Astrocytic Kir4.1 channels regulate locomotion by orchestrating neuronal rhythmicity in the spinal network.

Neuronal rhythmogenesis in the spinal cord is correlated with variations in extracellular K+ levels ([K+ ]e ). Astrocytes play important role in [K+ ]e homeostasis and compute neuronal information. Yet it is unclear how neuronal oscillations are regulated by astrocytic K+ homeostasis. Here we identify the astrocytic inward-rectifying K+ channel Kir4.1 (a.k.a. Kcnj10) as a key molecular player for neuronal rhythmicity in the spinal central pattern generator (CPG). By combining two-photon calcium imaging with electrophysiology, immunohistochemistry and genetic tools, we report that astrocytes display Ca2+ transients before and during oscillations of neighboring neurons. Inhibition of astrocytic Ca2+ transients with BAPTA decreases the barium-sensitive Kir4.1 current responsible of K+ clearance. Finally, we show in mice that Kir4.1 knockdown in astrocytes progressively prevents neuronal oscillations and alters the locomotor pattern resulting in lower motor performances in challenging tasks. These data identify astroglial Kir4.1 channels as key regulators of neuronal rhythmogenesis in the CPG driving locomotion.

The widely used antihistamine mepyramine causes topical pain relief through direct blockade of nociceptor sodium channels.

Mepyramine, a first-generation antihistamine targeting the histamine H(1) receptor, was extensively prescribed to patients suffering from allergic reactions and urticaria. Serious adverse effects, especially in case of overdose, were frequently reported, including drowsiness, impaired thinking, convulsion, and coma. Many of these side effects were associated with the blockade of histaminergic or cholinergic receptors. Here we show that mepyramine directly inhibits a variety of voltage-gated sodium channels, including the Tetrodotoxin-sensitive isoforms and the main isoforms (Nav1.7, Nav1.8, and Nav1.9) of nociceptors. Estimated IC50 were within the range of drug concentrations detected in poisoned patients. Mepyramine inhibited sodium channels through fast- or slow-inactivated state preference depending on the isoform. Moreover, mepyramine inhibited the firing responses of C- and Aß-type nerve fibers in ex vivo skin-nerve preparations. Locally applied mepyramine had analgesic effects on the scorpion toxin-induced excruciating pain and produced pain relief in acute, inflammatory, and chronic pain models. Collectively, these data provide evidence that mepyramine has the potential to be developed as a topical analgesic agent.

Analgesic Effects of Topical Amitriptyline in Patients With Chemotherapy-Induced Peripheral Neuropathy: Mechanistic Insights From Studies in Mice.

Oral amitriptyline hydrochloride (amitriptyline) is ineffective against some forms of chronic pain and is often associated with dose-limiting adverse events. We evaluated the potential effectiveness of high-dose topical amitriptyline in a preliminary case series of chemotherapy-induced peripheral neuropathy patients and investigated whether local or systemic adverse events associated with the use of amitriptyline were present in these patients. We also investigated the mechanism of action of topically administered amitriptyline in mice. Our case series suggested that topical 10% amitriptyline treatment was associated with pain relief in chemotherapy-induced peripheral neuropathy patients, without the side effects associated with systemic absorption. Topical amitriptyline significantly increased mechanical withdrawal thresholds when applied to the hind paw of mice, and inhibited the firing responses of C-, Aß- and Aδ-type peripheral nerve fibers in ex vivo skin-saphenous nerve preparations. Whole-cell patch-clamp recordings on cultured sensory neurons revealed that amitriptyline was a potent inhibitor of the main voltage-gated sodium channels (Nav1.7, Nav1.8, and Nav1.9) found in nociceptors. Calcium imaging showed that amitriptyline activated the transient receptor potential cation channel, TRPA1. Our case series indicated that high-dose 10% topical amitriptyline could alleviate neuropathic pain without adverse local or systemic effects. This analgesic action appeared to be mediated through local inhibition of voltage-gated sodium channels. PERSPECTIVE: Our preliminary case series suggested that topical amitriptyline could provide effective pain relief for chemotherapy-induced peripheral neuropathy patients without any systemic or local adverse events. Investigation of the mechanism of this analgesic action in mice revealed that this activity was mediated through local inhibition of nociceptor Nav channels.