The antidepressant-like effect of vagus nerve stimulation is mediated through the locus coeruleus

Highlights

• VNS has an antidepressant-like effect in the forced swim test.

• Lesioning the LC abolishes this antidepressant-like effect of VNS.

• Our results support a key role for the LC in the antidepressant mechanism of VNS.

Abstract

It has been shown that vagus nerve stimulation (VNS) has an antidepressant-like effect in the forced swim test. The mechanism of action underlying this effect is incompletely understood, but there is evidence suggesting that the locus coeruleus (LC) may play an important role. In this study, noradrenergic LC neurons were selectively lesioned to test their involvement in the antidepressant-like effect of VNS in the forced swim test.

Forced swim test behavior was assessed in rats that were subjected to VNS or sham treatment. In half of the VNS-treated animals, the noradrenergic neurons from the LC were lesioned using the selective neurotoxin DSP-4 [N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine hydrochloride], yielding three experimental arms: sham, VNS and DSP-4-VNS (n = 8 per group). Furthermore, the open field test was performed to evaluate locomotor activity. A dopamine-β-hydroxylase immunostaining was performed to confirm lesioning of noradrenergic LC neurons.

VNS significantly reduced the percentage of immobility time in the forced swim test compared to sham treatment (median: 56%, interquartile range: 41% vs. median: 75%, interquartile range: 12%). This antidepressant-like effect of VNS could not be demonstrated in the DSP-4-VNS group (median: 79%, interquartile range: 33%). Locomotor activity in the open field test was not different between the three treatment arms. The absence of hippocampal dopamine-β-hydroxylase immunostaining in the DSP-4-treated rats confirmed the lesioning of noradrenergic neurons originating from the brainstem LC.

The results of this study demonstrate that the noradrenergic neurons from the LC play an important role in the antidepressant-like effect of VNS.

Introduction

Major depressive disorders are highly prevalent, widely distributed in the population and usually associated with substantial symptom severity and role impairment (Kessler et al., 2003). While depressive symptoms can be effectively treated with antidepressant drugs or psychotherapy in the majority of patients, up to 20% of patients fail to respond to standard interventions (Fava, 2003). These drug-refractory patients are candidates for treatment with neurostimulation therapies such as vagus nerve stimulation (VNS).

VNS is an extracranial neurostimulation technique, where the cervical region of the left vagus nerve is stimulated by means of a helical electrode, connected to a subclavicularly-implanted pulse generator. It is a well-established, safe and effective add-on therapy for refractory epilepsy (Ben-Menachem, 2002). The initial rationale for using VNS for the treatment of refractory depression was based on mood improvements observed in epilepsy patients treated with VNS, irrespective of the effects of VNS on seizure frequency (Elger et al., 2000, Harden, 2002). Several clinical studies have subsequently confirmed the therapeutic efficacy of VNS for treatment resistant depression (Rush et al., 2000, Rush et al., 2005a, Rush et al., 2005b, Sackeim et al., 2001, Marangell et al., 2002, George et al., 2005, Nahas et al., 2005, Schlaepfer et al., 2008, Bajbouj et al., 2010, Cristancho et al., 2011), but the mechanism of action is still unknown.

The forced swim test is one of the most commonly used and validated experimental assays to assess depression-like behavior in rodents. During the test, animals are placed in a cylinder filled with water from which they cannot escape. Mobile escape-related behavior (defined as forepaw movements along the side of the cylinder and swimming throughout the cylinder) and immobile passive behavior (defined as the lack of whole body movement, except for small efforts to keep the head above water) are scored blindly on videotaped images of the test. A reduction in immobile passive behavior is reflective of an antidepressant-like effect of the investigated intervention (Porsolt et al., 1977, Slattery and Cryan, 2012). To rule out the possibility that the effects in the forced swim test are caused by an overall change in locomotor activity, an open field test can be performed. Krahl et al. showed that VNS produces an antidepressant-like effect in the forced swim test in rats with the same efficacy as electroconvulsive shock therapy and the tricyclic antidepressant desipramine (Krahl et al., 2004).

Since the 1960s there has been a strong emphasis on the role of noradrenaline both in the pathogenesis of depressive disorders and in the mechanism of action of antidepressants (Schildkraut, 1965, Moret and Briley, 2011). This largely results from the fact that many of the first generation antidepressants, the tricyclics, increase the synaptic concentration of noradrenaline (Cryan et al., 2002). There is extensive evidence demonstrating that VNS also enhances the noradrenergic neurotransmission through the activation of the locus coeruleus (LC) (Florin-Lechner et al., 1996, Hassert et al., 2004, Groves et al., 2005, Dorr and Debonnel, 2006, Roosevelt et al., 2006, Follesa et al., 2007, Manta et al., 2009, Manta et al., 2012b, Raedt et al., 2011), which is the main source of cortical noradrenaline (Krahl and Clark, 2012). Therefore, we investigated the hypothesis that the VNS-induced antidepressant-like effect in the forced swim test is mediated through activation of the LC and subsequent release of noradrenaline. For this purpose LC neurons were lesioned using DSP-4 [N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine hydrochloride], a highly selective neurotoxin for the noradrenergic axons originating from the LC (Fritschy and Grzanna, 1989, Fritschy and Grzanna, 1991).