Single unit activity at ventromedial medulla level in the awake, freely moving rat: effects of noxious heat and light tactile stimuli onto convergent neurons.

In this study, we recorded single unit activity at the ventromedial medulla (VMM) level in the awake, freely moving rat. In agreement with previous work under the same conditions, we found a vast majority of neurons which possess heterosensory and heterosegmental inputs ('convergent'). These units are activated either by auditory or mechanical innocuous and noxious stimuli applied all over the body surface. The activation threshold of these neurons is very low since light stimulation such as air puff produce intense bursts. In addition to this highly represented neuronal class, we also find another consistent VMM group of neurons which fire in relation to precise or generalized body movements. The main result of the present work is that, in addition to auditory and mechanical inputs, a relatively high proportion of VMM convergent neurons are activated by noxious heat pulses between 43 and 51 degrees C. In this range, it was possible to obtain stimulus-response functions with 2 degrees C steps only when a skin twitch reflex produced by the heat was present, also encoding the temperature intensity. In comparison to the VMM activations produced by an intense noxious heat pulse such as 51 degrees C, either auditory or controlled light touch stimuli induced a more robust response in terms of maximum frequency of discharge. Differential properties of VMM neurons in relation to innocuous and noxious information were also found using repetitive stimulation: although a strong and fast habituation of the 51 degrees C responses was observed, this phenomenon was not present for light touch induced activations. We propose that these differential properties might reflect separate pathways reaching the VMM, the one carrying innocuous information possibly relayed through the dorsal column nuclei. Although obtaining stimulus-response functions might implicate the VMM convergent neurons in the sensory-discriminative aspect of pain, their massive heterosensory and heterosegmental inputs favor a role in more general processes such as alertness or stress. Also, due to massive convergent properties, the involvement of this neuronal class in specific bulbospinal descending control systems of nociceptive information is questionable, Finally, our results obtained in the awake, freely moving rat strongly differ from the anesthetized preparation in that we found neither nociceptive specific units nor neurons inhibited by noxious peripheral stimulations largely described in this approach.