Elevations in gonadotrophin concentrations and milt volumes as a result of spawning behavior in the goldfish

High levels of circulating prostaglandin F2α associated with ovulation stimulate female sexual receptivity and spawning behavior in the goldfish (Carassius auratus)

This study tested the hypothesis that blood-borne prostaglandin F_2α (PGF_2α) produced at the time of ovulation by female goldfish, a typical scramble-spawning, egg-laying cyprinid fish, functions as a hormone which stimulates female sexual receptivity, behavior, and pheromone release, thereby synchronizing female mating behavior with egg availability. We conducted 5 experiments. First, we tested whether PGF_2α is found in the blood of female fish and if it increases at the time of ovulation. Using gas chromatography-mass spectrometry, we found that circulating PGF_2α was approximately 1 ng/ml prior to ovulation, increased over 50-fold within 3 h of ovulation and returned to preovulatory values after spawning and egg release. Ovulated fish also released over 2 ng/h of PGF_2α and 800 ng/h of 15-keto-PGF_2α, a metabolite of PGF_2α - both compounds with known pheromonal function. Second, we tested how closely levels of circulating PGF_2α tracked the timing of ovulation by sampling fish at the time of ovulation and discovered that PGF_2α increased within 15 min of ovulation, peaked after 9 h, and fell to basal levels as fish spawned and released their eggs. Third, we tested whether an interaction between eggs and the reproductive tract serves as a source of circulating PGF_2α and its relationship with female sexual receptivity by injecting ovulated eggs (or an egg-substitute) into the reproductive tract of females stripped of ovulated eggs. We found both of these treatments elicited measurable increases in plasma PGF_2α as well as female sexual behavior. A fourth experiment showed that indothemacin, a PG synthase inhibitor, blocked both PGF_2α increase and female sexual behavior in egg-substitute-injected fish. Finally, we tested the relationship between the expression of female behavior and PGF_2α in PGF_2α-injected fish and found that circulating PGF_2α levels closely paralleled behavior, rising within 15 min and peaking at 45 min. Together, these experiments establish that PGF_2α functions as a behavioral blood-borne hormone in the goldfish, suggesting it likely has similar activity in other related, externally-fertilizing fishes.

Rapid sex steroid effects on reproductive responses in male goldfish: Sensory and motor mechanisms

Contribution to Special Issue on Fast effects of steroids. Although we have learned a great deal about the molecular mechanisms through which sex steroids rapidly affect cellular physiology, we still know little about the links between those mechanisms and behavioral output, nor about their functional consequences in natural contexts. In this review, we first briefly discuss the contexts associated with rapid effects of sex steroids on reproductive behaviors and their likely functional outcomes, as well the sensory, motor, and motivational mechanisms associated with those effects. We then discuss our recent studies on the rapid effects of testosterone in goldfish. Those studies indicate that testosterone, through its aromatization and the subsequent activation of estrogen receptors, rapidly stimulates physiological processes related to the release of milt/sperm through likely influences on motor pathways, as well as behavioral responses to female visual stimuli that may reflect, in part, influences on early stages of sensory processing. Such motor and sensory mechanism are likely important for sperm competition and mate detection / tracking, respectively, in competitive mating contexts. We also present preliminary data on rapid effects of testosterone on responses to pheromones that may not involve estrogen receptors, suggesting a dissociation in the receptor mechanisms that mediate behavioral responses in different sensory modalities. Lastly, we briefly discuss the implications of our work on unresolved questions about rapid sex steroid neuromodulation in fish.

Rapid modulation of gene expression profiles in the telencephalon of male goldfish following exposure to waterborne sex pheromones

Sex pheromones rapidly affect endocrine physiology and behaviour, but little is known about their effects on gene expression in the neural tissues that mediate olfactory processing. In this study, we exposed male goldfish for 6h to waterborne 17,20βP (4.3 nM) and PGF2α (3 nM), the main pre-ovulatory and post-ovulatory pheromones, respectively. Both treatments elevated milt volume (P=0.001). Microarray analysis of male telencephalon following PGF2α treatment identified 71 unique transcripts that were differentially expressed (q<5%; 67 up, 4 down). Functional annotation of these regulated genes indicates that PGF2α pheromone exposure affects diverse biological processes including nervous system functions, energy metabolism, cholesterol/lipoprotein transport, translational regulation, transcription and chromatin remodelling, protein processing, cytoskeletal organization, and signalling. By using real-time RT-PCR, we further validated three candidate genes, ependymin-II, calmodulin-A and aldolase C, which exhibited 3-5-fold increase in expression following PGF2α exposure. Expression levels of some other genes that are thought to be important for reproduction were also determined using real-time RT-PCR. Expression of sGnRH was increased by PGF2α, but not 17,20βP, whereas cGnRH expression was increased by 17,20βP but not PGF2α. In contrast, both pheromones increase the expression of glutamate (GluR2a, NR2A) and γ-aminobutyric acid (GABAA γ2) receptor subunit mRNAs. Milt release and rapid modulation of neuronal transcription are part of the response of males to female sex pheromones.

