The aim of the current study was to examine the effects of deleting the A-type voltage-gated K+ channel Kv4.2 on multiple stress- and other 'emotion'-related phenotypes. We found a complex set of significant phenotypic alterations in Kv4.2 mice. These included increased exploratory activity in novel environments, test-specific decreases in anxiety-like behavior, increased fear response to auditory stimuli, and an exaggerated corticosterone response to stress.
Consistent with previous studies , an initial observational battery showed no gross alterations in various measures of health or sensory function in the Kv4.2 knockout; however, this analysis did reveal increased rearing in the KO mice, suggestive of heightened levels of exploratory behavior. Similarly, exploratory locomotion in a novel open field was robustly increased in the KO mice, at least over a 30 minute session (longer sessions were not examined). These genotype effects did not seem to reflect generalized locomotor hyperactivity in the mutants, as evidenced by normal levels of activity in the familiar and low-stress environment of the home cage. Rather, these data suggest that Kv4.2 deletion produced an augmented behavioral reaction to novel and/or stressful environments.
This behavioral phenotype may be related to the neural abnormalities that have been reported in Kv4.2 KO mice. For example, these mice have been found to exhibit neuronal and dendritic hyperexcitability due to increased axonal back-propagation . Interestingly in this context, we have previously identified a similar profile in the same behavioral assays after deletions of other genes involved in regulating neuronal excitability, such as components of the glutamate signaling system [16, 37, 38]. This overlap indicates that genetic loss of molecules involved in regulating neuronal excitability and synaptic plasticity produces some convergent phenotypic effects on novelty-driven behavioral reactivity. Although hyperlocomotion in a novel environment is often taken as a measure relevant to schizophrenia [39, 40], the Kv4.2 KO mice in the present study did not show changes in another schizophrenia-relevant measure, prepulse inhibition of startle, unlike our findings in previous studies of glutamate deletions. Therefore, without additional experiments to interrogate this issue further, it would be premature discuss the Kv4.2 KO phenotype in terms of potential relevance to that disease.
Novelty-induced hyperactivity can confound interpretation of behavior in tests for anxiety-like behavior due to non-specific increases in activity . However, we found that Kv4.2 KO mice were no different from WT controls in the light/dark exploration test and showed increased open-arm exploration in the elevated plus maze, which was not associated with increased general locomotor activity in this test (as measured by the number of closed-arm entries). This suggests a test-specific anxiolytic-like phenotype in the KO mice, albeit with the caveat that the classic conflict-based assays for anxiety-like behavior cannot unequivocally parse an anxiety-like decrease in avoidance from a novelty-driven increase in approach . Notwithstanding, the test-specific nature of this phenotype is notable, and echoes earlier studies showing that the elevated plus maze can be particularly sensitive to certain gene mutations, possibly because it is inherently more stressful than ostensibly similar tests such as the light/dark exploration . This would generally fit with a profile of exaggerated behavioral responses of Kv4.2 KO mice, particularly under conditions of strong environmental 'provocation.'
Consistent with this interpretation, the KO mice showed increased unconditioned freezing to an auditory stimulus, and an augmented corticosterone response to a single exposure to the forced-swim test. By contrast, the behavioral response (immobility) to forced swim, a very stressful and 'provocative' situation, was normal in the KO mice. Increases in immobility produced by repeated, brief, forced-swim exposures were also similar between KO mice and WT controls. Thus, increased behavioral reactivity in the mutants does not generalize to altered immobility during various forms of forced-swim exposure. One explanation is enhanced tonic inhibitory transmission  served to mitigate the penetrance of the phenotype under forced-swim conditions.
The absence of changes in forced-swim tests in the current study differs from the previous finding by Lockridge et al. that these mice showed increased immobility in the forced-swim test . Indeed, whereas the earlier report found, as we did, novel open-field hyperactivity, some other differences were seen in the earlier study, including no change in elevated plus maze anxiety-like behavior. Other than methodological variations, the principal salient factor potentially explaining these apparent discrepancies is genetic background. Kv4.2 KO mice were bred on a C57BL/6J background for the current study and a 129S6 background for the previous study. Genetic background can have a profound influence on the penetrance and expression of emotion-related phenotypes in mutant mice, because of epistatic interactions between a mutation and modifier genes [21, 22]. It seems likely that such interactions shaped the phenotypic profile of the Kv4.2 KO mice, and it would be interesting to explore this further by identifying the responsible modifiers.
Another important avenue for future studies will involve assessment of Kv4.2 KO mice for learning and memory. The mutants have enhanced hippocampal synaptic plasticity [6, 10], which is associated with altered synaptic expression of synaptic proteins that are strongly implicated in learning by mutant studies [23, 43–45], such as the NMDA receptor GluN2A and GluN2B subunits [11–13]. In the current study we found that the KO mice displayed normal amygdala-mediated fear learning and mPFC-mediated fear extinction. Given the localization of synaptic alterations to the hippocampus, it will be valuable to supplement these findings by assessing Kv4.2 KO mice for hippocampal-dependent forms of learning.