Elsevier

Brain Research

Volume 954, Issue 2, 8 November 2002, Pages 247-255
Brain Research

Research report
Histamine H1 receptors in rat dorsal raphe nucleus: pharmacological characterisation and linking to increased neuronal activity

https://doi.org/10.1016/S0006-8993(02)03352-8Get rights and content

Abstract

In this work we studied the presence of histamine H1 receptors in the rat dorsal raphe nucleus (DRN) and the effect of their activation on the activity of presumed serotonergic DRN neurones. [3H]-Mepyramine bound to DRN membranes with best-fit values of 107±13 fmol/mg protein for maximum binding (Bmax) and 1.2±0.4 nM for the equilibrium dissociation constant (Kd). In DRN slices labelled with [3H]-inositol and in the presence of 10 mM LiCl, histamine stimulated the accumulation of [3H]-inositol phosphates ([3H]-IPs) with maximum effect 172±6% of basal and EC50 3.2±1.3 μM. [3H]-IPs accumulation induced by 100 μM histamine (162±5% of basal) was markedly, but not fully blocked by the selective H1 antagonist mepyramine (300 nM; 64±6% inhibition). The simultaneous addition of mepyramine and the selective H2 antagonist ranitidine (10 μM) abolished histamine-induced [3H]-IPs accumulation. The presence of H2 receptors was confirmed by [3H]-tiotidine binding and by the determination of histamine-induced [3H]-cyclic AMP formation. Extracellular single-unit recording in brain stem slices showed that the exposure to histamine resulted in a marked increase in the firing rate of DRN presumed serotonergic neurones (471±10% of basal), that was dependent on the concentration of the agonist (EC50 4.5±0.3 μM). The action of histamine was not affected by the H2 antagonist tiotidine (2 μM) but was fully prevented by 1 μM mepyramine. Taken together, our results indicate that histamine modulates the firing of DRN presumed serotonergic neurones through the activation of H1 receptors coupled to phosphonositide hydrolysis.

Introduction

Histamine is a modulator in the mammalian central nervous system where it regulates, through both pre- and postsynaptic mechanisms, a variety of central responses and functions such as wakefulness, feeding, drinking, the neuroendocrine system, body temperature, analgesia and motor activity [31], [37], [42]. Histaminergic neurones are located in the hypothalamus from where they send diffuse projections to almost all brain regions [42]. The actions of histamine are mediated by three well-defined receptors (H1, H2 and H3), characterised by their pharmacology and signal transduction mechanisms [13], although the molecular cloning of a fourth histamine receptor was recently reported [24], [27], [29], [44].

Both neurochemical and electrophysiological studies indicate that the activity of histaminergic neurones is maximal during periods of wakefulness and in rats the intracerebroventricular administration of H1-receptor agonists increases wakefulness at the expense of slow-wave and paradoxical sleep, an effect that can be blocked by H1 antagonists. Inhibition of histamine synthesis increases deep slow-wave sleep and decreases wakefulness in cats and a similar effect is observed in rats. Conversely, inhibition of histamine degradation increases wakefulness [37]. These results support the idea that endogenous histamine, mainly through the activation of H1 receptors, is critically involved in wakefulness.

On the other hand, the activity of serotonergic neurones located in the raphe nuclei is highest during periods of waking arousal and absent during rapid-eye-movement (REM) sleep [7]. With the use of antibodies against histamine, moderate to dense networks of histaminergic fibres have been detected in guinea-pig and rat raphe nuclei [37], and [3H]-mepyramine binding has been showed by autoradiography in guinea-pig raphe nuclei [32]. A possible mechanism, among others, for histamine to regulate wakefulness would be thus the modulation of the activity of raphe serotonergic neurones. We report herein that the dorsal raphe nucleus (DRN) expresses histamine H1 receptors coupled to phosphoinositide hydrolysis, whose activation results in increased activity of presumed serotonergic neurones.

Section snippets

Animals

Rats (male, Wistar strain, 150–200 g), bred in Cinvestav facilities, were used throughout. Animals were maintained under constant room temperature (23 °C) and light–dark cycle (12:12 h); with food and water ad libitum. All procedures were in accordance to the Guide for the Care and Use of Laboratory Animals of the Mexican Council for Animal Care as approved by the Cinvestav’s Animal Care and Use Committee. All efforts were made to minimise animal suffering, to use only as many animals as were

[3H]-Mepyramine binding

The specific binding of [3H]-mepyramine to membranes from the DRN (Fig. 1) did not differ significantly from binding to a single site (Hill coefficient, nH, 1.1±0.1). The best-fit values for the equilibrium dissociation constant (Kd) and maximum binding (Bmax) were 1.2±0.4 nM and 107±13 fmol/mg protein, respectively (means±S.E.M. from the combined data from four experiments).

Histamine-induced [3H]-inositol phosphate ([3H]-IPs) accumulation

To allow for differences in the size of the slices, [3H]-IPs accumulation was expressed as the ratio {[3H]-IPs /[3H]-IPs+[

Discussion

The rat brain contains ∼20 000 serotonergic neurones, most of which (∼60%) are located in the DRN, from where they send projections to numerous parts of the diencephalon, basal ganglia, limbic system and cortex [7], [15]. The tegmental pedunculopontine, tegmental laterodorsal, locus coeruleus and parabrachial nuclei of the upper brain stem conform the so-called REM-on cholinergic–cholinoceptive neuronal system implicated in the generation of REM sleep and phasic related events such as

Acknowledgements

Supported by Cinvestav and CONACyT (grant 37354-N to J.-A.A.-M.). We are grateful to A. Nuñez and A. Sierra for technical assistance, Dr. J. Quevedo for helpful discussions and CONACyT for a predoctoral scholarship to AB.

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