Elsevier

Brain Research

Volume 812, Issues 1–2, 23 November 1998, Pages 113-120
Brain Research

Research report
Escapable and inescapable stress differentially alter extracellular levels of 5-HT in the basolateral amygdala of the rat

https://doi.org/10.1016/S0006-8993(98)00960-3Get rights and content

Abstract

The effects of escapable and yoked inescapable electric tailshocks on extracellular levels of serotonin (5-HT) in the basolateral amygdala were measured by in vivo microdialysis. Inescapable, but not escapable, shock increased extracellular 5-HT in the amygdala relative to restrained controls. Basal levels of 5-HT were elevated 24 h after inescapable shock, and previously inescapably shocked subjects exhibited an exaggerated 5-HT response to two brief footshocks. Levels of extracellular 5-HIAA did not follow any particular pattern and were not correlated with the changes in 5-HT.

Introduction

The degree of behavioral control which an organism has over a stressor is among the most potent variables in determining the behavioral and physiological impact of the stressor 4, 37, 57. The control dimension has most often been investigated by comparing escapable (controllable) and yoked inescapable (uncontrollable) electric shocks [45]. These two stressors, even though physically identical, have quite different effects on subsequent behavior [37]. For example, exposure to inescapable shock (IS) produces later poor escape learning in a different situation, whereas initial exposure to escapable shock (ES) does not [47]. The mechanisms responsible for IS-induced interference with escape learning have been intensively explored for many years, but the mechanisms responsible for other sequelae of IS have received less attention.

It has more recently been shown that stressor controllability interacts with fear and anxiety. IS potentiates later fear conditioning, and ES does not. Exposure to footshock in a novel context produces conditioned fear to that context as measured by freezing [15], and Maier [38]found such conditioning to be enhanced in animals that had been previously exposed to IS, but not enhanced in animals given ES. Similarly, IS produces increased anxiety as measured by reductions in social interaction, but prior ES does not [54]. Conditioned fear and anxiety are often viewed as being related [11], and these two effects of IS do share a number of characteristics. For example, both have a similar time course and persist for 3–4 days after IS 38, 54, and both are sensitive to blockade by benzodiazepines 41, 44, 54. However, the mechanism(s) by which IS, but not ES, potentiate fear conditioning and augment anxiety as measured by social interaction are not well understood.

The amygdala is generally regarded as a key structure in the mediation of conditioned fear and anxiety 11, 20, 35. This conclusion is supported by findings indicating that: (a) lesion of the amygdala interferes with the development and expression of behavioral (e.g., Ref. [7]), endocrine/autonomic 31, 50, and neurochemical [18]indices of conditioned fear; (b) stimulation of the amygdala produces fear responses 5, 13; and (c) stimuli that induce conditioned fear activate the amygdala [8]. Although the data are less consistent, manipulations of the amygdala have also been reported to alter behavior in animal models developed to measure anxiety [21]. It is therefore noteworthy that lesions in the basolateral region and the central nucleus of the amygdala also block the potentiation of fear conditioning produced by IS [40], thereby implicating the amygdala in IS-induced potentiation of fear conditioning.

Numerous systems and transmitters modulate amygdalar function [2], but serotonin (5-HT) may be especially important as a mediator of IS-induced potentiation of fear and anxiety (see below). A number of different 5-HT receptor subtypes have been localized in the amygdala, particularly in basolateral regions (e.g., Refs. 9, 14, 56, 59). Furthermore, the amygdala receives dense projections from midbrain raphe nuclei, particularly the dorsal raphe nucleus (DRN) [36]. 5-HT injection into the amygdala evokes anxiogenic effects in a number of different test situations 24, 28, and infusion of 5-HT1A receptor agonists exerts into basolateral regions of the amygdala exert anxiogenic action in punished responding [28]and social interaction [19]paradigms. 5-HT has not been extensively studied in fear conditioning paradigms, but appears to play a minor role at best 12, 40. However, it is possible that even though not directly involved in the mechanisms that mediate fear conditioning, 5-HT could modulate the magnitude of conditioned fear expression.

As already noted, the DRN is the major site of 5-HT cells that project to the amygdala, and manipulations of the DRN have yielded data generally consistent with the notion that 5-HT augments anxiety. Lesions of the DRN can have an anxiolytic effect in some models of anxiety [17]. DRN 5-HT neurons express somatodendritic 5-HT1A receptors whose activation inhibits activity of the 5-HT neurons, and microinjection of 5-HT1A agonists into the DRN has anxiolytic action in the social interaction 26, 29, Vogel conflict [25], plus maze [16], ultrasonic vocalization [52], and elevated T-maze [20]tests of anxiety. Conversely, intra-DRN microinjection of agents that increase DRN 5-HT activity can have anxiogenic effects [30].

These DRN manipulations have had a similar effect on the potentiation of subsequent fear conditioning produced by IS. DRN lesions [40]and intra-DRN microinjection of 5-HT1A agonists [42]block IS-induced potentiation of fear conditioning. Furthermore, microinjection of agents that increase DRN 5-HT activity mimic the behavioral effects of IS [39]. These data suggest that IS might potentiate subsequent fear conditioning and produce anxiogenic effects because it activates DRN 5-HT neurons to a greater degree than does ES, thereby causing a greater release of 5-HT in the amygdala than does ES.

This mediating mechanism requires that IS lead to increased levels of 5-HT in the amygdala, relative to ES. In vivo microdialysis experiments have found that some stressors do produce increased extracellular levels of 5-HT in the amygdala 1, 32, 51, but other stressors have had no effect on 5-HT efflux in this structure [1], and some have even led to reduced levels of extracellular 5-HT [33]. It is unknown whether IS would produce increased levels of 5-HT in the amygdala, and whether the controllability of the stressor would modulate any increase in 5-HT efflux that might be observed. Therefore, the purpose of the present experiment was to measure extracellular levels of 5-HT during IS and ES. Potentiation of fear conditioning has been typically observed after exposure to 2 footshocks 24 h later. For this reason 5-HT efflux was also measured under these conditions.

Section snippets

Materials and methods

The subjects were 20 Male Sprague Dawley rats (Harlan Labs), 280–380 g, maintained in groups of 4–6 in plastic cages on a 12/12 h light–dark cycle. Food and water were freely available at all times. Experimental procedures were in accordance with protocols approved by the University of Colorado Institutional Animal Care and Use Committee.

The rats were allowed at least one week after arrival in the laboratory before surgery. Rats were anesthetized with 60 mg/kg sodium pentobarbital i.p., and

Results

There were no significant 5-HT baseline differences across treatment groups on Day 1 of the experiment. 5-HT levels in pg/5μl were: R=0.134±0.021, ES=0.173±0.031 and IS=0.126±0.018. Fig. 1 shows the effect of the experimental treatments on 5-HT concentration in dialysates from the BLAMYG. The process of removing the subjects from the Plexiglas bowls, moving them to another room, and placing them in the wheel-turn boxes produced a nearly three fold increase in extracellular 5-HT concentrations

Discussion

The amygdala is a key integrative site regulating anxiety and fear. Disruption of amygdalar function reduces or eliminates fear and anxiety responses to unconditioned threat [7], and interferes with the development and expression of fear conditioning [27]. The amygdala is also important in the potentiation of fear conditioning produced by prior IS [40]. Thus it is possible that uncontrollability potentiates the fear conditioning that is produced by the stressor because it exaggerates the

Acknowledgements

We would like to thank Julie Barter for excellent technical assistance. This work was supported by MH50479 and MH00314 to S.F.M.

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