Tryptophan and tonic immobility in chickens: Effects of dietary and systemic manipulations.

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Abstract
Consistent with a serotonergic-midbrain raphe model of tonic immobility, 4 experiments designed to affect changes in serum tryptophan produced reliable effects on the duration of the response in chickens. Systemic injections of tryptophan, the dietary precursor to serotonin [5-hydroxytryptamine], led to a dose-dependent increase in immobility, with optimal effects being observed within 30 min after injection. Dietary depletion of endogenous tryptophan served to attenuate the duration of immobility, and a diet completely free of tryptophan, but supplemented with niacin, practically abolished the reaction. Dietary replacement served to reinstate the response. In a 5th experiment, tryptamine, an alternative metabolic by-product of tryptophan, had no effect on immobility. The data are discussed in light of evidence showing serotonergic involvement in tonic immobility.