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G V Callard

Publications and source records attributed to G V Callard.

At least 37 records · Page 2Linked to original sources
PubMed1993

Stage-related changes in steroid-converting enzyme activities in Squalus testis: synthesis of biologically active metabolites via 3 beta-hydroxysteroid dehydrogenase/isomerase and 5 alpha-reductase.

To investigate the relationship between steroidogenesis and spermatogenesis, two key enzymes of androgen biosynthesis, 3 beta-hydroxysteroid dehydrogenase/isomerase (3 beta-HSD) and 5 alpha-reductase, were compared at premeiotic (PrM), meiotic (M), and postmeiotic (PoM) stages. Staged tissues were obtained by dissection from the testis of the spiny dogfish Squalus acanthias, in which spermatogenesis is a simple diametric progression. Microsomal 3 beta-HSD activity was measured by conversion of [3H]dehydroepiandrosterone (DHEA) to androstenedione (AE). Reaction constants were: Km = 3.2 microM and Vmax = 243 pmol/min/mg protein. 3 beta-HSD increased progressively with maturation, resulting in three- to four-fold higher levels in PoM than in PrM stages. Absolute values and stage-related differences were the same, whether microsomes were derived from whole testis or from isolated spermatocysts (germ cell/Sertoli cell units), thus supporting microscopic studies showing that Sertoli cells are the primary steroidogenic elements of dogfish testis. In vitro conversion of [3H]testosterone to [3H]dihydrotestosterone (DHT) was used to estimate 5 alpha-reductase activity. Apparent substrate affinity was similar to that of 3 beta-HSD (Km = 2.9 microM), but maximal product yields were two to three orders of magnitude lower (Vmax = 208 fmol/min/mg protein). Also, the stage-related pattern of 5 alpha-reductase activity (PrM > PoM >> M) differed from that of 3 beta-HSD (PoM >> M > PrM).(ABSTRACT TRUNCATED AT 250 WORDS)

3-Oxo-5-alpha-Steroid 4-Dehydrogenase↗
PubMed1993

Molecular and cellular physiology of aromatase in the brain and retina.

Due to exceptionally high brain aromatase activity, teleost fish are advantageous for studying neural aromatase regulation, localization, and physiology. To determine the molecular mechanism of enhanced expression, we have isolated, cloned and sequenced a 3 kb full-length aromatase cDNA from a goldfish (Carassius auratus) brain library using a human placental aromatase cDNA as probe. The deduced sequence of goldfish aromatase is 510 amino acids (predicted Mw, 58 kDa) with 69% overall sequence similarity, when compared to human placental aromatase, and higher homologies in presumptive functional domains. A major 3 kb mRNA species was abundant in brain and low or non-detectable in non-neural tissues, reflecting the order of enzyme activities. To determine the cellular basis of high enzyme activity in goldfish brain, a human placental aromatase antibody was used to immunolocalize labeled cells. This antibody immunoprecipitated a single 56 kDa in vitro translation product of goldfish brain poly(A+)RNA and revealed discrete clusters of intensely stained neurons, processes, and terminals concentrated in, but not limited to, reproductive brain centers. Close proximity of aromatase- and androgen receptor-positive neurons in certain regions provides anatomic evidence of a functional relationship between direct and indirect pathways of neural androgen action. Aromatase-positive neurons and fibers formed interconnected networks in novel loci (e.g. retina-->optic tract-->optic tectum), and catalytic activity was confirmed biochemically in these tissues, indicating that neuroestrogen may have a role in visual input and integration. Availability of goldfish-specific nucleotide and antibody probes will facilitate further studies using this model.

Animals↗
PubMed1993

Immunocytochemical and biochemical evidence for aromatase in neurons of the retina, optic tectum and retinotectal pathways in goldfish.

