Atchley D, Hankosky ER, Gasparotto K, Rosenkranz JA. Pharmacological enhancement of calcium-activated potassium channel function reduces the effects of repeated stress on fear memory. Behav Brain Res 232: 37-43
Repeated stress impacts emotion, and can induce mood and anxiety disorders. These disorders are characterized by imbalance of emotional responses. The amygdala is fundamental in expression of emotion, and is hyperactive in many patients with mood or anxiety disorders. Stress also leads to hyperactivity of the amygdala in humans. In rodent studies, repeated stress causes hyperactivity of the amygdala, and increases fear conditioning behavior that is mediated by the basolateral amygdala (BLA). Calcium-activated potassium (K(Ca)) channels regulate BLA neuronal activity, and evidence suggests reduced small conductance K(Ca) (SK) channel function in male rats exposed to repeated stress. Pharmacological enhancement of SK channels reverses the BLA neuronal hyperexcitability caused by repeated stress. However, it is not known if pharmacological targeting of SK channels can repair the effects of repeated stress on amygdala-dependent behaviors. The purpose of this study was to test whether enhancement of SK channel function reverses the effects of repeated restraint on BLA-dependent auditory fear conditioning. We found that repeated restraint stress increased the expression of cued conditioned fear in male rats. However, 1-Ethyl-2-benzimidazolinone (1-EBIO, 1 or 10 mg/kg) or CyPPA (5 mg/kg) administered 30 min prior to testing of fear expression brought conditioned freezing to control levels, with little impact on fear expression in control handled rats. These results demonstrate that enhancement of SK channel function can reduce the abnormalities of BLA-dependent fear memory caused by repeated stress. Furthermore, this indicates that pharmacological targeting of SK channels may provide a novel target for alleviation of psychiatric symptoms associated with amygdala hyperactivity.
"In contrast to the effects of apamin, blocking M-type K+ channels depolarized IL neurons and reduced fear expression at the beginning of extinction training (Santini and Porter, 2010) suggesting that different K+ channels in IL regulate fear expression and extinction plasticity. However this relationship may be altered by stress, since systemic activation of SK channels reduces conditioned fear expression after repeated stress (Atchley et al., 2012). "
[Show abstract][Hide abstract] ABSTRACT: Fear extinction correlates with increased infralimbic (IL) neuronal excitability. Since small conductance Ca(2+)-dependent K(+) (SK) channels modulate neuronal excitability and certain types of learning and memory, pharmacological modulation of SK channels could be used to regulate IL excitability and fear extinction. To test this, we first determined the effect of blocking SK channels with apamin on the intrinsic excitability of IL pyramidal neurons in brain slices. In whole-cell patch-clamp recordings, apamin increased the number of spikes evoked by a depolarizing current pulse, increased the firing frequency, and reduced the fast afterhyperpolarizing potential (fAHP) indicating that blockade of SK channels could be used to enhance the intrinsic excitability of IL neurons. Next, we assessed whether SK channels in IL regulate extinction of conditioned fear by infusing apamin into IL of fear conditioned rats prior to extinction training. Apamin infusion did not affect conditioned freezing at the beginning of the extinction session or within-session extinction. However, the following day, apamin-infused rats showed significantly less conditioned freezing. To further examine the importance of SK channels in IL in fear extinction, we assessed the effect of the SK channel activator DCEBIO on IL neuronal excitability and fear extinction. Activation of SK channels with DCEBIO decreased the number of evoked spikes, reduced the firing frequency, and enhanced the fAHP of IL neurons. Infusion of DCEBIO into IL prior to fear extinction impaired recall of fear extinction without affecting acquisition of extinction. Taken together, these findings suggest that SK channels are involved in regulating IL excitability and extinction-induced plasticity. Therefore, SK channels are a potential target for the development of new pharmacological treatments to facilitate extinction in patients suffering from anxiety disorders.
"When the BLA and HPC circuits are activated repeatedly by stress, passive fear responses are enhanced to a similar degree after acute stress (Rodriguez Manzanares et al., 2005; Daviu et al., 2010; McGuire et al., 2010). In addition, repeated stress enhances both cued and contextual fear learning (Conrad et al., 1999; Atchley et al., 2012). It can be concluded that short duration, acute stressors, as well as repeated stressors can enhance learning during tasks with an emotional component. "
[Show abstract][Hide abstract] ABSTRACT: The basolateral amygdala (BLA) and ventral subiculum (vSub) of the hippocampus convey emotion and context information, respectively, to the nucleus accumbens (NAc). Using in vivo extracellular recordings from NAc neurons, we examined how acute and repeated restraint stress alters the plasticity of the vSub and BLA afferent pathways. High-frequency (HFS) and low-frequency (LFS) stimulation was applied to the vSub to assess the impact on NAc responses to vSub and BLA inputs. In addition, iontophoretic application of the dopamine D2-antagonist sulpiride was used to explore the role of dopamine in the NAc in mediating the effects of stress on plasticity. Acute and repeated restraint caused disparate effects on BLA- and vSub-evoked responses in the NAc. Following repeated restraint, but not after acute restraint, HFS of the vSub failed to potentiate the vSub-NAc pathway while instead promoting a long-lasting reduction of the BLA-NAc pathway and these effects were independent of D2-receptor activity. In contrast, LFS to the vSub pathway after acute restraint resulted in potentiation in the vSub-NAc pathway while BLA-evoked responses were unchanged. When sulpiride was applied prior to LFS of the vSub after acute stress, there was a pronounced decrease in vSub-evoked responses similar to control animals. This work provides new insight into the impact of acute and repeated stress on the integration of context and emotion inputs in the NAc. These data support a model of stress whereby the hippocampus is inappropriately activated and dominates the information processing within this circuit via a dopaminergic mechanism after acute bouts of stress.
The International Journal of Neuropsychopharmacology 06/2013; 16(09):1-13. DOI:10.1017/S1461145713000618 · 4.01 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Most cardiovascular researchers are familiar with intermediate-conductance KCa3.1 and small-conductance KCa2.3 channels because of their contribution to endothelium-derived hyperpolarization (EDH). However, to immunologists and neuroscientists these channels are primarily known for their role in lymphocyte activation and neuronal excitability. KCa3.1 is involved in the proliferation and migration of T cells, B cell, mast cells, macrophages, fibroblasts and dedifferentiated vascular smooth muscle cells and is, therefore, being pursued as a potential target for use in asthma, immunosuppression, and fibroproliferative disorders. In contrast, the three KCa2 channels (KCa2.1, KCa2.2 and KCa2.3) contribute to the neuronal medium afterhyperpolarization and, depending on the type of neuron, are involved in determining firing rates and frequencies or in regulating bursting. KCa2 activators are accordingly being studied as potential therapeutics for ataxia and epilepsy while KCa2 channel inhibitors like apamin have long been known to improve learning and memory in rodents. Given this background, we review the recent discoveries of novel KCa3.1 and KCa2.3 modulators and critically assess the potential of KCa activators for the treatment of diabetes and cardiovascular diseases by improving endothelium-derived hyperpolarizations.
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