Characterization of the effects of omega-conotoxin GVIA on the responses of voltage-sensitive calcium channels

Algogen-specific pain processing in mouse spinal cord: differential involvement of voltage-dependent Ca(2+) channels in synaptic transmission

1. The effects of intrathecal (i.t.) administration of N-, P/Q- or L-type voltage-dependent Ca(2+)-channel blockers were tested in two pain models involving bradykinin (BK)- and alpha,beta-methylene ATP (alpha,beta meATP)-induced activation of primary afferent neurons in mice. 2. The nociceptive response (amount of time spent licking and biting the hindpaw) induced by intraplantar injection of BK (500 pmol mouse(-1)) was significantly attenuated by both omega-conotoxin GVIA (N-type blocker) and calciseptine (L-type) but not by omega-agatoxin IVA (P/Q-type). 3. The nociceptive response induced in a similar way by alpha,beta meATP (100 nmol) was significantly inhibited by both the above N- and P/Q-type Ca(2+)-channel blockers but not by the L-type blocker. 4. The nociceptive responses elicited by BK and alpha,beta meATP were dose-dependently inhibited by a tachykinin-NK1-receptor antagonist (L-703,606) and an N-methyl-D-aspartate (NMDA)-receptor antagonist (D-AP5), respectively. 5. Intrathecal administration of substance P (SP) (1.8 nmol) or NMDA (350 pmol) elicited algesic responses, such as licking, biting and scratching of the hindquarters. The SP-induced algesic behaviour was significantly inhibited by the L-type blocker but not by the N-type. The NMDA-induced response was not affected by either the N- or the P/Q-type blocker. 6. These findings suggest that BK and ATP most likely excite different types of sensory neurons in the periphery and that within the spinal cord the former stimulates peptidergic transmission regulated by presynaptic N- and postsynaptic L-type Ca(2+) channels, while the latter stimulates glutamatergic transmission regulated by presynaptic N- and P/Q-type channels.

Multiple calcium channels control neurotransmitter release from rat postganglionic sympathetic nerve terminals

1. Intracellular recording techniques were used to study neurotransmitter release mechanisms in postganglionic sympathetic nerve terminals of the rat isolated anococcygeus muscle. 2. Low concentrations of the N-type calcium channel blocker omega-conotoxin GVIA (omega-CgTX GVIA) irreversibly abolished excitatory junction potentials (EJPs) evoked by trains of < or = five stimuli at 10 Hz. When the frequency of stimulation was increased (10-50 Hz) trains of stimuli evoked EJPs even in the presence of 1 microM omega-CgTX GVIA. We have termed this omega-CgTX GVIA-resistant release 'residual release'. EJP amplitude in the presence of omega-CgTX GVIA depended on both the frequency and number of stimuli in a train. 3. Residual release was inhibited by the P-type calcium channel blocker omega-agatoxin IVA (100 nM). However, even in the presence of both toxins, longer trains of stimuli could still evoke neurotransmitter release. 4. Residual release was abolished by omega-conotoxin MVIIC and by the non-specific calcium channel antagonist omega-grammotoxin SIA. Therefore, it would appear that a heterogeneous population of calcium channels is involved in mediating neurotransmitter release from these sympathetic nerve terminals.

Differential regulation of evoked peptide release by voltage-sensitive calcium channels in rat sensory neurons

To determine whether the sensitizing action of prostaglandins on sensory neurons are due to modulation of voltage-sensitive calcium channels (VSCC) we examined the effects of inhibiting these channels on PGE2-induced enhancement of evoked peptide release from isolated dorsal root ganglion neurons. The inhibitory effects of the VSCC blockers on stimulated release were dependent upon the type of chemical agent used to evoke the release. Bradykinin-stimulated release of immunoreactive substance P (iSP) and calcitonin gene-related peptide (iCGRP) was attenuated by the N-type VSCC blocker, omega-conotoxin GVIA (100 nM), but was unaffected by blockade of L-type (1 microM nifedipine) or P-type (200 nM omega-agatoxin IVA) VSCC. In contrast, potassium-stimulated release of peptides was inhibited by nifedipine, but not by omega-conotoxin GVIA or omega-agatoxin IVA. None of the VSCC blockers tested attenuated capsaicin-stimulated release of iSP and iCGRP. The combination of 1 microM nifedipine and 100 nM omega-conotoxin GVIA reduced the whole cell calcium current 89% +/- 1.7%. Administration of 100 nM PGE2 potentiated bradykinin- and capsaicin-evoked peptide release by 2-3-fold. Neither nifedipine nor omega-conotoxin GVIA attenuated the PGE2-mediated potentiation of bradykinin-evoked release, and neither omega-conotoxin GVIA nor omega-agatoxin IVA blocked the potentiation of capsaicin-evoked release induced by PGE2. These results indicate that the sensitizing actions of PGE2 as measured by enhanced peptide release, are not mediated by L-, N-, or P-type VSCC.

