Characterisation of SB-221420-A - a neuronal Ca(2+) and Na(+) channel antagonist in experimental models of stroke

For progression to clinical trials in stroke, putative neuroprotective compounds should show robust efficacy post-ischaemia in several experimental models of stroke. This paper describes the characterisation of (+)(1S, 2R)-cis-1-[4-(1-methyl-1-phenylethyl)phenoxy]-2-methylamino indane hydrochloride (SB-221420-A), a Ca(2+) and Na(+) channel antagonist. SB-221420-A inhibited (IC(50)=2.2 microM) N-type voltage-operated Ca(2+) channel currents in cultured superior cervical ganglion neurons, which were pretreated with 10 microM nimodipine to block L-type voltage-operated Ca(2+) channel currents. In dorsal root ganglion neurons pretreated with 1 microM omega-conotoxin GVIA to block N-type voltage-operated Ca(2+) channel currents, SB-221420-A inhibited the residual Ca(2+) current with an IC(50) of 7 microM. SB-221420-A also inhibited Na(+) currents in dorsal root ganglion neurons with an IC(50) of 8 microM. In rats, the pharmacokinetic profile of SB-221420-A shows that it has a half-life of 6.4 h, a high volume of distribution, is highly brain penetrating, and has no persistent metabolites. Following bilateral carotid artery occlusion in gerbils, SB-221420-A significantly reduced the level of ischaemia-induced hyperlocomotor activity and the extent of hippocampal CA1 cell loss compared to the ischaemic vehicle-treated group. SB-221420-A was also effective in focal models of ischaemia. In the mouse permanent middle cerebral artery occlusion model, SB-221420-A (10 mg/kg) administered intravenously, post-ischaemia significantly (P<0.05) reduced lesion volume compared to the ischaemic vehicle-treated group. In the normotensive rat permanent middle cerebral artery occlusion model, SB-221420-A (10 mg/kg) administered intravenously over 1 h, beginning 30 min postmiddle cerebral artery occlusion, significantly (P<0.05) reduced lesion volume from 291+/-16 to 153+/-30 mm(3), compared to ischaemic vehicle-treated controls when measured 24 h postmiddle cerebral artery occlusion. Efficacy was maintained when the same total dose of SB-221420-A was infused over a 6-h period, beginning 30 min postmiddle cerebral artery occlusion. SB-221420-A also significantly (P<0.05) reduced lesion volume following transient middle cerebral artery occlusion in normotensive rats and permanent middle cerebral artery occlusion in spontaneously hypertensive rats (SHR). Investigation of the side effect profile using the Irwin screen in mice revealed that, at neuroprotective doses, there were no overt behavioural or cardiovascular changes. These data demonstrate that robust neuroprotection can be seen post-ischaemia with SB-221420-A in both global and focal ischaemia with no adverse effects at neuroprotective doses, and indicate the potential utility of a mixed cation blocker to improve outcome in cerebral ischaemia.

A comparison of the relative activities of a number of GABAB antagonists in the isolated vas deferens of the rat

1. A series of GABAB receptor antagonists were tested against (+/-)-baclofen for activity on the presynaptic GABAB receptor in the rat vas deferens. 2. All the antagonists tested caused a rightward shift in the concentration-response curve to (+/-)-baclofen. 3. pA2 values calculated from full Schild analysis were as follows: phaclofen, pA2 = 4.3; delta-amino valeric acid, pA2 = 4.4; 3-aminopropyl(diethoxymethyl)phosphinic acid (CGP 35348), pA2 = 5.0; 3-amino-propyl(n-hexyl)phosphinic acid (3-APHPA), pA2 = 4.5. 4. These results show that none of the above compounds possess potent antagonist activity at the GABAB receptor (i.e. pA2 > 6) in this peripheral tissue. In addition, the more recently available phosphinic acid antagonists, appear to offer no great advance over the GABAB antagonists previously available.

