Precise Localization of α7 Nicotinic Acetylcholine Receptors on Glutamatergic Axon Terminals in the Rat Ventral Tegmental Area

169

110

In this paper, we report on the pharmacological and functional profile of SSR180711 (1,4-Diazabicyclo[3.2.2]nonane-4-carboxylic acid, 4-bromophenyl ester), a new selective a7 acetylcholine nicotinic receptor (n-AChRs) partial agonist. SSR180711 displays high affinity for rat and human a7 n-AChRs (K i of 2274 and 1471 nM, respectively). Ex vivo 3 [H]a-bungarotoxin binding experiments demonstrate that SSR180711 rapidly penetrates into the brain (ID 50 ¼ 8 mg/kg p.o.). In functional studies performed with human a7 n-AChRs expressed in Xenopus oocytes or GH4C1 cells, the compound shows partial agonist effects (intrinsic activity ¼ 51 and 36%, EC 50 ¼ 4.4 and 0.9 mM, respectively). In rat cultured hippocampal neurons, SSR180711 induced large GABA-mediated inhibitory postsynaptic currents and small a-bungarotoxin sensitive currents through the activation of presynaptic and somato-dendritic a7 n-AChRs, respectively. In mouse hippocampal slices, the compound increased the amplitude of both glutamatergic (EPSCs) and GABAergic (IPSCs) postsynaptic currents evoked in CA1 pyramidal cells. In rat and mouse hippocampal slices, a concentration of 0.3 mM of SSR180711 increased long-term potentiation (LTP) in the CA1 field. Null mutation of the a7 n-AChR gene totally abolished SSR180711-induced modulation of EPSCs, IPSCs and LTP in mice. Intravenous administration of SSR180711 strongly increased the firing rate of single ventral pallidum neurons, extracellularly recorded in anesthetized rats. In microdialysis experiments, administration of the compound (3-10 mg/kg i.p.) dosedependently increased extracellular acetylcholine (ACh) levels in the hippocampus and prefrontal cortex of freely moving rats. Together, these results demonstrate that SSR180711 is a selective and partial agonist at human, rat and mouse a7 n-AChRs, increasing glutamatergic neurotransmission, ACh release and LTP in the hippocampus.

Section: Discussionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

SSR180711, a Novel Selective α7 Nicotinic Receptor Partial Agonist: (1) Binding and Functional Profile

Biton

Bergis

Galli

et al. 2006

169

110

“…This mechanism is supported by our previous demonstration that local administration of MLA in the VTA reduces food-induced dopamine release in the nucleus accumbens (Schilström et al, 1998b). Moreover, a recent anatomical study demonstrated that a7 nAChRs in the VTA may be located presynaptically at sites indicative of a paracrine mode of signaling (Jones and Wonnacott, 2004).…”

Section: Discussionmentioning

confidence: 99%

Galantamine Enhances Dopaminergic Neurotransmission In Vivo Via Allosteric Potentiation of Nicotinic Acetylcholine Receptors

Schilström

Ivanov

Wiker

et al. 2006

115

Clinical studies suggest that adjunct galantamine may improve negative and cognitive symptoms in schizophrenia. These symptoms may be related to impaired dopaminergic function in the prefrontal cortex. Indeed, galantamine has been shown to increase dopamine release in vitro. Galantamine is an allosteric modulator of nicotinic acetylcholine receptors (nAChRs) and, at higher doses, an acetylcholine esterase (AChE) inhibitor. We have previously shown that nicotine, through stimulation of nAChRs in the ventral tegmental area (VTA), activates midbrain dopamine neurons and, hence, potentiation of these receptors could be an additional mechanism by which galantamine can activate dopaminergic pathways. Therefore, the effects of galantamine (0.01-1.0 mg/kg s.c.) on dopamine cell firing were tested in anaesthetized rats. Already at a low dose, unlikely to result in significant AchE inhibition, galantamine increased firing activity of dopaminergic cells in the VTA. The effect of galantamine was prevented by the nAChR antagonist mecamylamine (1.0 mg/kg s.c.), but not the muscarinic receptor antagonist scopolamine (0.1 mg/kg s.c.), and it was not mimicked by the selective AChE inhibitor donepezil (1.0 mg/kg s.c.). Our data thus indicate that galantamine increases dopaminergic activity through allosteric potentiation of nAChRs. Galantamine's effect was also prevented by the a7 nAChR antagonist methyllycaconitine (6.0 mg/kg i.p.) as well as the N-methyl-Daspartate antagonist CGP39551 (2.5 mg/kg s.c.), indicating a mechanism involving presynaptic facilitation of glutamate release. In parallel microdialysis experiments, galantamine was found to increase extracellular levels of dopamine in the medial prefrontal cortex. These results may have bearing on the enhancement of negative and cognitive symptoms in schizophrenia.

“…Nonetheless, acute ethanol can also reduce the postsynaptic effects of NMDA and non-NMDA receptors mediating excitation (see Crews et al, 1996)Fanother change that would reduce excitatory drive and allow greater effectiveness of GABA released by ethanol. As with GABA transmission, a number of presynaptic mechanisms can influence glutamate release (see Brown et al, 2004;Jones and Wonnacott, 2004). Since ethanol increases neurosteroids, another aspect that should receive future attention is the possibility that pregnenolone sulfate may enhance spontaneous glutamate release (see Meyer et al, 2002), while reducing release of GABA (Mtchedlishvili and Kapur, 2003).…”

Section: Ethanol On Excitatory Drivementioning

confidence: 99%

A Conceptualization of Integrated Actions of Ethanol Contributing to its GABAmimetic Profile: A Commentary

Criswell

Breese

2005

Early behavioral investigations supported the contention that systemic ethanol displays a GABAmimetic profile. Microinjection of GABA agonists into brain and in vivo electrophysiological studies implicated a regionally specific action of ethanol on GABA function. While selectivity of ethanol to enhance the effect of GABA was initially attributed an effect on type-1-benzodiazepine (BZD)-GABA A receptors, a lack of ethanol's effect on GABA responsiveness from isolated neurons with this receptor subtype discounted this contention. Nonetheless, subsequent work identified GABA A receptor subtypes, with limited distribution in brain, sensitive to enhancement of GABA at relevant ethanol concentrations. In view of these data, it is hypothesized that the GABAmimetic profile for ethanol is due to activation of mechanisms associated with GABA function, distinct from a direct action on the majority of postsynaptic GABA A receptors. The primary action proposed to account for ethanol's regional specificity on GABA transmission is its ability to release GABA from some, but not all, presynaptic GABAergic terminals. As systemic administration of ethanol increases neuroactive steroids, which can enhance GABA responsiveness, this elevated level of neurosteroids is proposed to magnify the effect of GABA released by ethanol. Additional factors contributing to the degree to which ethanol interacts with GABA function include an involvement of GABA B and other receptors that influence ethanol-induced GABA release, an effect of phosphorylation on GABA responsiveness, and a regional reduction of glutamatergic tone. Thus, an integration of these consequences induced by ethanol is proposed to provide a logical basis for its in vivo GABAmimetic profile.