Serotonin-2C receptor agonists decrease potassium-stimulated GABA release in the nucleus accumbens

Abstract

The serotonin 5-HT2C receptor has shown promise in vivo as a pharmacotherapeutic target for alcoholism. For example, recently, a novel 4-phenyl-2-N,N-dimethylaminotetralin (PAT) drug candidate, that demonstrates 5-HT2C receptor agonist activity together with 5-HT2A/2B receptor inverse agonist activity, was shown to reduce operant responding for ethanol after peripheral administration to rats. Previous studies have shown that the 5-HT2C receptor is found throughout the mesoaccumbens pathway and that 5-HT2C receptor agonism causes activation of ventral tegmental area (VTA) GABA neurons. It is unknown what effect 5-HT2C receptor modulation has on GABA release in the nucleus accumbens core (NAcc). To this end, microdialysis coupled to capillary electrophoresis with laser-induced fluorescence was used to quantify extracellular neurotransmitter concentrations in the NAcc under basal and after potassium stimulation conditions, in response to PAT analogs and other 5-HT2C receptor modulators administered by reverse dialysis to rats. 5-HT2C receptor agonists specifically attenuated stimulated GABA release in the NAcc while 5-HT2C antagonists or inverse agonists had no effect. Agents with activity at 5-HT2A receptors had no effect on GABA release. Thus, in contrast to results reported for the VTA, current results suggest 5-HT2C receptor agonists decrease stimulated GABA release in the NAcc, and provide a possible mechanism of action for 5HT2C -mediated negative modulation of ethanol self-administration.

Keywords: 5-HT2C receptor; GABA; capillary electrophoresis; microdialysis; nucleus accumbens.

Figure 1

Coronal sections showing microdialysis probe placement within the NAcc. Lines indicate the active dialysis regions. Numbers below the figure represent the position of the slice relative to bregma. Figure was adapted from Paxinos and Watson, 2005.

Figure 2

Agonists for the 5-HT2C receptor decreased potassium-stimulated GABA release in the NAcc. The concentration of neurotransmitter in the dialysate is displayed over time. Grey shaded areas represent perfusion with 50 mM potassium containing aCSF. Black bar indicates when 50 μM 5-HT2C receptor agonist was added to the aCSF. (A, B) (−)-trans-PAT attenuated potassium stimulated GABA release but did not affect taurine release. Potassium stimulated GABA release was also attenuated by the 5-HT2C agonists (C) (−)-trans-p-Cl-PAT, (D) m-Cl-6-OMe-PAT, and (E) Ro60-0175. (F) The 5-HT2 receptor inverse agonist (−)- trans-CAT had no significant effect on potassium stimulated GABA release. The data shown are mean values ± SEMs for n = 3 in each panel. * indicates p < 0.05 by AUC comparison.

Figure 3

Ketanserin blocks 5-HT2C agonist induced attenuation of potassium stimulated GABA release. The concentration of GABA in the dialysate is displayed over time. Grey shaded areas represent perfusion with 50 mM potassium containing aCSF. White bar signifies when 50 μM ketanserin was present. Black bar indicates when 50 μM 5-HT2C receptor agonist was present. (A) Ketanserin alone has no effect on potassium stimulated GABA release. However, when constantly perfused throughout the experiment, ketanserin blocks (B) (−)-trans-PAT, (C) (−)-trans-p-Cl-PAT, and (D) m-Cl-6-OMe-PAT from attenuating potassium stimulated GABA release. The data shown are mean values ± SEMs for n = 3 in each panel. * indicates p < 0.05 by AUC comparison.

Figure 4

Mepyramine, an H1 antagonist, fails to block 5-HT2C agonist induced attenuation of potassium stimulated GABA release. Grey shaded areas represent perfusion with 50 mM potassium containing aCSF. White bar signifies when 50 μM mepyramine was present. Black bar indicates when 50 μM (−)-trans-PAT was present. (A) Mepyramine alone has no effect on potassium stimulated GABA release. (B) When constantly perfused throughout the experiment, mepyramine fails to block (−)-trans-PAT from attenuating potassium stimulated GABA release. The data shown are mean values ± SEMs for n = 3 in each panel. *indicates p < 0.05 by AUC comparison.