ENCODING OF CORNEAL INPUT IN 2 DISTINCT REGIONS OF THE SPINAL TRIGEMINAL NUCLEUS IN THE RAT - CUTANEOUS RECEPTIVE-FIELD PROPERTIES, RESPONSES TO THERMAL AND CHEMICAL-STIMULATION, MODULATION BY DIFFUSE NOXIOUS INHIBITORY CONTROLS, AND PROJECTIONS TO THE PARABRACHIAL AREA

To determine whether corneal input is processed similarly at rostral a nd caudal levels of the spinal trigeminal nucleus, the response proper ties of second-order neurons at the transition between trigeminal subn ucleus interpolaris and subnucleus caudalis (Vi/Vc) and at the transit ion between subnucleus caudalis and the cervical spinal cord (Vc/C-1) were compared. Extracellular single units were recorded in 68 Sprague- Dawley rats under chloralose or urethan/chloralose anesthesia. Neurons that responded to electrical stimulation of the cornea at the Vi/Vc t ransition region (n = 61) and at laminae I/II of the Vc/C-1 transition region (n = 33) were classified regarding 1) corneal mechanical thres hold; 2) cutaneous mechanoreceptive field, if present; 3) electrical i nput characteristics (A and/or C fiber); 4) response to thermal stimul ation; 5) response to the small-fiber excitant, mustard oil (MO), appl ied to the cornea; 6) diffuse noxious inhibitory controls (DNIC); and 7) projection status to the contralateral parabrachial area (PEA). On the basis of cutaneous receptive field properties, neurons were classi fied as low-threshold mechanoreceptive (LTM), wide dynamic range (WDR) , nociceptive specific (NS), or deep nociceptive (D). All neurons reco rded at the Vc/C-1 transition region were either WDR (n = 19) or NS (n = 14). In contrast, 54% of the Vi/Vc neurons had no cutaneous recepti ve field. Of those Vi/Vc neurons that had a cutaneous receptive field, 57% were LTM, 25% were WDR, and 18% were D. All Vc/C-1 neurons respon ded to noxious thermal and MO stimulation. Only 22 of 47 and 13 of 19 Vi/Vc corneal units responded to thermal or MO stimulation, respective ly. At the Vc/C-1 transition region, 12 of 17 neurons demonstrated DNI C, whereas at the Vi/Vc transition region, DNIC was present in only 4 of 26 neurons. Of 15 Vc/C-1 corneal units, 12 could be antidromically activated from the contralateral PBA (average latency 6.29 ms, range 1 .8-26 ms). None of 22 Vi/Vc corneal units tested could be antidromical ly activated from the PEA. These findings suggest that neurons in lami nae I/II at the Vc/C-1 transition and at the Vi/Vc transition process corneal input differently. Neurons in laminae I/II at the Vc/C-1 trans ition process corneal afferent input consistent with that from other o rofacial regions. Corneal-responsive neurons at the Vi/Vc transition r egion may be important in motor reflexes or in recruitment of descendi ng antinociceptive controls.