Psychopharmacology (2004) 176: 362–368 DOI 10.1007/s00213-004-1910-y ORIGINAL INVESTIGATION Fabrício Araújo Moreira . Francisco Silveira Guimarães Benzodiazepine receptor and serotonin 2A receptor modulate the aversive-like effects of nitric oxide in the dorsolateral periaqueductal gray of rats Received: 2 December 2003 / Accepted: 14 April 2004 / Published online: 22 July 2004 # Springer-Verlag 2004 Abstract Rationale: Escape reactions induced by electrical stimulation of the dorsolateral periaqueductal gray (dlPAG) are inhibited by local administration of benzodiazepine (BZ) or serotonin (5-HT) receptor ago- nists. Nitric oxide (NO) is a gas messenger that may mediate aversive behaviors. NO donors injected into the dlPAG induce escape reactions. Objectives: To test the hypothesis that the escape reactions induced by a NO donor in the dlPAG would be attenuated by pre-treatment with BZ-receptor or 5-HT-receptor agonists. Methods: Male Wistar rats with cannulae aimed at the dlPAG received microinjections of vehicle (0.2 μl), the BZ midazolam maleate (80 nmol), the 5-HT 1A -receptor ago- nist 8-OH-DPAT (8 nmol or 16 nmol) or the 5-HT 2A/2C - receptor agonist DOI (16 nmol) 10 min before the administration of the NO donor SIN-1 (150 nmol). Behavioral observation took place immediately after the last injection in an open arena over a 10-min period. Results: SIN-1 induced escape reactions characterized by running and jumps. Pre-treatment with DOI, but not 8- OH-DPAT, partially inhibited the effects of SIN-1. Pre- treatment with midazolam maleate, however, completely prevented the effects of the NO donor. Conclusion: The results suggest that the aversive-like effects of NO donor in the dlPAG may be modulated by the BZ and 5-HT 2A/2C receptors. Keywords Nitric oxide . Flight . Anxiety . Midazolam . 8- OH-DPAT . DOI Introduction Nitric oxide (NO) is a gas messenger synthesized in neurons by the Ca 2+ /calmodulin-dependent enzyme neu- ronal nitric oxide synthase (nNOS). This enzyme synthe- sizes NO from L-arginine and may be activated by the calcium influx induced by glutamate-receptor activation (Garthwaite et al. 1988, 1989). NO may diffuse to pre- synaptic and post-synaptic neurons (Edelman and Gally 1992; Garthwaite 1991; Snyder and Bredt 1991) and activate the enzyme guanylate cyclase (GC), mediating the glutamate-linked enhancement of cGMP (Bredt and Snyder 1989; Garthwaite et al. 1988; Knowles et al. 1989). The enzyme nNOS is significantly present in the periaqueductal gray matter (PAG). This mesencephalic structure has been separated along its rostro-caudal axis into dorsomedial, dorsolateral (dlPAG), lateral and ven- trolateral columns (Carrive 1993). In the PAG, nNOS immunoreactive neurons are localized mainly in the dlPAG (Onstott et al. 1993). The dorsal columns of the PAG are proposed to be part of a neural substrate responsible for the elaboration of active aversive behaviors (Graeff 1981, 1994). Animals in a shuttle-box quickly learn to perform operant conditioning tasks in order to terminate the electrical stimulation of these columns (Delgado et al. 1954; Olds and Olds 1963; Sandner et al. 1987). Electrical or chemical stimulation of the dorsal PAG in freely moving animals leads to escape behaviors characterized by running and jumping (Bandler and Carrive 1988; Krieger and Graeff 1985). In terms of face validity and pharmacological predictability, these mani- festations of unconditioned fear have been proposed as an animal model of panic attacks (Deakin and Graeff 1991; Jenck et al. 1995; Schenberg et al. 2001). Several neurotransmitters have been proposed as mediators or modulators of the behavioral changes induced by dlPAG stimulation. Local injection of drugs that enhance or mimic gamma-aminobutyric acid (GABA)-mediated neurotransmission, such as benzodia- zepines (BZs), inhibits the escape behavior induced by dlPAG electrical stimulation (Audi and Graeff 1984). F. A. Moreira . F. S. Guimarães (*) Department of Pharmacology, FMRP, Campus USP, 14049-900 Ribeirão Preto, SP, Brazil e-mail: fsguimar@fmrp.usp.br Tel.: +55-16-6023209 Fax: +55-16-6332301