Intra-axonal recordings in rat dorsal column axons: membrane hyperpolarization and decreased excitability precede the primary afferent depolarization.

Intra-axonal recordings were obtained in the dorsal columns of the rat lumbosacral spinal cord. Dorsal root or dorsal column stimulation at levels subthreshold for the impaled axon elicited a prolonged depolarization corresponding to the primary afferent depolarization (PAD). The depolarization was preceded by a brief hyperpolarizing potential during which excitability was decreased. The hyperpolarization corresponds temporally to the extracellularly recorded DRP IV component of the dorsal root potential described by Lloyd and McIntyre, and may represent the intracellular correlate of this potential. Possible mechanisms for this hyperpolarization include electrical interactions between neuronal elements, a biphasic GABA response, or attenuation of background afferent axonal depolarization.