Calcium channel antagonist peptides define several components of transmitter release in the hippocampus

The use of subtype-selective voltage-sensitive calcium channel (VSCC) antagonists has established that neurotransmitter release in mammalian brain is mediated by N-like and P-like VSCCs, and that other subtypes also contribute significantly. To determine the roles presynaptic VSCCs play in nervous system function and to evaluate the therapeutic potential of their selective inhibition, it is necessary to define further the contributions of VSCC subtypes to neurotransmitter release. The novel conopeptide, SNX-230 (omega-conopeptide MVIIC), has revealed a new VSCC subtype, the Q-type, in cerebellar granule cells. We have compared the effects of SNX-230 on release of tritiated D-aspartate ([3H]D-Asp; a non-metabolizable analog of glutamate), gamma-aminobutyric acid ([3H]GABA), and norepinephrine ([3H]NE) from rat hippocampal slices to those of the N-type VSCC blocker, SNX-111 (omega-conopeptide MVIIA), and the P-type blocker, omega-agatoxin-IVA (AgaIVA). SNX-230 blocks both [3H]D-Asp and [3H]GABA release completely, whereas AgaIVA blocks them potently but partially and SNX-111 has no effect. These results suggest that glutamate and GABA release are mediated by two VSCC subtypes, a P-type and another, perhaps Q-like. SNX-111 blocks [3H]NE release potently but partially, while SNX-230 blockade is complete, consisting of one very potent phase and one less potent phase. AgaIVA also blocks [3H]NE release potently but partially. These results suggest that at least two VSCC subtypes, an N-type and a novel non-N-type, mediate NE release. Pair-wise combinations of the three ligands indicate that at least three pharmacologically distinct components comprise [3H]NE release in the hippocampus.

Effect of omega-agatoxin-IVA on autonomic neurotransmission

omega-Agatoxin-IVA, a peptide from the venom of the funnel-web spider Agelenopsis aperta and a P type Ca2+ channel inhibitor, was examined for effects on responses to nerve stimulation in isolated autonomic neuroeffector preparations from the rabbit, guinea-pig and rat. Ca(2+)-dependent, tetrodotoxin sensitive, noradrenergic excitatory responses of rabbit pulmonary artery, rat vas deferens, and anococcygeus muscles, and cholinergic guinea-pig myenteric plexus preparations (all highly sensitive to the N type Ca2+ channel inhibitor omega-conotoxin-GVIA) were unaffected by omega-agatoxin-IVA (100 nM). Similarly, the neurogenic response of rat bladder, which has cholinergic, and non-adrenergic non-cholinergic (NANC) excitatory components, and the NANC inhibitory response of rat jejunum (atropine 0.5 microM- and guanethidine 5.0 microM-treated), which are partially sensitive and insensitive to omega-conotoxin-GVIA, respectively, were unaffected by omega-agatoxin-IVA (100 nM). Neurogenic NANC inhibitory responses of the guinea-pig taenia caecum, and rat anococcygeus muscles (atropine- and guanethidine-treated, and tone raised with prostaglandin F2 alpha), were also insensitive to omega-agatoxin-IVA. These results suggest that P type Ca2+ channels, if present, play an insignificant role in supplying the Ca2+ necessary for neurotransmitter release in the peripheral autonomic nervous system.

N- and P-type Ca2+ channels are involved in acetylcholine release at a neuroneuronal synapse: only the N-type channel is the target of neuromodulators

Cholinergic transmission in an identified neuro-neuronal synapse of the Aplysia buccal ganglion was depressed by application of a partially purified extract of the funnel-web-spider venom (FTx) or of its synthetic analog (sFTx). This specific blocker of voltage-dependent P-type Ca2+ channels did not interfere with the effect of the N-type Ca2+ channel blocker omega-conotoxin, which could further decrease synaptic transmission after a previous application of FTx. Similar results were obtained when the reversal order of application of these two Ca2+ channel blockers was used. Both P- and N-type Ca2+ currents trigger acetylcholine release in the presynaptic neuron. The neuromodulatory effects of FMRF-amide, histamine, and buccalin on transmitter release disappeared after the blockade of the N-type Ca2+ channels but remained still effective in the presence of FTx. These results indicate that only N-type Ca2+ channels appear to be sensitive to the neuromodulators we have identified.

