The role of afferents to the locus coeruleus in the handling stress-induced increase in the release of noradrenaline in the medial prefrontal cortex: a dual-probe microdialysis study in the rat brain

Abstract

This study was aimed to identify the neuronal pathways that mediate the handling stress-induced increase in the release of noradrenaline in the medial prefrontal cortex of the rat brain. For that purpose a microdialysis probe was implanted in the vicinity of the locus coeruleus and a second probe was placed in the ipsilateral medial prefrontal cortex. Receptor specific antagonists acting on the α₂-adrenoceptor (50 μM idazoxan), GABA_A (50 μM bicuculline), GABA_B (100 μM (3,4-Dichlorophenyl)methyl]propyl](diethoxymethyl) phosphonic acid; CGP 52432), acetylcholine (10 μM atropine), corticotropin releasing factor (CRF) (100 μM butyl-ethyl-[2,5-dimethyl-7-(2,4,6-trimethyl-phenyl)-7H-pyrrolo[2,3-d]pyrimidin-4-yl]-amine; CP-154,526), NMDA glutamate (300 μM (±)-3(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid; CPP) and non-NMDA glutamate receptors (500 μM 6,7-dinitroquinoxaline-2,3-dione; DNQX) were infused into the locus coeruleus by retrograde dialysis, whereas extracellular noradrenaline was recorded in the ipsilateral medial prefrontal cortex. During infusion of the various compounds rats were gently handled for 10 min. Infusion of idazoxan potentiates the handling-induced increase in the release of noradrenaline in the medial prefrontal cortex. The infusions of, atropine, bicuculline, CGP 52432 and DNQX were without effect on the handling response. Infusion of the NMDA receptor antagonist CPP or the non-peptide CRF receptor antagonist CP-154,526 suppressed the stimulation of noradrenaline during stress. It is concluded that α₂-adrenoceptors, NMDA glutamate receptors and CRF receptors modify the handling stress response of locus coeruleus neurones. The data suggest no major role for glutamatergic, GABAergic, or cholinergic afferents to the locus coeruleus in mediating the stress response.

Introduction

Numerous anatomical, electrophysiological and behavioural studies indicate that the noradrenergic pathway that originates in locus coeruleus is involved in the regulation of transfer of information through sensory circuits during periods of stress, arousal and selective or sustained attention Foote et al., 1983, Abercrombie and Jacobs, 1987, Abercrombie et al., 1988, Aston-Jones et al., 1991, Page and Valentino, 1994.

A range of arousing and stressful stimuli have been reported to phasically activate locus coeruleus noradrenergic neurones and to increase noradrenaline metabolism and turnover in brain areas, such as the hippocampus, cortex and hypothalamus. Recently a series of microdialysis studies have confirmed that the release of noradrenaline in these structures is increased during stress (for review, Westerink, 1995).

There is anatomical and electrophysiological evidence that excitatory and inhibitory pathways originating in the nucleus paragigantocellularis, and inhibitory projections originating in the nucleus prepositus hypoglossi, play a major role in the regulation of locus coeruleus activity Aston-Jones et al., 1991, Chiang and Aston-Jones, 1993.

In the afferental pathways to the locus coeruleus a variety of neurotransmitters have been identified. Several studies were performed to identify whether these transmitters participate in the stress-induced activation of locus coeruleus neurons. For that purpose receptor-specific compounds have been administered systemically to animals during stressful conditions Ida et al., 1985, Ida et al., 1990, Rosetti et al., 1990, Tanaka et al., 1991, Murase et al., 1992. This approach has provided evidence for participation of GABA_A and NMDA glutamate receptors in controlling locus coeruleus activity during stress. However, because of the systemic approach, the location of these receptor sites remained unknown.

A second approach was followed by local application of compounds directly to the locus coeruleus, whereas stressors or noxious stimuli were applied. The results of these experiments provided evidence for participation of the α₂-adrenoceptor (Astier and Aston-Jones, 1989), the non-NMDA glutamate receptor (Ennis et al., 1992), the GABA_A receptor (Aston-Jones et al., 1991) and the corticotropin releasing factor (CRF) receptor Valentino et al., 1991, Emoto et al., 1993, Smagin et al., 1997 in the activation of the locus coeruleus by sensory and stressful stimuli.

In these studies different types of stress were studied, ranging from immobilisation, colon distension and hemodynamic stress (hypotension), which makes the results not directly comparable. It is evident that additional information is needed for a better understanding of the neurochemical interactions that participate in the activation of the locus coeruleus.

Here, we have applied the dual-probe microdialysis technique to identify receptor subtypes involved in the effect of stress on noradrenaline release in the medial prefrontal cortex. For that aim one microdialysis probe was implanted in the vicinity of the locus coeruleus and the second probe was implanted the medial prefrontal cortex. In this approach extracellular noradrenaline in the medial prefrontal cortex is considered as index of locus coeruleus activity. Receptor specific drugs were applied to the locus coeruleus by retrograde microdialysis, whereas extracellular noradrenaline was recorded in the medial prefrontal cortex. During the microdialysis session a stressful condition was applied to the rat. Ten min handling was chosen to activate the locus coeruleus, as it represents a mild and reproducible condition that includes elements of novelty, fear and stress.