Activation of histamine type 2 receptors enhances intrinsic excitability of medium spiny neurons in the nucleus accumbens

Abstract

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.

Acute restraint stress impairs histamine type 2 receptor ability to increase the excitability of medium spiny neurons in the nucleus accumbens

“…Coronal slices (300 μm) containing the nucleus accumbens were prepared as previously described (Aceto et al, 2022;D'Ascenzo et al, 2009). C57/BL/6 J mice (P28-40) were anesthetized with isoflurane and euthanized by cervical dislocation and decapitated.…”

“…Western blot experiments were performed as previously described (Aceto et al, 2022). Briefly, tissues were lysed in ice-cold lysis buffer (NaCl 150 mM,Tris-HCl 50 mM pH 7.4, EDTA 2 mM) containing 1% Triton X-100, 0.1% SDS, 1 × protease inhibitor cocktail (Sigma-Aldrich, St Louis, MO, USA), 1 mM sodiumorthovanadate (Sigma-Aldrich) and 1 mM sodiumfluoride (Sigma-Aldrich).…”

“…To evaluate whether H2R is expressed in both NAc MSN subpopulations we assessed H2R, D1R and D2R mRNA levels using a protocol that combines whole-cell patch-clamp recordings with high-quality single cell RNA analysis by ddPCR (single-cell qRT-ddPCR) (Cadwell et al, 2016;Possieri et al, 2021). As shown in Supplementary In naive mice, enhanced MSN excitability after H2R activation was associated with: i) decreased latency to fire, ii) decreased action potential afterhyperpolarization (AHP), and iii) increased action potential half-width (Aceto et al, 2022).…”

“…In the context of the neuronal signaling pathways activated by histamine receptors, we recently showed that histaminergic transmission in the NAc increases medium spiny neurons (MSNs) intrinsic excitability through histamine type 2 receptor (H2R), the prevailing postsynaptic histamine receptor expressed in NAc MSNs (Aceto et al, 2022). In particular, this study showed that pharmacological activation of these receptors strongly increases the evoked-firing through a mechanism that requires protein kinase A (PKA) signaling and downregulation of Kv4.2mediated currents (Aceto et al, 2022).…”

“…In the context of the neuronal signaling pathways activated by histamine receptors, we recently showed that histaminergic transmission in the NAc increases medium spiny neurons (MSNs) intrinsic excitability through histamine type 2 receptor (H2R), the prevailing postsynaptic histamine receptor expressed in NAc MSNs (Aceto et al, 2022). In particular, this study showed that pharmacological activation of these receptors strongly increases the evoked-firing through a mechanism that requires protein kinase A (PKA) signaling and downregulation of Kv4.2mediated currents (Aceto et al, 2022). Building on this evidence and considering that histamine-containing TMN neuron output is increased during acute stress (Ito, 2000;Manz et al, 2021), we posited that alterations of intrinsic excitability induced by H2R signaling could be a causative link and a converging mechanism underlying anxiety-like behaviors in stress animal models.…”