Blockade of tachykinin NK3 receptor reverses hypertension through a dopaminergic mechanism in the ventral tegmental area of spontaneously hypertensive rats

Abstract

Background and purpose: Intracerebroventricularly injected tachykinin NK(3) receptor (R) antagonists normalize mean arterial blood pressure (MAP) in spontaneously hypertensive rats (SHR). This study was pursued to define the role played by NK(3)R located on dopamine neurones of the ventral tegmental area (VTA) in the regulation of MAP in SHR.

Experimental approach: SHR (16 weeks) were implanted permanently with i.c.v. and/or VTA guide cannulae. Experiments were conducted 24 h after catheterization of the abdominal aorta to measure MAP and heart rate (HR) in freely behaving rats. Cardiovascular responses to i.c.v. or VTA-injected NK(3)R agonist (senktide) and antagonists (SB222200 and R-820) were measured before and after systemic administration of selective antagonists for D(1)R (SCH23390), D(2)R (raclopride) or non-selective D(2)R (haloperidol), and after destruction of the VTA with ibotenic acid.

Key results: I.c.v. or VTA-injected SB222200 and R-820 (500 pmol) evoked anti-hypertension, which was blocked by raclopride. Senktide (10, 25, 65 and 100 pmol) elicited greater increases of MAP and HR when injected in the VTA, and the cardiovascular response was blocked by R-820, SCH23390 and haloperidol. VTA-injected SB222200 prevented the pressor response to i.c.v. senktide, and vice versa, i.c.v. senktide prevented the anti-hypertension to VTA SB222200. Destruction of the VTA prevented the pressor response to i.c.v. senktide and the anti-hypertension to i.c.v. R-820.

Conclusions and implications: The NK(3)R in the VTA is implicated in the maintenance of hypertension by increasing midbrain dopaminergic transmission in SHR. Hence, this receptor may represent a therapeutic target in the treatment of hypertension.

Figure 1

Identification of the VTA as a microinjection site following post-mortem histological examination of microinjected Evans's blue. A rat was considered to be correctly injected when a black spot was seen in the VTA without any evidence of haemorrhage or necrosis. Diagram was modified from the atlas of Paxinos and Watson (1998). VTA, ventral tegmental area; SNR, substantia nigra reticular; ml, medial lemniscus; RMC, red nucleus magnocellular; MS, microinjection site. Scale bar: 0.5 mm.

Figure 2

Time-course effects on changes in mean arterial blood pressure (ΔMAP) and heart rate (ΔHR) induced by i.c.v. (A) or ventral tegmental area (VTA) (B) injection of 500 pmol of SB222200 or its inactive enantiomer SB222201. Areas under the curves (AUC) were measured for a period of 0–10 h for i.c.v and 0–8 h for VTA (small insets). Values represent the mean ± SEM of n rats. Significant difference from aCSF is indicated by *P < 0.05; **P < 0.01; ***P < 0.001.

Figure 3

Time-course effects on changes in mean arterial blood pressure (ΔMAP) and heart rate (ΔHR) induced by i.c.v. injection of (A) SB222200 (500 pmol) and (B) R-820 (500 pmol) before and 30 min after treatment with the D₂R antagonist raclopride (0.16 mg kg⁻¹, i.v.) or the D₁R antagonist SCH23390 (0.2 mg kg⁻¹, i.v.). Areas under the curves (AUC) were measured for a period of 0–8 h (SB222200) or 0–72 h (R-820) as shown in small insets. Values represent the mean ± SEM of n rats. Significant difference from aCSF values (*) or NK₃R antagonist alone (†) is indicated by *P < 0.05, **††P < 0.01; ***†††P < 0.001.

Figure 4

Time-course effects on changes in mean arterial blood pressure (ΔMAP) (A) and heart rate (ΔHR) (B) induced by R-820 (500 pmol) injected into the ventral tegmental area (VTA) before and 30 min after treatment with the D₂R antagonist raclopride (0.16 mg kg⁻¹, i.v.). Areas under the curves (AUC) were measured for a period of 0–5 h (small insets). Values represent the mean ± SEM of n rats. Significant difference from aCSF values (*) or R-820 (†) is indicated by *†P < 0.05; **††P < 0.01; ***†††P < 0.001.

