Locus coeruleus noradrenergic neurons phase-lock to prefrontal and hippocampal infra-slow rhythms that synchronize to behavioral events

Dissecting the Hippocampal Regulation of Approach-Avoidance Conflict: Integrative Perspectives From Optogenetics, Stress Response, and Epigenetics

Psychiatric disorders are multifactorial conditions without clear biomarkers, influenced by genetic, environmental, and developmental factors. Understanding these disorders requires identifying specific endophenotypes that help break down their complexity. Here, we undertake an in-depth analysis of one such endophenotype, namely imbalanced approach-avoidance conflict (AAC), reviewing its significant dependency on the hippocampus. Imbalanced AAC is a transdiagnostic endophenotype, being a feature of many psychiatric conditions in humans. However, it is predominantly examined in preclinical research through paradigms that subject rodents to conflict-laden scenarios. This review offers an original perspective by discussing the AAC through three distinct lights: optogenetic modulation of the AAC, which updates our understanding of the hippocampal contribution to behavioral inhibition; the impact of environmental stress, which exacerbates conflict and strengthens the stress-psychopathology axis; and inherent epigenetic aspects, which uncover crucial molecular underpinnings of environmental (mal) adaptation. By integrating these perspectives, in this review we aim to underline a cross-species causal nexus between heightened hippocampal activity and avoidance behavior. In addition, we suggest a rationale to explore epigenetic pharmacology as a potential strategy to tackle AAC-related psychopathology. This review assumes greater significance when viewed through the lens of advancing AAC-centric diagnostics in human subjects. Unlike traditional questionnaires, which struggle to accurately measure individual differences in AAC-related dimensions, new approaches using virtual reality and computer games show promise in better focusing the magnitude of AAC contribution to psychopathology.

Frequency Specific Optogenetic Stimulation of the Locus Coeruleus Induces Task-Relevant Plasticity in the Motor Cortex

The locus ceruleus (LC) is the primary source of neocortical noradrenaline, which is known to be involved in diverse brain functions including sensory perception, attention, and learning. Previous studies have shown that LC stimulation paired with sensory experience can induce task-dependent plasticity in the sensory neocortex and in the hippocampus. However, it remains unknown whether LC activation similarly impacts neural representations in the agranular motor cortical regions that are responsible for movement planning and production. In this study, we test whether optogenetic stimulation of the LC paired with motor performance is sufficient to induce task-relevant plasticity in the somatotopic cortical motor map. Male and female TH-Cre + rats were trained on a skilled reaching lever-pressing task emphasizing the use of the proximal forelimb musculature, and a viral approach was used to selectively express ChR2 in noradrenergic LC neurons. Once animals reached criterial behavioral performance, they received five training sessions in which correct task performance was paired with optogenetic stimulation of the LC delivered at 3, 10, or 30 Hz. After the last stimulation session, motor cortical mapping was performed using intracortical microstimulation. Our results show that lever pressing paired with LC stimulation at 10 Hz, but not at 3 or 30 Hz, drove the expansion of the motor map representation of the task-relevant proximal FL musculature. These findings demonstrate that phasic, training-paired activation of the LC is sufficient to induce experience-dependent plasticity in the agranular motor cortex and that this LC-driven plasticity is highly dependent on the temporal dynamics of LC activation.

Rhythmic oscillations in the midbrain dopaminergic nuclei in mice

Introduction

Dopamine release in the forebrain by midbrain ventral tegmental nucleus (VTA) and substantia nigra pars compacta (SNc) neurons is implicated in reward processing, goal-directed learning, and decision-making. Rhythmic oscillations of neural excitability underlie coordination of network processing, and have been reported in these dopaminergic nuclei at several frequency bands. This paper provides a comparative characterization of several frequencies of oscillations of local field potential and single unit activity, highlighting some behavioral correlates.

Methods

We recorded from optogenetically identified dopaminergic sites in four mice training in operant olfactory and visual discrimination tasks.

Results

Rayleigh and Pairwise Phase Consistency (PPC) analyses revealed some VTA/SNc neurons phase-locked to each frequency range, with fast spiking interneurons (FSIs) prevalent at 1-2.5 Hz (slow) and 4 Hz bands, and dopaminergic neurons predominant in the theta band. More FSIs than dopaminergic neurons were phase-locked in the slow and 4 Hz bands during many task events. The highest incidence of phase-locking in neurons was in the slow and 4 Hz bands, and occurred during the delay between the operant choice and trial outcome (reward or punishment) signals.

Discussion

These data provide a basis for further examination of rhythmic coordination of activity of dopaminergic nuclei with other brain structures, and its impact for adaptive behavior.