We have studied the effect of histamine H(3) receptor (H(3)R) activation on the depolarization-evoked release of labeled neurotransmitters from slices of the rat olfactory bulb (rOB). The presence of pre-synaptic H(3)Rs was evidenced by the specific binding of the H(3)R ligand N-α-[methyl-(3)H]histamine to membranes from rOB synaptosomes (maximum binding, B(max), 106 ± 19 fmol/mg protein; dissociation constant, K(d), 0.68 ± 0.11 nM) which was inhibited by selective H(3)R ligands (immepip, (R)(-)-α-methylhistamine (RAMH) and clobenpropit) with affinities similar to those previously reported for H(3)Rs expressed in other rat brain areas. Perfusion of rOB slices with the selective H(3)R agonist RAMH (0.1 and 1 μM) had no effect on the release of [(3)H]-γ-aminobutyric acid ([(3)H]-GABA), [(3)H]-d-aspartate, [(3)H]-dopamine or [(3)H]-5-hydroxytryptamine ([(3)H]-5-HT) evoked by depolarization with high K(+) (20 or 40 mM). [(3)H]-Noradrenaline release induced by 20 mM K(+) was reduced in a modest but significant manner by RAMH (94.9 ± 1.7% and 83.1 ± 2.1% of control release at 0.1 and 1 μM, respectively). The effect of 1 μM RAMH was blocked by the selective H(3)R antagonist/inverse agonist clobenpropit (5 μM). When tested alone clobenpropit and a second H(3)R antagonist/inverse agonist, ciproxifan (both at 1 μM) significantly increased K(+)-evoked [(3)H]-noradrenaline release to 119.4 ± 4.2% and 120.0 ± 3.7% of K(+) alone, respectively. Ciproxifan (1 μM) had no effect on the depolarization-evoked release of the other labeled neurotransmitters. These data indicate that H(3)Rs with constitutive activity modulate noradrenaline release in rOB, presumably through a pre-synaptic action. This article is part of a Special Issue entitled 'Post-Traumatic Stress Disorder'.
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