Discussion
The initiation, maintenance, and termination of aggressive behavior activates complex neurobiological circuits, among which the serotonergic system is strongly involved (Niederkofler et al., 2016; Takahashi & Miczek 2014). In the current study, using a RVI paradigm, we studied aggression behavior induced by pCPA and the relation with serotonergic activity, TPH2 and GABAAα1 mRNA expression in the OB.
It is well accepted that when evaluating social interaction, it becomes necessary to discard locomotor activity alterations. (Miczek et al., 2013; Takahashi & Miczek 2014). A locomotor activity alteration in the resident could prevent the correct measurement of aggressive behavior, in particular the aggression latency parameter. Interestingly, 5,7-dihydroxytryptamine lesions, used to deplete central 5-HT did not affect locomotor activity (Vergner et al., 1988; Hole et al., 1977). Nonetheless, previous works utilizing pCPA 1000 mg/kg doses found a substantial decrease in locomotor activity (Dringenberg et al., 1995; Matte & Tornow, 1978). Significantly reduced locomotor activity was also seen following chronic treatment with lower dosage of pCPA (100 mg/kg) (Keleta et al., 2007 Kubala et al., 2008). Our data show that pCPA i.p. did not change locomotor activity. It is possible that an acute administration of pCPA at lower doses does not affect this parameter.
According to Koolhaas et al. (2013) and Takahashi & Miczek (2014), to establish an animal aggressive behavior model, species specific qualitative and quantitative parameters of aggression must be considered. Therefore, when establishing the behaviors to evaluate, we took into account parameters that reconciled both characteristics. We measured aggressive behavior as a quantitative parameter and aggressive latency as a qualitative one (Miczek et al., 2013). Stunder et al. (2015) show decreased aggressive latency, as a parameter of desadaptative aggression, and nonaggressive behaviors in mice treated with pCPA. Our data show that the administration of pCPA increased aggressive behavior and decreased aggressive latency. These findings are consistent with others, in which central 5-HT depletion caused aggression increased (Valzelli et al., 1981; Vergner et al., 1986). This might indicate that our model, with pCPA, was effective to induce aggression, both in a qualitative and quantitative sense. Furthermore, we did not find changes in nonsocial activity and social activity after pCPA administration. 5-HT low-levels might affect social interactions, e.g. in studies where social isolation is used to produce aggressive behavior models (Goodell et al., 2017) or in maternal aggression models (Toth et al., 2012). Thus, this result could be explained by our model and experimental design.
Serotonergic innervation originating in the raphe nuclei towards the different brain structures has one of its main synaptic center in the OB (Locki 1985; Steinfeld et al., 2015). Our results show that after pCPA administration, the concentration of 5-HT in the OB was significantly decreased pointing to higher aggressive behavior. In this way, our model provides evidence to the serotonergic deficiency hypothesis and aggression (Kravitz & Huber, 2003; Miczek et al., 2004; Mongillo et al., 2014; Takahashi et al., 2011; Niederkofler et al., 2016). In addition, 5-HT modification has been associated with changes in its major metabolite, 5-HIAA (Stenfors & Ross, 2004). In humans (Sharma et al., 2021; Stanley et al., 2000) and monkeys (Zajicek et al., 2000), higher aggressiveness and low cerebrospinal 5-HIAA levels were associated. Furthermore, mutual decreased 5-HT and 5-HIAA were observed in models of pCPA aggression (Keleta et al., 2007; Kubala et al., 2008). However, the decrease in 5-HT concentration, following local or systemic administration of substances that affect its release from nerve terminals, does not always affect 5-HIAA concentration in the same way (Auerbach et al., 1989; Kalén at al., 1988). In our model, pCPA administration did not cause significant modifications in the concentration of 5-HIAA. It is possible that systemic depletion did not affect the metabolite as well as 5-HT, because others (Dringenberg et al., 1995; Matte & Tornow, 1978) administered higher doses or performed a chronic treatment (Keleta et al., 2007; Kubala et al., 2008). We also showed that serotonergic metabolism (5-HIAA/5-HT) decreased in treated animals, which is in accordance with previous reported data (Hritcu et al., 2007; Koe & Weissman, 1966). These findings suggest that pCPA may cause alterations in the OB serotonergic innervation, enhancing aggression in our model.
Since pCPA animals exhibited increased aggression and 5-HT decreased, we hypothesized that these differences are accompanied by differences in a key serotonergic gene, TPH2. TPH2 mRNA expression is frequently found in raphe complex neurons in rodents (Malek et al., 2005; Patel et al., 2004; Pelosi et al., 2015; Walther et al., 2003) and a very low expression has been found in other rats’ brain areas, (Patel et al., 2004). Interestingly, we found that pCPA animals showed increased TPH2 mRNA expression in the OB. Therefore, our results provide evidence for TPH2 mRNA expression presence in the OB. It has been also found that after postnatal programming with pCPA, TPH2 mRNA expression decreases in raphe nuclei (Trujillo et al., 2021). In this way, despite that our results are opposite they could be indicating a compensatory mechanism in a brain area receiving 5-HT depleted innervation.
In addition, pCPA decreases the protein expression of GABAA α1 receptors (Wang et al., 2020) and GABAAα1 is expressed in OB (Panzanelli et al., 2005). Interestingly, our results showed a higher expression of GABAAα1 subunit after pCPA administration. These results may indicate increased levels of GABAAα1 subunit mRNA in OB, due to its high synthesis demand caused by the serotonergic decrease as a consequence of pCPA administration.