Classical conditioning of front paw and hind paw footshock induced analgesia (FSIA): Naloxone reversibility and descending pathways

Opiate and non-opiate footshock induced analgesia (FSIA) can be differentially elicited dependent upon the body region shocked. As measured by the spinally-mediated tail flick test, hind paw shock produces non-opiate analgesia whereas front paw shock produces opiate analgesia. The present series of experiments utilized cord lesions and transections to identify descending and intraspinal pathways mediating front paw and hind paw FSIA. The results of these studies indicate that front paw shock leads to activation of supraspinal sites which mediate analgeshi via descending pathways lying solely within the dorsolateral funiculus (DLF) of the spinal cord; direct intraspinal pathways are not involved. Hind paw FSIA is also mediated by a descending DLF pathway but is unlike front paw FSIA in that it involves intraspinal pathways as well. This work provides further parallels between the analgesias produced by morphine, electrical brain stimulation and environmental stimuli.