Social behavior is a complex, innate motivational behavior frequently disrupted in neurodevelopmental and neuropsychiatric disorders. A key aspect of social behavior is the preference for novelty over familiarity, crucial for assessing new threats and opportunities. This novelty preference (NP) involves habituation with repeated exposures, a fundamental form of behavioral plasticity. While mesolimbic dopamine (DA) neurotransmission is implicated in social behavior and reward processing, the precise activity patterns mediating NP and the underlying circuitry remain unclear. Previous research has implicated the interpeduncular nucleus (IPN) in familiarity signaling and NP. This study aimed to elucidate the real-time dynamics of mesolimbic DA and the IPN's role in regulating NP, providing a detailed analysis of DA activity patterns and identifying a circuit potentially relevant to disorders involving impaired social novelty responses.

Existing literature establishes a link between mesolimbic DA, originating in the ventral tegmental area (VTA) and projecting to the ventral striatum, and social behavior in both humans and animal models. Mesolimbic DA is generally understood to compute reward prediction error (RPE) signals, updating the value of future events and driving learning. Positive social experiences are considered rewarding events where DA signals predict social cues and learning aspects of sociability. However, the exact activity patterns during the transition from novel to familiar social cues, the specific aspects of social interaction encoded by DA, and the real-time dynamics of DA during NP remain unclear. Prior work from the authors established the IPN as a neuroanatomical substrate for familiarity signaling in NP, but the dynamics of IPN neurons and their functional output projections remained unexplored. This study builds upon this foundation, focusing on the causal relationship between IPN activity and the regulation of mesolimbic DA signals during NP.

The study employed a combination of techniques in mice. A three-chamber sociability test was used to assess social behavior, measuring investigation time, bout number, and bout length. Fiber photometry, using GCaMP expression in dopamine transporter (DAT) Cre mice, recorded VTA DAergic neuronal activity during the sociability test. Similarly, the release of DA in the nucleus accumbens (NAc) was measured using the genetically encoded DA sensor dLight1.2. Optogenetic manipulations were employed to stimulate VTA DAergic neurons using AAV5 Cre-dependent Chrimson, and to inhibit the IPN→LDTg pathway using AAV2-retro Cre-dependent halorhodopsin (NpHR3.0). Viral tracing techniques were used to map the projections of IPN GAD2 neurons, and electrophysiological recordings in acute brain slices examined synaptic connections between the IPN and LDTg cholinergic neurons. Immunostaining and microscopy confirmed the expression of various markers in the targeted brain regions. Behavioral data, photometry recordings, and electrophysiological recordings were analyzed using statistical methods including t-tests and ANOVAs.

The study's key findings are as follows: 1. **VTA DAergic neuronal activity and NAc DA release encode the initial response to social novelty and bout length:** VTA DAergic neuron activity and NAc DA release significantly increased during the first interaction with a novel conspecific, correlating positively with the duration of the interaction. This response habituated with repeated exposures. 2. **Mesolimbic DA activity underlies NP by modulating the length of novel social interaction:** Optogenetic stimulation of VTA DAergic neurons increased the investigation time for novel social stimuli, primarily by prolonging the duration of interaction bouts. 3. **IPN GAD2 neurons are inhibited by social novelty and activated during the termination of familiar social investigations:** IPN GAD2 neuronal activity decreased during investigations of novel social stimuli but increased during the termination of familiar social interactions. 4. **The IPN→LDTg GAD2 projecting neurons are inhibited by social novelty and encode the offset of familiar social investigations:** The IPN→LDTg circuit showed activity patterns similar to overall IPN GAD2 neuron activity. 5. **Termination of familiar social interactions during social NP requires IPN→LDTg circuit activity:** Silencing the IPN→LDTg circuit prolonged familiar interaction bouts, disrupting the NP ratio. 6. **LDTg ChAT+ neurons projecting to the VTA receive GABAergic inputs from the IPN:** Electrophysiological recordings revealed monosynaptic inhibitory connections from IPN neurons to LDTg ChAT+ neurons projecting to the VTA. 7. **Activation of the IPN→LDTg circuit during social NP reduces the length of social novelty exploration by decreasing NAc DA release:** Photostimulation of the IPN→LDTg circuit reduced novel social investigation time and NAc DA release.

This study demonstrates that mesolimbic DA signaling, particularly in the VTA and NAc, is crucial for encoding social novelty and driving the duration of interaction. The initial response to novelty is strong and correlates with bout length, but habituates with familiarity. The IPN→LDTg circuit acts as a dynamic regulator of this DA response, influencing the balance between exploration of novel and familiar social stimuli. The IPN's inhibitory influence on the LDTg, which in turn projects to the VTA, provides a mechanism for modulating DA activity. Disruption of this circuit leads to altered social exploration patterns, highlighting its critical role in controlling social novelty preference. This circuit mechanism may have implications for understanding social motivation deficits in neurodevelopmental and neuropsychiatric disorders.

This study provides a comprehensive analysis of the neural circuits underlying social novelty preference, demonstrating the crucial role of mesolimbic DA and the IPN→LDTg pathway. The dynamic interaction between DA signaling and the IPN's inhibitory control over the LDTg finely tunes the balance between exploration of novel and familiar social stimuli. Future research could explore the specific roles of different neuronal subtypes within this circuit, examine the influence of other neurotransmitters, and investigate the potential for therapeutic interventions targeting this pathway to address social impairments in neurological and psychiatric disorders.

The study primarily focused on male mice, limiting the generalizability to females. While the study provided strong evidence for the role of the IPN→LDTg circuit, further investigations could explore the specific contributions of different neuronal subpopulations within this pathway. The optogenetic manipulations were performed in a closed-loop manner, which might not fully reflect the natural dynamics of the circuit. Finally, further research is required to translate these findings to human populations.