Testosterone rapidly increases ejaculate volume and sperm density in competitively breeding goldfish through an estrogenic membrane receptor mechanism

The social environment can have dramatic influences on reproductive behavior and physiology in many vertebrate species. In males, interactions with conspecifics affect physiological processes that increase an individual's ability to compete for mates. For example, in some species, males rapidly adjust the number of sperm they ejaculate in response to sociosexual cues from male and female conspecifics, however, little is known about the physiological mechanisms mediating this behavior. In goldfish, as in many vertebrates, social cues also drive transient surges of the gonadal hormone testosterone (T), which induces rapid effects on cellular processes via its conversion to estradiol (E2). We asked whether such surges rapidly influence ejaculate quantity and quality by experimentally manipulating peripheral levels of T and E2. We show that male goldfish injected with T increased ejaculate (milt) volume and sperm density within just 1 hr. Furthermore, increases in expressible milt were dependent on the conversion of T to E2 by the enzyme aromatase, required activation of estrogen receptors α and β, and were also elicited by BSA-conjugated E2, which acts on cell membrane-bound estrogen receptors. Together, these findings represent a novel steroid mechanism for the social modulation of sperm output over the short time scales that characterize reproductive encounters, and thus demonstrate a previously undescribed functional consequence of rapid estrogen signaling mechanisms. We suggest that such mechanisms may play a critical role in the enhancement of physiological and behavioral processes that increase reproductive success in competitive mating contexts.

Studies in zebrafish reveal unusual cellular expression patterns of gonadotropin receptor messenger ribonucleic acids in the testis and unexpected functional differentiation of the gonadotropins

This study aimed to improve, using the zebrafish model, our understanding of the distinct roles of pituitary gonadotropins FSH and LH in regulating testis functions in teleost fish. We report, for the first time in a vertebrate species, that zebrafish Leydig cells as well as Sertoli cells express the mRNAs for both gonadotropin receptors (fshr and lhcgr). Although Leydig cell fshr expression has been reported in other piscine species and may be a common feature of teleost fish, Sertoli cell lhcgr expression has not been reported previously and might be related to the undifferentiated gonochoristic mode of gonadal sex differentiation in zebrafish. Both recombinant zebrafish (rzf) gonadotropins (i.e. rzfLH and rzfFSH) stimulated androgen release in vitro and in vivo, with rzfFSH being significantly more potent than rzfLH. Forskolin-induced adenylate cyclase activation mimicked, whereas the protein kinase A inhibitor H-89 significantly reduced, the gonadotropin-stimulated androgen release. Therefore, we conclude that both FSH receptor and LH/choriogonadotropin receptor signaling are predominantly mediated through the cAMP/protein kinase A pathway to promote steroid production. Despite this similarity, other downstream mechanisms seem to differ. For example, rzfFSH up-regulated the testicular mRNA levels of a number of steroidogenesis-related genes both in vitro and in vivo, whereas rzfLH or human chorionic gonadotropin did not. Although not fully understood at present, these differences could explain the capacity of FSH to support both steroidogenesis and spermatogenesis on a long-term basis, whereas LH-stimulated steroidogenesis might be a more acute process, possibly restricted to periods during which peak steroid levels are required.

Treated wastewater effluent reduces sperm motility along an osmolality gradient

Many toxic effects of treated wastewater effluent on organismal and reproductive health have been documented. However, the physicochemical environment of treated wastewater effluent frequently differs considerably from that of its receiving waters and may affect organismal function independently of toxic effects. Teleost sperm, for example, may be affected by the higher osmolality of treated wastewater, as this sperm is activated for a brief period of time following ejaculation due to the sudden decrease in osmolality of its surrounding environment. In this study, we examined the effects of treated wastewater effluent on sperm motility to test the hypothesis that the higher osmolality of effluent compared to river water will adversely affect sperm activation in a concentration-dependent relationship. Treated wastewater effluent was collected on 5 days from the outflow of the Metropolitan Wastewater Treatment Plant, St. Paul, Minnesota, and from an upstream site on the Mississippi River. Milt aliquots collected from goldfish were diluted in an isotonic extender solution and subsequently activated in either deionized water, 100%, 50%, or 10% effluent, a synthetic ion mixture, or river water. Sperm motility and velocity were assessed at 15-s intervals for 1 min using a computer assisted sperm analyzer. Significant differences in performance parameters were found only at 15 s, with sperm motility and velocity declining rapidly at later sampling times. Predictably, deionized water resulted in the greatest activation of sperm motility, while motility exhibited a concentration-dependent decline in 10%, 50%, and 100% treated wastewater effluent. Interestingly, Mississippi River water and a synthetic ion mixture with an osmolality comparable to 50% effluent both resulted in the least amount of sperm activation. However, sperm activation in river water varied between collection days during the study. River water and 100% effluent both had low sperm activation characteristics despite a 10-fold difference in osmolality between these two treatments (1 and 10 mOsmol kg(-1), respectively). Results of this study indicate a concentration-dependent decrease in sperm motility in treated wastewater effluent as well as significant fluctuations of sperm activation in Mississippi River water. This study illustrates the complexity of assessing the effects of treated wastewater effluents and the difficulty of determining appropriate reference sites for such studies.

