Effects of Hypophysectomy on Galaninergic Neurons in the Rat Hypothalamus

Abstract

To understand better the relationship between hypothalamic galaninergic neurons and the pituitary gland, we studied the effects of hypophysectomy on hypothalamic galanin (GAL) content and distribution by radioimmunoassay and immunohistochemistry, and on GAL mRNA by Northern blot analysis. Three weeks after hypophysectomy, performed at 5 or 8 weeks of age, the hypothalamic concentrations of GAL and GAL mRNA were reduced by 30-50% in both male and female rats, compared to age- and sex-matched controls. Similar reverse-phase HPLC retention times of hypothalamic GAL were observed in intact and hypophysectomized rats. The reduction of hypothalamic GAL concentration following hypophysectomy was time-dependent, as peptide levels were unaffected one week after surgery. Immunohistochemistry showed regional differences in the effect of hypophysectomy on galaninergic neurons. In the hypophysiotropic hypothalamus, the scarce GAL immunoreactivity normally observed in the arcuate nuclei was no longer detectable in hypophysectomized rats, and the intense GAL immunoreactivity of the external zone of the median eminence progressively decreased and completely disappeared 3 and 6 weeks after hypophysectomy. In contrast, in the neurohypophyseal system, there was an increase of GAL labelling of the perikarya and emerging axons in the supraoptic and lateral-paraventricular nuclei, 1 and 3 weeks after hypophysectomy, that disappeared 6 weeks after hypophysectomy. An increase of GAL immunoreactivity was also observed in the internal zone of the median eminence 1 week but not 3 weeks after hypophysectomy. We conclude that hypophysectomy reduces the content of GAL and GAL mRNA in the rat hypothalamus. These changes are time-dependent and clearly detected after 3 weeks. The neurohypophyseal and hypophysiotropic galaninergic systems respond differently to hypophysectomy. In the neurohypophyseal system, the transient increase of GAL immunoreactivity could be related to axonal injury, diabetes insipidus or decreased neurosecretion. In contrast, the marked decrease of the GAL immunoreactivity found in the hypophysiotropic system, mainly in the external zone of the median eminence, accounts for the overall changes in hypothalamic peptide content and likely results from the loss of feedback up-regulation by pituitary or pituitary-dependent factors.

Pituitary-Dependent Hormonal Regulation of Galaninergic Neurons in the Rat Hypothalamus

“…As might be expected from this localization, GAL has been reported to regulate the secretion o f several anterior pitu itary hormones, stimulating, directly or indirectly, the release o f growth hormone (GH) [12,13], prolactin (PRL) [12] and luteinizing hormone [14], and inhibiting the release o f corticotropin [15], Since feedback regulatory loops are key features o f most endocrine systems, pituitary or pituitary-dependent hor mones could in turn regulate the synthesis and secretion of GAL by hypothalamic neurons. This hypothesis is support ed by our recent observation that hypophysectomy in male or female rats causes significant reductions o f hypothalam ic GAL and GAL mRNA concentrations [16], More pre cisely, both galaninergic networks are affected in a different way by pituitary removal. In the neurohypophyseal system, hypophysectomy is followed by a transient increase o f GAL peptide and mRNA contents in the SON and 1PVN [16][17][18].…”

“…This hypothesis is support ed by our recent observation that hypophysectomy in male or female rats causes significant reductions o f hypothalam ic GAL and GAL mRNA concentrations [16], More pre cisely, both galaninergic networks are affected in a different way by pituitary removal. In the neurohypophyseal system, hypophysectomy is followed by a transient increase o f GAL peptide and mRNA contents in the SON and 1PVN [16][17][18]. Inversely, in the hypophysiotropic sytem, there is a marked decrease o f GAL labeling in the AN and in the external zone o f ME [16], A decrease o f hypothalamic and ME GAL content has also been reported in thyroidectomized rats and was reversible after thyroid hormone replacement [19].…”

“…Parallelism was observed between the curves o f tracer displacement by serial dilutions of hypothalamic samples and by synthetic rat GAL. Reverse-phase HPLC showed that the GAL immunoreactivity present in our hypothalamic extracts of intact and hypophysecto mized rats coeluted with synthetic rat GAL-29 [16], Serum concentrations of rat GH were determined by RIA using the reagents generously provided by the NIDDK RPHDP. Results are expressed in terms of the RP-2 standard.…”

“…In the neurohypophyseal system, hypophysectomy is followed by a transient increase o f GAL peptide and mRNA contents in the SON and 1PVN [16][17][18]. Inversely, in the hypophysiotropic sytem, there is a marked decrease o f GAL labeling in the AN and in the external zone o f ME [16], A decrease o f hypothalamic and ME GAL content has also been reported in thyroidectomized rats and was reversible after thyroid hormone replacement [19]. Likewise, estrogens appear to regulate the expression o f GAL peptide and mRNA in a specific population of hypothalamic gonadotropin-releasing hor mone (GnRH)-producing neurons [20][21][22], The study o f hypothalamic galaninergic neurons in hypophysectomized rats receiving various hormonal treatments should provide additional clues to understand whether and how the pituitary modulates the activity o f hypothalamic GAL-producing neurons.…”

Thyroid Hormone-Dependent Regulation of Galanin Synthesis in Neurons and Glial Cells after Colchicine Administration

“…Galanin gene expression and peptide synthesis in vivo are also under thyroid hormone control in distinct brain regions and in the pituitary gland. Thus, galanin content dramatically decreases in the hypothalamus of adult thyroidectomized and hypophysectomized rats [23, 24, 25]. Its synthesis is almost completely suppressed in the paraventricular nucleus [26]and in the pituitary gland [23]after thyroidectomy, and it is restored by thyroxine treatment [23, 27].…”