Neuropeptide Y interaction with the adrenergic transmission line: a study of its effect on alpha-2 adrenergic receptors

Neuropeptide Y and alpha-melanocyte-stimulating hormone: interaction in obesity and possible role in the development of hypertension

AIM

Obesity and hypertension frequently coexist and both represent important risk factors for cardiovascular disease. The mechanisms implicated in the regulation of food intake have not been completely elucidated. Recent data suggests that peripheral and central neuropeptides play an important role in the maintenance of energy balance. More specifically, leptin, neuropeptide Y (NPY) and alpha-melanocyte-stimulating hormone (a-MSH) appear to be implicated in the pathogenesis of obesity and also contribute to the development of hypertension in obesity.

METHODS

Analysis of the pertinent bibliography published in PubMed database.

RESULTS

Leptin is produced in the adipose tissue directly correlated with fat tissue mass. Leptin acts on two distinct neural populations in the hypothalamus: the first expresses the orexigenic peptides NPY and agouti-related protein (AgRP), the second pro-opiomelanocortin (POMC). The activation of POMC neurons increases the production of the anorexigenic hormone a-MSH and inhibits the release of NPY and AgRP. In addition, the hypothalamus integrates the neuroendocrine systems with the autonomic nervous system and controls the activity of the latter. Stimulation of hypothalamic nuclei elicits sympathetic responses including blood pressure elevation. Both NPY and a-MSH appears to be implicated in the hypothalamic regulation of sympathetic nervous system (SNS) activity.

CONCLUSION

Alterations in leptin, NPY and a-MSH are frequently observed in obesity and might stimulate SNS activity, contributing to the development of hypertension in obese patients. These neuropeptides might provide a pathophysiologic link between excess weight and hypertension. However, more research is needed before the pharmacologic manipulation of these complex neuroendocrine systems can be applied in the treatment of obesity and hypertension.

Neuropeptides as synaptic transmitters

Neuropeptides are small protein molecules (composed of 3-100 amino-acid residues) that have been localized to discrete cell populations of central and peripheral neurons. In most instances, they coexist with low-molecular-weight neurotransmitters within the same neurons. At the subcellular level, neuropeptides are selectively stored, singularly or more frequently in combinations, within large granular vesicles. Release occurs through mechanisms different from classical calcium-dependent exocytosis at the synaptic cleft, and thus they account for slow synaptic and/or non-synaptic communication in neurons. Neuropeptide co-storage and coexistence can be observed throughout the central nervous system and are responsible for a series of functional interactions that occur at both pre- and post-synaptic levels. Thus, the subcellular site(s) of storage and sorting mechanisms into different neuronal compartments are crucial to the mode of release and the function of neuropeptides as neuronal messengers.

Neuropeptide Y and peptide YY inhibit excitatory synaptic transmission in the rat dorsal motor nucleus of the vagus

Pancreatic polypeptides (PPs) such as neuropeptide Y (NPY) and peptide YY (PYY) exert profound, vagally mediated effects on gastrointestinal (GI) motility and secretion. Whole-cell patch clamp recordings were made from brainstem slices containing identified GI-projecting rat dorsal motor nucleus of the vagus (DMV) neurons to determine the mechanism of action of PPs. Electrical stimulation of nucleus tractus solitarii (NTS) induced excitatory postsynaptic currents (EPSCs) that were reduced in a concentration-dependent manner by NPY and PYY (both at 0.1-300 nM) in 65 % of the neurons. An increase in the paired-pulse ratio without changes in the postsynaptic membrane input resistance or EPSC rise and decay time suggested that the effects of PPs on EPSCs were due to actions at presynaptic receptors. The Y1 and Y2 receptor selective agonists [Leu31,Pro34]NPY and NPY(3-36) (both at 100 nM) mimicked the inhibition of NPY and PYY on the EPSC amplitude. The effects of 100 nM NPY, but not PYY, were antagonized partially by the Y1 receptor selective antagonist BIBP3226 (0.1 micro M). In addition, the inhibition of the EPSC amplitude induced by NPY, but not PYY, was attenuated partially by pretreatment with the alpha2 adrenoceptor antagonist yohimbine (10 micro M), and occluded partially by the alpha2 adrenoceptor agonist UK14,304 (10 micro M) as well as by pretreatment with reserpine. Pretreatment with a combination of BIBP3226 and yohimbine almost completely antagonized the NPY-mediated effects on EPSCs. Contrary to the inhibition of EPSCs, perfusion with PPs had no effect on the amplitude of inhibitory postsynaptic currents (IPSCs) and a minimal effect on a minority of DMV neurons. Differences in the receptor subtypes utilized and in the mechanism of action of NPY and PYY may indicate functional differences in their roles within the circuitry of the dorsal vagal complex (DVC).

