Involvement of the paraventricular (PVN) and arcuate (ARC) nuclei of the hypothalamus in the central noradrenergic regulation of liver cytochrome P450

Abstract

The present study was aimed at assessing the influence of noradrenergic innervation of the paraventricular nucleus (PVN) and the arcuate nucleus (ARC) of the brain hypothalamus on cytochrome P450 expression in the liver. DSP-4, a neurotoxin specific to noradrenergic nerve terminals, was administrated locally into the PVN or ARC. One week after neurotoxin injection, the levels of neurotansmitters (noradrenaline/dopamine/serotonin) were measured in the middle part of the hypothalamus, hormone concentrations were estimated in blood plasma, and the activity and the protein levels of CYP isoforms were measured in the liver. A significant decrease in noradrenaline level in the hypothalamus was observed after DSP-4 injection into the PVN or ARC. The levels of dopamine or serotonin remained unchanged or slightly lowered. Simultaneously, significant changes in the plasma concentration of growth hormone were found; its elevation in PVN-lesioned rats and a drop in ARC-lesioned ones. There were no changes in the plasma concentration of the thyroid hormones or corticosterone. The activity and protein levels of isoforms CYP2C11, CYP3A and CYP2A rose in the liver of PVN-lesioned rats, but the activity and protein level of CYP2C11 fell in ARC-lesioned animals such a tendency being also observed in the case of CYP3A. Our study shows that noradrenergic innervation of the PVN and ARC of the hypothalamus exerts an opposite effect on the regulation of cytochrome P450 in the liver. These findings may be important for pharmacological experiments and pharmacotherapy with neuroactive drugs, since cytochrome P450 is responsible for the metabolism of steroids and the majority of drugs.

Introduction

Cytochrome P450 (CYP) is regulated by such endogenous hormones as the growth hormone, corticosterone, thyroid hormones or sex hormones via the growth hormone, which remain under control of the central nervous system [1]. The catecholaminergic systems of the brain have been shown to be engaged in the physiological regulation of cytochrome P450 in the liver [2], [3], [4], [5]. The hypothalamus, in particular the paraventricular nucleus (PVN) and the arcuate nucleus (ARC) which are involved in the regulation of pituitary hormone secretion, seem to play an important role in the central regulation of cytochrome P450.

The PVN is an important integrative part of the hypothalamus. It receives inputs from other regions of the brain and its neurons play an essential role in neuroendocrine and autonomic regulation. Neurons from the PVN project to the posterior lobe of the pituitary gland, the median eminence and a variety of sites in the brain stem and spinal cord. The parvocellular PVN consists of neuroendocrine and preautonomic neurons [6]. The neuroendocrine neurons secrete the corticotropin-releasing hormone – CRH, the thyrotropin-releasing hormone – TRH or somatostatin – SRIH. Their axons project to the external layer of the median eminence where these hormones are released into small portal vessels, reaching the pituitary and affecting hormone secretion from its anterior lobe. This nucleus contributes to the HPA axis, thyroid axis, reproductive axis, as well as to the growth, development and regulation of body fluid balance, gastrointerstinal and cardiovascular functions [7].

The arcuate nucleus is also a part of the periventricular zone of the hypothalamus. The parvocellular ARC contains growth hormone-releasing hormone (GHRH) neurons, which project to the external zone of the median eminence where GHRH is released into the capillary system, thus reaching the pituitary where it stimulates the secretion of the growth hormone (GH). GHRH neurons receive input from dopaminergic, serotonergic, adrenergic and enkephalinergic neurons. The ARC communicates with different brain regions including the pituitary gland, limbic system, some thalamic nuclei, the midbrain periaqueductal gray, autonomic nuclei of the brainstem and the hypothalamus using at least 15 neurotransmitters and neuropeptides and different neuronal phenotypes. The ARC sends its signals to the PVN and other nuclei of the hypothalamus and is involved in energy metabolism [8]. The strongest neuronal input of the ARC is mainly from the PVN. An interaction between the ARC and PVN has been proven neuroanatomically [9], [10], [11].

The neuroendocrine mechanisms controlling GH secretion have been extensively reviewed [12], [13]. GH secretion is stimulated by GHRH and inhibited by somatostatin. Somatostatin cells are found mostly in the periventricular region of the anterior hypothalamus (in the periventricular subnucleus PeV within the PVN). Their axons project mainly to the median eminence, but also to GHRH neurons which provide an inhibitory mechanism. Somatostatin inhibits GHRH secretion, while GHRH stimulates the secretion of somatostatin in the hypothalamus [14], [15], [16], [17]. Somatostatin release is regulated by a variety of factors such as, e.g. neurotransmitters and neuropeptides.

The hypothalamus is densely innervated by noradrenergic neurons [18]. The noradrenergic fibers are present in the periverticular zone and project mainly to the median eminence. Most of them originate from A1 (68%) and A2 (26%) cell groups of the medulla, a few stem from the locus coeruleus (6%) [19]. The A2 input is predominantly distributed in the medial part of the PVN, a region rich in CRH and TRH cells. A body of evidence indicates that noradrenaline synapses are in direct contact with CRH and TRH neurons. They form asymmetric types of axo-somatic and axo-dendritic synapses [20]. The periventricular area of the PVN is innervated by the locus coeruleus (an A6 neuronal group) [21]. Electrophysiological studies show that stimulation of A1, A2 and central noradrenergic bundle usually leads to excitation of the activity of neuroendocrine neurons, whereas A6 stimulation often causes their inhibition [22], [23], [24]. It has been demonstrated that CRH neurons in the PVN have α₁- and α₂-adrenoreceptors [25], [26]. Stimulation of α₁-adrenoreceptors positively regulates CRH secretion and CRH gene expression [27], [28]. TRH release and biosynthesis is stimulated by α₁- and β-adrenoceptors, respectively [29], [30], [31].

Noradrenergic nerves terminals (originating mainly from the A1 neuronal group) are also present in the arcuate nucleus. The secretion of the growth hormone in the pituitary is stimulated by α₂-adrenoceptors by increasing the secretion of GHRH in the ARC, or inhibiting the release of somatostatin in the PVN. Activation of α₁- or β-adrenoceptors inhibit the secretion of the growth hormone [29], [32], [33], [34], [35].

Our recent study showed that intracerebroventricular injection of the specific noradrenergic neurotoxin DSP-4 reduced GH and T₄ levels and enhanced corticosterone serum concentration, leading to a diminished expression of CYP2C11 and CYP3A and an elevated expression of CYP1A [5]. The purpose of the present study was to estimate the role of the two hypothalamic nuclei PVN and ARC in the central noradrenergic regulation of cytochrome P450 in the liver.