Effects of melatonin, p-chlorophenylalanine, and ?-methylparatyrosine on plasma gonadotropin level and ovarian activity in the catfish, Heteropneustes fossilis: A study correlating changes in hypothalamic monoamines

Abstract

The effects of (ip, 10 injections over 20 days) of melatonin (75 μg 100 g(-1) BW), the serotonin (5-HT)-synthesis blocker, para-cholorophenylalanine (p-CPA, 10 mg 100g(-1 BW)) and the catecholamine-synthesis blocker, α-methylparatyrosine (α-MPT, 10 mg 100 g(-1) BW) on gonadotropin (GTH) secretion and ovarian activity were studied in (Heteropneustes fossilis) during late preparatory to early prespawning (April-May). The treatments resulted in significant reductions of plasma GTH and estradiol-17β levels, the gonadosomatic index, frequency distribution of vitellogenic and postvitellogenic oocytes, and ovarian and serum (32)p-labelled alkali-labile phosphoprotein (a marker of vitellogenic activity). Most of the oocytes were nonvitellogenic or had undergone atretic changes. The hepatic (32)-phosphoprotein content increased significantly over the saline control value. The effects were similar and pronounced in the p-CPA and melatonin-treated groups but were moderate in the α-MPT-treated group. Hypothalamic 5-HT content and turnover were significantly inhibited in the p-CPA and melatonin-treated groups but the content and turnover of catecholamines were not. The α-MPT treatment decreased significantly the content and turnover of dopamine (DA), noradrenaline (NA), and adrenaline (A) but did not influence the 5-HT content or turnover. These results suggest that 5-HT, NA and A are stimulatory to GTH secretion and that melatonin may act on the serotonergic system to inhibit the pituitary-gonadal axis.

Role of photoperiod and melatonin in the regulation of ovarian functions in Indian carp Catla catla: basic information for future application

“…The mechanism by which exogenous melatonin exerts its regulatory actions on the gonads is not clearly known. There is a possibility that melatonin interacts with the brain-pituitary-gonadal axis (Amano et al 2000), or with one or more of a variety of peripheral and/or central sites, including the serotonergic system of the hypothalamus (Senthilkumaran and Joy 1995), brain (Khan and Thomas 1996), pituitary gland (Khan and Thomas 1996;Gaildrat and Falcon 2000), and also directly on the gonads (Amano et al 2000). Obviously, evidence from further study is essential before ruling out any such possibilities in a hitherto unstudied fish like carp.…”

“…We also reported ) that pineal functions in carp were stimulated following exposure to continuous illumination (LL), but inhibited under the influence of continuous darkness (DD). A study on H. fossilis indicated the importance of the serotonergic system in the action of melatonin on the pituitary-gonadal axis (Senthilkumaran and Joy 1995). Thus, further studies on the possible role of the pineal gland and/or melatonin in the regulation of the hypothalamus-pituitary-gonadal axis in carp would be worthwhile.…”

Estradiol-17β modulates dose-dependently hypothalamic tyrosine hydroxylase activity inhibited by α-methylparatyrosine in the catfish Heteropneustes fossilis

“…Previous studies in teleosts have shown that hypothalamic CA activity is influenced by the circulating levels of E 2 [21,22,42,43]. In H. fossilis, ovarian E 2 varies seasonally and exerts feedback effects on hypothalamic CA activity depending on the season and concentration of the steroid administered [20,23,31,32]. These studies indicate that E 2 interacts with the hypothalamic CA system at different enzyme-catalyzed metabolic steps (DbH, PNMT and COMT) to alter its activity, which, in turn, regulates LH secretion [44].…”

“…Gonadectomy inhibited mRNA levels in TH neurons and E 2 replacement partially reversed the effect [30]. Senthilkumaran and Joy [31] showed that a-MPT inhibited differentially CA content and turnover depending on the reproductive phase. The changes in CA activities following the a-MPT treatment have impaired the functioning of the neuroendocrine-gonadal axis and decreased spawning activity [31].…”

“…Ovariectomized and E 2 -replaced catfish model, as described earlier [18,20,26,31,32] was used in the present study. The results indicate that E 2 indeed reverses the effect of the drug at low concentrations.…”