Social Experience during Adolescence Shapes Maturation of Parvalbumin-Positive Interneurons in the Left Orbitofrontal Cortex

Social experience during adolescence is integral for the maturation of the prefrontal cortex (PFC) and the development of social behaviors in both humans and rodents (for review, see Bicks et al., 2015). Furthermore, a study by Kuniishi et al. (2022) showed that social isolation during adolescence resulted in changes in the connectivity between the basolateral amygdala and orbitofrontal cortex (OFC), a subregion of the PFC that plays a critical role in social cognition (for review, see Amodio and Frith, 2006).

During adolescence, we also see the maturation of parvalbumin (PV)-positive interneurons and perineuronal nets (PNNs) in the PFC (Huang et al., 1999; Juraska and Drzewiecki, 2020). PV⁺ neurons are GABAergic interneurons that modulate neural circuit activity by gating the excitability of pyramidal neurons. The maturation of PV⁺ neurons is also activity dependent (Sugiyama et al., 2008), which means experience affects PV⁺ neuron development within the neural circuit, fine-tuning its activity. PNNs are extracellular matrix structures that primarily surround PV+ neurons and are responsible for stabilizing neural connections, inhibiting new connections, and preventing receptor movement on the cell membrane (Pizzorusso et al., 2002). In addition, studies on autism and schizophrenia, two cases where social cognition is impaired (for review, see Bicks et al., 2015), show that in the dorsolateral PFC there is a decrease in the density of PV⁺ neurons and PNNs (Enwright et al., 2016).

Although adolescence is a sensitive period for development of PFC, it is unknown whether behavioral abnormalities caused by adolescent social isolation are the result of dysregulated PV development. To address this issue, Jeon et al. (2023) investigated how social isolation affects PV neuron development during adolescence and how this might affect sociability in mice during adulthood. To do this, they used a combination of behavioral tests, in vivo electrophysiology, optogenetics, and clustered regularly interspaced short palindromic repeats (CRISPR)-Cas9 approaches to investigate the effects of social isolation on adult social behavior and PV development in the OFC.

In their study Jeon et al. (2023) classified social isolation as the mouse being in a cage alone from postnatal day (P)21 to (P)80. This time frame of social isolation encompasses before puberty onset, through adolescence, and into early adulthood. Control animals were group housed their entire lives. After these different housing conditions, Jeon et al. (2023) quantified mice sociability by measuring how much time the mice spent interacting with another mouse compared with an object and how long they sniffed the other mouse (social sniffing). It was found that social isolation caused an abnormal increase in social approaches and social sniffing (labeled as hypersociability) in female mice, but not in male mice, in the sociability test when compared with the group-housed mice. In addition, the estrus cycle did not affect these results. This suggests that long-term juvenile social interactions play an important role in adult sociability behavior in female mice. This led the researchers to investigate what neural mechanisms might underlie this increased sociability after social isolation.

To understand the neural mechanisms driving this isolation-induced increase in sociability, Jeon et al. (2023) investigated how social isolation affects PV and PNN development in the OFC. Jeon et al. (2023) found a reduced number of PV⁺ cells in the left OFC (OFC_L) of socially isolated female mice compared with group-housed mice, although no difference was found in the right OFC (OFC_R). Interestingly, the number of PNNs never changed between the experimental groups, even when PV⁺ neurons decreased. In a subsequent experiment, Jeon et al. (2023) even knocked down PNNs with chABC, an enzyme that degrades PNNs by breaking down sulfide side chains of proteoglycans, which resulted in no difference in social behavior or PV⁺ neuron number.

Jeon et al. (2023) also addressed precisely when during social isolation PV⁺ neurons were affected. To investigate this further, they broke social isolation into two time frames. A group of mice was socially isolated from P21 to P50 and then returned to a group-housed setting afterward represented social isolation during early puberty (titled re-group-housed group). Another group, which was isolated from P50 to P80, represented social isolation during late adolescence. Jeon et al. (2023) then quantified PV⁺ neurons and social sniffing. They found an increase in social sniffing and a decrease in PV⁺ neurons in the social isolation during the late adolescence group but not in the re-group-housed group. These findings suggest that social isolation during late adolescence was responsible for increased sociability in adulthood along with the decrease in PV⁺ neurons.

