Pre-synaptic histamine H₃ receptors modulate glutamatergic transmission in rat globus pallidus

The globus pallidus, a neuronal nucleus involved in the control of motor behavior, expresses high levels of histamine H(3) receptors (H(3)Rs) most likely located on the synaptic afferents to the nucleus. In this work we studied the effect of the activation of rat pallidal H(3)Rs on depolarization-evoked neurotransmitter release from slices, neuronal firing rate in vivo and turning behavior. Perfusion of globus pallidus slices with the selective H(3)R agonist immepip had no effect on the release of [(3)H]-GABA ([(3)H]-γ-aminobutyric acid) or [(3)H]-dopamine evoked by depolarization with high (20 mM) K(+), but significantly reduced [(3)H]-d-aspartate release (-44.8 ± 2.6% and -63.7 ± 6.2% at 30 and 100 nM, respectively). The effect of 30 nM immepip was blocked by 10 μM of the selective H(3)R antagonist A-331440 (4'-[3-[(3(R)-dimethylamino-1-pyrrolidinyl]propoxy]-[1,1-biphenyl]-4'-carbonitrile). Intra-pallidal injection of immepip (0.1 μl, 100 μM) decreased spontaneous neuronal firing rate in anaesthetized rats (peak inhibition 68.8±10.3%), and this effect was reversed in a partial and transitory manner by A-331440 (0.1 μl, 1 mM). In free-moving rats the infusion of immepip (0.5 μl; 10, 50 and 100 μM) into the globus pallidus induced dose-related ipsilateral turning following systemic apomorphine (0.5 mg/kg, s.c.). Turning behavior induced by immepip (0.5 μl, 50 μM) and apomorphine was partially prevented by the local injection of A-331440 (0.5 μl, 1 mM) and was not additive to the turning evoked by the intra-pallidal injection of antagonists at ionotropic glutamate receptors (0.5 μl, 1 mM each of AP-5, dl-2-amino-5-phosphonovaleric acid, and CNQX, 6-nitro-7-sulphamoylbenzo[f]quinoxaline-2,3-dione). These results indicate that pre-synaptic H(3)Rs modulate glutamatergic transmission in rat globus pallidus and thus participate in the control of movement by basal ganglia.

Pre-synaptic histamine H3 receptors regulate glutamate, but not GABA release in rat thalamus

We have investigated the presence of histamine H(3) receptors (H(3)Rs) on rat thalamic isolated nerve terminals (synaptosomes) and the effect of their activation on glutamate and GABA release. N-alpha-[methyl-(3)H]histamine ([(3)H]-NMHA) bound specifically to synaptosomal membranes with dissociation constant (K(d)) 0.78+/-0.20 nM and maximum binding (B(max)) 141+/-12fmol/mg protein. Inhibition of [(3)H]-NMHA binding by histamine and the H(3)R agonist immepip fit better to a two-site model, whereas for the H(3)R antagonist clobenpropit the best fit was to the one-site model. GTPgammaS (30 microM) decreased [(3)H]-NMHA binding by 55+/-4% and made the histamine inhibition fit better to the one-site model. Immepip (30 nM) induced a modest, but significant increase (113+/-2% of basal) in [(35)S]-GTPgammaS binding to synaptosomal membranes, an effect prevented by clobenpropit (1 microM) and by pre-treatment with pertussis toxin. In thalamus synaptosomes depolarisation-induced, Ca(2+)-dependent glutamate release was inhibited by histamine (1 microM, 25+/-4% inhibition) and immepip (30 nM, 38+/-5% reduction). These effects were reversed by clobenpropit (1microM). Conversely, immepip (up to 1 microM) had no effect on depolarisation-evoked [(3)H]-GABA release. Extracellular synaptic responses were recorded in the thalamus ventrobasal complex by stimulating corticothalamic afferents. H(3)R activation reduced by 38+/-7% the glutamate receptor-mediated field potentials (FPs), but increased the FP2/FP1 ratio (from 0.86+/-0.03 to 1.38+/-0.05) in a paired-pulse paradigm. Taken together, our results confirm the presence of H(3)Rs on thalamic nerve terminals and show that their activation modulates pre-synaptically glutamatergic, but not GABAergic neurotransmission.

Homologous down-regulation of histamine H3 receptors in rat striatal slices

Preincubation of striatal slices with the selective histamine H3-receptor agonist immepip (100 nM) decreased the specific binding of N-alpha-[methyl-3H]-histamine ([3H]-NMHA) to membranes obtained from the treated slices. The binding decrease was significant after 5 min, remained at similar reduced levels between 5- and 30-min incubations with agonist, and only a partial recovery was observed after 90-min washout (34, 41, and 44% at 90, 120, and 150 min, respectively). Saturation analysis showed a significant decrease in both receptor density (-44% +/- 9%) and affinity (dissociation constant, Kd 1.15 +/- 0.23 nM from 0.59 +/- 0.17 nM). The effect of immepip was mimicked by histamine and the H3 agonists imetit and R-alpha-methylhistamine, and was blocked by the H3 antagonist thioperamide. The reduction in [3H]-NMHA binding was fully and partially prevented by incubation at 4 degrees C and in hypertonic medium, respectively, but not by the endocytosis inhibitor phenylarsine oxide (10 microM). None of the following protein kinase inhibitors, Ro-318220 and Gö-6976 (PKC), H-89 (PKA) and staurosporine (general inhibitor) prevented the effect of immepip. In [3H]-adenine-labeled slices the preincubation with immepip (100 nM, 15 min) prevented the inhibitory effect of H3 receptor activation on forskolin-induced [3H]-cAMP accumulation (99% +/- 9% vs. 76% +/- 4% of control values). Taken together our results indicate that agonist binding promotes the down-regulation of striatal H3 receptors resulting in a significant loss of function.