The effect of omega conotoxin GVIA, a peptide modulator of the N-type voltage sensitive calcium channels, on motor responses produced by activation of efferent and sensory nerves in mammalian smooth muscle

Nitric oxide is the mediator of tachykinin NK3 receptor-induced relaxation in the circular muscle of the guinea-pig ileum

The tachykinin NK3 receptor agonist, senktide, produces concentration-dependent contraction of the circular muscle of the guinea-pig ileum (EC50 2.59 nM). In the presence of the blocker of neuronal type of voltage-sensitive calcium channels, omega-conotoxin (0.1 microM), the contractile response to a low concentration of senktide was converted to an inhibitory effect on spontaneous activity of the ileum. This inhibitory effect was further enhanced in the presence of atropine (1 microM) and was abolished by tetrodotoxin (1 microM), indicating its neural origin. In the presence of atropine and omega-conotoxin, the inhibitory response to senktide (1 nM) was greatly inhibited or even abolished by L-nitroarginine (30 microM), its effect being prevented by L-arginine but not by D-arginine (300 microM in each case). Apamin (0.1 microM) failed to significantly affect the inhibitory response to senktide. Apamin enhanced spontaneous activity of the preparation while L-nitroarginine had no effect. Neither apamin nor L-nitroarginine affected the inhibitory response to isoprenaline. These findings indicate that inhibition of circular muscle activity produced through NK3 receptor stimulation in the guinea-pig ileum is mediated through a neuronal pathway involving nitric oxide or a nitric oxide-like substance(s) generation.

Inhibition of endogenous acetylcholine release by blockade of voltage-dependent calcium channels in enteric neurons of the guinea-pig colon

The effects on acetylcholine release from the guinea-pig colon of the N-type calcium channel blocker omega-conotoxin GVIA (omega-conotoxin), the L-type calcium channel blocker nifedipine and the putative blocker of T-type channels, flunarizine, have been investigated. Endogenous basal acetylcholine release and electrically (1 Hz, 1 ms, 450 mA)-evoked overflow in the presence of cholinesterase inhibitor were studied. omega-Conotoxin (1-10 nM) and nifedipine (0.03-3 microM) dose-dependently inhibited basal and electrically-evoked acetylcholine release. Maximal inhibition of basal or electrically-evoked acetylcholine release was about 40% for nifedipine and about 75% for omega-conotoxin. The potency of nifedipine was inversely related to the external calcium concentration: its EC50 value in low-calcium medium (0.5 mM) was as low as 12 nM. Flunarizine inhibited acetylcholine release only at concentrations higher than 0.2 microM. Our results are consistent with an involvement of N- and L-type calcium channels in the control of the endogenous acetylcholine release from the guinea-pig colon.

Pharmacological characterization of voltage-sensitive Ca2+ channels in autonomic nerves

Hololena curta venom a potent inhibitor of voltage sensitive Ca2+ channels and neurotransmitter release in mammalian brain, and synthetic funnel web spider toxin an inhibitor of P channels, were examined for their activity on autonomic nerves. Hololena curta (0.5 to 5.0 micrograms venom protein/ml) potently inhibited motor responses of the cholinergic guinea pig ileum myenteric plexus and the adrenergic rat anococcygeus muscle. Synthetic funnel web spider toxin was inactive at concentrations up to 100 microM. Hololena curta inhibited K+, and electrically evoked release of tritium from labeled superfused tissues. Furthermore, K(+)-contracted rat aorta was not relaxed by Hololena curta thereby precluding effects of Hololena curta on postjunctional L type smooth muscle Ca2+ channels. The pattern of effects of Hololena curta on peripheral autonomic nerves was similar to the N channel inhibitor omega-conotoxin GVIA. These results suggest that Hololena curta venom constituents block Ca2+ channels in peripheral autonomic nerves. The study failed to establish the presence of functional P type Ca2+ channels on these peripheral autonomic nerves and further suggests that N type channels may be exclusively responsible for supplying the Ca2+ necessary for neurotransmitter release in these nerves.

Effects of stimulus intensity on the inhibition by omega-conotoxin GVIA and neomycin of K(+_-evoked [3H]norepinephrine release from hippocampal brain slices and synaptosomal calcium influx

The effects of various K+ concentrations on the inhibition of [3H]norepinephrine release from rat hippocampal brain slices and evoked synaptosomal 45Ca2+ influx by omega-conotoxin GVIA (omega-CgTx) and neomycin were examined. K+ (15-75 mM) caused a concentration-dependent release of [3H]norepinephrine that was greater than 90% dependent on extracellular calcium. The ability of omega-CgTx to inhibit [3H]norepinephrine release was optimal at 25 mM K+ and was reduced substantially at higher concentrations of K+. omega-CgTx maximally inhibited [3H]norepinephrine release by 49% (15 mM K+), 58% (25 mM K+), 22% (50 mM K+), and 12% (75 mM K+). In contrast, neomycin caused a concentration-dependent and virtually complete inhibition of [3H]norepinephrine release at all concentrations of K+, with IC50 values of 210 microM (15 mM K+), 150 microM (25 mM K+), 450 microM (50 mM K+), and 1500 microM (75 mM K+). omega-CgTx (1 microM) had little effect (10% or less inhibition) on hippocampal synaptosomal 45Ca2+ influx at any concentration of K+, whereas 3 mM neomycin caused at least 75% inhibition of 45Ca2+ influx, with the largest inhibition (96%) occurring at 25 mM K+. The results suggest that increasing stimulus intensity decreases the contribution of N-type voltage-sensitive calcium channels (VSCC) in mediating K(+)-evoked release of [3H]norepinephrine. The comparative absence of omega-CgTx-sensitive synaptosomal 45Ca(2+)-influx sites suggests that N-type calcium channels are a small subset of channels in rat hippocampal synaptosomes. The demonstration that neomycin can inhibit omega-CgTx-sensitive and -insensitive neurotransmitter release and calcium influx suggests that neomycin may block N-type VSCC as well as non-N-type VSCC.

Actions of neomycin on neuronal L-, N-, and non-L/non-N-type voltage-sensitive calcium channel responses

The effects of neomycin on neuronal voltage-sensitive calcium channel (VSCC) responses were investigated by evaluating its effects on calcium-dependent neuronal responses that are sensitive and insensitive to the N-type voltage-sensitive calcium channel antagonist omega-conotoxin GVIA and the L-type VSCC antagonist nitrendipine. Chick synaptosomal 45Ca2+ influx and K(+)-evoked release of [3H]norepinephrine from chick cortical brain slices were omega-conotoxin GVIA sensitive and nitrendipine insensitive, suggesting that these responses are mediated predominantly by N-type VSCC. The K(+)-evoked increase of intracellular calcium in cortical neurons and the K(+)-evoked release of [3H]norepinephrine from rat brain cortical slices was partially sensitive to omega-conotoxin GVIA and nitrendipine, suggesting that these responses are mediated by N-, L- and non-L/non-N-type VSCC. Rat synaptosomal 45Ca2+ influx and the K(+)-evoked release of [3H]D-aspartate from rat hippocampal slices were completely insensitive to omega-conotoxin GVIA and nitrendipine, suggesting that these responses were mediated predominantly by non-L/non-N-type VSCC. Neomycin caused a concentration-dependent and virtually complete inhibition of all response parameters, with IC50 values ranging from 90 to 400 microM. The results suggest that neomycin is a nonselective inhibitor of neuronal responses mediated by L-, N-, and non-L/non-N-type VSCC.

