N-acetylation regulates the behavioral activity of alpha-melanotropin in a multineurotransmitter neuron

Suppression of basal and stress-induced prolactin release and stimulation of luteinizing hormone secretion by alpha-melanocyte-stimulating hormone

alpha-MSH and beta-endorphin, both synthesized from a common precursor, have opposite behavioral actions. In order to determine if these peptides have opposite effects on pituitary function, basal LH secretion and basal and stress-induced prolactin release were studied in adult male rats after intraventricular injection of alpha-MSH. Each rat also received intraventricular saline in order to serve as its own control. 18 micrograms alpha-MSH stimulated plasma LH from 16.5 +/- 2.5 (SEM) ng/ml to a peak of 27.2 +/- 4.0 and 26.0 +/- 4.9 ng/ml at 5 and 10 min, and suppressed prolactin from 3.5 +/- 0.7 ng/ml to 1.3 +/- 0.1 and 1.2 +/- 0.1 ng/ml at 15 and 30 min. Intraventricular alpha-MSH also significantly blunted the prolactin rise associated with the stress of swimming. 10 and 20 min after the onset of swimming, prolactin levels in rats pretreated with alpha-MSH were significantly diminished: 7.4 +/- 1.5 and 6.5 +/- 2.0 ng/ml vs 23.8 +/- 3.6 and 15.2 +/- 2.8 after normal saline. Similarly, des-acetyl alpha-MSH which is the predominant form of alpha-MSH in the hypothalamus, diminished the stress-induced prolactin rise from 18.4 +/- 5.3 and 11.2 +/- 3.4 ng/ml at 10 and 20 min to 10.0 +/- 2.4 and 5.5 +/- 1.6 ng/ml. We conclude that centrally administered alpha-MSH stimulates LH and suppresses basal and stress-induced prolactin release in male rats. These actions are opposite to those previously shown for beta-endorphin and suggest that alpha-MSH may antagonize the effects of beta-endorphin on pituitary function.

Induction of the intermediate pituitary by stress: synthesis and release of a nonopioid form of beta-endorphin

beta-Endorphin in the intermediate lobe of the pituitary gland is posttranslationally modified to produce opioid inactive peptides. Whether these are metabolites or biologically relevant products has not been known. It was found that repeated stress induces increased biosynthesis and release of beta-endorphin-like substances from the intermediate lobe of rats and that opioid-inactive N-acetylated beta-endorphin-(1-31) is selectively made and liberated. The possible role of this nonopioid product and the selective release of peptide forms are discussed.

Proopiomelanocortin peptide immunocytochemistry in rhesus monkey brain

The immunocytochemical distribution of proopiomelanocortin (POMC) peptides (beta-endorphin, ACTH, alpha-MSH, 16K fragment) was studied in the brain of the rhesus monkey (Macaca mulatta). Some animals were administered colchicine intracerebroventricularly prior to sacrifice to enhance the visualization of perikaryal immunoreactivity. Immunoreactive perikarya are localized to hypothalamic infundibular nucleus, giving rise to several distinct projections. Rostral projections extend through midline diencephalic and preoptic areas, and enter the telencephalon. Along this course, immunoreactive fibers are seen in midline hypothalamic and preoptic nuclei, nucleus of the diagonal band, olfactory tubercle, nucleus accumbens, bed nucleus of stria terminalis, septum, and other limbic structures in telencephalon. Caudal to the anterior commissure, some fibers ascend dorsally to enter the midline thalamus, which they innervate. Lateral projections of the infundibular perikarya course through the medial-basal hypothalamus, dorsal to the optic tracts, and enter the amygdala region where they innervate more medially situated amygdaloid nuclei. Caudal projections of the POMC neurons also extend through midline diencephalon, some coursing along a periventricular path to innervate midline hypothalamic and thalamic nuclei. This projection extends into the mesencephalic substantia grisea centralis and may also contribute to the innervation of more dorsally situated nuclei in the pons and medulla, such as the parabrachial nuclei and nucleus tractus solitarius. Other caudal projections originating in the hypothalamus course through the ventral tegmentum of mesencephalon and pons and may contribute to the innervation of midline raphe and other ventrally situated nuclei in the pons and medulla. The distribution of immunoreactive perikarya and fibers in the brain of rhesus monkey is strikingly similar to that found in the rat brain. However, subtle differences appear to exist in the innervation patterns of particular brain regions.

