GABA inhibits excitatory neurotransmission in rat pelvic ganglia

Excitatory GABAA receptor in autonomic pelvic ganglion neurons innervating bladder

Major pelvic ganglia (MPG) are relay centers for autonomic reflexes such as micturition and penile erection. MPG innervate the urogenital system, including bladder. γ-Aminobutyric acid (GABA) is the major inhibitory neurotransmitter in the mammalian central nervous system, and may also play an important role in some peripheral autonomic ganglia, including MPG. However, the electrophysiological properties and function of GABAA receptor in MPG neurons innervating bladder remain unknown. This study examined the electrophysiological properties and functional roles of GABAA receptors in bladder-innervating neurons identified by retrograde Dil tracing. Neurons innervating bladder showed previously established parasympathetic properties, including small membrane capacitance, lack of T-type Ca(2+) channel expression, and tyrosine-hydroxylase immunoreactivity. GABAA receptors were functionally expressed in bladder innervating neurons, but GABAC receptors were not. GABA elicited strong depolarization followed by increase of intracellular Ca(2+) in neurons innervating bladder, supporting the hypothesis GABA may play an important role in bladder function. These results provide useful information about the autonomic function of bladder in physiological and pathological conditions.

The spinal pharmacology of urinary function: studies on urinary continence in the unanaesthetized rat

The volume-evoked micturition reflex (VEMR) is under the control of a complex vesico-spino-bulbo-spino-vesical reflex arc. When functional this system provides for the storage and retention of urine and its subsequent efficient expulsion by virtue of a joint contraction of the bladder and synergic relaxation of the urethral sphincter. Transection of the spinal cord results in an initial disruption of this organization (areflexia) followed by a time-dependent change in the characteristics of the functioning of this reflex system. The growth of knowledge of the pharmacology of spinal systems has yielded considerable information on the potential spinal neurotransmitter systems and their associated receptors. Given the possible role of such systems in mediating and modulating the VEMR, a reasonable approach has been to investigate the effects of spinally administered agonists and antagonists in unanaesthetized animals in which the VEMR can be examined. Thus, it appears that the initial state of bladder distension is signalled by larger (A type) afferent fibres. After spinal injury and the loss of this supraspinal control, smaller unmyelinated C fibres play a predominant role in controlling this reflex. On stimulation these C fibres release peptides (VIP, CCK, substance P, CGRP) and excitatory amino acids (glutamate). Studies in this laboratory have shown that whereas administration of these peptides is without effect in normal intact rats, the antagonists for glutamate and VIP receptors (but not CCK) produce a dose-dependent increase in spontaneous bladder contractions with a corresponding decrease in the volume required to evoke a VEMR. Other spinal systems, such as those for opioids and GABA, are known to exert modulatory effects upon spinal somatomotor reflex arcs. In the spinal cord these agonists (mu/delta and GABAA/B) produce discrete changes in the VEMR in intact and spinally transected animals. Thus these studies may provide insight into the coordinated mechanisms which govern the VEMR and may also allow the development of pharmacological approaches to managing the dysfunctional bladder.

Expression of GABAA receptor β2/3 subunits in the rat major pelvic ganglion

Several pharmacological and physiological studies have suggested that GABA(A) receptors (GABA(A) Rs) may exist in the rat major pelvic ganglion (MPG), a large coalescent pelvic ganglion that contains both sympathetic and parasympathetic components which innervates pelvic organs. However, the presence of GABA(A) R in the MPG has never been demonstrated directly by morphological studies. In the present study, we used immunohistochemistry to demonstrate the existence of GABA(A) R beta2/3 subunits for the first time in the rat MPG. We also analyzed the neurochemical properties of MPG neurons expressing GABA(A) R beta2/3 subunits. GABA(A) R beta2/3-immunoreactive (-IR) neurons occupied 27.4+/-7.0% of the whole neuronal population, and many of these (77.6%) were co-localized with tyrosine hydroxylase (TH). Likewise, most (86.5%) of TH-IR neurons were GABA(A) R beta2/3-positive. GABA(A) R beta2/3 subunits were also expressed in a few VIP- or NOS-IR neurons, the cholinergic or non-adrenergic, non-cholinergic (NANC) neurons. These results suggest that GABA(A) Rs are involved in the modulation of most sympathetic, noradrenergic neurons and also a subset of VIP and NOS neurons of the rat MPG.

