Temporal synergism of neurotransmitter ‐ affecting drugs influences seasonal conditions in sparrows

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Abstract
Daily injections of corticosterone and prolactin have been shown previously to either stimulate or inhibit reproductive and migratory indices in sparrows as a function of the time interval between injection times (Meier and Ferrell, 1978). The present studies were performed to test whether the hormonal effects can be duplicated by timed injections of drugs which influence brain neurotransmitter activities in a manner similar to that of the hormones. DOPA (catecholamine precursor) injections at 12 h after daily injections of 5‐HTP (serotonin precursor) was stimulatory for gonadal growth and body fat stores in white‐throated sparrows (Zonotrichia albicollis) whereas other time relations (3, 8, or 18 h intervals between daily injections of 5‐HTP and DOPA) were not. The 8‐h relation was inhibitory for fat stores and reproductive indices. In house sparrows (Passer domesticus), DOPA injections 16 h after daily injections of 5‐HTP reduced the sizes of photoperiodically‐stimulated oviducts whereas an 8‐h relation had no effect. These effects are similar to those produced by hormone injections; they indicate that 5‐HTP can be substituted for corticosterone injections and DOPA can be substituted for prolactin. Both the hormones and the drugs are thought to reorganize two circadian neural oscillations which interact temporally to control seasonal conditions.