Craig A. Johnston

Effect of Acute Ether Stress on Monoamine Metabolism in Median Eminence and Discrete Hypothalamic Nuclei of the Rat Brain and on Anterior Pituitary Hormone Secretion

This study was designed to correlate the endocrine responses elicited by acute ether stress with the changes in metabolism of several monoamines in discrete nuclei of the rat brain. Concentrations of norepinephrine (NE), dopamine (DA), and 5-hydroxytryptamine (5-HT) and also of the specific metabolites of NE, DA, and 5-HT, 3-methoxy-4-hydroxyphenylethylene glycol, 3,4-dihydroxyphenylacetic acid, and 5-hydroxyindole-3-acetic acid, respectively, were concurrently measured in microdissected nuclei using high-performance liquid chromatography with electrochemical detection. The ratio of the metabolites to their respective amines was used as an estimate of the metabolism of NE, DA, and 5-HT. Acute exposure to ether vapors induced, within 5-15 min, large increments in plasma levels of adrenocorticotropic hormone (ACTH), beta-endorphin, and prolactin (PRL), and decrements in the levels of plasma growth hormone (GH). Significant increases in NE metabolism were observed in the rostral (ANr) and caudal (ANc) divisions of the arcuate nucleus, as well as in the paraventricular (PVN) and dorsomedial nuclei, 15 min after ether stress. A significant decrease in 5-HT metabolism was observed in the PVN, supraoptic nucleus, and ANc, whereas significant increases in 5-HT metabolism were detected in the suprachiasmatic nucleus and ANr. DA metabolism selectively increased in the ANr. The present results indicate that the acute changes in ACTH, beta-endorphin, PRL, and GH release induced by ether exposure are temporally correlated with increases in NE metabolism in many hypothalamic nuclei; a selective increase in DA metabolism restricted to the ANr, and differential effects on 5-HT metabolism, probably reflecting selective activation or inhibition of different populations of 5-HT neurons.

Physiological Role of Peptides and Amines on the Regulation of ACTH Secretion

Neural mechanisms regulating ACTH and @-endorphin release represent one of the more complex neuroendocrine regulatory system(s). Several hypothalamic peptides have been identified and determined to have clear-cut stimulatory effects on the release of ACTH from the anterior pituitary (AP).’” Corticotropin releasing factor (CRF), arginine vasopressin (AVP), oxytocin (OT), and angiotensin II (All) are among the most relevant neural peptides involved in the release of the proopiomelanocortin (P0MC)-derived peptides from the AP. The recently isolated C R F is by far the most potent releaser of ACTH, both in vivo and in vitraM The following order of potency of corticotropin releasing activity (CRA) was determined6 in vitro; CRF>AVP>OT>AII. In vivo, the potency of C R F is also the highest, but the potency If other peptides is more difficult to evaluate since in addition to the direct ACTH-releasing activity at the pituitary level, certain peptides may enhance the release of C R F and/or AVP. This indeed has been shown to be the case for AII, which although a weak ACTH secretagogue in vitro, can effectively stimulate ACTH release in v ~ v o . ~ ~ In addition to the peptidergic systems mentioned above, central aminergic systems are also known to be involved in the regulation of AP release of ACTH,” primarily by modulating the release of hypothalamic peptide hormones with intrinsic CRA. Attention has been focused on direct pituitary effects of some aminergic neurotransmitters. Epinephrine (EPI)”,12 and serotonin (5-HT)I3 enhance release of ACTH in vitro in a concentration-dependent fashion. Norepinephrine has been reported to have both inhibitory and facilitatory a c t i o n ~ ’ ~ . ~ ’ on ACTH release. Dopamine seems to have primarily inhibitory effects on ACTH secretion. Lastly, the monoamine, acetylcholine, is acknowledged to be stimulatory to ACTH The large number of peptidergic and aminergic neurotransmitters with direct or indirect ACTH-releasing activity suggests a multifactorial regulation of ACTH secretion. Unraveling the complex interactions between the different elements and the relative contribution of each factor to the overall secretory response may prove to bc a formidable task. Nevertheless, an evaluation of the possible physiological role of each substance with CRA may help understand the actions and interactions of the different agents in the regulation of ACTH. In the pages below, we shall discuss the available information concerning the physiological role of CRF, AVP, All , EPI, opioid peptides, and 5-HT on ACTH regulation.

Effect of ventral noradrenergic bundle lesions on concentrations of monoamine neurotransmitters and metabolites in several discrete areas of the rat brain.

