Pro-opiomelanocortin mRNA and peptide co-expression in the developing rat pituitary

Cell Type-Specific Sexual Dimorphism in Rat Pituitary Gene Expression During Maturation

The most obvious functional differences between mammalian males and females are related to the control of reproductive physiology and include patterns of GnRH and gonadotropin release, the timing of puberty, sexual and social behavior, and the regulation of food intake and body weight. Using the rat as the best-studied mammalian model for maturation, we examined the expression of major anterior pituitary genes in five secretory cell types of developing males and females. Corticotrophs show comparable Pomc profiles in both sexes, with the highest expression occurring during the infantile period. Somatotrophs and lactotrophs also exhibit no difference in Gh1 and Prl profiles during embryonic to juvenile age but show the amplification of Prl expression in females and Gh1 expression in males during peripubertal and postpubertal ages. Gonadotrophs exhibit highly synchronized Lhb, Fshb, Cga, and Gnrhr expression in both sexes, but the peak of expression occurs during the infantile period in females and at the end of the juvenile period in males. Thyrotrophs also show different developmental Tshb profiles, which are synchronized with the expression of gonadotroph genes in males but not in females. These results indicate the lack of influence of sex on Pomc expression and the presence of two patterns of sexual dimorphism in the expression of other pituitary genes: a time shift in the peak expression during postnatal development, most likely reflecting the perinatal sex-specific brain differentiation, and modulation of the amplitude of expression during late development, which is secondary to the establishment of the hypothalamic-pituitary-gonadal and -thyroid axes.

Impaired neonatal survival of pro-opiomelanocortin null mutants

Intercrosses of heterozygous pro-opiomelanocortin (POMC) mice result in homozygous null progeny at lower frequencies than expected. Genotyping offspring at pre-, peri-, and postnatal stages revealed that over half of homozygous null mutants die in the early postnatal stages. To investigate the reasons for this early postnatal lethality, we analyzed in detail different parameters in the initial hours after birth. POMC null mutants born to heterozygous dams presented at birth with corticosterone levels no different from wildtype littermates, were euglycemic, and had normal liver glycogen stores. However, already 30 min after birth corticosterone levels dropped by 80% and were undetectable thereafter, while corticosterone levels in wildtype animals increased during postnatal hours. Circulating adrenaline was almost below detection 1h after birth. Blood glucose levels fell sharply in all genotypes within 30 min after birth; however, wildtype and heterozygous pups overcame hypoglycemia within an hour, while mutant pups stayed hypoglycemic. The depletion of liver glycogen stores in mutant pups was significantly less efficient compared to their littermates in the hours after birth. POMC null mutant mice born to POMC null mutant dams completely lack corticosterone and die of the expected respiratory dysfunction. In contrast, POMC null mutant mice born to heterozygous dams do not die of respiratory problems, but rather due to hypoglycemia. Our studies confirm an essential involvement of POMC peptides and of adrenal glucocorticoids and catecholamines on glucose homeostasis critical for early postnatal survival.

Aversive stimulation during the stress-hyporesponsive period does not affect the number of corticotroph cells in neonatal male rats

Immunohistochemistry was used to evaluate the effects of neonatal handling and aversive stimulation during the first 10 days of life on the number of corticotrophs in the anterior lobe of the pituitary of 11-day-old male Wistar rats. Since adult rats handled during infancy respond with reduced corticosterone secretion in response to stressors and with less behavior inhibition in novel environments, we assumed that neonatal stimulation could affect pituitary morphology during this critical period of cell differentiation. Three groups of animals were studied: intact (no manipulation, N = 5), handled (N = 5) and stimulated (submitted to 3 different aversive stimuli, N = 5). The percentage of ACTH-immunoreactive cells in the anterior lobe of the pituitary (number of ACTH-stained cells divided by total number of cells) was determined by examining three slices per pituitary in which a minimum of 200 cells were counted by two independent researchers. Although animals during the neonatal period are less reactive to stress-like stimulation in terms of ACTH and corticosterone secretion, results showed that the relative number of ACTH-stained cells of neonatal handled (0.25 +/- 0.01) and aversive stimulated (0.29 +/- 0.03) rats was not significantly different from intact (0.30 +/- 0.03) animals. Neonatal stimulation may have a differential effect on the various subpopulations of corticotroph cells in the anterior pituitary.

Region- and stage-specific patterns of melanocortin receptor ontogeny in rat central nervous system, cranial nerve ganglia and sympathetic ganglia

Observations on developmental actions of melanotropic peptides in nervous system have been difficult to interpret in the absence of data on receptor ontogeny. We investigated binding of [125I]Nle4,D-Phe7-alpha-MSH ([125I]NDP) in developing Long Evans rats from gestational day (E) 13 by quantitative autoradiography. Regional [125I]NDP binding characteristics were assessed by competition experiments in early postnatal brain. The study revealed region- and stage-specific, often transient ontogenetic patterns. Sympathetic ganglia exhibit high [125I]NDP binding from E13, with a peak in superior cervical ganglion at E16-E18. The first central [125I]NDP binding sites transiently appear in parts of thalamus between E13 and E15. The early fetal period is characterized by prominent peaks of receptor density in somatosensory and viscerosensory nuclei (trigeminal sensory nuclei, solitary tract nucleus), paralleled by receptor expression in 5th, 7th, 9th and 10th cranial nerve ganglia. During late fetal life, receptor density peaks in dorsal motor nucleus of vagus and inferior olive; binding sites transiently appear in cerebellum. Caudate-putamen, nucleus accumbens, olfactory tubercle and septohippocampal nucleus show a high perinatal maximum. Starting with late fetal piriform cortex, [125I]NDP binding peaks sequentially in cerebral cortical areas, with highest levels in entorhinal cortex. Preoptic, septal, hypothalamic and amygdaloid areas known for elevated receptor densities in adulthood, exhibit a slow, peri- and postnatal receptor ontogeny. Temporal relations to regional developmental processes support the idea of a role of melanocortins during ontogeny.

Multifactorial regulation of the hypothalamic-pituitary-adrenal axis during development

The hypothalamic-pituitary-adrenal system shows an overall diminished responsiveness throughout ontogeny. Thus, during this period, the sensitivity of the adrenal gland to ACTH is markedly reduced. Furthermore, basal and stress-induced concentrations of corticosterone (CORT), ACTH and hypothalamic secretagogues remain at very low levels. Both structural immaturity and active inhibitory processes appear to underlie this overall hyporesponsiveness. The available data indicate that the characteristic developmental pattern of the HPA system results from multiple regulatory factors acting in conjunction at various levels of the axis. The primary rate-limiting steps, however, are probably at the brain and adrenal levels. The ultimate "goal" appears to be to keep CORT levels within the narrow range of concentrations required for normal development.