Pharmacological characterization of tachykinin-stimulated inositol phospholipid hydrolysis in peripheral tissues

Pharmacology of the lower urinary tract: basis for current and future treatments of urinary incontinence

The lower urinary tract constitutes a functional unit controlled by a complex interplay between the central and peripheral nervous systems and local regulatory factors. In the adult, micturition is controlled by a spinobulbospinal reflex, which is under suprapontine control. Several central nervous system transmitters can modulate voiding, as well as, potentially, drugs affecting voiding; for example, noradrenaline, GABA, or dopamine receptors and mechanisms may be therapeutically useful. Peripherally, lower urinary tract function is dependent on the concerted action of the smooth and striated muscles of the urinary bladder, urethra, and periurethral region. Various neurotransmitters, including acetylcholine, noradrenaline, adenosine triphosphate, nitric oxide, and neuropeptides, have been implicated in this neural regulation. Muscarinic receptors mediate normal bladder contraction as well as at least the main part of contraction in the overactive bladder. Disorders of micturition can roughly be classified as disturbances of storage or disturbances of emptying. Failure to store urine may lead to various forms of incontinence, the main forms of which are urge and stress incontinence. The etiology and pathophysiology of these disorders remain incompletely known, which is reflected in the fact that current drug treatment includes a relatively small number of more or less well-documented alternatives. Antimuscarinics are the main-stay of pharmacological treatment of the overactive bladder syndrome, which is characterized by urgency, frequency, and urge incontinence. Accepted drug treatments of stress incontinence are currently scarce, but new alternatives are emerging. New targets for control of micturition are being defined, but further research is needed to advance the pharmacological treatment of micturition disorders.

Tachykinins and tachykinin receptors: effects in the genitourinary tract

Tachykinins (TKs) are a family of peptides involved in the central and peripheral regulation of urogenital functions through the stimulation of TK NK1, NK2 and NK3 receptors. At the urinary system level, TKs locally stimulate smooth muscle tone, ureteric peristalsis and bladder contractions, initiate neurogenic inflammation and trigger local and spinal reflexes aimed to maintain organ functions in emergency conditions. At the genital level, TKs are involved in smooth muscle contraction, in inflammation and in the modulation of steroid secretion by the testes and ovaries. TKs produce vasodilatation of maternal and fetal placental vascular beds and appear to be involved in reproductive function, stress-induced abortion, and pre-eclampsia. The current data suggest that the genitourinary tract is a primary site of action of the tachykininergic system.

Urinary bladder contraction and relaxation: physiology and pathophysiology

The detrusor smooth muscle is the main muscle component of the urinary bladder wall. Its ability to contract over a large length interval and to relax determines the bladder function during filling and micturition. These processes are regulated by several external nervous and hormonal control systems, and the detrusor contains multiple receptors and signaling pathways. Functional changes of the detrusor can be found in several clinically important conditions, e.g., lower urinary tract symptoms (LUTS) and bladder outlet obstruction. The aim of this review is to summarize and synthesize basic information and recent advances in the understanding of the properties of the detrusor smooth muscle, its contractile system, cellular signaling, membrane properties, and cellular receptors. Alterations in these systems in pathological conditions of the bladder wall are described, and some areas for future research are suggested.

Tachykinins as modulators of the micturition reflex in the central and peripheral nervous system

In the normal urinary bladder, tachykinins (TKs) are expressed in a population of bladder nociceptors that is sensitive to the excitatory and desensitizing effects of capsaicin (i.e., capsaicin-sensitive primary afferent neurons (CSPANs)). Several endobiotics or xenobiotics excite CSPANs and release TKs and other mediators at both the peripheral and spinal cord level. The peripheral release of TKs determines a set of responses (known as neurogenic inflammation) that includes vasodilatation, plasma protein extravasation, smooth muscle contraction and stimulation of afferent nerves. Following chronic inflammation, both immune cells and capsaicin-resistant sensory neurons can de novo express TKs: whether these pools of TKs are releasable and contribute to inflammatory processes is presently unsettled. At the spinal cord level, the release of TKs contributes in determining an altered pattern of vesicourethral reflexes in response to nociceptive stimulation of the bladder by conveying: (a) the afferent transmission to supraspinal sites, and (b) descending or sensory inputs to the sacral parasympathetic nucleus (SPN). Recent evidence also attribute a synergetic role of TKs in the supraspinal modulation of the sensory arm of the micturition reflex. The overall available information suggests that TK receptor antagonists may affect bladder motility/reflexes which occur during different pathological states, while having little influence on the normal motor bladder function.

