of the Rat Display Highly Correlated Firing Patterns

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Abstract
Recent work in this laboratory has identified two classes of putative nociceptive modulating neurons in the rostral ventromedial medulla (RVM) of the rat: “off-cells,” which pause beginning just prior to the tail flick response (TF) evoked by noxious heat, and “on-cells,” which accelerate shortly before the occurrence of the TF. In the unstimulated, lightly anesthetized rat, the spontaneous firing pattern of individual on- and off-cells consists of alternating periods of silence and activity lasting from several seconds to a few minutes. In the present study, simultaneous recordings were made from pairs of TF-related neurons, and the relationships among the firing patterns of cells within a class and between cells of different classes were determined. All cells of a given class showed fluctuations in spontaneous discharge that were in phase. On the other hand, there was a striking reciprocity of firing between the two cell classes, such that a decrease in activity of cells of one class was accompanied by an increase in activity of cells of the other class. These observations point to the existence of integrating mechanisms that coordinate the activity of all members of each class of TF-related neurons. Thus, the pattern of activity of any single on- or off-cell provides a useful index of the excitability of all cells of that class. Moreover, because of the highly reciprocal nature of the firing of the two classes, it is possible to infer the current state of both cell populations from the pattern of activity of any single TF-related neuron.