Double immunostaining reveals distinctions among opioid peptidergic neurons in the medial basal hypothalamus

We adapted an uncommon peroxidase substrate, ortho-dianisidine, to double immunostaining methodology in order to analyze the degree of neuronal coexistence of pro-opiomelanocortin-derived peptides and to compare the distribution of neurons containing beta-endorphin immunoreactivity (ir) with those containing enkephalin-ir and dynorphin-ir in the medial basal hypothalamus. Double immunostaining demonstrated co-localization of beta-endorphin-ir, adrenocorticotropic hormone-ir, and gamma 3-melanocyte stimulating hormone-ir in medial basal hypothalamus (MBH) neurons. However, while all perikarya containing beta-endorphin-ir invariably contained adrenocorticotropic hormone-ir, not all of these cell bodies contained gamma 3-melanocyte stimulating hormone-ir. In contrast, separate neuronal cell bodies containing beta-endorphin-ir, enkephalin-ir, and dynorphin-ir were distributed throughout the rostral-caudal extent of the MBH. Fibers containing enkephalin-ir closely surrounded cell bodies containing beta-endorphin-ir, suggesting axosomatic contacts between these two opioid peptidergic neuronal populations.

Biosynthesis of multiple forms of beta-endorphin in the reptile intermediate pituitary

Fractionation of the beta-endorphin-sized material from freshly dissected reptile intermediate pituitaries by ion exchange chromatography on sulfopropyl Sephadex (SP) revealed at least three distinct forms of immunoreactive beta-endorphin. These forms eluted at 0.25 M NaCl, 0.28 M NaCl, and 0.32 M NaCl and represent respectively, 6%, 65% and 29% of the total immunoreactivity. Only the 0.28 M NaCl peak and the 0.32 M NaCl peak exhibited naloxone reversible opiate bioactivity when tested in the isolated guinea pig ileum bioassay system; taking into account the molar amount of immunoreactive peptides the 0.32 M NaCl peak was 6 fold more potent than the 0.28 M NaCl peak. Intermediate pituitaries in culture were incubated with either [3H]tyrosine, [3H]arginine, or [35S]methionine for periods up to 24 hours and beta-endorphin-sized peptides were prepared by immunoprecipitation and gel filtration. Fractionation of the labeled beta-endorphin-sized peptides by ion exchange chromatography yielded profiles nearly identical to the immunoassay analyses of freshly dissected tissue. Further analysis of the major labeled forms of reptile beta-endorphin by chromatography on Sephadex G-50 equilibrated in 6 M guanidine HCl indicated that the 0.32 M NaCl peak had an apparent molecular weight of 3500 +/- 100 and the 0.28 M NaCl peak had an apparent molecular weight of 3200 +/- 100. Furthermore, pulse/chase experiments showed that the 0.32 M NaCl peak was the precursor for the 0.28 M NaCl peak. These results coupled with the relative opiate bioactivities of the major argue that the principal post-translational modification of reptile beta-endorphin is COOH-terminal proteolytic cleavage.

Neurofilaments contain alpha-melanocyte-stimulating hormone (alpha-MSH)-like immunoreactivity