Different ion channel mechanisms between low concentrations of capsaicin and high concentrations of capsaicin and nicotine regarding peptide release from pulmonary afferents

Vagal nerve stimulation (1 Hz for 1 min), capsaicin (10(-8) M and 10(-6) M), resiniferatoxin (3 x 10(-10) M) and nicotine (10(-4) M) evoked a non-cholinergic bronchoconstriction in the isolated perfused guinea-pig lung preparation. Simultaneously there was an increase in the perfusate levels of calcitonin gene-related peptide-like immunoreactivity, suggesting release from sensory nerves. Both the bronchoconstriction and peptide release evoked by a low concentration of capsaicin (10(-8) M) and that evoked by nerve stimulation were depressed by tetrodotoxin, suggesting involvement of Na+ channel dependent depolarization. Since the effects of capsaicin (10(-8) M) and vagal nerve stimulation were inhibited by omega-conotoxin but not influenced by nifedipine, the Ca(2+)-channel dependent is probably of N-type. Furthermore, the capsaicin analogue resiniferatoxin also evoked omega-conotoxin sensitive peptide release and bronchoconstriction. At the higher capsaicin concentration (10(-6) M), the functional response was only slightly inhibited by omega-conotoxin or tetrodotoxin indicating that capsaicin at this concentration evoked peptide release and functional effects through other mechanisms, probably involving Ca2+ fluxes in the non-selective cation channel associated with the proposed capsaicin receptor. The nicotine (10(-4) M) evoked peptide release and bronchoconstriction were only marginally influenced by omega-conotoxin or tetrodotoxin. It is concluded that the ion-channel mechanisms underlying the peptide releasing properties of antidromic nerve stimulation and low concentrations of capsaicin are similar and depend on action potential propagation, whereas capsaicin in high, toxic concentration and nicotine mainly act via receptor operated channels.

The effect of Ca2+ channel modulators on vagally induced bronchoconstriction in the guinea-pig

The effects of N- and L-type voltage operated calcium channel (VOCC) antagonists were examined on the bronchoconstriction induced by vagal stimulation in artificially respired guinea-pigs. Vagal stimulation produced a reproducible and consistent bronchoconstrictor response which corresponded to an increase in pulmonary inflation pressure equivalent to (10.4 +/- 1.0%) of the maximum. This vagally induced rise in pulmonary inflation pressure was reduced (54% P less than 0.001) by pretreatment with atropine (1 mg/kg i.v.) and almost completely blocked by pretreatment with capsaicin (54.5 mg/kg s.c.) and atropine. omega-Conotoxin GVIA (CgTx) (5-20 micrograms/kg i.v.) caused a dose and time-related inhibition of the vagal response but did not affect either methacholine or substance P (SP)-induced bronchoconstriction. Combination studies with CgTx, atropine and capsaicin pretreatment revealed that CgTx effectively blocked both the atropine-sensitive cholinergic component and the capsaicin-sensitive non-adrenergic non-cholinergic (NANC) component of the vagal response. Selective L-type VOCC antagonists nicardipine, diltiazem and verapamil, at doses which had significant cardiovascular effects, did not reduce the rise in pulmonary inflation pressure to vagal stimulation. This study indicates that N-type VOCCs are important in controlling the release of neurotransmitters from both the cholinergic and NANC neurones within the airways of guinea-pigs.

Haemodynamic and humoral effects of omega-conotoxin GVIA in normotensive and spontaneously hypertensive rats

Omega-conotoxin GVIA, a 27-amino acid peptide, has been shown to be a potent and selective inhibitor of N-type voltage-operated calcium channels (VOCCs). A single intravenous dose of 10 micrograms/kg conotoxin slowly lowered blood pressure by 41.3 +/- 4.4 and 73.3 +/- 4.6 mm Hg in conscious Wistar-Kyoto rats (WKY) and spontaneously hypertensive rats (SHR) respectively without changing the heart rate. Plasma renin activity was significantly increased after conotoxin. In anaesthetized rats, conotoxin (3 and 10 micrograms/kg) lowered blood pressure and heart rate and produced a marked increase in renal vascular conductances. Baroreceptor heart rate reflex experiments using methoxamine and sodium nitroprusside before and after treatment with conotoxin showed that conotoxin almost totally abolished the sympathetic component of the reflex without affecting the vagal tone to the heart in both rat strains. Because conotoxin does not affect directly the vasculature and heart contractile properties, we suggest that the control of presynaptic calcium influx and of neurotransmitter release mostly depends on conotoxin-sensitive N-type VOCCs in the peripheral sympathetic system of the rat.

Vasoactive intestinal polypeptide induces neurogenic contraction of guinea-pig ileum. Involvement of acetylcholine and substance P

The effect and mode of action of vasoactive intestinal polypeptide (VIP), a peptidergic neuromodulator in the gastrointestinal nervous system, were investigated in isolated muscle strips of the guinea-pig ileum. VIP induced concentration-dependent (20 nM-1 microM) contractions of longitudinal ileal strips. TTX (1 microM), a mixture of atropine (3 microM) and spantide (30 microM), a mixture of atropine (3 microM) and omega-conotoxin GVIA (100 nM), somatostatin (60 nM) and dynorphin (100 nM) abolished the effect of VIP. In most cases a small relaxation became evident. Desensitization to substance P in the presence of atropine prevented VIP-induced contraction. A partial inhibition was observed in the presence of atropine (3 microM), spantide (30 microM), omega-conotoxin GVIA (100 nM), beta-endorphin (265 nM), met-enkephalin (1100 nM) and a mixture of spantide (30 microM) and omega-conotoxin GVIA (100 nM). The action of VIP was not significantly modified by guanethidine (3 microM) or hexamethonium (150 microM). In circular ileal strips VIP (10-300 nM) caused concentration-dependent relaxations through a direct myogenic effect. These results indicate that the VIP produced contractions of the guinea-pig ileum are exclusively neurally mediated and involve a cholinergic as well as a noncholinergic-nonadrenergic (NANC) pathway. It is concluded that besides acetylcholine (Ach) VIP releases the peptidergic transmitter substance P from postganglionic nerve fibers of myenteric plexus. Opioid peptides and somatostatin modulate the activity of cholinergic and peptidegic nerves in the guinea-pig ileum. The release of substance P appears to depend completely on N-type voltage sensitive calcium channels.

Antitussive effects of Ca2+ channel antagonists

The effects of Ca2+ channel antagonists on the capsaicin-induced cough reflex in guinea pigs were studied. Intraperitoneal injection of nifedipine, verapamil and flunarizine in doses that ranged from 0.3 to 3.0 mg/kg decreased the number of coughs in a dose-dependent manner. These Ca2+ channel antagonists exhibited antitussive effects in the following order of potency: flunarizine = verapamil greater than nifedipine. Pretreatment with a low dose of nifedipine (0.3 mg/kg), which by itself had no significant effect on the number of coughs, markedly increased the antitussive effects of morphine, dihydrocodeine and dextromethorphan. These data suggest that Ca2+ channels play an important role in the regulation of the cough reflex.