Modulation by protein kinase C of the enhanced responsiveness to tachykinins in ovalbumin-sensitized guinea pig alveolar macrophages

As previously reported, alveolar macrophages (AMs) from ovalbumin-sensitized guinea pigs present an enhanced responsiveness to tachykinins but not to N-formylmethionyl-leucyl-phenylalanine (fMLP). We have investigated the biochemical mechanisms underlying this varied responsiveness to tachykinins. The protein kinase C (PKC) activator phorbol 12-myristate 13-acetate (PMA) induced a larger superoxide anion (O2-) production in AMs from sensitized guinea pigs, as did tachykinins. Pretreatment of AMs with pertussis toxin abolished tachykinin-evoked respiratory burst, had no effect on PMA-evoked O2- production and strongly inhibited fMLP-evoked one, with no appreciable variation between control or sensitized AMs. Staurosporine and its derivative cgp 41251, significantly decreased PMA- and tachykinin-evoked O2- production in both populations, being more potent in control AMs, but exerted little effects against fMLP. Pretreatment of AMs with PMA significantly inhibited fMLP-, PMA- and tachykinin-evoked O2- production in both control and sensitized AMs. fMLP, substance P (SP), neurokinin A (NKA) and the NK2 agonist [beta-Ala8]-NKA(4-10) dose-dependently increased [3H] phorbol 12, 13 dibutyrate (PDBu) binding to control and sensitized AMs. While fMLP exerted similar effects in both populations, dose-response curves for SP1 NKA and the NK2 receptor agonist were shifted leftwards (1, 4 and 3 orders of magnitude, respectively) in sensitized AMs. These results indicate a possible PKC involvement in the enhanced responsiveness to tachykinins in actively sensitized AMs.

The mammalian tachykinin receptors

The tachykinins (TKs) are a family of small peptides which share the common C-terminal sequence Phe-X-Gly-Leu-MetNH2. Three peptides of this family, substance P, neurokinin A and neurokinin B, have an established role as neurotransmitters in mammals. 2. Three receptors for TKs have been cloned: they are G-protein coupled receptors with seven putative transmembrane spanning segments and have been termed NK1 (substance P-preferring), NK2 (neurokinin A-preferring) and NK3 (neurokinin B-preferring). 3. Synthetic agonists are available to selectively stimulate only one receptor, while natural TKs can act as full agonist at each one of the three receptors, albeit at different concentrations. 4. A number of potent and selective antagonists, both peptide and nonpeptide in nature, have recently been developed. 5. The introduction of these ligands has revealed an unforeseen pharmacological heterogeneity of NK1, NK2 and NK3 receptors which appears largely, if not exclusively, linked to the existence of species homologues of the three receptors.

Involvement of septide-sensitive tachykinin receptors in inositol phospholipid hydrolysis in the rat urinary bladder

The selective NK2 agonist [Lys5-MeLeu9,Nle10]NKA(4-10) markedly stimulated [3H]inositol monophosphate (PI1) formation in prisms from the rat urinary bladder. This response was blocked by the NK2 antagonist SR 48968. Senktide (NK3 agonist) was inactive. Septide, a short SP analogue, and the NK1 agonists [Pro9]SP and [Sar9,Met(O2)11]SP also stimulated [3H]IP1 formation and several NK1 tachykinin antagonists (RP 67580, CP 96345, GR 82334, and [D-Pro9,t beta-BPr10,Trp11]SP) were more potent in blocking the septide than the [Pro9]SP response. GR 82334 was the most discriminative. SR 48968 (10(-6) M shifted the [Pro9]SP dose-response curve but did not modify the septide dose-response curve. Septide had a low affinity for [3H][Pro9]SP binding sites, suggesting further that septide and NK1 agonists act on different receptors. Finally, both [Pro9]SP and [Sar9,Met(O2)11]SP blocked the septide-evoked response, acting as partial agonists at the septide-sensitive tachykinin receptors.

Non-adrenergic, non-cholinergic control of the urinary bladder

In the urinary, bladder, ATP is an excitatory neuromuscular transmitter, possibly a cotransmitter with acetylcholine from postganglionic parasympathetic nerves, which activates P2X-purinoceptors. The synthesis of prostaglandins is closely linked to the activation of P2X-purinoceptors, and these compounds make a significant contribution to non-cholinergic neurogenic responses. Many neuropeptides, such as NPY, VIP, somatostatin, SP and CGRP, are found in nerves innervating the lower urinary tract, but it is unlikely that any is a neuromuscular transmitter in the detrusor; rather, they may act as potent modulators of sympathetic and parasympathetic transmission. Modulatory actions are shown by GABA par excellence; this compound is also well represented in vesicular neurons and, via activation of GABAA- or GABAB-receptors, can potentiate or inhibit parasympathetic transmission. Although not discussed in depth in this review, the urinary bladder shows extraordinary plasticity in expression of nerves and of their transmitters and receptors under pathophysiological conditions, including pregnancy and ageing as well as disease states. Finally, the accessibility of the urinary bladder and the enormous range of chemoreceptors that it possesses has led to its being used extensively for pharmacological investigations of transmitter and drug receptors and their subclasses.

