Background: Alpha(2)-Adrenergic agonists such as clonidine and dexmedetomidine are known to produce sedation and analgesia in humans. The sedative effect of these agents is thought to occur through supraspinal pathways, involving the locus ceruleus (LC) and its projections in rats. While the antinociceptive response to alpha(2) agonists, given intrathecally, is mediated predominantly in the spinal cord, other sites of action have not been systematically studied. The authors examined whether alpha(2)-adrenergic receptors in the LC mediate an antinociceptive effect.
Methods: For administration of different drugs into the LC, guide cannulas were placed with their tips in the LC in male Sprague-Dawley rats. Dexmedetomidine (3.5 micrograms/0.2 microliter) was microinjected into the LC through the cannula, or given systemically by intraperitoneal injecton (50 micrograms/kg). The antinociceptive effect of dexmedetomidine was measured using the tail-flick latency response. To determine the sites through which dexmedetomidine injection into the LC produces antinociception, the authors examined whether this response could be perturbed by the specific alpha(2)-adrenergic antagonists atipamezole and L659,066 and pertussis toxin administered either into the LC or intrathecally before injection of dexmedetomidine systemically or directly into the LC. To eliminate the possibility that drug administered in one site (LC or intrathecal) could reach the other site, the dispositional characteristics of radiolabeled dexmedetomidine (LC) or atipamezole (intrathecal) were studied.
Results: Dexmedetomidine placed into the LC produces a dose-dependent increase in the tail-flick latency. This antinociceptive effect was blocked by pertussis toxin and by the alpha(2) antagonists atipamezole and L659,066 placed in the LC. Intrathecal administration of atipamezole and pertussis toxin also blocked the antinociceptive effect of dexmedetomidine placed in the LC. (3)H-dexmedetomidine introduced into the LC did not reach the spinal cord in pharmacologically active concentrations; also, intrathecally administered (3)H-atipamezole did not reach the LC in appreciable amounts. The systemic administration of dexmedetomidine produced an increase in tail-flick latency, and this effect was attenuated by the injection of atipamezole and L659,066 into the LC.
Conclusions: Part of the mechanism by which dexmedetomidine produces an antinociceptive effect is by an action directly on the LC, demonstrated by these studies in which antinociception produced by injection of this drug into the LC can be blocked by specific alpha(2) antagonists injected into the LC. Furthermore, the action of dexmedetomidine in the LC in turn may result in an increase in activation of alpha(2) adrenoceptors in the spinal cord, because the antinociceptive effect of LC dexmedetomidine injection also can be blocked by intrathecal injection of antipamezole and pertussis toxin.