Tachykinins and calcitonin gene-related peptide as co-transmitters in local motor responses produced by sensory nerve activation in the guinea-pig isolated renal pelvis

Electrical field stimulation of circular muscle strips from the guinea-pig isolated renal pelvis produces a frequency-dependent positive inotropic effect of the spontaneous contractions which is unaffected by atropine and guanethidine and abolished by tetrodotoxin or in vitro capsaicin desensitization. Omega conotoxin fraction GVIA markedly inhibited the response to low frequencies of stimulation but had only a partial or minor inhibitory effect at higher frequencies. Tachykinins produce a concentration-dependent positive inotropic effect, neurokinin A being more potent than substance P. On the other hand, rat alpha calcitonin gene-related peptide (CGRP) inhibited spontaneous contractions of the renal pelvis. MEN 10,376 a neurokinin A (4-10) analog, antagonized the positive inotropism produced by neurokinin A, without affecting the response to KCl, and suppressed the positive inotropic response produced by electrical field stimulation. In the presence of MEN 10,376, a negative inotropic response was produced by electrical field stimulation which was antagonized by the C-terminal fragment (8-37) of human alpha calcitonin gene-related peptide (hCGRP). hCGRP (8-37) antagonized the negative inotropic effect of exogenously administered CGRP without affecting inhibition by isoprenaline. Application of capsaicin (10 microM) produced a marked increase in the outflow of substance P-, neurokinin A- and CGRP-like immunoreactivities from the superfused guinea-pig renal pelvis. Substance P-, neurokinin A- and CGRP-like immunoreactivities were also detected in tissue extracts of the renal pelvis by radioimmunoassay. These experiments indicate that peptide release from peripheral endings of capsaicin-sensitive primary afferents represents the major type of nerve-mediated response affecting motility of the guinea-pig isolated renal pelvis. Tachykinins and CGRP act as physiological antagonists and the excitatory action of tachykinins prevails over the inhibitory action of CGRP. Local modulation of renal pelvis motility by sensory nerves could facilitate removal of irritants present in the urine, protecting the kidney during obstruction and ureteral antiperistalsis.

Variable α2-adrenoceptor-mediated inhibition of bronchoconstriction and peptide release upon activation of pulmonary afferents

In the present study, we evaluated the possible regulation by alpha 2-receptor agonists (SKF 35886 and UK 14304) of peptide release and functional responses upon sensory nerve activation in the guinea-pig lung. The peptide release and bronchoconstriction caused by antidromic vagal nerve stimulation (low frequency, 1 Hz), and a low concentration of capsaicin (10(-8) M) and resiniferatoxin (3 x 10(-10) M) were attenuated by alpha 2-adrenoceptor agonists. The effects of capsaicin and nicotine in high concentrations and high frequency nerve stimulation (10 Hz) were influenced to a much smaller extent by alpha 2-adrenoceptor stimulation. The calcitonin gene-related peptide release evoked by bradykinin but not the functional effects was inhibited by alpha 2-adrenoceptor activation. It is concluded that alpha 2-adrenoceptor stimulation mainly inhibits the release of mediator and/or the bronchoconstriction caused by moderate activation of sensory nerves. It is necessary to measure mediator release directly to reveal prejunctional effects and not to rely only on indirect functional evidence.

Different pathways by which extracellular Ca2+ promotes calcitonin gene-related peptide release from central terminals of capsaicin-sensitive afferents of guinea pigs: effect of capsaicin, high K+ and low pH media

Different modes by which Ca2+, entering the nerve terminal, promotes transmitter secretion as well as the ability of protons to release neuropeptides, have been shown in peripheral endings of capsaicin-sensitive afferents. We have studied these two aspects in the central endings of these neurons by measuring the release of calcitonin-gene related peptide-like immunoreactivity (CGRP-LI) from slices of the dorsal half of the guinea pig spinal cord. Although capsaicin (1 microM) released both CGRP-LI and substance P-like immunoreactivity (SP-LI), CGRP-LI was chosen as the sole suitable marker of peptides released from central terminals of capsaicin-sensitive afferents, since after in vitro desensitization to capsaicin (1 microM capsaicin for 20 min), high K+ (80 mM) failed to evoke CGRP-LI release, whereas SP-LI release was still observed. The capsaicin (1 microM)-evoked CGRP-LI release was entirely dependent on extracellular Ca2+. It was unaffected by 0.3 microM tetrodotoxin (TTX), slightly reduced by 0.1 microM omega-conotoxin (CTX) and blocked by 10 microM Ruthenium red (RR). The Ca(2+)-dependent K+ (80 mM)-evoked CGRP-LI release was unaffected by TTX, markedly reduced by CTX and only moderately inhibited by RR. Low pH (pH 5) produced a remarkable increase in CGRP-LI outflow that was abolished after exposure to capsaicin, reduced by about 50% in Ca(2+)-free medium and unaffected by TTX (0.3 microM). The Ca(2+)-dependent component of the proton-evoked CGRP-LI release was abolished in the presence of RR (10 microM) and slightly inhibited by CTX (0.1 microM).(ABSTRACT TRUNCATED AT 250 WORDS)

Omega conotoxin and prejunctional modulation of the biphasic response of the rat isolated urinary bladder to single pulse electrical field stimulation

1. Single pulse electrical field stimulation (EFS) produces a biphasic response of muscle strips of the rat isolated urinary bladder consisting of an early and a late contraction which were atropine-resistant and atropine-sensitive, respectively. Repeated application of desensitizing doses of the P2 purinoceptor agonist, alpha, beta-methylene ATP (mATP) inhibited the early response while leaving unaffected the late component. 2. Omega conotoxin (CTX, 0.1 microM) inhibited both the early and the late response either in control conditions or after enhancement by physostigmine (0.1 microM). The effect of CTX was, in both cases, more pronounced on the late than the early response to EFS. CTX (0.1 microM) failed to affect contraction produced by ATP or acetylcholine at concentrations (0.3 mM and 0.5 microM) which produced a response similar to that to EFS. 3. The effect of physostigmine was more intense for the late than the early response and was abolished by atropine. In the presence of CTX, physostigmine enhanced both the early and the late components of the mechanical response to EFS. 4. Nifedipine (0.1-1 microM) reduced to a similar extent both the early and late responses. Bay K 8644 (1 microM) produced a marked enhancement of the response to EFS, which, however, did not have a distinct late peak. In the presence of Bay K 8644, either atropine (3 microM) or tetrodotoxin (1 microM) had minor inhibitory effects indicating the myogenic origin of the response. 5. Neurokinin A (0.1-1 nM) enhanced both the early and late responses to EFS without affecting the contraction produced by exogenous acetylcholine or ATP. A consistent potentiation was evident also in the presence of CTX and for the early response, in the presence of atropine. Clonidine (3 microM) inhibited the response to EFS either in the absence or the presence of physostigmine. The inhibitory effect of clonidine, shown previously to depend upon activation of prejunctional alpha 2-adrenoceptors, was still observed in presence of CTX or atropine. 6. It is concluded that CTX-sensitive voltage dependent calcium channels play a more important role in determining the cholinergic rather than the non-cholinergic, putatively purinergic, component of the biphasic response of the rat bladder to single pulse EFS. The action of CTX is likely to be exerted on N-type rather than L-type (dihydropyridine-sensitive) calcium channels. Prejunctional modulation (enhancement by neurokinin A, inhibition by clonidine) occurs even in the presence of CTX-sensitive channels blockade.

