Subcellular localization of hypothalamic luteinizing hormone

Luteinizing hormone receptor mediates neuronal pregnenolone production via up-regulation of steroidogenic acute regulatory protein expression

The functional consequences of luteinizing hormone/human chorionic gonadotropin signaling via neuronal luteinizing hormone/human chorionic gonadotropin receptors expressed throughout the brain remain unclear. A primary function of luteinizing hormone (LH) in the gonads is the stimulation of sex steroid production. As LH can cross the blood-brain barrier, present in cerebrospinal fluid and is expressed by neuronal cells, we tested whether LH might also modulate steroid synthesis in the brain. Treatment of differentiated rat primary hippocampal neurons and human M17 neuroblastoma cells with LH (100 mIU/mL) resulted in a twofold increase in pregnenolone secretion in both cell types, suggesting an increase in P450scc-mediated cleavage of cholesterol to pregnenolone and its secretion from neurons. To explore how LH might regulate the synthesis of pregnenolone, the precursor for steroid synthesis, we treated rat primary hippocampal neurons with LH (0, 10 and 100 mIU/mL) and measured changes in the expression of LH receptor and steroidogenic acute regulatory protein (StAR). LH induced a rapid (within 30 min) increase in the expression of StAR, but induced a dose-dependent decrease in LH receptor expression. Consistent with these results, the suppression of serum LH in young rats treated with leuprolide acetate for 4 months down-regulated StAR expression, but increased LH receptor expression in the brain. Taken together, these results indicate that LH induces neuronal pregnenolone production by modulating the expression of the LH receptor, increasing mitochondrial cholesterol transport and increasing P450scc-mediated cleavage of cholesterol for pregnenolone synthesis and secretion.

Extrahypothalamic brain prolactin: characterization and evidence for independence from pituitary prolactin

Prompted by reports of immunohistochemical localization of a prolactin-like immunoreactivity (PLI) within the rat brain, a study was undertaken to define the immunologic and biologic characteristics of this material in extrahypothalamic regions of the rat brain. Ninety-seven percent recovery of rat prolactin standard, added to homogenates of brain parts, insured that neuronal tissue did not interfere with the radioimmunoassay for rat prolactin. PLI was consistently found in the cerebellum, thalamus, brainstem (pons-medulla), hippocampus, cerebral cortex and caudate. Examination of the elution profile of each of the extrahypothalamic brain parts from Sephadex G-75 columns showed that, although a small amount of brain PLI elutes in the vicinity of the anterior pituitary prolactin marker, the bulk of brain-based PLI migrates with the void volume and as late eluting, low molecular weight material. While increasing amounts of brain extracts progressively displaced more 125I-prolactin from antibody binding, the displacement curve was not parallel to that produced by the addition of increasing amounts of anterior pituitary prolactin standards of rat origin. Extracts of various brain parts from hypophysectomized animals, analyzed for biologic activity in the Nb2 lymphoma cell assay, revealed prolactin-like bioactivity, but the bioactivity/immunoreactivity ratio for some of the brain parts was significantly lower than that for pituitary prolactin. Hypophysectomy, which led to the expected fall in serum prolactin to undetectable levels, and restraint stress, which resulted in a statistically significant 4-fold rise in serum prolactin, caused no change in prolactin concentrations in extrahypothalamic brain parts, indicating that brain PLI is regulated independently of pituitary prolactin and of circulating serum prolactin levels.

Subcellular distribution of hypothalamic prolactin-like immunoreactivity

Prompted by interest in immunohistochemical reports of prolactin-like immunoreactivity (PLI) in the rat hypothalamus, we investigated and have reported that an immunoreactive and bioactive prolactin-like material can be extracted from the rat hypothalamus. In the present communication the subcellular distribution of this protein is reported. Using a sensitive and specific radioimmunoassay for rat prolactin and a standardized procedure for subcellular fractionation of neuronal tissue, we have found that 90% of hypothalamic PLI is particulate-bound with only 10% remaining in the S4 or cytosolic fraction. Almost 80% of the particulate-bound PLI is found in the P2 fraction containing myelin, synaptosomes and mitochondria. When P2 is further fractioned on a discontinuous sucrose density gradient, approximately 66% of the P2-associated PLI was found in subfractions rich in synaptosomes and poor in myelin and mitochondria. Such findings support the probability that hypothalamic PLI functions trans-synaptically as a neuromodulator in the brain.

In vitro LH release from the hypothalamus-pars tuberalis; effects of gonadotropin-releasing hormone (GnRH), beta-endorphin and biogenic amines

Immunoreactive and bioactive luteinizing hormone (LH) has been shown to be widely distributed in the rodent central nervous system (CNS), particularly in the hypothalamus. Subcellular localization of this LH in fractions rich in synaptosomes and in vitro release by potassium-induced depolarization suggests that this peptide may act in trans-synaptic neuromodulatory roles. Furthermore, a variety of experiments have proved that this brain-based LH is not of pituitary origin. In the in vitro studies reported here characterization of brain-based LH release, in response to gonadotropin-releasing hormone (GnRH), beta-endorphin, and biogenic amines, was examined. Adult male rats were sacrificed by decapitation, hypothalami removed, quartered, and incubated in Krebs Ringer's Bicarbonate (KRB). High potassium concentration and GnRH induced release of LH from these hypothalamic explants in short-term culture and serotonin significantly inhibited release of LH from these explants. In contrast, however, beta-endorphin, norepinephrine, dopamine, and acetylcholine, agents known to modulate pituitary LH release, had no effect on the release of LH from hypothalamic tissues in vitro. Furthermore, beta-endorphin did not alter potassium-induced release of LH from the hypothalamus. Whereas there are some similarities between LH release from the pituitary and from the CNS, the differences reported here suggest that hypothalamic LH does not simply serve as a supplemental source for LH in the general circulation but more likely subserves an entirely different role(s) than its pituitary counterpart, presumably acting in a neuromodulatory fashion within the brain.