Histamine H₃ receptor activation inhibits glutamate release from rat striatal synaptosomes

Abstract

The release of glutamate from striatal synaptosomes induced by depolarisation with 4-aminopyridine (4-AP) was studied by a method based on the fluorescent properties of the NAPDH formed by the metabolism of the neurotransmitter by glutamate dehydrogenase.

Ca²⁺-dependent, depolarisation-induced glutamate release was inhibited in a concentration-dependent manner by the selective histamine H₃ agonist immepip. Best-fit estimates were: maximum inhibition 60±10% and IC₅₀ 68±10 nM. The effect of 300 nM immepip on depolarisation-evoked glutamate release was reversed by the selective H₃ antagonist thioperamide in a concentration-dependent manner (EC₅₀ 23 nM, K_i 4 nM).

In fura-2-loaded synaptosomes, the increase in the intracellular concentration of Ca²⁺ ([Ca²⁺]_i) evoked by 4-AP-induced depolarisation (resting level 167±14 nM; Δ[Ca²⁺]_i 88±15 nM) was modestly, but significantly reduced (29±5% inhibition) by 300 nM immepip. The action of the H₃ agonist on depolarisation-induced changes in [Ca²⁺]_i was reversed by 100 nM thioperamide.

Taken together, our results indicate that histamine modulates the release of glutamate from corticostriatal nerve terminals. Inhibition of depolarisation-induced Ca²⁺ entry through voltage-dependent Ca²⁺ channels appears to account for the effect of H₃ receptor activation on neurotransmitter release. Modulation of glutamatergic transmission in rat striatum may have important consequences for the function of basal ganglia and therefore for the control of motor behaviour.

Introduction

Histamine is a neuromodulator in the mammalian central nervous system, where it regulates, via both pre- and post-synaptic mechanisms, a variety of central responses and functions, such as wakefulness, feeding, drinking, the neuroendocrine system, body temperature, analgesia and motor activity (Wada et al., 1991, Schwartz et al., 1991, Onodera et al., 1994). The cell bodies of histaminergic neurones are located in the hypothalamus, from where they send diffuse projections to almost all brain regions (Wada et al., 1991). The actions of histamine are mediated by three well-defined receptors (H₁, H₂ and H₃), characterised by their pharmacology and signal transduction mechanisms (Hill et al., 1997), although the molecular cloning of a fourth histamine receptor was recently reported (Oda et al., 2000, Liu et al., 2001, Nguyen et al., 2001, Zhu et al., 2001).

Histamine H₃ receptors in the CNS appear to be present primarily on nerve terminals, where they regulate the release of histamine itself and of other neurotransmitters, such as acetylcholine, dopamine, noradrenaline and 5-hydroxytryptamine (Hill et al., 1997). In the striatum, which has the highest density of H₃ receptors in both human and rat brain (Martı́nez-Mir et al., 1990, Cumming et al., 1991, Pollard et al., 1993), and in the substantia nigra pars reticulata, lesioning studies have shown that these receptors are present predominantly on the GABAergic projection neurones (Pollard et al., 1993, Ryu et al., 1994). In both the striatum and the substantia nigra H₃ receptor activation leads to a marked and selective inhibition of the component of depolarisation-induced release of γ-aminobutyric acid (GABA) that is dependent on concomitant D₁ receptor stimulation (Garcı́a et al., 1997, Arias-Montaño et al., 2001). This action in the basal ganglia, a group of subcortical nuclei intimately involved in the control of movement (Gerfen and Wilson, 1996), suggests that histamine might have an important role in disorders of motor control.

Nigrostriatal (dopaminergic) and corticostriatal (glutamatergic) pathways (Gerfen and Wilson, 1996) provide the major synaptic inputs to the striatum. We have recently shown that striatal synaptosomes are endowed with H₃ receptors (Molina-Hernández et al., 2000a) and selective lesioning of dopaminergic neurones with 6-hydroxydopamine indicates that a minor fraction (∼20%) of presynaptic H₃ receptors are located on nigrostriatal terminals, where they appear to modulate dopamine synthesis (Molina-Hernández et al., 2000a) and release (Schlicker et al., 1993). However, Doreulee et al. (2001) have recently shown that in rat striatum H₃ receptor activation depresses the amplitude of extracellular field potentials evoked by corticostriatal stimulation. This suggests that a significant fraction of striatal H₃ receptors could be located on corticostriatal afferents and regulate glutamate release. We provide here direct evidence for H₃ receptor-mediated inhibition of glutamate release from isolated striatal nerve terminals. A preliminary account of some of these results has been presented to the Society for Neuroscience (Molina-Hernández et al., 2000b).