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.

Interleukin-1beta causes a biphasic response in neurons of rat major pelvic ganglia

The effect of interleukin-1beta (IL-1beta) on peripheral autonomic neurons was examined with intracellular microelectrodes, in vitro. Recombinant human IL-1beta (6-300 pM) produced a depolarization, associated with decrease in input resistance, followed by a hyperpolarization, associated with increase in input resistance, in neurons of rat major pelvic ganglia (MPG). IL-1beta 163-171 (10-100 pM), the active domain of human IL-1beta, also produced a biphasic response. The IL-1beta-induced responses reversed polarity at the equilibrium potential for Cl-. The IL-1beta-induced responses were blocked by picrotoxin (100 microM) but not by bicuculline (20 microM). Imidazole-4-acetic acid (14AA, 100 microM), a GABA(C) receptor antagonist, reduced the IL-1beta-induced responses. The results suggest that the IL-1beta-induced biphasic response is mediated through GABA(C) receptors in rat MPG neurons.

Role of GABAA and GABAC receptors in the biphasic GABA responses in neurons of the rat major pelvic ganglia

The role of gamma-aminobutyric acid-A (GABAA) and GABAC receptors in the GABA-induced biphasic response in neurons of the rat major pelvic ganglia (MPG) were examined in vitro. Application of GABA (100 microM) to MPG neurons produced a biphasic response, an initial depolarization (GABAd) followed by a hyperpolarization (GABAh). The input resistance of the MPG neurons was decreased during the GABAd, whereas it was increased during the GABAh. The GABAd could be further separated into the early component (early GABAd) with a duration of 27 +/- 5 s (mean +/- SE; n = 11) and the late component (late GABAd) with a duration of 109 +/- 11 s (n = 11). The duration of the GABAh was 516 +/- 64 s (n = 11). The effects of GABA (5-500 microM) in producing the depolarization and the hyperpolarization were concentration-dependent. GABA (5-30 microM) induced only late depolarizations. The early component of the depolarization appeared when the concentration of GABA was >50 microM. Muscimol produced only early depolarizing responses. Baclofen (100 microM) had no effect on the membrane potential and input resistance of MPG neurons. Bicuculline (60 microM) blocked the early GABAd but not the late GABAd and the GABAh. Application of picrotoxin (100 microM) with bicuculline (60 microM) blocked both the late GABAd and the GABAh. CGP55845A (3 microM), a selective GABAB receptor antagonist, did not affect the GABA-induced responses. cis-4-Aminocrotonic acid (CACA, 1 mM) and trans-4-aminocrotonic acid (TACA, 1 mM), selective GABAC receptor agonists, produced late biphasic responses in the MPG neurons. The duration of the CACA responses was almost the same as those of the late GABAd and GABAh obtained in the presence of bicuculline. Imidazole-4-acetic acid (I4AA, 100 microM), a GABAC receptor antagonist, depressed the late GABAd and the GABAh but not the early GABAd. I4AA (100 microM) and picrotoxin (100 microM) also suppressed the biphasic response to CACA. The early GABAd and the late GABAd were reversed in polarity at -32 +/- 3 mV (n = 7) and -38 +/- 2 mV (n = 4), respectively, in the Krebs solution. The reversal potential of the GABAh was -34 +/- 2 mV (n = 4) in the Krebs solution. The reversal potentials of the late GABAd and the GABAh shifted to -20 +/- 3 mV (n = 5) and -22 +/- 3 mV (n = 5), respectively, in 85 mM Cl- solution. These results indicate that the late GABA(d) and the GABAh are mediated predominantly by bicuculline-insensitive, picrotoxin-sensitive GABA receptors, GABAC (or GABAAOr) receptors, in neurons of the rat MPG.

