Differential effects of histamine H(3) receptor inverse agonist thioperamide, given alone or in combination with the N-methyl-d-aspartate receptor antagonist dizocilpine, on reconsolidation and consolidation of a contextual fear memory in mice.

Albeit there is no doubt that histamine and its H(3) receptors participate in several aspects of learning and memory, such as memory consolidation, nothing is known about their potential involvement in memory reconsolidation. On the basis of previous reports of pro-cognitive effects of histamine H(3) receptor inverse agonists (which augment histamine release), we investigated to what extent the most representative of them, thioperamide, is able to facilitate reconsolidation of a contextually-conditioned fear memory in C57BL/6J mice. We also examined the effects of thioperamide on the stark disruptive effect that the non-competitive N-methyl-d-aspartate (NMDA) antagonist dizocilpine (MK-801) typically exerts on both reconsolidation and consolidation. Post-training systemic injections (i.p.) of thioperamide facilitated consolidation at 10 and 20 mg/kg and reversed amnesia induced by an i.p. injection of 0.12 mg/kg dizocilpine at 5, 10 and 20 mg/kg. Importantly, none of the five thioperamide doses (2.5, 5, 10, 20 and 30 mg/kg) given right after reactivation (reexposure to the context in which training took place 48 h earlier) affected reconsolidation, whereas all similarly given doses of dizocilpine (0.03, 0.06 and 0.12 mg/kg) disrupted it more or less equally. By contrast, thioperamide was able to unambiguously reverse the deficit in reconsolidation induced by 0.12 mg/kg dizocilpine at 10 and 20, but not 5 mg/kg. This is the first demonstration of an involvement of the interactive articulation between histamine and NMDA receptors in the mechanisms of memory reconsolidation, which seems to be indifferent to an increase of brain histamine per se. The results suggest a qualitatively different participation of histaminergic signalling in the mechanisms of reconsolidation and consolidation. The precise circuits within which these interactions take place are yet to be identified.