Diurnal variations of plasma gonadotrophins in male domestic ducks during the sexual cycle

Steroid metabolism in the brain: From bird watching to molecular biology, a personal journey

Since Arnold Adolph Berthold established in 1849 the critical role of the testes in the activation of male sexual behavior, intensive research has identified many sophisticated neurochemical and molecular mechanisms mediating this action. Studies in Japanese quail demonstrated the critical role of testosterone action and of testosterone aromatization in the sexually dimorphic medial preoptic nucleus in the activation of male copulatory behavior. The development of an immunohistochemical visualization of brain aromatase in quail then allowed further refinement in the localization of the sites of neuroestrogens production. Testosterone aromatization is required for the activation of both appetitive and consummatory aspects of male sexual behavior. Brain aromatase activity is modulated by steroid-induced changes in the transcription of the corresponding gene but also more rapidly by phosphorylation processes. Sexual interactions with a female also rapidly regulate brain aromatase activity in an anatomically specific manner presumably via the release and action of endogenous glutamate. These rapid changes in estrogen production modulate sexual behavior and in particular its motivational component with latencies ranging between 15 and 30min. Brain estrogens seem to act in a manner akin to a neurotransmitter or at least a neuromodulator. More recently, assays of brain estradiol concentrations in micropunched samples or in dialysis samples obtained from behaviorally active males suggested that aromatase activity measured ex vivo might not be an accurate proxy to the rapid changes in local neuroestrogens production and concentrations. Studies of brain testosterone metabolism are thus not over and will keep scientists busy for a little longer. Elsevier SBN Keynote Address, Montreal.

Neuroendocrine regulation of gonadotropin secretion in seasonally breeding birds

Seasonally breeding birds detect environmental signals, such as light, temperature, food availability, and presence of mates to time reproduction. Hypothalamic neurons integrate external and internal signals, and regulate reproduction by releasing neurohormones to the pituitary gland. The pituitary gland synthesizes and releases gonadotropins which in turn act on the gonads to stimulate gametogenesis and sex steroid secretion. Accordingly, how gonadotropin secretion is controlled by the hypothalamus is key to our understanding of the mechanisms of seasonal reproduction. A hypothalamic neuropeptide, gonadotropin-releasing hormone (GnRH), activates reproduction by stimulating gonadotropin synthesis and release. Another hypothalamic neuropeptide, gonadotropin-inhibitory hormone (GnIH), inhibits gonadotropin synthesis and release directly by acting on the pituitary gland or indirectly by decreasing the activity of GnRH neurons. Therefore, the next step to understand seasonal reproduction is to investigate how the activities of GnRH and GnIH neurons in the hypothalamus and their receptors in the pituitary gland are regulated by external and internal signals. It is possible that locally-produced triiodothyronine resulting from the action of type 2 iodothyronine deiodinase on thyroxine stimulates the release of gonadotropins, perhaps by action on GnRH neurons. The function of GnRH neurons is also regulated by transcription of the GnRH gene. Melatonin, a nocturnal hormone, stimulates the synthesis and release of GnIH and GnIH may therefore regulate a daily rhythm of gonadotropin secretion. GnIH may also temporally suppress gonadotropin secretion when environmental conditions are unfavorable. Environmental and social milieus fluctuate seasonally in the wild. Accordingly, complex interactions of various neuronal and hormonal systems need to be considered if we are to understand the mechanisms underlying seasonal reproduction.

Plasma prolactin and luteinizing hormone levels of pinealectomized and enucleated turkey hens

Plasma levels of LH and prolactin were determined in pinealectomized (PX) and/or bilateral ocular enucleated (EX) adult turkey hens maintained in either constant (24LL) or diurnal (16L:8D) lighting. Blood samples were obtained at the following times over a 24-hr period: onset, middle, and end of light period, middle and end of dark period. In experiment 1, the effects of PX alone were evaluated in photorefractory hens exposed to two different photoperiods (24LL and 16L:8D). In experiment 2, the effects of PX and EX were determined in egg-laying and photorefractory hens exposed to a 16L:8D daily photoperiod. Neither PX nor EX significantly altered LH or prolactin levels of egg laying or photorefractory hens at any of the sampling times of the day in either experiment. In experiment 1, LH levels were very similar in both 16L:8D and 24LL but prolactin levels were significantly lower in 24LL than 16L:8D. It was concluded that the pineal gland does not play an essential role in maintaining circulating LH and prolactin levels in constant light or a photoperiod of 16L:8D.

Changes in pituitary responsiveness to luteinizing hormone-releasing hormone during an annual cycle in the domestic duck, Anas platyrhynchos L

On four occasions during an annual cycle, 5--7 male domestic ducks were injected with two different doses (5 and 20 micrograms) of synthetic luteinizing hormone-releasing hormone (LHRH) to study the possible changes in responsiveness of the pituitary. The luteinizing hormone (LH) and the follicle-stimulating hormone (FSH) were measured in the plasma samples collected after these injections. The induced release of LH changes from one period of the year to another, being minimum in March at the height of the reproductive season. The LHRH injection also induces the release of some FSH but only in limited amounts. The changes in pituitary responsiveness to LHRH are negatively correlated to changes in the circulating LH level (it is high when the plasma LH is low and vice versa). This suggests that the hypothalamic synthesis and release of LHRH must also change during the year.