James L. Blank

Diurnal Variation in 5-Hydroxyindole-Acetic Acid Output in the Suprachiasmatic Region of the Siberian Hamster Assessed by in vivo Microdialysis: Evidence for Nocturnal Activation of Serotonin Release

In vivo brain microdialysis was used to characterize the daily pattern of 5-hydroxyindole-acetic acid (5-HIAA) release in the region of the suprachiasmatic nuclei (SCN) in freely behaving male Siberian hamsters housed under 16L:8D. A marked diurnal variation in the concentration of extracellular 5-HIAA was apparent, with peak levels (147 +/- 5% of the daily mean; p < 0.05) occurring 2-3 h after lights-off. Smaller nocturnal rises in extracellular 5-HIAA were observed in the posterior hypothalamus and preoptic area (128 +/- 4 and 123 +/- 8% of the daily mean, respectively; both p < 0.05 vs. average daytime levels). Tryptophan loading increased 5-HIAA in SCN microdialysates by 44 +/- 6%, and this response was enhanced by localized perfusion with tetrodotoxin (TTX; 5 microM). Localized applications of KCl (150 mM) or veratridine (100 microM) decreased 5-HIAA by 62 +/- 5 or 49 +/- 11%, respectively. The effect of KCl was not significantly affected by specific calcium channel blockers. Perfusion with TTX markedly decreased SCN 5-HIAA during the dark phase, but had little effect during the light phase (42 +/- 8 vs. 12 +/- 5% suppression, respectively; p < 0.01). Addition of serotonin (3 microM) to the perfusate significantly stimulated 5-HIAA output. This treatment increased the release of 5-HIAA more during the dark than during the light phase (61 +/- 8 vs. 25 +/- 5%, respectively; p < 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)

Differential testicular responses to short daylength in deer mice are reflected by regional and morphological differences in the GnRH neuronal system

Male deer mice (Peromyscus maniculatus) respond to short photoperiod with a range of spermatogenic responses. About one-third of all short day exposed mice exhibit a complete cessation of spermatogenesis (reproductive responsive), while about an equal number remain reproductively competent (reproductive nonresponsive). These differential spermatogenic responses are mirrored by endocrine measures; reproductive responsive males exhibit reduced circulating levels of testosterone and luteinizing hormone, while reproductive nonresponsive males exhibit long day levels of both hormones. A variety of evidence indicates that despite individual differences in reproductive responses, all short day exposed mice detect and respond to the change in photoperiod at the level of the hypothalamus, irrespective of gonadal response. We investigated whether deer mice exhibiting disparate gonadal responses to short days differed in aspects of the GnRH neuronal system, as revealed by immunocytochemistry. Reproductively mature males were maintained on either long (16L:8D) or short (8L:16D) photoperiod for 8 weeks. Thereafter, regional and morphological differences in GnRH-containing neurons were compared among long day (n = 8) and short day reproductive responsive (n = 8) and nonresponsive (n = 8) deer mice. Results demonstrate both an effect of photoperiod and reproductive state on the number, location and morphology of immunoreactive-GnRH neurons. Short days caused immunoreactive GnRH cell number to increase in both short day phenotypes relative to long day mice. Significant differences among treatment groups in the numbers of cells per region were confined to the lateral hypothalamus and lateral preoptic area. Both short day phenotypes exhibited an increase in cell number in the lateral preoptic area, compared to long day mice.(ABSTRACT TRUNCATED AT 250 WORDS)

Depletion of olfactory bulb norepinephrine by 6-OHDA disrupts chemical cue but not social recognition responses in male rats

In the present experiment, 6-OHDA was infused directly into the olfactory bulb (OB) to produce a localized neurotoxic lesion. Habituation/dishabituation behavioral tests were then conducted to measure recognition responses to chemical cues (urine as a stimulus) and to social stimuli (ovariectomized rat as a stimulus). Infusion of 6-OHDA resulted in a near complete depletion of OB-norepinephrine (NE), whereas it had little effect (15% reduction) on OB dopamine (DA) contents. Nor were any significant effects on hypothalamic, hippocampal, olfactory tubercle, and corpus striatal NE and DA contents observed. Behaviorally, dishabituation responses to chemical cues were greatly impaired, however, there was relatively little effect on social behavior dishabituation responses. These results demonstrate that 6-OHDA can be used to produce a near complete but localized depletion of OB-NE. This treatment impairs dishabituation responses to chemical cues but not social stimuli indicating that OB-NE appears necessary for processing of chemical cue, but not social memory recognition process.

Phenotypic variation in seasonal adjustments of testis size, body weight, and food intake in deer mice: role of pineal function and ambient temperature

Abstract We investigated pineal function as well as reproductive and energetic characteristics in male deer mice (Peromyscus maniculatus) that differentially respond to short photoperiod with full, partial or no gonadal regression. In mice at both high (23 °C) and low temperature (1 °C), these phenotypic differences in reproductive responses to short days were not reflected by differences in urinary excretion of 6-sulphatoxy-melatonin, the main metabolite of pineal melatonin. Neither duration nor amplitude or phase-angle of nocturnal peaks in 6-sulphatoxymelatonin significantly differed between reproductive phenotypes at either temperature. Differences in testis size were, however, associated with different energy requirements. In gonadally regressed males only, food intake and body weight were significantly (P < 0.01) reduced by up to 29% and 13% respectively. Chronic cold exposure (5 °C) had no effect on the proportion of males undergoing testicular regression under short days, but caused a general elevation in body weights among all mice (P < 0.05). Phenotypic differences in body weight and food intake were maintained in the cold. Together, these results suggest that within-population variation of reproductive responses in male deer mice is based on post-pineal differences in the regulation of gonadal function, and that phenotypic characteristics in reproductive and energetic responses to short days are largely unaffected by ambient temperature.

