Tuberoinfundibular dopaminergic neurons and lactotropes in young and old female rats

Impact of very old age on hypothalamic dopaminergic neurons in the female rat: a morphometric study

Dopaminergic neurons of the A(12) (tuberoinfundibular dopaminergic system) and A(14) (periventricular dopaminergic system) hypothalamic areas exert a tonic inhibitory control of prolactin secretion. Tuberoinfundibular dopaminergic system neuron function is known to decline during aging in rats, but little is known about the impact of extreme age on neuron number and morphology in the two systems. We morphometrically assessed the neurons of the tuberoinfundibular dopaminergic system and the periventricular dopaminergic system in female rats 6 (young, Y), 24 (old, O), and 30-32 (senescent, S) months old. Serial coronal sections of fixed hypothalami were immunohistochemically labeled for tyrosine hydroxylase, and immunoreactive perikarya from the A(12) and A(14) areas were quantitatively characterized and compared among the three age groups. Radioimmunoassay was used to measure serum prolactin. The number of A(12) tyrosine hydroxylase-immunoreactive perikarya showed a steady decline with age, whereas the number of A(14) tyrosine hydroxylase-immunoreactive perikarya remained stable from young to old age but showed a sharp drop in the senescent rats. In the old rats, tyrosine hydroxylase-immunoreactive neuronal area (A(12) = 135.37 and A(14) = 158.79 microm(2)) was significantly higher than that of young (A(12) = 72.56 and A(14) = 99.7 microm(2)) and senescent animals (A(12) = 95.5 and A(14) = 106.5 microm(2)). Densitometric assessment of median eminence tyrosine hydroxylase immunoreactivity revealed a steady age-related reduction of tyrosine hydroxylase content in the median eminence. Serum prolactin levels increased steadily with age. We conclude that, in the female rat, aging brings about a progressive loss of both tuberoinfundibular dopaminergic system and periventricular dopaminergic system neurons, which becomes more conspicuous at extreme ages.

Effect of age on immunocytochemical staining characteristics of adenohypophyseal cells in Mongolian pony mares and stallions

OBJECTIVE

To determine the effect of age on immunocytochemical staining characteristics of adenohypophyseal cells of Mongolian pony mares and stallions.

ANIMALS

35 Mongolian ponies.

PROCEDURE

Pituitary glands from 1- to 22-year-old horses of both sexes were collected at a commercial slaughterhouse in China and allocated into 7 groups according to age and sex: prepubertal stallions (n = 5; 1 to 2 years old), young stallions (6; 3 to 7 years old), middle-aged stallions (4; 10 to 12 years old), old stallions (5; 15 to 22 years old), young mares (3; 3 to 7 years old), middle-aged mares (5; 10 to 12 years old), and old mares (7; 15 to 22 years old). Pituitary glands were processed for microscopy, and sections were immunocytochemically stained for various pituitary hormones. The percentages and cell areas of 6 types of adenohypophyseal cells were determined by use of morphometry.

RESULTS

Age-related alterations in adenohypophyseal cells were detected in horses > 15 years old. Percentage of somatotrophs decreased with age regardless of sex, and percentage of lactotrophs increased with age in stallions. In old mares, percentage of follicle-stimulating hormone cells increased, whereas cell area of follicle-stimulating hormone and luteinizing hormone cells decreased. Differences between sexes for percentage of somatotrophs and lactotrophs, and cell area of follicle-stimulating hormone and luteinizing hormone cells were not evident in old horses.

CONCLUSIONS AND CLINICAL RELEVANCE

Differences among percentages and cell areas of pituitary cell types in old horses may be associated with degeneration of the sex organs.

Luteinizing hormone-releasing hormone (LHRH) neurons maintained in hypothalamic slice explant cultures exhibit a rapid LHRH mRNA turnover rate

Evidence indicates that neuropeptide gene expression is tightly coupled to biosynthesis and secretion. Moreover, rhythmic gene expression often accompanies rhythmic secretion. Luteinizing hormone-releasing hormone (LHRH) neurosecretion, which regulates gonadal function, is pulsatile, with interpulse intervals of approximately 1 hr and pulse decays of <30 min in rats. As a basis for a rapid fall in peptide secretion, we hypothesize that LHRH mRNA levels rapidly decay. To address this hypothesis, we examined LHRH mRNA turnover in primary postnatal LHRH neurons maintained in long-term hypothalamic/preoptic area slice explant cultures, using in situ hybridization histochemistry (ISHH). Relative LHRH mRNA content per cell was quantitated by single-cell analysis after transcription inhibition with 5, 6-dichloro-1-D-ribofuranosyl-benzimidazole (DRB) or actinomycin D. Cultures were maintained in serum-free medium with tetrodotoxin to suppress spontaneous electrical activity and hence assess only intrinsic cellular activity. A plot of LHRH mRNA level per cell versus DRB treatment time showed a rapid initial decay of LHRH mRNA (t1/2, 5-13 min), followed by a slower decay rate (t1/2, 329-344 hr). LHRH cell number after drug treatment as determined by immunocytochemistry did not change. Comparison of mammalian LHRH mRNA 3'-untranslated regions showed two conserved regions. These data indicate that, in primary LHRH neurons, LHRH mRNA has an intrinsically high rate of turnover and a mRNA stabilization component. Foremost, decay of LHRH mRNA, the fastest reported for a neuropeptide to date, corresponds to the decay of LHRH peptide pulses.

Sexual dimorphism in the age changes of the pituitary lactotrophs in rats

It is known that aging is associated with alterations in hypothalamic and pituitary functions. In the present study, we have undertaken a quantitative immunohistochemical assessment of the lactotroph cell population as well as prolactin (PRL) secretion, in male and female rats of different ages. Pituitaries from young (3 months), old (20 months) and senescent (29 months) male and female Sprague-Dawley rats were processed for the immunohistochemical detection of lactotrophs. Serum PRL was measured by a homologous RIA. Additionally, the in vitro PRL secretory activity was estimated by perifusion of pituitary cells from senescent animals. Analysis of morphometric parameters revealed age-related changes of PRL cell population in animals of both sexes. The cell density (CD), surface density (SD) and volume density (VD) decreased with age in both male and female rats. However, CD as well as SD appeared to have increased in females when compared to males, either in young or old animals, while VD was higher only in old females. The pituitaries of senescent females displayed chromophobic microadenomas on a background of diffuse PRL cell hyperplasia. Prolactin serum levels showed a marked increase with age in females, but only a modest elevation in males. In senescent females, PRL production per cell was reduced. We conclude that in rats, there exists a clear sexual dimorphism in the age-related changes of pituitary PRL cells.