The mechanism of action of peppermint oil on gastrointestinal smooth muscle. An analysis using patch clamp electrophysiology and isolated tissue pharmacology in rabbit and guinea pig

An investigation of the mechanism of peppermint oil action was performed using isolated pharmacological preparations from guinea pig large intestine and patch clamp electrophysiology techniques on rabbit jejunum. Peppermint oil relaxed carbachol-contracted guinea pig taenia coli (IC50, 22.1 micrograms/mL) and inhibited spontaneous activity in the guinea pig colon (IC50, 25.9 micrograms/mL) and rabbit jejunum (IC50, 15.2 micrograms/mL). Peppermint oil markedly attenuated contractile responses in the guinea pig taenia coli to acetylcholine, histamine, 5-hydroxytryptamine, and substance P. Peppermint oil reduced contractions evoked by potassium depolarization and calcium contractions evoked in depolarizing Krebs solutions in taenia coli. Potential-dependent calcium currents recorded using the whole cell clamp configuration in rabbit jejunum smooth muscle cells were inhibited by peppermint oil in a concentration-dependent manner. Peppermint oil both reduced peak current amplitude and increased the rate of current decay. The effect of peppermint oil resembled that of the dihydropyridine calcium antagonists. It is concluded that peppermint oil relaxes gastrointestinal smooth muscle by reducing calcium influx.

Phosphinic acid analogues of GABA are antagonists at the GABAB receptor in the rat anococcygeus

CGP 35348 (3-aminopropyl(diethoxymethyl)phosphinic acid) and 3-aminopropyl(n-hexyl)phosphinic acid (3-APHPA) were tested in the rat anococcygeus muscle against CGP 27492 (3-aminopropylphosphinic acid), a selective GABAB agonist, for their antagonist activity. Their antagonist potency was compared with that of 2-hydroxysaclofen. The pA2 values for CGP 35348, 3-APHPA and 2-hydroxysaclofen were 5.38, 4.86, 4.45 respectively in the rat anococcygeus muscle. These results confirm the previous reports of GABAB antagonist activity for these compounds and show a marginal improvement in potency over 2-hydroxysaclofen.

3-Aminopropylphosphinic acid--a potent, selective GABAB receptor agonist in the guinea-pig ileum and rat anococcygeus muscle

1. 3-Aminopropylphosphinic acid, a gamma-aminobutyric acid (GABA) analogue, was tested for activity on guinea-pig isolated ileum and rat isolated anococcygeus muscle preparations. The effects of 3-aminopropylphosphinic acid were compared with those of GABA and baclofen. 2. In the electrically stimulated ileum, 3-aminopropylphosphinic acid, like GABA and baclofen, caused a concentration-dependent inhibition of the cholinergic twitch contraction, the IC50 value being 1.84 +/- 0.23 microM (n = 12). Unlike GABA, but like baclofen, 3-aminopropylphosphinic acid did not produce an initial contraction. 3. The inhibitory effects of 3-aminopropylphosphinic acid and baclofen in the guinea-pig ileum were not significantly antagonized by bicuculline (10 microM), phentolamine plus propranolol (both 1 microM), yohimbine (1 microM), naloxone (1 microM), impromidine (1 microM) or 8-phenyltheophylline (10 microM). The inhibitory effects of 3-aminopropylphosphinic acid, but not of baclofen, were however antagonized by phaclofen (500 microM). In addition the effects of 3-aminopropylphosphinic acid were abolished by baclofen desensitization in the guinea-pig ileum. 4. 3-Aminopropylphosphinic acid, GABA and baclofen reduced the twitch contraction evoked by electrical field stimulation in the rat anococcygeus muscle. The IC50 for 3-aminopropylphosphinic acid inhibition of the anococcygeus contraction was 0.89 +/- 0.15 microM (n = 8). 5. It is concluded that 3-aminopropylphosphinic acid is a potent, selective GABAB agonist, being seven times more potent than baclofen in the guinea-pig ileum and five times more potent than baclofen in the rat anococcygues muscle preparations.

GABA immunoreactivity and 3H-GABA uptake in mucosal epithelial cells of the rat stomach

GABA, best known as a neurotransmitter in the central nervous system, is also present in various peripheral tissues including the gastrointestinal tract, where there is strong evidence that GABA acts as a transmitter in some intrinsic myenteric neurones. Several studies indicate that the gastric mucosa is one of the sites of action of GABA in the gut. Highly specific anti-GABA antibodies have been used to detect endogenous GABA in the mucosa of the rat gastrointestinal tract, and 3H-GABA uptake followed by autoradiography has been used to localise cells with uptake sites for exogenous GABA. It was found that although GABA immunoreactive nerve fibres are essentially absent from this site, some mucosal cells are strongly GABA-immunoreactive. These cells are common in the pyloric stomach and upper part of the small intestine. The autoradiographic experiments provide evidence that these cells also possess high-affinity GABA uptake sites. These observations raise the possibility that in the gastrointestinal tract GABA acts as a gut hormone in a subpopulation of mucosal endocrine cells, in addition to its role as an enteric neurotransmitter.

