Characteristics of the trigeminal depressor response in cats.

We studied the effects of electrical stimulation of the inferior alveolar nerve (IAN) on cardiovascular responses in cats. There was statistical correlation between cardiovascular response and prestimulus mean arterial blood pressure (MABP) and heart rate (HR). A trigeminal depressor response (TDR) was induced when the prestimulus MABP and HR were above 95 mm Hg and 140 beats/min, respectively. We investigated further to identify the vasomotor regulating center and neural transmitters involved in TDR. In the medulla, electrical stimulation of the dorsomedial medulla, the infratrigeminal nucleus (IFT), and the rostral ventrolateral medulla (RVLM) induced a vasopressor response. We confirmed that neurons in the RVLM were retrogradely labeled by wheat germ agglutinin-conjugated horseradish peroxidase injection into the nucleus intermediolateralis of the spinal cord. The vasopressor response induced by IFT stimulation was similar to that induced by IAN stimulation. Vasodepressor responses were induced when the caudal ventrolateral medulla, the nucleus tractus solitarius, the lateral tegmental field, the trigeminal nucleus interpolaris, the trigeminal spinal tract, and the paramedian reticular nucleus were stimulated. These responses, however, were not similar to the vasodepressor response induced by IAN stimulation but were similar to the cardiovascular response induced by vagal afferent stimulation. After spinalization or lesion of the RVLM, MABP and HR decreased and TDR completely disappeared. Inhibitory synaptic ligands and receptors were localized using immunohistochemical techniques. Neurons immunopositive for adrenaline, noradrenaline, and gamma-aminobutyric acid (GABA), and adrenaline alpha(2A), GABA(A), GABA(B), and glycine receptors were distributed along the sympatho-reflexive route including the RVLM and IFT. These results suggest that TDR could be induced as negative feedback to sympathetic hyperactivity whenever MABP and HR are high, because of the inhibitory control of the RVLM. Copyright 2004 Wiley-Liss, Inc.