Effects of diverse omega-conopeptides on the in vivo release of glutamic and gamma-aminobutyric acids

omega-Conopeptides are antagonists of subtypes of neuronal calcium channels. Two omega-conopeptides, SVIB and MVIIC, have recently been identified which have a novel specificity for these ionophores. We have tested the actions these peptides, as well as the more selective MVIIA, on the release of glutamic acid and gamma-aminobutyric acid (GABA) in the hippocampus in vivo. For the assay of peptide effects on release, we used microdialysis to deliver multiple pulses of elevated potassium to the brain extracellular fluid. Peptide effects were quantitated from the decrement of the release with peptide perfused through the probes, in comparison to that in control experiments. Synthetic MVIIC caused a 40-50% decrement in the release of both glutamate and GABA at a probe concentration of about 200 nM. Synthetic SVI-B caused a 50% block at about 20-40 microM, while about 200 microM of MVIIA was required for 50% block. Chromatographic experiments showed that differences in potency between MVIIC and MVIIA were not explained by differential degradation. Blockade of release was also observed in the thalamus. MVIIC provides a tool for investigating the role of calcium mediated release of glutamate and GABA in physiological and pathological processes in the mammalian brain in vivo.

Hippocampal noradrenaline release is modulated by gamma- and delta-hexachlorocyclohexane isomers: which mechanisms are involved?

The differential effects of gamma- and delta-hexachlorocyclohexane isomers on 25 mM K(+)-evoked release of [3H]noradrenaline were studied in hippocampal slices treated with selected agents to activate or block L- and N-type Ca2+ and Na+ voltage-sensitive ion channels, Cl- transport and Ca(2+)-dependent protein activity. At maximally effective concentrations, the L- and N-type Ca2+ channel blockers nifedipine and omega-conotoxin, respectively, and the Na+ channel antagonist tetrodotoxin did not modify the enhancement of K(+)-evoked [3H]noradrenaline release induced by gamma-hexachlorocyclohexane. Likewise, under activation of protein kinase C by phorbol 12,13-dibutyrate (PDB) or inhibition of calmodulin by N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide (W-7), the stimulatory effect of gamma-hexachlorocyclohexane remained almost unchanged. The Cl- transport blocker 4,4-diisothiocyanato-stilbene-2,2'-disulfonic acid (DIDS) significantly reduced the effect of gamma-hexachlorocyclohexane on [3H]noradrenaline release. The enhanced release in the presence of Bay K 8644, the L-type Ca2+ channel activator, was significantly inhibited by nifedipine but not by delta-hexachlorocyclohexane. The combination of omega-conotoxin and tetrodotoxin with delta-hexachlorocyclohexane did not alter the [3H]noradrenaline release effects of each agent alone. Activation of protein kinase C in the presence of delta-hexachlorocyclohexane resulted in a reduction of the delta isomer effect and in a potentiation of the PDB effect. W-7 did not further facilitate the inhibition induced by delta-hexachlorocyclohexane alone. These data suggest that hexachlorocyclohexane isomers may modify K(+)-evoked [3H]noradrenaline release by interacting with presynaptic molecular processes involving changes in Cl- membrane permeability and intracellular Ca2+ homeostasis.

Effect of longitudinal muscle-myenteric plexus removal and indomethacin on the response to tachykinin NK-2 and NK-3 receptor agonists in the circular muscle of the guinea-pig ileum

1. The effect of removal of the longitudinal muscle-myenteric plexus (LM-MP) and/or indomethacin (10 microM) on the response to the tachykinin NK-2 receptor selective agonist, [beta Ala8]NKA(4-10), or to the NK-3 receptor selective agonist, senktide, was investigated by measuring mechanical activity (isotonic recording) of circular muscle (ring preparation) of the guinea-pig ileum. 2. Indomethacin (10 microM) increased the percentage of ileal rings displaying spontaneous activity, either intact or LM-MP-free. The response to senktide (10 nM and 1 microM) was lower in LM-MP-free than in intact ileal rings, either in the absence or presence of indomethacin. The response to a low concentration (10 nM) of [beta Ala8] NKA (4-10) was enhanced in LM-MP-free rings and by indomethacin. 3. In intact ileal rings, the response to senktide was unaffected by atropine (3 microM) alone or by the tachykinin NK-2 receptor antagonist MEN 10,376 (10 microM) alone while it was reduced by the combined administration of the two antagonists. The response to senktide was greatly reduced by tetrodotoxin (TTX, 1 microM). Senktide-induced contractions (10 nM) were also reduced by the blocker of N-type voltage-sensitive calcium channels, omega-contoxin (CTX, 0.1 microM). 4. In about 30% of preparations tested, an inhibitory response (decrease in spontaneous activity) to 10 nM senktide, was disclosed in CTX-treated intact ileal rings. This inhibitory effect was TTX-sensitive. 5. In LM-MP-free ileal rings, the response to senktide was abolished or reduced by atropine and MEN 10,376, alone or in combination, and was also reduced or abolished by TTX and CTX. 6. The response to [beta Ala8]NKA (4-10) was inhibited by MEN 10,376, in both intact and LM-MP-free ileal rings while it was unaffected by atropine, TTX or CTX. 7. These results indicate that indomethacin pretreatment induces a regular background activity for studying the motor response to tachykinins in the circular muscle of the ileum, probably by blocking the formation of relaxant prostanoids. A further increase in sensitivity to direct smooth muscle stimulation (NK-2 receptor agonist) can be obtained by removal of the LM-MP. The response to NK-3 receptor stimulation is diminished but not abolished by removal of the LM-MP, suggesting that NK-3 receptors are located on neuronal bodies of myenteric neurons, but possibly also at other sites (possibly, nerve terminals).(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of calcium antagonists on hypoxic and NMDA injury in rat hippocampal slices

