Cardiovascular and behavioural effects of intracerebroventricularly administered tachykinin NK3 receptor antagonists in the conscious rat

1. In the conscious rat, three tachykinin NK3 receptor antagonists, namely SR142801 ((S)-(N)-(1-(3-(1-benzoyl-3-(3 ,4-dichlorophenyl)piperidin-3-yl)propyl)-4-phenylpiperidin-4-yl)-N -methylacetamide), R820 (3-indolylcarbonyl-Hyp-Phg-N(Me)-Bzl) and R486 (H-Asp-Ser-Phe-Trp-β-Ala-Leu-Met-NH2) were assessed against the intracerebroventricular (i.c.v.) effects induced by senktide, a selective NK3 receptor agonist, on mean arterial blood pressure (MAP), heart rate (HR) and motor behaviour. 2. Senktide (10-650 pmol per animal; i.c.v; n = 4-16) at the lowest dose caused a significant fall in MAP (-10 ± 6 mmHg), while at the highest doses (100 and 650 pmol), senktide caused a rise in MAP (9 ± 3 and 12 ± 1 mmHg, respectively) when compared to vehicle. The intermediate doses (25 and 65 pmol) had no effect on MAP. The highest two doses caused a tachycardia of 62 ± 15 and 88 ± 8 beats min-1, respectively. The dose of 65 pmol had a biphasic effect on HR, an initial bradycardia of 47 ± 12 beats min-1 followed by a tachycardia of 46 ± 14 beats min-1. The lowest doses caused either a rise of 52 ± 10 beats min-1 (25 pmol) or no effect (10 pmol) on HR. All doses of senktide caused similar increases in face washing, sniffing and wet dog shakes except at the dose of 100 pmol, when wet dog shakes were more than double those observed with the other doses. 3. The antagonist SR142801 (100 pmol-65 nmol per animal; i.c.v.; n = 6-8) caused increases in MAP at the highest two doses (6.5 and 65 nmol) while HR, dose-dependently, increased (23 ± 6 to 118 ± 26 beats min-1) and the onset dose-dependently decreased. The (R)-enantiomer, SR142806 (100 pmol-65 nmol per animal; i.c.v.; n = 6-8) only caused rises in MAP (13 ± 2 mmHg) and HR (69 ± 11 beats min-1) at the highest dose. These drugs had no apparent effect on behaviour, except for the highest dose of SR142801 which increased sniffing. The agonist R820 (650 pmol-6.5 nmol per animal; i.c.v.; n = 6) had no effect on MAP or HR and only increased sniffing behaviour at 6.5 nmol. At 650 pmol (n = 6), R486 had no effect on any variable, but at 3.25 nmol, i.c.v. (n = 4) a delayed tachycardia and a significant increase in all behavioural variables were observed. 4. The cardiovascular responses induced by 6.5 nmol SR142801 and 25 pmol senktide were inhibited by R820 (6.5 nmol, 5 min earlier i.c.v.). In contrast, R820 failed to affect the central cardiovascular and behavioural responses induced by 10 pmol [Sar9, Met(O2)11]substance P, a NK1 receptor selective agonist. The senktide-induced behavioural changes were not inhibited by R820 (6.5 nmol, i.c.v.) while R486 (650 pmol, i.c.v.) blocked both the cardiovascular and behavioural responses to 25 pmol senktide. A mixture of antagonists for NK1 (RP67580; 6.5 nmol) and NK2, (SR48968; 6.5 nmol) receptors injected i.c.v. did not affect the cardiovascular response to SR142801. Cross-desensitization was shown between the central responses to SR142801 and senktide, but not between SR142801 and [Sar9, Met(O2)11]substance P. 5. The antagonists SR142801 and SR142806 (6.5-650 nmol kg-1, n = 5-7), given i.v., did not evoke any cardiovascular or behavioural changes, except a delayed bradycardia for SR142806 (650 nmol kg-1), and also failed to inhibit the increase in MAP evoked by senktide (4 nmol kg-1, i.v.). However, at the highest dose, both drugs slightly reduced the senktide-induced tachycardia. 6. Although the present data are consistent with the in vitro pharmacological bioassays and binding data, showing that SR142801 is a poor antagonist at rat peripheral NK3 receptors, they suggest that SR142801 has a partial agonist action at these receptors centrally. A separation of the cardiovascular and behavioural effects mediated by central NK3, receptor activation was achieved with SR142801 and R820 but not with R486. These results could be explained by the existence of NK3 receptor subtypes in the rat or by the differential activation and inhibition of the same receptor protein linked to the production of different second messengers. Differences in the pharmacokinetic or pharmacodynamic properties of the antagonists cannot be excluded at this time.