Prolactin cell activity in female and male Syrian hamsters: an apparent sexually dimorphic response to light deprivation and pinealectomy

Absence of pineal-independent mediation of seasonal differences in suprachiasmatic nucleus AVP and VIP mRNA expression in Siberian hamsters

Assessment of seasonal variations in expression of brain neuropeptide mRNA is complicated by concurrent circadian variations. Because entrainment of suprachiasmatic nucleus (SCN) based rhythms differs in long versus short day lengths, valid seasonal comparisons must be made at equivalent circadian phases. We used a novel experimental design which permitted sampling at identical circadian phases of animals exhibiting opposite seasonal reproductive responses to the same intermediate day length. This allowed us to test whether seasonal changes in arginine vasopressin (AVP) and vasoactive intestinal peptide (VIP) mRNA expression in the SCN occur in the absence of the pineal gland. Juvenile Siberian hamsters were gestated and maintained postnatally in either a long photoperiod (16 h light/day) or short photoperiod (10 h light/day). At the time of weaning (18 days of age), the hamsters were pinealectomized and either transferred to a new photoperiod (10-, 16- or 14-h light/day) or left in the original photoperiod. Hamsters from 10L had substantially smaller and lighter testes than those from 16L. If photoperiodic modulation of AVP and VIP mRNA expression occurs in the absence of the pineal, then transfer of pinealectomized hamsters from a longer (16L) or shorter (10L) photoperiod to an intermediate photoperiod (14L) should result in a differential response with respect to SCN AVP and VIP mRNA expression but not testis size. When sampled at an identical circadian phase (3 h after lights on) in 14L there was no difference in the expression of AVP or VIP mRNA in the SCN between animals previously housed in long versus short day lengths. In contrast to a previous study that did not carefully control for circadian phase, the present findings suggest that seasonal photoperiodic control of SCN neuropeptide mRNA expression depends upon the pineal gland. In addition, the present findings demonstrate a significant, negative correlation between AVP mRNA expression in the SCN and the length of the daily active phase (alpha).

Relationship between norepinephrine release in the hypothalamic paraventricular nucleus and circulating prolactin levels in the Siberian hamster: role of photoperiod and the pineal gland

The impact of norepinephrine (NE) and its metabolite, 3-methoxy4-hydroxyphenylglycol (MHPG), on circulating prolactin (PRL) was evaluated in the paraventricular region of the hypothalamus as a function of photoperiod and integrity of the pineal gland. In Experiment 1, whole tissue content of NE and MHPG was assessed in male and female hamsters that had been pinealectomized or sham-pinealectomized and exposed to long or short photoperiods for 5 weeks. The results revealed a marginal effect of photoperiod in males, but no overall effects of surgery. Because analysis of whole tissue content can be complicated by concurrent changes in synthesis and storage rates, Experiment 2 was conducted using microdialysis to assess extracellular levels of NE and MHPG in female hamsters. Pinealectomy completely prevented the short-day-induced suppression of luteinizing hormone, but it only partially prevented the effects of short days on PRL. Furthermore, both NE and MHPG levels were significantly elevated in short-day-exposed pinealectomized and sham-operated animals. These results suggest that NE release within the paraventricular nucleus inhibits the circulating PRL levels and is one mechanism by which direct photic information can influence the neuroendocrine axis independently of the pineal melatonin signal.

Effects of chronic melatonin administration on adrenal medulla catecholamine metabolism in adult male golden hamsters

The role of melatonin on adrenal medullary function was evaluated in adult golden (Syrian) hamsters injected daily at either 0800 or 1600 with melatonin (25 micrograms/hamster) or vehicle (0.1% ethanol in saline). Sixty days after initiation of treatment and 16 or 24 hr after the last injection of melatonin, animals were killed by decapitation and blood and tissue collected. Plasma prolactin levels were reduced by chronic administration of melatonin only when animals were treated at 1600. Tyrosine hydroxylase (TH) activity in the adrenal medulla was not changed by either morning or afternoon administration of melatonin. Phenylethanolamine-N-methyl transferase (PNMT) was changed by afternoon administration of melatonin. Morning and afternoon melatonin injections led to the reduction of both adrenal monoamine oxidase (MAO) and catechol-O-methyl transferase (COMT) activities although the afternoon injections led to a greater magnitude of change. Norepinephrine and epinephrine contents were significantly reduced by morning melatonin administration but were not changed by afternoon administration of the hormone. These data suggest that melatonin has direct and possibly indirect effects on adrenal medullary function in the hamster. However, further studies will be required to characterize the effects of melatonin on catecholamine synthesis and release and the physiological significance of these effects.