The effects of various calcium antagonists, acting at the different neuronal calcium channels, were studied towards two models of in vitro neuronal injury in rat hippocampal slices. In particular, the influence of the drugs were tested on the electrical failure induced by treatment of hippocampal slices with hypoxia or high concentrations of the excitatory amino acid N-methyl-D-aspartate (NMDA). The L-type calcium antagonists, nifedipine (100 microM) and diltiazem (100 microM) or the T-type calcium antagonist amiloride (100 microM) failed to significantly affect the recovery from the CA1 electrical failure induced by both hypoxia or NMDA (50 microM). The N-type calcium antagonists, omega-conotoxin GVIA (0.5 microM) and neomycin (300 microM) significantly (P < 0.01) increased the probability of the recovery of the CA1 population spike after hypoxia but not after NMDA (50 microM). The glutamate antagonist dizocilipine (50 microM), tested for comparison, significantly (P < 0.01) increased the probability of the recovery of the CA1 population spike after hypoxia and NMDA (50 microM). The results suggest an involvement of calcium channels especially of N-type in the genesis of hypoxic but not NMDA neuronal injury.

Effects of omega-conotoxin on adrenergic, cholinergic and NANC neurotransmission in the rabbit urethra and detrusor

1. The effects of omega-conotoxin GVIA (an inhibitor of N-type voltage-operated calcium channels; VOCCs) were compared on adrenergic, cholinergic and non-adrenergic, non-cholinergic (NANC) responses induced by electrical field stimulation (EFS) in the rabbit urethra and detrusor. 2. EFS induced a relaxation in urethral smooth muscle and lamina propria precontracted by arginine vasopressin (AVP). The relaxation was abolished by tetrodotoxin (TTX) or the nitric oxide (NO) synthase inhibitor N omega-nitro-L-arginine. omega-Conotoxin inhibited the relaxation induced by EFS, but not that elicited by the NO donor 3-morpholino-sydnonimin. The inhibition, however, decreased with increasing frequencies of stimulation. Nimodipine, tetramethrin and nickel did not affect the omega-contoxin-resistant relaxation in lamina propria, suggesting that neuronal L or T VOCCs were not involved in the response. 3. EFS contracted urethral smooth muscle at resting tension. The contractions were virtually abolished by TTX or prazosin. omega-Conotoxin effectively inhibited the contractile responses to EFS, but not those to exogenous noradrenaline. An omega-conotoxin-resistant contraction was, however, observed at high frequencies of stimulation. 4. The detrusor responded with frequency-dependent contractions upon EFS. A TTX-resistant contraction less than 10% of controls remained at 30 Hz stimulation. At a stimulation frequency of 10 Hz, scopolamine reduced the EFS-induced contraction by 71%. omega-Conotoxin inhibited the responses in both the absence and presence of scopolamine. The inhibition decreased with increasing frequencies of stimulation (examined in the absence of scopolamine). omega-Conotoxin did not affect the contractile responses to carbachol or adenosine 5'-triphosphate. 5. The adrenergic contraction (25 Hz) and NANC relaxation (10 Hz) in the urethra, and cholinergic and NANC contractions (10 Hz) in the detrusor were inhibited concentration-dependently by omega-conotoxin.The adrenergic contraction in the urethra was 10 times and the cholinergic contraction in the detrusor was three times more sensitive to omega-conotoxin than the NANC responses.6. These results suggest that NANC neurotransmission is less inhibited by omega-conotoxin than transmission mediated by adrenergic and cholinergic nerves in the rabbit lower urinary tract. In the urethra a marked omega-conotoxin-resistant component of the NANC relaxation was observed which increased with increasing stimulation frequencies and was unaffected by inhibitors of L and T type VOCCs. This raises the question whether VOCCs of a type other than L, T, and N is involved in the mediation of this response.

Calcium-channel blockers and gastrointestinal motility: basic and clinical aspects

Several calcium-channel blockers currently in use for the treatment of cardiovascular disorders have recently been tested for their effects on gastrointestinal motility. The rationale for this approach centers on the concept that calcium-channel blockers are at least as potent in inhibiting intestinal smooth muscle as in relaxing vascular smooth muscle. This review will give an outline of the most recent findings on the role of calcium and calcium channels in smooth muscle and neuronal function in the digestive system. It will also consider the mechanisms by which calcium-channel blockers may affect gastrointestinal motility and assess potential clinical applications in gastroenterology. The main goal for researchers in this field will be the development of gut-selective agents, with no cardiovascular side effects.

