The excitability and functional plasticity of specific brain circuits in cortical, limbic, and midbrain regions is thought to mediate behavioral responses to environmental threats, and to enable adaptation to stressors. At the cellular level, voltage- and calcium-activated potassium (K+) channels provide an important means of modulating neuronal activity and synaptic plasticity. Their function is further refined via the presence of multiple Kv primary and auxiliary subunits that differ in their voltage-dependence, post-translational regulation, sub-cellular localization and regional pattern of expression in the brain to accomplish distinct physiological functions.
A-type K+ channels composed of Kv4 subunits transmit a rapidly activating and inactivating current [1
]. Kv4.2 mRNA is expressed in the periphery and in various brain regions involved in mediating stress-related behaviors, including the medial prefrontal cortex (mPFC), hippocampus and hypothalamus [2
]. In CA1 hippocampal neurons, Kv4.2 protein is primarily localized on dendrites [3
] where they exert their strongest functional effects [4
]. Gene deletion of Kv4.2 (Kcnd2
) in mice eliminates most of the A-type K+ current in hippocampal and cortical neurons [5
], and increases back-propagation of the action potential from the axon to the somatodendritic region [6
]. These and other effects of Kv4.2 knockout (KO) may be countered to some extent by compensatory increases in the expression of other Kv subunits [8
], and increased inhibitory transmission in the hippocampus [9
]. Notwithstanding, loss of Kv4.2 function produces a net increase in neuronal, and particularly dendritic, hyperexcitability in the mPFC and hippocampus, and possibly other brain regions in which this subunit is expressed.
Synaptic plasticity is also altered by Kv4.2 deletion, as evidenced by the enhanced CA1 hippocampal long-term potentiation (LTP) and increased threshold for long-term depression (LTD) seen in Kv4.2 (KO) mice [6
]. Further demonstrating an important contribution to hippocampal plasticity, Kv4.2 channels are internalized during LTP induction, and Kv4.2 overexpression or deletion alters synaptic expression of N-Methyl-D-aspartate (NMDA) receptor subunits (GluN2A, GluN2B) to respectively constrain or promote hippocampal LTP [11
]. Conversely, NMDAR throughput decreases hippocampal Kv4.2 through degradation [13
] whereas concurrently causing an increase in Kv4.2 translation [15
]. The reciprocal relationship between Kv4.2 and NMDA receptor subunits is particularly intriguing in the context of prior studies showing that gene KO of GluN2A [16
] or GluN2B [17
] disrupts anxiety-related behaviors and adaptations to stress.
Behaviorally, Kv4.2 KO mice are viable and appear grossly normal [18
]. However, Kv4.2 KO mice show augmented nociceptive responses to thermal and mechanical stimulation [8
] and increased sensitivity to the pro-convulsive effects of kainate, with no apparent increase in spontaneous seizures [19
]. Furthermore, in a recent study assessing Kv4.2 KO mice on a 129S6/SvEvTac (129S6) background on a range of behavioral assays, Lockridge et al.
found that the KO mice had reduced grip strength on a wire-hang test and increased locomotor activity in a brightly illuminated open field, but not in a dimly illuminated one [20
]. Although the KO mice displayed an increased tendency to enter into the aversive central area of the open field, consistent with an anxiolytic-like phenotype, the mutants were no different from the wild-type (WT) controls in the increased plus-maze test for anxiety-such as behavior. This study also found that Kv4.2 KO mice showed increased immobility in the forced-swim test (FST) (but not the tail-suspension test) for 'depression-related' behavior, and were insensitive to the anti-immobility effects of the antidepressant fluoxetine, but not imipramine or desipramine, in the FST. In addition, ex vivo
slice physiological recordings indicated that a form of 5-hydroxytryptamine (5-HT)2A receptor-mediated excitatory synaptic transmission in mPFC pyramidal neurons was attenuated in Kv4.2 KO mice after a single forced swim exposure, but only after repeated exposures in WT controls. Based on these findings, Lockridge et al.
concluded that Kv4.2 deletion produces abnormal behavioral responses to stress, and suggested that this might be related to excessive 5-HT release.
The aim of the present study was to confirm and extend the characterization of Kv4.2 KO mice in stress- and other 'emotion'-related phenotypes. KO mice and WT littermate controls were compared for a battery of sensory and neurological tests (including hot-plate nociception, acoustic startle and prepulse inhibition of startle, and home-cage locomotion), exploratory and anxiety-related behaviors (novel open field, light/dark exploration, elevated plus-maze tests), pavlovian fear conditioning and extinction, and 'depression-related' behavioral responses (single and repeated inescapable forced-swim exposure). Phenotyping was conducted after the mutants were backcrossed onto a C57BL/6J background strain. This is important because genetic background can strongly influence the phenotype of mutant mice [21
], and the aforementioned studies by Lockridge and colleagues tested the KO mice on a different genetic background (129S6).