Neuropeptide Y and peptide YY neuronal and endocrine systems

An extensive system of neuropeptide Y (NPY) containing neurons has recently been identified in the central and peripheral nervous system. In addition, NPY and a structurally related peptide, peptide YY (PYY), containing endocrine cells have been identified in the periphery. The NPY system is of particular interest as the peptide coexists with catecholamines in the central and sympathetic nervous system and adrenal medulla. Evidence has been presented which indicates that NPY may play important roles in regulating autonomic function.

Corticotropin-releasing factor binding to peripheral tissue and activation of the adenylate cyclase-adenosine 3',5'-monophosphate system

Specific binding sites for rat corticotropin-releasing factor (rCRF) are present in rat adrenal medulla, ventral prostate, spleen, liver, kidney, and testis and bovine chromaffin cells in culture. Maximal binding of [125I]rCRF occurred within 25 min at 4 C and was saturable. Scatchard analysis of rCRF binding to rat adrenal membranes and bovine chromaffin cells revealed the existence of two classes of binding sites. One class had a relatively higher apparent affinity and lower number of binding sites, whereas the other class had a relatively lower affinity and higher number of binding sites. CRF induced a dose-related increase in rat adrenal membrane adenylate cyclase activity and cAMP levels in bovine chromaffin cells. Nanomolar concentrations of rCRF maximally stimulated adenylate cyclase activity in rat adrenal membranes and maximally increased cAMP levels in bovine chromaffin cells to 86% and 130% above control values, respectively. The demonstration of specific CRF-binding sites in a variety of peripheral tissues and the finding that activation of specific CRF-binding sites in adrenal tissue stimulates the adenylate cyclase-cAMP system suggest that CRF may have an important regulatory role in various peripheral tissues.

Selective regulation of carboxypeptidase peptide hormone-processing enzyme during enkephalin biosynthesis in cultured bovine adrenomedullary chromaffin cells

Bovine adrenomedullary chromaffin cells in culture were incubated with reserpine or forskolin, two agents acting through different mechanisms, which increase cellular [Met]enkephalin levels by 2-fold after 72 h. Cells were harvested and chromaffin granules were purified on a linear sucrose gradient. After reserpine treatment, carboxypeptidase-processing enzyme specific activity in chromaffin granule fractions was stimulated 1.9-fold, and Co2+-stimulated carboxypeptidase specific activity was stimulated 3-fold. The increase in enzyme activity was dependent on the time of reserpine treatment. Forskolin, on the other hand, had no significant effect on carboxypeptidase activity. The differential effects of reserpine and forskolin suggest that the carboxypeptidase-processing enzyme may be selectively regulated during periods of elevated enkephalin formation. Kinetic studies revealed that in cells exposed to reserpine, the Km value for [Met]enkephalin-Arg6 for the Co2+-stimulated carboxypeptidase activity was lowered to 0.136 from 0.447 mM, but there was no change in the Km values of the non-Co2+-stimulated carboxypeptidase activity from reserpine and control groups. Cellular levels of immunoreactive carboxypeptidase-processing enzyme, measured by a radioimmunoassay method, were not altered after reserpine treatment. These data suggest that while the total number of carboxypeptidase enzyme molecules remained constant, there may be a conversion of existing enzyme molecules to a more active form which displays a higher affinity for [Met]enkephalin-Arg6 in the presence of Co2+.

Methionine and leucine enkephalin in rat neurohypophysis: different responses to osmotic stimuli and T2 toxin

Specific radioimmunoassays were used to measure the effects of hypertonic saline (salt loading), water deprivation, and trichothecene mycotoxin (T2 toxin) on the content of methionine enkephalin (ME), leucine enkephalin (LE), alpha-neoendorphin, dynorphin A, dynorphin B, vasopressin, and oxytocin in the rat posterior pituitary. Concentrations of vasopressin and oxytocin decreased in response to both osmotic stimuli and treatment with T2 toxin, but the decrease was greater with osmotic stimulations. Similarly, concentrations of LE and dynorphin-related peptides declined after salt loading and water deprivation; LE concentrations also decreased after treatment with T2 toxin. The concentration of ME decreased after water deprivation, did not change after salt loading, and increased after T2 toxin treatment. The differentiating effects of these stimuli on the content of immunoreactive LE and ME are consistent with the hypothesis that LE and ME may be localized in separate populations of nerve endings with different roles in the posterior pituitary.

