Therapeutic intervention for schizophrenia relies on blockade of dopamine D2 receptors in the associative striatum; however, there is little evidence for baseline overdrive of the dopamine system. Instead, the dopamine system is in a hyper-responsive state due to excessive drive by the hippocampus. This causes more dopamine neurons to be in a spontaneously active, hyper-responsive state. Antipsychotic drugs alleviate this by causing depolarization block, or excessive depolarization-induced dopamine neuron inactivation. Indeed, both first- and second-generation antipsychotic drugs cause depolarization block in the ventral tegmentum to relieve positive symptoms, whereas first-generation drugs also cause depolarization in the nigrostriatal dopamine system to lead to extrapyramidal side effects. However, by blocking dopamine receptors, these drugs are activating multiple synapses downstream from the proposed site of pathology: the loss of inhibitory influence over the hippocampus. An overactive hippocampus not only drives the dopamine-dependent positive symptoms, but via its projections to the amygdala and the neocortex can also drive negative and cognitive symptoms, respectively. On this basis, a novel class of drugs that can reverse schizophrenia at the site of pathology, i.e., the hippocampal overdrive, could be effective in alleviating all three classes of symptoms of schizophrenia while also being better tolerated.

Background: Blockade of cannabinoid CB1 or vanilloid TRPV1 receptors in the ventromedial prefrontal cortex of rats respectively increases or decreases the conditioned emotional response during re-exposure to a context previously paired with footshocks. Although these mechanisms are unknown, they may involve local modulation of glutamatergic and nitrergic signaling. Aim: We investigated whether these mechanisms are involved in the reported effects of CB1 and TRPV1 modulation in the ventromedial prefrontal cortex. Methods: Freezing behavior and autonomic parameters were recorded during the conditioned response expression. Results: The CB1 receptors antagonist NIDA, or the TRPV1 agonist capsaicin (CPS) in the ventromedial prefrontal cortex increased the conditioned emotional response expression, and these effects were prevented by TRPV1 and CB1 antagonism, respectively. The increased conditioned emotional response evoked by NIDA and CPS were prevented by an NMDA antagonist or a neuronal nitric oxide synthase inhibitor. A nitric oxide scavenger or a soluble guanylate cyclase inhibitor prevented only the NIDA effects and the CPS effect was prevented by a non-selective antioxidant drug, as nitric oxide can also induce reactive oxygen species production. Conclusion: Our results suggest that CB1 and TRPV1 receptors in the ventromedial prefrontal cortex differently modulate the expression of conditioned emotional response through glutamatergic and nitrergic mechanisms, although different pathways may be involved.

The prefrontal cortex (PFC) is critical for regulating stress responses through top‐down control over limbic and subcortical structures. The PFC undergoes a prolonged developmental process that only reaches maturation during adulthood, causing it to be highly sensitive to environmental insults during neurodevelopment, such as adolescence. During this critical period, synaptic pruning, the maturation of inhibitory GABAergic interneurons, and the refinement of dopaminergic transmission collectively establish the excitatory‐inhibitory balance necessary for adaptive behavior. Impairment of the PFC due to developmental disruptions increases susceptibility to maladaptive stress responses. These responses can, in turn, contribute to the development of major depressive disorder and schizophrenia. In depression, a dysfunctional PFC fails to effectively inhibit the amygdala, which contributes to hyperactivity in stress‐related circuits, hypodopaminergic states, and anhedonia. In schizophrenia, a neurodevelopmental PFC dysfunction would precipitate hippocampal circuit disruption driven by stress. The inability of an immature PFC to regulate the amygdala response to stress would trigger an increased excitatory drive to the ventral hippocampus, which is proposed to underlie the excessive limbic drive, hippocampal hyperactivity, and a hyperdopaminergic state. In addition, the activation of the mesocortical dopaminergic system by stress facilitates the PFC response to stress, both during adulthood and adolescence. A dopamine (DA)‐induced unregulated stress response disrupts the excitatory and inhibitory transmission within the PFC, which plays a critical role in its function. Understanding the interplay between stress and PFC activity/maturation to regulate the circuit toward adaptive or maladaptive outcomes offers critical insights for early intervention and prevention. Early changes in the PFC could underlie vulnerability to unregulated stress response and its consequent effect in contributing to schizophrenia and depression. In this way, early intervention may limit the impact and prevent further circuit dysregulation leading to pathological states. image