Differential regulation of prelimbic and thalamic transmission to the basolateral amygdala by acetylcholine receptors

The amygdalar anterior basolateral nucleus (BLa) plays a vital role in emotional behaviors. This region receives dense cholinergic projections from basal forebrain which are critical in regulating neuronal activity and synaptic transmission. Cholinergic signaling in BLa is thought to occur through both a slow mode of volume transmission as well as a rapid, phasic mode. However, the relative effect of each mode of signaling in BLa is not understood. Here, we used electrophysiology and optogenetics in mouse brain slices to compare regulation of afferent input from cortex and thalamus to the BLa by these two modes of transmission. Phasic ACh release evoked by single pulse stimulation of cholinergic terminals had a biphasic effect on glutamatergic transmission at cortical input, producing rapid nicotinic receptor-mediated facilitation followed by slower muscarinic receptor (mAChR)-mediated depression. In contrast, tonic elevation of ACh through application of the cholinesterase inhibitor physostigmine suppressed glutamatergic transmission at cortical inputs through mAChRs only. This suppression was not observed at thalamic inputs to BLa. In agreement with this pathway-specificity, the mAChR agonist, muscarine more potently suppressed transmission at inputs from prelimbic cortex (PL) than thalamus. Muscarinic inhibition at PL input was dependent on presynaptic M4 mAChRs, while at thalamic input it depended upon M3 mAChR-mediated stimulation of retrograde endocannabinoid signaling. Muscarinic inhibition at both pathways was frequency-dependent, allowing only high frequency activity to pass. These findings demonstrate complex cholinergic regulation of afferent input to BLa that depends upon the mode of ACh release and is both pathway specific and frequency dependent.

The mitochondrial calcium uniporter regulates mitochondrial mass and dendritic localization in distinct hippocampal circuits

CA2 is an understudied subregion of the hippocampus that is critical for social memory. Previous studies identified multiple components of the mitochondrial calcium uniporter (MCU) complex as selectively enriched in CA2, however the functional significance of this enrichment remains unclear. The MCU complex regulates calcium entry into mitochondria, which in turn regulates mitochondrial transport and localization to active synapses. We found that MCU is strikingly enriched in CA2 distal apical dendrites, precisely where CA2 neurons receive entorhinal cortical input carrying social information. Further, MCU-enriched mitochondria in CA2 distal dendrites are larger compared to mitochondria in CA2 proximal apical dendrites and neighboring CA1 apical dendrites. Genetic knockdown of MCU in CA2 resulted in smaller mitochondria in CA2 distal dendrites, indicating that MCU expression plays a role in regulating mitochondrial mass in CA2. MCU overexpression in neighboring CA1 led to larger mitochondria preferentially in proximal dendrites compared to distal dendrites and GFP controls. Our findings demonstrate that mitochondria are molecularly and structurally diverse across hippocampal cell types and circuits, and that MCU expression cell-autonomously regulates mitochondrial mass, but layer-specific dendritic localization depends on cell type. Our data support the idea that CA2 mitochondria are functionally distinct from CA1 mitochondria, which may confer unique synaptic and circuit properties underlying CA2 function in social memory.

A striatal circuit balances learned fear in the presence and absence of sensory cues

During fear learning, defensive behaviors need to be finely balanced, to allow animals to return to normal behaviors after the termination of threat-indicating sensory cues. Nevertheless, the circuits underlying such balancing are largely unknown. Here, we investigate the role of direct (D1R+) - and indirect (Adora+) pathway neurons of the amygdala-striatal transition zone (AStria) in fear learning. In-vivo Ca2+ imaging revealed that fear learning increased the responses of D1R+ AStria neurons to an auditory CS, given that the animal moved. In Adora+ neurons, fear learning also induced a differential activity during freezing and movement, albeit with little influence of the CS. In-vivo optogenetic silencing during the training day showed that plasticity in D1R+ AStria neurons contributes to auditory-cued fear memories, whereas Adora+ neurons suppressed learned freezing when no CS was present. Circuit tracing experiments identified cortical input structures to the AStria, and recording of optogenetically-evoked EPSCs at the corresponding projection revealed different forms of long-term plasticity at synapses onto D1R+ and Adora+ AStria neurons. Taken together, direct- and indirect pathways neurons of the AStria show differential signs of in-vivo and ex-vivo plasticity after fear learning, and balance defensive behaviors in the presence and absence of aversively motivated sensory cues.