The effect of omega-conotoxin GVIA on synaptic transmission within the nucleus accumbens and hippocampus of the rat in vitro

1. The actions of two calcium channel antagonists, the N-channel blocker omega-conotoxin GVIA (omega-CgTx) and the L-channel antagonist nisoldipine, on synaptic transmission were investigated in the hippocampus and nucleus accumbens of the rat in vitro. 2. omega-CgTx (100 nM for 10 min) produced a marked and irreversible reduction of focally evoked population spikes and intracellularly recorded excitatory postsynaptic potentials (e.p.s.ps) in the nucleus accumbens, which could not be overcome by increasing the stimulus strength. 3. Nisoldipine (10 microM for 10 min) had no effect on population spikes in the nucleus accumbens or the CA1 of the hippocampus. 4. In the hippocampus, population spikes were not irreversibly reduced by omega-CgTx (100 nM for 10 min) but rather, multiple population spikes were produced along with spontaneous synchronous discharges. This indicated that inhibitory synaptic transmission was being preferentially reduced. 5. Intracellular recordings demonstrated that omega-CgTx powerfully reduced inhibitory synaptic transmission in an irreversible manner and that excitatory transmission was also reduced but to a lesser extent. Unlike excitatory transmission in the nucleus accumbens and inhibitory transmission in the hippocampus, increasing the stimulus strength overcame the reduction of hippocampal excitatory transmission. 6. It is concluded that omega-CgTx-sensitive calcium channels are involved in the calcium entry that precedes the synaptic transmission in all these synapses. The apparent lower sensitivity of the hippocampal excitatory fibres to omega-CgTx may indicate that calcium entry that promotes transmitter release at central synapses may be mediated by pharmacologically distinct calcium channels.

Prejunctional modulatory action of neuropeptide Y on responses due to antidromic activation of peripheral terminals of capsaicin-sensitive sensory nerves in the isolated guinea-pig ileum

1. The effect of neuropeptide Y (NPY) on motor responses produced by activation of capsaicin-sensitive primary afferents in the guinea-pig isolated ileum was determined by use of capsaicin itself and electrical mesenteric nerve stimulation as stimuli. 2. NPY inhibited or suppressed the cholinergic contractile response produced by electrical mesenteric nerve stimulation while leaving the contractile response to a threshold concentration of capsaicin. 3. NPY had no effect on motor responses produced by a submaximal concentration of substance P, the putative endogenous mediator of the 'efferent' function of sensory fibres in this preparation. 4. It is concluded that NPY exerted a prejunctional inhibitory action on transmitter release from peripheral endings of capsaicin-sensitive nerves at interneuronal synapses.

The neurotransmitter role of calcitonin gene-related peptide in the rat and guinea-pig ureter: effect of a calcitonin gene-related peptide antagonist and species-related differences in the action of omega conotoxin on calcitonin gene-related peptide release from primary afferents

In the rat and guinea-pig isolated ureter electrical field stimulation of intrinsic nerves (10 Hz for 10 s) produces transient inhibition of evoked (20 mM KCl or 0.1-1 microM neurokinin A) rhythmic contractions by releasing transmitter(s) from peripheral endings of capsaicin-sensitive primary afferents. The C-terminal fragment of human calcitonin gene-related peptide (8-37) blocked the inhibitory effect of electrical field stimulation as well as that produced by exogenous calcitonin gene-related peptide, while leaving unaffected the inhibitory response to isoprenaline. Human calcitonin gene-related peptide (8-37) was devoid of any inhibitory activity of its own but enhanced the amplitude and frequency of KCl-evoked rhythmic contractions in the rat ureter, probably by antagonizing the inhibitory effect of endogenous calcitonin gene-related peptide released by KCl. Omega conotoxin fraction GVIA, a peptide which possesses a potent blocking activity of N-type voltage-sensitive calcium channels, prevented the inhibitory response to electrical stimulation in the guinea-pig ureter, while leaving the response unaffected in the rat ureter. Conotoxin had no effect toward the inhibition produced by exogenous calcitonin gene-related peptide indicating its prejunctional site of action, demonstrated previously in the guinea-pig ureter [Maggi et al. (1990) Neurosci, Lett. 114, 203-206]. Dermorphin, an amphibian peptide with potent agonist activity on mu-type opioid receptors, inhibited the response to electrical stimulation in the guinea-pig ureter but had no effect in the rat ureter.(ABSTRACT TRUNCATED AT 250 WORDS)

GABA agonists and omega conotoxin GVIA modulate responses to nerve activation of the perfused rat mesentery

The modulatory actions of gamma-aminobutyric acid (GABA) receptor agonists and omega-conotoxin GVIA (CTX) on sympathetic and sensory nerves were examined on contractile responses of the perfused rat mesentery to transmural nerve stimulation (TNS). GABA and baclofen, a selective GABAB receptor agonist, significantly inhibited vasoconstrictor responses to TNS, while muscimol, a selective GABAA receptor agonist, had no effect. In the guanethidine treated and methoxamine-contracted mesentery, TNS caused a vasodilator response which was unaffected by GABA. CTX (10(-8) M) markedly suppressed the vasoconstrictor response to TNS, but did not affect vasodilator responses. These findings suggest that in the rat mesentery: (1) GABA receptors modulate the activity of sympathetic nerves via prejunctional GABAB receptors, but do not influence sensory nerves, and (2) calcium channels which participate in sympathetic nerve activation have different properties than calcium channels in capsaicin-sensitive sensory nerves.

Treatment with conotoxin, an 'N-type' calcium channel blocker, in neuronal hypoxic-ischemic injury

Therapeutic efficacy of calcium channel blockers in stroke remains controversial, but previously used agents bind almost exclusively to L-type calcium channels. The newly-discovered N-type calcium channel is specific to neurons, and therapy involving blockade of this site has not been previously attempted. We assessed the neuroprotective effect of omega-conotoxin GVIA (CgTx), a blocker of N-type calcium channels, using both in vitro hypoxic injury to rat cortical neurons and an in vivo model of reversible spinal cord ischemia in the rabbit. In cell cultures, CgTx inhibited hypoxia-induced 45Ca accumulation and neuronal injury minimally, compared to the NMDA antagonist ketamine. In vivo, the duration of spinal cord ischemia which produced permanent paraplegia in 50% of control animals (ET50) was 24.0 +/- 2.6 min. Animals treated 2 h prior to ischemia with 0.5 nmol CgTx in the subarachnoid space had an ET50 of 26.9 +/- 1.8 min (P = 0.36). Animals treated 24 h prior to ischemia (all had persistent systemic tremor) had a ET50 of 28.9 +/- 1.8 min (P = 0.13). We conclude that pharmacologic modulation of the N-type calcium channel does not provide a significant protective effect against neuronal hypoxic-ischemic injury.

Effects of omega-conotoxin GVIA on autonomic neuroeffector transmission in various tissues

