The three steroidal components of the goldfish preovulatory pheromone signal evoke different behaviors in males

L-Kynurenine, an amino acid identified as a sex pheromone in the urine of ovulated female masu salmon

Many animals employ sex pheromones to find mating partners during their reproductive seasons. However, most sex pheromones of vertebrates remain to be identified. Over the past 20 years, steroids and prostaglandins have been identified as sex pheromones in several fishes. These pheromones are broadly termed "hormonal pheromones" because they or their precursors act as hormones in these fishes. Hitherto, no other type of sex pheromone has been unambiguously identified in teleost fish. Here we report the identification of a "nonhormonal pheromone" in teleost fish. The urine of the reproductively mature female masu salmon (Oncorhynchus masou) contains a male-attracting pheromone. Bioassay-guided fractionation yielded an active compound that was identical to L-kynurenine in spectral and chromatographic properties. L-Kynurenine is a major metabolite of L-tryptophan in vertebrates. This pheromone elicits a male-specific behavior at even picomolar concentrations; its electrophysiological threshold is 10(-14) M. L-Kynurenine is a reasonable substance for female masu salmon to advertise their readiness for mating.

Gender distinction in neural discrimination of sex pheromones in the olfactory bulb of crucian carp, Carassius carassius

Studies on projection of the sensory neurons onto the olfactory bulb in fish have revealed a clear subdivision into spatially different areas that each responded specifically to different classes of odorants. Amino acids induce activity in the lateral part, bile salts induce activity in the medial part, and alarm substances induce activity in the posterior part of the medial olfactory bulb. In the present study, we demonstrate a new feature of the bulbar chemotopy showing that neurons specifically sensitive to sex pheromones are located in a central part of the ventral olfactory bulb in crucian carp. Extensive single-unit recordings were made from these neurons, stimulating with four sex pheromones, 17,20beta-dihydroxy-4-pregnen-3-one, 17,20beta-dihydroxy-4-pregnen-3-one-20-sulfate, androstenedione, and prostaglandin F(2alpha), known to induce specific reproductive behaviors in males of carp fish. All substances were applied separately to the sensory epithelium at a concentration of 10(-9) M. Of the 297 neurons recorded in males, the majority (236 or 79.5%) responded exclusively to one of the four sex pheromones and thus showed a high specificity. Of the 96 neurons recorded from the olfactory bulb in females, only 1 unit showed such a specific activation. These findings reflect remarkable differences between males and females in the discriminatory power of the olfactory neurons toward these sex pheromones. The gender differences are discussed in relation to behavior studies, expression of olfactory receptors, and the convergence of sensory neurons onto the secondary neurons in the olfactory bulb.

Steroidogenesis in ovarian follicles of chub mackerel, Scomber japonicus

We incubated different radiolabeled steroid precursors with intact chub mackerel ovarian follicles to clarify the synthetic pathways of steroid hormones during vitellogenesis and following final oocyte maturation (FOM). During vitellogenesis, estradiol-17beta (E2) was synthesized from pregnenolone via 17-hydroxypregnenolone, 17-hydroxyprogesterone, androstenedione, and testosterone. The physiological significance of the intermediate metabolites of E2 in the ovarian follicles was examined by comparing follicular steroidogenesis between gonochoric and hermaphroditic fish species. After vitellogenesis, the steroidogenic pathway shifted from E2 to maturation-inducing hormone (MIH) production owing to the inactivation of 17,20-lyase and the activation of 20 beta-hydroxysteroid dehydrogenase. Of the new steroids produced during FOM, 17,20beta-dihydroxy-4-pregnen-3-one (17,20beta-P) was most effective at inducing germinal vesicle breakdown in vitro. Circulating levels of 17,20beta-P increased specifically around the time of germinal vesicle migration, while another FOM-specific 20beta-hydroxylated progestin, 17,20beta,21-trihydroxy-4-pregnen-3-one, was present at consistently low levels during FOM. These results indicate that 17,20beta-P is the MIH of chub mackerel.