A corticothalamic circuit trades off speed for safety during decision-making under motivational conflict

Decisions to act while pursuing goals in the presence of danger must be made quickly but safely. Premature decisions risk injury or death whereas postponing decisions risk goal loss. Here we show how mice resolve these competing demands. Using microstructural behavioral analyses, we identified the spatiotemporal dynamics of approach-avoidance decisions under motivational conflict. Then we used cognitive modelling to show that these dynamics reflect the speeded decision-making mechanisms used by humans and non-human primates, with mice trading off decision speed for safety of choice when danger loomed. Using calcium imaging and functional circuit analyses, we show that this speed-safety trade off occurs because increases in paraventricular thalamus (PVT) activity increase decision caution, thereby increasing approach-avoid decision times in the presence of danger. Our findings demonstrate that a discrete brain circuit involving the PVT and its prefrontal cortical input dynamically adjusts decision caution during motivational conflict, trading off decision speed for decision safety when danger is close. They identify the corticothalamic pathway as central to cognitive control during decision-making under conflict.

Biophysical and Architectural Mechanisms of Subthalamic Theta under Response Conflict

The cortico-basal ganglia circuit is needed to suppress prepotent actions and to facilitate controlled behavior. Under conditions of response conflict, the frontal cortex and subthalamic nucleus [STN] exhibit increased spiking and theta band power, which are linked to adaptive regulation of behavioral output. The electrophysiological mechanisms underlying these neural signatures of impulse control remain poorly understood. To address this lacuna, we constructed a novel large-scale, biophysically principled model of the subthalamopallidal [STN-Globus Pallidus externus (GPe)] network, and examined the mechanisms that modulate theta power and spiking in response to cortical input. Simulations confirmed that theta power does not emerge from intrinsic network dynamics but is robustly elicited in response to cortical input as burst events representing action selection dynamics. Rhythmic burst events of multiple cortical populations, representing a state of conflict where cortical motor plans vacillate in the theta range, led to prolonged STN theta and increased spiking, consistent with empirical literature. Notably, theta band signaling required NMDA, but not AMPA, currents, which were in turn related to a triphasic STN response characterized by spiking, silence and bursting periods. Finally, theta band resonance was also strongly modulated by architectural connectivity, with maximal theta arising when multiple cortical populations project to individual STN "conflict detector" units, due to an NMDA-dependent supralinear response. Our results provide insights into the biophysical principles and architectural constraints that give rise to STN dynamics during response conflict, and how their disruption can lead to impulsivity and compulsivity.

Patterns of Unilateral and Bilateral Projections From Layers 5 and 6 of the Auditory Cortex to the Inferior Colliculus in Mouse

The auditory cortex sends massive projections to the inferior colliculus, but the organization of this pathway is not yet well understood. Previous work has shown that the corticocollicular projection emanates from both layers 5 and 6 of the auditory cortex and that neurons in these layers have different morphological and physiological properties. It is not yet known in the mouse if both layer 5 and layer 6 project bilaterally, nor is it known if the projection patterns differ based on projection location. Using targeted injections of Fluorogold into either the lateral cortex or dorsal cortex of the inferior colliculus, we quantified retrogradely labeled neurons in both the left and right lemniscal regions of the auditory cortex, as delineated using parvalbumin immunostaining. After dorsal cortex injections, we observed that approximately 18–20% of labeled cells were in layer 6 and that this proportion was similar bilaterally. After lateral cortex injections, only ipsilateral cells were observed in the auditory cortex, and they were found in both layer 5 and layer 6. The ratio of layer 5:layer 6 cells after lateral cortex injection was similar to that seen after dorsal cortex injection. Finally, injections of different tracers were made into the two inferior colliculi, and an average of 15–17% of cells in the auditory cortex were double-labeled, and these proportions were similar in layers 5 and 6. These data suggest that (1) only the dorsal cortex of the inferior colliculus receives bilateral projections from the auditory cortex, (2) both the dorsal and lateral cortex of the inferior colliculus receive similar layer 5 and layer 6 auditory cortical input, and (3) a subpopulation of individual neurons in both layers 5 and 6 branch to innervate both dorsal cortices of the inferior colliculus.