[Role of voltage-dependent ion channels in epileptogenesis]

The aim of this review is to gather information in favour of the involvement of voltage-dependent ion channels in epileptogenesis. Although, up to now, no study has shown that epilepsy is accompanied by a modification in the activity to these channels, the recently acquired knowledge of their physiology allows to presume would favor their involvement in epileptogenesis. The results from electrophysiological studies are as follows: a persistent sodium current increases neuronal excitability whereas potassium currents have an inhibitory role. In particular, calcium-dependent potassium current are involved in the post-hyperpolarization phases which follows PDS. Calcium currents are also involved in the genesis of the "bursting pacemaker" activity displayed by the neurons presumed to be inducers of the epileptic activity. Biochemical data has shown that as a consequence of epileptic activity, sodium and calcium channels are down regulated. This down-regulation could be a way to reduces neuronal hyperexcitability. Pharmacological data demonstrate the drugs which activate calcium channels or which inhibit potassium channels have a convusilvant effect. On the contrary, agents which block calcium or sodium channels or which properties. Among the latter ones, some antiepileptic drugs can be found. In summary situations which lead to increase in calcium and sodium currents and/or to an inhibition in potassium currents are potentially epileptogenic.

Calcium modulatory properties of 2,6-dibutylbenzylamine (B25) in rat isolated vas deferens, cardiac and smooth muscle preparations

1. In rat isolated vas deferens the new compound 2,6-dibutylbenzylamine (B25) evoked a series of repeating rhythmic contractions. Concentration-response curves constructed for this effect were bell-shaped, indicating a biphasic effect for this compound. By contrast, B25 depressed heart contractility without any visible positive inotropic or chronotropic activity. 2. Experiments with tetrodotoxin, reserpine, capsaicin, alpha-adrenoceptor blocking compounds and other agents permit us to exclude a release of neuromediators or a direct stimulation of post-synaptic receptors to account for the rhythmic effect of B25 in the rat vas deferens. 3. In the same tissue, the increase in 45Ca2+ uptake, the voltage-dependency as well as the dependence of the B25-induced rhythmic activity upon the external calcium concentration indicate a direct activation of voltage-sensitive calcium channels (VSCC). 4. Verapamil paradoxically stimulated the rhythmic effect of B25 in the rat vas deferens. La3+ was inactive while nifedipine was a weak inhibitor. By contrast Ni2+ and Mn2+ ions were good inhibitors (IC50 < 10(-4) M), suggesting that a possible opening of T-type VSCC underlies rhythmic effect of B25. 5. In radioligand binding studies competition experiments with [3H]-nitrendipine indicated that only at high concentrations was B25 able to interact with dihydropyridine-sensitive binding sites of heart and vas deferens smooth muscle. 6. B25 (3-30 microM) counteracted the inhibitory effects of omega-conotoxin GVIA in field-stimulated rat vas deferens.

Calcium channels at the adrenergic neuroeffector junction in the rabbit ear artery

Neurotransmitter release is dependent on influx of Ca2+ through voltage-operated calcium channels (VOCCs). These channels may be divided into L, N, T and P subtypes. To investigate the subtypes of VOCC involved in transmitter release from adrenergic nerves in the isolated rabbit ear artery, the effects of some subtype selective VOCC antagonists were examined on contractile responses induced by electrical field stimulation (EFS), and exposure to an isosmolar (low Na+, normal Cl- content) or a hyperosmolar (normal Na+, high Cl- content) 60 mM K+ solution. Tetrodotoxin (TTX) and the L channel blocker nimodipine were present in the latter experiments to inhibit sodium-dependent action potential discharge and the direct contractile effect of K+ depolarization on the smooth muscle cells. Prazosin abolished the contractile effect of EFS, indicating that the response was elicited by activation of adrenergic nerves. The EFS-induced contractions were concentration-dependently inhibited by the N channel blocker omega-conotoxin (pIC50 = 9.0) and the proposed L channel blocker T-cadinol (pIC50 = 4.5), while nimodipine and the T channel blocker tetramethrin had no effect. The isosmolar and hyperosmolar K+ solutions induced a prazosin-sensitive contraction, amounting to 46% and 10% of the response to 10(-5) M noradrenaline (NA), respectively. omega-Conotoxin inhibited the contractile response to the hyperosmolar K+ solution, but not that to the isosmolar K+ solution. T-cadinol preferentially inhibited the response to the hyperosmolar K+ solution. Tetramethrin had no effect on contractions induced by either type of K+ solution.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Effects of site-specific acetylation on omega-conotoxin GVIA binding and function

