Elsevier

Neuropharmacology

Volume 52, Issue 3, March 2007, Pages 949-957
Neuropharmacology

Activation of the brain 5-HT2C receptors causes hypolocomotion without anxiogenic-like cardiovascular adjustments in mice

https://doi.org/10.1016/j.neuropharm.2006.10.012Get rights and content

Abstract

The present study evaluated whether hypolocomotion elicited by subcutaneous administration of the non-specific 5-HT/preferential 5-HT2C receptor agonist mCPP during novelty exposure was due to an enhanced anxiety-like state. The effects of mCPP on exploratory behavior during exposure to a new environment (novelty) were studied in male C57BL/6N mice. Subcutaneous injection of mCPP (1 and 3 mg/kg) and the preferential 5-HT2C receptor agonist MK212 (0.7 and 1 mg/kg) induced hypolocomotion during novelty exposure. The selective 5-HT2C receptor antagonist SB242084 (0.3 mg/kg) reversed the mCPP-induced hypolocomotion into hyperlocomotion. In contrast, MK212 induced hypolocomotion that was blocked by SB242084, indicating a specific 5-HT2C receptor involvement. When injected intracerebroventricularly, mCPP (30 μg) elicited hypolocomotion, whereas the same dose mildly increased locomotion when injected into the dorsal hippocampus. Since anxiety affects autonomic functions, effects of mCPP on cardiovascular function were studied by radio-telemetry in the home cage of unrestrained mice. Subcutaneous injection of mCPP (3 mg/kg) had no significant effect on heart rate and mean arterial blood pressure. In summary, in view of lack of autonomic effects, and the lack of hypoactivity upon forebrain stimulation, the hypolocomotion induced by systemic mCPP cannot be explained by an enhanced anxiety-like state.

Introduction

Assessment of anxiety-like states in rodents has been based on exploratory activity and place preference paradigms such as the elevated plus maze, open field or the dark–light box. However, changes in baseline activity may not simply be related to an altered emotional state of increased anxiety-like behavior but can reflect changes in risk assessment or sedative-like effects. It is therefore critical to complement behavioral assessment with physiological measures such as autonomic reactivity. Thus, changes in emotional states such as anxiety and fear are accompanied by adjustments of autonomic responses such as cardiovascular function (reviewed in Berntson et al., 1998).

Several 5-hydroxytryptamine (5-HT; serotonin) receptor subtypes including the 5-HT2C receptors have been implicated in anxiety disorders (Graeff, 1993). The evidence for a role of the 5-HT2C receptors is based on studies using the 5-HT agonists MK212 and mCPP with preferential or non-preferential affinity for 5-HT2C receptors, respectively. These two compounds induce hypolocomotion in rats (Kennett and Curzon, 1988, Lucki et al., 1989, Gleason and Shannon, 1998) that has been suggested to be indicative of an enhanced anxiety-like state (Kennett et al., 1989, Griebel et al., 1997). In line with this evidence, injections of mCPP caused activation of c-Fos expression in rat brain areas such as the central nucleus of the amygdala, the bed nucleus of the stria terminalis, and the paraventricular nucleus of the hypothalamus (Singewald et al., 2003, Salchner and Singewald, 2006), that are part of the “fear circuits” (Fendt and Fanselow, 1999). On the other hand, non-anxiogenic effects of mCPP are also reported in the rat (e.g. Gommans et al., 1999). In mice, activation of 5-HT2C receptors by mCPP has been reported to produce varying results, ranging from anxiolytic-like to anxiogenic-like effects (Dhonnchadha et al., 2003, Griebel et al., 1997, Walker et al., 2005). The anxiolytic-like action of mCPP was observed at relatively low dose of 0.25 mg/kg, while doses of 1–4 mg/kg were used in most studies. Therefore, the 5-HT2C receptor-mediated anxiogenic effects are difficult to interpret (see Walker et al., 2005) since (1) the behavioral tests have been based on locomotor activity, (2) different tests with different aspects of fear and anxiety have been used, (3) there are species- and strain-dependent differences in the expression of 5-HT2C and other 5-HT receptor subtypes, and (4) the 5-HT receptor agonists used in the above studies lack selectivity for 5-HT2C receptors.