From social behavior to neural circuitry: steroid hormones rapidly modulate advertisement calling via a vocal pattern generator

Across vertebrates, androgens are rapidly elevated within minutes in response to aggressive or reproductive stimuli, yet it is unclear what the causal relationship is between fast androgen elevation and the ongoing (minute-by-minute) expression of behavior. This study tested the hypothesis that rapid increases in plasma steroid levels induce similarly rapid increases in both vocal behavior and the neurophysiological output of a central pattern generator that governs vocal behavior. In Gulf toadfish (Opsanus beta), males call to attract females to their nesting sites, and both males and females vocalize in aggressive interactions. Previous field experiments with males showed that simulated territorial challenges produce rapid and concurrent elevations in ongoing calling behavior and circulating levels of the teleost-specific androgen 11-ketotestosterone (11kT), but not the glucocorticoid cortisol. The current field experiments showed that non-invasive (food) delivery of 11kT, but not cortisol, induced an elevation within 10 min in the ongoing calling behavior of males. Electrophysiological experiments revealed that intramuscular injections of either 11kT or cortisol, but neither testosterone nor 17-beta-estradiol, induced increases within 5 min in the output of the vocal pattern generator in males, whereas only cortisol had similarly fast effects in females. The field behavioral results support predictions generated by the challenge hypothesis and also parallel the 11kT-dependent modulation of the vocal pattern generator in males. The cortisol effect on the vocal pattern generator in both sexes predicts that glucocorticoids regulate vocalizations in non-advertisement contexts. Together, these experiments provide strong support for the hypothesis that surges in circulating steroid levels play a causal role in shaping rapid changes in social behavior (vocalizations) through non-genomic-like actions on neural (vocal motor) circuits that directly encode behavioral patterning.

Rapid elevations in both steroid hormones and vocal signaling during playback challenge: a field experiment in Gulf toadfish

It is well known that plasma androgens are rapidly released in response to aggressive or sexual stimuli in a broad array of vertebrates. However, experimental work on behavioral functions of rapid androgen elevation is rare. A combination of field-based behavioral experiments and lab-based neuroendocrinological approaches is beginning to show how steroid hormones rapidly regulate the expression of vocal communication signals in Gulf toadfish (Opsanus beta). Male toadfish emit multiharmonic "boatwhistles" and shorter-duration, broadband "grunts" during intraspecific communication. Neurophysiology experiments demonstrate that androgens and glucocorticoids rapidly modify vocal motor patterning in male toadfish. In this study, we simulated territorial intrusions (vocal "challenges") with acoustic playbacks to toadfish in the field, and observed simultaneous, rapid (within 5-20 min) changes in vocalizations and steroid hormones. Both plasma androgens and vocal activity increased following the presentation of pure tones that mimic the duration of natural boatwhistles (275 ms), while they remained unchanged following playbacks of tone stimuli that mimic the duration of grunts (75 ms) or the upper-range of boatwhistles (475 ms). Circulating glucocorticoids were elevated in calling vs. non-calling males but were unaffected by playback stimuli, suggesting a role in the energetics of vocalization. These results strongly suggest that one function of rapid androgen elevation in response to social challenge is to mediate similarly rapid changes in territorial vocal signaling. Given the conserved organization of neuroendocrine and vocal motor systems, rapid steroid action on vocalization mechanisms may be true of other vocal vertebrates as well, including birds and mammals.

Milt production in goldfish: regulation by multiple social stimuli

Previous studies in goldfish (Carassius auratus) demonstrate that milt (sperm and seminal fluid) volume is increased both by a preovulatory steroidal pheromone, 17 alpha,20 beta-dihydroxy-4-pregnen-3-one (17,20 beta-P), that acts by increasing serum concentrations of gonadotropin II (GTH-II), and by a post-ovulatory prostaglandin (PG) pheromone, that acts by stimulating sexual interactions. Here, we show that male goldfish also increase milt volume when isolated for 24 h, or when placed with another male injected with human chorionic gonadotropin (hCG) for 12 h. In contrast to the milt increase induced by pheromonal 17,20 beta-P, the milt increase following isolation or exposure to hCG-injected males was not associated with increased serum GTH-II. Serum growth hormone also was unaffected, although serum testosterone increased in isolated males. The absence of GTH-II increase following isolation or exposure to hCG-injected males and the long latency time for these two types of responses, suggests these effects are mediated by a novel mechanism as yet undescribed. The present findings suggest that sperm competition has selected for complex mechanisms regulating milt production and fertility in goldfish: males regulate milt production not only in response to stimulatory pheromonal cues from ovulatory females, but also in response to unknown stimulatory and inhibitory cues from male competitors.