Immunohistochemical localisation and electrophysiological actions of GABA in prevertebral ganglia in guinea-pig

Immunohistochemical techniques were used to detect the presence of a GABA-like material in prevertebral sympathetic ganglia in guinea-pigs. Varicose, immunolabelled nerve fibres were observed in close proximity to sympathetic neurones in inferior mesenteric ganglia and coeliac ganglia. Non-varicose, immunolabelled nerve fibres were observed in lumbar colonic nerves and superior coeliac nerves, i.e. in nerve bundles peripheral to prevertebral ganglia. Immunolabelling was also present in neurones in the myenteric plexus and in nerve fibres in the circular muscle of the colon, as shown previously (Hills et al., Neuroscience, 22 (1987) 301-312). However, GABA-like immunoreactivity was not observed in the cell bodies of prevertebral ganglia nor in splanchnic nerves central to prevertebral ganglia. It was concluded from these results that prevertebral ganglia in guinea-pig receive a GABAergic innervation from neurones peripheral to the ganglia, possibly from GABA-containing neurones in the myenteric plexus of the gastrointestinal tract. Intracellular recordings were made from sympathetic neurones in the inferior mesenteric ganglion (IMG). Application of GABA onto the IMG caused a slow depolarisation of sympathetic neurones, during which there was a marked decrease in the input resistance of IMG cells. Application of GABA also depressed excitatory postsynaptic potentials (EPSPs) and action potentials in sympathetic neurones excited by cholinergic nerve fibres in the lumbar colonic nerves. The reversal potential of the GABA-induced slow depolarisation was -37 mV, a value close to the chloride equilibrium potential for sympathetic neurones. The actions of GABA were reversibly reduced by the GABAA antagonist, bicuculline, and were modulated in a predictable manner by substituting chloride ions with methane-sulphonate ions. These results indicated that GABA, and presumably GABAergic nerves in prevertebral ganglia, modulate the excitability of sympathetic neurones by acting on GABAA receptors linked to a chloride ionophore.

Substance P-mediated membrane currents in voltage-clamped guinea pig inferior mesenteric ganglion cells

Responses to substance P (SP) and to hypogastric nerve stimulation were recorded from voltage-clamped guinea pig inferior mesenteric ganglion (IMG) neurons, and compared with those to muscarine. Muscarine produced a voltage-dependent inward current accompanied by a reduced input conductance and inhibition of IM a time- and voltage-dependent K+-current (Brown and Adams: Nature 283:673-676, 1980). SP also produced an inward current, accompanied by a fall in input conductance (20 out of 31 cells) or a rise in input conductance (7 out of 31 cells). The fall in input conductance was not accompanied by an inhibition of M-current (unlike frog ganglia: Adams et al.: British Journal of Pharmacology 79:330-333, 1983) or an inhibition of the inward rectifier current (unlike globus pallidus neurons: Stanfield et al.: Nature 315:498-501, 1985). Repetitive hypogastric nerve stimulation (10-20 Hz, 2-10 s) produced a slow inward postsynaptic current lasting 1-3 min, with decreases or increases of input conductance matching those produced by SP. The postsynaptic current did not show a consistent or reproducible change in amplitude on varying the holding potential between -90 and -25 mV. It is concluded that SP and hypogastric stimulation produce complex and variable changes in ionic conductance in IMG neurons.

An immunohistochemical study of the distribution of enteric GABA-containing neurons in the rat and guinea-pig intestine

gamma-Aminobutyric acid (GABA) antiserum was applied to sections of rat and guinea-pig intestine which were subsequently processed to reveal any immunoreactivity using either fluorescence or peroxidase techniques. Immunopositive fibres were demonstrated in stomach, duodenum, ileum and colon of rat and guinea-pig intestine. Myenteric ganglia and nerve bundles in the circular muscle contained immunopositive nerve fibres, while the longitudinal muscle, submucosa and mucosa were only rarely innervated. In favourable sections, immunopositive fibres could be seen running from the myenteric plexus into the circular muscle, thus suggesting that the GABA-immunopositive nerves in the circular muscle originate from neurons in the myenteric plexus. In both rat and guinea-pig, immunoreactive nerve cell bodies were most numerous in the myenteric plexus of the colon. In the rat, immunopositive fibres in the circular muscle were most abundant in the ileum, whereas in the guinea-pig it was the colon circular muscle that was most richly innervated. The results demonstrate that neurons which show GABA immunoreactivity are present along the length of the gastrointestinal tract. Their distribution in both myenteric ganglia and circular muscle is heterogeneous both within and between the two species studied. It is probable that this heterogeneity reflects the diversity and specificity of function of this class of enteric neurons.

