SK2 Channels: Key Circuit Determinant for Stress-Induced Amygdala Dysfunction

Anxiety disorders are one of the most common mental illnesses, encompassing general anxiety disorder, social phobia, post-traumatic stress disorder and obsessive-compulsive disorder, amongst others. Decades of research have significantly improved our understanding of the neural dysfunctions associated with anxiety disorders in both human studies and preclinical models. However, numerous patients remain treatment-resistant, as the limitations of current therapies persist (1). Further studies focused on investigating the neurobiological substrate underlying anxiety disorders and more specifically, neural circuit function and dysfunction, will allow for the establishment of new treatment options.

Anxiety disorders are multifactorial. Among multiple hereditary and environmental factors, stress exposure has emerged as a major risk factor for the development of mood disorders, resulting in an increasing number of preclinical studies using stress-induced models for anxiety Adaptive or anticipatory responses to a stressful environment are initiated through comparison of environmental stimuli to either innate program or memory and thus rely heavily on limbic and associated circuits in brain structures such as cortical brain areas, the thalamus, the hippocampus (HPC), the striatum and the amygdala complex (AMG) (2). The dysfunctional connectivity between these brain areas has been suggested by clinical imaging studies to support pathological behaviors such as anxiety.

Although the AMG is involved in processing positive emotions and reward, it is also well known for playing a crucial role in fear conditioning and for regulating stress-induced adaptive and pathological behaviors, including anxiety. Clinical imaging studies show that patients suffering from anxiety disorders display altered AMG activity and connectivity, and increased volume (3). Further, preclinical studies consistently show that AMG lesions reduce fear, emotional responses and anxiety-like behaviors, positioning the AMG as a major structure in the expression of anxiety behaviors (2).

The AMG is part of the limbic system and is composed of multiple, highly interconnected nuclei situated in the temporal lobe. The AMG can be broken down into three main groups: the basomedial AMG (BMA), the central AMG (CeA) and the basolateral complex (BLA). The BLA, and more precisely the lateral part of the BLA, is a major signal entrance of the AMG. The BLA interprets sensory information from the thalamus and cortical structures and in turn, makes synaptic connections within the AMG itself, projecting to the prefrontal cortex (PFC), HPC, and the nucleus accumbens (NAc). The BLA is composed of GABA inhibitory interneurons and projecting glutamatergic principal neurons (PN). The current view sees the BLA as a heterogeneous, but central hub of circuits that controls actions and reactions prompted by a challenging environment.

With recent advances in circuit-tracing and circuit-probing techniques, a more exact parsing of a selective neuronal pathway’s function in and contribution to naturalistic and pathological behaviors is now feasible. Combining transgenic tools, viral strategies and optogenetics, several efforts have successfully been made to dissect AMG circuit function, revealing a broad functional heterogeneity of BLA-specific projections (2,4). Optogenetic manipulations in naïve animals have established that BLA neurons projecting to the NAc (BLA-NAc) support positive reinforcement, while terminal stimulation of BLA projections to the CeA and ventral hippocampus (BLA-vHPC) are suggested to drive negative reinforcement and defensive behaviors (2,4). Specifically, BLA-vHPC PN terminal inhibition, mediated by halorhodopsin optomodulation, reduces anxiety-like behaviors. Conversely, BLA-vHPC PN activation via channelrhodopsin optomodulation promotes anxiety-like behaviors (2).

In a previous study, Zhang et al. (5) define differential morphological changes following stress exposure, depending upon BLA PN projection target. In a follow up study, Zhang et al. (6) investigate, in a projection-specific manner, BLA PN activity following acute or chronic stress exposure. While acute stress may engage and prime homeostatic processes and adaptive behaviors, chronic stress exposure often results in robust and pathological adaptations in neuronal activity and morphology, which ultimately drives anxiety-like behaviors and resulting mental disorders. In line with previous observations (2,5), Zhang et al. (6) establish that while all projection neurons from the BLA recover from acute restraint stress, BLA-vHPC PN but not BLA-mPFC become persistently active following chronic exposure to restraint stress.

