Abstract
Alteration in social behavior is one of the most debilitating symptoms of major depression, a stress related mental illness. Social behavior is modulated by the reward system, and gamma oscillations in the nucleus accumbens (NAc) seem to be associated with reward processing. In this scenario, the role of gamma oscillations in depression remains unknown. We hypothesized that gamma oscillations in the rat NAc are sensitive to the effects of social distress. One group of male Sprague-Dawley rats were exposed to chronic social defeat stress (CSDS) while the other group was left undisturbed (control group). Afterward, a microelectrode array was implanted in the NAc of all animals. Local field potential (LFP) activity was acquired using a wireless recording system. Each implanted rat was placed in an open field chamber for a non-social interaction condition, followed by introducing another unfamiliar rat, creating a social interaction condition, where the implanted rat interacted freely and continuously with the unfamiliar conspecific in a natural-like manner (see Supplementary Videos). We found that the high-gamma band power in the NAc of non-stressed rats was higher during the social interaction compared to a non-social interaction condition. Conversely, we did not find significant differences at this level in the stressed rats when comparing the social interaction- and non-social interaction condition. These findings suggest that high-gamma oscillations in the NAc are involved in social behavior. Furthermore, alterations at this level could be an electrophysiological signature of the effect of chronic social stress on reward processing.
Highlights
Humans as well as many other mammalian species exhibit social behaviors which imply several evolutionary advantages (Alexander, 1974)
The t test showed differences between the groups in depressive-like behaviors based on the sucrose preference test (t = 4.277, df = 10; P = 0.0016), which allows obtaining an indicator of anhedonia in the stressed rats (Figure 2F)
There was already some evidence about the participation of the nucleus accumbens (NAc) in social behaviors (Alkire et al, 2018; Warnell et al, 2018) to date, it was unknown that social interaction correlated with the increase in highgamma power (61–90 Hz, in the present study) in the NAc, which points to a possible brain oscillatory modulation based on social interaction, in that specific region, in high-gamma (Figure 6)
Summary
Humans as well as many other mammalian species exhibit social behaviors which imply several evolutionary advantages (Alexander, 1974). Social reward is crucial for emotional wellbeing; and impairment in this domain, is a key symptom in mood disorders (American Psychiatric Association, 2013). One of these conditions is depression; this disorder is characterized. Social Stress Disrupts Gamma Oscillations by “anhedonia” –the decreased reactivity to pleasurable stimuli – as well as by a deficiency in reward processing (Admon and Pizzagalli, 2015). Reward conditioning for both drug and social interaction, leads to an increase of the electrical network activity in the NAc (Kummer et al, 2015). Clinical studies suggest that the NAc reward responsivity is altered in depressive patients (Pizzagalli et al, 2009; Misaki et al, 2016), and there is evidence about the antidepressive effect of targeting the NAc with deep brain stimulation in patients suffering treatment-resistant depression (Schlaepfer et al, 2008; Nauczyciel et al, 2013)
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