A differential effect of naloxone on transmission of impulses in primary afferents to ventral roots and ascending spinal tracts.

Ventral root reflexes and ascending volleys to stimulation of group I muscle afferents, large diameter cutaneous afferents and unmyelinated primary afferents were examined in barbiturate anaesthetized spinal cats. Intravenous naloxone (0.05-0.10 mg/kg) increased reflexes to stimulation of all primary afferent types but of the ascending volleys, only those to stimulation of unmyelinated primary afferents were increased. Thus it appears that opioid peptides have differential effects on transmission of primary afferent impulses to supraspinal areas, an action possibly relevant to analgesia, in contrast to a non-selective suppression of transmission to motoneurones.