picrotoxinin and tramiprosate

In isolated segments of guinea-pig small intestine, gamma-aminobutyric acid (GABA) (3-300 microM), the GABAA receptor agonist 3-aminopropane sulphonic acid (3-APS) (3-300 microM) and ivermectin (1-300 microM) caused concentration-dependent nerve-mediated cholinergic contractions sensitive to tetrodotoxin (1 microM) and hyoscine (1 microM). The EC50 values were 30.2 +/- 4.3, 24.6 +/- 3.1 and 4.8 +/- 0.6 microM, respectively. Picrotoxinin (10 microM), an allosteric blocker of the Cl- channel associated with GABAA receptors, non-competitively antagonized the contractile response caused by each agonist. Like picrotoxinin, lindane (10, 30 microM) caused a dose-related shift to the right of the concentration-response curve to GABA, 3-APS and ivermectin with depression of the maximum response. SR 95531 (3 microM), a competitive antagonist of GABAA receptors, caused a parallel dextral shift of the concentration-response curve to ivermectin with an apparent single point pA2 value of 6.5. Our results suggest that ivermectin and lindane, two neurotoxic pesticides interfering with central GABAErgic transmission, exert agonist and non-competitive antagonist properties at the enteric GABAA receptor-ionophore complex. This peripheral complex can thus be considered as an additional target for the action of both these compounds.

Topics: Animals; Chloride Channels; Female; GABA-A Receptor Antagonists; Guinea Pigs; Hexachlorocyclohexane; In Vitro Techniques; Intestine, Small; Isometric Contraction; Ivermectin; Male; Membrane Proteins; Muscle, Smooth; Picrotoxin; Pyridazines; Receptors, GABA-A; Scopolamine; Sesterterpenes; Taurine; Tetrodotoxin

The application of 1.2 and 12.0 micrograms/side of the GABAA receptor agonist 3-aminopropane sulphonic acid bilaterally into the nucleus accumbens (Acb) of rats nonsignificantly depressed locomotor activity as assessed in automated Animex activity cages, while the highest dose (60 micrograms/side) significantly stimulated activity. The GABAA receptor antagonists picrotoxinin (0.0625 and 0.125 micrograms/saide) and bicuculline (0.895 micrograms/side) produced forward locomotion around the cage accompanied by a number of other behaviours. The GABAB agonist baclofen (0.023 and 0.092 micrograms/side) induced a short-lasting (18 min) locomotor depression. None of the GABAB antagonists tested (2-hydroxysaclofen 2.6 micrograms/side, two novel beta-(benzo[b]furan) analogues of baclofen 9G or 9H each 6.8 micrograms/side, 4-aminobutylphosphonic acid 1.32 micrograms/side and phaclofen 0.535 and 2 micrograms/side) significantly affected locomotor activity. In rats pretreated with reserpine and alpha-methyl-p-tyrosine, picrotoxinin (0.0625 and 0.125 micrograms/side) did not significantly alter locomotor activity. Furthermore, when picrotoxinin (0.0625 micrograms/side) was combined with either the selective dopamine (DA) D1 agonist SKF38393 or the selective D2 agonist quinpirole, no significant alteration in locomotor function occurred. When SKF38393 and quinpirole were coadministered, significant stimulation occurred which was further enhanced by the addition of picrotoxinin. It is concluded that GABAA receptors, together with D1 and D2 receptors, play a major role in modulating the control of motor function by the Acb of rats.

Topics: Animals; Baclofen; Dopamine; Dopamine Antagonists; GABA Antagonists; gamma-Aminobutyric Acid; Male; Motor Activity; Nucleus Accumbens; Picrotoxin; Rats; Rats, Inbred Strains; Receptors, Dopamine; Receptors, GABA-A; Sesterterpenes; Taurine

The guinea pig ileum myenteric plexus contains GABAA receptors linked to chloride ion channels which are pharmacologically similar to those in the central nervous system. The present study examined the reported ability of acute ethanol treatment to directly activate GABAA receptors or to increase GABAA agonist-mediated activation of the GABAA receptor in the myenteric plexus. Direct addition of ethanol to preparations of the guinea pig ileum longitudinal muscle had two effects. Immediately after ethanol (10-300 mM) was added to the tissue bath a concentration-related contractile response was observed which became maximal within 10 sec and then decayed over the next 60 sec. Contractile responses to higher concentrations of ethanol (greater than 100 mM) also were followed by a sustained reduction of longitudinal muscle tone. Contractions evoked by gamma-aminobutyric acid (GABA) and GABAA agonists, 3-aminopropane sulfonic acid (APSA) (3-100 microM) or muscimol (0.3-30 microM) developed maximally and decayed within 20 sec. Acetylcholine (0.01-10 microM) induced contractions were sustained over several minutes. Preincubation of tissue strips in ethanol (30 mM) for 1 min did not alter concentration relationships for GABA, muscimol or APSA contractile responses. Furthermore, addition of ethanol (10-100 mM) simultaneously with APSA, or 0.5, 2 or 5 min before the addition of APSA, also failed to consistently enhance contractile responses. Ethanol (30 mM) also did not alter desensitization-induced reductions in contractile responses to muscimol (3 microM) caused by preincubation of tissues with muscimol (1 microM). Finally, contractile responses to ethanol and APSA were completely blocked by atropine (0.1 microM) and tetrodotoxin (0.1 microM).(ABSTRACT TRUNCATED AT 250 WORDS)