Figure 5

Time-course effects on changes in mean arterial blood pressure (ΔMAP) and heart rate (ΔHR) induced by i.c.v. injection of 500 pmol R-820, once daily for a period of 5 days in spontaneously hypertensive rats. Values represent the mean ± SEM of four rats. Significant difference from pre-injection values is indicated by *P < 0.05; **P < 0.01.

Figure 6

Time-course effects on changes in mean arterial blood pressure (ΔMAP) and heart rate (ΔHR) following i.c.v. (A) or ventral tegmental area (VTA) (B) injection of four increasing doses (10, 25, 65 and 100 pmol) of senktide in spontaneously hypertensive rats. Areas under the curves (AUC) were measured for a period of 0–1 h (open columns for VTA and solid columns for i.c.v.). Values represent the mean ± SEM of six rats per injection site. Significant difference from aCSF values (*) or i.c.v. (†) is indicated by **††P < 0.01; ***†††P < 0.001.

Figure 7

Time-course effects on changes in mean arterial blood pressure (ΔMAP) and heart rate (ΔHR) produced by senktide (25 pmol) injected into the ventral tegmental area (VTA) prior to (first day) and 1 h after VTA injection of R-820 (second day). Each point represents the mean ± SEM of four rats. Significant difference from aCSF (*) or senktide (†) values is indicated by *†P < 0.05; **††P < 0.01; ***†††P < 0.001.

Figure 8

Time-course effects on changes in mean arterial blood pressure (ΔMAP) produced by i.c.v. senktide 65 pmol (baseline: 161 ± 5 mmHg) prior to and after treatment with (A) the D₁R antagonist SCH23390 (0.2 mg kg⁻¹ i.v.) and the D₂R antagonist raclopride (0.16 mg kg⁻¹ i.v.) (baseline: 167 ± 4 mm Hg); and (B) the non-selective D₂R antagonist haloperidol (10 mg kg⁻¹ s.c.) (baseline: 164 ± 2.4 mm Hg) in spontaneously hypertensive rats. In (C), the areas under the curves (AUC) measured for a period of 0–1 h are shown. Values represent the mean ± SEM of n rats. Significant difference from aCSF (*) or senktide (†) values is indicated by *†P < 0.05; **††P < 0.01; ***†††P < 0.001.

Figure 9

Time-course effects on changes in mean arterial blood pressure (ΔMAP) produced by treatments with the D₁R antagonist SCH23390 (0.2 mg kg⁻¹ i.v.), D₂R antagonist raclopride (0.16 mg kg⁻¹ i.v.) and the non-selective D₂R antagonist haloperidol (10 mg kg⁻¹ s.c.) in spontaneously hypertensive rats. Different time scales were used: in (A) (min), and (B) (h).Values represent the mean ± SEM of four rats. Significant difference from saline is indicated by *P < 0.05; **P < 0.01; ***P < 0.001.

Figure 10

Time-course effects on changes in mean arterial blood pressure (ΔMAP) produced by senktide injected i.c.v. on the (A) ipsilateral and (B) contralateral sides before and after SB222200 (500 pmol) injected into the ventral tegmental area (VTA). The anti-hypertensive effect elicited by the VTA injection of SB222200 (500 pmol) was measured before and after i.c.v. injection of senktide (65 pmol) on the ipsilateral (C) and contralateral (D) side in spontaneously hypertensive rats. Areas under the curves (AUC) for MAP are shown in (E and F). Values represent the mean ± SEM of n rats. Significant difference from aCSF (*) and senktide alone or SB222200 alone (†) is indicated by *†P < 0.05; **††P < 0.01; ***†††P < 0.001.

Figure 11

Time-course effects on changes in mean arterial blood pressure (ΔMAP) produced by i.c.v. injected 65 pmol senktide (A, B) or 500 pmol R-820 (C, D) in spontaneously hypertensive rats which underwent an ipsilateral (A,C) and contralateral (B,D) lesion of the ventral tegmental area with ibotenic acid (IBO), 5 days earlier or a sham-operation. Areas under the curves (AUC) for MAP are shown in (E,F). Each point represents the mean ± SEM of n rats. Significant difference from aCSF (*) or sham-operated rats (†) is indicated by *†P < 0.05; **††P < 0.01; ***†††P < 0.001.