Increased adult neurogenesis in mice with a permanent overexpression of the postsynaptic 5-HT_1A receptor

Keywords

• 5-HT_1A receptor;
• Adult neurogenesis;
• Overexpression;
• Transgenic mice;
• BrdU

1. Introduction

With a life-time prevalence of about 15% in high-income countries, depression represents a common mental disorder with an enormous social and economic impact [10]. To date, depression is among the leading causes of disability and disease burden [34]. Depressive disorder is characterized by low mood and/or anhedonia, combined with several cognitive and vegetative symptoms. In many cases, these symptoms can be reduced by selective serotonin (5-HT) reuptake inhibitors (SSRIs), which are generally recommended for first line-treatment in depressive disorders [4]. The efficacy of these drugs as well as studies in depressive patients led to the monoamine hypothesis of depression, which postulates a prominent pathophysiological role of decreased monoamine levels, especially 5-HT, in depression [1].

Among an impressive variety of identified 5-HT receptors, the 5-HT_1A receptor (5-HT_1AR) represents one of the most abundant subtypes expressed in the mammalian brain [25]. 5-HT_1ARs are located presynaptically as somatodendritic autoreceptors on 5-HT neurons, where their activation inhibits the firing rate of the serotonergic neurons, and are postsynaptically expressed in corticolimbic regions that are implicated in mood and emotion [2]. Over the last decades, they have been investigated intensely in the context of pathophysiology and treatment of depression. Indeed, animal studies as well as studies in depressive patients, including pharmacologic and genetic studies, propose that 5-HT_1ARs are important players in the mediation of antidepressant effects and antidepressant-like behavior [16].

More recent studies shed light on a possible involvement of altered adult neurogenesis in the pathophysiology of depression. For several drugs it has been demonstrated that adult hippocampal neurogenesis is required for the mediation of antidepressant-like behavioral effects in laboratory animals [26]. The proneurogenic effect seems to be, at least partly, mediated by 5-HT and some of its receptor subtypes with a proposed pivotal role for the 5-HT_1AR [19] ;  [28]. However, the exact underlying mechanisms remain unclear, mainly due to the diverse external and internal influences on neurogenesis and the complexity of the serotonergic system with its various receptors and locations [13].

In 2004, Kusserow et al. generated a transgenic mouse line with a permanent 5-HT_1AR overexpression (OE mice) in cortical, hippocampal and further limbic areas, while unaltered receptor densities were detected in the raphe nuclei, where serotonergic autoreceptors are located [14] ;  [20]. In the present study, we used this animal model to examine the role of postsynaptic 5-HT_1ARs on hippocampal adult neurogenesis in mice. Initially, we investigated morphometric parameters of the dentate gyrus (DG) and the prefrontal cortex (PFC), because of their relation to the serotonergic system and/or involvement in adult hippocampal neurogenesis and depression [7]; [19] ;  [31]. Afterwards, proliferation, differentiation and survival of new-born neurons of OE mice were analyzed.

2. Materials and methods

All experiments were done in accordance with the European Communities Council Directive (86/609/EEC) and in compliance with the German Animal Welfare Act (T0270/13; G0013/14). Maximum care was taken for optimizing welfare and to minimize the number of animals used.

2.1. Animals

The present experiments were carried out in a total of 72 transgenic mice (background: NMRI mice by Harlan-Winkelmann, Borchen, Germany) and NMRI WT mice of both genders, aged 10–13 weeks. In the OE animals, stable overexpression of postsynaptic 5-HT_1ARs, especially in the outer cortical layers and the CA3 region was demonstrated previously [14]. Furthermore, recent ^[35S]GTPγS binding studies revealed correct 5-HT_1AR coupling and functionality [23]. All mice were born and kept under controlled environmental conditions (23–25 °C, 50–60% humidity, 12 h light/dark cycle) and had free access to standard diet and water. To analyze proliferation and survival of newborn neurons, animals received intraperitoneal injections of bromodeoxyuridine (BrdU; 50 mg/kg body weight dissolved in 0.9% NaCl; Abcam) on three consecutive days (see Fig. 1). Animals were killed either one day (proliferation) or 21 days (survival) after the last injection aged 10 or 13 weeks, respectively.