thioperamide and nordimaprit

The effects of histamine on high-voltage-activated Ca2+ channels in the histaminergic neurons acutely dissociated from the rat tuberomammillary nucleus were investigated in the nystatin-perforated patch recording mode under voltage-clamp conditions. Histamine suppressed the high-voltage-activated Ca2+ channel currents in neurons which were positive for histidine decarboxylase with immunocytochemistry. The half-maximum inhibitory concentration and maximum inhibition were 2.6 x 10(-7) M and 16.6+/-1.90%, respectively. An H3 receptor agonist, R(-)-alpha-methylhistamine, mimicked the response to histamine, and thioperamide, an H3 receptor antagonist, inhibited the response to histamine. On the other hand, neither 2-methylhistamine, an H1 receptor agonist, nor dimaprit, an H2 receptor agonist, had a significant effect on the Ca2+ channel currents. Pretreatment with pertussis toxin blocked the inhibitory effect of histamine on Ca2+ channels, suggesting the involvement of Gi/Go proteins in the action of histamine. Omega-conotoxin-GVIA, omega-agatoxin-IVA, nicardipine, and omega-conotoxin-MVIIC blocked the high-voltage-activated Ca2+ channel currents by 15.6, 4.3, 27.1, and 31.2% of the total current, respectively, suggesting the existence of N-, P-, L-, and Q-type Ca2+ channels. A current that was insensitive to these blockers was also found. This residual current, "R-type", was completely suppressed by the addition of 200 microM Cd2+. Histamine significantly inhibited both the N- and P-type current components among these five types of Ca2+ channel currents. We concluded that histamine suppresses the N- and P-type Ca2+ channels in histaminergic neurons through an H3 receptor which is linked to a pertussis toxin-sensitive G-protein.

Topics: Animals; Calcium; Calcium Channels; Calcium Channels, N-Type; Depression, Chemical; Dimaprit; Histamine; Histamine Agonists; Histamine Antagonists; Ion Channel Gating; Mammillary Bodies; Methylhistamines; Nerve Tissue Proteins; Neurons; Nicardipine; omega-Agatoxin IVA; omega-Conotoxin GVIA; omega-Conotoxins; Patch-Clamp Techniques; Peptides; Pertussis Toxin; Piperidines; Rats; Rats, Wistar; Receptors, Histamine H3; Spider Venoms; Tuber Cinereum; Virulence Factors, Bordetella