The inhibitory effect of nociceptin on the micturition reflex in anaesthetized rats

1. We have investigated the effect of nociceptin on the micturition reflex evoked by distension or topical application of capsaicin on the urinary bladder of urethane-anaesthetized rats. 2. Nociceptin produced a dose-dependent (3-100 nmol kg(-1) i.v.) transient suppression of the distension-evoked micturition reflex: its effect was not modified by guanethidine (68 micromol kg(-1) s.c.) nor by bilateral cervical vagotomy, alone or in combination, and by naloxone (1.2 micromol kg(-1) i.v.). 3. Nociceptin (100 nmol/kg i.v.) slightly (about 30%) inhibited the contractions of the rat bladder produced by pre- or postganglionic electrical stimulation of the pelvic nerve. 4. Nociceptin almost totally abolished the reflex component of the response to topical capsaicin (1 microg in 50 microl). 5. In the rat isolated bladder, submaximal contractions produced by electrical field stimulation were slightly reduced (25+/-4% inhibition) by 1 microM nociceptin. Nociceptin did not affect the contraction of the rat bladder induced by acetylcholine (10 microM) or ATP (1 mM). 6. These findings indicate that nociceptin exerts a naloxone-resistant suppression of the volume-evoked micturition reflex which involves inhibition of transmitter release from postganglionic bladder nerves. An inhibitory effect on bladder afferent nerves is also suggested.

Effects of GABA-receptor stimulation and blockade on micturition in normal rats and rats with bladder outflow obstruction

The effects on micturition of the GABA receptor agonists muscimol (selective for GABAA-receptors) and baclofen (selective for GABAB-receptors), given intrathecally and intra-arterially to unanesthetized rats with and without bladder hypertrophy secondary to outflow obstruction, were studied by continuous cystometry. When given intrathecally in increasing doses, both muscimol (1 to 10 micrograms) and baclofen (0.1 to 5 micrograms) produced a dose-dependent inhibition of micturition with progressive increases in bladder capacity and residual volume, and a decrease in micturition pressure ending with urinary retention and dribbling incontinence. Generally, the effects were similar in normal rats and rats with bladder hypertrophy. In normal rats, bicuculline (blocking GABAA-receptors) decreased bladder capacity and micturition volume, and increased micturition pressure. In both types of rat, the effects of muscimol and baclofen were partly counteracted by bicuculline and baclofen, respectively. In rats with bladder hypertrophy, the amplitude of the spontaneous bladder contractions during filling was significantly increased after administration of muscimol and baclofen. When given intra-arterially in high doses, muscimol (2 to 16 mg./kg.) and baclofen (8 to 16 mg./kg.) produced effects on cystometric parameters and spontaneous bladder contractions similar to those obtained with intrathecal administration. These results suggest that muscimol and baclofen have insignificant peripheral effects on the lower urinary tract, but depress micturition by an effect on the central nervous system. They also suggest that the inhibitory GABA-ergic system does not play a role in the genesis of bladder hyperactivity in rats with outflow obstruction.

Effect of the GABAB antagonist, phaclofen, on baclofen-induced inhibition of micturition reflex in urethane-anesthetized rats

