Y2-receptor-mediated selective inhibition of slow, inhibitory postsynaptic potential in submucous neurones of guinea-pig caecum

Neuropeptide Y receptors; antisecretory control of intestinal epithelial function

This paper reviews the cellular localisation, mechanisms of release and intestinal absorptive actions of neuropeptide Y and its related peptides, peptide YY, pancreatic polypeptide and major fragments NPY(3-36) and PYY(3-36). While NPY is commonly found in inhibitory enteric neurons that can be interneurons, motor neurons or secretomotor-nonvasodilator in nature, its analogue, peptide YY in contrast, is located in neuroendocrine L-cells that predominate in the colorectal mucosa. Peptide YY is released from these cells when nutrients arrive in the small or large bowel, exerting paracrine as well as hormonal actions. Pancreatic polypeptide is found in relatively few, scattered intestinal endocrine cells, the majority of this peptide being produced by, and released from pancreatic islet F-cells in response to food intake. An introduction to the current pharmacology of this family of peptides is provided and the different types of neuropeptide Y (termed Y) receptors, their agonist preferences, antagonism, and preferred signalling pathways, are described. Our current understanding of specific Y receptor localisation within the intestine as determined by immunohistochemistry, is presented as a prelude to an assessment of functional studies that have monitored ion transport across isolated mucosal preparations. It is becoming clear that three Y receptor types are significant functionally in human colon, as well as particular rodent models (e.g. mouse) and these, namely the Y(1), Y(2) and Y(4) receptors, are discussed in detail. Their presence within the basolateral aspect of the epithelial layer (Y(1) and Y(4) receptors) or on enteric neurons (Y(1) and Y(2) receptors) and their activation by endogenous neuropeptide Y, peptide YY (Y(1) and Y(2) receptors) or pancreatic polypeptide (which prefers Y(4) receptors) results consistently in antisecretory/absorptive responses. The recent use of novel mouse knockouts has helped establish loss of specific intestinal functions including Y(1) and Y(2) receptor-mediated absorptive tone in colon mucosa. Progress in this field has been rapid recently, aided by the availability of selective antagonists and mutant mice lacking either one (e.g. Y(4)-/-, for which no antagonists exist at present) or more Y receptor types. It is therefore timely to review this work and present a rational basis for developing stable synthetic Y receptor agonists as novel anti-diarrhoeals.

The regulation of veratridine-stimulated electrogenic ion transport in mouse colon by neuropeptide Y (NPY), Y1 and Y2 receptors

1 Neuropeptide Y (NPY) is a prominent enteric neuropeptide with prolonged antisecretory effects in mammalian intestine. Veratridine depolarises neurons consequently causing epithelial anion secretion across mouse colon mucosa. Our aim was to characterise functionally, veratridine-stimulated mucosal responses and to determine the roles for NPY, Y(1), and Y(2) receptors in modulating these neurogenic effects. 2 Colon mucosae (with intact submucous innervation) from wild-type mice (+/+) and knockouts lacking either NPY (NPY-/-), Y(1)-/- or Y(2)-/- were placed in Ussing chambers and voltage clamped at 0 mV. Veratridine-stimulated short-circuit current (I(sc)) responses in +/+, Y(1) or Y(2) antagonist pretreated +/+ colon, Y(1)-/- and NPY-/- colon were insensitive to cholinergic blockade by atropine (At; 1 microM) and hexamethonium (Hex; 10 microM). Tetrodotoxin (TTX, 100 nM) abolished veratridine responses, but had no effect upon carbachol (CCh) or vasoactive intestinal polypeptide (VIP)-induced secretory responses. 3 To establish the functional roles for Y(1) and Y(2) receptors, +/+ tissues were pretreated with either the Y(1) or Y(2) receptor antagonist (BIBO3304 (300 nM) or BIIE0246 (1 microM), respectively) and veratridine responses were compared with those from Y(1)-/- or Y(2)-/- colon. Neither BIBO3304 nor Y(1)-/- altered veratridine-induced secretion, but Y(1) agonist responses were abolished in both preparations. In contrast, the Y(2) antagonist BIIE0246 significantly amplified veratridine responses in +/+ mucosa. Unexpectedly, NPY-/- colon exhibited significantly attenuated veratridine responses (between 1 and 5 min). 4 We demonstrate that electrogenic veratridine responses in mouse colon are noncholinergic and that NPY can act directly upon epithelia, a Y(1) receptor effect. The enhanced veratridine response observed in +/+ tissue following BIIE0246, indicates that Y(2) receptors are located on submucosal neurons and that their activation by NPY will inhibit enteric noncholinergic secretory neurotransmission. 5 We also demonstrate Y(1) and Y(2) receptor-mediated antisecretory tone in +/+ colon and show selective loss of each in Y(1) and Y(2) null colon respectively. In NPY-/- tissue, only Y(1)-mediated tone was present, this presumably being mediated by endogenous endocrine peptide YY. Y(2) tone was absent from NPY-/- (and Y(2)-/-) colon and we conclude that NPY activation of neuronal Y(2) receptors attenuates secretory neurotransmission thereby providing an absorptive electrolyte tone in isolated colon.