Sexually mature male goldfish release large quantities of androstenedione into the water where it functions as a pheromone

Previous studies have demonstrated that ovulatory female goldfish release a variety of sex steroids into the water where they function as a pheromonal blend dominated by C21 steroids that stimulates male hormone release, sperm production and behavior. This study investigated whether male goldfish might also release sex steroids with pheromonal activity. It found that spermiated male goldfish release substantial quantities of androstenedione (AD; about 50 ng/h) together with smaller (10-20 ng/h) quantities of several other related C19 steroids but only very small quantities (<5 ng/h) of C21 steroids. Further, when sexually aroused by females and/or their pheromones, males released even greater quantities of AD (up to 1 microg/h) while C21 steroid release rate changed little. This created a ratio of C19 to C21 steroids of about 50:1 that was dramatically different from that emitted by females (1:7). The male olfactory system was also found to be extremely sensitive to AD, detecting it to near picomolar concentrations. Together with previous studies that have shown water-borne AD to increase male aggressive behavior while suppressing responsiveness to female pheromones, this study establishes AD as a male pheromone in the goldfish. Because ovulating females also release AD but in the presence of C21 steroids, recognition of the male-derived steroid pheromone is presumably mixture dependent.

Visual sex discrimination in goldfish: seasonal, sexual, and androgenic influences

The olfactory signals used by goldfish for sexual and aggressive communication have been studied extensively, but little work has addressed the role of other sensory modalities in social communication in this species. We therefore investigated the role that visual stimuli play in sex discrimination and the ability of androgens, which masculinize courtship behavior, to affect behavioral responses toward female visual stimuli. We found that males selectively orient toward female visual stimuli during the breeding season but not outside it, whereas prostaglandin F2-alpha (PGF2alpha)-injected females do not differentially approach male and female visual stimuli, even during the breeding season. Implanting adult females with testosterone (T) and 11-ketotestosterone (KT), however, induced orientation responses toward female visual stimuli similar to those observed in males. These results indicate that visual sexual stimuli are likely important for reproductive signaling in goldfish, potentially helping males identify ovulating females from a distance in a shoal of fish, and that androgens can influence mechanisms associated with orientation responses toward such stimuli.

Evidence that 4-pregnen-17,20beta,21-triol-3-one functions as a maturation-inducing hormone and pheromonal precursor in the percid fish, Gymnocephalus cernuus

Behavioral, biochemical, and electrophysiological studies suggest that the trihydroxylated progestin steroid, 4-pregnen-17,20beta,21-triol-3-one (20beta-S) stimulates oocyte maturation and pheromone release in the Eurasian ruffe, a freshwater percid fish. Behavioral observations found that female ruffe undergoing oocyte maturation (OM) release a pheromonal cue that stimulates swimming activity and social interactions among conspecific males. Neither vitellogenic nor ovulated females released the cue. Pheromone production was directly associated with elevated plasma levels of 20beta-S in maturing female ruffe which in vitro incubation suggested to be a possible maturation-inducing hormone (MIH) in this species along with 4-pregnen-17,20beta-diol-3-one (17,20betaP). However, neither of these steroids appear to be the pheromone because electrophysiological and behavioral studies found them to lack olfactory (EOG) and behavioral activity. Instead, studies of the odor of steroid-injected fish suggest the pheromone is a metabolite of 20beta-S. In particular, inter-peritoneal injection of 20beta-S (but not 17,20betaP) consistently induced release of a urinary cue with strong behavioral activity. The pheromone may be a highly polar and novel metabolite because it could not be extracted using octadecylsilane resin (C18) which has proven effective for other teleost hormonal pheromones.

Peptide effects on social behavior: effects of vasotocin and isotocin on social approach behavior in male goldfish (Carassius auratus)

The authors measured the effects of centrally infused peptides on social approach behaviors in goldfish (Carassius auratus), a social teleost. Vasotocin (VT) inhibited approach responses toward the visual stimuli of conspecifics in the absence of aggressive or sexual olfactory contextual cues in males, and a V1 receptor antagonist stimulated such responses, at least in males that were not highly social in baseline conditions, as did isotocin (IT). In the absence of social stimuli, VT did not affect activity, therefore indicating that the inhibition was not the result of nonspecific effects on arousal or motor functioning. These experiments indicate that VT and IT induce opposite effects on social approach responses in male goldfish and that endogenous VT, at least, is associated with levels of sociality.

Hormonally derived sex pheromones in fish: exogenous cues and signals from gonad to brain

Living in a medium that can limit visual information but readily exposes the olfactory organ to hormonal compounds released by conspecifics, fish throughout their long evolutionary history have had both clear cause and ample opportunity to evolve olfactory responsiveness to these potentially important chemical cues (hormonal pheromones). Indeed, water-borne steroids, prostaglandins, and their metabolites are detected with great sensitivity and specificity by the olfactory organs of diverse fishes, and exert important effects on reproductive behavior and physiology in major taxa including carps (goldfish), catfishes, salmon, and gobies. Best understood are goldfish, where periovulatory females sequentially release a preovulatory steroid pheromone and a postovulatory prostaglandin pheromone that dramatically affect male behavior, physiology, and reproductive fitness. Although the diverse array of hormonal products released and detected by fish indicates clear potential for species-specific hormonal pheromones, olfactory recordings showing similar patterns of hormone detection among closely related species provide little evidence of selection for specificity. By demonstrating that the actions of sex hormones and related products are not limited to reproductive synchrony within the individual, the relatively recent discovery of hormonal pheromones has considerably expanded our understanding of fish reproductive function, while providing valuable model systems for future study of olfactory function and pheromone evolution.

