The voltage‐gated sodium (Nav) channel complex is comprised of pore‐forming α subunits (Nav1.1–1.9) and accessory regulatory proteins such as the intracellular fibroblast growth factor 14 (FGF14). The cytosolic Nav1.6 C‐terminal tail binds directly to FGF14 and this interaction modifies Nav1.6‐mediated currents with effects on intrinsic excitability in the brain. Previous studies have identified the FGF14 V160 residue within the FGF14 core domain as a hotspot for the FGF14:Nav1.6 complex formation. Here, we used three short amino acid peptides around FGF14 V160 to probe for the FGF14 interaction with the Nav1.6 C‐terminal tail and to evaluate the activity of the peptide on Nav1.6‐mediated currents. In silico docking predicts FLPK to bind to FGF14 V160 with the expectation of interfering with the FGF14:Nav1.6 complex formation, a phenotype that was confirmed by the split‐luciferase assay (LCA) and surface plasmon resonance (SPR), respectively. Whole‐cell patch‐clamp electrophysiology studies demonstrate that FLPK is able to prevent previously reported FGF14‐dependent phenotypes of Nav1.6 currents, but that its activity requires the FGF14 N‐terminal tail, a domain that has been shown to contribute to Nav1.6 inactivation independently from the FGF14 core domain. In medium spiny neurons in the nucleus accumbens, where both FGF14 and Nav1.6 are abundantly expressed, FLPK significantly increased firing frequency by a mechanism consistent with the ability of the tetrapeptide to interfere with Nav1.6 inactivation and potentiate persistent Na + currents. Taken together, these results indicate that FLPK might serve as a probe for characterizing molecular determinants of neuronal excitability and a peptide scaffold to develop allosteric modulators of Nav channels.

Although major depressive disorder (MDD) is highly prevalent, its pathophysiology is poorly understood. Recent evidence suggests that glycogen-synthase kinase 3β (GSK3β) plays a key role in memory formation, yet its role in mood regulation remains controversial. Here, we investigated whether GSK3β activity in the nucleus accumbens (NAc) is associated with depression-like behaviors and synaptic plasticity. We performed whole-cell patch-clamp recordings of medium spiny neurons (MSNs) in the NAc and determined the role of GSK3β in spike timing-dependent long-term potentiation (tLTP) in the chronic unpredictable mild stress (CUMS) mouse model of depression. To assess the specific role of GSK3β in tLTP, we used in vivo genetic silencing by an adeno-associated viral vector (AAV2) short hairpin RNA against GSK3β. In addition, we examined the role of the voltage-gated potassium Kv4.2 subunit, a molecular determinant of A-type K+currents, as a potential downstream target of GSK3β. We found increased levels of active GSK3β and augmented tLTP in CUMS mice, a phenotype that was prevented by selective GSK3β knockdown. Furthermore, knockdown of GSK3β in the NAc ameliorated depressive-like behavior in CUMS mice. Electrophysiological, immunohistochemical, biochemical, and pharmacological experiments revealed that inhibition of the Kv4.2 channel through direct phosphorylation at Ser-616 mediated the GSK3β-dependent tLTP changes in CUMS mice. Our results identify GSK3β regulation of Kv4.2 channels as a molecular mechanism of MSN maladaptive plasticity underlying depression-like behaviors and suggest that the GSK3β–Kv4.2 axis may be an attractive therapeutic target for MDD.

Histaminergic neurons are exclusively located in the hypothalamic tuberomammillary nucleus, from where they project to many brain areas including the nucleus accumbens (NAc), a brain area that integrates diverse monoaminergic inputs to coordinate motivated behaviours. While the NAc expresses various histamine receptor subtypes, the mechanisms by which histamine modulates NAc activity are still poorly understood. Using whole‐cell patch‐clamp recordings, we found that pharmacological activation of histamine 2 (H2) receptors elevates the excitability of NAc medium spiny neurons (MSNs), while activation of H1 receptors failed to significantly affect MSN excitability. The evoked firing of MSNs increased after seconds of local H2 agonist administration and remained elevated for minutes. H2 receptor (H2R) activation accelerated subthreshold depolarization in response to current injection, reduced the latency to fire, diminished action potential afterhyperpolarization and increased the action potential half‐width. The increased excitability was protein kinase A‐dependent and associated with decreased A‐type K+ currents. In addition, selective pharmacological inhibition of the Kv4.2 channel, the main molecular determinant of A‐type K+ currents in MSNs, mimicked and occluded the increased excitability induced by H2R activation. Our results indicate that histaminergic transmission in the NAc increases MSN intrinsic excitability through H2R‐dependent modulation of Kv4.2 channels. Activation of H2R will significantly alter spike firing in MSNs in vivo, and this effect could be an important mechanism by which these receptors mediate certain aspects of goal‐induced behaviours. Key points Histamine is synthesized and released by hypothalamic neurons of the tuberomammillary nucleus and serves as a general modulator for whole‐brain activity including the nucleus accumbens. Histamine receptors type 2 (HR2), which are expressed in the nucleus accumbens, couple to Gαs/off proteins which elevate cyclic adenosine monophosphate levels and activate protein kinase A. Whole‐cell patch‐clamp recordings revealed that H2R activation increased the evoked firing in medium spiny neurons of the nucleus accumbens via protein kinase A‐dependent mechanisms. HR2 activation accelerated subthreshold depolarization in response to current injection, reduced the latency to fire, diminished action potential medium after‐hyperpolarization and increased the action potential half‐width. HR2 activation also reduced A‐type potassium current. Selective pharmacological inhibition of the Kv4.2 channel mimicked and occluded the increased excitability induced by H2R activation.