Blockade of Alpha1 Adrenoreceptors in the Dorsal Raphe Nucleus
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Behavioural Brain Research 134 (2002) 387Á/392 www.elsevier.com/locate/bbr Research report Blockade of alpha1 adrenoreceptors in the dorsal raphe nucleus prevents enhanced conditioned fear and impaired escape performance following uncontrollable stressor exposure in rats Ruth E. Grahn a,*, S.E. Hammack a, M.J. Will b, K.A. O’Connor a, T. Deak c, P.D. Sparks a, L.R. Watkins a, S.F. Maier a a Department of Psychology, Campus Box 345, University of Colorado, Boulder, CO 80309, USA b Department of Psychology, WISPIC, 6001 Research Park Blvd, Rm 1026, Madison, WI 53719, USA c Department of Psychology, Terrence Deak, SUNY-Binghamton, Binghampton, NY 13902-6000, USA Received 16 August 2001; received in revised form 28 February 2002; accepted 28 February 2002 Abstract Previous research has shown that the effect of exposure to uncontrollable stressors on conditioned fear responding and escape behavior in rats is dependent on serotonergic neural activity in the dorsal raphe nucleus (DRN). The role that norepinephrine released in the DRN plays in producing the behavioral consequences of exposure to inescapable tail shock in rats was investigated in the present study. The selective alpha1 adrenoreceptor antagonist benoxathian was injected into the DRN before exposure to inescapable tail shock or before behavioral testing conducted 24 h later. Benoxathian prevented the impairment of escape responding produced by inescapable shock, but did not reverse this effect when given before testing. The enhancement of conditioned fear produced by prior inescapable shock was attenuated by benoxathian administered before inescapable shock or before behavioral testing. These results support the view that noradrenergic input to the DRN is necessary to produce the behavioral effects of inescapable tail shock. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Learned helplessness; Controllability; Stress; Anxiety; Depression; Serotonin; Animal models The impact of a stressful situation often depends on [25,31,40], dopamine [5], serotonin [24,30], gamma whether or not the stressor can be predicted, avoided, amino butyric acid [10,11], corticotropin releasing escaped or otherwise controlled by an individual. hormone [41], and adenosine [26] in the development Stressors that are not controllable have been shown to and expression of learned helplessness behavior. This have greater negative impact on behavior compared to neurochemical evidence has often been characterized in controllable stressors (see Maier, for a review [18]). combination with anatomical investigations, which have Behavioral effects and physiological effects that occur shown that the locus coeruleus [39,41], hippocampus only after uncontrollable, but not after controllable [5,31], cortex [35], hypothalamus [5], and dorsal raphe stressors, are referred to as learned helplessness effects nucleus [24] are important brain regions involved in [22]. learned helplessness behavior. Much work has been devoted to understanding the One recent hypothesis regarding the neural basis of neural basis of learned helplessness. This extensive work learned helplessness is focused on the role of serotonin has provided evidence for a role for acetylcholine [4], produced in the dorsal raphe nucleus (DRN) [18]. Maier endogenous opioid peptides [9], norepinephrine et al. have provided substantial evidence that exposure to an uncontrollable stressor (a series of 100 inescapable tail shocks) increases the activity of serotonin neurons in * Corresponding author. Present address: Box 5275, Connecticut the DRN to an extent such that they become transiently College, New London, CT 06320-4196, USA. Tel.: /1-860-439-2387; sensitized to subsequent input [24]. While the behavioral fax: /1-860-439-5300 E-mail address: [email protected] (R.E. Grahn). testing situation (a series of 30 mild escapable foot 0166-4328/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 6 - 4 3 2 8 ( 0 2 ) 0 0 0 6 1 - X 388 R.E. Grahn et al. / Behavioural Brain Research 134 (2002) 387Á/392 shocks) induces escape responses and conditioned fear the center to allow the subject to pass from one side to responding in naı¨ve rats, subjects exposed previously to the other. Bright illumination was provided by a 28 V an uncontrollable stressor show impaired escape re- bulb, and background noise was provided by ventilation sponding [33] and exaggerated fear responses [17]. These fans. The clear Plexiglas front and top of each shuttle learned helplessness effects are prevented and often box allowed for behavioral observation. reversed by manipulations that decrease serotonin Rats were implanted with a guide cannula approxi- neural activity in the DRN [14,20,21] and are produced mately 2 weeks prior to each experiment. The rats were by manipulations that increase serotonin neural activity anesthetized with 60 mg/kg ketamine and 13 mg/kg in the DRN [13,19]. Thus, increased serotonin neural xylazine. A 13 mm 26 ga stainless