N. E. Stacey

Endocrine changes during natural spawning in the white sucker, Catostomus commersoni: I. Gonadotropin, growth hormone, and thyroid hormones☆

White suckers (Catostomus commersoni; Cypriniformes, Teleosteii) spawning in a small stream in central Alberta were captured during different stages of their spawning migrations in 1981 and 1982, blood was sampled, and the fish were examined to determine their reproductive condition. Blood samples were analyzed for gonadotropin (GtH), growth hormone (GH), triiodothyronine (T3), and thyroxine (T4) by radioimmunoassay. GtH levels in both sexes were lowest prior to the onset of spawning, increased significantly in spawning males, females in which germinal vesicle migration had begun, and ovulated females and then dropped significantly in spent fish of both sexes. GH was lowest in prespawning females, increased significantly at ovulation, and remained high in spent females. In contrast, GH levels in males were relatively constant throughout spawning. In both sexes, highest T4 levels were found in prespawning fish, and T4 decreased significantly in spent fish. Although a similar decline was seen in T3 in 1981, in 1982 there were no T3 changes associated with changes in reproductive condition. No significant diurnal variations were detected in the levels of GtH or T3; T4 levels appeared to vary on a diurnal basis in prespawning males only. Spawning activity in both sexes therefore appears to be associated with increases in GtH occurring at ovulation in females and at the initiation of spawning in males. GH levels may also be related to reproductive condition in females, but not in males. The relationship of thyroid hormone levels to reproductive condition is less clear, however, and these levels may reflect both endocrine and environmental influences on thyroid function.

Endocrine changes during natural spawning in the white sucker, Catostomus commersoni ☆: II. Steroid hormones

Blood samples were taken from white suckers (Catostomus commersoni) during their annual spring spawning migration and analyzed by radioimmunoassay for gonadotropin (GtH), estradiol-17 beta (E2), testosterone (T), 11-ketotestosterone (11-KT), 17 alpha-hydroxyprogesterone (17-P), 17 alpha-hydroxy-20 beta-dihydroprogesterone (17, 20-P), androstenedione (A), and cortisol. GtH, 17-P, and 17,20-P levels were low in prespawning fish of both sexes, rose to their highest levels in ovulated females and spawning males, and then fell to low levels in spent fish. In females, E2, T, and A levels were high in prespawning fish and declined significantly at ovulation, dropping to lowest levels in spent fish. In males, 11-KT, T, and A levels were highest in prespawning fish, and lowest in spent fish. Cortisol levels were highest in spermiating males and ovulated females. Plasma profiles of the sex steroids in the white sucker are very similar to those observed in the rainbow trout (Salmo gairdneri).

Ovulatory surge of gonadotropin in the goldfish, Carassius auratus

The periovulatory pattern of circulating gonadotropin is described for the first time in a poikilothermic vertebrate. In sexually mature female goldfish (Carassius auratus) maintained on a 16L:8D photoperiod, ovulation occurs during the latter part of the dark phase, approximately 20 hr after the fish are warmed from 13 to 21°. Serum gonadotropin levels increase gradually during the first half of the light phase, and by the latter part begin to increase rapidly, reaching the peak of the gonadotropin surge by the onset of the dark phase. Gonadotropin levels remain high through the period of ovulation, and decrease markedly by the onset of the following light phase. The findings indicate that the goldfish provides a valuable model for the study of ovulation in teleosts.

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

Abstract This study directly tested the hypothesis that 17α,20β-dihydroxy-4-pregnen-3-one (17,20β-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 17,20β-P in female blood and 17,20β-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 17,20β-P release by ovulatory females commenced. Although these findings strongly support the hypothesis that 17,20β-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 17,20β-P release begins earlier than the data indicate or that other steroids known to have pheromonal activity are released before 17,20β-P.

Extreme olfactory sensitivity of mature and gonadally-regressed goldfish to a potent steroidal pheromone, 17α,20β-dihydroxy-4-pregnen-3-one

Summary1.Electrical responses (electro-olfactogram; EOG) were obtained from the olfactory epithelium of goldfish exposed to a variety of sex steroids thought to function as pheromones, severall-amino acids, and a bile acid. Responses of male and female goldfish with both mature and regressed gonads were compared.2.17α,20β-dihydroxy-4-pregnen-3-one (17,20P) and progesterone were potent odorants. 17,20P had a detection threshold of 10−13M and at a concentration of 10−8M evoked an EOG response two to three times that of 10−5Ml-serine. Progesterone was less stimulatory than 17,20P. This finding corroborates recent endocrinological and behavioral studies (Stacey and Sorensen 1986; Dulka et al. 1987) which indicate that 17,20P is a potent pheromone in mature male goldfish.3.Preliminary cross-adaptation experiments suggest that 17,20P is detected by receptor site(s) differing from those which detect bile acids andl-amino acids.4.The goldfish olfactory epithelium did not respond to etiocholanolone glucuronide, 17β-estradiol glucuronide, testosterone glucuronide and 17β-estradiol, which have also been hypothesized to function as pheromones in fish.5.The olfactory thresholds of goldfish tol-amino acids and taurocholic acid were similar to other species of fish: approximately 10−13M forl-serine and 10−9M forl-cysteine and taurocholic acid.6.The olfactory sensitivities of mature male, mature female and gonadally-regressed fish were similar to all compounds tested suggesting that differences in olfactory receptor function are not responsible for the fact that only mature males exhibit behavioral and endocrine responses to pheromonal 17,20P.

