3-aminopropylphosphinic-acid and phaclofen

gamma-Hydroxybutyric acid (GHB) is a naturally occurring compound that has the ability to induce generalized absence seizures when given to animals. GHB has been hypothesized to induce this effect via the postsynaptic gamma-aminobutyric acidB (GABAB) receptor. We sought to test this hypothesis by examining the affinity of GABAB agonists and antagonists for the [3H]GHB binding site, the affinity of GHB and a GHB antagonist for the [3H]GABAB binding site, and the effect of guanine nucleotides and pertussis toxin on both, using autoradiographic binding assays. GHB and its antagonist, NCS 382, did not compete for [3H]GABAB binding, nor did (-)-baclofen or the [3H]GABAB antagonists, CGP 35348 or SCH 50911, compete for [3H]GHB binding; however, the GABAB agonist 3-amino-propylphosphinic acid (3-APPA), and the GABAB antagonists phaclofen and 2-hydroxysaclofen (2-OH saclofen) did show a weak affinity for [3H]GHB binding in frontal cortex. GTP and the nonhydrolyzable GTP analogues, GTP gamma S and Gpp(NH)p, depressed [3H]GABAB binding throughout the brain, but increased [3H]GHB binding in frontal cortex and thalamus, those regions involved in GHB-induced absence seizures. Pertussis toxin significantly depressed [3H]GABAB binding throughout the brain, but attenuated [3H]GHB binding only in frontal cortex, and to a lesser degree than [3H]GABAB binding. The guanine nucleotide-induced changes in [3H]GHB and [3H]GABAB binding were due to a change in KD for both. Moreover, GTP gamma S reversed the ability of 3-APPA, phaclofen, and 2-OH saclofen to compete for [3H]GHB binding. These data do not support the hypothesis that GHB acts through the postsynaptic GABAB receptor to produce absence seizures. Rather, they raise the possibility either that the [3H]GHB binding site may be an isoform of the presynaptic GABAB receptor or that an independent GHB site is operative in the GHB model of absence seizures.

Topics: Animals; Autoradiography; Baclofen; Benzocycloheptenes; Binding Sites; Brain; GABA Agonists; GABA Antagonists; gamma-Aminobutyric Acid; Guanosine Triphosphate; In Vitro Techniques; Male; Organophosphorus Compounds; Rats; Rats, Sprague-Dawley; Receptors, GABA-B; Seizures; Sodium Oxybate; Stereoisomerism

The purpose of the present study was to investigate the role played by GABAB receptors in the regulation of gastric basal pepsinogen secretion in anaesthetized rats. Following parenteral administration, the GABAB receptor agonists (-)-baclofen and 3-aminopropylphosphinic acid (3-APPA) caused a dose-dependent increase in basal pepsinogen secretion which was associated with a parallel increment in acid output. The gastric stimulant effects induced by both agonists were not affected by intracerebroventricular injection of the GABAB receptor antagonists 2-hydroxy-saclofen, 3-aminopropyl(diethoxymethyl)phosphinic acid (CGP 35348) or phaclofen, whereas the excitatory actions were antagonized by intravenously administered 2-hydroxy-saclofen or CGP 35348, but not phaclofen. In addition, the (-)-baclofen-induced increases in both pepsinogen and acid output, were fully prevented by omeprazole or cimetidine, partly reduced by atropine and unaffected by pretreatment with capsaicin. When tested on rats undergoing bilateral cervical vagotomy, both (-)-baclofen and 3-APPA were still able to stimulate the basal pepsinogen and acid secretions, although at a lesser extent than in animals with intact vagus nerves. The stimulant actions elicited by (-)-baclofen in vagotomized rats were antagonized by 2-hydroxy-saclofen or CGP 35348, but not phaclofen. Moreover, these gastric excitatory effects were prevented by cimetidine or compound 48/80, while being unaffected by atropine. The present results show that peripheral GABAB receptors mediate an excitatory effect on gastric pepsinogen secretion which totally depends on an increase in acid output. It is also suggested that both vagal cholinergic and extravagal pathways, probably histaminergic in nature, take part in these GABAB receptor-mediated gastric stimulant actions.