Plasma gonadotropin II, sex steroids, and thyroid hormones in wild striped bass (Morone saxatilis) during spermiation and final oocyte maturation

The blood levels of gonadotropin II (GtH II), sex-steroid hormones, and thyroid hormones were determined in wild spermiating male striped bass (Morone saxatilis) in males and in females at various stages of final oocyte maturation (FOM), captured on their spawning grounds. The progression of spermiation was associated with increases in plasma GtH II and decreases in plasma testosterone (T), 11-ketotestosterone, and thyroxine (T4). Plasma triiodothyronine (T3) remained at high and relatively unchanged levels. Plasma levels of 17,20beta-dihydroxy-4-pregnen-3-one (17,20beta-P) and 17,20beta, 21-trihydroxy-4-pregnen-3-one (17,20beta,21-P), the proposed maturation-inducing steroids (MIS) in striped bass, were low and unchanged during the same period. It was concluded that low progestogen levels are adequate to induce spermiation in striped bass, and that higher levels may be associated with spawning behavior. In the females, based on the profiles of the studied hormones, FOM was separated into two phases. Early FOM, which included germinal vesicle (GV) migration and lipid-droplet coalescence, was associated with elevations in plasma GtH II, T, and estradiol 17beta. Late FOM, which included GV breakdown and yolk-globule coalescence, was associated with a further surge in plasma GtH II, increases in the levels of the two MIS, mainly 17, 20beta-P, and a drop in T4. Plasma T3 levels did not change during FOM. Examination of conjugated steroids demonstrated, in the males, a reduction in conjugated androgens at the peak of the spawning season and, in the females, a small increase in conjugated 17, 20beta-dihydroxylated and 5beta-reduced,3alpha-hydroxylated steroids after spawning. This is the most comprehensive report, to date, on the endocrine regulation of gonadal maturation in wild striped bass, demonstrating that a two-stage process of FOM is regulated by different endocrine signals, providing further evidence for the involvement of 17,20beta-P as a MIS in the females, and indicating that both males and females are in an euthyroid state during the spawning season.

Amplexus induces surge of luteinizing hormone in male toads, Bufo japonicus

At mating time in the wild, male toads (Bufo japonicus) exhibit a surge in circulating luteinizing hormone (LH) that is thought to result in spermiation. Experiments were performed to identify proximate control of this gonadotropin surge. Male toads migrating to a breeding pond were captured and put in plastic containers either with equal numbers of females or without females. All the males placed with the females clasped them in amplexus that lasted for about 12 hr. During this period, plasma LH and follicle-stimulating hormone levels increased progressively to about 25 and 2 times the initial levels, respectively, and then returned to the initial levels. No significant increases in plasma gonadotropin levels were observed in the males kept without females. These results suggest that the amplexus directly induced the LH surge, but do not preclude control by pheromonal or visual stimuli. To isolate effects of these factors, solitary male toads were kept with dummies of the female. The dummy was a block of "konnyaku," Japanese food whose component is mannan extracted from the root of a plant. Konnyaku is highly elastic, white in color, and virtually odorless. Thus, these dummies felt, but did not look or smell, realistic. All the males clasped their dummy and formed amplexus for 12 hr or more; an LH surge indistinguishable from that with the females was observed. It is concluded that the amplexus itself is the stimulus that induces the LH surge, and subsequent spermiation, in male toads. A strongly male-biased sex ratio, combined with a need to time spermiation precisely with actual mating, may have resulted in the evolution of this neuroendocrine reflex in male Bufo.

Changes in plasma levels of gonadotropins and sex steroids in the toad, Bufo japonicus, in association with behavior during the breeding season

Plasma levels of gonadotropins and sex steroids in Bufo japonicus were surveyed at different behavioral stages during the breeding period in March. All hormone levels were lower in torpid toads, which were found underground 1 week before the start of the breeding migration, than in active toads in the breeding season, although the levels were higher than those in the other months. LH and FSH levels in toads migrating toward a breeding pond were still low or only slightly elevated. Gonadotropin levels increased markedly when toads arrived at the pond and were highest for individuals actually in the pond. These LH and FSH surges occurred in both sexes, although peak levels were higher in females. Gonadotropin levels decreased in individuals of both sexes when they left the pond after breeding ("landed" individuals). LH and FSH levels in solitary males were significantly lower than those in amplexing males. In females, there was no significant difference in the level of any hormone between amplexing and solitary individuals. Circulating testosterone increased to the highest level in migrating males; this elevated level was maintained after arrival in the pond. DHT in males showed a pattern similar to that of testosterone. Both androgens decreased to the lowest level in landed males. In females, estradiol was dominant during migration, while progesterone became dominant shortly after arriving near or in the pond. This progesterone surge coincided with the LH surge. Androgen may stimulate amplexus behavior in male toads, and then amplexus seems to induce the LH surge that is necessary for spermiation. In females, however, amplexus seems to have no direct correlation with the gonadotropin surge, and the LH surge may induce the progesterone surge which causes ovulation.