The effect of intrathecal or intracerebroventricular administration of the GABAB receptor agonist, baclofen, on rhythmic contractions induced by distension of the urinary bladder (micturition reflex) was evaluated in urethane-anesthetized rats. Baclofen inhibited bladder motility acting at central nervous system sites (spinal and supraspinal) with a comparable potency. The inhibitory effect of i.t. baclofen (0.1-10 nmol) was blocked by i.t. phaclofen (200 nmol) while i.c.v. phaclofen did not affect i.c.v. baclofen (0.1-1 nmol). The inhibition of the micturition reflex induced by bladder distension observed after i.t. administration of baclofen was unaffected by systemic capsaicin pretreatment (50 mg/kg s.c., four days before). On the other hand, i.t. baclofen suppressed, in a phaclofen-sensitive manner, the reflex bladder contraction evoked by chemical stimulation (topical capsaicin) of capsaicin-sensitive bladder afferents. Intrathecal baclofen did not affect the hexamethonium-resistant tonic contraction produced by topical application of capsaicin on to the urinary bladder, which is ascribable to local peptide release from sensory nerves. Bladder motility inhibition by i.t. or i.c.v. baclofen (1 nmol) was unchanged by previous administration of p-chlorophenylalanine, indicating that the serotonergic pathways do not play a role in its action. Baclofen (100 microM) suppressed the release of calcitonin gene-related peptide-like immunoreactivity evoked by electrical field stimulation from the dorsal half of the rat spinal cord. This response was also abolished by in vitro capsaicin desensitization or tetrodotoxin, indicating that baclofen suppresses transmitter release from central endings of capsaicin-sensitive primary afferents. The present findings indicate that baclofen acts at both spinal and supraspinal sites to inhibit, with different mechanisms, the micturition reflex.(ABSTRACT TRUNCATED AT 250 WORDS)

Acrylamide-induced visceral neuropathy: evidence for the involvement of capsaicin-sensitive nerves of the rat urinary bladder

The mechanisms underlying the severe urinary retention induced by acrylamide intoxication were studied in detail in the rat. Subcutaneous treatment with acrylamide monomer (50 mg/kg daily for 10 days) almost completely impaired the micturition reflex, resulting in urinary retention. In fact, the ability to eliminate an oral water load was virtually abolished, while bladder filling with saline (transvesical cystometrogram) failed to activate reflex micturition. Instead, a picture of overflow incontinence resulted in urethane-anaesthetized rats, which was not reversed by intravenous administration of 4-aminopyridine. The nerve-mediated contractile response to field stimulation (0.1-20 Hz, 0.5 ms, 60 V) of the isolated bladder was unaffected, thus suggesting the integrity of bladder efferent innervation, and no evidence was found from in vitro experiments that the myogenic contractility of the bladder was depressed by acrylamide treatment. Conversely, the sensory nerve-mediated response to capsaicin was abolished and sensory nerve fibres of the bladder were selectively depleted of their content of substance P- and calcitonin gene-related peptide immunoreactivity following acrylamide treatment. In fact, concentrations of the same neuropeptides in other organs, including the adjoining ureters, were unaffected. As to the urethral segment, including the striated sphincter, the D-tubocurarine (0.2 mM)-sensitive urethral response to electrical stimulation (0.1 Hz, 0.1 ms, 20 V) was significantly reduced in acrylamide-treated animals. At the same level, neurofilament protein immunostaining revealed striking accumulations of neurofilament protein-like material in motor end-plates, thus indicating that neuromuscular junctions of the urethral striated sphincter were severely affected. Thus, the afferent arm of the micturition reflex was shown to be severely deranged by acrylamide intoxication, especially in its capsaicin-sensitive component. Since twitch-like contractions of the urethral striated sphincter are probably involved in promoting bladder voiding, a decreased efficiency of this mechanism could participate in the picture of urinary retention induced by acrylamide.

