Neurosteroid analogues. 12. Potent enhancement of GABA-mediated chloride currents at GABAA receptors by ent-androgens

Allopregnanolone (1) and pregnanolone (2), steroids containing a 17beta-acetyl group, are potent enhancers of GABA (gamma-aminobutyric acid) action at GABAA receptors. Their effects are enantioselective with the non-naturally occurring enantiomers (ent-1 and ent-2) being less potent. Androsterone (3) and etiocholanolone (4), steroids with a C-17 carbonyl group, are weak enhancers of GABA action at GABAA receptors. Unexpectedly, their enantiomers (ent-3 and ent-4) have been found to have enhanced, not diminished, activity at GABAA receptors. Furthermore, the C-17 spiro-epoxide analogues (ent-5 and ent-6) of ent-3 and ent-4, respectively, have activities comparable to those of steroids 1 and 2. The results indicate that some ent-steroids are potent modulators of GABAA receptors and might have clinical potential as GABAergic drugs of the future.

Acute actions of marine toxin latrunculin A on the electrophysiological properties of cultured dorsal root ganglion neurones

The effects of latrunculin A, isolated from the nudibranch Chromodoris sp., on the excitability of neonatal rat cultured dorsal root ganglion neurones were investigated using patch-clamp recording and Ca(2+) imaging techniques. Under current-clamp conditions, acute application of latrunculin A (100 microM) reversibly induced multiple action potential firing and significantly increased action potential duration. No significant effects on action potential peak amplitude, threshold of action potential firing, resting membrane potential and input resistance were observed. Under voltage-clamp conditions, significant and dose-dependent suppression of K(+) current was seen with 10-100 microM latrunculin A. Additionally, a significant difference between inhibition of the current measured at the peak and the end of a 100 ms voltage step was seen with 100 microM latrunculin A. Fura-2 fluorescence Ca(2+) imaging revealed that latrunculin A (100 microM) significantly inhibited Ca(2+) transients evoked by KCl-induced depolarisation in all neurones. In 36% of DRG neurones, latrunculin A alone had no effect on intracellular Ca(2+). In 64% of neurones, latrunculin A alone evoked a transient rise in intracellular Ca(2+). Moreover, latrunculin A (10-100 microM) significantly inhibited the mean high voltage-activated Ca(2+) current. The effects of latrunculin A on action potential firing and K(+) currents were attenuated by intracellular phalloidin, an indication that these effects are mediated through actin disruption.

The metabotropic glutamate (mGLU)2/3 receptor antagonist LY341495 [2S-2-amino-2-(1S,2S-2-carboxycyclopropyl-1-yl)-3-(xanth-9-yl)propanoic acid] stimulates waking and fast electroencephalogram power and blocks the effects of the mGLU2/3 receptor agonist ly379268 [(-)-2-oxa-4-aminobicyclo[3.1.0]hexane-4,6-dicarboxylate] in rats

The highly selective metabotropic glutamate (mGlu)2/3 receptor agonist LY379268 [(-)-2-oxa-4-aminobicyclo[3.1.0]hexane-4,6-dicarboxylate] completely suppresses rapid eye movement (REM) sleep and strongly depresses theta (6-10 Hz) and high-frequency (10-60 Hz) power in the waking and nonrapid eye movement (NREM) EEG, effects consistent with depressed brain excitation (arousal). We hypothesized the selective mGlu2/3 receptor antagonist LY341495 [2S-2-amino-2-(1S,2S-2-carboxycyclopropyl-1-yl)-3-(xanth-9-yl)propanoic acid] given alone would 1) increase arousal, producing sleep-wake EEG effects opposite those of LY379268, and 2) block/reverse the effects of LY379268 when the drugs are coadministered. Rats with implanted electrodes were injected with 1, 5, or 10 mg/kg LY341495 at hour 5.5 of the dark period. In the coadministration study the rats received the same dose of LY341495 followed 30 min later by 1 mg/kg LY379268. LY341495 alone increased waking by reducing NREM and REM sleep. LY341495 also depressed low-frequency and stimulated high-frequency EEG power. It produced a sharp spike in theta power in waking but not NREM sleep, a striking state-dependent difference in pharmacological response. These changes indicate that blocking mGlu2/3 receptors increases brain arousal. Moreover, they show that mGlu2/3 receptors actively support arousal even in the absence of heightened glutamate excitation. The coadministration experiment demonstrates that LY341495 is selective in vivo since it dose-dependently attenuates or reverses the sleep-wake EEG effects of the highly selective mGlu2/3 receptor agonist LY379268. The capacity of mGlu2/3 receptor agonists and antagonists to alter the sleep wake balance suggests they could be developed to enhance sleep or sustain arousal. Their opposing actions on theta EEG could test the putative role of these oscillations in memory consolidation.

