Central trigeminal effects of somatostatin and etorphine on adrenal and autonomic function in the cat

The influence of somatostatin and the potent μ-opiate receptor agonist etorphine on adrenal and autonomic responses mediated by trigeminal neurons was examined in chloralose-anesthetized cats. Microinjections of somatostatin (100 pmol) into laminae I-II of trigeminal subnucleus caudalis (V(c)) evoked increases in the adrenal secretion of catecholamines and adrenal blood flow without affecting arterial pressure, heart rate, or plasma adrenocorticotropic hormone. Injections of somatostatin into laminae III-IV of V(c) had no effect. Microinjections of etorphine (500 pmol) into laminae I-II of V(c) had no effect, whereas injections into laminae III-IV caused small increases in total adrenal blood flow and peripheral concentrations of norepinephrine. To assess local effects of these drugs on peripheral trigeminal nociceptor-evoked autonomic responses, corneal stimulation was presented before and after topical application of somatostatin or etorphine on the dorsal brain stem surface. Somatostatin did not affect the adrenal and autonomic responses evoked by noxious thermal or chemical irritant stimulation of the cornea. Topical etorphine blocked completely the increase in adrenal blood flow, arterial pressure, and heart rate after noxious thermal stimulation of the cornea. These results suggest that somatostatin and μ-opiate receptor agonists act by different mechanisms at the level of the spinal trigeminal complex to affect adrenal and autonomic function.