Changes in plasma levels of gonadal steroids during spawning behaviour in territorial male demoiselles Chromis dispilus (Pisces: Pomacentridae) sampled underwater

Males of the demoiselle Chromis dispilus (a species endemic to New Zealand) were hand-netted from nest territories by scuba divers and blood sampled in situ. Reproductive condition and behaviour prior to capture were compared with plasma levels of testosterone (T) and 17 alpha,20 beta dihydroxy-4-pregnen-3-one (17,20 beta P). Spawning cycles of about 7 days, consisting of 1-2 days of spawning activity followed by 4-5 days of brooding behaviour, were observed. Plasma levels of T and 17,20 beta P were elevated (4-6 and 1-1.5 ng.ml-1, respectively) in fish that were engaged in courtship display, spawning, or guarding freshly spawned eggs. Levels of both hormones fell to less than 1 (T) and less than 0.4 ng.ml-1 (17,20 beta P) at the end of spawning episodes and remained low through the subsequent brooding period. T levels increased slightly toward the end of the brooding phase. Samples taken at 2- to 3-hr intervals during daylight hours showed similar correlation of steroid levels and activity; however, there was a marked fall in 17,20 beta P in spawning fish with the onset of shelter seeking at dusk. Values returned to high levels with the resumption of spawning the next day. Plasma steroid levels were not consistently correlated with reproductive condition (degree of spermiation). It is concluded that there is a strong correlation between plasma T and 17,20 beta P levels and the male spawning cycle, but not brooding or territorial behaviour per se.

Alterations in gonadotropin-releasing hormone immunoactivities in discrete brain areas of male goldfish during spawning behavior

In male goldfish, spawning behavior and gonadotropin (GtH) secretion are stimulated during exposure to females which were induced to perform spawning behavior by treatment with prostaglandin F2 alpha (PG). In this study, exposure to PG-treated females for 1 or 2 h significantly increased serum GtH levels, and GtH-releasing hormone (GnRH) concentrations in the olfactory bulbs, telencephalon and hypothalamus, indicating that spawning behavior can influence cellular events of the brain GnRH neuronal system and pituitary GtH secretion in male goldfish. To study the role of the olfactory system in the hormonal responses to PG-treated females, bilateral sectioning of olfactory tracts (OTX), medial (mOTX) or lateral (lOTX) olfactory tract were carried out in male goldfish. Both OTX and mOTX, but not lOTX, blocked the behavioral response of male goldfish to PG-treated females and abolished the increases in serum GtH and brain GnRH levels, suggesting that alterations in pituitary GtH secretion and brain GnRH levels are associated with a pheromonal activation of spawning behavior through the medial olfactory tracts in male goldfish. This behavioral activation of the GnRH neuronal system provides a useful physiological model for studying the regulation of GnRH system in male goldfish.

Differing behavioral and endocrinological effects of two female sex pheromones on male goldfish

Ovulatory female goldfish sequentially release at least two sex pheromones: 17 alpha, 20 beta-dihydroxy-4-pregnen-3-one (17,20 beta P) and a mixture of F prostaglandins (PGFs). This study sought to determine whether these pheromones have different endocrinological and behavioral actions and whether the PGF pheromone, which is released by spawning females, is responsible for increasing the gonadotropin (GtH) and milt (sperm and seminal fluid) levels of spawning males. Grouped and isolated males were exposed to combinations of these pheromones, food odor, and spawning and nonspawning females. 17,20 beta P stimulated GtH increases in both grouped and isolated males but had only minor effects on behavior; because its principal function appears to be physiological it may be considered a "primer" pheromone. In contrast, exposue to the PGFs elicited large increases in sexual behavior but increased GtH only when fish were exposed as groups; this pheromone's principal action appears to be behavioral and it should be considered a "releaser" pheromone. Although males had increased GtH and milt levels after 1 hr of spawning, males allowed to interact with nonspawning females also had elevated GtH; thus, behavioral interactions appear capable of elevating GtH in the absence of either pheromone. The existence of an independent behavioral mechanism which stimulates GtH was supported by the fact that males exposed to 17,20 beta P while spawning had GtH levels much greater than males exposed to only one of these stimuli.

Serum 11-ketotestosterone and testosterone concentrations associated with reproduction in male bluegill (Lepomis macrochirus: Centrarchidae)

Male bluegill (Lepomis macrochirus) display a complex reproductive behavior involving two alternative life history pathways: delay of sexual maturation to become "parentals" or precocious maturation as "cuckolders." The purpose of our study was to investigate the association of two androgens, 11-ketotestosterone (11KT) and testosterone (T), with reproduction in these two types of males. Radioimmunoassay techniques were used to measure daily levels of the two androgens in the blood serum of parental male bluegill captured during the prespawning, spawning, and nesting periods throughout the reproductive season. Dramatic changes in the levels of 11KT and T were observed among parental males during these periods. Peaks occurred at the onset of spawning activity during each breeding bout. Compared to spawning parental males, spawning cuckolder males had significantly lower serum levels of 11KT. In contrast, the serum levels of T among parental and cuckolder males were not significantly different. These findings suggest that the elevated levels of 11KT are associated with the behaviors displayed by spawning parental males. The levels of T, however, seem to be associated with the occurrence of a phenomenon common to both parental and cuckolder males, such as development of gonads and/or spermiation.