Chemical modification of omega-conotoxin GVIA (omega-CgTXGVIA) was performed using nonsaturating concentrations of acetic anhydride to generate seven distinct derivatives. Following separation of these peptides using reverse-phase HPLC (RP-HPLC), their individual molecular weights were determined using fast bombardment mass spectrometry (FAB-MS). Three peptides contained a single acetylated amino group, three possessed two acetylated amino groups, and the last contained three acetylations. For each peptide, the specific site of acetylation was confirmed using a scheme of tryptic digestion, under nonreducing conditions, followed by RP-HPLC and FAB-MS. Biological profiles for each peptide were obtained by analyzing their capacity to displace native 125I-omega-CgTx GVIA binding to rat hippocampal membranes and to block K(+)-stimulated 45Ca2+ influx into chick brain synaptosomes. The data indicate that successive additions of acetyl moieties to omega-CgTx GVIA lead to a loss of both binding affinity and Ca2+ influx inhibitory potency. Within the monoacetylated series, acetylation of the amino terminal of Cys-1, as compared to the epsilon-amino group of either Lys-2 or Lys-24, leads to the greatest shift in potency. In summary, these results indicate that basic (i.e., primary amino) groups, which are brought into close proximity as a result of disulfide bridging, are important in the functional blockade of neuronal Ca2+ channels by omega-CgTx GVIA.

Omega-conotoxin increases sleep time following ethanol injection

omega-Conotoxin and ethanol produce similar actions on in vitro calcium channel functions. The present study was designed to determine their possible behavioral interaction. omega-Conotoxin injected ICV at either 0.1 microgram or 0.3 microgram, produced an increase in spontaneous and evoked tremor activity in male Sprague-Dawley rats. The tremor was present at 30 min and continued at least 4 h after injection. At 4 h post ICV injection, animals were given an IP injection of ethanol (3 g/kg body weight). Although no blood alcohol differences were observed between groups, rats injected with omega-conotoxin showed a concentration-dependent increase in sleep times: Saline controls slept for an average of 84.7 +/- 16.7 min, 0.1 and 0.3 microgram conotoxin treated animals slept for 121.3 +/- 16.2 and 211.1 +/- 30.7 min, respectively. These results extend the class of calcium channel blockers capable of producing a behavioral interaction with ethanol.

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.

Omega-conotoxin GVIA inhibits release of noradrenaline from rat hippocampal slices in the absence of extracellular calcium

Slices of hippocampus of the rat, preincubated with [3H]noradrenaline ([3H]NA), were used to investigated the effects of omega-conotoxin GVIA (omega-CTX) on the release of [3H]NA evoked by 3,4-diaminopyridine (3,4-DAP, 200 microM), veratridine (0.7 microM) or monensin (0.01 microM) in the absence of extracellular CA2+. The 3H outflow, evoked by 3,4-DAP or veratridine, was inhibited by tetrodotoxin (TTX) or omega-CTX but the 3H outflow evoked by monensin was neither affected by TTX nor by omega-CTX in Ca(2+)-free medium, containing 1 mM EGTA. The release response to 3,4-DAP or veratridine was also blocked by omega-CTX in a concentration-dependent manner in Ca(2+)-free medium, containing 2.5 mM Mg2+ and the blockade was still complete after washing for 20 min with omega-CTX-free medium. The findings suggest that, under these conditions, the toxin might also block sodium channels.

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.

HIV-1 envelope protein evokes intracellular calcium oscillations in rat hippocampal neurons

Treatment of single rat hippocampal neurons with 200 pM recombinant HIV-1 envelope glycoprotein, gp120, resulted in large increases in the intracellular free calcium concentration ([Ca2+]i) as measured with indo-1-based microfluorimetry. Three patterns of [Ca2+]i increases were observed: in one pattern the [Ca2+]i rose rapidly and transiently as a single peak, in a second pattern gp120 induced [Ca2+]i oscillations that subsided when the protein was removed, and in a third pattern the oscillations continued long after washout of gp120. Both single peak and oscillatory [Ca2+]i increases were completely blocked by the Ca2+ channel blocker nitrendipine (1 microM). The sustained oscillatory responses were also blocked completely and reversibly by the N-methyl-D-aspartate (NMDA) receptor antagonist CGS19755 (10 microM) and the Na+ channel blocker tetrodotoxin (1 microM). Complete block by antagonists of Ca2+, Na+, and NMDA-gated ion channels suggests that at least two cells are required to maintain the [Ca2+]i oscillations. We hypothesize that gp120 acts as an excitotoxin by increasing synaptic activity in the network of neurons established in primary culture.

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.

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.

A toxin (Aga-GI) from the venom of the spider Agelenopsis aperta inhibits the mammalian presynaptic Ca2+ channel coupled to glutamate exocytosis

Venom of the funnel web spider Agelenopsis aperta was fractionated and screened for activity against the mammalian presynaptic, voltage-dependent Ca2+ channel coupled to glutamate exocytosis. A purified toxin (Aga-GI) from this venom inhibits glutamate exocytosis evoked by elevated potassium or by 4-aminopyridine but is without effect on ionomycin-evoked release. At the same time a partial inhibition of the depolarisation-evoked elevation of cytoplasmic free Ca2+ is seen. The toxin does not inhibit 4-aminopyridine- or potassium-evoked depolarisation, or block Ca(2+)-dependent, potassium-evoked [3H]noradrenaline release. The results indicate that the venom contains a toxin capable of inhibiting the presynaptic voltage-dependent Ca2+ channel coupled to glutamate exocytosis in the mammalian central nervous system. This channel is resistant to block by either omega-conotoxin GVIA or nifedipine. Thus Aga-GI is a novel tool with which to probe this elusive neuronal calcium channel.