An antiserum to alpha-melanocyte-stimulating hormone (alpha-MSH) was found to contain antibodies to at least two types of determinants on the alpha-MSH peptide: one is present only on the free peptide, the other is shared with neurofilaments. Immunoblots from mouse brain showed the neurofilament crossreactivity to be located on proteins in the Mr 140,000 range. The neurofilament-crossreactive portion of the antiserum could be selectively absorbed out with a cytoskeletal preparation, which abolished all affinity of the antiserum to the retina but did not affect the labeling pattern in the pituitary. Absorptions with desacetyl-alpha-MSH and corticotropin seemed to indicate that the determinant shared with neurofilaments is not located at either end of the alpha-MSH peptide, but somewhere in between. The immunohistochemical labeling of the retina with the alpha-MSH antiserum was compared to the labeling with monoclonal antibodies against Mr 200,000 neurofilaments. In the adult retina the alpha-MSH-like immunoreactivity was found to be slightly more widespread; most consistently it was detectable in cell bodies of large ganglion cells, whereas the heavy neurofilament subunit was absent from somata and proximal axons of these cells. In the developing mouse brain, expression of the heavy subunit was found to lag 2-3 wk behind expression of the Mr 140,000 proteins. This confirms previous reports of a more restricted distribution and late expression of high molecular weight neurofilaments as compared to the lower subunits.

Behavioral effects of [4-norleucine, 7-D-phenylalanine]-alpha-melanocyte-stimulating hormone

The behavioral effects of alpha-melanocyte stimulating hormone (alpha-MSH) were compared to an alpha-MSH analogue that had a norleucine substituted for methionine in the four position and a D-phenylalanine substituted for L-phenylalanine in the seven position. [Nle4, D-Phe7]-alpha-MSH has previously been shown to be a superpotent agonist on melanocytes [17]. The present experiments indicate that [Nle4, D-Phe7]-alpha-MSH is equipotent to alpha-MSH in inducing grooming when administered intraventricularly. In contrast, the analogue has the opposite effect of alpha-MSH on performance of a visual discrimination task. alpha-MSH improves visual performance whereas [Nle4, D-Phe7]-alpha-MSH attenuates such performance. The contrasting activities of [Nle4, D-Phe7]-alpha-MSH on the physiological processes described suggest that this analogue may interact with three distinct melanotropin receptors in different ways. On melanocyte receptors the melanotropin analogue is a superagonist, on CNS melanotropin receptors involved in grooming it is equipotent to alpha-MSH, and on CNS receptors involved in attention, learning and memory [Nle4, D-Phe7]-alpha-MSH may be an antagonist of endogenous melanotropin.

Co-occurrence of immunoreactive corticotropin-like and alpha-melanotropin-like material in pituitary cells: differences between young and adult rats

We examined to which extent adrenocorticotropin (ACTH)-like substances are cleavable into alpha-melanotropin (MSH)-like material by application of exogenous enzymes on pituitary sections prior to immunocytochemical stainings. In pro-opiomelanocortin (POMC) cells of the anterior lobe from developing rats (1 to 21 postnatal days) at least one of two cells contained alpha-MSH-like immunoreactivity besides ACTH-like immunostaining. Pretreatment of these tissue sections with tryptic enzymes enhanced remarkably alpha-MSH-like immunostaining. In contrast, alpha-MSH-like immunoreactivity was almost not detectable in anterior lobes from adult rats, also after treatment with enzymes. After adrenalectomy (ax) POMC cells in anterior lobes behaved similarly: a few days after the operation almost all ACTH cells contained also alpha-MSH immunostaining, whereas after more than two weeks alpha-MSH immunostaining was nearly absent, also after application of enzymes. In contrast to rats, in young and adult guinea pig pituitaries all ACTH-like immunoreactivity was prevented by treatment with tryptic enzymes. These observations are consistent with the assumption that occurrence of ACTH- and alpha-MSH-like substances in rat pituitaries is regulated by a protection mechanism against proteolytic cleavage in the ACTH sequence which appears with delay in differentiating and maturing cells.

Thymosin alpha 1-like peptides: localization and biochemical characterization in the rat brain and pituitary gland

Using a radioimmunoassay for thymosin alpha 1, endogenous thymosin-like peptides were characterized in the rat brain and pituitary gland. Thymosin alpha 1-like peptides were present in high concentrations in hypothalamus and pituitary extracts. These peptides were characterized using gel filtration techniques and the main peak of immunoreactive thymosin had a molecular weight similar to that of thymosin alpha 1 (3108 daltons). Using HPLC techniques, one main peak of immunoreactivity was present in brain extracts, whereas two peaks were present in pituitary extracts, one of which coeluted with thymosin alpha 1. The discrete regional distribution of thymosin alpha 1-like peptides was investigated and the highest densities of immunoreactive thymosin were present in the median eminence and arcuate nucleus of the hypothalamus, as well as the neurointermediate lobe of the pituitary. Due to the anatomical proximity of immunoreactive thymosin to loci containing known releasing factors and hormones, thymosin alpha 1-like peptides may function as neuroendocrine regulatory agents.