Receptor assay-guided isolation of anti-GABAergic insecticidal alkaloids from a fungal culture

The gamma-aminobutyric acid (GABA) receptor bears sites of action for insecticides. To discover GABA receptor-directed insecticides in natural products, fungal culture extracts were screened for their ability to inhibit specific binding of the radiolabeled noncompetitive antagonist [3H]1-(4-ethynylphenyl)-4-n-propyl-2,6,7-trioxabicyclo[2.2.2]octane to housefly head membranes. The screening efforts led to the isolation of two alkaloids from Aspergillus terreus: PF1198A (alantrypinone) and PF1198B (serantrypinone), which had IC50 values of 0.34 and 2.1 microM, respectively, in this assay. These compounds were ca. 47-61-fold selective for housefly vs rat GABA receptors. Both compounds showed insecticidal activity against Myzus persicae in the range of 100-500 ppm. Binding assay-guided screening should provide significant opportunities for the identification of novel and selective insecticides.

Selective decreases of nicotinic acetylcholine receptors in PC12 cells exposed to fluoride

In an attempt to elucidate the mechanism by which excessive fluoride damages the central nervous system, the effects of exposure of PC12 cells to different concentrations of fluoride for 48 h on nicotinic acetylcholine receptors (nAChRs) were characterized here. Significant reductions in the number of binding sites for both [3H]epibatidine and [125I]alpha-bungarotoxin, as well as a significant decrease in the B(max) value for the high-affinity of epibatidine binding site were observed in PC12 cells subjected to high levels of fluoride. On the protein level, the alpha 3 and alpha 7 subunits of nAChRs were also significantly decreased in the cells exposed to high concentrations of fluoride. In contrast, such exposure had no significant effect on the level of the beta 2 subunit. These findings suggest that selective decreases in the number of nAChRs may play an important role in the mechanism(s) by which fluoride causes dysfunction of the central nervous system.

Differential nicotinic receptor expression in monkey basal ganglia: effects of nigrostriatal damage

Our previous work showed that there were marked declines in (125)I-alpha-conotoxin MII labeled nicotinic receptors in monkey basal ganglia after nigrostriatal damage, findings that suggest alpha3/alpha6 containing nicotinic receptors sites may be of relevance to Parkinson's disease. We now investigate whether there are differential changes in the distribution pattern of nicotinic receptor subtypes in the basal ganglia in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-lesioned animals compared to controls to better understand the changes occurring with nigrostriatal damage. To approach this we used (125)I-alpha-conotoxin MII, a marker for alpha3/alpha6 nicotinic receptors, and (125)I-epibatidine, a ligand that labels multiple nicotinic subtypes. The results demonstrate that there were medial to lateral gradients in nicotinic receptor distribution in control striatum, as well as ventromedial to dorsolateral gradients in the substantia nigra, which resembled those of the dopamine transporter in these same brain regions. Treatment with MPTP, a neurotoxin that selectively destroys dopaminergic nigrostriatal neurons, led to a relatively uniform decrease in nicotinic receptor sites in the striatum, but a differential effect in the substantia nigra with significantly greater declines in the ventrolateral portion. Competition analysis in the striatum showed that alpha-conotoxin MII sensitive sites were primarily affected after lesioning, whereas multiple nicotinic receptor populations were decreased in the substantia nigra. From these data we suggest that in the striatum alpha3/alpha6 nicotinic receptors are primarily localized on dopaminergic nerve terminals, while multiple nicotinic receptor subtypes are present on dopaminergic cell bodies in the substantia nigra. Thus, if activation of striatal nicotinic receptors is key in the regulation of basal ganglia function, alpha3/alpha6-directed nicotinic receptor ligands may be more relevant for Parkinson's disease therapy. However, nicotinic receptor ligands with a broader specificity may be more important if receptors in the substantia nigra play a dominant role in controlling nigrostriatal activity.