Topical versus systemic capsaicin desensitization: specific and unspecific effects as indicated by modification or reflex micturition in rats

The aim of this study was to determine the acute and delayed effect of topical application of high concentrations of capsaicin on the rat urinary bladder on micturition reflex and compare the effects of "topical" bladder desensitization with those produced by systemic (subcutaneous administration) capsaicin desensitization. On acute application, capsaicin (1-3%) produced a transient bladder contraction, not observed in capsaicin-pretreated rats. After a transient increase in excitability of the micturition reflex, topical capsaicin suppressed micturition and overflow incontinence ensued which was reverted by intravenous injection of 4-aminopyridine. Topical capsaicin also abolished reflex micturition in rats which had been systemically treated with capsaicin as adults (50 mg/kg, 7 days before) and reduced significantly the neurogenic bladder contractions produced by intravenous dimethylphenylpiperazinium or neurokinin A, while the direct (myogenic) response to neurokinin A was unaffected. In rats whose bladder was pre-exposed to 1-3% topical capsaicin (7 days before) the micturition reflex was affected in a manner which is qualitatively and quantitatively similar to that observed in rats treated with capsaicin as adults, e.g. increase in bladder capacity with no change in voiding efficiency. Topical capsaicin desensitization of the rat urinary bladder was shown to produce a selective impairment of bladder sensory nerves without any sign of desensitization in other areas of the body using both functional (hot plate, wiping, plasma extravasation) and neurochemical (determination of substance P-like immunoreactivity) assays. Systemically administered capsaicin (7 days before) had little effect on reflex micturition at 12.5 mg/kg but the change in bladder capacity produced at a dose of 25 mg/kg was comparable with that produced at 350 mg/kg. These findings provide evidence that selective desensitization of peripheral terminals of capsaicin-sensitive nerves of the rat urinary bladder inactivates their sensory and "efferent" function in a manner similar to that observed after systemic capsaicin desensitization in adult rats. The functional deficit of reflex micturition produced in this way can be overcome by increasing the stimulus to void. By contrast, neonatal capsaicin desensitization produced a long lasting abolition of reflex micturition. These data are in keeping with the hypothesis that adult versus neonatal capsaicin desensitization may be used as a tool to distinguish between two sets of sensory nerves in the rat urinary bladder.(ABSTRACT TRUNCATED AT 400 WORDS)

The effect of 4-aminopyridine on micturition reflex in normal or capsaicin-desensitized rats

4-aminopyridine (4-AP) produced a dose-related (0.15-2 mg/kg i.v.) potentiation of the voiding cycle of the urinary bladder and increased frequency of micturition in urethane-anesthetized rats. In bladders containing a subthreshold amount of fluid for eliciting reflex micturition 4-AP (1-3 mg/kg i.v.) activated a series of high-amplitude, hexamethonium-sensitive rhythmic bladder contractions. In rats desensitized to capsaicin as newborns, reflex micturition was almost abolished: in these animals i.v. 4-AP did not affect bladder voiding unless at high doses (1-2 mg/kg), at which a reversal from anesthesia occurred. This was accompanied by a prompt micturition. In unanesthetized rats, neither the 4-AP-induced convulsions nor the behavioral response (assessed in an open field) to 4-AP were affected by neonatal capsaicin desensitization. Daily urine production of capsaicin-pretreated animals did not differ from that of controls. However, when measurements were made during daytime, almost no spontaneous urine emission was found in capsaicin-treated rats. On the rat isolated urinary bladder, 4-AP potentiated the response to field stimulation in preparations from both vehicle- and capsaicin-pretreated animals. These findings indicate that 4-AP has a potent excitatory action on bladder voiding in rats and support the hypothesis that in this species 'conscious' bladder voiding can be initiated through capsaicin-resistant mechanisms.

Localization of L-glutamate decarboxylase immunoreactivity in the major pelvic ganglion and in the coeliac-superior mesenteric ganglion complex of the rat

