GABA_B receptor antagonist CGP-36742 enhances somatostatin release in the rat hippocampus in vivo and in vitro

Abstract

Here, we show the modulation of somatostatin functions in the hippocampus by the orally active ‘cognition enhancer’ GABA_B receptor antagonist, (3-aminopropyl)n-butylphosphinic acid (CGP-36742), both in vivo and in vitro. Using high-pressure liquid chromatography-coupled electrospray mass spectrometry, we measured a two-fold increase in the extracellular level of somatostatin to CGP-36742 application in the hippocampus of anaesthetised rats. The basal release of [¹²⁵I]somatostatin in the synaptosomal fraction was increased by CGP-36742 in concentrations lower than 1 μM. Simultaneous measurement of [¹⁴C]Glu and [³H]γ-aminobutyric-acid ([³H]GABA) showed that CGP-36742 increased their basal release. However, prior [¹²⁵I]somatostatin application suppressed the increase in the basal release of [¹⁴C]Glu and induced a net decrease in the basal release of [³H]GABA. Somatostatin application had a similar effect. In slices, CGP-36742 increased the postsynaptic effect of somatostatin on CA1 pyramidal cells. These results suggest a pre- and postsynaptic functional ‘cross-talk’ between coexisting GABA_B and somatostatin receptors in the rat hippocampus.

Introduction

GABA_B receptors are often located presynaptically on axon terminals and are commonly believed to modulate the release of different neurotransmitters Bonanno and Raiteri, 1993, Bowery, 1993. Isolation of GABA_B1 and GABA_B2 receptor proteins Kaupmann et al., 1997, White et al., 1998 revealed the heterodimeric structure of active GABA_B receptors Marshall et al., 1999, White et al., 1998, Bowery and Enna, 2000, Bowery et al., 2002. Although molecular biology has not revealed subtype-multiplicity of GABA_B receptors as yet, several lines of evidence suggest that pharmacologically distinct GABA_B receptor subtypes may exist Bonanno and Raiteri, 1993, Kerr and Ong, 1996, Deisz et al., 1997, Bonanno et al., 1999, Ong et al., 2001.

The orally active GABA_B receptor antagonist, (3-aminopropyl)n-butylphosphinic acid (CGP-36742), has been demonstrated to be active both at pre- and postsynaptic GABA_B receptors, including autoreceptors regulating the release of γ-aminobutyric acid (GABA) in the cerebral cortex Olpe et al., 1993, Froestl et al., 1995 and in the hippocampus as well (Pozza et al., 1999). CGP-36742-sensitive receptors have been suggested to mediate Glu release in other central areas in the central nervous system, also Teoh et al., 1996, Nyitrai et al., 1999. Furthermore, in superfused rat and human cerebrocortical synaptosomes, CGP-36742 was shown to antagonise the baclofen-induced inhibition of the release of the cyclopeptide, and has been suggested to act selectively on a pharmacologically distinct presynaptic GABA_B receptor subtype that regulates the release of somatostatin Bonanno and Raiteri, 1993, Gemignani et al., 1994, Bonanno et al., 1999. CGP-36742 is reported to facilitate memory in a social recognition test in rats (Mondadori et al., 1996) and to improve cognitive performance in different species Mondadori et al., 1993, Froestl et al., 1995, so it has been selected as a drug candidate for the treatment of cognition deficits Bittiger et al., 1992, Froestl et al., 1995.

Somatostatin and GABA are co-transmitters of interneurons that synapse with pyramidal cells (Freund and Buzsáki, 1996), and each subtype of somatostatin receptor is present in the hippocampus of rats (for a review see Selmer et al., 2000). Somatostatin receptors are suggested to modulate GABAergic synaptic transmission in the thalamus (Leresche et al., 2000) and both Gluergic and GABAergic transmission in the hippocampus Boehm and Betz, 1997, Gardette et al., 1995, Scharfman and Schwartzkroin, 1989, Tallent and Siggins, 1997, Xie and Sastry, 1992.

Somatostatin is reported to have promnesic effect in animal models of dementia (Matsuoka et al., 1995) as well as a facilitatory role in long-term potentiation Nakata et al., 1996, Kaneko et al., 1997. Acute stress and dexamethasone have been shown to induce the rapid synthesis and release of somatostatin from the dentate gyrus hilus (Arancibia et al., 2001). It may be conjectured that somatostatin levels are decreased in Alzheimer's disease patients Davies et al., 1980, Grouselle et al., 1998, suggesting that increased levels of somatostatin may help to restore memory function. How somatostatin affects the neural mechanisms of memory formation, however, is far from being understood. Therefore, we studied the effect of GABA_B receptor antagonist, CGP-36742, on somatostatin level and functions in the rat hippocampus since these changes might correlate with the ‘cognition enhancer’ property of the compound. To investigate this possibility, we used a strategy of comparing the ability of CGP-36742 (i) to modify hippocampal somatostatin levels in vivo and in vitro and (ii) to influence pyramidal cell excitability by affecting the postsynaptic effect of somatostatin. In addition, we measured the effect of CGP-36742 and somatostatin receptor activation on basal GABA and glutamate releases from nerve endings in vitro.