Using an animal model in which neural aromatase is apparently overexpressed (the goldfish, Carassius auratus) and an anti-human placental antibody which specifically crossreacts with goldfish brain aromatase, aromatase-immunoreactive neuronal cell bodies and fibers have been localized within the retina. These include a subset of horizontal cells, bipolar cells, and amacrine cells of the inner nuclear layer, some fibers of the outer and inner synaptic layers and certain cells of the ganglion cell layer; photoreceptors were never labeled. Some ganglion cell projections to the brain via the optic nerve and optic tract were aromatase-positive, as were small neurons of the stratum periventriculare (SPV) and fibers of two other strata of the optic tectum. Aromatase activity, as measured by [3H]androgen by tissue homogenates and cell cultures, confirmed the presence of aromatase in retina and in brain regions containing the optic tectum. This localization of the rate-limiting enzyme in estrogen biosynthesis suggests that neuroestrogen derived from circulating androgen m ay modulate transmission and integration of visual information important for reproduction in this species.

Animals↗
PubMed1992

Autocrine and paracrine role of steroids during spermatogenesis: studies in Squalus acanthias and Necturus maculosus.

Due to a cystic mode of spermatogenesis and zonal testes, the spiny dogfish shark (Squalus acanthias) and salamander (Necturus maculosus) are advantageous for studying the relationship between steroidogenesis and germ cell development. Direct analysis of steroidogenic enzymes and steroid receptors in staged tissue samples, together with light and electron microscopic observation, suggests that androgens and estrogens produced by Sertoli cells or Leydig cells immediately before or after spermiation serve as intratesticular regulators of events specific to premeiotic stages of spermatogenesis. These correlative data are consistent with limited observations in mammalian testis and provide a conceptual framework for direct testing of functional interactions between spermatogenesis and steroidogenesis in future studies.

Animals↗
PubMed1991

Culture of intact Sertoli/germ cell units and isolated Sertoli cells from Squalus testis: I. Evidence of stage-related functions in vitro.

As part of an ongoing program of research using the testis of the dogfish shark (Squalus acanthias) to characterize morphologic and functional changes during spermatogenesis, we have developed procedures for culturing intact spermatocysts (germ cell/Sertoli cell clones) and isolated Sertoli cells from premeiotic, meiotic, and postmeiotic stages of development. Phase contrast and light microscopy confirmed the stage and cellular composition of spermatocysts and showed that they retained their closed, spherical configuration for at least 15 d in culture. Stage-related variations in [3H]thymidine incorporation (premeiotic much greater than meiotic = postmeiotic) were observed, a pattern that was the same quantitatively and qualitatively after one or seven days of culture. [3H]Leucine-labeled protein synthesis was twofold greater in cultures with premeiotic spermatocysts than in cultures with more mature stages, whether medium or cysts were analyzed. Sertoli cells isolated from spermatocysts of different stages differed in size, shape, cytological appearance, ability to form flattened monolayers, and rate of DNA synthesis. One day after seeding, [3H]thymidine labeling of Sertoli cells corresponded to the pattern obtained with intact spermatocysts (premeiotic much greater than meiotic = postmeiotic); however, 7 days in culture effected a 40- to 200-fold increase in this parameter and altered the stage-dependent pattern (premeiotic = meiotic greater than postmeiotic). Also, when [3H]leucine-labeled macromolecules secreted by Sertoli cells from premeiotic versus meiotic stages were analyzed by polyacrylamide gel electrophoresis (PAGE), banding patterns differed. Initial results demonstrate the feasibility and potential of this in vitro system for studying qualitative and quantitative changes during spermatogenesis.

Animals↗
PubMed1991

Production and secretion of an interleukin-1-like factor is stage-dependent and correlates with spermatogonial DNA synthesis in the rat seminiferous epithelium.