Androgen increases olfactory receptor response to a vertebrate sex pheromone

Although it is well known that responses to ethologically-relevant odors are influenced by endocrine factors, it has not been clear whether these hormonal effects might be mediated at the level of the peripheral sensory neurons. During an investigation of hormonal pheromones in South-East Asian Cyprinids, we observed that in adult male Puntius schwanenfeldi, an androgen-dependent sex character was correlated with electro-olfactogram response to a putative sex pheromone (15-keto-prostaglandin-F2α). As secondary sex characteristics are androgen-dependent in male teleosts, this observation suggested a functional relationship between androgen and peripheral olfactory receptor response. We therefore investigated this possibility using androgen implants.In laboratory-raised juveniles, androgen treatment increased the magnitude and sensitivity of electro-olfactogram response to prostaglandin without affecting responses to other odors. Furthermore, androgen-treated juveniles performed pheromone-dependent sex behavior in the presence of a prostaglandin-injected stimulus fish. For the first time in vertebrates, the present data demonstrate hormone-induced plasticity of primary chemosensory neuronal responsiveness to an ethologically relevant compound.

Extreme olfactory specificity of male goldfish to the preovulatory steroidal pheromone 17α,20β-dihydroxy-4-pregnen-3-one

Summary1.To determine the specificity of the goldfish (Carassius auratus) olfactory system to the reproductive pheromone 17α,20β-dihydroxy-4-pregnen-3-one (17α,20βP), and to determine if related sex steroids might also function as pheromones, electro-olfactogram (EOG) responses were recorded from mature male goldfish.2.Of the 24 steroids tested, 17α,20βP was the most stimulatory. It had a detection threshold of 10−12M and at a concentration of 10−8M elicited an EOG response 3 times that elicited by 10−5M L-serine3.17α,20β,21-triol-4-pregnen-3-one, a metabolite of 17α,20βP, was the only other highly stimulatory steroid. Its threshold was 10−11M.4.In cross-adaptation experiments EOG responses to all 21-carbon steroids were inhibited during adaptation to 17α,20βP: responses to this pheromone are transduced by a single receptor/transduction mechanism.5.To verify the biological relevance of EOG recording whole animal responsiveness was determined by measuring blood gonadotropin. When goldfish were placed into homogeneous steroid solutions endocrine responsiveness strongly correlated with EOG recording. However, when steroids were added to aquaria containing fish, responses were less specific indicating that transient wisps of steroids trigger endocrine responses.6.Although the extreme sensitivity and specificity of the goldfish olfactory system to 17α,20βP gives it the potential to serve as a highly specific cue, realization of this potential is probably determined by the dynamics of pheromone exposure.

Sex pheromones selectively stimulate the medial olfactory tracts of male goldfish

The olfactory tracts of teleost fish are comprised of medial and lateral sub-tracts which previous studies suggest convey responses to pheromones and food odors respectively. This study tested this possibility by recording electrical responses from the medial and lateral tracts of male goldfish exposed to sex pheromones and food odors. Only the medial olfactory tract responded to pheromones and both tracts responded to an L-amino acid and crude food odor. These findings verify earlier studies of peripheral olfactory sensitivity to pheromones and confirm that pheromonal information is carried within the medial tracts. They also suggest that the neural processes responsible for food recognition are more complex than previously supposed.

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

In many male mammals and birds, exposure to sexual stimuli results in acute elevations of circulating luteinizing hormone (LH) and testosterone (T); a similar phenomemon has now been observed in the male goldfish (Carassius auratus). Mature males placed with either a receptive female or stimulus pairs of spawning goldfish had gonadotrophin (GtH) concentrations and expressible milt (sperm) volumes that were significantly greater than those of males kept in all-male groups. This stimulatory effect lasted from 20 min to at least 2 hr for GtH (20 degrees) and from less than or equal to 1 hr to greater than or equal to 24 hr for milt (14 degrees). When males were separated from the spawning pair by either a solid or perforated clear partition, no elevations of GtH or milt levels occurred. In contrast, these values increased in males placed in contact with a spawning pair, even when that pair contained no female, but a male induced to perform female sexual behavior by treatment with prostaglandin. These results suggest that, in goldfish, access to a spawning situation is necessary for rapid elevations in GtH and milt. Furthermore, it appears that the males must be sexually active in order for these physiological changes to occur, as males that failed to engage in courtship behavior with a spawning pair had GtH and milt values not different from isolated fish. This suggests that male sexual behavior and elevations in milt and GtH are concurrent events that share a common activation pathway in the brain. The increase in milt may be due to both neurally and hormonally mediated events that ensure milt availability for imminent spawning activity.

Ventral telencephalic lesions: effects on bisexual behavior, activity, and olfaction in the male goldfish

Previous work had demonstrated that male spawning behavior in goldfish (Carassius auratus) was reduced by lesions in the sex steroid-binding area of the ventral telencephalon, the area ventralis telencephali pars supracommissuralis (Vs) and the posterior area ventralis telencephali pars ventralis (pVv). The present study measured the effects of Vs—pVv lesions on other behavioral parameters in male goldfish. On Day 0, mature males were either sham lesioned or lesioned in the Vs—pVv and tested for male spawning behavior on Day 11 and female spawning behavior on Day 15; female behavior can be induced in males by the injection of 250 ng/g prostaglandin F2alpha (PG). On Day 18, spontaneous activity and feeding in response to a food odor was measured in the sexually inactive lesioned fish and a random subset of the sham lesioned animals. As in the previous work, Vs—pVv lesions were effective in reducing male courtship and spawning behavior. In addition, lesioned males that failed to show male sexual behavior also failed to show female spawning behavior when injected with PG. When fish were isolated, less swimming activity was seen in the sexually inactive lesioned fish than in the shams. All behavior was not indiscriminately reduced, however, as feeding behavior evoked by a food odor was unaffected by Vs—pVv lesions. The lesions may have disrupted the processing of stimuli that regulate social activities, particularly for male behavior, the processing of critical olfactory cues. In this regard, parallels with the mammalian amygdala were noted.