Cornea-responsive medullary dorsal horn neurons: Modulation by local opoids and projections to thalamus and brain stem

Previously, it was determined that microinjection of morphine into the caudal portion of subnucleus caudalis mimicked the facilitatory effects of intravenous morphine on cornea-responsive neurons recorded at the subnucleus interpolaris/caudalis (Vi/Vc) transition region. The aim of the present study was to determine the opioid receptor subtype(s) that mediate modulation of corneal units and to determine whether opioid drugs affected unique classes of units. Pulses of CO₂ gas applied to the cornea were used to excite neurons at the Vi/Vc ('rostral' neurons) and the caudalis/upper cervical spinal cord transition region (Vc/C1, 'caudal' neurons) in barbiturate-anesthetized male rats. Microinjection of morphine sulfate (2.9-4.8 nmol) or the selective mu receptor agonist D-Ala, N-Me-Phe, Gly-ol-enkephalin (DAMGO; 1.8-15.0 pmol) into the caudal transition region enhanced the response in 7 of 27 (26%) rostra units to CO₂ pulses and depressed that of 10 units (37%). Microinjection of a selective delta {[D-Pen^2,5] (DPDPE); 24 - 30 pmol } or kappa receptor agonist (U50488; 1.8-30.0 pmol) into the caudal transition region did not affect the CO₂-evoked responses of rostral units. Caudal units were inhibited by local DAMGO or DPDPE but were not affected by U50,488H. The effects of DAMGO and DPDPE were reversed by naloxone (0.2 mg/kg iv). Intravenous morphine altered the CO₂-evoked activity in a direction opposite to that of local DAMGO in 3 of 15 units, in the same direction as local DAMGO but with greater magnitude in 4 units, and in the same direction with equal magnitude as local DAMGO in 8 units. CO₂-responsive rostral and caudal units projected to either the thalamic posterior nucleus/zona incerta region (PO/ZI) or the superior salivatory/facial nucleus region (SSN/VII). However, rostral units not responsive to CO₂ pulses projected only to SSN/VII and caudal units not responsive to CO₂ projected only to PO/ZI. It was concluded that the circuitry for opioid analgesia in comeal pain involves multiple sites of action: Inhibition of neurons at the caudal transition region, by intersubnuclear connections to modulate rostral units, and by supraspinal sites. Local administration of opioid agonists modulated all classes of corneal units. Comeal stimulus modality was predictive of efferent projection status for rostral and caudal units to sensory thalamus and reflex areas of the brain stem.