The importance of 5-hydroxytryptamine turnover for the analgesic effect of morphine in the chicken

A new method for testing analgesia in young chicks is described. It consists of shining a light on the chick's head till the chick senses the heat and moves. The report in the literature that chickens require very large doses of morphine to produce analgesia was only partly confirmed. Three strains (called A, B &; C) were tested. Only 57% of the chicks showed analgesia after morphine 100mg/kg in strain C, whereas all chicks responded to 30mg/kg in strain B and to 15mg/kg in strain A. The subcutaneous ED₅₀ was 80 mg/kg in C, 17.5 mg in B and 5.6 mg in A. Except for the first days of life, the analgesic responses are not age-dependent (tested up to 48 days). The relationship between 5-hydroxytryptamine turnover in the brain and analgesic effect of morphine was examined in the three strains. Only strain A showed an increase in 5-hydroxyindole acetic acid (of 31%, P < 0.01, in the cerebral hemispheres, and of 23%, P < 0.05, in the optic lobes) and in this respect resembled the mammals.It can be concluded that the phenomenon of the interaction of morphine with 5-hydroxytryptamine-containing neurones, which enhances the analgesic effect of morphine in the mammalian brain, occurs in some strains of chicks but is absent in others. The strain in which it was found had a sensitivity to morphine which approached that seen in mammals.