Functional consequences of neuropeptide Y Y 2 receptor knockout and Y2 antagonism in mouse and human colonic tissues

1 Neuropeptide Y (NPY), peptide YY (PYY) and pancreatic polypeptide (PP) differentially activate three Y receptors (Y(1), Y(2) and Y(4)) in mouse and human isolated colon. 2 The aim of this study was to characterise Y(2) receptor-mediated responses in colon mucosa and longitudinal smooth muscle preparations from wild type (Y(2)+/+) and knockout (Y(2)-/-) mice and to compare the former with human mucosal Y agonist responses. Inhibition of mucosal short-circuit current and increases in muscle tone were monitored in colonic tissues from Y(2)+/+ and Y(2)-/- mice+/-Y(1) ((R)-N-[[4-(aminocarbonylaminomethyl)phenyl)methyl]-N(2)-(diphenylacetyl)-argininamide-trifluoroacetate (BIBO3304) or Y(2) (S)-N(2)-[[1-[2-[4-[(R,S)-5,11-dihydro-6(6H)-oxodibenz[b,e]azepin-11-yl]-1-piperazinyl]-2-oxoethyl]cyclopentyl]acetyl]-N-[2-[1,2-dihydro-3,5(4H)-dioxo-1,2-diphenyl-3H-1,2,4-triazol-4-yl]ethyl]-argininamide (BIIE0246) antagonists. 3 Predictably, Y(2)-/- tissues were insensitive to Y(2)-preferred agonist PYY(3-36) (</=100 nM), but unexpectedly Y(4)-preferred PP responses were right-shifted probably as a consequence of elevated circulating PP levels, particularly in male Y(2)-/- mice (Sainsbury et al., 2002). 4 BIBO3304 and BIIE0246 elevated mucosal ion transport, indicating blockade of inhibitory mucosal tone in Y(2)+/+ tissue. While BIBO3304 effects were unchanged, those to BIIE0246 were absent in Y(2)-/- mucosae. Neither antagonist altered muscle tone; however, BIIE0246 blocked NPY and PYY(3-36) increases in Y(2)+/+ basal tone. BIBO3304 abolished residual Y(1)-mediated NPY responses in Y(2)-/- smooth muscle. 5 Tetrodotoxin significantly reduced BIIE0246 and PYY(3-36) effects in Y(2)+/+ mouse and human mucosae, but had no effect upon Y-agonist contractile responses, indicating that Y(2) receptors are located on submucosal, but not myenteric neurones. 6 Tonic activation of submucosal Y(2) receptors by endogenous NPY, PYY or PYY(3-36) could indirectly reduce mucosal ion transport in murine and human colon, while direct activation of Y(2) receptors on longitudinal muscle results in contraction.

Presynaptic calcium channels mediating synaptic transmission in submucosal neurones of the guinea-pig caecum

1. Intracellular recording techniques were used to examine the voltage-activated calcium channels mediating neurotransmitter release from nerve terminals of extrinsic, sympathetic origin and intrinsic (enteric) origin innervating submucosal neurones of the guinea-pig caecum. 2. The noradrenergic slow inhibitory postsynaptic potential (IPSP) was abolished by superfusion of omega-conotoxin (omega-CTX) GVIA (3-300 nM), with an apparent IC50 of 8.6 nM. Superfusion of omega-CTX MVIIC (500 nM) also suppressed the amplitude of slow IPSPs, but both omega-agatoxin IVA (100 nM) and nicardipine (1-10 microM) were ineffective. The hyperpolarization induced by exogenous noradrenaline was not affected by omega-CTX GVIA (100 nM). 3. In contrast to the slow IPSP, the amplitude of the cholinergic fast excitatory postsynaptic potential (EPSP) was partially inhibited, but not abolished, by omega-CTX GVIA (0.1-1 microM). Furthermore, omega-agatoxin IVA (0.1-1 microM) or omega-CTX MVIIC (0.1-1 microM) also affected the fast EPSP, but nicardipine (1-10 microM) was ineffective. In combination, omega-CTX GVIA (100 nM) and omega-agatoxin IVA (100 nM) inhibited the fast EPSP by 74 +/- 6 %; the residual fast EPSP was not affected by omega-CTX MVIIC (100 nM). The fast EPSP was completely abolished by low Ca2+, high Mg2+ Krebs solution or Krebs solution containing Co2+ (2 mM) and Cd2+ (400 microM). The depolarization induced by exogenous acetylcholine was not affected by either omega-CTX GVIA (100 nM), omega-agatoxin IVA (100 nM) or omega-CTX MVIIC (100 nM). 4. Taken together, these results suggest that, in the submucosal plexus of the guinea-pig caecum, release of noradrenaline from extrinsic nerve terminals is regulated by N-type calcium channels, whereas release of acetylcholine from intrinsic nerve terminals involves several types of calcium channel.

