INTRODUCTION

1. CONTEXT FEAR CONDITIONING INHIBITS ACTIVE DEFENSIVE BEHAVIOR ELICITED BY ELECTRICAL STIMULATION OF DORSAL PERIAQUEDUCTAL GRAY: RELATIONSHIP BETWEEN ANXIETY AND PANIC ATTACK J.Landeira-Fernandez1,2*; R.L.Nogueira2,4; V.Magierek3; P.L.Ramos2; N.G.Silveira-Filho3 1. Departamento de Psicologia, PUC/RIO, Rio de Janeiro, Brazil; 2. Curso de Psicologia, Universidade Estacio de Sa, Rio de Janeiro, Brazil; 3. Departamento de Psicobiologia, Universidade de Sao Paulo, Sao Paulo, Brazil; 4. Curso de Psicologia, Universidade de Ribeirao Preto, Ribeirao Preto, Brazil INTRODUCTION Different patterns of animal defensive behaviors have been employed as useful tools for investigating and understanding the underlying mechanisms for different anxiety disorders. Accordingly, flight behavior evoked by electrical stimulation of the dorsal portion of the periaqueductal gray (DPAG) has been proposed as a model of panic attacks (5,6), whereas freezing response to contextual cues previously associated with electrical footshocks as an animal model of anxiety. The present study employed these two animals to investigate the relationship between anxiety and panic attack. This is an important issue since experiments with human patients have lead to conflicting results. There are evidences supporting the view that anxiety might either facilitate or inhibit the occurrence of panic attack. METHODS Male albino Wistar rats were anaesthetized and a bipolar electrode was implanted into the DPAG. Five days after the surgery, each animal was placed inside the experimental chamber for a 6 min habituation period. After that, DPAG was electrically stimulated in order to determine the animal’s DPAG aversive baseline threshold to evoke a vigorous escape response. Following the DPAG electrical stimulation, one group of animals (DPAG/SHOCK) was submitted to the context fear conditioning procedure. Conditioning consisted in the presentation of 5 unsignaled two-sec 1 mA electrical footshocks with a 1 min intershock interval. A no-shock control group (DPAG/NO SHOCK) had exactly the same procedure as the first group with the exception that no footshock was delivered. Testing session took place two hours later. During this phase, all animals were reexposed to the experimental chamber and freezing behavior was recorded for 5 min. Freezing behavior was scored through a time-sample procedure. For each 2 s, an experimenter, who was blind to the experimental conditions, rated the animal's behavior as freezing or activity. Freezing was defined as absence of visible body movements, except the activity necessary for respiration. After the freezing scoring period, a new DPAG aversive threshold to induce an escape response was determined. At the end of the experiment, animals were sacrificed and the brains were removed for histological analysis. DISCUSSION Results from the present experiment clearly indicate that contextual cues previously associated with footshock induced defensive freezing behavior and inhibited active defensive reaction evoked by electrical stimulation of the DPAG. Since context fear conditioning is a model of anxiety whereas electrical stimulation of the DPAG is related to panic attack, it is concluded that an increase in anxiety might cause a decrease in panic attack. In accordance with this view, it has been reported that relaxation therapy employed to reduce anxiety symptoms may precipitate panic attacks. Moreover, the frequency of panic attacks is higher at the beginning of agoraphobia, when there is little anticipatory anxiety, as compared with the late phase, when anxiety has fully developed. Pharmacological results also support the suggestion that enhancement of anxiety inhibits the occurrence of panic attacks. Panic disorder patients treated with serotonergic reuptake blocking drugs show a decrease in panic attack but an increase in anxiety. Therefore, it appears that activation of neural circuitry involved in anxiety might in fact inhibit the incidence of panic attacks. RESULTS A representative histological section showing the location of the electrode tip is presented in Figure 1. Histological examination of the brain slices indicated that all electrode tips were located inside or at the borders of DPAG. A- AVERSIVE CONTEXT CONDITIONING B- ELECTRICAL CURRENT THRESHOLD Figure 1- Photomicrograph showing a typical example of a stimulation electrode site (arrow). Aq, aqueduct of Sylvius; DPAG, dorsal periaqueductal gray; DlPAG, dorsolateral periaqueductal gray; VlPAG, ventrolateral periaqueductal gray; VPAG, ventral periaqueductal gray. The histological section was located 7.6 mm posterior to bregma. Figure 2a presents the mean (+ SEM) of freezing behavior during the 5-min testing session 2 hours after rats were exposed (DPAG / SHOCK) or not (DPAG / NO SHOCK) to 5 unsignaled footshocks. The results are clear cut and indicate that animals exposed to contextual cues that were previously associated with electrical footshocks displayed more freezing than control non-shocked animals (t(20)=5,8; p<0.001). The high levels of freezing among the animals in the DPAG/SHOCK group is in accordance with previous findings which report that unsignaled footshock reliably leads to context fear conditioning (7). Figure 2b presents the mean (+ SEM) change in DPAG electrical current aversive threshold between test and baseline phases of the experiment. As can be observed, current threshold difference (D) between testing and baseline phases was greater among animals previously exposed contextual cues previously associated with electrical footshocks when compared to no-shock control group (t(20) 2,3; p<0,05). The fact that DPAG/NO SHOCK group presented a negative D-threshold indicates that some animals in this group presented a lower current aversive threshold during the test phase as compared to their baseline aversive threshold. The opposite pattern was observed among the animals in the DPAG/SHOCK group. Therefore, these results indicate that context fear conditioning was able to inhibit active flight reactions evoked by electrical stimulation of the DPAG. Acknowledgements This work was supported by CNPq (Proc. No. 94/5933-2), University of Estácio de Sá (UNESA), University of Riberão Preto (UNAERP) and AFIP. Figure 2- (A) – Mean percent (+ SEM) of time spent engaged in freezing behavior during testing session 2 hours after rats were exposed (DPAG / SHOCK) or not exposed (DPAG / NO SHOCK) to 5 unsignaled footshocks. (B) – Mean (+ SEM) of the difference (D) between testing and baseline DPAG electrical current threshold to induce an escape behavior. * p<0.05.