There exists limited information on the role of 5-HT2C receptors in autonomic regulation. Systemic injections of mCPP induced an increase in mean arterial blood pressure (MAP) and heart rate (HR) in conscious rats (Bagdy et al., 1989), and similar results were obtained after intracerebroventricular injection of low doses of 31.5 and 63 μg per rat of 300 g body weight (Ferreira et al., 2005). In addition, the 5-HT2B/2C receptor antagonist SB200646A did not produce any detectable effects on HR and MAP in anesthetized rats (Knowles and Ramage, 1999). So far, no detailed investigation of the effects of mCPP on autonomic function in mice has been reported.

The aim of the present study was to determine whether the hypothesized anxiogenic-like effects of 5-HT2C receptor activation were limited to hypolocomotion or extends to autonomic modulation. The functional role of 5-HT2C receptors was investigated by experiments with injections of MK212 or mCPP in combination with the selective 5-HT2C receptor antagonist SB242084 (Kennett et al., 1997). To study the modulatory role of the brain 5-HT2C receptors in exploratory activity, mice were given subcutaneous, intracerebroventricular or intrahippocampal injections of mCPP and exposed to a novel environment. To examine the role of 5-HT2C receptors in autonomic function, changes in HR and MAP that occurred in response to subcutaneous administration of mCPP were measured under baseline conditions by radio-telemetry in home cages. In addition, the changes in HR were determined in the fear conditioning task during exposure to a conditioned auditory stimulus (Stiedl and Spiess, 1997, Stiedl et al., 1999) that was previously associated with an aversive unconditioned stimulus. Based on previous reports of enhanced anxiety-like behavior due to 5-HT2C receptor activation, it was expected that mice would exhibit enhanced sympathetic tone expressed as increases in HR and MAP. In addition, facilitated fear conditioning and/or slower rate of extinction of conditioned fear was expected.

Section snippets

Animals

Experiments were performed in 185 male C57BL/6NCrlBR (C57BL/6N; Charles River, Germany) mice, aged 9–12 weeks. This strain of mice shows behavioral and autonomic responses similar to that of C57BL/6J mice (Stiedl et al., 1999) which are generally used to backcross genetically modified mice. Mice were initially housed in groups until they had reached the age of 8 weeks. Subsequently, they were individually housed in standard cages (type II) with free access to food and water. Mice were kept

Effects of subcutaneous administration of mCPP, MK212 and SB242084 on exploratory activity

Exploratory activity was measured after subcutaneous injection of mCPP and MK212 to confirm its previously reported hypoactive effects. Subcutaneous injection of the three ligands resulted in a significant effect on locomotor activity in C57BL/6N mice (F3,28 = 23.21, p < 0.0001; Fig. 1A). mCPP (3 mg/kg) induced hypoactivity when compared with saline-injected controls (F1,15 = 27.50, p < 0.0001). Significant hypoactivity was also elicited by injection of MK212 (1 mg/kg) (F1,16 = 34.58, p < 0.0001). Injection

Discussion

The present study confirms previous findings in the rat (e.g. Lucki et al., 1989) and the mouse (e.g. Gleason et al., 2001) of hypolocomotion induced by mCPP due to activation of 5-HT2C receptors in the brain. A novel finding is the differential effect of mCPP on locomotor activity by intracerebroventricular versus intrahippocampal administration. Our results show that the hypolocomotion induced by subcutaneous injection of mCPP is not mediated by 5-HT2C receptors in the dorsal hippocampus.

Acknowledgements

We thank Rene Jansen and Petra Baarendse for helpful discussion, Anja Ronnenberg and Anton Pieneman for excellent technical assistance and Philip Tovote for blood pressure transmitter implantation. This work was supported by the Vrije Universiteit Amsterdam, the Max Planck Society, the University of Saskatchewan and the Karolinska Institutet.

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    Parts of the results were presented at the joint 29th Goettingen Neurobiology Conference and 5th Conference of the German Neuroscience Society, Goettingen, Germany, 2003.

    1

    Current address: JSC Grindeks, Riga, Latvia.

    2

    Current address: Department of Physiology, School of Veterinary Medicine, Hannover, Germany.

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