The effect of calcium channel modulators on blood pressure and pressor responses to noradrenaline in the guinea-pig

Selective L-type voltage-operated calcium channel (VOCC) antagonists nicardipine, diltiazem and verapamil all produced pronounced falls in mean arterial blood pressure (MAP) in the anaesthetized artificially ventilated guinea-pig. This fall in MAP was associated with a significant reduction of the pressor response induced by i.v. noradrenaline. omega-Conotoxin GVIA (CgTx) a preferential N-type VOCC antagonist produced a significant time-dependent fall in MAP, similar in magnitude to the L-type VOCC antagonist, but did not affect the noradrenaline-induced pressor responses.

Alpha 2-adrenoceptor mediated inhibition of exocytotic noradrenaline release in the absence of extracellular Ca2+

The effect of the alpha 2-adrenoceptor agonist clonidine on 3,4-diaminopyridine (3,4-DAP)-evoked [3H]noradrenaline ([32H]NA) release in rat hippocampus slices was studied in the presence or absence (+1 mM EGTA) of extracellular Ca2+. 3H overflow (consisting mainly of unmetabolized [3H]NA) was evoked by addition of 100 microM 3,4-DAP for 10 min to the medium, which always contained 1 microM desipramine. Ligands for L-type voltage-sensitive Ca2+ channels (VSCC) did not affect the evoked [3H]NA release, whereas the preferential N-type VSCC antagonist omega-conotoxin was inhibitory, both in the presence and even more potently in the absence of Ca2+, suggesting an involvement of N-type VSCC in the mechanism of 3,4-DAP-evoked [3H]NA release. In the absence of extracellular Ca2+ the initial Na+ influx, which has been previously proposed to liberate Ca2+ from intracellular stores for the exocytotic process, most probably occurs via N-type VSCC. Clonidine inhibited the 3,4-DAP-evoked [3H]NA release in a concentration-dependent manner, both in the presence and even more potently in the absence of Ca2+; its effects were antagonized by yohimbine. In the presence of extracellular Ca2+ the clonidine effect was not changed by addition of omega-conotoxin. Similar effects of clonidine were found in slices from the rabbit hippocampus. Since the availability of Ca2+ from intracellular stores seems to predominate in the present model, our results lend some support to the suggestion that alpha 2-adrenoceptor activation might affect intracellular mechanisms of Ca2+ homeostasis.(ABSTRACT TRUNCATED AT 250 WORDS)

Smithkline Beecham Prize for Young Psychopharmacologists: A review of the relationship between calcium channels and psychiatric disorders

The symptoms and etiology of most major psychiatric disorders probably represent an underlying disturbance of neurotransmitter function. Understanding the mechanisms which control neurotransmitter function, and in particular neurotransmitter release, is therefore of considerable importance in determining the appropriate pharmacological treatment for these disorders. Calcium entry into neurons triggers the release of a wide range of neurotransmitters and recently our understanding of the mechanisms which control neuronal calcium entry has increased considerably. Neuronal calcium entry occurs through either voltage-sensitive or receptor-operated calcium channels. This article reviews the different subtypes of calcium channel, with particular reference to their structure; drugs which act upon them; and the possible function of the subtypes identified to date. In addition, it reviews the potential role of calcium channel antagonists in the treatment of a wide range of psychiatric disorders, and concludes that these drugs may have an increasing therapeutic role particularly in the treatment of drug dependence, mood disorders and Alzheimer's disease.

Effect of omega-conotoxin GVIA on neurotransmitter release at the mouse neuromuscular junction

The effect of omega-conotoxin GVIA (omega-CgTx) was studied on spontaneous, K(+)-induced and electrically evoked neurotransmitter release at the neuromuscular junction of mouse diaphragm. omega-CgTx decreased the frequency and amplitude of basal and K(+)-induced miniature end plate potentials. This effect was abolished by raising the extracellular Ca2+ concentration. omega-CgTx had no effect on the quantal content of the electrically evoked release in this preparation.

