Daily injections of L-dihydroxyphenylalanine (L-DOPA, dopamine precursor) given 4？h after 5-hydroxytryptophan (5-HTP, serotonin precursor) induced inhibitory responses in recrudescing gonad (in the first week of December) of Indian palm squirrel, a seasonally breeding subtropical animal. Other temporal relations (L-DOPA given at 0, 8, 12, 16, and 20？h after 5-HTP administration) did not show any effect on the recrudescing gonad. This inhibitory effect of 4？h was evident under short day length (6？:？18) group but was masked by the increasing day length of nature (NDL, late December onwards) and increased photoperiod of long day group (16？:？8). It is apparent that seasonal testicular recrudescence of Indian palm squirrel during short day length by 4？h relation of 5-HTP and L-DOPA is not a pharmacological effect but actually is an alteration of seasonality in this annually breeding mammal. It seems that endogenous mechanism controlling seasonal testicular recrudescence of Indian palm squirrel is reset by timed daily injections of these neurotransmitter drugs. It is suggested that in spite of different environmental factors (photoperiod, humidity, etc.) used by different species to time their annual reproduction, basic mechanism of seasonality appears to be the same, that is, the temporal synergism of neurotransmitter activity. 1. Introduction Injections of two neurotransmitter precursor drugs the 5-hydroxytryptophan (5-HTP, serotonin precursor) and L-dihydroxyphenylalanine (L-DOPA, dopamine precursor) are thought to reset the phase of two circadian neuroendocrine oscillations with serotonergic and dopaminergic components and thereby determine seasonality [1]. A mechanism for neuroendocrine regulation of seasonality was proposed in different species of animals like a teleost fish [2], white throated sparrow [3], Japanese quail [4, 5], spotted munia [6, 7], red headed bunting [8], lal munia [9], hamster [10], Indian palm squirrel [11, 12], Indian weaver bird [13], Indian garden lizard [14], and so forth. Temporal synergism of 5-HTP and L-DOPA was not only found to be gonadostimulatory by 12？h relationship in squirrel [11] but also the advancement of puberty in Japanese quail [15]. On the other hand, the 8？h relation inhibits gonadal growth and sometimes induces gonadal suppression [16]. Recently, Sethi and Chaturvedi [17] had reported that in mice 8？h temporal relation of 5-HTP and L-DOPA suppresses gonadal growth and 12？h relation accelerates gonadal growth, whereas other relations (0, 4, 16, and 20？h) were ineffective. Further, the inhibitory effects of an
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