Gender differences in tuberoinfundibular dopaminergic neuronal activity in a photoperiodic rodent (Mesocricetus auratus)

When compared to males, female hamsters showed higher concentrations of dihydroxyphenylacetic acid (DOPAC) in the median eminence. This gender difference may be related to the higher levels of prolactin seen in females, but it is not associated with sex differences in median eminence dopamine or in the number of arcuate dopaminergic neurons.

Photoperiodic and pineal influences on estrogen-stimulated behaviors in female Syrian hamsters

Three experiments investigated the effects of short photoperiod exposure on the estrogenic facilitation of locomotor activity and lordosis. In Experiment 1, ovariectomized female hamsters were administered exogenous estrogen to stimulate locomotor activity in running wheels. Estrogen was effective in the long photoperiod group but did not stimulate running-wheel activity in the short photoperiod group. In Experiment 2, the role of the pineal gland in mediating photoperiodic influences on female hamster behavior was examined. Both estrogen-induced locomotor activity and estrogen+progesterone-stimulated lordosis behavior were significantly reduced in short photoperiod females. Both these photoperiodic effects were absent in pinealectomized hamsters. Sham-pinealectomized, short photoperiod females expressed behavioral deficits; pinealectomized hamsters in the short photoperiod did not. Experiment 3 investigated lordosis only and used hormone injections rather than silastic implants to administer estrogen. The photoperiodic and pineal effects observed in Experiment 2 were replicated in Experiment 3. Additionally, the suppression of lordosis responsiveness by short photoperiod exposure was estrogen dose dependent. Photoperiodic effects were present when 2 micrograms estradiol cypionate was used but absent when higher estrogen doses were used. These findings are discussed in the context of other results that suggested photoperiodic effects on hamster lordosis were pineal independent.

Prolactin-dependent seasonal changes in pelage: role of the pineal gland and dopamine

The Siberian hamster displays seasonal changes in pelage that are dependent upon fluctuations in circulating prolactin levels. Pinealectomy prevented the decrease in serum prolactin and molt to the winter pelage displayed by castrated males housed under a short-day photoperiod. A dopaminergic antagonist, pimozide, enhanced prolactin levels in both pinealectomized and sham-operated animals under both long and short photoperiods. In the short-day animals, this effect of pimozide was associated with a prevention of the development of winter pelage. These results indicate that seasonal prolactin levels and related pelage changes are dependent upon the integrity of the pineal gland. However, basal prolactin levels under different photoperiod conditions appear to be only partly regulated by the actions of the dopaminergic system.

Photoperiodic responsiveness of hamsters with lesions of the lateral geniculate nucleus is related to hippocampal damage

We tested the hypothesis that the geniculohypothalamic tract is important for hamster photoperiodism. Adult male hamsters, maintained in a long photoperiod (LD 14:10), received either large bilateral neurotoxic lesions of the lateral geniculate nucleus (LGN) or sham lesions. One week later, half of the animals from each group were transferred to a short photoperiod (LD 8:16) where they were maintained for 15 weeks. Most lesions effectively destroyed the intergeniculate leaflet (IGL) and much of the lateral geniculate complex. They also caused substantial damage to the overlying hippocampus. The lesions had no effect on long-day animals, but significantly reduced the extent of testicular regression during short photoperiod exposure. This effect, however, appeared to be the result of hippocampal, rather than geniculate, damage. Four individuals with complete IGL lesions, but minimal hippocampal damage, underwent a pattern of regression that was indistinguishable from controls. Body weight was increased by lesions in short-day, but not long-day, animals. This effect was not related to the extent of hippocampal damage. We conclude that geniculate input to the suprachiasmatic nuclei is not essential for hamster photoperiodism and that hippocampal damage may modify the effect of short daylengths on testes size.

Influence of neonatally-administered monosodium glutamate on the neuroendocrine regulation of prolactin cell activity in adult Syrian hamsters

The effects of MSG on PRL cell activity in hamsters was assessed by injecting either MSG (8 mg/g body weight) or saline into male and female hamsters on day 8 of the neonatal period. One-hundred and three days later, the anterior pituitaries were removed for the analysis of PRL synthesis and storage. Serum PRL levels were also determined by radioimmunoassay as an index of in vivo PRL secretion. Blood levels of PRL were decreased by 74% in female and 88% in male hamsters treated with MSG as compared with vehicle controls. In females, glutamate induced a drop in PRL storage (90%) and synthesis (82%) while in male animals glutamate caused a non-significant decrease in PRL storage (48%) and synthesis (31%) as compared with controls. These results indicate that MSG-induced arcuate nucleus lesions inhibit PRL cell activity in the hamster.

