EFFECTS OF SYSTEMIC MORPHINE ON LAMINA-I SPINOTHALAMIC TRACT NEURONS IN THE CAT

Lamina I spinothalamic tract (STT) neurons are an integral component o f the central representation of pain and temperature and thus their se nsitivity to various analgesics needs to be examined. In the present s tudy, the effects of successive, cumulative doses (0.125-2.0 mg/kg) of intravenous morphine sulfate on the quantitative stimulus-response pr operties of nociceptive lamina I STT eels have been tested in the inta ct, barbiturate-anesthetized cat. Both nociceptive-specific (n = 7) an d multireceptive (heat, pinch and cold sensitive; n = 7) lamina I STT cells were inhibited in a dose-dependent manner. Parallel dose-depende nt effects on responses to noxious heat and pinch were generally obser ved that reduced ongoing discharge levels and the slopes of the stimul us-response functions. However, non-STT lamina I cells (n = 5) differe d significantly; the responses of one multireceptive (heat, pinch and cold-sensitive) cell and the responses to pinch of 3 of 4 wide dynamic range cells were not inhibited. In addition, two-thirds of the nocice ptive lamina I STT cells showed enhanced responses at the lowest dose of morphine (0.125 mg/kg). These results contrast with the varied effe cts of morphine reported for superficial dorsal horn cells with unchar acterized projections and they support the role of lamina I STT cells in pain. Furthermore, these observations are consistent with previous findings indicating that lamina I STT neurons are a distinct subpopula tion of lamina I cells. These results support previous evidence that o piatergic modulation of sensory activity in lamina I is functionally o rganized.