Noxious mechanical stimulation of the hind paws of the anaesthetized rat fails to elicit release of immunoreactive beta-endorphin in the periaqueductal grey matter.

As a test of the hypothesis that an animal responds to a severe peripheral painful stimulus by a central release of beta-endorphin, antibody microprobes were inserted stereotactically into the midbrain of urethane anesthetized rats. These microprobes bore antibodies to beta-endorphin immobilized to their outer surfaces. While microprobes were in the brain for periods of 10 to 30 min either no stimulus was delivered or alligator clamps were applied to both hind paws. Microprobes were then incubated with 125I-beta-endorphin. Quantitative image analysis of microprobe autoradiographs showed no differences between the no-stimulus and noxious-stimulus groups. Thus these experiments found no evidence for beta-endorphin release following a severe peripheral painful stimulus.