Possible disruption of pheromonal communication by humic acid in the goldfish, Carassius auratus

Humic acids are large, complex, organic molecules which are ubiquitous components of aquatic environments as products of degradation of plant material. In aqueous solution they form microvesicles. As many teleost pheromones are steroidal in nature, we hypothesised that they would preferentially dissolve in the organic, hydrophobic core of these vesicles instead of in water and therefore be unavailable for detection. This would have obvious and profound effects on many aspects of fish biology. To test this hypothesis we recorded electro-olfactogram (EOG) response of the goldfish (Carassius auratus) olfactory epithelium to the pheromones 17 alpha,20 beta-dihydroxy-4-pregnen-3-one (1720 beta-P), its sulphated conjugate (1720 beta-P-SO(4)) and prostaglandin F(2alpha) (PGF(2alpha)), all at 10(-11) to 10(-8) M, in the absence and presence of humic acids (1-1000 m x gl(-1)). At nearly all concentrations of humic acid tested, there was a significant attenuation of the amplitude of the initial (phasic) response to 1720 beta-P compared to 1720 beta-P alone. At higher concentrations of humic acid, the EOG response to 1720 beta-P was often completely obliterated, suggesting that the concentration of the pheromone available to the olfactory epithelium was below the threshold of detection. Exposure of the olfactory epithelium to humic acid did not cause any short-term loss of sensitivity to 1720 beta-P per se. Furthermore, simultaneous recording of electro-encephalograms from the olfactory bulb demonstrated that the nervous activity evoked by the same concentration of 1720 beta-P was less intense in the presence of humic acid than its absence. PGF(2alpha) is non-steroidal and much more soluble in water. In contrast to 1720 beta-P, only the higher concentrations of humic acid (100 and 1000 mg x l(-1)) significantly diminished the EOG amplitude. 1720 beta-P-SO(4) is detected via a distinct olfactory mechanism to the free form. Given that the sulphate group increases the water solubility, we predicted that the effect of humic acid would be reduced. However, the effect of humic acid on EOG amplitude in response to 1720 beta-P-SO(4) was similar to that of the free form. We suggest that the steroid portion of the molecule adsorbs onto the surface of the humic acid microvesicles and is still effectively unavailable for olfactory detection. In conclusion, humic acid may significantly reduce the concentration of 1720 beta-P and 1720 beta-P-SO(4) available for detection by Carassius auratus in natural environments. Furthermore, as many teleost pheromones are steroid derivatives, this phenomenon may be applicable to chemical communication systems in teleosts in general.

Isolation increases milt production in goldfish

Milt volume in goldfish is increased by female steroid and prostaglandin pheromones, by exposure to males with elevated gonadotropin levels, and by isolation from conspecifies. This study examined various aspects of the isolation effect on milt volume and serum gonadotropin II (GTH II). The latency of isolation-induced milt increase in this study (12-24 hr) was longer than the latencies to pheromone-induced milt increase in previous work (0.5-6.0 hr), was not affected by the time of day at which males were isolated, persisted for at least 72 hr in isolated males, and was terminated within 24 hr in males that were returned to groups. Isolated males maintained high milt production when separated from tank mates by a perforated barrier or when exposed to visual and odor cues from males in other tanks, suggesting that the unknown conspecific cues that maintain low milt production in groups operate at close range. Isolation appears to increase milt through a mechanism different from that mediating response to female pheromones because: (1) unlike female pheromones, which consistently increase serum GTH II, no isolation or regrouping treatment in this study affected male GTH II concentration; and (2) the effects of isolation and of exposure to the female pheromone 17 alpha, 20 beta-dihydroxy-4-pregnen-3-one (17,20 beta-P) are additive. Finally, males that were previously isolated or exposed to 17,20 beta-P increased milt (but not GTH II) in grouped males, suggesting that the effects of isolation and pheromone exposure can indirectly stimulate male conspecifics. Although the biological function of the isolation effect is not clear, we propose that it illustrates the effect of removal from inhibitory cues normally received from male conspecifies.