MORPHINE MICROINJECTED INTO THE NUCLEUS-TRACTUS-SOLITARIUS AND ROSTRAL VENTROLATERAL MEDULLARY NUCLEUS ENHANCES SOMATOSYMPATHETIC A-REFLEX AND C-REFLEX IN ANESTHETIZED RATS

The modulatory effects of morphine microinjected into localized areas of the brainstem on somatosympathetic A- and C-reflexes were examined in urethane-anesthetized rats. Somatosympathetic A- and C-reflexes wer e elicited in a branch of the inferior cardiac nerve by electrical sti mulation of myelinated (A) and unmyelinated (C) afferent fibers in the tibial nerve. Morphine (0.002-0.2 mu g/50 nl) was microinjected into the rostral, intermediate and caudal parts of the nucleus tractus soli tarius (NTS), the rostral ventrolateral medullary nucleus (RVLM), the caudal ventrolateral medullary nucleus (CVLM), the locus coeruleus (LC ), the raphe magnus (RM), the periaqueductal gray (FAG), and the accum bens nucleus (Acb). Microinjections of morphine (0.2 mu g) into the in termediate and caudal NTS produced significant augmentations of the A- and C-reflexes, C-reflexes being more markedly enhanced than A-reflex es. Microinjection of morphine (0.2 mu g) into the RVLM produced a pro minent increase in the C-reflex, the threshold dose for a significant increase being 0.02 mu g morphine. Microinjection of morphine up to 0. 2 mu g/50 nl into the other areas mentioned above had no significant e ffect on either reflex component. All opiate-induced increases of the reflex discharges could be reversed by intravenous application of nalo xone (2 mg/kg). The reflex augmentation induced by microinjection of m orphine into the NTS may be caused by suppressing inhibitory barorecep tor information or by enhancing excitatory chemoreceptor information i n the NTS. Augmentation of the C-reflex induced by microinjection of m orphine into the RVLM may be caused by facilitating C-reflex pathways or by suppressing inhibitory neural circuits involved in the C-reflex within the RVLM. (C) 1996 Elsevier Science Ireland Ltd.