It has been shown previously that the intact adult rat testis produces large amounts of an interleukin-1 (IL-1)-like growth factor. The present study has investigated whether this testicular IL-1-like factor (tIL-1) is produced and secreted differentially by the fourteen stages of the seminiferous epithelial cycle in the rat testis. Seminiferous tubule segments representing defined stages were identified by transillumination-assisted microscopy and isolated by microdissection. Pooled segments were either homogenized and extracted with aqueous buffer or incubated for 24 h to produce conditioned media (CM). The recovered material was then analysed for IL-1 bioactivity in a sensitive murine thymocyte proliferation assay. When divided into four stage groups, extracts of stages II-VI, IX-XII and XIII-I showed equally high IL-1 activity whereas stage group VII-VIII showed much lower activity. More detailed analysis with 10 different stage groups showed that tIL-1 activity was undetectable in extracts of substages VIIab and VIIcd. The same pattern was seen when CM from cultured tubular segments were analysed. Labelling of seminiferous tubules with tritiated thymidine in vitro and analysis by autoradiography revealed that DNA-synthesizing spermatogonia were absent in substages VIIb and VIIc and sparse in substages VIIa and VIId. The results show that tIL-1 activity is produced in a stage-dependent manner and suggest that tIL-1 might be involved in the regulation of spermatogonial proliferation in vivo.

Animals↗
PubMed1990

Shark testis model: stage-dependent functions and the regulation of spermatogenesis.

In mammals, a single Sertoli cell nurtures 3-4 successive generations of germ cells. Thus, it is not possible to study this cell type at a single spermatogenic stage. In the dogfish shark Squalus acanthias, a single cohort of Sertoli cells remains associated with a germ cell clone throughout its development. Moreover, different germ cell stages are topographically segregated within the testis and can be easily staged by transilluminationmicroscopy. Recently, we have developed methods for the isolation and culture of spermatocysts (Sertoli/germ cell units) and Sertoli cells only from pre-meiotic, meiotic, and post-meiotic stages of germ cell development. Here, we present data that illustrate the feasibility of using the Squalus testis model for characterizing stage-related biochemical changes in Sertoli cells.

Animals↗
PubMed1990

Aromatization mediates aggressive behavior in quail.

Although testosterone (T) stimulates aggressive and reproductive behaviors in males of many vertebrate species, it is now known that the full expression of T action in the brain requires aromatization to estradiol (E2) and subsequent interaction of locally formed E2 with nuclear estrogen receptors. In experiments reported here, we used a behavioral test which quantifies the response of an individual male Japanese quail (Coturnix coturnix japonica) to the visual stimulus of a conspecific. We have called this behavior aggression because it shares many features in common with traditional measures of aggression, e.g., predicting dominance and subordinance. Nevertheless, the behavior probably also combines a complex steroid-sensitive masculine behavior. The advantage of this test is that it allows the discrimination of individual differences in masculine behavior but avoids fighting and sexual encounters per se, thereby reducing effects of learning, a problem with previous tests of avian aggression. In addition, this test has been applied usefully to identify neuroendocrine correlates to male behavior. Using this test, the arousal of reproductively inactive males (hereafter referred to as aggression) is activated by administration of T or estradiol benzoate (EB), but not by 5 alpha-dihydrotestosterone (DHT). T-induced aggression was blocked by the aromatase inhibitor 4-hydroxyandrostenedione (OHA), an effect partially reversed by treatment with EB. In addition, OHA or the estrogen receptor blocker CI-628 reduced aggressiveness of reproductively active males whereas the androgen receptor blocker flutamide had no effect. Results with the 5 alpha-reductase inhibitor N,N-diethyl-4-methyl-3-oxo-4-aza-5 alpha-androstane-17 alpha-carboxyamide (4-MA) were equivocal. Additionally, treatment of reproductively inactive quail with T or E2 but not DHT increased aromatase activity in the hypothalamus-preoptic area (HPOA). We conclude, therefore, that T to E2 conversion is essential for the activation of aggressiveness in this species. Although locally formed estrogen exerts its effects on aggression in part by increasing activity of aromatase per se, analysis of the time course of behavioral induction or suppression by the various treatments suggests that the response has multiple components, including both short latency, receptor-independent and long latency, receptor-dependent events.