Prolactin messenger ribonucleic acid levels, prolactin synthesis, and radioimmunoassayable prolactin during the estrous cycle in the golden Syrian hamster

The purpose of this study was to observe the molecular dynamics of pituitary prolactin (PRL) gene expression during the estrous cycle of the Golden Syrian hamster. PRL messenger ribonucleic acid (mRNA) levels, PRL synthesis (3H-PRL in the incubation media or incubated pituitary after a 3 hr incubation with 3H-leucine), and radioimmunoassayable (RIA) PRL (in the incubation media or incubated pituitary after the 3 hr incubation) were measured in the morning (0930-1100 hr) on each day of the cycle. We observed that all of these PRL indices declined or did not change from Day 2 to Day 3 of the cycle. From Day 3 to Day 4 (proestrus), however, PRL mRNA levels increased 33-38% and media 3H-PRL increased 32-42%, while there were no significant changes in pituitary 3H-PRL, or RIA-PRL in the media or pituitary. From Day 4 to Day 1 (estrus) there was a reciprocal change in the levels of 3H-PRL in the pituitary vs. the media, with the former increasing 37-50% and the latter decreasing 25-32%. Pituitary RIA-PRL also increased 45-64% from Day 4 to Day 1 while media RIA-PRL did not change. These data are consistent with the following hypothesis: On the morning of proestrus (Day 4) in the hamster, PRL mRNA levels are elevated compared to those on Day 3, signaling an increase in PRL synthesis. This newly synthesized PRL is shunted into a "readily releasable" pool on the morning of Day 4 (contributing to the afternoon surge of serum PRL), and into a "preferentially stored" pool by the morning of Day 1 (for release in response to cervical stimulation and use as a luteotrophin to maintain early pregnancy should fertilization occur).

Photoperiodic modulation of sexual and aggressive behavior in female golden hamsters (Mesocricetus auratus): role of the pineal gland

Pinealectomized female hamsters (Mesocricetus auratus) housed in a short-day photoperiod were ovariectomized and tested for hormone-induced sexual receptivity in order to investigate the role of the pineal gland in the control of behavioral sensitivity to exogenous ovarian steroid hormones (Experiment 1). Behavioral sensitivity to hormones was further investigated in females maintained in a long-day photoperiod and rendered acyclic by daily administration of exogenous melatonin (Experiment 2). Female aggressive behavior was also monitored in all tests. Pinealectomy did not affect the reduced behavioral sensitivity to exogenous estrogen (E) induced by short days. These animals were also partially refractory to the effects of E when combined with low doses of progesterone. In addition, although melatonin administration mimicked the effects of short days on estrous cyclicity, the expression of hormone-dependent behaviors in these animals resembled the pattern displayed by control animals kept in long days. Thus, these findings suggest that the pineal gland plays a negligible role in the photoperiodic modulation of hormone-dependent sociosexual behaviors in female hamsters.

Pineal and ocular influences on male and female turkeys: plasma luteinizing hormone and prolactin levels during gonadal development

Plasma levels of prolactin (PRL) and luteinizing hormone (LH) were determined in pinealectomized (PX) and/or bilateral ocular enucleated (EX) turkey hens and toms during gonadal development. Measurements were obtained weekly for 6 weeks following photo-induced ovarian recrudescence in adult hens and biweekly from 12 to 34 weeks of age in sexually developing toms. Both hens and toms were maintained on a photoperiod of 16L:8D. Plasma PRL levels were significantly correlated with time (P less than 0.01) and were linear with significant slopes (P less than 0.01) in all treatment groups of hens and toms. However, the regression coefficients were positive for the hens and negative for the toms. Both PX and EX reduced PRL levels in the hens. PX did not significantly affect PRL levels of toms but EX depressed PRL levels. In hens LH was not correlated with weeks of photostimulation but in toms LH was positively correlated to age and linear with significant slopes in all treatment groups. There were no significant treatment effects on plasma LH levels of hens or toms. It was concluded that neither the pineal gland nor the eyes affect plasma LH levels of male turkeys during testicular development or female turkeys during photostimulated ovarian recrudescence. Although the eyes affected plasma PRL of both males and females during gonadal development, the pineal only influenced female PRL levels.