The impact of phosphorylated Pten at threonine 366 on cortical connectivity and behaviour

The lipid phosphatase Pten (phosphatase and tensin homologue on chromosome 10) is a key tumour suppressor gene and an important regulator of neuronal signalling. Pten mutations have been identified in patients with autism spectrum disorders, characterized by macrocephaly, impaired social interactions and communication, repetitive behaviour, intellectual disability, and epilepsy. Pten enzymatic activity is regulated by a cluster of phosphorylation sites at the C-terminus of the protein. Here we specifically focussed on the role of Pten T366 phosphorylation and generated a knock-in mouse line in which Pten T366 was substituted with alanine (PtenT366A/T366A). We identify that phosphorylation of Pten at T366 controls neuron size and connectivity of brain circuits involved in sensory processing. We show in behavioural tests that PtenT366/T366A mice exhibit cognitive deficits and selective sensory impairments, with significant differences in male individuals. We identify restricted cellular overgrowth of cortical neurons in PtenT366A/T366A brains, linked to increases in both dendritic arborization and soma size. In a combinatorial approach of anterograde and retrograde monosynaptic tracing using rabies virus, we characterize differences in connectivity to the primary somatosensory cortex of PtenT366A/T366A brains, with imbalances in long-range cortico-cortical input to neurons. We conclude that phosphorylation of Pten at T366 controls neuron size and connectivity of brain circuits involved in sensory processing and propose that PTEN T366 signalling may account for a subset of autism-related functions of Pten.

Role of 5-HT1A Receptor in Vilazodone-Mediated Suppression of L-DOPA-Induced Dyskinesia and Increased Responsiveness to Cortical Input in Striatal Medium Spiny Neurons in an Animal Model of Parkinson’s Disease

L-DOPA therapy in Parkinson’s disease (PD) is limited due to emerging L-DOPA-induced dyskinesia. Research has identified abnormal dopamine release from serotonergic (5-HT) terminals contributing to this dyskinesia. Selective serotonin reuptake inhibitors (SSRIs) or 5-HT receptor (5-HTr) agonists can regulate 5-HT activity and attenuate dyskinesia, but they often also produce a loss of the antiparkinsonian efficacy of L-DOPA. We investigated vilazodone, a novel multimodal 5-HT agent with SSRI and 5-HTr1A partial agonist properties, for its potential to reduce dyskinesia without interfering with the prokinetic effects of L-DOPA, and underlying mechanisms. We assessed vilazodone effects on L-DOPA-induced dyskinesia (abnormal involuntary movements, AIMs) and aberrant responsiveness to corticostriatal drive in striatal medium spiny neurons (MSNs) measured with in vivo single-unit extracellular recordings, in the 6-OHDA rat model of PD. Vilazodone (10 mg/kg) suppressed all subtypes (axial, limb, orolingual) of AIMs induced by L-DOPA (5 mg/kg) and the increase in MSN responsiveness to cortical stimulation (shorter spike onset latency). Both the antidyskinetic effects and reversal in MSN excitability by vilazodone were inhibited by the 5-HTr1A antagonist WAY-100635, demonstrating a critical role for 5-HTr1A in these vilazodone actions. Our results indicate that vilazodone may serve as an adjunct therapeutic for reducing dyskinesia in patients with PD.

Dense cortical input to the rostromedial tegmental nucleus mediates aversive signaling

The rostromedial tegmental nucleus (RMTg) encodes negative reward prediction error (RPE) and plays an important role in guiding behavioral responding to aversive stimuli. While initial studies describing the RMTg revealed the presence of cortical afferents, the density and distribution of this input has not been explored in detail. In addition, the functional consequences of cortical modulation of RMTg signaling are only just beginning to be investigated. The current study anatomically and functionally characterizes cortical input to the RMTg in rats. Findings from this work reveal dense input spanning the entire medial prefrontal cortex (PFC) as well as the orbitofrontal cortex and anterior insular cortex. Afferents were most dense in the dorsomedial subregion of the PFC (dmPFC), an area which has also been implicated in both RPE signaling and aversive responding. RMTg-projecting dmPFC neurons originate in layer V and collateralize extensively throughout the brain. In-situ mRNA hybridization further revealed that neurons in this circuit are predominantly D1 receptor-expressing with a high degree of D2 receptor colocalization. Optogenetic stimulation of dmPFC terminals in the RMTg drives avoidance, and cFos expression is enhanced in this neural circuit during exposure to aversive stimuli. Exposure to such aversive stimuli results in significant physiological and structural plasticity suggestive of a loss of top-down modulation of RMTg-mediated signaling. Altogether, these data reveal the presence of a prominent cortico-subcortical projection involved in adaptive behavioral responding and provide a foundation for future work aimed at exploring alterations in circuit function in diseases characterized by deficits in cognitive control over the balance between reward and aversion.