5-alpha Reductase Inhibitors↗
PubMed1990

Aggressive behavior in birds: an experimental model for studies of brain-steroid interactions.

1. Although testosterone (T) stimulates aggressiveness in males of many vertebrate species, it is now known that the full expression of T actions in the central nervous system requires aromatization to estradiol (E2) and subsequent binding of formed E2 to its receptor. 2. We have recently confirmed these as rate-limiting steps in the control of sex-related and individual differences in aggressiveness of the Japanese quail (Coturnix coturnix japonica). 3. In this review, we describe some of the neuroendocrine factors which control aggression with a focus on our recent studies in quail.

Aggression↗
PubMed1989

A specific androgen-binding protein (ABP) in Necturus testis and its zonal distribution.

The urodele amphibian Necturus maculosus has a zoned testis, which is advantageous for separating Leydig cells from germinal elements and for studying stage-dependent biochemical changes. Using [3H]testosterone (T) in a standard binding assay and dextran-coated charcoal (DCC) or Sephadex LH-20 to separate free and bound steroids, we identified an androgen-binding protein (ABP) in Necturus testis cytosols. This protein was of high affinity (Kd = 10(-9) M) and was saturable (Bmax = 10(-9) M) and specific for androgen (T; 5 alpha-dihydrotestosterone, DHT) but could be distinguished from the androgen receptor of Necturus testis by its relative abundance (300-550 fmol/mg protein), short half-time of dissociation (3 min at 22 degrees C), inability to adhere to DNA-cellulose, and absence from nuclear extracts. Additionally, when analyzed on sucrose gradients, the ABP of Necturus testis sedimented at 6-7 S in both low or high ionic strength buffers. In that estradiol (E2) is a poor competitor for T-binding, this protein resembles a sex steroid-binding protein previously identified in urodele serum but differs from the ABP and testosterone-estradiol-binding globulin (TEBG) of rodents, humans, goldfish, and sharks. It is differentially distributed within the testis, with the highest levels in immature lobular regions composed of Sertoli cells and germ cells in premeiotic stages and lower levels in regions composed primarily of Leydig cells. The cellular source and function of this protein in Necturus testis remain to be determined.

Androgen-Binding Protein↗
PubMed1989

Estrogen receptors in quail brain: a functional relationship to aromatase and aggressiveness.

Estradiol (E2) mediates many of the activational effects of testosterone (T) on masculine reproductive and aggressive behaviors. Using Japanese quail (Coturnix coturnix japonica) as an animal model, together with a newly devised procedure for quantifying aggressiveness, we recently showed that aggression is E2-dependent and that individual differences in behavioral intensity are correlated with aromatase in the hypothalamus/preoptic area (HPOA). In this study we characterized estrogen receptors (ER) in quail brain and tested the hypothesis that aromatase in brain regulates T-induced behavioral responsiveness by regulating the quantity of E2 available for receptor binding. Based on standard binding assays and Sephadex LH-20 chromatography, quail brain ER was shown to be estrogen-specific, of high affinity (Kd = 0.88 nM), and of limited capacity with highest concentrations in limbic brain areas (Bmax 23-27 fmoles/gm HPOA). In addition, this ER adhered to DNA-cellulose under activating conditions. The quantitative relationship between aromatization, ER, and aggressiveness was tested in reproductively inactive (nonaggressive) males by treatment with T +/- the aromatase inhibitor 4-hydroxyandrostenedione (OHA). After 5 days, T markedly stimulated aggressiveness, and elevated aromatase and nuclear (occupied) ER in HPOA. Simultaneous treatment with OHA blocked effects on aggressiveness and aromatase, and lowered nuclear ER, but increased cytosolic (empty) ER. Total ER (nuclear plus cytosolic) was higher after T treatment whether or not OHA was administered, suggesting that androgen per se induces ER in quail HPOA.(ABSTRACT TRUNCATED AT 250 WORDS)

Aggression↗
PubMed1989

Localization of aromatase in synaptosomal and microsomal subfractions of quail (Coturnix coturnix japonica) brain.

