Inhibition of the NMDA receptor/Nitric Oxide pathway in the dorsolateral periaqueductal gray causes anxiolytic-like effects in rats submitted to the Vogel conflict test

Abstract

Background: Several studies had demonstrated the involvement of the dorsolateral portion of periaqueductal grey matter (dlPAG) in defensive responses. This region contains a significant number of neurons containing the enzyme nitric oxide synthase (NOS) and previous studies showed that non-selective NOS inhibition or glutamate NMDA-receptor antagonism in the dlPAG caused anxiolytic-like effects in the elevated plus maze.

Methods: In the present study we verified if the NMDA/NO pathway in the dlPAG would also involve in the behavioral suppression observed in rats submitted to the Vogel conflict test. In addition, the involvement of this pathway was investigated by using a selective nNOS inhibitor, Nomega-propyl-L-arginine (N-Propyl, 0.08 nmol/200 nL), a NO scavenger, carboxy-PTIO (c-PTIO, 2 nmol/200 nL) and a specific NMDA receptor antagonist, LY235959 (4 nmol/200 nL).

Results: Intra-dlPAG microinjection of these drugs increased the number of punished licks without changing the number of unpunished licks or nociceptive threshold, as measure by the tail flick test.

Conclusion: The results indicate that activation of NMDA receptors and increased production of NO in the dlPAG are involved in the anxiety behavior displayed by rats in the VCT.

Figure 1

Microinjections sites. Photomicrograph of a coronal brain section showing an unilateral microinjection site in the dorsal periaqueductal gray and a histological localization of injection sites (200 nL) in diagrams based on the atlas of Paxinos and Watson [23]. The solid and the open circles represent injection sites inside of the dlPAG. Numbers represent distances from interaural level (mm). dPAG: dorsal periaqueductal gray; dlPAG: dorsolateral periaqueductal gray.

Figure 2

Vogel conflict test. Effects of LY235959 (4 nmol/200 nL), Nω-propyl-L-arginine (0.04 nmol/200 nL) and c-PTIO (1 nmol/200 nL) injected into the dlPAG of rats submitted to the Vogel test. Bars represent the mean ± SEM total number of punished licks in the 3 min session. Animals that received drugs outside the dlPAG were included as OUT groups. Asterisk indicates significant difference from vehicle (P < 0.05, ANOVA followed by the Dunnet's test; vehicle (veh, n = 10), LY235959 (n = 8), Nω-propyl-L-arginine (n = 7), c-PTIO (n = 9); OUT-LY235959 (n = 11), OUT-Nω-propyl-L-arginine (n = 5) and OUT-c-PTIO (n = 8).

Figure 3

Water consume test. Effect of LY235959 (4 nmol/200 nL), Nω-propyl-L-arginine (0.04 nmol/200 nL) and c-PTIO (1 nmol/200 nL) injected into the dlPAG in the water consumption test during training (Day 1) and test (Day 2). Vehicle (veh, n = 6) LY235959 (n = 6), Nω-propyl-L-arginine (n = 6), c-PTIO (n = 6).

Figure 4

Tail flick nociception test. Time course of the effects of vehicle (veh, n = 6), LY235959 (n = 6), Nω-propyl-L-arginine (n = 6), c-PTIO (n = 6) or Morphine 5 mg/kg (n = 4) in the tail flick test. Each point represents the mean ± S.E.M. for the latency of tail withdrawal. Asterisk indicates significant difference from vehicle (P < 0.05, ANOVA followed by the Dunnet's test).