Summary1.The effects of radiofrequency lesions of the ventral noradrenergic bundle (VNB) on monoamine and metabolite concentrations in several discrete areas of the rat hypothalamus were examined. Monoamines and metabolites were analyzed utilizing high-performance liquid chromatography coupled with electrochemical detection.2.VNB lesions decreased the concentrations of norepinephrine (NE) and 3-methoxy-4-hydroxyphenylethylene glycol in all areas examined except in the ventromedial nucleus (VMN). Dopamine and 3,4-dihydroxyphenylacetic acid concentrations were selectively decreased in the dorsomedial nucleus (DMN) and also slightly decreased in the medial forebrain bundle following VNB lesions. Serotonin and 5-hydroxyindole-3-acetic acid concentrations were not altered by VNB lesion in any area examined.3.The results indicate that the NE innervation to the hypothalamus is extensive and that NE in the VMN may not be derived from the VNB. The source of the DA innervation to the DMN may be located in or pass through the area affected by the VNB lesion.

Neurotoxin effects on oxytocin, vasopressin and somatostatin in discrete rat brain areas.

Monosodium-L-Glutamate (MSG) produces lesions to monoaminergic and peptidergic neurons in several brain areas. The present study examined the effect of neonatal MSG treatment on oxytocin (OXY), arginine-vasopressin (AVP) and somatostatin (SRIF) concentrations in several discrete brain areas of adult rats. OXY increased in the suprachiasmatic and arcuate nuclei and median eminence (ME) and decreased in the paraventricular nucleus of MSG-treated rats. MSG treatment caused AVP to increase in the arcuate nucleus and ME and decrease in the supraoptic nucleus. SRIF decreased following neonatal MSG treatment in both the ME and neurointermediate pituitary lobe. The results demonstrate that the effects of neonatal MSG treatment on neuropeptide content are not just limited to the arcuate nucleus. Furthermore, taken together with previous results, the data suggest that these changes may be indicative of functional deficits in the neuronal activity of some of these peptidergic neurons which, in turn, may be responsible for the abnormal secretion of several pituitary hormones observed in MSG-treated animals.

Differential effect of neurointermediate lobectomy on central oxytocin and vasopressin

Removal of the neurointermediate pituitary (NIL) affects the secretion of anterior pituitary (AP) hormones. It is not known if these effects are due to changes in central neuropeptide neurons. Two neuropeptides implicated in the neuroendocrine regulation of AP hormone secretion, and which are located in the NIL, are oxytocin and vasopressin. The present study evaluated whether removal of the NIL affected oxytocin and vasopressin concentrations in discrete brain areas containing cell bodies (paraventricular nucleus), fibers (arcuate nucleus), and/or terminals (median eminence) of these central neurons. Adult male rats underwent removal (NIL-X) or visualization (SHAM controls) of the NIL using a parapharyngeal approach. Oxytocin levels increased in the paraventricular nucleus and median eminence following NIL-X, whereas vasopressin concentrations were relatively unaffected by NIL-X. The data suggest that at least part of the influence of the NIL on AP hormone secretion may result from the ability of feedback from the NIL to differentially affect central neuropeptide neurons.

Orchidectomy Induces Temporal and Regional Changes in the Synthesis and Processing of the LHRH Prohormone in the Rat Brain

Recently, the sequence of the cDNA which encodes the LHRH-prohormone was elucidated from human placenta and human and rat hypothalamus and the corresponding amino acid sequence deduced (Seeburg et al., 1984 and Adelman et al., 1986). In addition to LHRH, the prohormone contains a 56 amino acid sequence, designated gonadotropin-releasing hormone associated peptide (GAP), which is attached to the C-terminus of the LHRH decapep-tide. Although not yet confirmed, the human GAP sequence has been reported to possess both gonadotropin-releasing and prolactin inhibiting activity (Nikolics et al., 1985). Additionally, a 13-amino acid fragment of the human GAP sequence (proLHRH 14–26) has been reported to stimulate gonadotropin release (Millar et al., 1986). Regardless of whether the non-LHRH portion of the LHRH prohormone contains biological activity, this sequence can serve as a valuable marker for studies of LHRH prohormone synthesis, processing and degradation. In order to initiate these types of studies, we have generated specific antisera (MC-1, 2 and 3) against a fragment of the human GAP sequence (proLHRH 38–66) and developed a radioimmunoassay procedure for the quantitation of GAP (Culler and Negro-Vilar, 1986). The antisera are specific for midportion sequences of the GAP molecule and do not cross-react with any other known brain peptide.