Habituation and dehabituation of the spinal polysynaptic reflex responses: modification by naloxone and opiates and their anatomical correlates.

The sustained inhibitory action of spinal endorphins could be responsible for the habituation of polysynaptic responses in the spinal cord. To test this hypothesis, acute spinalized unanesthetized cats (decerebrated and curarized) were used. Sural nerve electrical stimulation (0.2 Hz) was provided and a progressive decrease in the reflex response was found. Conversely, the field potential (lamina V) progressively increased during stimulation, reaching its maximum amplitude when ventral root response showed maximum habituation. The administration of naloxone (0.8-10.0 mg/kg) produced dehabituation or prevented habituation. The immunohistological results showed leu-enkephalin-like immunoreactive dot-like structures in close proximity to neurons of laminae VII, VIII and IX in the lumbo-sacral segment of the spinal cord. Our results suggest an involvement of opioid peptides in the habituation process.