Possible non-narcotic component to action of opiate peptides on tonic immobility

Chickens were tested in a tonic immobility paradigm after a single intraperitoneal injection of either 0.0, 0.1, 1.0, 10.0; 100.0, or 1000.0 microgram/kg of the potent opiate analog, (D-Ala2, F5Phe4)-Met-enkephalin-NH2. An inverted-U relationship was obtained, with 100 microgram/kg being the most effective in prolonging immobility. This dose was used in subsequent studies involving pretreatment with naloxone or diluent followed by treatment with diluent, (D-Ala2, F5Phe4)-Met-enkephalin-NH2 (a strong opiate), or (D-Phe4)-Met-enkephalin (a weak opiate). The results indicated that although naloxone had mixed effects in attenuating the duration of tonic immobility, even the analog with negligible opiate activity reliably potentiated the response. Therefore, a component of the effect of opiate peptides on tonic immobility could be due to a non-narcotic action.