Cardiovascular and behavioural effects of centrally administered tachykinins in the rat: characterization of receptors with selective antagonists.

1. The effects of intracerebroventricular (i.c.v.) injection of selective and potent NK1 (RP 67580), NK2 (SR 48968) and NK3 (R 486, [Trp7, beta-Ala8]NKA(4-10)) receptor antagonists were assessed on the cardiovascular and behavioural responses elicited by the i.c.v. injection of substance P (SP), neurokinin A (NKA) or [MePhe7]neurokinin B ([MePhe7]NKB) in the conscious freely moving rat. 2. SP, NKA and [MePhe7]NKB (5-650 pmol) evoked dose-dependent increases in mean arterial blood pressure (MAP) and heart rate (HR) with the rank order of potency SP > NKA > [MePhe7]NKB. The cardiovascular responses were accompanied by excessive face washing, grooming and wet dog shakes. 3. The cardiovascular effects and face washing behaviour induced by SP (25 pmol) were significantly reduced by the pre-injection (i.c.v., 5 min earlier) of RP 67580 (6.5 nmol). However, this antagonist failed to affect the central effects of 25 pmol NKA or [MePhe7]NKB. 4. The cardiovascular and behavioural responses (except for wet dog shakes) elicited by NKA (25 pmol) were significantly reduced by 6.5 nmol SR 48968. However, the latter antagonist had no effect on the SP or [MePhe7]NKB-mediated responses. 5. Both cardiovascular and behavioural effects produced by either SP or NKA (25 pmol) were completely abolished when rats were pretreated with a combination of RP 67580 (6.5 nmol) and SR 48968 (6.5 nmol), yet this combination of antagonists failed to modify the central effects of [MePhe7]NKB. 6. R 486 (6.5 nmol) inhibited the cardiovascular effects as well as wet dog shakes produced by [MePhe7]NKB, but it was inactive against the responses induced by either SP or NKA. 7. None of the tachykinin receptor antagonists or agonists caused motor impairment or respiratory distress. All antagonists blocked in a reversible manner and were devoid of intrinsic activity except R486 (6.5 nmol) which produced a transient increase of MAP and HR.8. These results suggest that the central effects of SP, NKA and [MePhe7]NKB are primarily mediated by central NK1, NK2 and NK3 receptors, respectively. However, a minor activation of NK2 receptors bySP and NK1 receptors by NKA was seen during blockade of both receptors. This study therefore supports the existence of functional NK1, NK2 and NK3 receptors in the adult rat brain.