CGP 55845 hydrochloride

IC50: 5 nM

CGP 55845 is a potent, selective GABAB receptor antagonist that abolishes agonist binding (pKi = 8.35) and blocks GABA and glutamate release (pEC50 values are 8.08 and 7.85 respectively). CGP 55845 prevents GABAB responses to baclofen (IC50 = 130 nM in an isoproterenol assay) and increases the hypoglycemic response to glucose in vitro. [1,2]

Presynaptic GABAB receptors seem to regulate the release of several neurotransmitters. Baclofen, the GABAB agonist, that prevents the release of GABA itself via autoreceptors, was 10 times more potent in antagonizing the inhibitory effect of (-)-baclofen on the release of GABA and of somatostatin-like immunoreactivity (SRIF-LI) than of glutamate. However, CGP 35348 was about 70 times more potent in preventing the effect of baclofen on glutamate and SRIF-LI than on GABA release.

In vitro: Antagonist CGP 55845A of the GABAB receptor in the presence of CNQX and d(2)-2-amino-5-phosphonovaleric acid prevented the inhibitory postsynaptic potential-B and paired-pulse depression. [3].

This secretion was cadmium sensitive, potentiated by CGP 55845, and blocked by ketanserin. Taken together these data suggest that CB receptors act as direct glucosensors, and that processing of hypoglycaemia utilizes similar neurotransmitter and neuromodulatory mechanisms as hypoxia [4]. The convulsant 4-aminopyridine (4-AP) and the GABAB receptor antagonist CGP 55845 both applied to adult guinea pig hippocampal to slices, resulting in eliciting giant GABA-mediated postsynaptic potentials (GPSPs) and epileptiform discharges. [5].

In vivo: So far, no study in vivo has been conducted.

Clinical trial: So far, no clinical study has been conducted.

References:
[1] Waldmeier PC, Wicki P, Feldtrauer JJ, Mickel SJ, Bittiger H, Baumann PA.  GABA and glutamate release affected by GABAB receptor antagonists with similar potency: no evidence for pharmacologically different presynaptic receptors. Br J Pharmacol. 1994 Dec;113(4):1515-21.
[2] Cunningham MD, Enna SJ.  Evidence for pharmacologically distinct GABAB receptors associated with cAMP production in rat brain. Brain Res. 1996 May 13;720(1-2):220-4.
[3] Deisz RA.  The GABA(B) receptor antagonist CGP 55845A reduces presynaptic GABA(B) actions in neocortical neurons of the rat in vitro. Neuroscience. 1999;93(4):1241-9.
[4] Zhang M, Buttigieg J, Nurse CA.  Neurotransmitter mechanisms mediating low-glucose signalling in cocultures and fresh tissue slices of rat carotid body. J Physiol. 2007 Feb 1;578(Pt 3):735-50. Epub 2006 Nov 23.
[5] Salah A, Perkins KL.  Effects of subtype-selective group I mGluR antagonists on synchronous activity induced by 4-aminopyridine/CGP 55845 in adult guinea pig hippocampal slices. Neuropharmacology. 2008 Jul;55(1):47-54. doi: 10.1016/j.neuropharm.2008.04.010. Epub 2008 Apr 23.

Effects of subtype-selective group I mGluR antagonists on synchronous activity induced by 4-aminopyridine/CGP 55845 in adult guinea pig hippocampal slices.[Pubmed:18538357]

Neuropharmacology. 2008 Jul;55(1):47-54.

