Elsevier

Behavioural Brain Research

Volume 153, Issue 1, 12 August 2004, Pages 233-239
Behavioural Brain Research

Research report
Inescapable shock activates serotonergic neurons in all raphe nuclei of rat

https://doi.org/10.1016/j.bbr.2003.12.020Get rights and content

Abstract

Animal studies examining the effects of stress upon brain serotonergic neurons have not presented a clearcut and consistent picture. One stressor that has been shown to exert a consistently strong effect on serotonin release and c-fos activation in the dorsal raphe nucleus of rats is a series of inescapable electrical shocks. Using immunohistochemical double labeling for c-fos activation and serotonin, we examined the effects of delivering 100 inescapable tailshocks to rats on serotonergic neuronal activation throughout the brainstem raphe system. This stimulus exerted a consistent and strong activation of the entire midline brain stem system of serotonergic neurons. The implications of these findings for animal models of human psychopathology are discussed.

Introduction

Evidence from a variety of sources implicates brain serotonin in the mammalian stress response. However, the experimental data from recent laboratory studies in animals is at best ambiguous, and sometimes contradictory. Much of this may, of course, be attributable to differences in the species (or strains), the basic methodology, and/or the experimental paradigm (or specific procedure).

Electrophysiological evidence from one of our laboratories indicates that a variety of environmental and physiological stressors or challenges do not significantly elevate serotonergic neuronal activity above that observed in an appropriate control condition (active waking). In various studies, this was observed in the dorsal raphe nucleus (DRN), median raphe nucleus (MRN) or in nucleus raphe magnus (NRM) of the domestic cat. Thus, tonic or phasic mildly painful stimuli, loud noise, physical restraint and a natural enemy (dog) all failed to increase neuronal activity above control level. Similarly, no significant change in neuronal activity was observed in response to a heated environment, the administration of a pyrogen, drug-induced increases or decreases in blood pressure, or insulin-induced hypoglycemia. This is despite the fact that all of these conditions evoked strong behavioral responses and/or physiological changes indicative of sympathetic activation [2], [6], [7], [8], [21].

Examination of recent neurochemical studies, limited to measuring serotonin levels using in vivo brain microdialysis in rats, often show a pattern of results similar to the electrophysiology studies described above. Results from one of our laboratories indicated that, when compared to a non-stressful active waking condition, little change in serotonin level is seen in forebrain structures (hippocampus, corpus striatum, amygdala and prefrontal cortex) of rats exposed to tail pinch, a cat, or to swimming or floating [18]. Results from other laboratories using these and other stressors range from no change in serotonin levels to very large increases, and in a few cases, decreases in serotonin levels [17]. Clearly, the changes in serotonin efflux that are observed depend on the stressor used and the brain region examined, a point made by Kirby et al. [11].

Finally, a number of studies have recently used induction of c-fos (an immediate early gene) as an index of neuronal activation by various stressors (reviewed in ref. [12]). Several of these studies have used this approach specifically to examine the activation of serotonergic neurons by counting Fos immunoreactive (FosIR) raphe neurons that are double labeled for serotonin. However, it is difficult to draw any generalizations across these studies because they often employ different species, use different stressors, and examine different groups of serotonergic neurons. One of the conditions that has consistently been found to strongly activate fos in serotonergic neurons of the DRN [9] and to increase extracellular serotonin levels in projection regions of the DRN [1], [3], [4] is inescapable electrical tailshocks. These studies have focused on the DRN because it is comprised of neurons that provide the majority of the serotonergic innervation of the forebrain in rats. However, serotonergic neurons in the other brainstem raphe nuclei can be assumed to be of equal biobehavioral importance. Therefore, we examined the effects of inescapable tailshocks (IS) on c-fos induction throughout the brainstem serotonergic raphe nuclei: DRN, MRN (provide most of the rest of the serotonergic input to the forebrain), NRM (input to the dorsal horn of the spinal cord—implicated in nociception), and nuclei raphe obscurus (NRO) and nuclei raphe pallidus (NRP) (provide the serotonergic input to the intermediolateral column and the ventral horn of the spinal cord—implicated in autonomic and motor regulation, respectively). In addition to its basic neurobiological importance, this study is of preclinical interest because of the relevance of the IS paradigm as an animal model for human psychopathology [13], [20].

Section snippets

Animals

Adult male Harlan Sprague–Dawley rats weighing 300–350 g were used. These animals were housed in pairs in standard Plexiglas cages, with food and water ad libitum, in a room on a 12/12 (h) light/dark cycle (on at 0700 h). All procedures were in accordance with NIH animal care guidelines and were conducted with approval of the University of Colorado Institutional Animal Care and Use Committee.

Experimental conditions

The experimental group (n=6) was exposed to IS, in a single session, 100 min long, which took place between

Results

Overall, compared to control, IS produced a statistically significant increase in both FosIR cells as well as double-labeled (fos+5-HT) cells across the raphe nuclei (P<0.001, ANOVA). There were approximately 10 times the number of FosIR cells in the mesencephalic raphe nuclei (DRN and MRN) of animals in the IS group as compared to animals in the control group (Fig. 1, top panels), and approximately 2–4 times the number of FosIR cells in the medullary raphe nuclei (MRN, NRP and NRO) of animals

Discussion

The most significant result in this study is the finding that neurons throughout the raphe serotonergic system are activated by inescapable electric shock. In one sense this is not surprising since this manipulation represents a potent behaviorally and physiologically activating stimulus (reviewed in ref. [14]). We believe that inescapable shock is a more potent/broader stressor than those to which laboratory animals are typically exposed (see Section 1). Since the present study did not compare

Acknowledgments

This research was supported by USPHS grants MH 50479 and MH 23433.

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