Direct evidence that 17 alpha,20 beta-dihydroxy-4-pregnen-3-one functions as a goldfish primer pheromone: preovulatory release is closely associated with male endocrine responses

This study directly tested the hypothesis that 17 alpha,20 beta-dihydroxy-4-pregnen-3-one (17,20 beta-P) is a goldfish preovulatory pheromone (pheromone released at peak levels during oocyte final maturation) which increases blood gonadotropin (GtH) and milt volume in males. During spontaneous ovulation, GtH and 17,20 beta-P in female blood and 17,20 beta-P released to the water increased dramatically 7-10 hr prior to ovulation, peaked 1-4 hr prior to ovulation, and then rapidly declined. Males held with these females, or exposed to their odors, had increased GtH levels and milt volumes at approximately the time when increased 17,20 beta-P release by ovulatory females commenced. Although these findings strongly support the hypothesis that 17,20 beta-P is a preovulatory female sex pheromone in goldfish which stimulates male GtH levels and milt production prior to spawning, the milt increases occurred earlier than predicted, suggesting either that preovulatory 17,20 beta-P release begins earlier than the data indicate or that other steroids known to have pheromonal activity are released before 17,20 beta-P.

Gonadotropin stimulation of K+ secretion and Na+ absorption by brook trout (Salvelinus fontinalis) sperm duct epithelium

The sperm duct epithelium from mature spermiating brook trout (Salvelinus fontinalis) was mounted in vitro to examine control of Na+ absorptive and K+ secretory transport. Na+ absorption (measured as the short-circuit current) and K+ secretion (measured using 86Rb+ as tracer) were stimulated by 3-isobutyl-1-methylxanthine and cyclic AMP while unstimulated tissues had no net ion transport. Purified chum salmon (Oncorhynchus nerka) Con AII carbohydrate-rich gonadotropin produced a rapid, sustained rise in Rb+ secretion and Na+ uptake in a log linear dose-dependent manner. Addition of gonadotropin to either apical (mucosal) or basolateral (serosal) sides evoked the response, but addition to the apical side produced the more rapid effect, indicating that receptors for the hormone are present on both sides of the transporting cells and suggesting that subepithelial tissue may slow the response to serosally added hormone. This is the first indication that gonadotropin may directly regulate ion transport functions of the blood-testis barrier of vertebrates and in this way regulate seminal plasma ionic composition.

Control of ion transport by the sperm duct epithelium of brook trout (Salvelinus fontinalis)

The sperm duct epithelium of brook trout (Salvelinus fontinalis), mountedin vitro in Ussing-style epithelial chambers actively absorbs Na(+) (measured as the short-circuit current, Isc) and secretes K(+) (measured using(86)Rb(+) as tracer). Dibutyryl-cyclic-adenosine monophosphate (db-cAMP) and 3-isobutyl-1-methylxanthine (IMX) produce a rapid, sustained stimulation of both ion transport processes, but the hormone connected to the response is unknown. Purified sockeye salmon CON A2 gonadotropin (GtH) produces a dose-dependent, rapid and sustained rise in Na(+) uptake and K(+) secretion. The time course, electrophysiological and transport characteristics are similar to those evoked by IMX. Carbohydrate-poor (chum salmon CON A1) GtH is ineffective. Pretreatment of fish with 17α,20β-dihydroxy-4-pregnen-3-one (17α,20β-P) significantly increases milt volume but is without effect on resting or stimulated (IMX + db-cAMP) levels of sperm duct ion transport. This is the first indication of a direct, rapid action of GtH on ion transport by the vertebrate blood-testis barrier. The results suggest direct involvement of GtH in control of later stages of sperm maturation.

Endocrine changes during gonadal maturation and spawning in the orange roughy (Hoplostethus atlanticus Collett), a teleost from the midslope waters off New Zealand