Metabolism and mode of action of cis- and trans-3-pinanones (the active ingredients of hyssop oil)

1. Hyssop oil is an important food additive and herbal medicine and the principal active ingredients are (-)-cis- and (-)-trans-3-pinanones. No information is available on their metabolism or specific mode of action. 2. The metabolites of cis- and trans-3-pinanones were examined from mouse and human liver microsomes and human recombinant P4503A4 with NADPH and on administration to mouse by gas chromatography/chemical ionization mass spectrometry comparison with standards from synthesis. 3. The major metabolite of cis-3-pinanone in each P450 system and in brain of the i.p.-treated mouse in quantitative studies was 2-hydroxy-cis-3-pinanone, and two minor metabolites were hydroxypinanones other than 2-hydroxy-trans-3-pinanone and 4S-hydroxy-cis-3-pinanone. The urine from oral cis-3-pinanone treatment examined on a qualitative basis contained conjugates of metabolites observed in the microsomal systems plus 2,10-dehydro-3-pinanone. 4. Trans-3-pinanone was metabolized more slowly than the cis-isomer in each system to give hydroxy derivatives different than those derived from cis-3-pinanone. 5. Cis- and trans-3-pinanones and hyssop oil act as gamma-aminobutyric acid type A (GABAA) receptor antagonists based on inhibition of 4'-ethynyl-4-n-[2,3-(3)H(2)]propylbicycloorthobenzoate ([(3)H]EBOB) binding in mouse brain membranes (IC(50) of 35-64 microM) and supported by tonic/clonic convulsions in mouse (i.p. LD(50) 175 to >250 mg kg(-1)) alleviated by diazepam. The cis-3-pinanone metabolites 2-hydroxy-cis-3-pinanone and 2,10-dehydro-3-pinanone exhibit reduced toxicity and potency for inhibition of [(3)H]EBOB binding.

Modulation Of [35S]-tert-butylbicyclophosphorothionate binding by somatostatin in rat hypothalamus

1. The present study was designed to assess the effect of the tetradecapeptide somatostatin on the GABA(A) receptor complex in the rat hypothalamus. 2. GABA(A) receptors were labelled with [35S]-tert-butylbicyclophosphorothionate (TBPS), which binds in or near the chloride channel, and binding as assessed by in vitro quantitative autoradiography using a computer-assisted image analysis system. 3. Somatostatin inhibited the binding of [35S]-TBPS to the convulsant site of the hypothalamic GABA(A) receptor complex of rat slide-mounted hypothalamic structures in a concentration-dependent manner with an affinity in the micromolar range (10(-6) to 3 x 10(-6) mol/L). Somatostatin appeared to mimic the effects of the neurosteroid 5alpha-pregnane-3alpha ol-one (5alpha3alphaP), GABA and picrotoxin on [35S]-TBPS binding in the rat hypothalamus in all structures examined. Furthermore, GABA or muscimol (a GABA(A) receptor agonist), when added to the incubation medium, enhanced the capacity of somatostatin to inhibit [35S]-TBPS binding, with an IC50 of 10(-7) mol/L. However, incubation with bicuculline (a GABA(A) receptor antagonist) led to the abolition of the inhibitory effect of somatostatin on [35S]-TBPS specific binding in rat hypothalamus. 4. The present results demonstrate the presence of a modulatory effect of somatostatin on the GABA(A) receptor complex in rat hypothalamic structures. Furthermore, the data suggest that somatostatin allosterically modifies [35S]-TBPS binding through a mechanism similar to that of GABA. Taken together, these results provide evidence for the presence of somatostatin- GABA interactions in rat hypothalamus.

A Link between cholesterol levels and phenobarbital induction of cytochromes P450

Squalestatin1 (SQ1), a potent inhibitor of squalene synthase produced a dose-dependent induction of cytochromes P450 CYP2H1 and CYP3A37 mRNAs in chicken hepatoma cells. The effect of SQ1 was completely reversed by 25-hydroxycholesterol. Bile acids elicited an induction of CYP3A37 and CYP2H1 mRNA. Bile acids also reduced the phenobarbital induction of CYP2H1 but not of CYP3A37 mRNA. The effects of SQ1 and its reversal by 25-hydroxycholesterol and the effects of bile acids were reproduced in reporter gene assays with a phenobarbital-responsive enhancer unit of CYP2H1. These data suggest that an endogenous molecule related to cholesterol homeostasis regulates induction of drug-inducible CYPs.