Costorage and coexistence of neuropeptides in the mammalian CNS

The term neuropeptides commonly refers to a relatively large number of biologically active molecules that have been localized to discrete cell populations of central and peripheral neurons. I review here the most important histological and functional findings on neuropeptide distribution in the central nervous system (CNS), in relation to their role in the exchange of information between the nerve cells. Under this perspective, peptide costorage (presence of two or more peptides within the same subcellular compartment) and coexistence (concurrent presence of peptides and other messenger molecules within single nerve cells) are discussed in detail. In particular, the subcellular site(s) of storage and sorting mechanisms within neurons are thoroughly examined in the view of the mode of release and action of neuropeptides as neuronal messengers. Moreover, the relationship of neuropeptides and other molecules implicated in neural transmission is discussed in functional terms, also referring to the interactions with novel unconventional transmitters and trophic factors. Finally, a brief account is given on the presence of neuropeptides in glial cells.

Actions of neuropeptide Y on the rat adrenal cortex

Although several studies have demonstrated the presence of neuropeptide Y (NPY) in nerves supplying the mammalian adrenal cortex, its function in this tissue remains unclear, with reports of both stimulatory and inhibitory effects on aldosterone secretion apparently depending on the tissue preparation used. In the present study the effects of NPY on rat adrenal capsular tissue were investigated. NPY significantly stimulated aldosterone secretion in a dose-dependent manner, and this effect was abolished by atenolol, a beta1-adrenergic antagonist. NPY also stimulated the release of catecholamines from intact rat adrenal capsular tissue with the same dose-dependent relationship as the stimulation of aldosterone release. These observations suggest that the actions of NPY may be mediated by the local release of catecholamines from chromaffin cells within adrenal capsular tissue, as we have previously described for vasoactive intestinal peptide. The second part of this study concerned the NPY receptor subtype mediating the actions of NPY on the adrenal cortex. It was found that peptide YY stimulated aldosterone release with a comparable potency to NPY, whereas pancreatic polypeptide (PP) was without effect. The Y1 selective NPY analog Leu31Pro34NPY had a greater effect on aldosterone release than the Y2 selective analog NPY18-36. Studies using the specific Y1 receptor antagonist BIBP 3226 showed significant attenuation of the aldosterone response to NPY, but no effect on the response to added norepinephrine. Binding studies carried out using [125I]NPY revealed the presence of a single population of NPY-binding sites with a Kd of 12.25 nmol/liter and a binding capacity of 623 fmol/mg protein. Competition studies revealed displacement of [125I]NPY specific binding by NPY, peptide YY, and Leu31Pro34NPY, but not by other peptides. Messenger RNA analysis revealed the presence of messenger RNA coding for both the Y1 receptor and the Y4 receptor, but not the other subtypes. Taken together these data suggest that the effects of NPY on the rat adrenal cortex are mediated by the Y1 receptor subtype.

Increased potency of neuropeptide Y to antagonize alpha2-adrenoceptor function in the nucleus tractus solitarii of the spontaneously hypertensive rat