Orange roughy were sampled at all stages of the reproductive cycle by trawling at depths of 700-1200 m off the coast of New Zealand. Blood samples were collected from live fish, and changes in plasma levels of gonadal steroids were correlated with gonadal development and spawning. Plasma androgens were low in spent and regressed fish of both sexes and increased during gametogenesis to peak early in the spawning period at 6.6 and 9.4 ng.ml-1 for males and females, respectively. Androgen levels dropped to near basal levels over successive days during the first week of spawning in both prespawning and ovulated or spermiated fish. Falls in plasma androgens were not accompanied by increasing levels of plasma 17 alpha,20 beta-dihydroxy-4-pregnen-3-one (17 alpha,20 beta P) which remained at or near assay detection limits throughout. 11-Deoxycortisol (11-DOC) was present in the plasma of both sexes but did not change in concert with reproductive development. 17 beta-Estradiol was present in low concentrations (maximum 0.9 ng.ml-1 plasma) in recrudescing females, whereas estrone was detectable but not elevated at any stage. 17 alpha-Hydroxyprogesterone was not detectable in the plasma of any fish at any time. It is suggested that neither 17 alpha,20 beta P nor 11-DOC were active as maturation-inducing steroids (MIS) in orange roughy and that androgens may act as MIS. If the recorded fall in androgens was due to synthesis of another steroid from a common precursor, then the unidentified steroid does not appear to have been involved in stimulating final gamete maturation, but may have some action in initiating spawning behavior.

Seasonal changes in reproductive condition and plasma levels of sex steroids in the blue cod,Parapercis colias (Bloch and Schneider) (Mugiloididae)

Gonad and plasma samples were taken from blue cod captured throughout the reproductive cycle, gonad condition was assessed, and plasma levels of 17α-hydroxyprogesterone (17αOHP), 17α,20Β-dihydroxy-4-pregnen-3-one (17α,20ΒP), testosterone (T), 17Β-estradiol (E2) and estrone (E1) were measured by radioimmunoassay. It was confirmed that spawning occurred over an extended period in late winter and spring, with individual fish being involved in multiple spawning events. Plasma levels of T were bimodal in both sexes with peaks (maximum of 6.0 ng.ml(-1)) occurring 2 months prior to, and also during the early part of the spawning period. 17α,20ΒP was elevated in males (2.1 ng.ml(-1)) in mid-spermatogenesis coinciding with the first T peak (4.9 ng.m.(-1)). 17α,20ΒP was detectable but not significantly elevated (0.6-1.2 ng.ml(-1)) at any sample time in females. E2 was elevated in mature females (1.0 ng.ml(-1)) early in the spawning period but remained at assay detection limits (0.3 ng.ml(-1)) at all other sample times. Neither 17αOHP nor E1 were detectable in the plasma of either sex. It is suggested that bimodal increases in sex steroids prior to spawning may be a feature of species with rapid recrudescence.

Central regulation of reproduction in teleosts

As in other vertebrates, reproduction in teleosts depends upon interactions taking place along the brain-pituitary-gonads axis. At the central level, these interactions involve at least three types of factors:A gonadotrophin-releasing factor which has recently been isolated from chum salmon brain extracts. This decapeptide, whose structure is (Trp(7)-Leu(8))-LHRH, appears to have a widespread distribution among teleosts, and is less active that LHRH or LHRH analogues in releasing gonadotrophin from the teleost pituitary. Immunohistochemical and quantitative studies have demonstrated that Gn-RH neurons are mainly located in the ventral telencephalon and the preoptic area, while projections are found in the entire brain and the pituitary gland.A gonadotrophin release-inhibiting factor has been demonstrated in the anterior preoptic region of the goldfish and a large set of data suggests that dopamine has GRIF activity in goldfish, and in other teleost species, by direct action on the gonadotrophs. Accordingly, a dopaminergic preoptico-hypophyseal pathway could be demonstrated in the goldfish brain.Sex steroids exert, depending on the dosages, either a negative feedback in sexually mature fish or a positive feedback in immature fish. Such a positive feedback is caused by estrogens and aromatizable androgens. Accordingly, the brain of teleosts contains high levels of aromatase activity in particular in the telencephalon and anterior hypothalamus. The distribution of estrogens concentrating cells within the brain is consistent with possible interactions with Gn-RH or catecholaminergic neurons at the level of certain brain territories.These data are discussed in relation with the functional significance of different brain areas where interactions between these different factors possibly take place, in particular the terminal nerve, the ventral telencephalon, the preoptic area and nucleus lateralis tuberis.

Pheromone from ovulatory female goldfish induces gonadotropin surge in males

Characterization was made of the female stimuli which induce male gonadotropin (GtH) surge during spawning in the goldfish. In the presence of ovulatory females, olfactory tract sectioned (OTX) males failed to show sexual behavior or the GtH surge, while sham-operated (sham) males spawned and showed a GtH surge. When OTX and sham males were separated from ovulatory females with an opaque partition but with water circulating between male and female compartments, the GtH surge occurred only in sham males, although they could not court with the females. These results indicate that an olfactory stimulus (pheromone) from ovulatory females is essential to the occurrence of male GtH surge. Pheromone(s) from ovulatory female goldfish thus functions both as a releaser stimulating sexual behavior and as a primer inducing GtH surge in males.

