Central opiate receptor blockade by naloxone impairs thalamic and hypothalamic autoregulation in the cat

Abstract

 Experiments were carried out in order to determine the effects of centrally induced opiate receptor blockade on the autoregulation of the regional cerebral blood flow (rCBF, measured by the H₂-gas clearance technique) of the anesthetized, ventilated cats. General opiate receptor blockade was induced by intracere- broventricularly (i.c.v.) injected naloxone. Changes of the lower limit of hypothalamic, thalamic and white matter blood flow autoregulation were studied by reducing the systemic mean arterial pressure (MAP) in a stepwise manner to 80 mmHg, 60 mmHg and 40 mmHg by hemorrhage. Centrally administered naloxone (10 μg/10 μl/kg, i.c.v.) resulted in an abolishment of the autoregulatory mechanisms in the hypothalamic and thalamic regions but caused no such changes in the white matter. These observations suggest that 1.) intracerebral opiate receptors and/or central opioid peptiderg mechanisms play a significant role in the maintenance of the thalamic and hypothalamic blood flow autoregulation during hemorrhagic hypotension and 2.) the involvement of the endogenous opioid peptide system in rCBF autoregulatory mechanisms is regionally different.