The regulation by neuropeptide Y of alpha2-adrenoceptors in the nucleus tractus solitarii was evaluated in the adult normotensive Wistar Kyoto rat and the adult spontaneously hypertensive rat. The microinjection of a submaximal dose of l-noradrenaline (800 pmol in 50 nl) alone into the nucleus tractus solitarii produced a significant reduction in the mean arterial blood pressure in either strain. The threshold dose (1 pmol in 50 nl) of neuropeptide Y(1-36) for the vasodepressor response in the Wistar Kyoto rat was five times higher than that (0.2 pmol in 50 nl) in the spontaneously hypertensive rat. Furthermore, neuropeptide Y(1-36) at 0.2 pmol in 50 nl could significantly counteract the vasodepressor response to l-noradrenaline (800 pmol in 50 nl) in the spontaneously hypertensive rat, but not in the Wistar Kyoto rat, in which 1 pmol in 50 nl of neuropeptide Y(1-36) must be employed to counteract the vasodepressor response to l-noradrenaline (800 pmol in 50 nl), although the vasodepressor responses are of a similar magnitude. The in situ hybridization and quantitative receptor autoradiographical experiments showed that the alpha2A-adrenoceptor messenger RNA levels and the B(max) value of the alpha2-adrenoceptor agonist [3H]p-aminoclonidine binding sites measured in the nucleus tractus solitarii of the spontaneously hypertensive rat were substantially lower than those in the Wistar Kyoto rat. The quantitative receptor autoradiographical results were consistent with the cardiovascular results and showed that in the spontaneously hypertensive rat, neuropeptide Y(1-36) at 1 nM led to a significant increase in the K(d) value of [3H]p-aminoclonidine binding sites. In the Wistar Kyoto rat, neuropeptide Y(1-36) produced this effect only at 10 nM. The present study provides evidence for an increase of the potency of neuropeptide Y(1-36) to antagonistically modulate alpha2-adrenoceptors in the nucleus tractus solitarii of the spontaneously hypertensive rat. This enhanced antagonistic action may partly be related to a reduction in the number of alpha2A-adrenoceptors in the nucleus tractus solitarii of the spontaneously hypertensive rat, since a decrease has been observed in the alpha2A-adrenoceptor messenger RNA levels and the alpha2-adrenoceptor binding sites in the spontaneously hypertensive rat. This increased potency of neuropeptide Y(1-36) to antagonize alpha2-adrenoceptor function in the nucleus tractus solitarii of the spontaneously hypertensive rat may contribute to the development of high blood pressure in this hypertensive strain.

Neurotransmitter interactions in the stomatogastric system of the spiny lobster: one peptide alters the response of a central pattern generator to a second peptide

Two of the peptides found in the stomatogastric nervous system of the spiny lobster, Panulirus interruptus, interacted to modulate the activity of the cardiac sac motor pattern. In the isolated stomatogastric ganglion, red-pigment-concentrating hormone (RPCH), but not proctolin, activated the bursting activity in the inferior ventricular (IV) neurons that drives the cardiac sac pattern. The cardiac sac pattern normally ceased within 15 min after the end of RPCH superfusion. However, when proctolin was applied within a few minutes of that time, it was likewise able to induce cardiac sac activity. Similarly, proctolin applied together with subthreshold RPCH induced cardiac sac bursting. The amplitude of the excitatory postsynaptic potentials from the IV neurons to the cardiac sac dilator neuron CD2 (1 of the 2 major motor neurons in the cardiac sac system) was potentiated in the presence of both proctolin and RPCH. The potentiation in RPCH was much greater than in proctolin alone. However, the potentiation in proctolin after RPCH was equivalent to that recorded in RPCH alone. Although we do not yet understand the mechanisms for these interactions of the two modulators, this study provides an example of one factor that can determine the "state" of the system that is critical in determining the effect of a modulator that is "state dependent," and it provides evidence for yet another level of flexibility in the motor output of this system.

The neuropeptide Y antagonist PYX2 decreases lordosis behavior

Neuropeptide Y (NPY) has been localized to noradrenergic neurons and both the noradrenergic system and NPY play a facilitatory role in the control of luteinizing hormone-releasing hormone (LHRH) and luteinizing hormone (LH) release. The present experiments examined whether NPY also plays a role in the control of lordosis. Adult female guinea pigs were ovariectomized (ovx) and implanted with a cannula into the lateral ventricle. In Experiment 1, intracerebroventricular (ICV) administration of the NPY antagonist PYX2 (0, 0.5, 2.0 or 10.0 μg) caused a dose-dependent decrease in lordosis behavior in ovx, estrogen and progesterone-primed guinea pigs. In addition to an effect on the mean lordosis response, PYX2 also decreased the percent of animals showing lordosis and the maximum lordosis response. In Experiment 2, NPY administration (25 μg, ICV) 30 min after PYX2 (2 μg, ICV) to ovx estrogen and progesterone-primed females significantly reversed the effect of the PYX2. Because the NPY antagonist PYX2 reversibly decreased lordosis behavior this suggests that NPY plays a facilitatory role in the control of lordosis behavior.