Numerous human imaging studies have supported the notion that healthy cognitive and emotional responses require coordinated activity of a distributed brain macrocircuit. In line with this, Zhang et al. (6) show that chronic stress exposure induces disequilibrium between brain areas pivotal for the maintenance of adaptive behaviors: BLA-vHPC subcircuit strengthening over BLA-NAc and BLA-mPFC subcircuits. Noteworthy BLA PN collaterals are limited but BLA PNs do locally modulate neighboring neurons (4). Moreover, neuronal activity within the vHPC, NAc and mPFC are also directly disturbed in response to stress exposure. It is thus, unlikely that the imbalanced BLA PN subcircuit has a restrained impact solely onto the BLA-vHPC circuit, but is more likely to be associated with broader macrocircuit dysfunction. Future studies dissecting the specific contribution of pathological neuroadaptations of one circuit over others may provide further crucial, detailed information in the development of therapies aimed at macrocircuit homeostasis.

Altered neuronal activity may emerge from extrinsic and/or intrinsic modifications and, in part, is dependent on the presence or absence of specific ion channels. How a sole subpopulation, i.e. BLA-vHPC PN, displays persistent, altered activity is a key component in depicting circuit-specific dysfunctions induced by stress and ultimately the identification of new therapeutic targets. Potassium (K⁺) channels, including small-conductance calcium-activated K⁺ channels (SK), are a key determinant of neuronal excitability. Upon intracellular calcium increase (e.g. action potential calcium influx) SK channels undergo a conformational change leading to an increased K⁺ current and hyperpolarization of the neuron and subsequent reduction of excitability. Consequently, a reduction in SK channel function increases neuronal excitability. Three main SK channel families are expressed in the central nervous system (SK1, SK2 and SK3). By combining pharmaceutical tools and electrophysiological approaches, Zhang et al. (6) isolate SK2 channel hypofunction as the key contributor of BLA-vHPC PN hyperexcitability, following chronic restraint stress exposure.

Using an elegant circuit-specific, virally mediated overexpression of SK2 channels, Zhang et al. (6) causally link SK2 channel expression in BLA-vHPC PNs to anxiety-like behavior following chronic stress exposure. Indeed, circuit-specific overexpression of the SK2 channel prevents chronic restraint stress-induced anxiety-like behavior. Importantly, this result not only provides the cellular mechanism of BLA-vHPC PN hyperactivity, but also provides information regarding potential discrepancies with previous experiments (7). An earlier set of experiments (7), shows that while SK2 overexpression in the entire BLA reduces anxiety-like behavior in the stress-naïve animal, accompanied by a reduction in BLA PN stress-induced dendritic hypertrophy, it does not fully prevent the expression of anxiety-like behavior following stress exposure. In light with the results of the Zhang et al. (6) study, the partial prophylactic effect of SK2 overexpression could be explained by the lack of circuit-specificity in the earlier study.

As a key component of the action potential afterhyperpolarization (AHP) in several types of neurons, SK2 channels are also expressed at the dendritic level of BLA PNs where they gate excitatory, sensory input (8). Consequently, decreased SK2 channel function following stress exposure will alter baseline activity of a neuron but also its capacity to properly integrate sensory signal inputs. Interestingly, blockade of SK channels increases HPC neuron excitability, facilitates HPC synaptic plasticity and improves memory encoding (9). Taken together with the observations of Zhang et al. (6), an overall SK specific channelopathy may magnify abnormal BLA-vHPC excitability and connectivity.

Tremendous efforts have been performed to depict the biological signature encoding anxiety behaviors, but our knowledge of the pathological mechanisms remains incomplete. Within this context, the Zhang et al. (6) study is intriguing at two main levels: 1) it defines AMG circuit-specific dysfunction associated with pathological behavior, and thus could infer on clinical studies; and 2) it reveals a selective cellular mechanism that could lead to future treatment development.

Over the past decade, clinical and preclinical studies have attempted to determine new therapeutic approaches. Although the development of more selective drugs for precise receptor or molecule targeting is highly promising and ongoing, drugs targeting the prototypical anxiety-associated GABA-benzodiazepine system and SSRI drugs are foremost in term of use and success rate. Benzodiazepine and SSRI have a broad impact on the central nervous system. Thus, targeting SK channels, a broadly expressed key physiological modulator of neuronal activity, may offer therapeutic promises. Future studies investigating the precise impact of existing SK2 channel positive allosteric modulators (10) on macrocircuit function and anxiety behavior may provide very interesting results, based on the aforementioned discussion.