α1-Adrenoceptor activation is involved in the central N-methyl-d-aspartate-induced adrenomedullary outflow in rats

Acute intracerebroventricular injection of chemerin-9 increases systemic blood pressure through activating sympathetic nerves via CMKLR1 in brain

Chemerin is an adipocytokine involved in inflammation and lipid metabolism via G protein-coupled receptor, chemokine-like receptor (CMKLR)1. Since the important nuclei regulating pressure (BP) exist in the brain, we examined the effects of acute intracerebroventricular (i.c.v.) injection of chemerin-9 on systemic BP and explored underlying mechanisms. We examined the effects of acute i.c.v. injection of chemerin-9 (10 nmol/head) on systemic BP by a carotid cannulation method in the control or CMKLR1 small interfering (si) RNA-treated Wistar rats (0.04 nmol, 3 days, i.c.v.). We examined protein expression of CMKLR1 around brain ventricles by Western blotting. We examined the effects of acute i.c.v. injection of chemerin-9 on serum adrenaline by a high performance liquid chromatography. In the control siRNA-treated rats, chemerin-9 significantly increased mean BP, which reached a peak at 2 to 4 min after injection. On the other hand, in the CMKLR1 siRNA-treated rats, chemerin-9 did not affect the mean BP. Protein expression of CMKLR1 specifically in subfornical organ (SFO) and paraventricular nucleus (PVN) from the CMKLR1 siRNA-treated rats decreased compared with the control siRNA-treated rats. In the control siRNA-treated rats, chemerin-9 increased serum adrenaline level. On the other hand, in the CMKLR1 siRNA-treated rats, chemerin-9 did not affect the serum adrenaline level. Further, pretreatment with prazosin, an α-adrenaline receptor blocker, significantly prevented the pressor responses induced by chemerin-9. In summary, we for the first time demonstrated that chemerin-9 stimulates the sympathetic nerves via CMKLR1 perhaps expressed in SFO and PVN, which leads to an increase in systemic BP.

Thromboxane A2 in the paraventricular hypothalamic nucleus mediates glucoprivation-induced adrenomedullary outflow

Glucoprivation stimulates a rapid sympathetic response to release and/or secrete catecholamines into the bloodstream. However, the central regulatory mechanisms involving adrenoceptors and prostanoids production in the paraventricular hypothalamic nucleus (PVN) that are responsible for the glucoprivation-induced elevation of plasma catecholamines are still unresolved. In this study, we aimed to clarify whether glucoprivation-induced activation of noradrenergic neurons projecting to the PVN can induce α- and/or β-adrenergic receptor activation and prostanoids production in the PVN to elevate plasma catecholamine levels. We examined the effects of α- and β-adrenergic receptor antagonists, a cyclooxygenase inhibitor, a thromboxane A synthase inhibitor, and a PGE₂ subtype EP₃ receptor antagonist on intravenously administered 2-deoxy-D-glucose (2-DG)-induced elevation of noradrenaline in the PVN and plasma levels of catecholamine in freely moving rats. In addition, we examined whether intravenously administered 2-DG can increase prostanoids levels in the PVN microdialysates. Intracerebroventricular (i.c.v.) pretreatment with phentolamine (a non-selective α-adrenergic receptor antagonist) suppressed the 2-DG-induced increase in the plasma level of adrenaline, whereas i.c.v. pretreatment with propranolol (a non-selective β-adrenergic receptor antagonist) suppressed the 2-DG-induced elevation of the plasma level of noradrenaline. I.c.v. pretreatment with indomethacin (a cyclooxygenase inhibitor) and furegrelate (a thromboxane synthase inhibitor) attenuated the 2-DG-induced elevations of both noradrenaline and adrenaline levels. Furthermore, 2-DG administration elevated the thromboxane B₂ level, a metabolite of thromboxane A₂ in PVN microdialysates. Our results suggest that glucoprivation-induced activation of α- and β-adrenergic receptor in the brain including the PVN and then thromboxane A₂ production in the PVN, which are essential for the 2-DG-induced elevations of both plasma adrenaline and noradrenaline levels.

Changes in hypothalamic neurotransmitter and prostanoid levels in response to NMDA, CRF, and GLP-1 stimulation

Determination of neuroactive substances, such as neurotransmitters and prostanoids, in the extracellular space of the living brain is a very important technique in neuroscience. The hypothalamic paraventricular nucleus (PVN) is one of the most important autonomic control centers in the brain. Recently, we demonstrated that thromboxane (Tx) A2 in the PVN may play an important role in adrenomedullary outflow evoked by N-methyl-D-aspartate (NMDA), corticotrophin-releasing factor (CRF), and glucagon-like peptide-1 (GLP-1) stimulation using microdialysis sampling and liquid chromatography-ion trap tandem mass spectrometry (LC-ITMS(n)). In the present study, we investigated whether centrally administered NMDA, CRF, and GLP-1 can release five neurotransmitters, acetylcholine (ACh), histamine, glutamate (Glu), γ-aminobutyric acid (GABA), and serotonin (5-HT), along with six prostanoids, TxB2, prostaglandin (PG) E2, PGD2, 15-deoxy-∆(12,14) (15d)-PGJ2, 6-keto-PGF1α, and PGF2α in rat PVN microdialysates after optimization of LC-ITMS(n) conditions . All stimulations increased the levels of 5-HT, TxB2, PGE2, and PGF2α (except for 5-HT stimulated with GLP-1). Only NMDA increased the levels of ACh, Glu, and GABA. Treatment with dizocilpine maleate (MK-801), a noncompetitive NMDA receptor antagonist, attenuated the NMDA-induced increase in the levels of neuroactive substances except for Glu. This is the first study to use LC-ITMS(n) to analyze neurotransmitters and prostanoids in the same microdialysates from rat PVN.

Centrally administered isoproterenol induces sympathetic outflow via brain prostaglandin E2-mediated mechanisms in rats

Brain β-adrenoceptor stimulation can induce elevations of plasma levels of noradrenaline. However, there have been no detailed studies related to signaling pathways downstream of β-adrenoceptors responsible for central sympathetic outflow. In the present study, we pharmacologically examined the possibility that centrally administered isoproterenol can induce elevations of plasma noradrenaline levels in a brain prostaglandin-dependent manner. In addition, we also examined whether or not intracerebroventricular administration of isoproterenol could release endogenously synthesized prostaglandin (PG) E2 in the hypothalamic paraventricular nucleus (PVN) by using the brain microdialysis technique combined with liquid chromatography-ion trap tandem mass spectrometry (LC-ITMS(n)). Under urethane anesthesia, a femoral venous line was inserted for infusion of saline and a femoral arterial line was inserted for collecting blood samples. Next, animals were placed in a stereotaxic apparatus for application of test agents. Catecholamines in the plasma were extracted by alumina absorption and were assayed by high-performance liquid chromatography with electrochemical detection. Quantification of PGE2 in rat PVN microdialysates was performed by the LC-ITMS(n) method. We demonstrated that centrally administered isoproterenol-induced elevations of plasma noradrenaline could be mediated via activation of β-adrenoceptors and the downstream phospholipase A2-cyclooxygenase pathway. Furthermore, PGE2 in the PVN and the PGE2 receptor EP3 subtype appear to play an important role in the process. Our results suggest that central isoproterenol-induced sympathetic outflow is mediated via brain PGE2 in a PGE2 receptor EP3 subtype-dependent manner.