Photoperiodic Control of Oestrous Cycles in Syrian Hamsters: Mediation by the Mediobasal Hypothalamus
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Journal of Neuroendocrinology, 2002, Vol. 14, 294–299 Photoperiodic Control of Oestrous Cycles in Syrian Hamsters: Mediation by the Mediobasal Hypothalamus D. Lewis,* D. A. Freeman,* J. Dark,* K. E. Wynne-Edwards‡ and I. Zucker*† Departments of *Psychology and †Integrative Biology, University of California, Berkeley, CA, USA. ‡Department of Biology, Queen’s University, Kingston, ON, Canada. Key words: melatonin, prolactin, refractoriness, pars tuberalis, reproduction. Abstract To assess whether the mediobasal hypothalamus (MBH) is necessary for photoperiodic control of oestrous cycles and prolactin secretion, we tested intact female Syrian hamsters (controls) and those that had sustained unilateral or bilateral lesions of the MBH. All hamsters displayed 4-day oestrous cycles postoperatively in the long-day photoperiod (14 h light/day); control females and those with unilateral MBH damage ceased to undergo oestrous cycles approximately 8 weeks after transfer to a short-day photocycle (10 h light/day), whereas 12 of 15 females with bilateral MBH lesions continued to generate 4-day oestrous cycles throughout 22 weeks in short days. Serum prolactin concentrations were either undetectable or low in all hamsters 8 or 14 weeks after the transfer to short-day lengths, but increased above long-day baseline values by week 22. We conclude that melatonin-binding sites in the MBH mediate suppression of oestrous cycles but not prolactin secretion by short-day lengths; recovery of prolactin secretion in females during prolonged exposure to short-day lengths reflects development of refractoriness to melatonin in a substrate distinct from the MBH. These findings suggest that separate neural pathways mediate photoperiodic control of gonadotropin and prolactin secretion in female hamsters. Synchronization of seasonal reproductive rhythms by day tissue of male hamsters; animals with bilateral lesions of this length is well documented for male and female mammals (1). brain region (MBHx) failed to undergo testicular involution Day length information is transduced in the nervous system when challenged with short-day lengths or long-duration by melatonin acting on several localized target tissues (2). The melatonin infusions, each of which induced testicular regres- duration of nocturnal melatonin secretion is highly correlated sion in neurologically intact controls. Both treatments, with the length of the night (3, 4), and determines whether however, did depress prolactin concentrations in MBHx animals adopt the winter or summer phenotype. In Syrian males. The authors concluded that ‘an intact MBH is essen- hamsters, elevated melatonin secretion for >8 h per night pro- tial for melatonin to exert its photoperiodic control over motes gonadal regression, whereas melatonin durations
Mediobasal hypothalamus and photoperiodism 295
PVN most likely interfere with photoperiodic control of female variance of 27.0%. All determinations falling below the lowest standard were
rounded up to that limiting concentration of 0.80 ng/ml before analyses. The
reproduction by eliminating or degrading pineal secretory
two determinations which fell above the highest standard were rounded down
activity (17) rather than by influencing target tissues that to a concentration of 400 ng/ml. Duplicate determinations were averaged to
decode the duration of nightly melatonin signals. yield a sample prolactin concentration in ng/ml.
The aim of the present study was to determine whether
the MBH structures that mediate photoperiodic control of Surgical procedures
reproduction in male hamsters perform a similar role in Brain lesions
females. Specifically, we tested the hypothesis that the Surgery was performed under deep anaesthesia induced by a ketamine cocktail
(21 mg ketamine, 2.4 mg xylazine and 0.3 mg acepromazine/ml injected ip in
suppression of oestrous cycles and prolactin secretion by a dose of 0.34 ml per 100 g body mass) and supplemented as needed with
short-day lengths would not occur in females lacking isoflurane vapors. Lesions aimed at the MBH were made using a Radionics
melatonin target tissues in the MBH. Model RFG-4 A Research RF Lesion Generator system (coordinates: 1.4 mm
anterior to bregma, 0.4 mm lateral to midline and 7.4 mm below dura, with
incisor bar set 5 mm above the interaural line) (Radionics, Burlington,
Methods MA, USA). Current was delivered with an electrode tip temperature of 80 uC
for 15 s per lesion. In one group of animals, the lesions were placed bilaterally
Young adult female Syrian hamsters (Mesocricetus auratus; Hsd:Han:AURA) and in another unilaterally. During sham operations, the electrode was
were obtained from Harlan Sprague-Dawley (Madison, WI, USA) and lowered to a depth of 0.3 mm above the MBH and no current was passed.
maintained in our laboratory in a 14 : 10 light : dark cycle (14 h light/day; lights At the end of the experiment, hamsters were administered a lethal dose of
on at 03.30 h). Food and water were available ad libitum and room temperature pentobarbital sodium and perfused transcardially with 0.9% NaCl (950 ml)
was maintained at 22t2 uC. Animals were housed individually in poly- followed by 10% formalin in phosphate-buffered saline (50 ml). Brains were
propylene cages on pine shavings. After an acclimation interval of 24 days, removed and transferred to 50 ml of a 15% sucrose/10% formalin/phosphate-
oestrous cycles were monitored for each female by daily visual inspection of the buffered saline solution overnight, sliced on a freezing microtome at 40 mm,
vagina. The interval between successive appearances of stringy postestrous stained with cresyl violet and examined microscopically by two individuals
vaginal discharge (18) was used to determine oestrous cycle length. After who were unaware of the oestrous cycle or prolactin data. All procedures were
hamsters had displayed several consecutive 4-day oestrous cycles, they were approved by the Animal Care and Use Committee of the University of
assigned to one of the surgical treatment conditions described below. After California at Berkeley.
approximately 3 weeks of postoperative testing in long days, they were
transferred to an 8 : 16 light : dark cycle (lights on at 09.30 h) for the remainder
Statistical analysis
of the study.