Do antiepileptics phenytoin, carbamazepine, and loreclezole show GABA(A) receptor subtype selectivity in rat brain sections?

[35S]t-Butylbicyclophosphorothionate ([35S]TBPS), a convulsant site ligand of GABA(A) receptors, was used in autoradiography with rat brain sections to test suggested receptor subtype-selective actions of antiepileptics phenytoin, carbamazepine and loreclezole on native GABA(A) receptors. At maximal 100 microM concentration, both phenytoin and carbamazepine decreased [35S]TBPS binding only by 20%, indicating that their low potency and efficacy prevents their use as alpha1 subunit-identifying compounds. Ten microM loreclezole did not affect the binding, but a further increase in loreclezole concentration strongly decreased it. The action of loreclezole, assumed to reflect beta2/3 subunit-containing receptors, varied from brain region to region, but the effects were unrelated to the regional expression profiles of beta subunit variants. We conclude that in autoradiographic [35S]TBPS binding assay neither carbamazepine, phenytoin nor loreclezole are useful tools in characterizing brain regional heterogeneity of GABA(A) receptors in rats and that only loreclezole exhibits high, pharmacologically relevant efficacy.

Implications of selective type II IMP dehydrogenase (IMPDH) inhibition by the 6-ethoxycarbonyl-3,3-disubstituted-1,5-diazabicyclo[3.1.0]hexane-2,4-diones on tumor cell death

It was shown previously that three 1,5-diazabicyclo[3.1.0]hexane-2,4-diones selectively inhibited human Type II IMP dehydrogenase (IMPDH) from Tmolt4 cell leukemia [Barnes et al., Biochemistry 2000;39:13641-50]. The agents acted as competitive inhibitors of this isoform, yet when tested against human Type I at concentrations ranging from 0.5 to 500 microM, Type I was not inhibited. This study focuses on the antineoplastic activity and cellular effects of one of these agents and two new derivatives containing ethoxycarbonyl substitution at position C6. Agents were studied for antiproliferative activity in human Tmolt4 leukemia (EC(50) 3.3 to 9.2 microM) and alterations in the levels of enzymes involved with cellular metabolism, including DNA and RNA syntheses due to IMPDH inhibition. Results reported here demonstrate that 6-ethoxycarbonyl-3,3-disubstituted-1,5-diazabicyclo[3.1.0]hexane-2,4-diones are effective inhibitors of DNA synthesis (30-66% inhibition) due to reductions in dGTP pool levels. Collectively, the three agents proved to be selective inhibitors of human IMPDH Type II activity (K(i) 11-33 microM), leading to cytotoxicity in a number of suspended and solid tumor lines, notably MCF-7 (EC(50) 0.7 to 6.0 microM). In addition, negative cytotoxic actions of these agents on WI-38 cell growth, a normal rapidly growing human line, suggest that specific targeting of Type II IMPDH would help to eliminate cell killing in lines where Type I predominates. Furthermore, effects of agents on DNA synthesis and cell death could be reversed by the addition of exogenous guanosine to the medium. Results from in vitro studies suggest that the 6-ethoxycarbonyl-3,3-disubstituted-1,5-diazabicyclo[3.1.0]hexane-2,4-diones may be used as effective isozyme-selective chemotherapeutic agents.

Cotinine binding to nicotinic acetylcholine receptors in bovine chromaffin cell and rat brain membranes