Topics: Animals; Atropine; Bicuculline; Chlorides; Ethanol; Female; Guinea Pigs; Ileum; In Vitro Techniques; Male; Muscimol; Muscle Contraction; Pentobarbital; Picrotoxin; Receptors, GABA-A; Sesterterpenes; Taurine; Tetrodotoxin

1. The effects of gamma-aminobutyric acid (GABA), 3-aminopropane sulphonic acid (3-APS) and baclofen on spontaneous, electrically-induced and propulsive motility were investigated in rabbit distal colon. 2. In unstimulated longitudinal (LMPs) and circular muscle strip preparations (CMPs) 3-APS (10-200 microM) and GABA caused a clear-cut relaxation susceptible to desensitization. Baclofen (10-200 microM) caused relaxation in a minority (30%) of preparations. The 3-APS response was sensitive to tetrodotoxin (TTX; 1 microM), SR 95531 (a novel competitive GABAA-receptor antagonist) (10 microM), picrotoxinin (30 microM), and insensitive to hyoscine (1 microM) and to a combination of prazosin (1 microM) and propranolol (1 microM). The baclofen response was antagonized by 5-aminovaleric acid (DAVA, 500 microM), TTX and hyoscine and resistant to GABAA-receptor and adrenoceptor blockade. GABAA-receptors were therefore associated with non-adrenergic non-cholinergic (NANC) inhibitory nerve activation while GABAB-receptors were involved in depression of cholinergic tone of smooth muscle. GABA (10-200 microM) elicited both above mentioned effects. 3. In LMPs, baclofen (10-200 microM) dose-dependently inhibited submaximal responses to both cholinergic and NANC inhibitory nerve stimulation. This effect was resistant to SR 95531 and picrotoxinin and prevented by DAVA and baclofen desensitization. GABA (10-200 microM) mimicked the action of baclofen. GABA inhibitory effects persisted in the presence of GABAA-receptor blockade. 4. In segments of distal colon, GABA and baclofen (1-200 microM), but not 3-APS (1-200 microM), dose-dependently decreased the velocity of propulsion of an intraluminally-distended balloon. This effect was antagonized by DAVA and GABA or baclofen desensitization and resistant to SR 95531 and picrotoxinin. These antagonists per se had no effect on propulsion. In preparations in which propulsion was slowed by hyoscine (1 microM), baclofen caused no consistent further depression of propulsive activity. 5. Our results show that GABAA- and GABAB-receptors are present in rabbit colon. GABAA-receptor stimulation activates NANC inhibitory nerves without apparently affecting propulsion. GABAB-receptors are associated with a reduction of neural (mainly cholinergic) activity subserving muscular tone and peristalsis and appear to be located on both cholinergic and NANC inhibitory nerves. However, the persisting propulsive activity during suppression of GABAA- a

Topics: Amino Acids; Amino Acids, Neutral; Animals; Autonomic Nervous System; Baclofen; Colon; gamma-Aminobutyric Acid; Gastrointestinal Motility; In Vitro Techniques; Male; Muscle Relaxation; Muscle, Smooth; Picrotoxin; Rabbits; Receptors, GABA-A; Sesterterpenes; Taurine

The gamma-aminobutyric acid (GABA)-induced contractile responses in the guinea-pig isolated ileum, maintained in Krebs-bicarbonate solution (pH 7.4, 37 degrees C), were significantly potentiated by inhibitors of GABA uptake, with a greater potentiation of the responses in the presence of (+/-)-cis-3-aminocyclohexane-carboxylic acid (ACHC) greater than L-2,4-diaminobutyric acid (DABA) greater than (+/-)-nipecotic acid greater than beta-alanine, whilst simultaneous addition of DABA with beta-alanine caused a greater potentiation of the GABA-induced responses than did nipecotic acid with beta-alanine, or any of the uptake blockers applied alone. The concentration-response curves for the GABA-induced ileal contraction were shifted to the left in the presence of the uptake inhibitors, this shift being more prominent over the lower concentration range of GABA (1-20 microM). By contrast, contractile responses to muscimol or 3-amino-1-propanesulphonic acid (3APS) were not potentiated by the uptake blockers, neither were their concentration-response curves altered. Bicuculline methochloride shifted the GABA concentration-response curve to the right, whilst picrotoxinin both shifted the concentration-response curve for GABA to the right and depressed the maximum response. In the presence of the uptake inhibitors, the rightward shift of the concentration-response curves for GABA induced by bicuculline was less than that induced by bicuculline alone. The rightward shift with picrotoxinin was similarly reduced in the presence of the uptake inhibitors, without altering the depression of the maximum by picrotoxinin. Bicuculline caused a rightward shift of the concentration-response curves for 3APS and muscimol, with the curve for 3APS most affected. Picrotoxinin similarly shifted the concentration-response curves for 3APS and muscimol but depressed the maximum, with the curve for 3APS again being most affected. None of the inhibitors of GABA uptake influenced the concentration-response curves for 3APS or muscimol in the presence of bicuculline or picrotoxinin. 5. In conclusion, a saturable GABA uptake system is present in the enteric nervous system of the guinea-pig intestine, where neuronal GABA uptake appears to predominate over glial uptake.

Topics: Animals; Bicuculline; Female; gamma-Aminobutyric Acid; Guinea Pigs; Ileum; In Vitro Techniques; Male; Muscimol; Muscle Contraction; Muscle, Smooth; Picrotoxin; Sesterterpenes; Taurine