Long-term reproductive cycles in rams after pinealectomy or superior cervical ganglionectomy

Lincoln, G. A.; Libre, Eric; Merriam, George R.

doi:10.1530/jrf.0.0850687

Cited by 50 publications

(34 citation statements)

References 17 publications

(16 reference statements)

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“…The testes regressed during long days and became maximally redeveloped during short days. This cycle is similar to that observed in normal rams exposed to this lighting regime (12,15).…”

Section: Testiculur Cyclesupporting

confidence: 86%

Luteinizing Hormone Responses to N‐Methyl‐D,L‐Aspartate During a Photoperiodically‐lnduced Reproductive Cycle in the Ram

Lincoln

1991

J Neuroendocrinology

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Aspartate and glutamate represent a major class of excitatory amino-acid neurotransmitters in the vertebrate brain, and stimulation of glutaminergic receptors by the specific agonist N-methyl-D,L-aspartate (NMDA) induces luteinizing hormone-releasing hormone (LHRH) and LH secretion. In this study, we used NMDA as a probe to investigate the hypothalamic regulation of LH secretion at different stages of a reproductive cycle in Soay rams induced by exposure to alternating 16-week periods of long and short days. NMDA (20 mg/kg) injected intravenously caused an acute increase in blood plasma concentrations of LH, with a significant increase within 2 to 4 min, and peak levels within 20 min. The magnitude of the response changed markedly during the cycle; it was greatest in the sexually inactive phase under long days (with a linear dose-response over the range 1 to 20 mgikg NMDA) and least in the sexually active phase under short days (only 20 mg/kg NMDA produced a small response). Pituitary responsiveness to a standard dose of LHRH (5.3 ng/kg iv) varied only slightly at the different stages and could not account for the observed changes in the LH response to NMDA. There were the expected changes in the endogenous pattern of LH secretion during the cycle, with high frequency LH pulses occurring during the sexually active phase. There was an inverse relationship between the frequency of LH pulses and the LH response to NMDA. Pretreatment of rams with an LHRH antagonist (36 pg/kg iv; Syntex, RS18286) totally blocked the LH response to both NMDA and LHRH, while in a separate study, treatment with dexamethasone (synthetic glucocorticoid, 133.4 pglkg iv) partially blocked the LH responses. The overall results indicate that NMDA stimulates LHRH secretion in the ram and the response is independent of the activation of the hypothalamo-pituitary-adrenal axis. The variation in the LH response during the reproductive cycle may reflect changes in the releasable stores of LHRH in the median eminence. These appear to be depleted in the sexually active phase, when LHRH is secreted in pulses at high frequency. The variation in the response to NMDA may also reflect changes in release of endogenous excitatory amino-acids, which act via NMDA receptors to form part of the physiological regulation of the reproductive cycle in the ram.The dicarboxylic amino-acids aspartate and glutamate represent a major class of excitatory neurotransmitters in the vertebrate brain ( I ) . These amino-acids have excitatory effects through at least three classes of receptors of which the N-methyl-D,Laspartate (NMDA) receptor class is best characterized, and these receptors are apparently involved in the neural control of reproduction (2). Studies in the rat have shown that the administration of NMDA causes the release of luteinizing hormone (LH) from the anterior pituitary gland by inducing the secretion of LHreleasing hormone (LHRH) from the hypothalamus (3, 4), and there is evidence that both NMDA and non-NMDA excitatory amino-acid receptors are in...

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“…The testes regressed during long days and became maximally redeveloped during short days. This cycle is similar to that observed in normal rams exposed to this lighting regime (12,15).…”

Section: Testiculur Cyclesupporting

confidence: 86%

Luteinizing Hormone Responses to N‐Methyl‐D,L‐Aspartate During a Photoperiodically‐lnduced Reproductive Cycle in the Ram

Lincoln

1991

J Neuroendocrinology

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“…A phenomenon which may or may not be related is seen in rams in which melatonin secretion has been abolished by pinealectomy or cranial cervical ganglionectomy. Such animals come into breeding condition several weeks earlier than their intact controls (Lincoln et al, 1989). It is possible that the suspension of melatonin secretion caused by the continuous light of the arctic summer has a similar effect.…”

Section: Discussionmentioning

confidence: 99%

Relationship between Conception Date and Latitude in Muskoxen

Flood

Tedesco

1997

Ran

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It has been suggested that muskoxen calve earlier with increasing latitude but the available data do not seem to have been thoroughly analyzed. Therefore, estimated conception dates (K=657) from a wide range of latitudes were compared. The results indicate that conceptions occur about a month earlier in the arctic than at temperate latitudes. This conclusion is consistent with reported observations of mating behaviour and calving season.

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“…Whereas there is a high degree of redundancy built into circadian entrainment pathways, it appears that melatonin is both necessary and sufficient for entrainment of seasonal photoperiodic phenomena to the annual cycle of daylength. Removal of the pineal gland or ablation of circadian input to the pineal abolish both the normal pattern of melatonin synthesis and seasonal responses (Lincoln et al 1989). Conversely, exogenous melatonin delivered by injection or infusion can mimic the seasonal effects of changing photoperiod (Elliott et al 1989, Maywood et al 1990, Bartness et al 1993, Tamarkin et al 1977.…”

Section: Melatonin Synthesis and Physiological Rolesmentioning

confidence: 99%

What is the role of melatonin within the anterior pituitary?

Hazlerigg

2001

Journal of Endocrinology

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The pineal hormone, melatonin, is uniquely defined by its role as hormonal time, but the processes whereby cells extract temporal information from the melatonin signal are not understood. Melatonin receptors are expressed in the pars tuberalis (PT) and, during fetal and perinatal life, in the pars distalis (PD). Functional studies suggest that the PT mediates the seasonal effects of melatonin on prolactin secretion, whilst the PD may be involved in photoperiodic programming of the developing gonadotrophic axis. To understand these effects at the cellular level we need to know the phenotype of melatonin-responsive cells. This review summarises current understanding in this area, and highlights present shortcomings. A case is presented for exploring the hypothesis that there is a functional association between melatonin receptor expression and cell differentiation in the anterior pituitary.

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Long-term reproductive cycles in rams after pinealectomy or superior cervical ganglionectomy

Cited by 50 publications

References 17 publications

Luteinizing Hormone Responses to N‐Methyl‐D,L‐Aspartate During a Photoperiodically‐lnduced Reproductive Cycle in the Ram

Luteinizing Hormone Responses to N‐Methyl‐D,L‐Aspartate During a Photoperiodically‐lnduced Reproductive Cycle in the Ram

Relationship between Conception Date and Latitude in Muskoxen

What is the role of melatonin within the anterior pituitary?

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