TRANSCRANIAL ELECTRICAL-STIMULATION WITH LIMOGES CURRENTS DECREASES HALOTHANE REQUIREMENTS IN RATS - EVIDENCE FOR THE INVOLVEMENT OF ENDOGENOUS OPIOIDS

Transcutaneous cranial electrical stimulation with Limoge's currents has been shown to facilitate anesthesia/analgesia in surgical patients. However, the neurobiologic substrate of this effect remains unknown. The present study was designed to analyze the influence of transcranial electrical stimulation (TCES) on halothane requirements in rats and the contribution of the central endogenous opioid, alpha-2-adrenergic and 5-hydroxytryptamine (5-HT1 and 5-HT2) serotonergic systems to this effect. The influence of TCES on the MAC of halothane (MACH) and its reversibility by a subcutaneous 2 mg/kg naloxone injection were first determined in 20 rats using a randomized blinded protocol. MACH was decreased markedly in stimulated animals (TCES, n = 10) in comparison with sham-operated nonstimulated rats (controls, n = 10): MACH = 0.60 +/- 0.15, mean +/- SD, versus 1.07 +/- 0.05 vol%, P < 0.001. In TCES animals, naloxone administration restored MACH values to the levels of controls but failed to affect MACH in controls. The influence of the duration of TCES applied prior to MACH determination was further investigated in 30 animals. The magnitude of MACH reduction was significantly increased with the cumulative duration of stimulation. For each duration of stimulation tested, administration of a 5-mu-g intracerebroventricular (icv) dose of the enkephalinase inhibitor thiorphan significantly enhanced TCES effects (P < 0.05). Finally, the icv administration of a 15-mu-g naloxone dose appeared to reverse completely the MACH reduction elicited by TCES (n = 8, P < 0.01). In contrast, neither the alpha-2-adrenergic (norepinephrine) blocker yohimbine (30-mu-g, n = 8) nor the nonselective serotonergic (5-HT) antagonist methysergide (30-mu-g, n = 8) or the selective 5-HT2 antagonist ritanserin (30-mu-g, n = 8) given icv were effective in this regard. Taken together, these results indicate that TCES significantly reduces halothane requirements in rats and support the involvement of the endogenous opioid, but not monoaminergic systems, in this effect.