Co-application of the convulsant 4-aminopyridine (4-AP) and the GABA(B) receptor antagonist CGP 55845 to adult guinea pig hippocampal slices elicits giant GABA-mediated postsynaptic potentials (GPSPs) and epileptiform discharges. Here we tested the effects of the group I metabotropic glutamate receptor (mGluR) subtype-selective antagonists LY 367385 (mGlu1, 100 microM), MPEP (mGlu5, 10 microM), and MTEP (mGlu5, 500 nM) on this synchronous activity. Electrophysiological field recordings were performed in the CA3 region of hippocampal slices from adult guinea pigs. The mGlu5 receptor antagonists increased GPSP rate, but the mGlu1 receptor antagonist did not. This ability of mGlu5 receptor antagonists to increase the rate of GPSPs indicates that enough endogenous glutamate is released under these conditions to activate group I mGluR; nevertheless, co-application of a mGlu1 receptor antagonist (LY 367385 or JNJ 16259685) and MPEP did not decrease pre-existing epileptiform activity. Furthermore, co-application of LY 367,385 and MPEP did not prevent the emergence of epileptiform activity. When ionotropic glutamate receptor (iGluR) antagonists were present, neither MPEP nor the group I mGluR agonist DHPG changed GPSP rate, suggesting that pyramidal cell-to-interneuron iGluR-mediated synaptic connections are involved in the rate change mechanism. In contrast to the lack of effect of group I mGluR antagonists on epileptiform activity in the 4-AP/CGP 55845 model, group I mGluR antagonists blocked the emergence of longer epileptiform events and decreased the overall amount of synchronous activity in the GABA(A) antagonist/4-AP model. In conclusion, in the 4-AP/CGP 55845 model, enough glutamate was released to activate group I mGluRs and affect GPSP rate via mGlu5 receptors; however, this group I mGluR activation was not required for the generation of the epileptiform activity.

Neurotransmitter mechanisms mediating low-glucose signalling in cocultures and fresh tissue slices of rat carotid body.[Pubmed:17124268]

J Physiol. 2007 Feb 1;578(Pt 3):735-50.

The mammalian carotid body (CB) is a polymodal chemosensor which can detect low blood glucose (hypoglycaemia), leading to increased afferent discharge and activation of counter-regulatory autonomic pathways. The underlying neurotransmitter mechanisms are unknown and controversy surrounds whether the action of low glucose is direct or indirect. To address this, we used a coculture model containing functional chemosensory units of rat CB receptor (type I) cell clusters and afferent petrosal neurones (PN). During perforated-patch, whole-cell recordings, low glucose (0-2 mM) stimulated sensory discharge in cocultured PN. When the background P(O2) was lowered to levels typical of arterial blood (approximately 90 mmHg), robust PN chemoexcitation could be induced by physiological hypoglycaemia (3.3-4 mM glucose). These sensory responses were reversibly inhibited by a combination of purinergic (suramin, 50 microM) and nicotinic (mecamylamine, 1 microM) receptor blockers, suggesting that transmission depended on corelease of ATP and ACh. Hypoglycaemic responses were additive with those evoked by hypoxia or hypercapnia; further, they could be potentiated by the GABAB receptor blocker (CGP 55845) and inhibited by 5-HT2A receptor blockers (ketanserin or ritanserin). During paired simultaneous recordings from a PN and a type I cell in an adjacent cluster, the afferent PN response coincided with type I cell depolarization, which was associated with a decrease in input resistance. In fresh tissue slices of rat CB, low glucose stimulated ATP secretion as determined by the luciferin-luciferase assay; this secretion was cadmium sensitive, potentiated by CGP 55845, and inhibited by ketanserin. Taken together these data indicate that CB receptors act as direct glucosensors, and that processing of hypoglycaemia utilizes similar neurotransmitter and neuromodulatory mechanisms as hypoxia.

The GABA(B) receptor antagonist CGP 55845A reduces presynaptic GABA(B) actions in neocortical neurons of the rat in vitro.[Pubmed:10501448]

Neuroscience. 1999;93(4):1241-9.