Immunohistochemical demonstration of GABAergic neurons in the enteric nervous system

Application of a highly specific antiserum against GABA to whole-mount preparations of the guinea pig and rat myenteric plexus resulted in discrete and unambiguous immunolabeling of a subpopulation of myenteric neuronal cell bodies and fibers. The anti-GABA antiserum, which was raised against GABA conjugated by glutaraldehyde to BSA, was applied to glutaraldehyde-fixed whole mounts and subsequently visualized using the peroxidase-antiperoxidase method. In the guinea pig ileum and colon, immunoreactive varicose nerve fibers and scattered nerve cell bodies were found within the myenteric plexus. Immunostained fibers were also seen in the tertiary plexus and in the circular muscle, running parallel to the muscle bundles. GABA immunoreactivity in these intramuscular nerves was most pronounced in the colon. In the rat, immunoreactive fibers were prominent throughout the myenteric plexus. They formed dense networks within the myenteric ganglia, which also contained immunopositive nerve cell bodies, and ran between them in the interconnecting nerves. Some immunoreactive nerve fibers were seen in the circular muscle. Control experiments using non-immune sera or adsorbed anti-GABA antiserum showed no staining. These results add a definitive support to our previous suggestion that GABA serves as an autonomic neurotransmitter in vertebrates. In addition to the present immunohistochemical evidence, this hypothesis is supported by biochemical, autoradiographic, transmitter release, electrophysiological, and pharmacological studies on the enteric nervous system of several species. It is now important to determine the functional role of GABAergic neurons within the complex neuronal circuitry that controls gut functions.

Evidence that the P1-purinoceptor in the guinea-pig taenia coli is an A2-subtype

The effects of 5'-N-ethylcarboxamidoadenosine (NECA), L-NECA, 2-chloroadenosine, N6-phenylisopropyladenosine (L-PIA and D-PIA), cyclohexyladenosine (CHA), and adenosine were examined on the guinea-pig taenia coli. All the analogues except L-NECA caused relaxations; the order of potency for the series was: NECA greater than 2-chloroadenosine greater than L-PIA greater than CHA greater than D-PIA greater than adenosine. L-PIA was twice as potent as D-PIA in inducing relaxations of the guinea-pig taenia coli. Adenosine and its analogues that induce relaxation all caused a slow membrane hyperpolarization; differences in the rates of hyperpolarization and latencies were apparent, although not statistically significant. The duration of the response to adenosine was significantly less than that for any adenosine analogue. Ion studies, using the sucrose gap, revealed that responses to the analogues were attenuated in elevated extracellular potassium or reduced extracellular chloride. 8-Phenyltheophylline, a potent P1-purinoceptor antagonist, caused a rightward shift of all the adenosine and analogue concentration-response curves. Dipyridamole, an adenosine uptake inhibitor, potentiated the relaxations to adenosine but had no significant effect on the relaxations induced by the analogues. It is concluded that NECA, 2-chloroadenosine, L-PIA, CHA, D-PIA and adenosine mediate their relaxant effects via an extracellular P1-purinoceptor which displays characteristics of the A2-subtype as determined by the rank order of agonist potency. Electrophysiological analysis of the responses to each of the analogues did not reveal any marked differences in the modes of action even between NECA and L-PIA (preferential A2- and A1-receptor agonists, respectively).