The subcellular distribution patterns of aromatase, 5 alpha- and 5 beta-reductase in the hypothalamus/preoptic area of Japanese quail were studied using standard methods of centrifugation, and fractional constituents were identified by marker enzymes. Aromatase was concentrated 8-fold in the 100,000 g pellet (P3) along with a 3-fold enrichment in the microsomal marker NADPH-cytochrome c reductase (NCR) a result consistent with glandular tissues. In addition, aromatase was enriched 2-fold in the 11,000 g pellet (P2) and, owing to its large size, this fraction accounted for more total activity than P3. Although P2 contained the mitochondrial marker succinate dehydrogenase (SDH), treatment with Triton X-100 to solubilize membranes and release occluded enzymes increased measured NCR and the cytosol marker lactate dehydrogenase (LDH) 2- and 4-fold, respectively--evidence that this fraction was composed of mitochondria plus synaptosomes (pinched-off nerve terminals). To further explore the location of aromatase in the 11,000 g fraction, P2 was exposed to hypotonic buffer, a treatment known to cause lysis of synaptosomes, and then separated into three fractions P2a (11,000 g pellet), P2b (100,000 g pellet) and P2s (100,000 g supernatant). Aromatase colocalized with the microsomal marker NCR (13- and 4-fold increase, respectively) in the 100,000 g (P2b) pellet which was, however, devoid of mitochondrial enzyme activity. We infer from this that a significant portion of aromatase in brain is associated with smooth membranes present inside synaptosomes. 5 beta-Reductase in quail brain subfractions was enriched 6-fold in the 100,000 g supernatant together with a 4-fold enrichment in the cytosolic marker LDH.(ABSTRACT TRUNCATED AT 250 WORDS)

3-Oxo-5-alpha-Steroid 4-Dehydrogenase↗
PubMed1989

Aromatase activity in quail brain: correlation with aggressiveness.

Testosterone (T) triggers aggressive behavior in males of many vertebrate species; however, the neural and hormonal basis of individual differences in the frequency or intensity of aggressive behavior is still debated. Using the Japanese quail (Coturnix coturnix japonica), a species in which individuals exhibit a wide range of aggressiveness in nature and the laboratory, together with a newly devised test procedure for quantifying aggressiveness, we recently demonstrated that aggression is estrogen dependent. Here we extend these studies by testing the hypothesis that aromatization in brain is a rate-limiting step in the expression of individual differences in aggressiveness. Using procedures previously validated for this species, aromatase and 5 alpha- and 5 beta-reductase activities were estimated in selected brain regions of reproductively active male quail by measuring conversion of [3H]androstenedione to [3H]estrone, [3H]5 alpha-androstanedione, and [3H]5 beta-androstanedione, respectively. In Exp 1, behaviorally inexperienced test birds were killed 90 sec after a single behavioral test. Aggressiveness of individuals in this group, as determined by pecking and locomotor activity in response to visualization of a conspecific, ranged 3- to 4-fold from high to low. Aromatase activity in the posterior hypothalamus (PHYP) was significantly higher in males rated high for aggressiveness than in animals rated low (1.04 vs. 0.59 pmol/h.mg protein; P less than 0.02). Similar differences were observed in the anterior hypothalamus/preoptic area (AHPOA) but were not significant. In Exp 2, sexually mature males were behaviorally tested eight times over 22 days and killed 24 h after the final test. Aggressiveness varied 5-fold from high to low, although the rating in a given bird remained constant with time and repeat testing. Aromatase activity in the AHPOA was significantly greater in birds rated high for aggressiveness than in low aggressiveness birds (3.77 vs. 2.80 pmol/h.mg protein; P less than 0.02). In addition, when AHPOA aromatase in all birds was plotted against behavioral intensity, there was a 2-fold variation and a significant positive correlation (r = 0.556; P less than 0.02). Similar differences were observed in PHYP, but these were of borderline significance. By contrast, aromatase levels outside the AHPOA and PHYP were unrelated to behavior. Moreover, in both Exp 1 and 2, 5 alpha- and 5 beta-reductase activities in AHPOA, PHYP, and other brain regions; plasma T, 5 alpha-dihydrotestosterone, and total estrogens; and relative testicular weights were not consistently related to aggression.(ABSTRACT TRUNCATED AT 400 WORDS)

