RESPONSES OF RAT NUCLEUS SUBMEDIUS NEURONS TO ENKEPHALINS APPLIED WITH MICROPRESSURE

The purpose of this study was to determine what effects leucine-enkeph alin and D-Ala2-D-Leu5-enkephalin have on both the background and natu rally evoked activity of thalamic nucleus submedius neurons responsive to mechanical cutaneous stimulation. Thirty-five neurons in the nucle us submedius were fully characterized during single-unit extracellular recordings as nociceptive, low-threshold mechanoreceptive (LTM) or un responsive. Micropressure was used to apply the opioids. Eighteen neur ons were inhibited; 13 of these were nociceptive and one was LTM. Six units were activated; two of these were nociceptive and three were LTM . The remaining 11 units were unaffected. Opioid responses were tested for antagonism by naloxone in 12 neurons; eight of these responses we re antagonized by naloxone. Statistical analyses indicated that the ef fects of enkephalins on nociceptive neurons were selective for neurona l modality. The opioids also altered the response of some nociceptive neurons to receptive field stimulation. The presence of nociceptive ne urons in the nucleus submedius that are selectively inhibited by opioi ds provides additional support for the involvement of submedius neuron s in nociception. The results of this study suggest that this involvem ent is more than merely transmission of nociceptive input, since the o pioids may be selectively modulating the type of information that is t ransmitted to the cortex.