(Met)enkephalin and carboxypeptidase processing enzyme are co-released from chromaffin cells by cholinergic stimulation

A carboxypeptidase B-like enzyme is involved in processing of proenkephalin in adrenal medulla. Nicotine stimulated the co-release of this enzyme with (Met)enkephalin pentapeptide from bovine chromaffin cells in primary culture. The ratio of enzyme activity/immunoreactivity was determined for the released carboxypeptidase to provide an index of the level of enzyme activity per unit number of enzyme molecules. The ratio for the Co++-stimulated carboxypeptidase secreted into the cell culture medium upon nicotinic stimulation was 10.1 +/- 1.02 (pmol Met-enkephalin formed per ng carboxypeptidase immunoreactivity), while the Co++-stimulated carboxypeptidase in the soluble and membrane components of purified chromaffin granules had lower ratios of 5.46 +/- 0.70 and 1.07 +/- 0.13, respectively. Hexamethonium, a nicotinic receptor antagonist, blocked the nicotine-induced release of the carboxypeptidase processing enzyme and (Met)enkephalin. These data suggest that a pool of carboxypeptidase enzyme molecules at a high state of activation are present in functionally mature granules whose contents are released by nicotinic receptor stimulation.

Influence of acute, subchronic and chronic treatment with neuroleptic (haloperidol) on enkephalins and their precursors in the striatum of rat brain

We examined the effects of chronic, subchronic and acute treatment with haloperidol on the ME, the MERGL and enkephalin precursor concentrations in rat brain. The changes affected primarily the striatum. The ME content was greatly increased by the treatment, the precursor level was decreased by the haloperidol treatment. The specific mRNA for proenkephalin A increased. For these reasons, we conclude that the effect of haloperidol increase both the biosynthesis and the processing of precursors of enkephalins in the striatum.

Cholecystokinin innervation of the ventral striatum: a morphological and radioimmunological study

Immunocytochemistry, radioimmunological assay after surgical cuts, anterograde degeneration and retrograde tracing of fluorescent dyes were used in order to elucidate the cholecystokinin-containing afferents to the ventral striatum (nucleus accumbens, olfactory tubercle and ventral part of the caudate-putamen). In agreement with the report by Hökfelt et al., midbrain cholecystokinin-containing cells supply the posteromedial parts of the nucleus accumbens and olfactory tubercle, as well as the subcommissural part of caudate-putamen. Brainstem cholecystokinin afferents also reach more rostral parts of the ventral striatum including the rostrolateral olfactory tubercle. The ascending cholecystokinin axons enter the medial forebrain bundle at the meso-diencephalic border and maintain a rough medial to lateral topography at the caudal diencephalon. A second major cholecystokinin pathway, with possible origin in the piriform and medial prefrontal cortices and/or the amygdala, projects to the subcommissural caudate-putamen, the olfactory tubercle, the lateral part of the nucleus accumbens and the dorsal part of the bed nucleus of stria terminalis. Finally, the rostral part of the dorsal caudate-putamen receives a substantial cholecystokinin innervation from the basolateral amygdala and possibly from the neocortex. According to radioimmunological data, the descending telencephalic cholecystokinin system accounts for about 60% of all cholecystokinin in the rostral forebrain. The combined use of morphological and biochemical methods provided evidence for a partially overlapping distribution and possible interaction between an ascending brainstem and descending telencephalic cholecystokinin fiber systems within the striatum and related rostral forebrain areas.

Multiple populations of neuropeptide-containing cells in cultures of the bovine adrenal medulla

Cell populations containing vasoactive intestinal polypeptide (VIP), enkephalins, and catecholamines were identified in bovine adrenal medullary cultures by immunofluorescence and radioimmunoassay. Addition of forskolin to the culture medium increased the cellular levels of both VIP and the enkephalins. These changes resulted from an increase in the number of VIP-positive cells and an increase in cellular enkephalin content.

Neurotrophic action of VIP on spinal cord cultures

Vasoactive intestinal peptide-like immunofluorescence was observed in 3-5% of the neurons in 4 week old dissociated cultures from fetal mouse spinal cord and dorsal root ganglion. Radioimmunoassay indicated that VIP was spontaneously released into the culture medium. This release was inhibited by tetrodotoxin (TTX). Previous studies indicated that 50-60% of the spinal cord neurons die between days 7 and 21 in vitro. Blockade of electrical activity with TTX during days 8-15 resulted in a 30% decrease in the number of neurons as compared to control cultures. Addition of VIP (0.1 nM) to TTX-treated cultures prevented neuronal cell death. When VIP alone was added to the cultures, no significant difference in the number of neurons from controls was observed. The possibility that VIP influences cholinergic neurons was tested by measuring choline acetyltransferase (CAT) activity at various periods during development. A 48 hour treatment with 0.1 nM VIP increased CAT activity by 50%. The CAT stimulation was observed only during a period in development when naturally occurring neuronal cell death was taking place. The increases in CAT activity were dose dependent within a range of 10(-12) M to 10(-10) M VIP; however, higher concentrations of VIP attenuated the increases in enzyme activity.