Sexually dimorphic effects of melatonin on prolactin cell function in male and female syrian hamsters

The purpose of this study was to determine and compare the effects of chronic melatonin treatment in vivo on several aspects of prolactin (PRL) cell activity (PRL synthesis, storage, and release) in vitro in male and female Syrian hamsters. Adult male and female hamsters were maintained on long photoperiod and treated with daily late afternoon injections of melatonin (25 micrograms) or vehicle for 11 weeks. Melatonin treatment resulted in an 85% and 65% decrease in serum PRL levels in male and female hamsters, respectively. There was a similar 79% and 64% decrease in PRL release in vitro from pituitaries of male and female hamsters respectively, treated with melatonin in vivo. Total stored PRL was 75% lower in male hamsters and 62% lower in female hamsters receiving melatonin. The synthesis of PRL by pituitaries from melatonin-treated male hamsters was reduced by 65%, whereas in melatonin-treated females it was decreased by 58%. Both nanomolar and micromolar doses of melatonin in vitro caused a modest but significant decrease (14-19%) in the amount of PRL stored in and released from normal male pituitaries without affecting synthesis. The inhibitory effects of melatonin on PRL cell function appear to be more pronounced in male than in female hamsters suggesting a sexually dimorphic response to this pineal hormone. While melatonin's PRL-inhibitory effects appear to be exerted primarily via indirect neuroendocrine mechanisms, a secondary component of its overall regulation of PRL processing may involve direct pituitary mechanisms as well.

Pineal and photoperiodic influences on fat deposition, pelage, and testicular activity in male meadow voles

Pinealectomy completely prevented gonadal regression as well as reduction in body weight and white adipose tissue content of the gonadal and retroperitoneal fat deposits in male meadow voles transferred from long to short day lengths. Pineal influences on pelage characteristics depended on which parameter was assessed. For instance, the increase in guard hair length observed in short-day control voles was blocked by pinealectomy; however, a similar increase in underhair length was unaffected by removal of the pineal gland. Photoperiod-dependent changes in fat deposition, testicular activity, and guard hair length presumably rely on altered pineal secretory activity to transduce the effects of day length on the neuroendocrine axis; however, mechanisms independent of pineal activity may be capable of mediating photoperiodic control of underhair growth.

Photoperiodic sensitivity of prepubertal female Fisher 344 rats

The laboratory rat is thought to be a poor model for study of the photoperiodic control of reproduction; however, this has only been investigated in a few rat strains. The purpose of the present investigation was to determine if the neuroendocrine-reproductive system of the Fisher 344 (F344) rat, an inbred strain, is sensitive to light deprivation. All treatments were performed on 28-day-old female F344 rats and the animals maintained for 8 weeks in a 14:10 light:dark cycle. Blinding resulted in a 65% (P less than 0.01) reduction in uterine weight and a 25% (P less than 0.01) decrease in ovarian weight. Accompanying these reductions in blinded animals were significant inhibitions of anterior pituitary weight, serum prolactin levels, and pituitary prolactin synthesis as measured in vitro. Pinealectomy of the blinded animals prevented all of these effects. Additionally, when olfactory bulbectomy, a procedure known to sensitize rats to the effects of photoperiod, was combined with blinding, the results did not differ significantly from that found with blinding alone. From these data we conclude that 1) the neuroendocrine-reproductive system of the prepubertal F344 female rat is highly sensitive to light deprivation; 2) light deprivation produces its antigonadotrophic effect through the pineal gland; and 3) olfactory bulbectomy does not potentiate the effects of blinding in the F344 rat.

Evidence for dopamine receptor-mediated inhibition of prolactin cell function in the female Syrian hamster

Since the exact nature of the hypothalamic regulation of prolactin (PRL) cell activity in the photosensitive female Syrian hamster is unknown, the present investigation was designed to determine whether dopamine (DA), a physiological PRL-inhibitory hormone in the rat, inhibits the synthesis and release of female hamster PRL in vitro via a DA receptor-mediated mechanism. Anterior pituitary glands from long photoperiod-exposed adult female Syrian hamsters were incubated in the presence of increasing concentrations of DA (5 nM, 500nM and 50 microM) in Kreb's Ringer Bicarbonate medium for 2 hours following a preincubation period of 1 hour in medium not containing DA. While PRL synthesis, as measured by the incorporation of 3H-leucine into PRL, was unaffected by DA, the release of immunoreactive (RIA)-PRL into the medium was inhibited by 55% and 53% by 500 nM and 50 microM DA, respectively; however, these same concentrations of DA inhibited the release of 3H-PRL into the medium by only 25% and 23%, respectively. The DA receptor blocker, pimozide (PIM) was effective in blocking the PRL-inhibitory effects of DA (500 nM) on both RIA- and 3H-PRL release in vitro. These are the first data suggesting that DA directly inhibits PRL release from female hamster anterior pituitary glands via a DA receptor-mediated mechanism.