PARABRACHIAL AREA AND NUCLEUS RAPHE MAGNUS-INDUCED MODULATION OF ELECTRICALLY-EVOKED TRIGEMINAL SUBNUCLEUS CAUDALIS NEURONAL RESPONSES TO CUTANEOUS OR DEEP A-FIBER AND C-FIBER INPUTS IN RATS

The aim of this study was to determine whether trigeminal subnucleus caudalis (Vc) neurons show differential inhibitory effects produced by parabrachial area (PBA) or nucleus raphe magnus (NRM) conditioning stimulation (CS) on their A-fiber versus C-fiber inputs electrically evoked from cutaneous and deep tissues. A total of 55 Vc neurons in urethane/chloralose-anesthetized rats were functionally classified as low-threshold mechanoreceptive (LTM; n = 12), nociceptive (wide-dynamic-range, WDR, and nociceptive-specific, NS; n = 11), deep (D; n = 6), or nociceptive convergent neurons receiving skin and deep inputs (S + D; n = 26). Most neuronal responses were tested for both PBA and NRM CS (20-40 mu A) delivered 50 msec prior to test stimulation (1.1-1.5 x threshold) of their mechanoreceptive field (RF) or the hypoglossal nerve (XII). Inhibition of responses to both cutaneous and deep A-fiber and C-fiber inputs occurred in over two-thirds of the Vc neurons tested in each neuronal class, and facilitation of responses was only occasionally found. In general, both PBA (ipsi- and contralateral) and NRM CS produced profound inhibition of electrically evoked responses in all neuron types, e.g., responses reduced to mean values of 50-53% of control in LTM neurons, 35-46% in WDR, NS, and S + D neurons, and 61-63% in D neurons. No significant difference in the magnitude of their inhibitory effects was found between PBA and NRM, and between Vc neuronal responses to A- and C-fiber inputs irrespective of their origin from skin or deep tissues. However, in a group of 6 S + D Vc neurons, responses elicited by deep inputs were more powerfully inhibited by PBA stimulation than those elicited by cutaneous inputs, although the magnitude of NRM-induced inhibition of the responses to deep and cutaneous inputs were comparable. These data confirm earlier findings that Vc neuronal responses to cutaneous A- and C-fiber inputs are subject to PBA- and NRM-induced descending modulation, but further reveal that responses to deep A- and C-fiber inputs are also subject to descending modulation from PBA and NRM.