Abstract
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.
Highlights
Injections of two neurotransmitter precursor drugs the 5hydroxytryptophan (5-HTP, serotonin precursor) and Ldihydroxyphenylalanine (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]
Seasonal gonadal recrudescence of Indian palm squirrel starts from approximately mid-December, attaining full breeding condition by January which is maintained till June/July [11]
Results indicate that daily administration of dopaminergic precursor drug (L-DOPA) given 4 h after serotonergic precursor drug (5-HTP) induce inhibiting responses in sexually recrudescing testis unlike control squirrels
Summary
Injections of two neurotransmitter precursor drugs the 5hydroxytryptophan (5-HTP, serotonin precursor) and Ldihydroxyphenylalanine (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 different phase relationship has been reported in the circadian hypothalamic serotonin and dopamine rhythms of breeding versus nonbreeding quail under both control and simulated conditions [19, 20] and in spawning and nonspawning fish [21]. In view of these previous studies, present long term study was conducted in recrudescing/preparatory phase of annual gonadal cycle to study the interaction, if any, between the external (photoperiod) and internal (neuroendocrine entrainment by neurotransmitter precursor drug) factors. Since 4 h relation is thought to be gonadosupressive, its effect was determined during the period when initiation of seasonal gonadal growth takes place (in first week of December) and continued up to fully active reproductive phase (June) to observe the effect of neurotransmitter drugs
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