Importance of the Locus coeruleus and involvement of alpha-adrenergic receptors in the post-decapitation reflex in the rat.

The latency, duration, hindlimb kick frequency, and total activity components of the post-decapitation reflex (PDR) were measured in the rat using a movement-sensitive transducer. Reduction of brain and spinal cord norepinephrine (NE) caused by neonatal administration of 6-hydroxydopamine (6-OHDA) or 5,7-dihydroxytryptamine, which also reduced brain serotonin, decreased all components of the PDR. Depletion of serotonin or dopamine alone reduced the vigor of the reflex, suggesting that these pathways can influence the PDR but are not essential for the response. Lesions of neurons in the Locus coeruleus, made electrolytically or with 6-OHDA, decreased the intensity of the PDR, with the 6-OHDA-induced lesion being more effective. Depletion of forebrain NE terminals with 6-OHDA did not alter the PDR, consistent with a critical involvement of spinal noradrenergic fibers. The PDR was also decreased by phentolamine and prazosin, but not by propanolol, suggesting an involvement of alpha-adrenergic receptors in the response. This hypothesis was further supported by the finding that the efficacy of a variety of drugs (such as tricyclic antidepressants, phenothiazines, and anti-hypertensive compounds) for blocking the reflex was apparently related to their affinity for alpha-adrenergic receptors. Thus, the PDR is dependent on noradrenergic fibers in the spinal cord and may provide a simple screen for drugs with suspected alpha-adrenergic blocking properties or for agents that disrupt the function of central noradrenergic fibers.