Signal transduction mechanisms of recombinant bovine neurokinin-2 receptor stably expressed in baby hamster kidney cells

The bovine neurokinin-2 (NK-2) receptor gene was stably transfected into Baby hamster kidney (BHK-21) fibroblasts and one recombinant clone expressing 17,700 high-affinity [125I]neurokinin A (NKA) binding sites/cell characterized further. [125I]NKA binding was displaced by unlabeled NKA with an IC50 of 8.26 +/- 2 nM (n = 5) and with the rank order of potency NKA > neurokinin B (NKB) > Substance P (SP) confirming pharmacological characteristics of an NK-2 receptor subtype. Stimulation with NKA resulted in a rapid and dose-dependent increase in inositol 1,4,5-trisphosphate (IP3) levels (EC50 = 32 +/- 10 nM; n = 7) which was paralleled by a transient biphasic rise in intracellular free calcium concentration [Ca2+]i (EC50 = 35 +/- 20 nM; n = 3). In addition to phosphoinositide (PI) hydrolysis and Ca2+ mobilization, NKA was found to stimulate both cyclic AMP formation (EC50 = 1.02 +/- 0.26 microM; n = 7) and [3H]arachidonic acid mobilization (EC50 = 0.65 +/- 0.45 microM; n = 4). Interestingly, cyclic AMP levels also rose after addition of an exogenous arachidonic acid metabolite, prostaglandin E2 (PGE2) (EC50 = 11.5 +/- 2 microM). Similar observations of NKA-induced IP3 production, Ca2+ mobilization, arachidonic acid liberation, and cAMP formation have been made previously following expression of the bovine NK-2 receptor in Chinese hamster ovary (CHO) epithelial cells. The present results suggest that activation of NK-2 receptors leads to characteristic and reproducible intracellular second messenger responses in a subclass of cell types which includes fibroblasts and epithelial cells irrespective of their genetic and phenotypic background.

Tachykinin-stimulated inositol phospholipid hydrolysis and taurine release from human astrocytoma cells

The activation of NK1 receptors on U373 MG human astrocytoma cells by substance P (SP) and related tachykinins was accompanied by an increase in taurine release and an accumulation of inositol phosphates. Both of these effects could be inhibited by spantide, a SP receptor antagonist. The relative potency of tachykinins in stimulating 3H-inositol phosphate accumulation correlated very well with their effects in stimulating the release of [3H]-taurine and inhibition 125I-Bolton-Hunter reagent-conjugated SP binding. The effect on [3H]taurine release was mimicked by a protein kinase C (PKC) activator, phorbol 12-myristate 13-acetate (PMA). The inactive phorbol ester analogue 4-alpha-phorbol 12,13-didecanoate, however, was without effect. Both SP- and PMA-induced releases of [3H]-taurine were markedly inhibited by staurosporine, a potent PKC inhibitor. Pretreatment of U373 MG cells with 10 microM PMA for 19 h to down-regulate PKC activity also markedly inhibited both SP- and PMA-induced releases of [3H]-taurine. Treatment of cells with 100 nM SP induced a time-dependent translocation of PKC from the cytosolic fraction to the membrane fraction. These findings are consistent with the hypothesis that an activation of NK1 receptors on U373 MG cells results in the release of inositol phosphates and activation of PKC, which in turn may regulate the release of taurine.

Comparison of the effects of neuropeptide K and neuropeptide gamma with neurokinin A at NK2 receptors in the hamster urinary bladder

Neuropeptide K (NPK) and neuropeptide gamma (NP gamma) are two endogenous N-terminally extended forms of neurokinin A (NKA). Here, we compared their effects with those of NKA on 125I-NKA binding, phosphatidylinositol (PI) turnover and smooth muscle contraction in the hamster urinary bladder. NPK, NP gamma and NKA were equipotent in competing 125I-NKA from NK2 receptors in crude hamster bladder membranes. All three peptides stimulated PI turnover by approximately 750% with similar potency. In a third series of experiments, these peptides had similar efficacy in inducing a dose-dependent contraction of bladder smooth muscle. The NK2 receptor selective antagonist L-659,877 (cyclo[Leu-Met-Gln-Trp-Phe-Gly]) inhibited the stimulation of PI turnover and bladder contractions induced by all three tachykinins. The present results show that NKA, NPK and NP gamma display a similar biological profile. The N-terminal extensions of NPK and NP gamma appear not to influence binding of these peptides to NK2 receptors, NK2 receptor mediated stimulation of PI turnover, or smooth muscle contraction in hamster urinary bladder.