Most amphibians lack morphologically distinguishable sex chromosomes, but a number of experimental techniques have shown that amphibian sex determination is controlled genetically. The few studies suggesting that environment influences sex determination in amphib- ians have all been conducted at temperatures outside of the range normally experienced by the species under study, and these effects probably do not occur under natural conditions. No sex- determining genes have been described in amphibians, and sex differentiation can be altered by treatment with exogenous steroid hormones. The effects of sex steroids vary extensively between species, and a variety of steroids can alter the sex ratios of treated larvae. The role of endogenous sex steroids in gonadal differentiation has not been fully explored; thus the natural role of ste- roids in amphibian gonadal differentiation is unknown. Sex steroid receptors have not been exam- ined in amphibian gonads, and the mechanism of steroid action on the gonad is unclear. In addition to steroids, the thyroid hormones may play a role in gonadal differentiation. Pituitary gonado- trop(h)ins affect gonadal growth, but not differentiation or maturation of gonads. In addition to the issue of resolving the mechanisms underlying hormone action in gonadal differ- entiation, other debates concerning interactions between the developing gonads and the invading germ cells, and even the origin of the medullary and cortical portions of the developing gonads, remain unresolved. Studies examining links between sex determination and gonadal differentiation are needed. In addition, examinations of variation in steroidal effects on gonadal development in a phylogenetic context are lacking. J. Exp. Zool. 281:373n399, 1998. © 1998 Wiley-Liss, Inc. Amphibians are excellent models to study sex differentiation, for a number of reasons. Genetic sex-determining systems and sex chromosomes have evolved a number of times in this class, and gonadal differentiation (although under genetic control) is responsive to experimental manipula- tions. In addition, as laboratory animals, amphib- ians are ideal; because females produce up to 10,000 eggs per clutch in some species, large sample sizes can be obtained. Also, large num- bers of individuals can be treated easily in ex- perimental manipulations because larvae are typically aquatic and readily absorb hormones and other chemicals added to rearing water. Despite their accessibility as study animals, relatively few recent studies have addressed sex determination and differentiation in amphibians. 1

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Sex determination and primary sex differentiation in amphibians: Genetic and developmental mechanisms

The substantia nigra (SN) is a midbrain center composed of dopaminergic (DA-) and gamma aminobutyric acid (GABA)ergic (GABA-) neurons. In this study, we investigated the topographical relationship between both cell populations and their chemical profile by using single and double immunostaining for tyrosine hydroxylase (TH), glutamic acid decarboxylase (GAD), cholecystokinin (CCK), calretinin (CR), calbindin (CB), parvalbumin (PV), and nitric oxide synthase (NOS). Our results showed that DA-cells are arranged in two bands, one rostrodorsal that corresponds to the SN pars compacta (SNC), and another caudoventral that corresponds to the SN pars reticulata (SNR) and emits cell bridges that make contact with the rostrodorsal one. In the SNR, GABA-cells are arranged in dorsoventrally elongated clusters that occupy DA-cell free regions. According to cytoarchitectural, topographical, and chemical criteria, we identified ten different cell groups: five dopaminergic ones, and five GABAergic ones. Within DA-cells, we found a cell group in the dorsomedial portion of the SNC which contains CCK, CR, and CB (dmSNC); DA-cells in the SN pars lateralis (SNL) which also contain CCK, CR and CB; DA-cells in the rostral half of the SNC containing CCK and CR (rSNC); DA-cells in the SNR and the caudal half of the SNC which only express CR (cSNC-SNR), and a DA-cell group in the lateral part of the SNC that contains none of the markers studied (lSNC). Within GABA-cells, we distinguished: large GABA-cells in the SNL that contain PV; large GABA-cells in the rostrolateral part of the SNR containing PV and NOS (rlSNR), small GABA-cells in the caudomedial part of the SNR containing PV (cmSNR), and two groups of small GABA-cells in the rostromedial portion of the SNR, one of them containing CR (rmcSNR), and the other containing NOS (rmnSNR). These data suggest that over a compartmental and complementary organization, DA- and GABA-nigral cells form a mosaic of neurochemically different subnuclei which probably differ in their physiological and pharmacological properties and vulnerability to aggression.

Compartmental organization and chemical profile of dopaminergic and GABAergic neurons in the substantia nigra of the rat.

Melatonin in non-mammalian vertebrates: Physiological role in reproduction?

Somatostatin receptor gene expression in neuroblastoma

Characterization of spontaneous and decalin-induced hyaline droplets in kidneys of adult male rats

Injection of cadmium chloride in a toad increases both theΔ5-3β-hydroxy-steroid dehydrogenase and 17β-hydroxysteroid dehydrogenase activities in Bidder's organ.

Effect of cadmium chloride on steroidogenic enzymes in the Bidder's organ of the toad (Bufo melanostictus).

Adult male rats were given injections of oestradiol-17 beta (50 micrograms/100 g body wt per day) for 7 days. When they were killed 14 days after the last injection, serum levels of gonadotrophins and testosterone and weights of accessory sex organs were less, testicular 17 beta-hydroxysteroid dehydrogenase (17 beta-HSD) activity was suppressed and spermatogenesis was inhibited. Administration of alpha 2u-globulin (1.5 mg/day) for 14 days to oestrogen-treated rats and for 10 days to control rats resulted in increased concentrations of gonadotrophins and testosterone in the serum. Accessory sex organ weight and spermatogenesis appeared to be normal while 17 beta-HSD activity increased in oestrogen-treated rats after treatment with alpha 2u-globulin. It was concluded that alpha 2u-globulin has an effect on testicular function in oestrogenized rats by inducing gonadotrophin and testosterone synthesis.

Effect of alpha 2u-globulin on serum concentration of gonadotrophins and testicular activity in oestrogen-treated rats.

The relationship between the pineal gland and testes of the toad was investigated in breeding and hibernating seasons. Pinealectomy increased spermatogenesis in hibernating toads but it failed to exert any effect during the breeding season. The pineals were found to be active during hibernation and atrophied in the breeding season. It is speculated that the cyclic pineal activity might be responsible for the seasonal variation of spermatogenesis in the toad.

Influence of Pineal Gland on Spermatogenesis of Toad (Bufo melanostictus)

Treatment of adult male rats with oestradiol-17 beta (50 microgram/100 g body wt per day for 7 days) resulted 14 days later in reduced levels of alpha 2 micro-globulin in serum and inhibition of spermatogenesis. Administration of corticosterone (2 mg/day) for 14 days to oestrogen-treated rats raised the concentration of alpha 2 micro-globulin in the serum and prevented spermatogenic degeneration. It is concluded that endogenous alpha 2 micro-globulin induced by corticosterone may play a role in spermatogenesis in oestrogenized rats.

Effect of corticosterone on serum concentrations of alpha 2 micro-globulin and on spermatogenesis in oestrogen-treated rats.

Injection of cadmium chloride in toad increased Leydig cell size (area) and Δ5-3β-hydroxysteroid dehydrogenase activity in the testis.

N. M. Biswas

Papers

Effect of cadmium chloride on steroidogenic enzymes in the Bidder's organ of the toad (Bufo melanostictus).

Effect of alpha 2u-globulin on serum concentration of gonadotrophins and testicular activity in oestrogen-treated rats.

Influence of Pineal Gland on Spermatogenesis of Toad (Bufo melanostictus)

Effect of corticosterone on serum concentrations of alpha 2 micro-globulin and on spermatogenesis in oestrogen-treated rats.

Testicular Leydig cells and 277-1277-1277-1dehydrogenase in cadmium-treated toads (Bufo melanostictus)