Nicotinic receptor stimulation activates enkephalin release and biosynthesis in adrenal chromaffin cells

Neuroendocrine cells release a portion of their stored secretory hormone content when exposed to tissue-specific secretagogues. In the case of the adrenal medulla, catecholamines and enkephalin peptides, as well as other secretory proteins, are secreted in response to acetylcholine, which is released onto cholinergic receptors on chromaffin cells upon splanchnic nerve stimulation in vivo. Secretagogue stimulation thus depletes intracellular stores of exportable hormone. We were interested to know whether the signal for exportable hormone release might also function as a signal for compensatory hormone repletion by enhancing the biosynthesis of the released hormone(s). Accordingly, we have investigated the effect of nicotinic receptor stimulation on Met-enkephalin peptide biosynthesis and expression of proenkephalin messenger RNA in primary cultures of bovine chromaffin cells. Our results, reported here, suggest a model for stimulus-secretion-synthesis coupling in which nicotinic receptor occupancy activates two pathways. One pathway, dependent on calcium and not mimicked by increased intracellular cyclic AMP, leads to exocytotic hormone release; the other, probably via a calcium-dependent increase in intracellular cyclic AMP, leads to a compensatory increase in intracellular enkephalin through activation of transcription of the proenkephalin structural gene.

[Leu5]enkephalin inhibits norepinephrine-induced contraction of rat aorta

Leucine-enkephalin produces a dose related decrease in the contractile response of spirally cut strips of rat aorta to norepinephrine (NE). The maximal effect (55.0 +/- 1.0 percent reduction in developed tension) occurs at a leucine-enkephalin concentration of 10(-10) M. The attenuation of contractile response is reversed by naloxone.

Characterization of enkephalins and related peptides in rat hypophysial portal blood

Rat hypophysial portal blood, collected from the pituitary stalk, was extracted and enkephalins were assayed by different RIA. Met-Enk-IR and Leu-Enk-IR levels were 1635 +/- 470 pg/ml and 125 +/- 50 pg/ml, respectively. Using HPLC characterization, the presence in portal blood of Met-Enk, Leu-Enk, proenkephalins fragments and dynorphin1-17 has been demonstrated. An unidentified Met-Enk-IR peptide has also been found.

Gonadotropin-releasing hormone (Gn-RH) in striped mullet (Mugil cephalus), milkfish (Chanos chanos), and rainbow trout (Salmo gairdneri): comparison with salmon Gn-RH

Immunoreactive gonadotropin-releasing hormone (Gn-RH) was extracted from brains of striped mullet, milkfish, rainbow trout, and chum salmon with acetone/HCl and petroleum ether. High pressure liquid chromatography and cross-reactivity studies show mullet, milkfish, and trout brains to contain a peptide chromatographically and immunologically identical to synthetic salmon Gn-RH, while the mammalian form of Gn-RH is detectable in none of these fishes. Gn-RH is present in immature 7-month-old and 4-year-old milkfish. A second immunoreactive peptide is separable by HPLC in all the fish studied. This "early-eluting" form of Gn-RH is unlikely to be a precursor; its cross-reactivity with antisera R-42 and #185 suggests that any modification is in the C-terminal region. Several possible roles for this peptide are advanced.

Two peptidases that convert 125I-Lys-Arg-(Met)enkephalin and 125I-(Met)enkephalin-Arg6, respectively, to 125I-(Met)enkephalin in bovine adrenal medullary chromaffin granules

Two peptidases which convert 125I-Lys-Arg-ME and 125I-ME-Arg6, respectively, to 125I-ME, have been identified and characterized in bovine adrenomedullary chromaffin granules. The former is referred to as a secretory granule peptidase (SGP) and the latter as a carboxypeptidase B-like enzyme (CPB-like) [7] which is here further characterized. SGP cleaved 125I-Lys-Arg-ME to produce only 125I-ME and was localized in chromaffin granules which contained Co2+-stimulated CPB-like activity, ME, and catecholamines. Both the SGP and the CPB-like enzymes appear to be thiol-metalloproteases. While the CPB-like enzyme seems likely to be involved in processing the enkephalin precursors [7], SGP may function as a trypsin-like or aminopeptidase enzyme in secretory granules.