Electrophysiology of neurochemically identified submucosal neurones of the guinea-pig intestine

A number of electrophysiological studies have shown that neurones in the submucous plexus are endowed with three major types of synaptic potentials in response to nerve stimulation: a fast EPSP, a slow IPSP, and a slow EPSP. Combined electrophysiological and immunohistochemical studies enabled analysis of the types of neurochemically identified neurones which receive each type of synaptic input. This short review briefly summarizes the results obtained from these studies.

Neuropeptide Y hyperpolarizes submucosal neurons of the guinea-pig descending colon

Effects of neuropeptide Y (NPY) on submucosal neurons of the guinea-pig descending colon were investigated electrophysiologically by means of intracellular electrophysiological recordings. NPY (100 nM) induced a marked and prolonged hyperpolarization, accompanied by a decrease in input resistance in most (90%) neurons. This NPY-induced hyperpolarization was diminished and augmented by membrane hyperpolarization and depolarization, respectively. The NPY-hyperpolarization was not affected by exposure to either calcium-free solutions or the alpha2-adrenoceptor antagonist, idazoxan (1 microM). When more than one peptide was applied to a neuron, NPY, PYY and Pro34-NPY were equipotent, whilst NPY13-36 was less potent. It was concluded that NPY hyperpolarized submucosal neurons of the guinea-pig descending colon, possibly via a direct action on postsynaptic Y1-receptor and increasing potassium conductance.

Neuropeptide Y in submucosal ganglia: regional differences in the innervation of guinea-pig large intestine

Since information about possible regional differences in the innervation of the guinea-pig large intestine is incomplete, a comparative study was made of the occurrence of neurones and nerve fibres of the submucosa showing immunoreactivity (IR) to neuropeptide Y (NPY) and vasoactive intestinal polypeptide (VIP). In addition, a quantitative analysis was made of submucosal neurones in regions of guinea-pig large intestine selected for probable differences in their function. There were two principal findings: First, the density of NPY-IR neurone somata was high in the ascending colon (mean +/- SEM 3148 +/- 464 neurones/cm2; n = 5 animals) and progressively declined in an anal direction, the descending colon having 348 +/- 125 neurones/cm2 (in the same 5 animals); immunoreactive cell bodies were rare in the rectum. The reduced density was also reflected in a fall in the number of NPY-IR neurones/ganglion from 3.0 +/- 0.3 in the ascending colon to 0.5 +/- 0.2 in the descending colon. Second, varicose NPY-IR intraganglionic fibres were a conspicuous feature of the duodenum, caecum, transverse colon, descending colon and rectum, but not of the ileum, ascending colon or distal spiral. Moreover, in the descending colon and rectum the fibres were arranged in a loose 'cobweb' structure around non-NPY-IR neurone somata; in the caecum, there was an apparent paucity of NPY-IR somata but the exceptionally dense intraganglionic varicose fibre network may have obscured NPY-IR somata. In all regions, fibre baskets were rare. In the ascending colon, only 25 +/- 5% of ganglia (compared to 92 +/- 2% of ganglia in the descending colon) showed any intraganglionic nerve fibres; furthermore, when they occurred, these were not of the 'cobweb' type but, rather, they gave the ganglia a speckled appearance. In very immature fetuses at a stage of development when no neuropeptide somata could be found in either the myenteric or submucosal plexuses, many NPY-IR nerve fibres were present in the submucosa with a distribution similar to that of adult guinea pigs. With respect to the density of VIP-IR neurones in the large intestine, there was only a 40% reduction in the number of neurones/cm2 from proximal to distal colon, in contrast to the corresponding 90% reduction in the density of NPY-IR neurones. The number of VIP-IR neurones/ganglion (6.4) and the proportion of ganglia with VIP-IR fibres (> 90%) were constant. It is concluded that the striking regional dissimilarities in (i) the occurrence of NPY-IR neurone somata and (ii) in the disposition of intraganglionic NPY-IR nerve fibres indicate potentially important regional differences in the functions of neuropeptide Y as an antisecretory peptide in the local regulation of chloride transport in the mucosa and as a modulator of ganglionic transmission, respectively.