Electrophysiology of barbiturate withdrawal in the spinal cord.

Cats were made physically dependent on sodium pentobarbital using the "maximally tolerable" dosing technique. All animals treated this way receive equieffective doses chronically and all become severly dependent. After 5 weeks of treatment each cat displayed withdrawal signs indicative of severe physical dependence. At specific times after the last dose, electrophysiological measurements of spinal cord segmental reflex function were made. Under brief volatile anesthesia, a C1 spinal section was performed. The lumbar spinal cord was exposed by laminectomy and a hindleg was dissected to provide peripheral nerves for stimulation. Recordings were taken from ventral roots L7 and S1 that had been cut near their exits through the dura. Following single, supramaximal sciatic nerve shocks, monosynaptic (2N) responses were not altered during withdrawal, but both the amplitude and duration of polysynaptic response and of afterdischarge were increased withdrawing cats. The rate of 2N synaptic recovery, measured by a paired stimuli technique, was found to be increased during withdrawal. The 2N pathway was able to transmit more effectively during repetitive stimulation since there was less decrement in response during transmission. The relationship between post-tetanic potentiation and tetanic frequency was shifted toward lower frequencies. The size of the motor neuron pool, estimated by maximum post-tetanic potentiation, and the 2N discharge zone were not altered. Background discharge ("noise") was increased during withdrawal.