Parabrachial area and nucleus raphe magnus inhibition of corneal units in rostral and caudal portions of trigeminal subnucleus caudalis in the rat

The cornea has been used extensively as a means to selectively stimulate tr igeminal nociceptive neurons. The aim of this study was to determine the ef fects of descending modulatory control pathways on corneal unit activity by comparing the effects of conditioning stimulation of the pontine parabrach ial area (PBA CS) and nucleus raphe magnus (NRM CS). Electrical stimulation of the cornea at A- and C-fiber intensities was used to activate neurons i n two regions of the trigeminal spinal nucleus, the subnucleus interpolaris /caudalis transition (Vi/Vc, 'rostral units') and laminae I-II at the subnu cleus caudalis/cervical cord transition (Vc/C1, 'caudal units'), in chloral ose-anesthetized rats. Corneal units were further classified according to c onvergent cutaneous receptive field properties and PEA projection status. N one of 48 rostral and 23/28 caudal units projected to the ipsilateral or co ntralateral PEA. PEA CS inhibited the cornea-evoked responses (<75% change from control) of approximately 65% of rostral and caudal units regardless o f neuronal class. For rostral corneal units, PEA CS inhibited A- and C-fibe r input equally (15 +/- 3 and 18 +/- 14% of-control, respectively), whereas among caudal units, A-fiber input was inhibited more than C-fiber input (2 6 +/- 5 and 64 +/- 120/0 of control, respectively, P < 0.01). The magnitude of NRM CS inhibition on cornea-evoked activity of both rostral and caudal units was not different from that seen after PEA CS. Glutamate microinjecti ons into PEA also inhibited rostral and caudal corneal units (6/9 tested). These results indicate that corneal input to rostral and caudal units is mo dified by activation of descending controls from the PEA and NRM. The signi ficance for processing corneal sensory information is discussed in terms of functional differences between rostral and caudal neurons. (C) 2000 Intern ational Association for the Study of Pain. Published by Elsevier Science B. V. All rights reserved.