S-38093

Histamine H3 receptors are mainly expressed onCNSneurons, particularly along the nociceptive pathways. The potential involvement of these receptors in pain processing has been suggested using H3 receptor inverse agonists.

The antinociceptive effect of S 38093, a novel inverse agonist of H3 receptors, has been evaluated in several neuropathic pain models in rat and compared with those of gabapentin and pregabalin.

While S 38093 did not change vocalization thresholds to paw pressure in healthy rats, it exhibited a significant antihyperalgesic effect in the Streptozocin‐induced diabetic (STZ) neuropathy model after acute and chronic administration and, in the chronic constriction injury (CCI) model only after chronic administration, submitted to the paw‐pressure test. Acute S 38093 administration at all doses tested displayed a significant cold antiallodynic effect in a model of acute or repeated administration of oxaliplatin‐induced neuropathy submitted to cold tail immersion, cold allodynia being the main side effect of oxaliplatin in patients. The effect of S 38093 increased following chronic administration (i.e. twice a day during 5 days) in theCCIandSTZmodels except in the oxaliplatin models where its effect was already maximal from the first administration The kinetics and size of effect of S 38093 were similar to gabapentin and/or pregabalin. Finally, the antinociceptive effect of S 38093 could be partially mediated by α2 adrenoreceptors desensitization in the locus coeruleus.

These results highlight the interest of S 38093 to relieve neuropathic pain and warrant clinical trials especially in chemotherapeutic agent‐induced neuropathic pain.

S 38093, a new H3 antagonist/inverse agonist, displays antiallodynic and antihyperalgesic effect in neuropathic pain, especially in oxaliplatin‐induced neuropathy after chronic administration. This effect of S 38093 in neuropathic pain could be partly mediated by α2 receptors desensitization in the locus coeruleus.

S 38093, a novel histamine H3 receptor inverse agonist, was tested in a series of neurochemical and behavioral paradigms designed to evaluate its procognitive and arousal properties. In intracerebral microdialysis studies performed in rats, S 38093 dose-dependently increased histamine extracellular levels in the prefrontal cortex and facilitated cholinergic transmission in the prefrontal cortex and hippocampus of rats after acute and chronic administration (10mg/kg i.p.). Acute oral administration of S 38093 at 0.1mg/kg significantly improved spatial working memory in rats in the Morris water maze test. The compound also displayed cognition enhancing properties in the two-trial object recognition task in rats, in a natural forgetting paradigm at 0.3 and 1mg/kg p.o. and in a scopolamine-induced memory deficit situation at 3mg/kg p.o. The property of S 38093 to promote episodic memory was confirmed in a social recognition test in rats at 0.3 and 1mg/kg i.p. Arousal properties of S 38093 were assessed in freely moving rats by using electroencephalographic recordings: at 3 and 10mg/kg i.p., S 38093 significantly reduced slow wave sleep delta power and induced at the highest dose a delay in sleep latency. S 38093 at 10mg/kg p.o. also decreased the barbital-induced sleeping time in rats. Taken together these data indicate that S 38093, a novel H3 inverse agonist, displays cognition enhancing at low doses and arousal properties at higher doses in rodents.