Proceedings of a conference on steroid hormones as trophic agents in neural development. UCLA, October 9 & 10, 1984

The electric communication system includes both special muscle-derived cells or electrocytes that produce species-typical electric signals, or electric organ discharges (EODs), and specialized sensory receptors, or electroreceptors, that encode the electric fields set up by EODs. Steroid hormones can influence the characteristic properties of both EODs and electroreceptors. Steroids appear to directly effect the anatomy and physiology of the electrocytes that generate an EOD. In contrast, the steroid effect on electroreceptors may be predominantly via an indirect mechanism whereby changes in the spectral characteristics of the EOD appear to induce changes in the spectral sensitivity of electroreceptors. Continued studies of electrosensory and electromotor systems will offer insights into the cellular bases for the development and evolution of steroid-sensitive pathways in the vertebrate nervous system.

Endocrine changes associated with spawning behavior and social stimuli in a wild population of rainbow trout (Salmo gairdneri). I. Males

Rainbow trout (Salmo gairdneri) were collected from a natural spawning population at Pennask Lake, B.C. Blood samples were taken from male trout at different stages of spawning and assayed by radioimmunoassay for gonadotropin (GtH), testosterone (T). 11-ketotestosterone (11-KT), and 17 alpha-hydroxy-20 beta-dihydroprogesterone (17,20-P). Testosterone and 11-KT were detected in the plasma of males in spawning condition: 11-KT was at a higher concentration than T and declined to undetectable levels in postspawning fish. There were striking increases in plasma GtH and 17,20-P in males actively courting nest building females. The increase in 17,20-P was evidently not dependent upon an elevation of the pituitary hormone: 17,20-P remained low in males placed with sexually inactive, unovulated females even though GtH levels rose above levels measured in males isolated from females. The significance of the social control of hormone levels is not clear. There is some evidence that 17,20-P and/or GtH may cause an increase in quantity of the milt, and thus the endocrine response to a sexually active female may serve to synchronize maximum male fertility with oviposition readiness in the female.

Gonadotropin surge during spawning in male goldfish

Changes in plasma hormone levels during spawning were studied in male goldfish. Blood samples were analyzed for gonadotropin (GtH), testosterone, 11-ketotestosterone (11-KT), and 17 alpha,20 beta-dihydroxy-4-pregnen-3-one (17 alpha,20 beta-diOH-P) in males, and for GtH in females. Spawning was induced by raising water temperature from 12 to 20 degrees in sexually mature goldfish kept in pairs. Plasma GtH levels in males showed a marked increase (GtH surge) which was synchronous with the preovulatory surge in females, and peaked at the onset of spawning. Plasma levels of testosterone and 17 alpha, 20 beta-diOH-P increased almost at the same time as the GtH surge in males, whereas 11-KT levels remained low during the spawning. The small increase in GtH in males could be induced in the absence of females by raising the water temperature, but the levels were significantly enhanced by the presence of ovulatory females. A large amount of milt was observed during spawning, and the amount was correlated with plasma levels of GtH. Increased levels of GtH may be involved in milt production. We can propose that the synchronous GtH surge in both sexes causes ovulation and milt production to occur at the same time, favoring a higher rate of fertilization of the eggs.

Endocrine changes associated with spawning behavior and social stimuli in a wild population of rainbow trout (Salmo gairdneri). II. Females

Rainbow trout (Salmo gairdneri) were collected from a natural spawning population at Pennask Lake, B.C. Blood samples taken from female trout at different stages of spawning were assayed by radioimmunoassay for gonadotropin (GtH), estradiol-17 beta (E2), androgens, including testosterone (T), and 17 alpha-hydroxy-20 beta-dihydroprogesterone (17,20-P). Plasma levels of androgen and estradiol were highest in females sampled shortly before ovulation ("green" females) and declined in ovulated and sexually active females, reaching lowest levels in postspawning fish. Concentrations of 17,20-P rose markedly in ovulated females allowed to dig nests and interact with sexually active males. Plasma GtH levels were similar in green unovulated females and ovulated fish prevented from spawning, but showed a marked increase in actively nest building ovulated fish. The results demonstrate that social stimuli affect plasma levels of 17,20-P and perhaps GtH. The functional significance of the endocrine responses to social factors is not clear, but it is suggested that increased hormone levels may contribute to an acceleration or synchronization of breeding, or be responsible for causing and maintaining more vigorous sexual activity.

Sperm duct contractions mediate centrally evoked sperm release in goldfish

In order to determine the peripheral mechanisms underlying sperm release (SR) in goldfish, the contractile activity of the sperm ducts (SD) and testes were monitored during SR responses evoked by electrical stimulation of the brain. Electrical stimulation of the brain triggered testicular and SD contractions, and SR, while electrical stimulation of the genital nerve branch to the SD evoked only SD contractions and SR. Centrally activated SD contractions and SR were blocked by sectioning the SD genital nerve, while testicular contractions were unaffected. Testicular contractions do not appear necessary for centrally evoked SR since the response can be elicited from preparations in which the testes were separated from the SD. The results indicate that SR in goldfish is primarily mediated by the SD and not the testes. Testicular contractions may, however, serve to load the SD with milt. The functional significance of the central pathway(s) associated with SD and testicular contractions are discussed.