3-Oxo-5-alpha-Steroid 4-Dehydrogenase↗
PubMed1989

Aromatase, 5 alpha- and 5 beta-reductase in brain, pituitary and skin of the sex-role reversed Wilson's phalarope.

While intrasexual competition for mates is generally considered to be an androgen-dependent characteristic of reproductively active males, in the Wilson's phalarope (Phalaropus tricolor) it is the female that acquires the brighter nuptial plumage and aggressively competes for access to the less aggressive males. Despite this pronounced sex-role reversal, circulating sex steroid hormones of breeding phalaropes are similar to those of avian species displaying traditional male-female reproductive roles. To investigate whether these behavioural and morphological steroid-dependent differences may be due to differences in target organ metabolism of circulating androgen, [3H]androstenedione in the presence of an NADPH-generating system was incubated with homogenates of brain, pituitary and skin of male and female Wilson's phalaropes collected from a naturally breeding population. Oestrone, 5 alpha-androstanedione and 5 beta-androstanedione were measured as endpoints of aromatization, 5 alpha-reduction and 5 beta-reduction respectively. Aromatase activity in the anterior hypothalamus/preoptic area (AHPOA) and posterior hypothalamus was greater in breeding males with high circulating concentrations of testosterone than in females, and activity in the AHPOA was greater in breeding than in non-breeding males (with low circulating testosterone). Aromatase levels did not differ in septum, archistriatum, hyperstriatum or pituitary. 5 alpha- and 5 beta-reductase were detected in all neuroendocrine tissues sampled and although there were no significant male-female differences, 5 alpha-reductase was greater in the AHPOA of breeding than of nonbreeding males.(ABSTRACT TRUNCATED AT 250 WORDS)

3-Oxo-5-alpha-Steroid 4-Dehydrogenase↗
PubMed1988

Aromatase is concentrated in the proximal pars distalis of tilapia pituitary.

Aromatase has been identified in the telostean, avian, and mammalian pituitaries, although its cellular location(s) is not yet certain. To address this question, experiments were performed in tilapia (Oreochromis mossambicus), a species which has been well characterized with respect to the intraglandular distribution of the different pituitary cell types. To estimate aromatase, glands were microdissected into rostral pars distalis (RPD), proximal pars distalis (PPD), and neurointermediate lobe (NIL) and organs were cultured in the presence of [3H]androstenedione for 16-24 hr. [3H]Estrogen products were isolated and quantified after ether extraction, hydrolysis with glucuronidase-sulfatase, thin-layer chromatography, and phenolic partition. Authentic estrone or estradiol-17 beta were produced by all pituitary regions and also by the urophyseal region of the spinal cord. Aromatase was two to five times higher in PPD than in RPD or NIL and similar to activity in adjacent hypothalamus-preoptic area (HPOA). Much lower estrogen yields were obtained in cultures of cerebellum, urophysis, and other cord regions. Since the PPD contains most of the somatotropes, these data are consistent with earlier studies implicating GH3/GH4 cell strains as an enriched enzyme source, although its presence in other cell types cannot be ruled out. The unusually high and localized aromatase in tilapia pituitary renders this species a useful model for studying the targets and functional importance of estrogen as a parahormone in the pituitary.

Androstenedione↗
PubMed1988

In vivo steroid regulation of aromatase and 5 alpha-reductase in goldfish brain and pituitary.