"Joining peptide" of pro-opiomelanocortin. II. Interspecies heterogeneity of the joining peptide fragment

The use of an antiserum raised against the joining peptide sequence -23 to -14 of bovine pro-opiomelanocortin (POMC) enabled the detection of related immunoreactive sequences of peptides in bovine, porcine, mouse and guinea-pig pituitaries, as well as in mouse brain and cerebral cortex, guinea-pig cerebral cortex, and bovine hypothalamus. Gel chromatography of pituitary extracts (Sephadex G-75 and Bio-Gel P-4) indicated the presence of several immunoreactive joining peptide fragments ranging in the molecular weight range (Mr) of 1,500 to 2,300. Furthermore, high molecular weight (Mr greater than 22,500) immunoreactive-precursor from bovine anterior pituitary was readily digested with trypsin into an immunoreactive fragment of approximately Mr 1,500. Analyses of these immunoreactive peptides by reverse-phase high-performance liquid chromatography (HPLC) led to their resolution into six distinct peptides. The only apparent correspondence in the elution profiles of immunoreactive peptide profiles between different mammalian species was the identification of a similar fragment (Mr 2,000) from bovine and guinea-pig pituitaries. Thus, we conclude that immunoreactivity to the joining peptide region of POMC from various mammalian species exhibits a degree of heterogeneity in its composition. The relatively low levels of immunoreactivity in comparison to that of ACTH also suggest that the joining peptide domain may be further processed. The hormonal status of the joining peptide region remains to be determined.

Action of peptidases in brain synaptic membranes on the NH2-terminus of adrenocorticotropin using ACTH-(1-16)-NH2 as a model substrate

The action of brain peptidases on NH2-terminal sequences of adrenocorticotropin was studied by incubation of ACTH-(1-16)-NH2 under different pH conditions. Profiles of metabolites and time course of product formation were obtained by HPLC analysis of the digests. Fragments of ACTH-(1-16)-NH2 were isolated and characterized by their amino acid composition and NH2-terminal groups. Both at pH 7.4 and pH 8.5 the following fragments were found: ACTH-(3-16)-NH2, ACTH-(4-16)-NH2, ACTH-(5-16)-NH2, and ACTH-(7-16)-NH2. At pH 7.4 the major products were ACTH-(4-16)-NH2 and ACTH-(7-16)-NH2, while the peptide ACTH-(3-16)-NH2 was the main metabolite at pH 8.5. The nature of identified peptides and the time course of their formation demonstrates that aminopeptidase activities predominate in the conversion of the NH2-terminus of adreno-corticotropin and related peptides by brain synaptic membranes.

Pituitary blood flow

The direction of pituitary blood flow, the amount of pituitary blood flow, its regional control, and the role of the median eminence microcirculation are the subjects of this review. Present concepts of pituitary blood flow are focused almost entirely on its direction and arouse from studies of pituitary vascular anatomy performed almost 50 years ago. The development of new anatomic techniques has led to a reappraisal of pituitary angioarchitecture, stimulated physiological studies to clarify the pattern of blood flow within the entire gland, and led to a reappraisal of accepted concepts of directional pituitary blood flow. The availability of techniques to accurately measure organ blood flow has permitted study of pituitary blood flow; and, when combined with knowledge of pituitary anatomy, the application of these techniques promises to provide a means to develop insight into control of the mechanisms by which chemical messengers are delivered to the pituitary to control its function. New anatomic techniques promise to develop new understanding of the three-dimensional arrangement of median eminence microvasculature and yield new concepts of blood flow regulation within the median eminence that can be tested by physiological means.