Use-dependent depression of inhibitory postsynaptic potentials was investigated with intracellular recordings and the paired-pulse paradigm in rat neocortical neurons in vitro. Pairs of stimuli invariably reduced the second inhibitory postsynaptic potential-A (GABA(A) receptor-mediated inhibitory postsynaptic potential) of a pair; at interstimulus intervals of 500 ms, the amplitude of the second inhibitory postsynaptic potential-A was considerably smaller than the first (36.2 +/- 6.2%, n= 17). Decreasing the interstimulus interval reduced the second inhibitory postsynaptic potential-A further and with interstimulus intervals shorter than 330 ms the compound excitatory postsynaptic potential-inhibitory postsynaptic potential response reversed from a hyperpolarizing to a depolarizing response. The depression of the inhibitory postsynaptic potential-A exhibited a maximum at interstimulus intervals near 150 ms and recovered with a time constant of 282 +/- 96.2 ms. Elimination of excitatory transmission by the application of 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and D(-)-2-amino-5-phosphonovaleric acid yielded an essentially unaltered time-course of paired-pulse depression (maximum depression near 150 ms, time constant of recovery 232 +/- 98 ms). The polarity change of the compound excitatory postsynaptic potential response at shorter interstimulus intervals was abolished in the presence of CNQX and D(- )-2-amino-5-phosphonovaleric acid. CNQX and D(-)-2-amino-5-phosphonovaleric acid also reduced the apparent depolarizing shift of the reversal potential between the first and second inhibitory postsynaptic potential-A from about 6 mV to less than 2 mV. Application of the GABA(B) receptor antagonist CGP 55845A in the presence of CNQX and (-)-2-amino-5-phosphonovaleric acid abolished the inhibitory postsynaptic potential-B and paired-pulse depression. Under these conditions, the amplitude of the second inhibitory postsynaptic potential was, on average, about 90% of the first, i.e. reduced by about 10%. The second inhibitory postsynaptic potential-A was approximately constant at interstimulus intervals between 100 and 500 ms. It is concluded that paired-pulse depression of cortical inhibition is predominantly mediated by presynaptic GABA(B) receptors of GABAergic interneurons. The abolition of net inhibition at interstimulus intervals near 330 ms may facilitate spread of excitation and neuronal synchrony during repetitive cortical activation near 3 Hz. This use-dependent depression of inhibition may contribute to highly synchronized slow electroencephalogram activity during spike-and-wave or delta activity.

Evidence for pharmacologically distinct GABAB receptors associated with cAMP production in rat brain.[Pubmed:8782915]

Brain Res. 1996 May 13;720(1-2):220-4.

Gamma-Aminobutyric acid-B (GABAB) receptors mediate a variety of cellular functions, suggesting the possibility of pharmacologically and molecularly distinct receptors. To explore this possibility a number of GABAB receptor agonists and antagonists were examined for their ability to influence cAMP production in rat brain cerebral cortical slices. While the agonists did not differentiate between receptors associated with the augmentation of isoproterenol-induced cAMP production and those mediating inhibition of forskolin-stimulated second messenger accumulation, significant differences were noted between the potencies of some antagonists to inhibit these GABAB receptor-mediated responses. The results suggest at least two pharmacologically distinct subclasses of GABAB receptors regulate cAMP production in brain.

GABA and glutamate release affected by GABAB receptor antagonists with similar potency: no evidence for pharmacologically different presynaptic receptors.[Pubmed:7889310]

Br J Pharmacol. 1994 Dec;113(4):1515-21.

1. The effects of a series of nine GABAB receptor antagonists of widely varying potencies on electrically stimulated release from cortical slices of [3H]-GABA in the absence or presence of 10 microM of the GABAB agonist, (-)-baclofen and of endogenous glutamate in the presence of (-)-baclofen were compared. 2. The concentrations of the compounds half maximally increasing [3H]-GABA release (EC50's) at a stimulation frequency of 2 Hz correlated well with the IC50 values obtained from the inhibition of the binding of the agonist, [3H]-CGP 27492, to GABAB receptors in rat brain membranes (rank order of potency: CGP 56999 A > or = CGP 55845 A > CGP 52432 > or = CGP 56433 A > CGP 57034 A > CGP 57070 A > or = CGP 57976 > CGP 51176 > CGP 35348). 3. Likewise, the concentrations causing half-maximal increases of [3H]-GABA in the absence or presence of (-)-baclofen, and of endogenous glutamate in the presence of (-)-baclofen, correlated well with each other. Reports in the literature suggesting the CGP 35348 exhibits a 70 fold preference for inhibition of (-)-baclofen's effects on glutamate over [3H]-GABA release, and that CGP 52432 shows a 100 fold preference in the opposite sense, could not be confirmed in our model. 4. Therefore, our results suggest that, if there are pharmacological differences between GABAB autoreceptors and GABAB heteroreceptors on glutamatergic nerve endings in the rat cortex, they are not revealed by this series of compounds of widely different potencies. 5. In particular, our results with CGP 35348 and CGP 52432 do not support the hypothesis that GABAB autoreceptors and GABAB heteroreceptors on glutamatergic nerve endings represent subtypes with different pharmacology.