The localization of L-glutamate decarboxylase (GAD), the GABA-synthesizing enzyme, was studied in the rat major pelvic ganglion and in the coeliac-superior mesenteric ganglion complex by indirect immunofluorescence technique with a specific antiserum raised in rabbits. GAD immunoreactivity was demonstrated in small cells of these ganglia. The GAD-immunoreactive small cells were 10-20 microns in diameter and formed clusters or occurred as solitary cells. The principal neurons were non-reactive but they were surrounded by immunoreactive processes. Studies on colocalization of GAD with tyrosine hydroxylase (TH), the rate-limiting enzyme of the catecholamine synthesis, in the major pelvic ganglion and in the coeliac-superior mesenteric ganglion complex indicated that all GAD-immunoreactive small cells were also labelled with TH. In the major pelvic ganglion all TH-immunoreactive SIF cells were also immunoreactive for GAD. However, in the coeliac-superior mesenteric ganglion complex there occurred TH-immunoreactive small cells which showed no immunoreactivity to GAD. It is suggested that the small GAD-immunoreactive cells represent small intensely fluorescent (SIF) cells.

Neuroeffector mechanisms in the voiding cycle of the guinea-pig urinary bladder

1. The effect of drugs expected to modify cholinergic (atropine, physostigmine) or GABA-ergic (GABA, picrotoxin, bicuculline methiodide, BMI) neurotransmission were investigated on the voiding cycle of the urinary bladder in urethane-anaesthetized guinea-pigs. 2. Transvesical saline infusion elicited a series of regular voiding cycles which were abolished by topical tetrodotoxin as well as by topical or intravenous (i.v.) hexamethonium, thus indicating their reflex neurogenic origin. 3. Either i.v. or topical atropine decreased amplitude and duration of micturition contraction, reduced efficiency of bladder voiding and, in about 30% of preparations, suppressed micturition (overflow incontinence). Either topical or i.v. physostigmine potentiated, in an atropine-sensitive manner, voiding efficiency. 4. GABA was administered during the ascending limb of the voiding cycle: i.v. GABA inhibited while topical GABA increased efficiency of the first voiding cycle. Both direction and intensity of the effect of GABA were dependent upon initial pressures. The second and following voiding cycles after GABA administration were inhibited transiently. 5. Either topical or i.v. picrotoxin or BMI potentiated the efficiency of the voiding cycle of the guinea-pig bladder. In these preparations either i.v. or topical GABA (administered during the ascending limb of the voiding cycle) produced only inhibitory effects on voiding efficiency. 6. These findings indicate that, in guinea-pigs, bladder voiding is largely dependent upon an intact cholinergic neurotransmission. Although amplitude of the initial voiding contraction is largely atropine-resistant, its duration and, consequently, degree of bladder voiding, are strongly influenced by blockade (atropine) or potentiation (physostigmine) of cholinergic neurotransmission. Endogenous GABA-ergic mechanism modulate, possibly at peripheral level, the voiding efficiency of the guinea-pig bladder. GABA exerts a complex neuromodulatory influence on voiding function, its overall effect being mainly influenced by resting parameters.

The effects of baclofen on spinal and supraspinal micturition reflexes in rats

1. The effect of (+/-)-baclofen on micturition reflexes was investigated in urethane-anaesthetized rats. A 'low' dose of (+/-)-baclofen (0.5 mg/kg i.v.) barely affected the early phase of the transurethral cystometrogram (CMG) which involves activation of a spinal vesico-vesical excitatory reflex. 2. At a higher dose (2.5 mg/kg i.v.) (+/-)-baclofen suppressed both the spinal and supraspinal components of the bladder response to transurethral saline filling. 3. When the bladder was filled by the transvesical route a series of regular voiding cycles was obtained which are due to activation of a supraspinal vesico-vesical excitatory reflex. In this model, voiding efficiency of the rat bladder was markedly reduced even after a low dose of (+/-)-baclofen (0.5 mg/kg) and almost suppressed at 2.5 mg/kg. 4. (+/)-Baclofen reduction of voiding efficiency was mainly ascribable to an inhibitory effect on the expulsive phase of the voiding cycle which, in rats, depends critically upon the activation of a reflex which induces a twitch-like contraction of urethral/periurethral skeletal muscles. 5. (+/-)-Baclofen produced a small inhibition of the pinching-induced somatovesical excitatory reflex. (+/-)-Baclofen (2.5 mg/kg i.v.) produced also a marked but transient inhibition of bladder contractions induced by preganglionic nerve stimulation. However the time course of this effect was markedly shorter as compared to the long lasting suppression of voiding cycle observed with this same dose of the drug.(ABSTRACT TRUNCATED AT 250 WORDS)