Because Jeon et al. (2023) found fewer PV⁺ neurons in the OFC_L after social isolation, the authors decided to test whether social sniffing was correlated with changes in neuronal activity in the OFC_L. In female mice, neuronal activity was measured using head-mounted probes, and it was found that in the group-housed mice there was an increase in OFC_L neuronal activity during social sniffing compared with the OFC_R, which suggests that OFC_L plays a specialized role in social investigation. This difference was not found in the socially isolated mice, and there were fewer OFC_L fast-spiking neurons in socially isolated mice compared with group-housed mice.

Then, Jeon et al. (2023) used CRISPR/Cas9-mediated knockdown of PV in the OFC_L to manipulate the presence of PV in the PV⁺ neurons during late adolescence. This knockdown of PV removes the fast-spiking GABAergic effects of the neuron without removing the entire interneuron (Cates et al., 2002). Interestingly, this decrease in PV⁺ neuron functionality resulted in increased social sniffing, which is similar to the social isolation treatment. Jeon et al. (2023) did a follow-up experiment and recorded neuronal activity in the CRISPR PV knockdown mice and found that this decrease in PV⁺ neuron functionality was accompanied by a general increase in neuronal activity in the OFC_L but a decrease in activity associated with sniffing. It appears that social sniffing causes PV⁺ neurons to fire their associated inhibitory signals, which decreases overall activity in the OFC, but the knockdown of PV in PV⁺ neurons removes this inhibitory activity and leads to a general increase in activity of the entire OFC as the excitatory pyramidal neurons no longer are having their activity gated by the PV⁺ neurons. Jeon et al. (2023) then used optogenetics to both activate and inhibit PV⁺ neurons in the OFC_L. It was found that activating PV⁺ neurons decreased sociability, whereas inhibiting PV⁺ neurons increased sociability. These results, along with the previous experiments, provide strong evidence that adolescent social experience is critical for the maturation of inhibitory circuits in the OFC_L, which shape adult social behavior in female mice (Fig. 1).

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Figure 1.

For female mice, social isolation during late adolescence results in a decrease in GABAergic inhibitory PV+ interneurons in the left orbitofrontal cortex in adulthood. Behaviorally, this results in an increase in sociability, labeled as hypersociability. This is in comparison with female mice that were group housed during late adolescence. It is thought this increase in sociability from social isolation is caused by a decrease of inhibition in the left orbitofrontal cortex because of fewer PV+ neurons. This results in an overall increase in excitatory signals from pyramidal cells (Pyr) and dysregulation of the function of the left orbitofrontal cortex. Figure created with BioRender.com.

Jeon et al. (2023) demonstrated how social experiences during adolescence are vital to shaping behaviors in adulthood along with the neural circuits responsible for producing these behaviors. Not only did they identify social isolation during late adolescence as a driver for the decrease in PV⁺ neurons in the OFC_L in adulthood but they showed that this reduction in PV⁺ neurons was responsible for hypersociability in adulthood. This finding is further reinforced by the use of CRISPR-mediated knockdown of the PV gene in the OFC_L to increase sociability. Moreover, neural activity of the OFC_L increased following the reduction of PV⁺ neurons and resulted in a dysregulation of the balance between inhibitory and excitatory neurons. Finally, the optogenetic manipulation of PV⁺ neurons really strengthens the argument that PV⁺ neurons in the OFC_L play a regulatory role in social interaction in adult female mice.

Despite there seemingly not being a key role of PNNs in shaping development of social investigation, it does not mean nothing was revealed regarding potential PNN changes during social isolation. In this article, they found that despite PV⁺ neurons decreasing in the OFC_L, PNNs did not change at all, which was surprising given that PNNs primarily surround cortical PV⁺ neurons and very little else (Pizzorusso et al., 2002). This might suggest there might be a compensatory response resulting in an increase in PNNs surrounding other cell types. Another explanation for no changes in PNNs being seen in the study is that the study may have been underpowered to detect the decrease in PNNs that accompanies the decrease in PV+ neurons. This would result in the decrease in PNNs not being statistically significant despite a real decrease in PNNs occurring. Analyzing the percentage of PNNs that are colabeled with a PV⁺ neuron would answer both of these questions. From there, further questions can be asked regarding the close relationship between PNN and PV⁺ neuron development.

Overall, this study adds to our growing knowledge of lateralization of brain function as well as sex differences in experience-dependent neural plasticity. Studies such as this further encourage the use of multiple techniques to verify results in a rigorous manner.