Studies suggest that catecholaminergic projections to the forebrain mediate reward-related learning and memory consolidation (Schultz, 2001, Berke, 2018, Ranjbar-Slamloo and Fazlali, 2020). However, recent evidence indicates that dopamine activity is also involved in establishing aversive-related memories (Castillo Díaz et al., 2022, Gil-Lievana et al., 2022), including taste aversion memory (Guzmán-Ramos et al., 2010, Osorio-Gómez et al., 2021, Gil-Lievana et al., 2022); highlighting its involvement in emotional information modulation. While dopamine neurotransmission is pivotal for emotional memory formation, it is not the only mechanism engaged. Additional mechanisms, such as glutamatergic neurotransmission, are also required (Prado et al., 2016, Russo and Nestler, 2013). Both dopaminergic and glutamatergic systems in the insular cortex (IC) are related to various cognitive processes, including reward expectation and decision-making (Ishii et al., 2015; Mizoguchi & Yamada, 2019)). Notably, both catecholaminergic and glutamatergic projections to the IC are essential for maintaining rewarding contextual (Gil-Lievana et al., 2020) and taste aversion memories (Guzmán-Ramos et al., 2010, Osorio-Gómez et al., 2021, Gil-Lievana et al., 2022). Catecholaminergic and glutamatergic neurons project to the agranular region of the anterior IC (Gehrlach et al., 2020, Gerfen and Clavier, 1979, Kayyal et al., 2019). The anterior agranular insula has been linked to pain relief (Coffeen et al., 2008, Jung et al., 2016, Gamal-Eltrabily et al., 2020), fear and anxiety (Shi et al., 2020), contextual and cue control of addictive behavior (Arguello et al., 2017, Sun et al., 2020), and is affected during the chronic consumption of addictive substances (Luo et al., 2021). Therefore, the IC is a multimodal hub that integrates emotional, motivational, gustatory, and olfactory information (Brunert and Rothermel, 2020).

The ventral tegmental area (VTA) activity is related to reward, motivation, drug addiction, social behaviors, and stress (Margolis et al., 2006, Bariselli et al., 2016). The dopaminergic neurons originated from the VTA project to several forebrain structures, comprising the IC and basolateral amygdaloid nucleus (BLA), among others (Gil-Lievana et al., 2020, Gil-Lievana et al., 2022, Ohara et al., 2003, Naqvi and Bechara, 2009). Modulation of the VTA-IC projection has been associated with enhancing stimulus salience and has a crucial role in preference and aversive behavior (Gil-Lievana et al., 2020, Gil-Lievana et al., 2022). This modulation is likely to regulate neuronal plasticity in the brain.

The IC receives fibers from the BLA (Gerfen and Clavier, 1979, Shi and Cassell, 1998, Uematsu et al., 2015). This projection has been implicated in the acquisition and maintenance of various behavioral tasks, such as conditioned taste aversion, place preference, or taste familiarity (Abe et al., 2020, Moraga-Amaro et al., 2014, Kayyal et al., 2019, Gil-Lievana et al., 2020, Chen et al., 2018, Chen et al., 2022, Rivera-Olvera et al., 2018, Rodríguez-Durán et al., 2017). Additionally, the BLA processes information on internal states through recurrent connections with mesencephalic and diencephalic nuclei, such as the parabrachial nucleus, the nucleus of the solitary tract, and the ventromedial hypothalamus, which are directly involved in the primary processing of visceral and metabolic information (Pitkänen et al., 2000, Gauriau and Bernard, 2002, King, 2006, Knapska et al., 2007).

The IC integrates internal and external stimuli to guide the behavior of approaching or distancing stimuli to maintain homeostasis (Ibrahim et al., 2019). The BLA and IC are believed to form a system that links external stimuli with representations of rewarding or aversively motivated learning (Mannella et al., 2016, Guzmán-Ramos et al., 2012). Additionally, the reciprocal connections between the IC and BLA have been linked to establishing the valence of taste stimuli (Kayyal et al., 2019). Due to its connections with the amygdala, the IC is regarded as a hub that integrates interoceptive information with emotional information (Gehrlach et al., 2020, Hsueh et al., 2023).

Long-term potentiation (LTP) is a well-known phenomenon observed in the IC, dependent on NMDA receptor activity (Escobar et al., 1998, Rodríguez-Durán et al., 2017). LTP has been proposed as a memory storage mechanism in various brain regions, embracing the hippocampus and neocortex (Izquierdo et al., 2008, Cascella and Al Khalili, 2022, Escobar and Derrick, 2007). In the BLA-IC pathway, LTP has been linked to the increased maintenance of conditioned taste aversion (Escobar and Bermúdez-Rattoni, 2000, Urrieta and Escobar, 2021). LTP can be induced with a variety of electrical stimulation protocols (Nakao et al., 2004, Albensi et al., 2007, Hernandez et al., 2005, Bliss and Cooke, 2011), as well as through training in specific memory tasks (Whitlock et al., 2006, Rioult-Pedotti et al., 2007, Zhu et al., 2011, Kim and Cho, 2017), sensory stimulation (Gambino et al., 2014, Spriggs et al., 2019), or pharmacologically (Shetty and Sajikumar, 2017, Rivera-Olvera et al., 2016). However, while there exist numerous protocols for inducing LTP, these approaches often fail to achieve the desired specificity level in targeting specific neurons and their distinct neurotransmitter profiles.

Optogenetics is a powerful tool for selectively inducing LTP in specific cell populations. For instance, optogenetic LTP (oLTP) can be induced through direct monosynaptic stimulation of fibers expressing a modified channel rhodopsin (ChR) receptor that enhances the channel kinetics or improves the response to high-frequency stimulation (HFS; Nabavi et al., 2014, Kim and Cho, 2017). Another approach is to simultaneously activate pre-synaptic terminals and post-synaptic neurons using optogenetics methods (Ma et al., 2018). Despite the successful application of LTP in various brain regions, it is essential to consider that these manipulations may lead to non-physiological responses and may not fully replicate the natural processes involved in synaptic plasticity and memory formation. Therefore, investigating the effects of concurrent optogenetic activation of glutamatergic and dopaminergic projections, mirroring the processes observed during taste-aversion memory formation (Guzmán-Ramos et al., 2010), in the induction of LTP within the neocortex may provide insights into the mechanistic basis of neuronal plasticity that underpins the formation of motivational memories.

In this study, we aimed to investigate the role of concomitant optogenetic activation of the glutamatergic BLA-IC and dopaminergic VTA-IC projections in inducing cortical plasticity. Our findings indicate that the simultaneous activation of these pathways leads to a slow-onset LTP in electrically evoked field EPSPs in the IC elicited by BLA stimulation; this LTP is intricately dependent on the cooperative involvement of both dopamine and glutamatergic receptors. These results suggest a synergic interaction between the dopaminergic and glutamatergic systems for IC-LTP induction and provide insights into the mechanisms underlying IC-related learning and memory.