1. The effects of omega-conotoxin GVIA (conotoxin), a potent inhibitor of neuronal N-type Ca2+ channels, have been examined on responses to stimulation of noradrenergic, cholinergic and non-adrenergic, non-cholinergic (NANC) nerves in a range of isolated tissues to investigate the role of conotoxin-sensitive Ca2+ channels in neurotransmission. 2. Contractions elicited by field stimulation of noradrenergic nerves in rat and mouse anococcygeus muscles, rabbit ear artery and rat vas deferens (epididymal portion) were inhibited by conotoxin. Responses to noradrenaline, and to adenosine triphosphate in the vas deferens, were not affected. 3. Positive chronotropic responses to field stimulation of noradrenergic nerves were inhibited by conotoxin in rat and mouse atria, but responses to noradrenaline and tyramine were not affected. 4. The stimulation-induced release of noradrenaline was inhibited by conotoxin in the rabbit ear artery and in rat and mouse atria. 5. Relaxations in response to stimulation of the noradrenergic perivascular mesenteric nerves were reduced or abolished by conotoxin in rat and rabbit jejunum. The response to noradrenaline in rat jejunum was not affected. 6. Contractions elicited by stimulation of cholinergic nerves were inhibited by conotoxin in rat jejunum and mouse ileum (perivascular mesenteric nerves), and in guinea-pig taenia caeci (field stimulation). Responses to acetylcholine in rat jejunum and mouse ileum were not affected. 7. Contractions elicited by stimulation of the cholinergic plus NANC pelvic nerves were inhibited by conotoxin in rabbit colon, and to a lesser extent in guinea-pig colon. The stimulation-induced contraction of the guinea-pig colon was inhibited by conotoxin by a greater proportion in the presence than in the absence of atropine. Responses to acetylcholine were not affected in the rabbit colon but were slightly reduced in the guinea-pig colon. 8. Relaxations in response to field stimulation of NANC nerves were inhibited by conotoxin in guinea-pig taenia caeci and rat gastric fundus strips, and in rat anococcygeus muscle when the tone was raised by guanethidine but not when it was raised by carbachol. The relaxations produced by sodium nitroprusside in the rat gastric fundus and anococcygeus were not affected. 9. Contractions of the rat bladder elicited by stimulation of the peri-urethral nerves, which are NANC- and cholinergically mediated, were relatively insensitive to inhibition by conotoxin. The response were almost completely abolished by tetrodotoxin. 10. The conotoxin-induced inhibitions of responses to nerve stimulation developed slowly and persisted after removal of conotoxin. The responses were almost completely abolished by tetrodotoxin. 10. The conotoxin-induced inhibitions of responses to nerve stimulation developed slowly and persisted after removal of conotoxin. 11. The inhibitory effect of conotoxin was inversely proportional to the frequency of stimulation (in several preparations) and to the Ca2+ concentration in the bathing solution (in rat vas deferens). These observations suggest that the inhibition by conotoxin of the Ca2+ influx required for excitation-secretion coupling in autonomic nerve terminals is not absolute, and can be overcome by repeated stimulation or by raising the Ca2 + concentration.

Apparent vascular to cardiac sympatholytic selectivity of omega-conotoxin GVIA in the pithed rat

The effects of omega-conotoxin GVIA (omega-CTX), a blocker of N-type voltage-operated calcium channels (VOCCs), were investigated in the pithed rat, omega-CTX (1.6 and 3.2 micrograms/kg i.v.) did not alter resting diastolic pressure or heart rate nor the pressor and chronotropic responses to noradrenaline injections (0.1-10 micrograms/kg). In contrast, the pressor responses to electrical stimulation of the whole spinal cord (0.2-6.4 Hz) were dose dependently reduced by omega-CTX whereas the concomitant tachycardia was less affected. When selective stimulation of the cardiac sympathetic outflow was applied, the resulting chronotropic response was more sensitive to omega-CTX. This result is discussed in the light of the possible interference of adrenal catecholamine release during whole spinal cord stimulation which is not sensitive to omega-CTX. These results provide in vivo evidence that omega-CTX is able to reduce sympathetic neurotransmission to the vasculature and the heart, presumably by blocking N-type VOCCs on pre- and post-ganglionic nerve terminals.

The polyamine spermine affects omega-conotoxin binding and function at N-type voltage-sensitive calcium channels

1. The effects of the polyamines, spermine and spermidine on neuronal N-type voltage-sensitive calcium channels were investigated using the binding and function of the ligand omega-conotoxin GVIA (omega-CT). 2. Spermine and spermidine enhanced (EC50 approximately 0.16 and 0.45 microM) and, at higher concentrations, inhibited (IC50 of 9 and 240 microM) the binding of [125I]omega-CT to rat hippocampal synaptosomes. 3. Spermine and, less potently, spermidine inhibited the neurotransmitter-mediated, omega-CT-sensitive, electrical-field-stimulated contractile responses of the rat vas deferens. 4. The polyamines also inhibited the phenylephrine-evoked contractile responses of the vas deferens with the same rank order, consistent with a postsynaptic mechanism of inhibition. 5. However, pre-exposure to spermine prevented the irreversible inhibition of vas deferens twitch responses by omega-CT (previously found to be presynaptic). The prevention of inhibition by omega-CT demonstrates that the neuronal binding of spermine and omega-CT is mutually exclusive. Thus spermine (and presumably spermidine at higher concentrations) appears to modulate the actions of omega-CT at N-type voltage-sensitive calcium channels.

Neurochemical evidence for the involvement of N-type calcium channels in transmitter secretion from peripheral endings of sensory nerves in guinea pigs

In the guinea pig ureter, substance P-(SP) and calcitonin gene-related peptide-(CGRP) like immunoreactivity (LI) were depleted by systemic capsaicin pretreatment, indicating that they are entirely stored in peripheral endings of primary afferent neurons. Electrical field stimulation (20 Hz, 60 V, 0.5 ms) evoked the simultaneous release of SP- and CGRP-LI from superfused guinea pig ureters which was abolished by tetrodotoxin (0.3 microM). omega-Conotoxin (0.1 microM), a potent blocker of N-type voltage-sensitive calcium channels, reduced by 50-70% the evoked release of both peptides. These findings provide direct neurochemical evidence indicating that conotoxin-sensitive calcium channels play a role in transmitter secretion evoked by antidromic invasion of peripheral terminals of capsaicin-sensitive primary afferents.

Neomycin and omega-conotoxin GVIA interact at a common neuronal site in peripheral tissues

1. The present study examined the interaction of omega-conotoxin GVIA (omega-CT) and aminoglycoside antibiotics on electrically evoked, nerve-mediated contractile responses in the rat vas deferens, guinea-pig ileum and guinea-pig left atria. 2. omega-CT caused a time- and concentration-dependent inhibition of the electrically evoked twitch responses of the rat vas deferens and guinea-pig ileum. Aminoglycoside antibiotics inhibited the twitch responses of these preparations with a rank order of potency: neomycin greater than gentamycin greater than kanamycin. omega-CT had no effect on the postjunctional contractile responses of either noradrenaline (vas deferens) or carbachol (ileum). However, at high concentrations neomycin and gentamycin caused significant postjunctional inhibition. The results suggest that omega-CT and aminoglycosides cause prejunctional inhibition in these preparations, with the aminoglycoside antibiotics exhibiting postjunctional inhibitory effects as well at high concentrations. 3. omega-CT caused a concentration- and frequency-dependent inhibition of the neuronally mediated field stimulation enhancement of electrically paced guinea-pig left atria. omega-CT had no effect on either the electrically paced contractile response that was elicited by direct muscle stimulation or the enhancement of the paced response caused by beta-adrenoceptor agonist stimulation. Neomycin caused a concentration-dependent inhibition of the electrically paced contractile response and inhibited the field stimulation response only at concentrations which caused pronounced inhibition of the paced response. Neomycin also caused insurmountable inhibition of responses elicited by beta-adrenoceptor agonist stimulation. Thus, omega-CT caused an exclusive prejunctional inhibition in guinea-pig left atria, whereas the substantial postjunctional effects of neomycin made it difficult to discern any prejunctional activity of neomycin in these experiments. 4. In the vas deferens, ileum and atria the inhibitory effects of omega-CT were long-lasting, whereas the effects of neomycin could be reversed upon wash-out. The disparate kinetics of omega-CT and neomycin allowed for the design of receptor protection studies to determine whether neomycin acts at a prejunctional site in common with omega-CT. The pre-equilibration of a competitive antagonist (neomycin) should prevent the irreversible antagonist (omega-CT) from gaining access to receptors. Pre-exposure of tissues with neomycin prevented the irreversible inhibition of omega-CT. These receptor protection studies suggest that omega-CT and neomycin interact at common neuronal sites in the rat vas deferens, guinea-pig ileum and guinea-pig atria. Neomycin, however, exhibits activity at postjunctional sites as well.