Topics: Anesthesia; Animals; Baclofen; Capsaicin; GABA Agonists; GABA Antagonists; GABA-B Receptor Agonists; GABA-B Receptor Antagonists; Gastric Mucosa; Male; Organophosphorus Compounds; p-Methoxy-N-methylphenethylamine; Pepsinogens; Perfusion; Rats; Rats, Wistar; Receptors, GABA-B; Vagotomy

Amacrine and ganglion cells in the amphibian retina contain GABAB, as well as GABAA, receptors. Baclofen, a GABAB agonist, hyperpolarizes the dark membrane potential of these third order neurons and makes their light responses more transient. GABAB receptors in the retina have a similar agonist profile to GABAB receptors described at other sites in the brain. Namely, preferential activation by the R-enantiomer of baclofen, and agonist sensitivity in the order 3-aminopropylphosphinic acid > baclofen >> 3-aminopropylphosphonic acid. The GABAB receptor was not activated by 4-aminobutylphosphonic acid. Several antagonists, such as phaclofen, saclofen, and 2-hydroxysaclofen, were ineffective in the amphibian retina. However, CGP35348 blocked the action of applied baclofen and produced effects on the light response that were opposite to those of baclofen. Applied agonists and antagonists support the hypothesis that GABAB receptors serve to regulate the balance of sustained and transient signals to the inner retina.

Topics: Ambystoma; Animals; Baclofen; Butylamines; Evoked Potentials; GABA-B Receptor Agonists; GABA-B Receptor Antagonists; In Vitro Techniques; Membrane Potentials; Necturus; Organophosphorus Compounds; Propylamines; Receptors, GABA-B; Retina; Retinal Ganglion Cells; Stereoisomerism; Structure-Activity Relationship

Release-regulating gamma-aminobutyric acidB (GABAB) autoreceptors were studied in synaptosomes from fresh specimens of human cerebral cortex. The K+ (12 mM)-evoked overflow of [3H]GABA was inhibited by the GABAB receptor agonists (-)-baclofen (EC50 = 1.48 microM) and 3-aminopropylphosphinic acid (3-APPA; EC50 = 0.034 microM). The effect of 10 microM (-)-baclofen was differentially reduced by the three GABAB receptor antagonists CGP 52432 ([3-[[(3,4-dichlorophenyl)methyl)amino]propyl]-(diethoxymethyl)- phosphinic acid), phaclofen and CGP 35348 (3-aminopropyl-(diethoxymethyl)- phosphinic acid). CGP 52432 was by far the most potent antagonist (IC50 = 0.09 microM). Phaclofen was about 700-fold less potent than CGP 52432 (IC50 = 70.0 microM) while CGP 35348 was ineffective up to 100 microM. The present results suggest that human and rat GABAB neocortical autoreceptors have similar pharmacological characteristics.

Topics: Adolescent; Adult; Aged; Animals; Baclofen; Benzylamines; Binding, Competitive; Brain Neoplasms; Cerebral Cortex; Female; GABA Agonists; GABA Antagonists; GABA-A Receptor Antagonists; GABA-B Receptor Agonists; GABA-B Receptor Antagonists; Humans; Male; Middle Aged; Organophosphorus Compounds; Phosphinic Acids; Rats; Receptors, GABA-B

gamma-Aminobutyric acid (GABA) autoreceptors regulating release of [3H]GABA have been characterized pharmacologically by using rat cerebral cortex and spinal cord synaptosomes exposed in superfusion to mild depolarization (9 mM KCl). In both regions GABA inhibited the K(+)-evoked overflow of [3H]GABA. The EC50 values amounted to 1.23 microM (cortex) and to 1.01 microM (spinal cord). Also the GABAB receptor agonist 3-aminopropylphosphonous acid (3-APPA) decreased the [3H]GABA overflow: EC50 values = 0.095 microM (cortex) and 0.078 microM (spinal cord). The classical GABAB receptor agonist (-)-baclofen was equipotent to GABA at the cortical autoreceptor (EC50 = 1.37 microM), whereas it was almost ineffective in the spinal cord (EC50 = 425 microM). (+)-Baclofen was extremely weak in both brain areas. Two GABAB receptor antagonists, phaclofen and (CGP 35348) [3-aminopropyl(diethoxymethyl)phosphinic acid] provided opposite results at the two autoreceptors examined. The IC50 values for phaclofen amounted to 47.9 microM (cortex) and to > 1000 microM (spinal cord), respectively. In contrast, CGP 35348 was almost ineffective at the cortex autoreceptors (IC50 > 300 microM), but was rather potent in the spinal cord (IC50 = 1.07 microM). To conclude, GABA autoreceptors in the cerebral cortex are classically sensitive to (-)-baclofen and to 3-APPA; they are also sensitive to phaclofen, but resistant to CGP 35348. Surprisingly, GABA autoreceptors in the spinal cord are poorly sensitive to (-)-baclofen, although they are as sensitive as the cortical autoreceptors to 3-APPA; moreover they are resistant to phaclofen, but highly sensitive to CGP 35348. The data show that pharmacologically distinct release-regulating GABAB autoreceptors exist in the central nervous system of a same animal species.

