Comparative study on the electrophysiological responses at thalamic level to different analgesic peptides

Using electrophysiological methods to detect the extracellular activity of single neurons in the thalamus of anaesthetized rats, their response to mechanical and thermal noxious stimuli were assessed before and after administration of 4 analgesic peptides of various types. Dermophin, a peptide extracted from frog's skin, was found to have an opioid-like antinociceptive activity antagonized by naloxone. Caerulein, which has a similar origin, failed to suppress the nociceptive responses of thalamic neurons evoked by peripheral stimuli. Calcitonin, a peptide found at brain level, induced an alteration of the increased firing characteristic of noxious stimuli, and its action was not reversed by naloxone. FK 33-824, a synthetic peptide, induced a morphine-like action when injected i.c.v. at a dosage 1000 times lower than that of morphine on a molar basis. It is concluded that electrophysiological investigations on peptides endowed with analgesic activity contribute greatly to a more precise profile of the peptides as candidate drugs in pain control.