Neurotransmission through dopamine D1 receptors is required for aversive memory formation and Arc activation in the cerebral cortex

Mechanisms and Functions of Activity-Regulated Cytoskeleton-Associated Protein in Synaptic Plasticity

Activity-regulated cytoskeleton-associated protein (Arc) is one of the most important regulators of cognitive functions in the brain regions. As a hub protein, Arc plays different roles in modulating synaptic plasticity. Arc supports the maintenance of long-term potentiation (LTP) by regulating actin cytoskeletal dynamics, while it guides the endocytosis of AMPAR in long-term depression (LTD). Moreover, Arc can self-assemble into capsids, leading to a new way of communicating among neurons. The transcription and translation of the immediate early gene Arc are rigorous procedures guided by numerous factors, and RNA polymerase II (Pol II) is considered to regulate the precise timing dynamics of gene expression. Since astrocytes can secrete brain-derived neurotrophic factor (BDNF) and L-lactate, their unique roles in Arc expression are emphasized. Here, we review the entire process of Arc expression and summarize the factors that can affect Arc expression and function, including noncoding RNAs, transcription factors, and posttranscriptional regulations. We also attempt to review the functional states and mechanisms of Arc in modulating synaptic plasticity. Furthermore, we discuss the recent progress in understanding the roles of Arc in the occurrence of major neurological disorders and provide new thoughts for future research on Arc.

Cerebrocortical activation following unilateral labyrinthectomy in mice characterized by whole-brain clearing: implications for sensory reweighting

Posture and gait are maintained by sensory inputs from the vestibular, visual, and somatosensory systems and motor outputs. Upon vestibular damage, the visual and/or somatosensory systems functionally substitute by cortical mechanisms called “sensory reweighting”. We investigated the cerebrocortical mechanisms underlying sensory reweighting after unilateral labyrinthectomy (UL) in mice. Arc-dVenus transgenic mice, in which the gene encoding the fluorescent protein dVenus is transcribed under the control of the promoter of the immediate early gene Arc, were used in combination with whole-brain three-dimensional (3D) imaging. Performance on the rotarod was measured as a behavioral correlate of sensory reweighting. Following left UL, all mice showed the head roll-tilt until UL10, indicating the vestibular periphery damage. The rotarod performance worsened in the UL mice from UL1 to UL3, which rapidly recovered. Whole-brain 3D imaging revealed that the number of activated neurons in S1, but not in V1, in UL7 was higher than that in sham-treated mice. At UL7, medial prefrontal cortex (mPFC) and agranular insular cortex (AIC) activation was also observed. Therefore, sensory reweighting to the somatosensory system could compensate for vestibular dysfunction following UL; further, mPFC and AIC contribute to the integration of sensory and motor functions to restore balance.

Photostimulation of Ventral Tegmental Area-Insular Cortex Dopaminergic Inputs Enhances the Salience to Consolidate Aversive Taste Recognition Memory via D1-Like Receptors

Taste memory involves storing information through plasticity changes in the neural network of taste, including the insular cortex (IC) and ventral tegmental area (VTA), a critical provider of dopamine. Although a VTA-IC dopaminergic pathway has been demonstrated, its role to consolidate taste recognition memory remains poorly understood. We found that photostimulation of dopaminergic neurons in the VTA or VTA-IC dopaminergic terminals of TH-Cre mice improves the salience to consolidate a subthreshold novel taste stimulus regardless of its hedonic value, without altering their taste palatability. Importantly, the inhibition of the D1-like receptor into the IC impairs the salience to facilitate consolidation of an aversive taste recognition memory. Finally, our results showed that VTA photostimulation improves the salience to consolidate a conditioned taste aversion memory through the D1-like receptor into the IC. It is concluded that the dopamine activity from the VTA into IC is required to increase the salience enabling the consolidation of a taste recognition memory. Notably, the D1-like receptor activity into the IC is required to consolidate both innate and learned aversive taste memories but not appetitive taste memory.

D1 Receptor Mediated Dopaminergic Neurotransmission Facilitates Remote Memory of Contextual Fear Conditioning

Dopaminergic neurotransmission via dopamine D1 receptors (D1Rs) is considered to play an important role not only in reward-based learning but also in aversive learning. The contextual and auditory cued fear conditioning tests involve the processing of classical fear conditioning and evaluates aversive learning memory. It is possible to evaluate aversive learning memory in two different types of neural transmission circuits. In addition, when evaluating the role of dopaminergic neurotransmission via D1R, to avoid the effects in D1R-mediated neural circuitry alterations during development, it is important to examine using mice who D1R expression in the mature stage is suppressed. Herein, we investigated the role of dopaminergic neurotransmission via D1Rs in aversive memory formation in contextual and auditory cued fear conditioning tests using D1R knockdown (KD) mice, in which the expression of D1Rs could be conditionally and reversibly controlled with doxycycline (Dox) treatment. For aversive memory, we examined memory formation using recent memory 1 day after conditioning, and remote memory 4 weeks after conditioning. Furthermore, immunostaining of the brain tissues of D1RKD mice was performed after aversive footshock stimulation to investigate the distribution of activated c-Fos, an immediate-early gene, in the hippocampus (CA1, CA3, dentate gyrus), striatum, amygdala, and prefrontal cortex during aversive memory formation. After aversive footshock stimulation, immunoblotting was performed using hippocampal, striatal, and amygdalar samples from D1RKD mice to investigate the increase in the amount of c-Fos and phosphorylated SNAP-25 at Ser187 residue. When D1R expression was suppressed using Dox, behavioral experiments revealed impaired contextual fear learning in remote aversion memory following footshock stimulation. Furthermore, expression analysis showed a slight increase in the post-stimulation amount of c-Fos in the hippocampus and striatum, and a significant increase in the amount of phosphorylated SNAP-25 in the hippocampus, striatum, and prefrontal cortex before and after stimulation. These findings indicate that deficiency in D1R-mediated dopaminergic neurotransmission is an important factor in impairing contextual fear memory formation for remote memory.

The Effects of Intravermis Cerebellar Microinjections of Dopaminergic Agents in Motor Learning and Aversive Memory Acquisition in Mice

The cerebellum receives dopaminergic innervation and expresses the five types of described dopaminergic receptors. The cerebellar function involves both motor movement and cognition, but the role of cerebellar dopaminergic system on these processes remain unclear. The present study explores the behavioral responses to intracerebellar microinjection of dopaminergic agents in motor and emotional memory. For this, naïve Swiss mice had their cerebellar vermis implanted with a guide canula, received a intravermis microinjection of Dopamine, D1-like antagonist SCH-23390 or D2-like antagonist Eticlopride, and underwent a behavioral analysis of motor learning (by a Rotarod and balance beam learning protocol) or aversive memory acquisition (by the inhibitory avoidance task). The mixed-effects analysis was used to evaluate groups performance, followed by Tukey’s post hoc when appropriated. In this study, Dopamine, SCH-23390 and Eticlopride at the doses used did not affected motor control and motor learning. In addition, the administration of Dopamine and SCH-233390 had no effects on emotional memory acquisition, but the animals that received the highest dose of Eticlopride had an improvement in aversive memory acquisition, shown by a suppression of its innate preference for the dark compartment of the inhibitory avoidance apparatus following an exposure to a foot shock. We propose that cerebellar dopaminergic D2 receptors seem to participate on the modulation of aversive memory processes, without influencing motor performance at the doses used in this study.