Attenuated dopaminergic tone in the paraventricular nucleus contributing to sympathoexcitation in rats with Type 2 diabetes

Abstract

The study was conducted to investigate the role for dopamine in the centrally mediated sympathoexcitatory response in rats with Type 2 diabetes (T2D). T2D was induced by a combination of high-fat diet (HFD) and low-dose streptozotocin (STZ). HFD/STZ treatment for 12-14 wk resulted in significant increase in the number of FosB-positive cells in the paraventricular nucleus (PVN) and rostral ventrolateral medulla (RVLM). In anesthetized rats, administration of exogenous dopamine (dopamine hydrochloride, 20 mM) in the PVN, but not in the RVLM, elicited decreases in renal sympathetic nerve activity (RSNA) and mean arterial pressure (MAP) in control rats and but not in the T2D rats. Blocking the endogenous dopamine with dopamine D1/D5 receptor antagonist SCH39166 (2 mM) in the PVN and RVLM, resulted in increases in RSNA, MAP, and heart rate (HR) in both control and T2D rats. These responses were significantly attenuated in T2D rats compared with control rats (PVN - ΔRSNA: 21 ± 10 vs. 44 ± 2%; ΔMAP: 7 ± 3 vs. 19 ± 6 mmHg, ΔHR: 17 ± 5 vs. 32 ± 4 bpm, P < 0.05). There were no significant increases in response to dopamine D2/D3 receptor antagonist raclopride application in the PVN and RVLM of both control and T2D rats. Furthermore, there were decreased dopamine D1 receptor and D2 receptor expressions in the PVN of T2D rats. Taken together, these data suggest that reduced endogenous dopaminergic tone within the PVN may contribute to the sympathoexcitation in T2D.

Keywords: central nervous system; dopaminergic; sympathetic nerve activity.

Fig. 1.

Plasma glucose during intraperitoneal glucose tolerance test in control and Type 2 diabetic (T2D) rats. Data are expressed as means ± SE. *P < 0.05 vs. control rats.

Fig. 2.

A: representative images of FosB staining in the paraventricular nucleus (PVN) (−1.8 mm from bregma), supraoptic nucleus (SON) (−1.8 mm from bregma), medial preoptic nucleus (MnPO) (−0.4 mm from bregma), subfornical organ (SFO) (−1.1 mm from bregma), and rostral ventrolateral medulla (RVLM) (−12.0 mm from bregma) from one rat/group (control and T2D). Scale bar = 50 μm. B: number of FosB-positive cells in the PVN, SON, MnPO, SFO, and RVLM in the two groups of rats. Data are expressed as means ± SE. *P < 0.05 compared with control.

Fig. 3.

The mean data of changes in renal sympathetic nerve activity (RSNA), mean arterial pressure (MAP), and heart rate (HR) after microinjections of dopamine hydrochloride (20 mM) into the PVN and RVLM in control and T2D rats. *P < 0.05 vs. control group.

Fig. 4.

RSNA, MAP, and HR responses to dopamine antagonists microinjected into the PVN and RVLM. A: segments of original recordings from individual rats from each experimental group showing responses of HR, MAP, integrated RSNA (Int.RSNA), and RSNA to D1/D5 antagonist SCH39166 microinjected into the PVN. B: mean data of changes in RSNA, MAP, and HR after microinjections of dopamine antagonists (D₁/D₅ antagonist SCH39166 and D₂/D₃ antagonist raclopride) into the PVN and RVLM in control and T2D rats. *P < 0.05 vs. control group. Ant: antagonist.

Fig. 5.

A: example of visualized bands of dopamine receptor (D₁ receptor and D₂ receptor) and GAPDH in the PVN and RVLM in control and T2D rats. B: mean data of band densities normalized by GAPDH. *P < 0.05 vs. control group.

Fig. 6.

A: immunofluorescent photomicrographs from the sections of the PVN region (bregma −1.8 mm) stained for D₁ receptor (red), microtubule associated protein 2 (MAP2 in green) and 4′,6-diamidino-2-phenylindole (DAPI in blue) in a control and a T2D rat. Scale bar = 100 μm. B: immunofluorescent photomicrographs from the sections of the PVN region (bregma −1.8 mm) stained for D₂ receptor (red), MAP2 (green), and DAPI (blue) in a control and a T2D rat. Scale bar = 100 μm. C: mean data of D₁ receptor and D₂ receptor, MAP2, and DAPI immunofluorescent signal in the PVN in control and T2D rats. M, magnocellular; D, dorsal cap; vP, ventrolateral parvocellular; 3V, third ventricle. *P < 0.05 vs. control group.

Fig. 7.

A–C: schematic representations of serial sections from the rostral (−1.4) to the caudal (−2.1) extent of the region of the PVN. The distance (in mm) posterior to bregma is shown for each section. Solid circles represent the sites of termination of an injection that is within the PVN region in control group. Plus signs represent that in T2D group. D: histological photo showing the injection site (arrow) in the PVN of one rat. AH, anterior hypothalamic nucleus; f, fornix; 3V, third ventricle; ox, optic tract; PVN, paraventricular nucleus; SO, supraoptic nucleus.