Omega-conotoxin GVIA is a potent inhibitor of sympathetic neurogenic responses in rat small mesenteric arteries

1. We have investigated the effects of the N-type calcium channel blocker, omega-conotoxin GVIA, on contractile responses to nerve stimulation, noradrenaline and KCl in rat small mesenteric arteries. In separate experiments, single and summated excitatory junctional potentials (e.j.ps) evoked by nerve stimulation were recorded with an intracellular electrode in the absence and presence of omega-conotoxin. 2. Electrical field stimulation of intramural sympathetic nerves (30 V; 0.25 ms pulse width; 3 s train length; 4-24 Hz) caused frequency-dependent contractions. Cumulative concentration-response curves for the contractions induced by noradrenaline and KCl were constructed in the same preparations. Stimulation at 0.2 Hz and 10 Hz induced respectively single e.j.ps without contractions and summated e.j.ps associated with a contractile response. 3. omega-Conotoxin (0.1 to 3 nM) inhibited markedly and in a concentration-dependent manner both the contractions and e.j.ps to electrical field stimulation. The concentration-response curves to exogenous noradrenaline and KCl remained unaffected. 4. The time-course for the effects of omega-conotoxin (0.3 to 3 nM) indicated a slow onset of action with at least one hour to achieve an equilibrium. 5. The experiments indicate that omega-conotoxin acts prejunctionally to inhibit sympathetic neurotransmission in rat small arteries presumably by inhibition of noradrenaline release. We suggest that omega-conotoxin could be a useful tool to study the control of vascular tone through the autonomic nervous system.

Opioid receptors and prejunctional modulation of capsaicin-sensitive sensory nerves in guinea-pig left atrium

1. In the isolated electrically driven left atria from reserpine-pretreated guinea-pigs and in presence of 1 microM atropine, electrical field stimulation (EFS) at 10 Hz produces a delayed positive inotropic response (DPIR) involving activation of capsaicin-sensitive afferents. 2. Opioids inhibited the DPIR with the following order of potency: dermorphin greater than [D-Ala2,N-MePhe4, Gly5-ol]-enkephalin (DAGO) greater than or equal to [D-Ala2,D-Leu5]-enkephalin (DADLE) greater than morphine greater than dynorphin A (1-13) greater than [D-Pen2,D-Pen5]-enkephalin (DPDPE). U-50488 was ineffective up to 10 microM. 3. Opioids also inhibited resting inotropism (3 Hz) with the following rank order of potency: DADLE greater than DAGO greater than U-50488 = dynorphin A (1-13) = morphine = DPDPE. 4. Both inhibition of the DPIR and inhibition of resting inotropism were prevented by 10 microM naloxone. 5. Neither dermorphin (0.1 microM) nor DAGO (0.3 microM) or DADLE (1 microM) inhibit responses produced by capsaicin (30 nM) or calcitonin gene-related peptide (3 nM). 6. These findings indicate that capsaicin-sensitive nerves in the guinea-pig atrium are endowed with mu opioid receptors which inhibit transmitter release when sensory nerve terminals are activated by EFS but not by capsaicin.

Effect of omega conotoxin on reflex responses mediated by activation of capsaicin-sensitive nerves of the rat urinary bladder and peptide release from the rat spinal cord

The aim of this study was to assess whether omega conotoxin fraction GVIA, a potent blocker of N- and L-type voltage-sensitive calcium channels, might interfere with reflex responses (micturition, blood pressure rise in spinal rats) produced by activation of capsaicin-sensitive sensory nerves of the rat urinary bladder. The effect of conotoxin was also investigated on reflex micturition persisting after capsaicin pretreatment. Following topical application onto the bladder, conotoxin did not affect the volume threshold to elicit micturition although it reduced the amplitude of volume-evoked bladder contractions. Likewise, topical conotoxin did not prevent the reflex rise in blood pressure elicited by sudden bladder distension or topical application of capsaicin onto the bladder. In contrast, topical lidocaine strongly prevented both reflex responses. After intrathecal administration, conotoxin produced a dose-dependent inhibition of volume-evoked bladder contractions and the cardiovascular reflex produced by mechanical or chemical stimulation of bladder nerves. Intrathecal conotoxin inhibited micturition also in rats pretreated with capsaicin (50 mg/kg s.c., 4 days before). Depolarization by high potassium (80 mM) produced release of both substance P- and calcitonin gene-related peptide-like immunoreactivity from superfused slices of the dorsal half of rat spinal cord. Capsaicin (1 microM) produced a similar effect, and a previous exposure to capsaicin prevented the effect of potassium. Conotoxin (0.1 microM) significantly reduced (about 50%) the potassium-induced release of neuropeptides from the dorsal half of the rat spinal cord. These findings indicate that conotoxin-sensitive calcium channels in the rat spinal cord play a role in the neurotransmission along reflex pathways activated by stimulation of capsaicin-sensitive nerves in the urinary bladder.(ABSTRACT TRUNCATED AT 250 WORDS)

The 'efferent' function of capsaicin-sensitive nerves: ruthenium red discriminates between different mechanisms of activation

We have investigated the ability of Ruthenium Red, an inorganic dye with calcium entry blocking properties, to interfere with the 'efferent' function of capsaicin-sensitive sensory nerves. These nerves were activated in the guinea-pig isolated bronchus (atropine in the bath) or left atria (reserpine-pretreated animals, atropine in the bath) by electrical field stimulation or with capsaicin. Both stimuli produced a contraction of the bronchus and a positive inotropic response in the atria, responses which are mediated by endogenous neuropeptides (tachykinins in the bronchus, calcitonin gene-related peptide in the atria) released from sensory nerves. Ruthenium Red (10 microM for 20 min in both cases) selectively inhibited the responses produced by the administration of capsaicin, while leaving the responses to electrical field stimulation unaffected. Likewise, the bronchoconstrictor response to exogenous neurokinin A and the atrial positive inotropic response to calcitonin gene-related peptide were unaffected by Ruthenium Red. A prejunctional site of action of Ruthenium Red was confirmed in release experiments where the dye strongly inhibited the capsaicin-evoked outflow of calcitonin gene-related peptide, which is taken as a marker of activation in sensory nerves. Together with other observations, these findings support the concept that there are two independent mechanisms for activating the 'efferent' function of sensory nerves, one of which is activated by capsaicin and is Ruthenium Red-sensitive but omega-conotoxin-resistant, while the other is activated by propagated action potentials (field stimulation) and is omega-conotoxin-sensitive and Ruthenium Red-resistant.

Effect of ruthenium red on responses mediated by activation of capsaicin-sensitive nerves of the rat urinary bladder

(1) Topical administration of Ruthenium Red (10-100 microM in saline) to the serosal surface of the urinary bladder in urethane-anesthetized rats prevented the motor response of the urinary bladder to topical administration of capsaicin and protected the sensory fibers from capsaicin desensitization, but had no effect on the volume-evoked contractions (micturition reflex). At 1 mM increased bladder capacity and decreased amplitude of micturition contraction were observed. (2) At 100 microM, topical Ruthenium Red prevented the blood pressure rise produced by topical administration of capsaicin onto the bladder but did not affect the blood pressure rise produced by sudden bladder distension in spinal rats. (3) After intrathecal administration, Ruthenium Red (80-800 ng/rat) produced a long lasting inhibition of the micturition reflex in urethane-anesthetized rats, this effect being evident in both vehicle- or capsaicin- (50 mg/kg s.c. 4 days before) pretreated rats. At 800 ng/rat, intrathecal Ruthenium Red did not affect the blood pressure rise produced by topical administration of capsaicin onto the rat bladder nor that produced by bladder distension. (4) These findings provide further evidence that Ruthenium Red acts quite selectively as a "capsaicin antagonist" preventing both reflex and "efferent" responses activated by peripherally administered capsaicin. By contrast, sensory impulse generation by a natural stimulus such as bladder distension is apparently unaffected by Ruthenium Red. The marked inhibition of the micturition reflex observed after intrathecal administration of Ruthenium Red does probably not involve an interaction with primary afferents in the spinal cord.

