¹Rudolf-Boehm-Institute of Pharmacology and Toxicology, University Leipzig, Leipzig, Germany

Correspondence: Dr H Kittner, Rudolf-Boehm-Institute of Pharmacology and Toxicology, University Leipzig, Härtelstrasse 16–18, D-04107 Leipzig, Germany. Tel: +49341 9724627; Fax: +49 341 9724609; E-mail: kith@medizin.uni-leipzig.de

Received 12 February 2002; Revised 1 July 2002; Accepted 23 July 2002.

Abstract

The widespread and abundant distribution of P2Y receptors in the mammalian brain suggests important functions for these receptors in the CNS. To study a possible involvement of the P2Y receptors in the regulation of fear and anxiety, the influences of the P2Y_1,11,12 receptor-specific agonist adenosine 5'-O-(2-thiodiphosphate) (ADPS), the P2X_1,3 receptor agonist ,-methylene ATP (,meATP), the unspecific P2 receptor antagonist pyridoxalphosphate-6-azopheny l-2',4'-disulfonic acid (PPADS), and the specific P2Y₁ receptor antagonist N⁶-methyl-2'-deoxyadenosine-3',5'-bisphosphate (MRS 2179) on the elevated plus-maze behavior of the rat were investigated. All tested compounds were given intracerebroventricularly (0.5 l). ADPS (50 and 500 fmol) produced an anxiolytic-like behavioral profile reflected by an increase of the open arm exploration. The anxiolytic-like effects were antagonized by pretreatment with PPADS (5 pmol) or MRS 2179 (5 pmol). Both compounds caused anxiogenic-like effects when given alone. Furthermore, the anxiolytic-like effects of ADPS could be antagonized by pretreatment with the nitric oxide synthase (NOS) inhibitor N^w-nitro-L-arginine methyl ester (L-NAME). In addition, the anxiogenic-like effects of PPADS were reversed by the pretreatment with L-arginine (500 pmol), which is the natural substrate for NOS, but not by D-arginine (500 pmol), which is not. Immunofluorescence staining revealed the presence of P2Y₁ receptors on neurons in different brain regions such as hypothalamus, amygdala, hippocampus and the periaqueductal gray. Furthermore, the colocalization of P2Y₁ receptors and neuronal NOS (nNOS) on some neurons in these regions could be demonstrated. The highest density of P2Y₁- and nNOS-immunoreactivity was detected in the dorsomedial hypothalamic nucleus. Taken together, the present results suggest that P2Y₁ receptors are involved in the modulation of anxiety in the rat. The anxiolytic-like effects after stimulation of P2Y₁ receptors seem to be in close connection with the related nitric oxide production.