Topics: Animals; Baclofen; Cerebral Cortex; gamma-Aminobutyric Acid; Male; Organophosphorus Compounds; Potassium; Rats; Rats, Wistar; Receptors, GABA-A; Spinal Cord; Synaptosomes

1. Intracellular recordings were made from neurons in striatum (caudate-putamen) and substantia nigra pars compacta in rat brain slices. Three GABAB agonists, baclofen, 3-aminopropylphosphinic acid (3-APPA) and 3-aminopropyl(methyl)phosphinic acid (SK&F 97541), depressed excitatory postsynaptic potentials (e.p.s.ps) mediated by glutamate in the striatum, and hyperpolarized neurones in the substantia nigra. The ability of 3-aminopropyl(diethyoxymethyl)phosphinic acid (CGP 35348), 3-aminopropyl (hexyl)phosphinic acid (3-APHPA) and phaclofen to antagonize these responses was assessed. 2. Striatal e.p.s.ps, studied in the presence of bicuculline (30 microns), were reduced in amplitude by 92% with 6,7-dinitroquinoxaline-2,3-dione (DNQX; 30 microns). These e.p.s.ps were depressed by up to 95% by SK&F 97541 and baclofen with EC50s of 0.092 microns and 1.25 microns respectively. The maximal effect of 3-APPA was 67% with an EC50 of 0.83 microns. Agonist concentration-effect data fitted a single-site logistic model. GABAB agonists were without effect on striatal neurone membrane potential, input resistance or depolarizations induced by applied glutamate. 3. The depression of striatal e.p.s.ps by SK&F 97541 was reversibly antagonized by CGP 35348, 3-APHPA and phaclofen with estimated equilibrium dissociation constants (KB) of 11.2 +/- 1.7 microns (n = 4), 13.3 +/- 0.4 microM (n = 3) and 405 +/- 43 microM (n = 3) respectively. CGP 35348 and 3-APHPA appeared to act competitively (Schild plot slopes of 0.99 and 1.01 respectively). 4. Nigral neurones were hyperpolarized by up to 25 mV by SK&F 97541 and baclofen with EC50s of 0.15 microns and 3.6 microns respectively. The maximum hyperpolarization by 3-APPA was only 84% that of the other agonists, with an EC50 of 9.0 microM. Agonist concentration-effect data fitted a single-site logistic model. 5. The SK&F 97541-induced hyperpolarization was reversibly antagonized by CGP 35348, 3-APHPA and phaclofen with estimated KBS of 17.6 + 4.4 (n = 3), 14.0 + 1.5 (n = 4), and >400 microM (n = 1) respectively. CGP 35348 appeared competitive (Schild plot slope of 0.99). Antagonists were also tested with baclofen as agonist, yielding similar KB estimates as for SK&F 97541. 6. It is concluded that at both the presynaptic and postsynaptic sites examined, SK&F 97541 was about 10 fold more potent than baclofen or 3-APPA. The antagonists CGP 35348 and 3-APHPA (KB 1O-20 microM) were about 20 fold more potent than phaclofen. The similarities

Topics: Animals; Baclofen; Bicuculline; Brain; Corpus Striatum; Electrophysiology; GABA-A Receptor Antagonists; In Vitro Techniques; Membrane Potentials; Neurons; Organophosphorus Compounds; Rats; Receptors, GABA-A; Substantia Nigra; Synapses

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.

Topics: Animals; Baclofen; Guinea Pigs; Ileum; In Vitro Techniques; Male; Muscle Contraction; Muscle, Smooth; Muscles; Organophosphorus Compounds; Parasympathetic Nervous System; Rats; Rats, Inbred Strains; Receptors, GABA-A