Alternative modes of enkephalin biosynthesis regulation by reserpine and cyclic AMP in cultured chromaffin cells

Exposure of bovine chromaffin cells in primary culture to 5 microM reserpine or 25 microM forskolin results in an increase in enkephalin peptide levels within 24-48 hr; 25 microM forskolin (or cholera toxin at 50 micrograms/ml) causes a 1.5- to 2-fold increase in enkephalin peptide levels, which is maximal after 48 hr of exposure and is totally blocked by addition of cycloheximide (0.5 microgram/ml). Reserpine (5 microM) elicits a 1.5- to 2-fold increase in enkephalin peptide levels within 24 hr, which is only partially blocked by cycloheximide. Chromatographic analysis of cellular extracts shows that forskolin increases levels of both [Met]enkephalin pentapeptide and high molecular weight enkephalin-containing peptides, while reserpine causes an increase in [Met]enkephalin pentapeptide and a concomitant decrease in high molecular weight enkephalin-containing peptides, suggesting enhanced conversion of enkephalin precursor(s) to the mature polypeptide hormone. Measurement of preproenkephalin messenger RNA (mRNAenk) by RNA blot hybridization with a cDNA probe for mRNAenk reveals that forskolin and cholera toxin cause a relatively rapid (less than 17 hr) 3- to 5-fold increase in mRNAenk, while exposure to reserpine elicits a gradual decrease in enkephalin mRNA (a 50%-80% decline) beginning within 24 hr and continuing over a 72-hr period. These results suggest that forskolin and reserpine differentially regulate enkephalin biosynthesis in cultured chromaffin cells, the former by increasing, presumably via a cAMP-dependent mechanism, cellular mRNA coding for preproenkephalin and the latter by a post-translational increase in proenkephalin processing.

Leucine-enkephalin increases norepinephrine-stimulated chronotropy and 45Ca++ uptake in guinea-pig atria

Norepinephrine (NE) (10(-5) M) increases the beating rate of isolated, spontaneously beating guinea-pig atria 78 +/- 3 beats per minute. Leucine-enkephalin (10(-7) M) increases the maximal chronotropic response to NE by 38%, i.e., by 30 beats per minute. In the presence of 10(-7) M leucine-enkephalin and 10(-7) M naloxone, the chronotropic response to NE is reduced to 38 +/- 3 beats per minute, a value observed in the presence of naloxone alone. Neither naloxone nor leucine enkephalin significantly altered the inotropic response to NE, or significantly altered the basal beating rate. Parallel effects were observed when 45Ca++ uptake by atrial tissue was examined. Incubation of atrial slices with 10(-5) M NE for 10 minutes minimally stimulated 45Ca++ uptake from 1.27 +/- 0.04 to 1.45 +/- 0.17 nmol/mg tissue. In the presence of 10(-7) M leucine-enkephalin, 45Ca++ uptake was increased to 1.95 +/- 0.14 nmol/mg tissue. 45Ca++ uptake was reduced to control values (1.19 +/- 0.09 nmol/mg tissue) in the presence of NE, leucine-enkephalin and naloxone (10(-7) M). The data are consistent with a leucine-enkephalin augmentation of NE-induced chronotropy in guinea-pig atria due to an enhancement of NE-dependent Ca++ accumulation. This effect of leucine-enkephalin is opposite that previously reported for rat atria, in which leucine-enkephalin inhibits both NE-induced positive chronotropy and Ca++ influx.

Leucine-enkephalin modulation of catecholamine positive chronotropy in rat atria is receptor-specific and calcium-dependent

Leucine-enkephalin (LE) at 10(-8) M reduces the maximum chronotropic response of isolated spontaneously beating rat atria to exogenously added (-)-norepinephrine (NE) by approximately 27%, with no effect on the NE ED50 (1.5 X 10(-7) M) for positive chronotropy. This modulatory effect of LE is completely blocked by addition of 10(-7) M naloxone, and seems to be catecholamine-receptor specific, since the positive chronotropic response to forskolin is unaltered in the presence of LE. Isoproterenol (ISO)-induced positive chronotropy is also attenuated by LE. This effect is markedly dependent on the extracellular calcium concentration: LE actually causes a greater than two-fold enhancement of the positive chronotropic effect of ISO at low (0.5 mM) extracellular calcium concentration. A possible role for enkephalins to modulate catecholamine action on the heart via an alteration of catecholamine-induced inward calcium flux is discussed.