Cotinine is the major metabolite of nicotine. It has nicotine-like biological activity, but its potency is low. We studied cotinine binding to nicotinic receptors labelled with [3H]epibatidine. In membranes from cultured bovine chromaffin cells [3H]epibatidine bound to two apparent sites with K(d) values of 93 and 1400 pM. The low-affinity binding represented two-thirds of the binding sites. In rat frontal cortex and hippocampus homogenate membranes, only one apparent binding site was detected. The Kd values were 40 and 62 pM, in frontal cortex and hippocampus, respectively. Nicotine displaced [3H]epibatidine 10 times more potently from the brain than from the chromaffin cell membranes, and cotinine had over two orders of magnitude lower affinity than nicotine. In addition, the competitive nicotinic receptor antagonists methyllycaconitine and dihydro beta-erythroidine displaced [3H]epibatidine (100 pM and 1 nM) from the chromaffin cell membranes. Alpha-bungarotoxin did not affect the binding of 100 pM [3H]epibatidine. However, upon labelling with 1 nM [3H]epibatidine alpha-bungarotoxin (10 nM to 10 microM) displaced one-sixth of the bound radioligand. Our results demonstrate that 100 pM to 1 nM [3H]epibatidine labels mostly neuronal heteropentameric nicotinic receptors in bovine chromaffin cell membranes, and that cotinine is a low-affinity nicotinic ligand both in the adrenal chromaffin cell and in the brain receptors.

Spinal mechanisms underlying A-85380-induced effects on acute thermal pain

Systemic administration of nicotinic receptor (nAChR) agonists is antinociceptive in models of acute pain whereas their intrathecal (i. t.) administration has been reported to be antinociceptive, nociceptive or without effect. It has been hypothesized that the action induced is dependent upon the subtype and location of the nAChR activated. In addition, there is considerable evidence that nAChR ligand-induced antinociception is mediated by other neurotransmitter systems via descending pathways from the brainstem to the spinal cord. The present study investigated the effects of i. t. and systemic administration of A-85380, a novel nAChR agonist, in the paw withdrawal model of acute thermal pain in the rat. Given i.t. , A-85380 (1 and 10 nmol/rat) decreased the latency to paw withdrawal by 2-4 s. This pronociception was accompanied by a spontaneous flinching behavior. Both of these effects were differentially blocked by i.t. pretreatment with the nAChR antagonists mecamylamine (10 nmol)>MLA (100 nmol)>DHbetaE (50% with 1000 nmol) but not by alpha-bungarotoxin (0% at 0.63 nmol). Given systemically, A-85380 (0.56 micromol/kg, i.p.) induced antinociception as indicated by an increased latency to paw withdrawal, an effect differentially altered by i.t. pretreatment with monoaminergic antagonists (100 nmol/rat). While mecamylamine and prazosin had no effect, scopolamine, methysergide and MDL 72222 partially antagonized and idazoxan completely antagonized A-85380-induced antinociception. Finally, as measured by in vivo microdialysis, levels of 5-HT, but not NE, in the i.t. space of the lumber region of the spinal cord were significantly increased following the systemic administration of A-85380. Together these data suggest that the nociceptive properties of spinally administered nAChR agents are not mediated by either an alpha(4)beta(2) or an alpha(7) subtype nAChR, whereas the antinociceptive properties of systemically-administered nAChR agents are mediated by descending noradrenergic, serotonergic and muscarinic inhibitory pathways.

The mGlu(2/3) receptor agonist LY379268 selectively blocks amphetamine ambulations and rearing

We have previously reported that the specific group II metabotropic glutamate receptor agonist LY379268 inhibited phencyclidine (PCP)-induced motor activations in rats, but had mixed effects on behaviors produced by amphetamine. Here, LY379268 (1 mg/kg subcutaneous (s.c.)) attenuated amphetamine-induced ambulations and rearing but did not alter amphetamine-evoked fine motor movements. Consistent with a mechanism involving mGlu(2/3) receptors, the inhibitory actions of LY379268 on ambulations and rearing were reversed by LY341495, a mGlu(2/3) receptor antagonist. These data further suggest antipsychotic actions of mGlu(2/3) receptor agonists with a low propensity for extra-pyramidal side effects.