Plasma neuropeptide Y and catecholamines in women and men with essential hypertension

Neuropeptide-Y (NPY) is a peptide proposed to modulate the effect of the sympathetic nervous system on blood pressure control and contribute to the development of essential hypertension. To assess the possible influence of gender on its role, we evaluated plasma NPY, noradrenaline (NA) and adrenaline (A) concentrations in men and women with essential hypertension. No difference in NPY concentration was found between genders, but NPY concentration was elevated in both hypertensive men and women. NA levels were similar in all investigated hyper- and normotensives, while A was increased only in hypertensive men. These results suggest various patterns of sympatho-adrenal activity in gender subgroups of patients with essential hypertension.

Selective modulation of the NPY receptors of the Y2 subtype by alpha 2 receptors in the nucleus tractus solitarii of the rat. A cardiovascular and quantitative receptor autoradiographical analysis

The modulation of neuropeptide Y (NPY) receptors by alpha 2 receptors in the nucleus tractus solitarii (Sol) of the rat was evaluated using quantitative receptor autoradiography and measurements of mean arterial blood pressure and heart rate. The receptor autoradiographical experiments showed that clonidine (10 nM), a selective alpha 2 receptor agonist, induced a 59% increase in the B0 value and a 47% decrease in the IC50 value of NPY(1-36) when competing for [125I]peptide YY ([125I]PYY)-binding sites in the presence of [Leu31, Pro34]NPY (100 nM), a selective NPY Y1 receptor agonist, to block the binding to NPY Y1 receptors. In contrast, when NPY(13-36) (300 nM), a selective NPY Y2 receptor agonist, was used to block the binding to NPY Y2 receptors, clonidine (1-30 nM) did not affect the B0 value and the IC50 value of NPY(1-36) when competing for [125I]PYY-binding sites, suggesting that the stimulation of alpha 2 receptors can selectively increase the affinity of NYP(1-36) for the NPY Y2 receptor. Microinjections of threshold doses of adrenaline or clonidine into the Sol not only counteracted the vasopressor action of a close to ED50 dose of coinjected NPY(13-36), but also changed the vasopressor and tachycardic response produced by NPY(13-36) into a vasodepressor and bradycardic response. However, threshold doses of adrenaline or of clonidine microinjected into the Sol did not modify the vasodepressor responses to a close to ED50 dose of NPY(1-36) or of [Leu31, Pro34]NPY.(ABSTRACT TRUNCATED AT 250 WORDS)

Neuropeptide Y (NPY)- and vasoactive intestinal peptide (VIP)-induced aldosterone secretion by rat capsule/glomerular zone could be mediated by catecholamines via beta 1 adrenergic receptors

The effects of two Neuropeptide Y (NPY) analogs (Y1- and Y2-type) and vasoactive intestinal peptide (VIP) on both catecholamine (adrenaline and noradrenaline) release and aldosterone production by rat adrenal capsule/glomerular zone, have been investigated in vitro. The adrenal capsule/glomerular zones, collected from adult male rats, were incubated in a medium (Krebs-Ringer bicarbonate buffer supplemented with glucose and bovine serum albumin) containing or not one of the following synthetic peptides: human Leu31,Pro34-NPY (an agonist of the Y1-type receptors), human/porcine NPY18-36 (an agonist of the Y2-type receptors) and VIP at the concentration of 10(-7) M, associated or not with 10(-7) M atenolol (a beta 1 adrenergic antagonist) or ICI-118,551 hydrochloride (a beta 2 adrenergic antagonist). The two NPY analogs as well as the VIP stimulated the release of catecholamines and of aldosterone. The beta 1 adrenergic antagonist, but not the beta 2 one, which failed to affect basal aldosterone production when given alone, prevented NPY18-36-, Leu31,Pro34-NPY- or VlP-induced aldosterone secretion. Present data support the hypothesis that adrenaline and/or noradrenaline could mediate the effects of both NPY and VIP on aldosterone secretion via beta 1 adrenergic receptors; alternatively, the steroidogenic effect of NPY or VIP could be related to direct interaction between NPY- or VIP-specific binding sites, present on the capsule/glomerular zone of the rat adrenal cortex, and beta 1 adrenergic receptors. Then the NPYergic, VIPergic and catecholaminergic innervation of the adrenal cortex, previously characterized by immunohistochemistry, may be a potent stimulatory element in the nervous control of the aldosterone secretion.