COMMON PATTERNS OF INCREASED AND DECREASED FOS EXPRESSION IN MIDBRAINAND PONS EVOKED BY NOXIOUS DEEP SOMATIC AND NOXIOUS VISCERAL MANIPULATIONS IN THE RAT

Immunohistochemical detection of the protein product (Fos) of the c-fo s immediate early gene was used to study neuronal activation in the ro stral pens and midbrain of halothane-anesthetised rats following noxio us deep somatic or noxious visceral stimulation. In animals exposed on ly to halothane anesthesia, Fos-like immunoreactive (IR) neurons were located in the midbrain periaqueductal gray matter, tectum, and parabr achial nucleus. Following noxious stimulation of hindlimb muscle, knee joint, vagal cardiopulmonary, or peritoneal nociceptors, there was, c ompared to halothane-only animals, a significant increase in the numbe rs of Fos-like (IR) cells in the caudal ventrolateral periaqueductal g ray and the intermediate gray lamina of the superior colliculus. Given the general agreement that increased Fos expression is a consequence of increased neuronal activity, the finding that a range of noxious de ep somatic and noxious visceral stimuli evoked increased neuronal acti vity in a discrete, caudal ventrolateral periaqueductal gray region is consistent with previous suggestions that this region is an integrato r of deep noxious evoked reactions. The noxious deep somatic and noxio us visceral manipulations also evoked, compared to halothane-only anim als, reductions in the numbers of Fos-like IR cells in the stratum opt icum of the superior colliculus and the unlaminated portion of the ext ernal subnucleus of the inferior colliculus. To our knowledge this is the first report of reductions in Fos-expression in the tectum evoked by noxious stimulation. In separate experiments, the effects of noxiou s deep somatic and noxious visceral manipulations on arterial pressure and heart rate were measured. The noxious visceral manipulations evok ed substantial and sustained falls in arterial pressure (15-45 mmHg), and heart rate (75-100 bpm), whereas the depressor and bradycardiac ef fects of the noxious deep somatic manipulations were weaker, not as su stained, or entirely absent. As similar distributions and numbers of b oth increased and decreased Fos-like IR cells were observed after each of the deep noxious manipulations, it follows that the deep noxious e voked increases and decreases in Fos expression were not secondary to the evoked depressor or bradycardiac effects. (C) 1996 Wiley-Liss, Inc .