Analgesic and toxic effects of ABT-594 resemble epibatidine and nicotine in rats

The present study directly compared the antinociceptive and toxic effects of the neuronal nicotinic receptor agonist ABT-594 ((R)-5-(2-azetidinylmethoxy)-2-chloropyridine) with (-)-nicotine and (+)-epibatidine. Like (-)-nicotine (0.8 and 1.6 mg/kg s.c.) and (+)-epibatidine (0.005 and 0.01 mg/kg s.c.), ABT-594 (0.05 and 0.1 mg/kg s.c.) increased response latencies in the hot-plate test in rats, indicating that it has antinociceptive activity. In contrast to (-)-nicotine and (+)-epibatidine, ABT-594 did not cause rotarod impairment at antinociceptive doses but did cause hypothermia and life-threatening adverse effects including seizures. ABT-594 (0.01 and 0.1 mg/kg i.v.) also produced a dose-dependent increase in blood pressure resembling that observed with (-)-nicotine (0.03, 0.1 and 0. 03 mg/kg i.v.) and (+)-epibatidine (0.001 and 0.003 mg/kg i.v.). Both the antinociceptive and toxic effects (convulsions and hypertension) were abolished by pretreatment with the brain penetrant neuronal nAChR antagonist mecamylamine (1 mg/kg s.c.; i.v. for cardiovascular studies), demonstrating that these actions of ABT-594 were mediated via activation of neuronal nicotinic receptors. Continuous infusion of ABT-594 (0.2 mg/kg per day s.c.) to rats for 7 days followed by challenge with mecamylamine (1 mg/kg i.p.) induced a nicotine-like abstinence syndrome suggesting that ABT-594 has nicotine-like dependence liability. These findings indicate that the acute safety profile of ABT-594 is not significantly improved over other nicotinic analgesics.

Modulation by bicuculline and penicillin of the block by t-butyl-bicyclo-phosphorothionate (TBPS) of GABA(A)-receptor mediated Cl(-)-current responses in rat striatal neurones

1. T-butyl-bicyclo-phosphorothionate (TBPS) is a prototypical representative of the cage-convulsants which act through a use-dependent block of the GABA(A)-receptor-ionophore complex. Using current recordings from cultured neurones of rat striatum the manner was investigated in which two antagonists, bicuculline and penicillin, presumably acting at the agonist binding site and in the ionic channel, respectively, modify the rate of block by TBPS. 2. Penicillin (5 or 10 mM) did not slow the rate of block by TBPS, but produced a significant enhancement of block rate, which, however, was inversely related to the degree of antagonism by penicillin of the GABA-induced current. 3. Bicuculline (10 microM) reduced the rate of block by TBPS. However, this effect was 3 fold weaker than its GABA-antagonistic action. The slowing of block rate and the current antagonism exhibited a biphasic, positive-negative relationship. Co-application of bicuculline (100 microM) in a concentration that produced nearly complete antagonism and TBPS (10 microM) resulted in a marked ( approximately 40%) reduction of subsequent GABA response amplitudes compatible with a direct, bicuculline-induced conformational change in the receptor required for the binding of and block by TBPS. 4. The lack of protection afforded by the channel blocker penicillin as well as the lack of correlation between bicuculline antagonism of the Cl(-)-current and its efficiency in protecting against TBPS block is evidence against an open channel blocking mechanism for TBPS. TBPS does, therefore, not appear to gain access to its binding site via the open pore but through alternative routes regulated from the agonist binding site.

Beta-estradiol attenuate amyloid beta-peptide toxicity via nicotinic receptors

A number of epidemiological studies suggest that estrogen therapy is linked to a reduced risk of developing Alzheimer's disease (AD). The present study was conducted to evaluate the effect of 17beta-estradiol on beta-amyloid (Abeta)-induced toxicity and was performed in rat pheochromocytoma PC 12 cells by measuring the mitochondrial activity. 17Beta-estradiol (10(-5), 10(-6) and 10(-8) M) attenuated Abeta(25-35)-induced toxicity in PC 12 cells. The neuroprotective effect of 17beta-estradiol (10(-5) M) was prevented in the presence of the nicotinic antagonists methyllycaconitine (MLA) and mecamylamine, suggesting an interaction probably via the alpha7 nicotinic receptor subtype. Chronic treatment with 17beta-estradiol (10(-10)-10(-5) M) alone did not change the number of [3H]epibatidine binding sites in human neuroblastoma SH-SY5Y cells and rat PC 12 cells, but significantly prevented the enhanced [3H]epibatidine binding in nicotine-treated PC 12 cells. This study demonstrates that 17beta-estradiol exerts neuroprotective effects which might involve interaction with the alpha7 nicotinic receptor subtype.