An atropine-like inhibitory effect of DMPP on rat isolated urinary bladder

DMPP inhibits the nerve-mediated contractions of the rat isolated bladder, its effect being greater in preparations from newborn (2 day old) than adult animals. This effect of DMPP was unaffected by hexamethonium. In preparations from adult animals the effect of DMPP increased with frequency of stimulation and was fully prevented by the presence of atropine. In bladders from newborn rats low concentrations of furthrethonium (FHR) (10 nM) activated a series of rhythmic contractions which were unaffected by tetrodotoxin and abolished by DMPP through an hexamethonium-insensitive action. On the other hand DMPP did not affect rhythmic contractions produced by a low concentration of eledoisin (60 nM). In bladders from adult rats FHR (10 microM) and KCI (30 mM) produced contractures of comparable magnitude. DMPP inhibited, in concentration-related manner the FHR-induced tonic contraction but had little effect on that produced by KCI. These findings indicate that in the rat bladder, DMPP antagonizes selectivity cholinergically-mediated contractions through a mechanism which is unaffected by hexamethonium or tetrodotoxin. An "atropine-like' activity of DMPP should be considered.

The nonstop transvesical cystometrogram in urethane-anesthetized rats: a simple procedure for quantitative studies on the various phases of urinary bladder voiding cycle

A new cystometric procedure that involves a nonstop transvesical (through a needle inserted into the bladder dome) infusion of saline in urethane-anesthetized rats is described. This permits the obtainment of a series of repetitive voiding cycles in both male and female rats. Recording at high-chart speed permits quantitative measurements of the various phases of the voiding cycle. Micturition was almost invariably associated with appearance, during a sustained increase of intravesical pressure, of a series of high-frequency oscillations, which were paralleled by a stream-like emission of the infused fluid. Micturition was not observed after topical tetrodotoxin, after bilateral transection of pelvic nerves, or in acute spinal rats. Intravenous hexamethonium produced a dose-related impairment of the voiding cycle, and, at high doses, suppressed micturition, and overflow incontinence ensued. The experimental procedure described herein appears to be suitable for physiopharmacological studies dealing with regulation of voiding cycle.

Pharmacological evidence for the existence of two components in the twitch response to field stimulation of detrusor strips from the rat urinary bladder

Isolated strips from the anterior dome of the rat urinary bladder respond to single pulse field stimulation with a contraction. Two distinct components of this contraction ("early' and "late') could be observed, both of which were unaffected by hexamethonium (10 microM) and almost abolished by tetrodotoxin (1 microM) indicating their dependence on neurotransmitter release from postganglionic nerve endings. Atropine (3 microM) inhibited the "late' component (over 60%) to a significantly greater extent than the "early' component (less than 10%) Amplitude of the "early' component was usually greater than that of the "late' component. There was almost no difference between the "early' and "late' component in respect of their relationship to stimulus strength and pulse duration. Physostigmine (0.03 microM) enhanced both components of the nerve-mediated contraction, although enhancement of the "late' component was much greater than that of the "early' one. Tetraethylammonium (TEA, 0.5-5 mM) enhanced, in a concentration-related manner, both "early' and "late' components of the nerve-mediated contraction. Following exposure to physostigmine or TEA (5 mM) both "early' and "late' components of contraction were almost completely inhibited by tetrodotoxin. Atropine inhibition was more evident on the "late' as compared to "early' component of contraction. These findings demonstrate the presence, in the twitch response of rat isolated urinary bladder to field stimulation, of two nerve-mediated components which exhibit a different susceptibility to atropine and physostigmine.