steel cannula was activity in the DRN appears to be necessary and positioned 1 mm above the DRN using coordinates sufficient to produce learned helplessness effects. from interaural reference at AP /0.7 mm, ML 0.0 mm, Further evidence for this serotonin hypothesis has DV /4.5 mm according to the Paxinos and Watson been provided by in vivo microdialysis experiments in atlas [29]. The cannula was secured with dental acrylic which increased extracellular levels of serotonin are applied to the cannula and screws anchored in the measured in the DRN [24] and in some of its projection surrounding skull. The cannula was kept patent by a regions [2,3] during and after the rat’s exposure to an stainless steel stylet. Animals were monitored daily and uncontrollable stressor. Lastly, direct evidence that an handled twice after surgery. uncontrollable stressor, more than a controllable stres- Injections were made by removing the stylet and sor, increases serotonin neural activity is provided by replacing it with a 14 mm microinjector made from 33 greater expression of the immediate early gene product ga stainless steel tubing. A total volume of 1 ml was Fos in serotonin neurons in the DRN under the injected slowly over a period of 30 s and the micro- uncontrollable stressor condition [15]. injector was left in place for 2 min while the rat was Given that increased serotonin neural activity in the gently restrained in a towel. Cannula placements were DRN is important for producing and maintaining verified histologically at the completion of the experi- learned helplessness behaviors, we have attempted to ment. Six subjects were excluded either because dye identify potential sources of excitatory influence that are injected through the cannula was evident in the ven- related to these behaviors produced by inescapable tricles or in brain sites other than the region of the shock (IS). Thus, glutamatergic input to the DRN was DRN. demonstrated to be a source of excitation related to the IS or restraint was applied approximately 10 min after effects of uncontrollable stressors [14]. Also, endogen- drug injections. IS consisted of 100 1.0 mA 5 s tail ous opiates within the DRN are involved in learned shocks delivered on average every 60 s while the control helplessness behavior [13,36]. The present study was condition was restraint without shocks. Rats were focused on exploring a third source of excitation, that returned to their home cage immediately after the 2-h provided by norepinephrine. There is evidence that norepinephrine has an excitatory effect on DRN ser- session. Conditioned fear and shuttle box escape per- otonin neurons [37], which depends on activation of formance was assessed in the same subjects 24 h after alpha1 adrenoreceptors [1]. This is especially interesting stressor pretreatment in the shuttle boxes. Conditioned because there are many studies supporting the view that fear was measured as the occurrence of freezing, defined norepinephrine is important in learned helplessness as the complete lack of movement except that required behavior [5,31,41], and its influence within the DRN for respiration. Rats were placed individually in the would help explain how norepinephrine and serotonin shuttle box and freezing observed for 10 min. The interact to produce learned helplessness behaviors. subjects were observed every 8 s by an observer unaware of group membership and scored as freezing or not Forty-five male naı¨ve SpragueÁ/Dawley rats obtained from Harlan Laboratories were used as subjects. They freezing. Each rat then received two 0.8 mA foot shocks were approximately 100 days old at the time of surgery through the grid floor of the box, each of which could be and were housed individually under a 12-h light/dark terminated by crossing to the other side of the box cycle with free access to standard rodent pellets and through an archway. Freezing behavior was then water. The experiment was conducted with the approval assessed for an additional 20 min as above. After the of the University of Colorado Institutional Animal Care observations were complete, the rats received three and Use Committee. IS or restraint was administered to further single-crossing escape trials (fixed ratio-1; FR- rats in Plexiglas tubes measuring 17.5 cm in length and 7 1) followed by 25 trials in which a back-and-forth cm in diameter. Two clip electrodes were positioned on crossing (FR-2) was required to terminate shock. Trials the distal portion of the tail. Behavioral testing occurred occurred on average every 60 s and shock intensity in shuttle boxes measuring 46/20.7/20 cm (L/W/ continued at 0.8 mA. Escape latency was recorded H). Foot shocks were delivered through a grid floor. automatically and each trial was terminated at 30 s if Each shuttle box included a stainless steel archway in no response was made. R.E. Grahn et al. / Behavioural Brain Research 134 (2002) 387Á/392 389 Fig. 1. Freezing was measured every 8 s for 20 min (15 observations in each 2-min block) following the presentation of two 0.8 mA scrambled foot shocks.