Epibatidine binding sites and activity in the spinal cord

Epibatidine has been shown to be the most potent nicotinic agonist in several neuronal nicotinic receptor preparations. Similar to other nicotinic agonists, intrathecal (-)-epibatidine elicits dose-dependent increases in pressor, heart rate and pain responses in rats, as well as an increase in latency to withdraw from a noxious thermal stimulus. The latter response requires higher doses and is of shorter duration, suggesting interaction with multiple subtypes of spinal nicotinic receptors. In the present study, we relate the binding properties of (+/-)-[3H]epibatidine in spinal cord membrane preparations to the cardiovascular and behavioral responses. Unlike (-)-[3H]cytisine or (-)-[3H]nicotine, (+/-)-[3H]epibatidine reveals two sites; the ratio of high affinity to low affinity sites is 2:1. The rank ordering of potencies of the nicotinic agonists in displacing (+/-)-[3H]epibatidine binding from spinal cord membranes correlates with the potencies in eliciting cardiovascular and behavioral responses upon spinal administration. The nicotinic receptor antagonists, alpha-lobeline, dihydro-beta-erythroidine and methyllycaconitine, also displayed similar rank ordering of potencies in displacing (+/-)-[3H]epibatidine, (-)-[3H]cytisine or (-)-[3H]nicotine binding to spinal nicotinic receptors. Virtually all the nicotinic analogs exhibited a Hill coefficient of less than one in competing with (+/-)-[3H]epibatidine to spinal cord membranes indicating their interaction with at least two classes of binding sites. Intrathecal (-)-epibatidine, in addition to eliciting an initial and subsequently a sustained pressor and tachycardic response, also exhibited a transient intervening bradycardia which coincided temporally with the duration of the analgesia. Repeated administration of (-)-epibatidine desensitized its responses as well as the cardiovascular and behavioral responses to spinal nicotine and cytisine. Intrathecal alpha-lobeline showed selectivity for blocking the analgesic response, whereas methyllycaconitine exhibited selectivity for the pressor and irritation responses. The NMDA receptor antagonist, AP-5, inhibited the pressor, tachycardic and irritation responses elicited by intrathecal (-)-epibatidine, confirming a role for spinal excitatory amino acids released by the nicotinic agonist.

High-affinity partial agonist imidazo[1,5-a]quinoxaline amides, carbamates, and ureas at the gamma-aminobutyric acid A/benzodiazepine receptor complex

A series of imidazo[1,5-a]quinoxaline amides, carbamates, and ureas which have high affinity for the gamma-aminobutyric acid A/benzodiazepine receptor complex was developed. Compounds within this class have varying efficacies ranging from antagonists to full agonists. However, most analogs were found to be partial agonists as indicated by [35S]TBPS and Cl- current ratios. Many of these compounds were also effective in antagonizing metrazole-induced seizures in accordance with anticonvulsant and possible anxiolytic activity. Selected quinoxalines displayed limited benzodiazepine-type side effects such as ethanol potentiation and physical dependence in animal models. Dimethylamino urea 41 emerged as the most interesting analog, having a partial agonist profile in vitro while possessing useful activity in animal models of anxiety such as the Vogel and Geller assays. In accordance with its partial agonist profile, 41 was devoid of typical benzodiazepine side effects.

Taurine allosterically inhibits binding of [35S]-t-butylbicyclophosphorothionate (TBPS) to rat brain synaptic membranes

The modulatory effects of taurine on [35S]-t-butylbicyclophosphorothionate (TBPS) binding to rat brain synaptic membranes were evaluated and compared with that of GABA. Taurine allosterically inhibited TBPS binding by interacting with a bicuculline-sensitive site, similar to GABA. Taurine was as effective as GABA but less potent. The potency of taurine inhibition of TBPS binding varied among brain regions with cerebellum > olfactory bulb > cortex, similar to that of GABA. Inhibition of TBPS binding to cortical membranes measured under nonequilibrium conditions yielded a dynamic biphasic inhibition curve that was similarly shaped for GABA and taurine. The effect of taurine on TBPS binding was pharmacologically specific in that beta-alanine and guanadinoethanesulfonate were as effective as taurine, while hypotaurine and alpha-aminoethylhydrogen sulfate were only partially effective at high concentrations, and isethionic acid was without effect. Taurine, similar to GABA, enhanced the effects of pentobarbital on TBPS binding when present at concentrations that were otherwise ineffective on their own. The results of these studies support the notion that taurine interacts with the GABA recognition site of the GABAA receptor complex.