3-amino-2-(4-chlorophenyl)-nitropropane is a new GABAB receptor agonist, more active peripherally

Differential effects of R-isovaline and the GABAB agonist, baclofen, in the guinea pig ileum

R-isovaline is a non-proteinogenic amino acid which produces analgesia in a range of nociceptive assays. Mediation of this effect by metabotropic receptors for γ-aminobutyric acid (GABA) and glutamate, demonstrated by previous work, may depend on the type of tissue or receptor system. The objective of this study was to assess the activity of R-isovaline acting at GABA_B and group II metabotropic glutamate receptors in guinea pig ileum, which is known to exhibit well-defined responses to GABA_B agonists such as baclofen. The effects of bath-applied R-isovaline and RS-baclofen were examined on electrically evoked contractions of guinea pig ileum and during GABA_B antagonism by CGP52432. In separate experiments, the group II metabotropic glutamate receptor agonist, LY354740 was applied to determine the functional presence of these receptors. R-isovaline (1-100mM) decreased the amplitude of ileal muscle contractions and increased tension. RS-baclofen reduced contraction amplitude, but decreased tension. CGP52432 did not prevent the effects of R-isovaline on contraction amplitude, but antagonized effects of RS-baclofen on contraction amplitude. The group II metabotropic glutamate receptor agonist, LY354740, produced no detectable effects on evoked contractions. R-isovaline differed significantly from RS-baclofen in its actions in the guinea pig ileum, indicated in particular by the finding that CGP52432 blocked only the effects of RS-baclofen. The ileal tissue did not respond to a group II metabotropic glutamate receptor agonist, previously shown to co-mediate R-isovaline analgesia. These findings raise the possibility of a novel therapeutic target at unknown receptors for R-isovaline-like compounds in the guinea pig ileum.

Behavioral effects of gamma-hydroxybutyrate, its precursor gamma-butyrolactone, and GABA(B) receptor agonists: time course and differential antagonism by the GABA(B) receptor antagonist 3-aminopropyl(diethoxymethyl)phosphinic acid (CGP35348)

Gamma-hydroxybutyrate (GHB) is used therapeutically and recreationally. The mechanism by which GHB produces its therapeutic and recreational effects is not entirely clear, although GABA(B) receptors seem to play an important role. This role could be complex, because there are indications that different GABA(B) receptor mechanisms mediate the effects of GHB and the prototypical GABA(B) receptor agonist baclofen. To further explore possible differences in underlying GABA(B) receptor mechanisms, the present study examined the effects of GHB and baclofen on operant responding and their antagonism by the GABA(B) receptor antagonist 3-aminopropyl(diethoxymethyl)phosphinic acid (CGP35348). Pigeons were trained to peck a key for access to food during response periods that started at different times after the beginning of the session. In these pigeons, GHB, its precursor gamma-butyrolactone (GBL), and the GABA(B) receptor agonists baclofen and 3-aminopropyl(methyl)phosphinic acid hydrochloride (SKF97541) decreased the rate of responding in a dose- and time-dependent manner. CGP35348 shifted the dose-response curve of each agonist to the right, but the magnitude of the shift differed among the agonists. Schild analysis yielded a pA(2) value of CGP35348 to antagonize GHB and GBL [i.e., 3.9 (3.7-4.2)] that was different (P = 0.0011) from the pA(2) value to antagonize baclofen and SKF97541 [i.e., 4.5 (4.4-4.7)]. This finding is further evidence that the GABA(B) receptor mechanisms mediating the effects of GHB and prototypical GABA(B) receptor agonists are not identical. A better understanding of the similarities and differences between these mechanisms, and their involvement in the therapeutic effects of GHB and baclofen, could lead to more effective medications with fewer adverse effects.

Enantioselective carbon-hydrogen insertion is an effective and efficient methodology for the synthesis of (R)-(-)-baclofen

A highly enantioselective methodology for the synthesis of the GABA(B) receptor agonist (R)-(-)-baclofen is described. This synthesis begins with p-chlorophenethyl alcohol and involves a catalytic carbon-hydrogen insertion reaction of a chiral dirhodium(II) carboxamidate with the corresponding diazoacetate (81% yield, 95% ee). Subsequent steps convert the intermediate gamma-lactone to (R)- (-)-baclofen in a 60% overall yield. The amount of catalyst required for the C-H insertion transformation is only 0.5 mol%.

Characterization of gamma-aminobutyric acid receptor GABAB(1e), a GABAB(1) splice variant encoding a truncated receptor

We have identified a splice variant encoding only the extracellular ligand-binding domain of the gamma-aminobutyric acid B (GABA(B)) receptor subunit GABA(B(1a)). This isoform, which we have named GABA(B(1e)), is detected in both rats and humans. While GABA(B(1e)) is a minor component of the total pool of GABA(B(1)) transcripts detected in the central nervous system, it is the primary isoform found in all peripheral tissues examined. When expressed in a heterologous system, the truncated receptor is both secreted and membrane associated. However, GABA(B(1e)) lacks the ability to bind the radiolabeled antagonist [(3)H]CGP 54626A, activate G-protein coupled inwardly rectifying potassium channels, or inhibit forskolin-induced cAMP production when it is expressed alone or together with GABA(B(2)). Interestingly, when co-expressed with GABA(B(2)), not only does the truncated receptor heterodimerize with GABA(B(2)), the association is of sufficient avidity to disrupt the normal GABA(B(1a))/GABA(B(2)) association. Despite this strong interaction, GABA(B(1e)) fails to disrupt G-protein coupled inwardly rectifying potassium activation by the full-length heterodimer pair of GABA(B(1a))/GABA(B(2)).

