Identification of the primary afferent fiber group and adequate stimulus initiating the trigeminal depressor response.

In anesthetized and immobilized rabbits we sought to identify the adequate stimulus and the primary afferent fiber group initiating hypotension and inhibition of sympathetic vasoconstrictor discharges in the trigeminal depressor response (TDR). In the first series of experiments we investigated the effects of electrical stimulation of different fiber groups of the infraorbital nerve on arterial pressure (AP) and renal sympathetic nerve activity (RNA). Stimulation of A-beta-fibers at frequencies between 1 and 200 Hz caused little or no reduction in AP or RNA. Recruitment of A-delta-fibers at stimulus frequencies between 1 and 30 Hz always resulted in falls in AP and RNA (by up to 30 mm Hg, and 75% of control, respectively). Decreases in AP and RNA were augmented to some extent by additional excitation of C-fibers over the same frequency range. In the second series of experiments we applied noxious or innocuous mechanical or thermal stimuli to the hairy skin of the upper lip. Hypotension and inhibition of RNA were elicited by any of the following noxious stimuli of the facial skin: (a) pricking by pins; (b) pinching by serrated forceps, (c) heating above 43% C by a contact thermoprobe; and (d) actual burning of the skin. In contrast, innocuous mechanical or thermal (below 40 degrees C) stimuli failed to diminish AP and RNA. These results indicate that noxious stimuli to the face initiate hypotension and inhibition of the sympathetic nerve activity in the TDR through activation of the A-delta primary afferent fiber group, alone or in combination with the C-fiber group.