Somatostatin-Expressing Neurons in the Ventral Tegmental Area Innervate Specific Forebrain Regions and Are Involved in Stress Response

Expanding knowledge about the cellular composition of subcortical brain regions demonstrates large heterogeneity and differences from the cortical architecture. Previously we described three subtypes of somatostatin-expressing (Sst) neurons in the mouse ventral tegmental area (VTA) and showed their local inhibitory action on the neighboring dopaminergic neurons (Nagaeva et al., 2020). Here, we report that Sst+ neurons especially from the anterolateral part of the mouse VTA also project far outside the VTA and innervate forebrain regions that are mainly involved in the regulation of emotional behavior, including the ventral pallidum, lateral hypothalamus, the medial part of the central amygdala, anterolateral division of the bed nucleus of stria terminalis, and paraventricular thalamic nucleus. Deletion of these VTASst neurons in mice affected several behaviors, such as home cage activity, sensitization of locomotor activity to morphine, fear conditioning responses, and reactions to the inescapable stress of forced swimming, often in a sex-dependent manner. Together, these data demonstrate that VTASst neurons have selective projection targets distinct from the main targets of VTA dopamine neurons. VTASst neurons are involved in the regulation of behaviors primarily associated with the stress response, making them a relevant addition to the efferent VTA pathways and stress-related neuronal network.

Pharmacological and optical activation of TrkB in Parvalbumin interneurons regulate intrinsic states to orchestrate cortical plasticity

Elevated states of brain plasticity typical for critical periods of early postnatal life can be reinstated in the adult brain through interventions, such as antidepressant treatment and environmental enrichment, and induced plasticity may be critical for the antidepressant action. Parvalbumin-positive (PV) interneurons regulate the closure of developmental critical periods and can alternate between high and low plasticity states in response to experience in adulthood. We now show that PV plasticity states and cortical networks are regulated through the activation of TrkB neurotrophin receptors. Visual cortical plasticity induced by fluoxetine, a widely prescribed selective serotonin reuptake inhibitor (SSRI) antidepressant, was lost in mice with reduced expression of TrkB in PV interneurons. Conversely, optogenetic gain-of-function studies revealed that activation of an optically activatable TrkB (optoTrkB) specifically in PV interneurons switches adult cortical networks into a state of elevated plasticity within minutes by decreasing the intrinsic excitability of PV interneurons, recapitulating the effects of fluoxetine. TrkB activation shifted cortical networks towards a low PV configuration, promoting oscillatory synchrony, increased excitatory-inhibitory balance, and ocular dominance plasticity. OptoTrkB activation promotes the phosphorylation of Kv3.1 channels and reduces the expression of Kv3.2 mRNA providing a mechanism for the lower excitability. In addition, decreased expression and puncta of Synaptotagmin2 (Syt2), a presynaptic marker of PV interneurons involved in Ca²⁺-dependent neurotransmitter release, suggests lower inputs onto pyramidal neurons suppressing feed-forward inhibition. Together, the results provide mechanistic insights into how TrkB activation in PV interneurons orchestrates the activity of cortical networks and mediating antidepressant responses in the adult brain.

Kainate receptors regulate development of glutamatergic synaptic circuitry in the rodent amygdala

Perturbed information processing in the amygdala has been implicated in developmentally originating neuropsychiatric disorders. However, little is known on the mechanisms that guide formation and refinement of intrinsic connections between amygdaloid nuclei. We demonstrate that in rodents the glutamatergic connection from basolateral to central amygdala (BLA-CeA) develops rapidly during the first 10 postnatal days, before external inputs underlying amygdala-dependent behaviors emerge. During this restricted period of synaptic development, kainate-type of ionotropic glutamate receptors (KARs) are highly expressed in the BLA and tonically activated to regulate glutamate release via a G-protein-dependent mechanism. Genetic manipulation of this endogenous KAR activity locally in the newborn LA perturbed development of glutamatergic input to CeA, identifying KARs as a physiological mechanism regulating formation of the glutamatergic circuitry in the amygdala.

Isoflurane produces antidepressant effects and induces TrkB signaling in rodents

A brief burst-suppressing isoflurane anesthesia has been shown to rapidly alleviate symptoms of depression in a subset of patients, but the neurobiological basis of these observations remains obscure. We show that a single isoflurane anesthesia produces antidepressant-like behavioural effects in the learned helplessness paradigm and regulates molecular events implicated in the mechanism of action of rapid-acting antidepressant ketamine: activation of brain-derived neurotrophic factor (BDNF) receptor TrkB, facilitation of mammalian target of rapamycin (mTOR) signaling pathway and inhibition of glycogen synthase kinase 3β (GSK3β). Moreover, isoflurane affected neuronal plasticity by facilitating long-term potentiation in the hippocampus. We also found that isoflurane increased activity of the parvalbumin interneurons, and facilitated GABAergic transmission in wild type mice but not in transgenic mice with reduced TrkB expression in parvalbumin interneurons. Our findings strengthen the role of TrkB signaling in the antidepressant responses and encourage further evaluation of isoflurane as a rapid-acting antidepressant devoid of the psychotomimetic effects and abuse potential of ketamine.

Familial Alzheimer's disease-linked presenilin-1 mutation M146V affects store-operated calcium entry: does gain look like loss?

Alzheimer's disease (AD) is a neurodegenerative disorder that leads to neuron death and synapse loss in the hippocampus and cortex, with consequent cognitive disability and dementia. Mutations in the presenilin-1 (PS1) gene lead to familial Alzheimer's disease (FAD). Here, we report that the expression of FAD-linked PS1 M146V mutant affects store-operated calcium channel activity (Isoc) in human neuroblastoma SK-N-SH cells. Electrophysiological measurements and calcium imaging experiments have revealed the emergent role of calcium sensor STIM2 in the inhibition of calcium release-activated calcium channel activity (Icrac) and enhancement of intracellular Ca(2+) stores content due to PS1 M146V mutant expression. In general, the results of this study suggest that the pathological inhibition of one type of store-operated calcium channels caused by FAD PS1 mutant expression may be accounted for by preceding gain of spontaneous activity of store-operated calcium channels driven by STIM2.

Homer regulation of native plasma membrane calcium channels in A431 cells

Homers are adapter proteins that play a significant role in the organization of calcium signaling protein complexes. Previous functional studies linked Homer proteins to calcium influx in nonexcitable cells. These studies utilized calcium imaging or whole-cell current recordings. Because of limited resolution of these methods, an identity of Homer-modulated ion channels remained unclear. There are several types of plasma membrane calcium influx channels in A431 cells. In the present study, we demonstrated that Homer dissociation resulted in specific activation of I(min) channels but not of I(max) channels in inside-out patches taken from A431 cells. In contrast, inositol 1,4,5-trisphosphate activated both I(min) and I(max) channels in inside-out patches. Short (1a) and long (1c) forms of Homer had different effects on I(min) channel activity. Homer 1a but not Homer 1c activated I(min) in the patches. This study indicates that I(min) channels are specifically regulated by Homer proteins in A431 cells.