Effects of Antidepressant Treatment on Inhibitory Avoidance Behavior and Amygdaloid b-Adrenoceptors in Rats Lynette C. Daws, Ph.D., Ray Lopez, Ph.D., and Alan Frazer, Ph.D. Chronic treatment of rats with a variety of antidepressants showed a marked deficit in retention. This effect was also b results in the down-regulation of 1-adrenoceptors in the observed after acute administration of these drugs, although b amygdaloid nuclei. The present study sought to determine if they did not down-regulate amygdaloid 1-adrenoceptors at this specific neurochemical effect caused an alteration in this time. It seems that the ability of these antidepressants to inhibitory avoidance conditioning, a behavior considered to potentiate the amnesic effect of scopolamine is unrelated to b b be mediated by -adrenoceptors in the amygdala. Rats their effect on 1-adrenoceptor number in the amygdala and b treated chronically with either desipramine (DMI) or that the extent of antidepressant-induced amygdaloid 1- b phenelzine (PHEN), which down-regulate 1-adrenoceptors adrenoceptor down-regulation is not sufficient, by itself, to in the amygdala, or fluoxetine (FLUOX), which does not do cause a deficit in an inhibitory avoidance task. this, did not exhibit a deficit in the retention of the [Neuropsychopharmacology 19:300–313, 1998] inhibitory avoidance task. However, when scopolamine was © 1998 American College of Neuropsychopharmacology. given prior to acquisition of the task in a dose that, by itself, Published by Elsevier Science Inc. did not affect retention, DMI- and PHEN-treated rats KEY WORDS: Inhibitory avoidance; Desipramine; ous reports emphasizing the putative role of the b Phenelzine; Fluoxetine; 1-Adrenoceptors; Amygdala amygdala in mediating the behavioral actions of antide- pressants (Horovitz 1966; Gorka et al. 1979; Broekkamp Chronic treatment of rats with a wide range of antide- and Lloyd 1981; Sarter and Markowitsch 1985; Duncan pressants produces a significant reduction in the num- et al. 1986; Beck and Fibiger 1995). It should be noted, ber of b-adrenoceptors in brain (Banerjee et al. 1977; see however, that the behavioral effects observed in these Heninger and Charney 1987). In particular, quantitative studies followed acute administration of antidepres- autoradiographic studies have shown that different sants; whereas, the down-regulation of b -adrenocep- types of antidepressants most consistently down-regu- 1 tors takes time (ca. 1 week) to occur (Ordway et al. 1991). late b -adrenoceptors in the amygdala (Ordway et al. 1 Thus, both acute and chronic effects of antidepressants 1988, 1991). This finding is of interest in light of previ- implicate the amygdala as a primary target of the phar- macological actions of this class of drugs. The purpose From the Department of Pharmacology (LCD, RL, AF), The Univer- of this study was to attempt to identify a behavioral sity of Texas Health Science Center at San Antonio, San Antonio, Texas; alteration that results from the down-regulation of and South Texas Veterans Health Care System (AF), Audie L. Murphy b Memorial Veterans Hospital Division, San Antonio, Texas, USA. amygdaloid -adrenoceptors. Address correspondence to: Lynette C. Daws, Department of b-adrenoceptors in the amygdala have been impli- Pharmacology, University of Texas Health Science Center at San cated in mediating inhibitory (passive) avoidance be- Antonio, 7703 Floyd Curl Drive, San Antonio, TX, 78284-7764, USA. Received October 23, 1997; revised January 16, 1998; accepted havior (e.g., Gallagher et al. 1977; Liang et al. 1986, 1990; January 20, 1998. Campeau and Davis 1995; Introini-Collison et al. 1996). NEUROPSYCHOPHARMACOLOGY 1998–VOL. 19, NO. 4 © 1998 American College of Neuropsychopharmacology Published by Elsevier Science Inc. 0893-133X/98/$–see front matter 655 Avenue of the Americas, New York, NY 10010 PII S0893-133X(98)00016-5 NEUROPSYCHOPHARMACOLOGY 1998–VOL. 19, NO. 4 Antidepressant Treatment and Behavior 301 Gallagher et al. (1977) observed that injection of into the illuminated compartment facing away from the b-adrenoceptor antagonists in the amygdala of rats pro- closed door. The rat was allowed a 10-second habitua- duced retrograde amnesia on an inhibitory avoidance tion period before the door was opened, allowing free task that was both time- and dose-dependent and ste- passage of the rat into the dark compartment. Once the reospecific. This work has been confirmed and ex- rat had entered the dark compartment (i.e., all four tended by McGaugh and his colleagues (Liang et al. paws being within the dark compartment), the door 1986; McGaugh 1989). Although there is a learning was closed behind the rat and a 0.5 mA shock delivered component to inhibitory avoidance behavior, there is for a 3-second duration. Approximately 5 seconds later, also an emotional component attributable