Inhibition of K(+)-evoked [3H]D-aspartate release and neuronal calcium influx by verapamil, diltiazem and dextromethorphan: evidence for non-L/non-N voltage-sensitive calcium channels

The effects of inhibitors of voltage-sensitive calcium channels (VSCC) on K(+)-evoked [3H]D-aspartate release from rat hippocampal slices and the K(+)-evoked increase in intracellular calcium in neocortical neurons in primary culture were examined. K+ caused a concentration-dependent release of [3H]D-aspartate that was approximately 85% dependent on the presence of extracellular calcium. Neither the marine snail toxin, omega-conotoxin GVIA, nor the dihydropyridine VSCC antagonist, nitrendipine, had any effect on K(+)-evoked release of [3H]D-aspartate. omega-Conotoxin GVIA and nitrendipine caused a relatively small (20-30%) inhibition of K(+)-evoked increase in intracellular calcium in neocortical neurons in primary culture. This suggests that K(+)-evoked [3H]D-aspartate release is not dependent on L- or N-type VSCC, whereas K(+)-evoked neuronal calcium influx was only partially dependent on L- and N-type VSCC. Verapamil, dextromethorphan and diltiazem caused a concentration-dependent inhibition of K(+)-evoked release of [3H]D-aspartate with IC50 values of 30, 100 and 120 microM, respectively. The K(+)-evoked increase in intracellular calcium was inhibited with essentially the same rank order of potency, but with slightly greater potencies (IC50 values for verapamil, diltiazem and dextromethorphan were 20, 50 and 50 microM, respectively). At 300 microM, neither verapamil, diltiazem nor dextromethorphan inhibited [3H]D-aspartate release evoked by the calcium ionophore ionomycin, suggesting that these compounds are not acting intracellularly to inhibit the ability of free cytosolic calcium to evoke release.(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.

Differential sensitivities of avian and mammalian neuromuscular junctions to inhibition of cholinergic transmission by omega-conotoxin GVIA

Nerve stimulation-induced contractions of the chick biventer cervicis muscle were slowly reduced by omega-conotoxin. However, omega-conotoxin had no effect on skeletal muscle function after i.v. injection in mice or on nerve stimulation-induced contractions of focally innervated muscle of the rat diaphragm or the rabbit proximal oesophagus, or the multiply innervated extra-ocular rectus muscle from rabbit. The lack of effect of omega-conotoxin on mammalian neuromuscular junctions was not due to the high safety factor in transmission or to a high local concentration of Ca2+ originating from the muscle, and could not be accounted for in terms of the operation of facilitatory or inhibitory feedback modulation of transmitter release from motoneurone terminals. It is concluded that the Ca2+ channels of mammalian motoneurone terminals differ from those of avian motoneurone terminals and other omega-conotoxin-sensitive nerve terminals.

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.

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.

Effects of galanin on smooth muscle and mucosa of porcine jejunum

The enteric neuropeptide galanin (GAL) increased the amplitude of spontaneous contractions in longitudinally oriented muscle strips and inhibited short-circuit current (Isc) elevations induced by transmural electrical stimulation (ES) of mucosal sheets from porcine jejunum in vitro. GAL-induced contractions (GAL EC50 = 9 nmol/l) were maximally 25% of those elicited by 10 mumol/l carbamylcholine and remained unaffected by atropine, tetrodotoxin, or tachyphylaxis to substance P. The presynaptic Ca2+ channel blocker, omega-conotoxin (0.1 mumol/l), inhibited GAL-induced contractions by 66%. GAL attenuated mucosal Isc elevations induced by ES with an IC50 = 13 nmol/l and at 0.1 mumol/l produced rapid decreases in basal Isc averaging 8 +/- 2 microA cm-1 in 77% of tissues examined. The alpha-adrenoceptor blocker phentolamine or the opiate antagonist naloxone did not alter tissue Isc responses to GAL. These results suggest that GAL modulates neuronal activity linked to secretomotor function in the porcine small intestine.

Inhibition of N-methyl-D-aspartate- and kainic acid-induced neurotransmitter release by omega-conotoxin GVIA

1. The role of voltage-sensitive calcium channels (VSCC) in N-methyl-D-aspartate (NMDA)- and kainic acid (KA)-evoked neurotransmitter release from rat cortical and hippocampal brain slices was evaluated by determining the effects of omega-conotoxin GVIA, an inhibitor of neuronal L- and N-type VSCC, and PN 200-110, a selective inhibitor of L-type VSCC. 2. Selective antagonists of the NMDA receptor ionophore complex, Mg2+, CPP and MK-801, inhibited NMDA- but not KA-evoked release of [3H]-noradrenaline from hippocampal and cortical brain slices. This suggests that cortical and hippocampal receptors are similar and that NMDA and KA act at distinct excitatory amino acid receptor subtypes. 3. [3H]-noradrenaline release induced by both NMDA and KA was similarly inhibited (approximately 30%) by omega-conotoxin GVIA. In contrast, PN 200-110 had no significant effect, although there was a tendency towards inhibition. 4. The results suggest that although NMDA- and KA-receptors are pharmacologically distinct, the N-type, but not the L-type, VSCC plays a small but significant role in neurotransmitter release induced by both NMDA and KA. It remains to be determined whether the N-type VSCC are involved in the physiological and/or pathological manifestations of excitatory amino acid receptor stimulation.