H3 agonist immepip markedly reduces cortical histamine release, but only weakly promotes sleep in the rat

Abstract

Presynaptic H3 receptors exert negative control on brain histamine synthesis and release and may thereby play a key role in the control of the sleep/wake cycle. This suggests that pharmacological stimulation by H3 receptor agonists may potentially decrease wakefulness and induce sleep. This study reports the effect of a potent and selective H3 agonist, immepip, on EEG assessed sleep/wake phases in Sprague–Dawley rats at doses that significantly modulate brain histamine release. Immepip injected intraperitoneally (i.p.) at 5 or 10 mg kg⁻¹ induced a sustained decrease in cortical histamine efflux as measured by in vivo microdialysis. In a separate experiment, rats were prepared for EEG/EMG recording and evaluated during the dark phase of their light/dark cycle. The results showed that the same i.p. doses of 5 and 10 mg kg⁻¹ of immepip was devoid of any significant impact on the sleep/wake phases (active awake, drowsiness and slow wave sleep), except for a slight, albeit significant, decrease in sleep onset latency. These results reveal that a marked H3 receptor agonist-mediated reduction in cortical histamine release is not corroborated by a significant sleep promoting effect and therefore question the hypnotic potential of H3 agonists.

Introduction

Histaminergic neurons originate from the tuberomammillary nucleus in the posterior hypothalamus and project widespread ascending and descending input to various brain areas involved in controlling arousal and wakefulness [1], [2]. Anatomical, physiological and pharmacological evidence support a role for histamine in the control of the sleep/wake cycle [1], [3] confirmed by more recent studies using knock out mice lacking histidine decarboxylase [4].

Histamine acts through four distinct H1–H4 receptor subtypes [5], [6]. The H3 receptor was originally discovered as a presynaptic autoreceptor regulating the release of histamine [7]. Subsequent studies revealed that it also acts as a heteroreceptor involved in the presynaptic regulation of the release of several neurotransmitters [8], [9]. Thus, theoretically, this receptor can play a role in the regulation of a variety of brain functions. In particular, a role in the modulation of arousal has been advanced [10], [11], [12]. From a therapeutic perspective, this suggests that H3 receptor agonists that decrease the release of histamine and other neurotransmitters important for the wake/sleep cycle may hold a potential as sedative/hypnotics [10], [11].

To evaluate in vivo the role of histamine H3 agonists in the modulation of sleep/wake states, the development of potent, specific and brain penetrating histamine H3 ligands is of major importance. Albeit very potent in vitro, the H3 agonists used to date were reported to lack selectivity or to poorly penetrates the blood–brain barrier. R-(α)-Methylhistamine, for example, has been shown to modulate sleep and waking in the rat when administered directly into the brain but not after systemic administration [11], [13]; see however [10]. BP 2.94, which was originally designed as a lipophilic pro-drug of R-(α)-methylhistamine was shown to slightly increase slow wave sleep after peripheral administration [12]. However, further studies revealed that this compound was not detected in the mice brain after oral administration [14], which question whether the effects observed were centrally-mediated. Finally, the brain penetrating histamine H3 agonist imetit [15] has been shown to possess non-histamine H3 receptor-mediated effects, in particular, a cardiovascular effect in the rat [16], [17]. Moreover, to our knowledge, this compound has not been used to study the consequences of histamine H3 stimulation on the sleep/wake cycle.

The H3 receptor agonist immepip has been described to possess high affinity and selectivity for H3 receptors in rodent tissue [18], [19] and to be about 3–20-fold more potent than R-(α)-methylhistamine in selected functional assays [20]. It has been described to readily cross the blood–brain barrier and drastically decrease brain neuronal histamine release in the rat [20]. Therefore, immepip appears to be a proper tool to evaluate in vivo the potential of H3 agonists as sedative/hypnotic drugs. To our knowledge, no such study has been performed so far with immepip.

The aim of the present study was to investigate the consequences on EEG assessed sleep/wake phases of intraperitoneal (i.p.) administration of the H3 agonist immepip in the rat at doses that significantly decrease neuronal histamine release. The reference benzodiazepine hypnotic flurazepam was used as a positive control [21] to assess the sensitivity of our EEG-sleep rat model, without any mechanistic kinship with immepip.

The protocol described below complies with the European Community guidelines for the use of experimental animals and was approved by the local ethics committee for laboratory animals according to Belgian law.