Suppression of annual testicular development in Indian Palm Squirrel,Funambulus pennanti by 8 hr temporal relationship of serotonin and dopamine precursor drugs

Summary Daily injections of 5-HTP (5-hydroxytryptophan, serotonin precursor) and L-DOPA (L-Dihydroxyphenylalanine, dopamine precursor) given 8 hour apart inhibited normal testicular growth in seasonally breeding Indian Palm Squirrel,Funambuluspennanti leading to complete gonadal atrophy, which was maintained till the end of the study. HCG administration induced higher degree of gonadal development but, when the two treatments (HCG + 8 hr relationship of 5-HTP and L-DOPA) were given simultaneously no significant difference was observed compared to control.Results indicate that induced suppression of gonadal function in a fashion similar to seasonal regression leading to non-breeding condition is the consequence of specific phase relationship (8 hr) between serotonergic and dopaminergic activities and not due to Serotonin or dopamine alone. This suggestion gets support from the finding that gonadal condition of squirrels receiving only 5-HTP (5-HTP control) or L-DOPA (DOPA control) was not different from control and exhibited normal testicular growth.It also seems that neurotransmitter precursor drugs given 8 hr apart possibly inhibited the activity of neuroendocrine-gonadal axis since this effect was overpowered by stimulatory effect of gonadotrophin. The findings strongly support the hypothesis that specific phase relation of circadian serotonergic and dopaminergic oscillations account for seasonal reproductive conditions.     

Elimination of testicular regression by 12-hr temporal relationship of serotonergic and dopaminergic activity in Indian Palm Squirrel,Funambulus pennanti

Summary Specific temporal relationship (12-hr) of serotonin and dopamine activity was induced by precursor drugs (5-HTP and L-DOPA) in Indian Palm SquirrelFunambulus pennanti during testicular regression phase. This treatment established breeding condition unlike control, which experienced complete atrophy of testes and accessory sex organs.It is obvious that seasonal testicular regression of Indian Palm Squirrel can be eliminated by 12hr relation of 5-HTP and L-DOPA, which is not a pharmacological effect but actually is an alteration of seasonality in this annually breeding subtropical mammal. It may be concluded that specific phase relation (12-hr) between circadian serotonergic and dopaminergic oscillations may induce breeding condition in regressing gonad of Indian Palm Squirrel during post-reproductive phase of annual gonadal cycle. It is also suggested that inspite of the different environmental factors (photoperiod, humidity etc.) used by different species to time their annual reproduction, basic regulatory mechanism of seasonality appears to be the same, i.e., temporal synergism of neurotransmitter activity.