Prejunctional modulatory action of neuropeptide Y on peripheral terminals of capsaicin-sensitive sensory nerves

1. We have determined the effect of neuropeptide Y (NPY) on motor responses produced by activation of capsaicin-sensitive primary afferents in the guinea-pig isolated left atria (reserpine-pretreatment, atropine in the bath) and bronchi (atropine and indomethacin in the bath) using capsaicin itself and electrical field stimulation as stimuli. 2. In both preparations, NPY inhibited or suppressed the response produced by electrical field stimulation while leaving the response to a submaximal concentration of capsaicin unaffected. 3. NPY had no effect on motor responses produced by a submaximal concentration of calcitonin gene-related peptide (atria) or neurokinin A (bronchi), the putative endogenous mediators of the responses produced by activation of the 'efferent' function of sensory fibres in these preparations. 4. We conclude that NPY exerts a prejunctional inhibitory action on transmitter release from peripheral endings of capsaicin-sensitive nerves. Failure of NPY to modulate responses activated by capsaicin provides further evidence for the existence of two independent modes of activation of the 'efferent' function of capsaicin-sensitive sensory nerves.

Inhibition of histamine release from rat hypothalamic slices by omega-conotoxin GVIA, but not by nilvadipine, a dihydropyridine derivative

Histamine release in response to 40 mM high K+-stimulation from the rat hypothalamic slice preparations perifused in vitro was significantly inhibited by 1.0 nM-1.0 microM omega-conotoxin GVIA, a peptide modulator of N- and L-type voltage-sensitive calcium channels, but not by similar concentrations of nilvadipine, a dihydropyridine derivative of L-type calcium channel antagonist. These results indicate that the voltage-sensitive calcium channel controlling histamine release from hypothalamic slices is omega-conotoxin-sensitive but dihydropyridine-insensitive.

GABAA receptor-mediated positive inotropism in guinea-pig isolated left atria: evidence for the involvement of capsaicin-sensitive nerves

1. Isolated left atria from reserpine-pretreated guinea-pigs, electrically driven (3 Hz) in the presence of atropine (1 microM), phentolamine (0.3 microM) and propranolol (1 microM), responded to a train of stimuli (10 Hz for 2.5s) with a delayed neurogenic positive inotropic response which was insensitive to hexamethonium (10 microM) but abolished by either tetrodotoxin (1 microM), omega-conotoxin (0.1 microM), in vitro capsaicin desensitization or desensitization to calcitonin gene-related peptide (CGRP). 2. In these experimental conditions, gamma-aminobutyric acid (GABA) produced a concentration-related (10 microM-1 mM) positive inotropic response similar to that produced by electrical field stimulation. The effect of GABA was competitively antagonized by bicuculline methiodide (10 microM), a GABAA receptor antagonist. 3. The selective GABAA receptor agonists, muscimol and homotaurine mimicked the positive inotropic effect of GABA while baclofen, the selective GABAB receptor agonist, did not. 4. The action of GABA (1 mM) was abolished by either tetrodotoxin (1 microM), omega-conotoxin (0.1 microM), in vitro capsaicin desensitization or desensitization to CGRP, while it was unaffected by hexamethonium. In contrast, the inotropic response to CGRP was unaffected by tetrodotoxin, omega-conotoxin, bicuculline methiodide, hexamethonium or in vitro capsaicin desensitization, but was abolished by CGRP desensitization. 5. In the spontaneously beating guinea-pig right atrium, GABA (1 microM) produced a small and transient positive chronotropic effect that was no longer observed after in vitro desensitization with capsaicin (1 microM). 6. In the guinea-pig isolated perfused heart from reserpine-pretreated animals (with atropine, phentolamine and propranolol in the perfusion medium), GABA (1 microM) produced a transient tachycardia and a small increase in coronary flow. Both capsaicin (1 microM) and CGRP (1 microM) produced marked tachycardias and increases in coronary flow. After exposure to capsaicin (1 microM), no effect of GABA could be detected. 7. We conclude that, in the guinea-pig heart, GABAA receptors, presumably located on the preterminal region of capsaicin-sensitive sensory nerves, initiate a conducted impulse (since it is tetrodotoxin-sensitive) which leads to transmitter release (endogenous CGRP-like material) by activation of omega-conotoxin-sensitive, voltage-sensitive calcium channels and a functional response.

Omega-conotoxin GVIA specifically blocks neuronal mechanisms in rat ileum

Omega Conotoxin is a peptide venom of the marine snail Conus geographus which has been postulated as a neural specific calcium channel blocker in several systems. In the present study in rat ileum strips omega-conotoxin GVIA specifically blocked neuronal responses evoked by electrical field stimulation in a dose-dependent manner, but did not block responses induced by carbachol or direct depolarization with KCl. On the other hand the dyhydropyridine calcium channel blocker nitrendipine (10(-7) M) showed no specificity and blocked electrical field stimulation as well as carbachol- and KCl-induced contractions. This indicates that contractions of intestinal smooth muscle which can be blocked by dihydropyridine calcium channel blockers or Ca2+-free medium are not reduced by the neurotoxin omega-conotoxin. Omega-conotoxin caused even a little increase of the responses elicited by depolarisation with KCL. Omega-conotoxin blocked however neural mediated contractions which might suggest that it interacts with a distinct neuronal calcium channel and, in addition to tetrodotoxin, could provide a useful pharmacological tool to discriminate between muscular and neural sites of action of excitatory or inhibitory agents.

Evidence for a dihydropyridine-sensitive and conotoxin-insensitive release of noradrenaline and uptake of calcium in adrenal chromaffin cells

1. It has been suggested that neuronal voltage-sensitive calcium channels (VSCC) may be divided into dihydropyridine (DHP)-sensitive (L) and DHP-insensitive (N and T), and that both the L and the N type channels are attenuated by the peptide blocker omega-conotoxin. Here the effects of omega-conotoxin on release of noradrenaline and uptake of calcium in bovine adrenal chromaffin cells were investigated. 2. Release of noradrenaline in response to 25 mM K+, 65 mM K+, 10 nM bradykinin or 10 microM prostaglandin E1 was not affected by omega-conotoxin in the range 10 nM-1 microM. 3. 45Ca2+ uptake stimulated by high K+ and prostaglandin was attenuated by 1 microM nitrendipine and enhanced by 1 microM Bay K 8644; these calcium fluxes were not modified by 20 nM omega-conotoxin. 4. With superfused rat brain striatal slices in the same medium as the above cell studies, release of dopamine in response to 25 mM K+ was attenuated by 20 nM omega-conotoxin. 5. These results show that in these neurone-like cells, release may be effected by calcium influx through DHP-sensitive but omega-conotoxin-insensitive VSCC, a result inconsistent with the suggestion that omega-conotoxin blocks both L-type and N-type neuronal calcium channels.