to its pairing the rat was removed from the chamber and placed into of a nonaversive stimulus with an aversive one. The a holding container until all cage mates has been pro- retention of memory caused by this pairing is manifest cessed. All rats were then returned to their home cage. as an increased latency for rats to enter the dark com- The latency for the rat to enter the dark compartment, partment of a two-compartment chamber after being i.e. acquisition latency, was recorded. Twenty-four hours shocked there in a training trial. It is possible to view later, retention was tested. This was done by placing the increased latency as a type of “freezing” behavior, a each rat into the illuminated compartment, as described behavior considered indicative of a state of fear or anxi- above, with the exception that the guillotine door was ety (Leaton and Borszcz 1985). This behavior may, already open. The latency to enter the dark compart- therefore, be particularly relevant to study, because ment was recorded as a measure of retention of the task. some antidepressants are efficacious in panic disorder Rats that did not enter the dark compartment within 10 (Sheehan et al. 1980; Murphy et al. 1985; Kahn et al. minutes were given a ceiling score of 600 seconds. To 1986), and some view anxiety as a central component of verify the involvement of amygdaloid b-adrenoceptors the depressive syndrome (Katz et al. 1984). The present in mediating inhibitory avoidance behavior, a separate study aims to characterize the effect of acute and group of rats received intra-amygdala administration of chronic treatment with antidepressant drugs on inhibi- propranolol or vehicle. tory avoidance behavior in rats and to correlate this with changes in b -adrenoceptor number in the amygdala. Surgery. Rats were anesthetized with sodium pento- 1 barbital (65 mg/kg intraperitoneally [IP]), placed into a stereotaxic frame, and bilaterally implanted with 22- MATERIALS AND METHODS gauge stainless steel guide cannulae into the amygdala. The tips were directed at the dorsal surface of the Animals amygdala complex (A.P. 22.3 mm, M.L. 14.6 mm, D.V. 2 Male Sprague–Dawley rats (270–350 g, Harlan, India- 7.5 mm; Paxinos and Watson 1986). Two skull screws napolis, IN) were housed either in groups of three, or served as anchors for the cranioplastic cement used to individually (postsurgery). The colony was maintained hold the cannulae in place. The patency of the cannulae on a 14:10 light/dark cycle with the lights on at 7:00 was maintained by inserting a 28-gauge stylet. a.m. All training was conducted between 0800 and 1300 Intra-Amygdala Drug Administration. Propranolol (34 6 8 hours. Room temperature was maintained at 23 2 C, or 68 nmol) or phosphate buffered saline (PBS, pH 7.4) and rats were provided with food and water ad libitum. was administered through a 28-gauge injector con- All animal procedures were in strict accordance with nected to a 10 ml Hamilton microsyringe by polyethyl- the NIH Guide for the Care and Use of Laboratory Ani- ene tubing (PE-20). The injector was designed so that, mals. All efforts were made to minimize both the num- when inserted into the cannula, the top protruded 1 mm ber of rats used and discomfort to the rats during exper- in the brain tissue. Both microsyringe and tubing were imental procedures. filled with fluid. Twenty minutes prior to acquisition training, one microliter of the drug solution was deliv- Inhibitory Avoidance Task ered sequentially into each amygdala. Each injection was given over a 60 s period. The injector was left in the Rats were trained on a one trial step-through inhibitory cannula for an additional 15 s to allow diffusion of the avoidance task. The inhibitory avoidance chamber was drug away from the injector tip. Stylets were replaced made from Plexiglas and was comprised of two com- immediately after termination of the injection. Reten- partments separated by a guillotine door. The smaller tion was tested 24 hours later. compartment (20 3 16 3 24 cm) was illuminated by a 60-watt light, mounted 50 cm above the Plexiglas base. Histology. At the conclusion of the experiment, one The base of the larger, dark compartment (30 3 20 3 20 microliter of Chicago blue dye was injected into the cm), consisted of a series of metal rods separated by 1 amygdala over a 60 s period to mark the site of the injec- cm, through which an electric shock could be delivered. tor tip. Rats were then decapitated, and the brain was On the day of acquisition training, each rat was placed rapidly removed and stored in 10% formalin for 24 302 L.C. Daws et al. NEUROPSYCHOPHARMACOLOGY 1998–VOL. 19, NO. 4 hours. The brain was frozen (2158C) prior to sectioning tested 24 hours later, after which the rats were sacri- (40 mm thick slices) through the cannula tracts. Sections ficed and brains stored as described above.