The proportion of animals that continued to undergo cycles in the 10 h light
cycle was compared by Fisher’s exact probability test or chi-square test where
Blood sampling appropriate. Changes in serum prolactin concentrations within groups over
1.0 ml of blood was withdrawn between 13.00 h and 15.00 h from the retro- time were analysed by paired t-tests (two-tailed) and serum prolactin con-
orbital sinus of each hamster after 8, 14 and 22 weeks of short-day treatment, centrations between groups at the different time points were analysed by t-tests
respectively, and assayed for serum concentrations of prolactin. In each case, for independent samples. Oestrous cycle measures were analysed by one-way
the hamster was anaesthetized with isoflurane vapors and blood was left to clot ANOVA using Statview 5 (SAS Institute Inc., Cary, NC, USA). Serum prolactin
overnight at 4 uC, centrifuged at 3500 r.p.m. for 20 min and serum samples concentrations as a function of stage of the oestrous cycle were analysed via
stored at x80 uC for subsequent radioimmunoassay. Baseline long-day pro- regression analysis. P296 Mediobasal hypothalamus and photoperiodism Oestrous cycle data are summarized in Table 1. Control in the unilateral MBHx than control females (P
Mediobasal hypothalamus and photoperiodism 297
70 females housed in the short photoperiod. These results suggest
c that photoperiodic control of follicle-stimulating hormone
60 (FSH) and luteinizing hormone (LH) secretion on the one
hand and prolactin on the other, are mediated by separate
neuroendocrine substrates. This confirms and extends findings
Serum prolactin (ng/ml)
50
on male Syrian hamsters (10, 11) and also supports the con-
clusion of Lincoln and Richardson (12), based on studies of
40 sheep, that the melatonin signal that encodes day length acts
in the hypothalamus to regulate the gonadotropin-gonadal
30 axis and to influence prolactin secretion within the pars
tuberalis of the pituitary gland. In rodents, the hypothalamus
20 a also has been implicated in photoperiodic control of prolactin
secretion; microdialysis of melatonin in a short-day pattern
to the suprachiasmatic nucleus (SCN) suppressed prolactin
10
concentrations of juvenile Siberian hamsters (2). In
b Siberian hamsters, SCN is both a termination point of the
0 retinohypothalamic tract that communicates day length
0 14 22
information to the endocrine system (23, 24) and contains
Weeks in short day lengths
high concentrations of melatonin-binding sites (25), but see
FIG. 2. Mean (tSEM) serum prolactin concentrations for five control also (26).
females during testing in the long-day photoperiod (week 0) and after The persistence of oestrous cycles in short-day hamsters
14 and 22 weeks in short day lengths. Time points with different letters with MBH lesions may be due to the elimination of melatonin
differ significantly (P298 Mediobasal hypothalamus and photoperiodism
concentrations (30), which override the effects of melatonin their oestrous cycles were less severely disrupted than those of
on gonadotropin secretion. Such a mechanism is unlikely to intact females. The location and extent of tissue damage in
account for continuation of reproduction in female MBHx these MBHx females did not differ in any obvious manner
hamsters; oestrous cycles are contingent on precise timing from that sustained by hamsters which continued to display
mechanisms and an LH surge every fourth day, and are oestrous cycles in short days. Variable outcomes after seem-
not compatible with chronically elevated gonadotropin ingly comparable neural insults are commonplace (34), and
concentrations. not presently understood.
The mediobasal hypothalamus is extensively connected In summary, the MBH is an essential component of the
to other medial hypothalamic areas with high densities of neural system by which day length controls gonadotropin
melatonin-binding sites (e.g. SCN) (10). The possibility that secretion necessary for generation of oestrous cycles, but
day length controls gonadotropin secretion via projections is not implicated in photoperiodic control of prolactin
from the DMH to the SCN is not supported by the observa- secretion.
tion that male SCNx Syrian hamsters undergo gonadal
involution during treatments with melatonin (17, 31), whereas
their MBHx counterparts do not (10, 11). In females, how-
ever, the issue remains unresolved and difficult to test Acknowledgements
because ablation of the SCN induces persistent vaginal and This research was supported by Grant MH-61171 and MH-11655 from the
behavioural oestrus (32). NIMH, Grant NS-30816 from the NIH and a grant from the Natural Sciences
and Engineering Research Council of Canada.
The respective contributions of MBH cell bodies versus
fibres of passage to photoperiodic control of oestrus also
remains unknown. The importance of cells bodies is suggested Accepted 20 December 2001
by the observation that microimplants of melatonin in
the MBH of ewes lead to increases in LH secretion, whereas
implants in other brain regions are ineffective (13). References
Serum prolactin concentrations of control females were
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