Gamma-aminobutyrate as an autonomic neurotransmitter: release and uptake of [3H]gamma-aminobutyrate in guinea pig large intestine and cultured enteric neurons using physiological methods and electron microscopic autoradiography

To provide further evidence that some enteric neurons use gamma-aminobutyrate (GABA) as a neurotransmitter, we have demonstrated a depolarization-induced release of [3H]GABA from isolated myenteric ganglia in culture, and from segments of large intestine containing the myenteric plexus. In addition, light and electron microscopic autoradiography has been employed to visualize the putative GABAergic neurons and their projections, both in cultured ganglia and in sections from the gut wall. Explant cultures of the guinea-pig myenteric plexus, containing only neurons and glia intrinsic to the gut, were incubated with 0.14 microM [3H]GABA, washed and then repeatedly depolarized by 62 mM K+. The depolarizations always evoked release of [3H]GABA. The evoked release was reversibly blocked by 5 mM Co2+, suggesting a dependence on Ca2+ influx, a characteristic of neuronal transmitter release. Strips of the guinea-pig taenia coli, containing the myenteric plexus, were incubated with 0.14 or 0.7 microM [3H]GABA, washed and subjected to electrical field stimulation. This caused release of [3H]GABA, which could be evoked successively on repeated stimulation. The release was of neuronal origin and Ca2+ dependent, since it was abolished by 3 microM tetrodotoxin and reversibly blocked by 10 mM Co2+. By combined electrophoresis and chromatography the released tritium was identified as being attached to GABA. Autoradiography, following incubation with low concentrations of [3H]GABA, was used to identify specifically putative GABAergic neurons. Light microscopic autoradiography of cultured ganglia, and electron microscopic autoradiography of sections from the taenia coli including the myenteric plexus, were in good agreement, showing a selective and heavy labelling over a sub-population of neurons, and light labelling over glial cells. The majority of neurons and the non-neural cells were unlabelled. The electron microscopic autoradiographs also showed heavy labelling over some, but not all, axons in the fine axon bundles that innervate the longitudinal muscle of the taenia. These results strongly support our previous suggestion that a population of myenteric neurons are GABAergic, although more work is needed before this is fully established. Our data also suggest that some of the putative GABAergic neurons project out to the gut musculature, where their axons, together with axons from other neurons, run in bundles to form the intramuscular nerves. We therefore propose that one of the roles of these neurons in the gut is the presynaptic modulation of transmitter released from neighbouring axons in these nerves.

Studies on the stereoselectivity of the P2-purinoceptor

ATP, 2-chloro-ATP, 2-methylthio-ATP, and their unnatural L-enantiomers, were synthesized and their effects tested on the guinea-pig taenia coli and urinary bladder, and the stimulated frog ventricle. The potent P2-purinoceptor agonists, 2-chloro-ATP and 2-methylthio-ATP were, respectively, 30 and 200 times more effective than ATP in relaxing the guinea-pig taenia, but approximately as effective as ATP in contracting the guinea-pig bladder and augmenting the force of contraction of the frog ventricle. A high degree of stereoselectivity was observed for relaxations of the guinea-pig taenia coli produced by the P2-purinoceptoragonists, and 2-methylthio-ATP was over 700 times more effective than its L-enantiomer. In contrast, stereoselectivity for contraction of the guinea-pig bladder was observed only at low concentrations with each pair of enantiomers, and a similar low stereoselectivity was displayed by the frog ventricle. These results show that P2-purinoceptors mediating inhibitory responses in the guinea-pig taenia coli can show a high degree of stereoselectivity, while P2-purinoceptors mediating excitatory responses in the guinea-pig bladder and in the frog ventricle show little stereoselectivity. The partial stereoselectivity of the P2-purinoceptor in smooth muscle contrasts with the absolute stereospecificity of P1-purinoceptors for adenosine on smooth muscle and autonomic nerve terminals and the absolute stereospecificity of the receptor for ADP on the human platelet.

The effects of vasoactive intestinal polypeptide on the electrical activity of guinea-pig intestinal smooth muscle

The effect of vasoactive intestinal polypeptide (VIP) on the electrical activity of the smooth muscle of the guinea-pig taenia coli has been examined using the single sucrose gap apparatus. VIP usually caused a slowly developing, long-lasting membrane hyperpolarisation, although sometimes it reduced the frequency of spontaneous spike discharge without hyperpolarising the membrane. In contrast, non-adrenergic, non-cholinergic nerve stimulation initiated rapid membrane hyperpolarisation followed by post inhibitory excitation. In the presence of apamin, the VIP response was little affected, whereas the responses to non-adrenergic, non-cholinergic nerve stimulation were either markedly antagonised or reversed to membrane depolarisation. Consideration is given to the possible role of either VIP or ATP as the neurotransmitter responsible for the inhibitory junction potential elicited in the guinea-pig taenia coli in response to non-adrenergic inhibitory nerve stimulation.