The pharmacology of adenylyl cyclase modulation by GABAB receptors in rat brain slices

GABAB receptor activation inhibits forskolin-stimulated adenylyl cyclase activity but augments noradrenaline-stimulated adenylyl cyclase activity. The present study investigated the pharmacology of these two GABAB receptor mediated responses. In a cross-chopped rat cortical slice preparation, it was confirmed that (-)baclofen inhibited forskolin-stimulated adenylyl cyclase activity and augmented noradrenaline-stimulated adenylyl cyclase activity. The potency of five further agonists was investigated (SKF97541, CGP47656, CGP44533, 3-APA and CGP44532). Of these agonists two compounds were significantly more potent as inhibitors of forskolin-stimulated adenylyl cyclase than as augmenters of noradrenaline-stimulated adenylyl cyclase activity, these were (-)baclofen (pEC50 = 6.07 +/- 0.29 and 5.04 +/- 0.17, respectively (p < 0.05)), and CGP47656 (pEC50 = 6.44 +/- 0.05 and 4.48 +/- 0.26, respectively (p < 0.05)). It is possible to explain this difference in potency by proposing that these compounds have low intrinsic efficacy, and the augmentation of noradrenaline-stimulated adenylyl cyclase has a low receptor reserve. In addition six antagonists (CGP49311A, CGP46381, CGP45024, CGP45397, CGP36742) were also tested for their ability to antagonize 10 microM (-)baclofen in these two assays. These antagonists ranged in potency as inhibitors of forskolin-stimulated adenylyl cyclase activity from CGP49311A (pEC50 = 5.45 +/- 0.30) to CGP36742 (pEC50 = 3.87 +/- 0.16). Each antagonist had similar potency in the two assays, suggesting that these two responses are mediated by pharmacologically similar receptors.

GABAB receptors

GABAB receptors are a distinct subclass of receptors for the major inhibitory transmitter 4-aminobutanoic acid (GABA) that mediate depression of synaptic transmission and contribute to the inhibition controlling neuronal excitability. The development of specific agonists and antagonists for these receptors has led to a better understanding of their physiology and pharmacology, highlighting their diverse coupling to different intracellular effectors through Gi/G(o) proteins. This review emphasises our current knowledge of the neurophysiology and neurochemistry of GABAB receptors, including their heterogeneity, as well as the therapeutic potential of drugs acting at these sites.

Differing actions of nitropropane analogs of GABA and baclofen in central and peripheral preparations

In the guinea-pig isolated ileum, both baclofen (5-100 microM) and gamma-aminobutyric acid (GABA; 5-100 microM) induced a bicuculline-insensitive depression of cholinergic twitch contractions which was reversibly and competitively antagonised by 3-amino-1-nitropropane (50-250 microM). 3-Amino-1-nitropropane (pA2 = 5.0 +/- 0.1) was twice as potent as 2-hydroxysaclofen (pA2 = 4.5 +/- 0.1), but was equipotent with 3-aminopropyl(P-diethoxymethyl)phosphinic acid (CGP 35348) (pA2 = 4.9 +/- 0.2), and did not show any partial agonist activity at these peripheral GABAA or GABAB receptor sites. In rat neocortical slices, 3-amino-1-nitropropane did not activate GABAB receptor sites or affect baclofen-induced suppression of spontaneous discharges. In the cat spinal cord, however, under in vivo conditions, the corresponding nitro analog of baclofen 3-amino-2-(4-chloro)-nitropropane was an agonist at GABAB receptor sites, although more than 60 times weaker than baclofen in depressing monosynaptic excitatory field potentials, whereas 3-amino-1-nitropropane was an extremely weak agonist at bicuculline-sensitive GABAA sites. The differing actions of 3-amino-1-nitropropane at peripheral and central GABAB receptor sites suggest heterogeneity among these receptors.

Gamma-guanidinobaclofen is a peripheral GABAB receptor agonist

In the guinea-pig isolated ileum, both baclofen and gamma-guanidinobaclofen elicited dose-dependent depression of cholinergic twitch contractions, sensitive to the GABAB receptor antagonists phaclofen and 2-hydroxysaclofen. gamma-Guanidinobaclofen was 5 times less potent than R,S-(+/-)-baclofen in depressing the contractions. The corresponding GABA analogs, guanidinoacetic acid, beta-guanidinopropionic acid and gamma-guanidinobutanoic acid were inactive. In rat neocortical slices maintained in Mg(2+)-free medium, baclofen (1-50 microM) reduced the amplitude and rate, whilst gamma-guanidinobaclofen (1 mM) has a very weak GABAB receptor agonist action, 100 times weaker than baclofen. gamma-Guanidinobaclofen is therefore a GABAB receptor agonist, more potent at peripheral than central GABAB receptors.