Our previous work fails to show any maternal effect with
Our previous work fails to show any maternal effect with blockade of 5-HT2A receptor, as acute and repeated treatment of MDL100907 does not alter maternal behavior at the behaviorally active doses (Chen et al., 2014). In the present study, MDL100907 pretreatment attenuated the maternal disruptive effect of TCB-2, confirming that the mechanism of action of TCB-2 is through stimulation of 5-HT2A receptor. A more interesting and unexpected finding is that both 5-HT2C agonist and antagonist potentiated the maternal disruptive effect of TCB-2. Although the 5-HT2A and the 5-HT2C receptors are closely related members of the G-protein-coupled receptors that share the highest degree of sequence homology (about 50% overall sequence identity) and cellular signaling pathways (Becamel et al., 2004), they often play opposing roles in various Exo1 functions and psychological processes. Overall evidence seems to suggest that activation of 5-HT2A receptors has a similar function to blockade of 5-HT2C receptors, whereas activation of 5-HT2C receptors is functionally equivalent to blockade of 5-HT2A receptors. These opposing effects between 5-HT2A and 5-HT2C receptors are found in the modulation of dopamine release and cell firing (Di Giovanni et al., 2002, Di Matteo et al., 2002, Ichikawa et al., 2001, Millan et al., 1998), in behavioral inhibition and impulsivity (Robinson et al., 2008, Winstanley et al., 2004), reversal learning (Boulougouris et al., 2008), head-twitch response (Vickers et al., 2001), sexual behavior (Popova and Amstislavskaya, 2002), as well as in drug-motivated behaviors (Filip et al., 2004, McMahon et al., 2001). Based on these observations, we originally expected to see the 5-HT2C antagonist SB240084 to enhance while the 5-HT2C agonist MK212 to reverse the maternal disruptive effect of TCB-2. Indeed, the behavioral enhancement effect of SB240084 was confirmed. SB240084 may do so to further increase TCB-2-induced dopamine release in the mPFC and NA and cell firing in the ventral tegmental area, causing disruption of behavioral organization (fragmentation) (Di Giovanni et al., 2002, Di Matteo et al., 2002, Ichikawa et al., 2001, Millan et al., 1998). In contrast, MK212, as a 5-HT2C agonist, was supposed to counteract the TCB-2-induced increase in dopamine release (Di Giovanni et al., 2000, Di Matteo et al., 2002) to reduce the maternal disruptive effect of TCB-2. The lack of such an effect is important, as it indicates that TCB-2 could disrupt maternal behavior through other mechanisms, such as by influencing the glutamatergic function in the mPFC, in addition to its known impacts on the VTA dopaminergic cells. Our microinjection work showing that intra-mPFC injection of TCB-2 suppressed pup retrieval, is consistent with this idea. This idea is also supported by the observation that the mPFC glutamatergic neurons do express 5-HT2A (Nocjar et al., 2015); lesions of the mPFC can cause pup retrieval deficits (Afonso et al., 2007); and inactivation or inhibition of neuronal activity in the mPFC disrupt maternal behavior (Febo et al., 2010). The direct interaction between the 5-HT2A and 5-HT2C in the mPFC has been reported in motor impulsivity (Anastasio et al., 2015). Existing evidence suggests that the mPFC glutamatergic neurons predominantly express 5-HT2A, whereas the mPFC GABAergic interneurons predominantly express 5-HT2C (Nocjar et al., 2015). Therefore, TCB-2 could stimulate the 5-HT2A receptors on the glutamatergic neurons to disrupt maternal behavior, while MK212 could stimulate the 5-HT2C receptors on the GABAergic interneurons to cause a disinhibition of mPFC glutamatergic neurons, leading to a further exacerbation of maternal disruption. This is an intriguing idea and worth further investigation. Our previous c-Fos studies show that the 5-HT2A/2C agonist DOI and antagonists clozapine and olanzapine, though not selective to 5-HT2C receptors, increased c-Fos expression in the mPFC (Zhao and Li, 2010, Zhao and Li, 2012). In the present study, although we did observe an increase in c-Fos expression in the mPFC by acute TCB-2, the effect did not reach a statistically significant level. The c-Fos results reveal several other brain sites where TCB-2 might have an action, including the vBNST, CeA and DR, all of which has been implicated in the regulation of certain aspect of maternal behavior (Barofsky et al., 1983, Bosch et al., 2010, Numan and Numan, 1995). The serotonergic neurons in the DR project to the VTA and NAc, where both 5-HT2A and 5-HT2C receptors have been found. Thus, the DR may regulate maternal motivation toward the young (Bridges, 2015). The CeA appears to be a crucial component for normal activation of maternal aggression circuitry and was shown to mediate the suppression of maternal care (Dulac et al., 2014). It has been shown that maternal aggression was increased after microinjection of the 5-HT2A/2C receptor agonist α-Methyl-5-hydroxytryptamine maleate into the amygdala (de Almeida et al., 2006). In addition, lesion of vBNST disrupts maternal behavior, especially pup retrieval (Numan and Numan, 1996), and maternal expression is associated with increased c-Fos expression and increased binding to arginine vasopressin V1a and oxytocin receptors in the BNST (Bosch et al., 2010, Numan and Numan, 1995).