Leucine enkephalin antagonizes norepinephrine-induced 45Ca++ accumulation in rat atria

Exposure of rat atrial slices to 10(-5) M norepinephrine (NE) for 10 minutes increases 45Ca++ accumulation from 1.64 +/- 0.10 to 2.23 +/- 0.06 nmol/mg tissue. In the presence of leucine enkephalin (10(-8) M), NE-stimulated 45Ca++ uptake is reduced to 1.44 +/- 0.10 nmol/mg tissue. The effect of leu-enkephalin is reversed in the presence of 10(-7) M naloxone, NE-stimulated 45Ca++ uptake being increased to 2.17 +/- 0.15 nmol/mg tissue. The results support a direct interaction of leu enkephalin with beta-agonist-stimulated Ca++ flux in rat atria, and correlate with the previously reported enkephalin antagonism of NE-induced positive chronotropy in the same tissue.

Cyclic adenosine monophosphate regulates vasoactive intestinal polypeptide and enkephalin biosynthesis in cultured bovine chromaffin cells

When bovine chromaffin cells are dissociated from the adult adrenal gland and placed in culture, they begin to synthesize vasoactive intestinal polypeptide (VIP); they also contain high levels of enkephalin peptides. The regulation of expression of VIP and enkephalin in these cells by cyclic nucleotides was examined. Exposure of cultured chromaffin cells to cholera toxin (CT), forskolin (F), isobutylmethylxanthine (IBMX), 8-bromo-cyclic AMP (8-Br-cAMP) or dibutyryl cyclic AMP (dbcAMP) increases VIP and met-enkephalin biosynthesis as measured by an increase in total (cellular + secreted) VIP and met-enkephalin in treated versus untreated cells. Enkephalin levels increase 1.5 to 3.0 -fold while VIP levels increase 10 to 20 -fold after 48 hours of exposure to the compounds above. Increased enkephalin levels are maximum by 48 hours of exposure; VIP levels are elevated by forskolin or cholera toxin already at 17 hours of exposure. Neither forskolin nor cholera toxin alter catecholamine levels in cultured cells even after 72 hours of exposure. Thus, VIP and ME expression in cultured chromaffin cells can be regulated by intracellular cAMP. Measurement of preproenkephalin messenger RNA (mRNAenk) by quantitative Northern blot hybridization with a preproenkephalin complementary DNA probe revealed that exposure of chromaffin cells to forskolin elicits a greater than two-fold increase in mRNAenk, suggesting that the regulation of neuropeptide expression by elevated intracellular cAMP occurs at a transcriptional locus.

Primary cultures of bovine chromaffin cells synthesize and secrete vasoactive intestinal polypeptide (VIP)

Dispersed cells of the bovine adrenal medulla express immunoreactive vasoactive intestinal polypeptide (VIP) after 24 hours in culture, although VIP could not be detected in extracts of bovine adrenal medulla or cortex. Immunoreactive VIP eluted from a reversed-phrase chromatography column with the same retention time as authentic porcine VIP1-28. VIP in chromaffin cells in culture appears to be contained in a secretory granule pool, since it, like methionine-enkephalin (met-enk) was released into the medium after exposure of cells to nicotine, carbachol, veratridine and elevated potassium in a dose-dependent manner. Dose-response curves for VIP and enkephalin release by the above secretagogues were similar but not identical. Enkephalins and VIP may either be contained in separate subpopulations of chromaffin cells or co-stored in the same cells.

The distribution of cholecystokinin and vasoactive intestinal peptide in rhesus monkey brain as determined by radioimmunoassay

The concentration of cholecystokinin (CCK) and vasoactive intestinal peptide (VIP) in dissected cortical and subcortical areas of four rhesus monkeys' brains was determined by radioimmunoassay (RIA). Cerebral cortical samples from one human brain are included for comparison. Preliminary data from two baboon brains are described. The results are similar to previous studies on rat (1-7), human (7-12), porcine (12,13), bovine (3) and guinea pig brains (14) and indicate that: 1) both CCK and VIP are widely distributed in cortical and subcortical areas in these species, 2) CCK is generally more abundant than VIP in primate brain, and 3) the distribution of CCK and VIP in the rat brain parallel those in infrahuman primate and human brain.

Intrinsic vasoactive intestinal polypeptide (VIP)-containing neurons in the baroreceptor nucleus of the solitary tract in rat

Vasoactive intestinal polypeptide (VIP)-immunoreactive cell bodies, dendrites and nerve terminals in the nucleus of the solitary tract have been identified with electron microscopic immunocytochemistry in rats. The relatively high concentration of VIP in this nucleus, which corresponds to the primary baroreceptor center, was depleted only insignificantly by uni- or bilateral transection of the solitary tract, which eliminates peripheral neuronal (via glossopharyngeal and vagal nerves) input to the nucleus. Both immunocytochemical and biochemical data suggest that the majority of VIP in the nucleus of the solitary tract is present in intrinsic neurons.