Resumo

Dangerous olfactory cues are able to activate neural structures related to defensive behaviors(DB)including the dorsolateral part of the periaqueductal gray matter (dlPAG). The chemical stimulation of dlPAG in rats evokes DB as the result of internal mood alterations. The present study was outlined to verify if the hypothesis that the aversive state induced by dlPAG chemical stimulation could be used as an unconditioned stimulus(US)in an olfactory fear conditioning (OFC) paradigm and therefore, investigate the anatomical pathway involved in this chemical stimulation-based fear conditioning (dlPAG-OFC). In Experiment I, male Wistar rats were microinjected into the dlPAG with NMDA 100 pmol (NA) or PBS (PA) and exposed to a chamber saturated with amylacetate odor (AMYL), for the conditioning session. Another two groups were used for model validation: PBS (PN) or NMDA (NN), without odor exposure. The expression phase consisted of 2 consecutive sessions: familiarization and test, performed in a box comprising an open and an enclosed communicating compartment. During the expression phase the DB was represented by: %time approaching the AMYL source (%AT) and %time hiding in the enclosed compartment (%HT). In Experiment 2, rats received saline or nadolol (beta-adrenergic blockade, 10 mg/Kg, IP) 30 min before the dlPAG-OFC. In Experiment 3, the efferent projections from the dlPAG were studied in rats following microinjections of the anterograde axonal tracer Phaseolus vulgaris-leucoagglutinin into the dlPAG. Furthermore, to test the hypothesis of a thalamic mediation of the dlPAG-OFC, rats underwent a bilateral iontophoretic NMDA lesions aimed at the ventral part of anteromedial thalamic nucleus (AMv) and the reuniens nucleus (RE). The protocols were approved according to the Animal Ethics Committee (23080.0055752/2006-64/UFSC). Regardless the behavioral measure, no statistical differences were detected among the groups’ performance during the familiarization session. ANOVA followed by Newman-Keuls test revealed that rats conditioned with NMDA in the dlPAG significantly (p<0.05) increased the DB during the test, spending less time approaching the AMYL (%AT= 12±1), and more time hiding (%HT= 75±2) than another groups (PA, PN, NN) (%AT=25±1; %HT=52±2). In Experiment 2, the dlPAG-OFC was not impaired by nadolol [NA: (%AT=18±2; %HT= 62±4); SAL :( %AT=19±3; %HT=64±5)]. In Experiment 3, the output of dlPAG was shown to be directed to the anterior hypothalamus nucleus (AHN) and subfornical region of the lateral hypothalamic area (LHAsf), both of which serving as putative relays to the thalamic sites. The AMv+Re lesion impaired the dlPAG-OFC acquisition decreasing the HT in the test session (%HT= 38±4) when compared with control group (%HT=63±5). In conclusion, the present study has confirmed our hypothesis that an enhanced activity of dlPAG can be used as an US biologically relevant, capable of generating an OFC. The results also highlights the pathway from the dlPAG to the AHN, and the AHN projection to the thalamus (AMv+RE) likely to carry critical information related to fear-like state related to learning and memory. Apart from the role of the dlPAG in the expression of DB, the present findings provide evidences that this brain area can also support OFC by signaling forebrain associative areas involved in fear mediation.