The full expression of testosterone (T) actions in neuroendocrine tissues requires aromatization and 5 alpha-reduction to estradiol (E2) and 5 alpha-dihydrotestosterone (DHT), respectively. Recently, we documented striking changes in aromatase and 5 alpha-reductase during the annual reproductive cycle of goldfish (Carassius auratus). To investigate possible regulatory effects of sex steroids, goldfish were implanted with hormone-filled silastic capsules for 2-5 weeks. Conversion of [3H]androstenedione to estrone or 5 alpha-androstanedione by homogenates of anterior hypothalamus/preoptic area, remaining telencephalon, and whole pituitary (PIT) was used to estimate aromatase and 5 alpha-reductase, respectively. Gonadosomatic index and plasma E2, T, and DHT were monitored as an index of reproductive status and capsule effectiveness. In reproductively inactive fish in which plasma steroids and aromatase were basal (October), E2 or T increased aromatase activity in brain of both sexes but stimulated activity in PIT of females only; DHT was not effective. In a subsequent experiment initiated close to the spawning peak and prior to the seasonal decline in plasma steroids and brain aromatase (April), T increased or maintained brain aromatase in a time-dependent manner. 5 alpha-Reductase activity was unaffected by steroid treatment in both reproductively active and inactive fish. These results indicate that variations in circulating steroids are responsible, at least in part, for changes in brain aromatase during the annual reproductive cycle of goldfish and provide the first evidence for steroid control of pituitary aromatase. The steroid specificity of the induction suggests that an estrogen receptor mechanism is involved.(ABSTRACT TRUNCATED AT 250 WORDS)

3-Oxo-5-alpha-Steroid 4-Dehydrogenase↗
PubMed1988

Changes in brain aromatase and 5 alpha-reductase activities correlate significantly with seasonal reproductive cycles in goldfish (Carassius auratus).

Aromatization and 5 alpha-reduction are known to be required for the full expression of testosterone actions in neuroendocrine tissues. Although aromatase and 5 alpha-reductase activities in brain and pituitary can be experimentally manipulated by castration and steroid replacement, naturally occurring variations during seasonal reproductive cycles have not been examined in any species. Goldfish (Carassius auratus) were selected for study because they exhibit exceptionally high levels of aromatase in both brain and pituitary, although 5 alpha-reductase levels resemble the vertebrate norm. Four animals of each sex were tested monthly through three breeding seasons (2.5 yr). Using previously validated techniques, the enzymes were assayed by product formation from [3H]androstenedione in homogenates of anterior hypothalamus-preoptic area (AHPOA), remaining telencephalon (TEL), whole pituitary, ovary, and testis. Seasonal variations in aromatase were most dramatic in the AHPOA of female fish, exhibiting a peak in April and May that was 6-fold higher than the nadir in July. As judged by changes in the appearance and weight of the gonads, maximal aromatase coincided with the spawning season, whereas low enzyme levels corresponded to reproductive inactivity. Seasonal variations were similar but of a lesser magnitude in the TEL of females and in the AHPOA and TEL of males (2- to 3-fold, peak to nadir). Both ovarian and testicular aromatase showed cyclic changes; however, activity was much lower than that in brain at all times of the year (4.5, 1.2, and 47.0 pmol/mg protein, maximal values in ovaries, testes, and AHPOA, respectively). Pituitary aromatase varied from 5-22 pmol/mg protein, but was not consistently correlated with season. Cyclic changes in 5 alpha-reductase were distinctly different from those in aromatase, with maximal values in both brain and pituitary occurring when fish were reproductively inactive. In general, circulating sex steroids were high when aromatase was high and low when reductase was maximal; however, there was no apparent causal relationship suggested by temporal changes in a given steroid. Variations in testosterone metabolism, by regulating the quantity and quality of active hormone in close proximity to receptor sites, may be responsible for the changes in feedback sensitivity and behavioral responsiveness that are known to occur in seasonal breeders.

3-Oxo-5-alpha-Steroid 4-Dehydrogenase↗