Modulatory action of galanin on responses due to antidromic activation of peripheral terminals of capsaicin-sensitive sensory nerves

Galanin inhibited, in a concentration-dependent manner (EC50 7.2 nM), the positive inotropic response produced by field stimulation of isolated left atria from reserpine-pretreated guinea-pigs (in the presence of atropine). These responses were shown to involve antidromic activation of capsaicin-sensitive primary afferents. On the other hand, galanin did not affect the inotropic response to capsaicin or calcitonin gene-related peptide, the putative endogenous mediator released from sensory nerves. Galanin-(1-10) was at least 10,000 times less potent than the parent peptide, while galanin-(25-29) was ineffective. Likewise, galanin inhibited the non-cholinergic contraction produced by field stimulation of the isolated guinea-pig bronchus but not the contraction produced by exogenous neurokinin A. These findings indicate a prejunctional neuromodulatory action of galanin on the excitability of peripheral terminals of capsaicin-sensitive sensory nerves.

Human isolated small intestine: motor responses of the longitudinal muscle to field stimulation and exogenous neuropeptides

(1) Longitudinal muscle strips from the human small intestine (jejunum/ileum) responded to electrical field stimulation (1-50 Hz) with frequency-related primary contractions which were largely atropine- (3 microM) sensitive. When the tone was raised by addition of galanin (0.3-1 microM), prostaglandin (PG) E2 (1-10 microM) or neurokinin A (NKA, 0.1 microM), a frequency-related relaxation was evident which was potentiated by atropine. All the responses to field stimulation were abolished by tetrodotoxin (1 microM), thus indicating their neural origin. (2) The atropine-sensitive primary contraction to field stimulation was virtually abolished by omega conotoxin fraction GVIA (CTX, 0.1-0.3 microM) while the relaxations were CTX-resistant. The field stimulation-induced relaxations, which were observed in the presence of atropine and guanethidine (3 microM), were also unaffected by apamin (0.1 microM). (3) NKA and substance P (SP) produced a concentration- (1 nM-1 microM for both peptides) related contraction, NKA being about 53 times more potent than SP. [Pro9]SP sulphone and [MePhe7]-NKB, selective agonists of the NK-1 and NK-3 receptor, respectively, were barely effective. On the other hand, [beta Ala8]NKA(4-10), a selective NK-2 receptor agonist, had a potent contractile activity, similar to that of NKA. (4) Galanin (1 nM-1 microM) produced an atropine- and tetrodotoxin-resistant concentration-related contraction of longitudinal muscle of human isolated small intestine. The response to galanin did not show any sign of fading and was particularly suitable to study the evoked relaxations.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition of transmitter release from sympathetic nerve endings by omega-conotoxin

1. The effects of the calcium channel blocker, omega-conotoxin, on sympathetic neuroeffector function in the guinea-pig vas deferens have been investigated using a combination of mechanical and electrophysiological recording techniques. 2. Biphasic contractions evoked by electrical field stimulation were irreversibly abolished by omega-conotoxin (10-100 nmol/L). 3. Electrically evoked excitatory junction potentials and currents were irreversibly blocked by omega-conotoxin (10-100 nmol/L). Spontaneous excitatory junction potentials and currents were unaffected by this treatment. 4. omega-Conotoxin did not block impulse propagation in the nerve terminals. However, in three of four experiments omega-conotoxin caused a decrease in the size of the nerve terminal impulse. 5. These findings support the suggestion the omega-conotoxin acts prejunctionally to inhibit sympathetic neuroeffector by interfering with depolarization-secretion coupling.

Further studies on the motor response of the human isolated urinary bladder to tachykinins, capsaicin and electrical field stimulation

1. Muscle strips from the dome of the human urinary bladder responded to field stimulation with contractions which were atropine- (3 microM) and tetrodotoxin- (1 microM) sensitive. These contractions were sensitive to omega conotoxin (CTX, 0.1 microM). The atropine- and tetrodotoxin-resistant contractions produced by field stimulation were totally unaffected by CTX. 2. DMPP (30-100 microM), a nicotinic agonist, produced transient bladder contractions which were hexamethonium- and atropine-sensitive. 3. Tachykinins produced a contraction of the human bladder. Among several synthetic tachykinin analogs only those having activity at the NK-2 receptor produced a consistent contractile response. 4. Either capsaicin (1 microM) or calcitonin gene-related peptide (10 nM-0.1 microM) had no motor effect. At 10 microM, capsaicin exerted a depressant effect on nerve-mediated contractions but this effect did not exhibit desensitization. 5. These findings provide evidence that NK-2 receptors are the main if not the sole mediators of the contractile response of the muscle from the dome of the human isolated bladder to tachykinins. 6. No evidence was found for a tachykininergic component in the excitatory response to field stimulation nor for motor responses mediated by capsaicin-sensitive nerves. 7. CTX-sensitive calcium channels are probably present on cholinergic nerve terminals in the human bladder muscle.

Distribution of the omega-conotoxin receptor in rat brain. An autoradiographic mapping

The distribution of [125I]omega-conotoxin GVIA binding sites, the putative voltage-sensitive calcium channels, was studied by an autoradiographic method in the rat brain. The toxin binding sites were distributed throughout the brain in a highly heterogeneous manner. The highest density of the binding sites was observed in the cerebral cortex, hippocampus, amygdaloid complex, substantia nigra, caudate putamen, superior colliculus, nucleus of the solitary tract, and the dorsal horn of the cervical spine. The glomerular layer of the olfactory bulb, molecular layer of the cerebellar cortex, and posterior lobe of the hypophysis showed intermediate density but the density was higher than in the surrounding areas. The globus pallidus, thalamic areas, inferior olive, and pontine nuclei showed low density, while no binding sites were observed in the white matter tract regions such as the internal and external capsule, corpus callosum, fimbria of the hippocampus, fornix, stria medullaris of the thalamus, and fasciculus retroflexus. This distribution of omega-conotoxin binding sites indicates that the toxin binding sites are localized in those areas of the brain enriched in synaptic connections. This distribution pattern resembles that reported for voltage-sensitive sodium channels but it differs from that of the binding sites of dihydropyridines and verapamil. These results suggest that omega-conotoxin recognizes different molecules from organic calcium channel antagonist binding sites and that omega-conotoxin-sensitive voltage-sensitive calcium channels are concentrated in the synaptic zones and play a key role in the excitation-secretion coupling of neurotransmitters.

Evidence for two independent modes of activation of the 'efferent' function of capsaicin-sensitive nerves

Field stimulation (10 Hz for 10 s, 0.5 ms pulse width, 60 V) of the guinea-pig isolated main bronchi (atropine plus indomethacin in the bath) produced reproducible contractions which were abolished by tetrodotoxin or in vitro capsaicin desensitization. These responses were almost abolished by omega-conotoxin GVIA (CTX), a peptide modulator of neuronal calcium channels which, however, did not affect the bronchial contraction due to neurokinin A or to capsaicin. Field stimulation (10 Hz for 2.5 s, 1 ms, 60 V) of the electrically driven, isolated guinea-pig left atria excised from reserpine-pretreated animals (atropine in the bath) produced a delayed positive inotropic response which was abolished by tetrodotoxin or in vitro capsaicin desensitization. This response was abolished by CTX, which did not affect the response to exogenous calcitonin gene-related peptide nor that to capsaicin. These findings indicate that CTX-sensitive mechanisms (presumably Ca channels regulating the release of transmitters) are activated upon antidromic invasion of sensory terminals and consequent production of the 'efferent' response while the activation of sensory nerve endings by capsaicin occurs through CTX-resistant mechanisms.