Effects of intrauterine 2,3,7,8-tetrachlorodibenzo-p-dioxin on the development and function of the gonadotrophin releasing hormone neuronal system in the male rat ☆

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is an environmental disrupter that continues to be generated from numerous industrial processes. In utero and lactational exposure of rats to levels of TCDD similar to those encountered by human populations have profound and persistent effects on growth, the reproductive axis and spatial learning and memory. TCDD is thought to act through the aryl hydrocarbon receptor, which displays crosstalk with estrogen-mediated genomic activation. An in vitro superfusion system was used to evaluate the effects of TCDD exposure on GnRH-release from hypothalamic explants at three developmental time points in male rats. Pregnant dams were treated with 5 microg/kg TCDD on gestational day 15, and male offspring displayed a marked reduction in GnRH release. However, total mediobasal hypothalamus/preoptic area (MBH/POA) GnRH content was significantly greater in dioxin-exposed animals. These results suggest deficits in release rather than production of GnRH. Confocal microscopy was used to characterize anatomical features of individual GnRH-positive neurons, as well as the organization of the neuronal network governing GnRH release. Differences in cellular structures were apparent in discrete regions of the GnRH neural network, specifically the lateral preoptic area and septal region. We propose that developmental reproductive effects in males treated in utero and lactionally with dioxin, results from a suppressive effect of TCDD on mechanisms governing GnRH release. These effects coincide with changes in growth and development, indicating that TCDD may induce a syndrome of effects by modifying hypothalamic structures regulating growth and reproductive development.

Role of gonadal steroids and inhibitory photoperiod in regulating body weight and food intake in deer mice (Peromyscus maniculatus).

Abstract We investigated the role of declining daylength and gonadal steroids on body weight and food intake in male deer mice (Peromyscus maniculatus). This species was chosen for study because individual males display different reproductive responses to inhibitory daylength. About one-third of all mice exposed to short days undergo testicular regression and exhibit reduced circulating levels of luteinizing hormone and testosterone (reproductive responsive males). In contrast, testicular function and circulating levels of both these hormones remain unaffected in an equal number of mice (reproductive nonresponsive males). Previous studies have shown that each phenotype exhibits a distinct set of metabolic responses to short days, including adjustments in body weight. These characteristics make deer mice a useful animal model in which to study the interactive effects of gonadal steroids and photoperiod on neural substrates regulating body weight. A gonadectomy/steroid replacement experimental model was used to test the role of testosterone in regulating body weight and food intake in short day-housed male deer mice. Among gonad-intact males, short daylength caused a decline in body weight in both reproductive responsive and nonresponsive individuals. However, reproductive responsive mice lost significantly more body weight than did nonresponsive mice. Furthermore, while the weight loss was accompanied by a significant reduction in food intake in responsive mice, the relatively minor weight loss in nonresponsive mice was not accompanied by a change in food intake. Because changes in body weight and food intake (data not shown) occurred nearly simultaneously during the 8-week exposure to short day-length, results suggest that modifications in body weight are not responsible for the decline in food consumed, and vice versa. Gonadectomized reproductive responsive mice lost the same amount of weight as intact responsive mice but ate significantly more food. Among nonresponsive males, gonadectomy led to significantly greater weight loss, relative to intact mice, but caused an increase in food intake per gram body weight. Steroid replacement prevented weight loss and increased food intake in both gonadal phenotypes. Despite the observations that food intake was steroid dependent and the magnitude of the effect differed between reproductive phenotypes, changes in food consumption do not fully explain the inhibitory effects of short days on body weight in either phenotype. Taken together with previous studies, these results suggest that reproductive quiescence confers significant metabolic benefits to individual deer mice by reducing the amount of daily energy requirements via a reduction in body weight. Conversely, maintenance of reproductive function during the nonbreeding winter season carries greater metabolic costs; these costs lead to increased amounts of food required to maintain body weight.

Short photoperiod and testosterone-induced modification of gnrh release from the hypothalamus of Peromyscus maniculatus

Seasonally breeding animals undergo numerous physiological changes in response to changes in the length of the photoperiod. In most warm-weather breeding rodents, these changes result in reproductive quiescence during short photoperiods. It has been hypothesized that this change is mediated by changes in the activity of gonadotropin-releasing (GnRH) hormone neurons of the hypothalamus. This study was designed to test whether there are changes in the releasable pool of GnRH in the hypothalamus in response to changes in photoperiod, the presence of gonadal steroids, or the responsiveness of the individual animal to photoperiodic changes. Male deer mice (Peromyscus maniculatus) were maintained on long or short day photoperiod and either left intact, castrated, or castrated with testosterone replacement. KCl-evoked GnRH release was measured from hypothalamic explants from each animal and compared between long and short days, between castrated, intact, and castrated with testosterone replacement animals, and between animals that did or did not show gonadal regression in response to short day treatment. There was a significant decline in evoked release of GnRH in short day housed animals when comparing photoperiod responsive mice to nonresponsive mice. In addition, both reproductively nonresponsive and long day-housed mice release less GnRH following castration than their intact counterparts. When castrated long day-housed mice were provided with long day levels of testosterone, evoked GnRH release was restored to intact levels. Taken together, the results of this study suggest that variation in testicular response to short days is most likely due to differences in the release of GnRH from the hypothalamus.