In the present study, we investigated the role of the central
cholinergic system in mediating the pressor effect of intracerebroventricularly administrated
U-46619, a
thromboxane A2 (TxA2) analog, in hemorrhaged hypotensive rats.
Hemorrhage was performed by withdrawing a total volume of 2.1 ml of blood per 100 g
body weight over a period of 10 min. Intracerebroventricular (i.c.v.) injection of
U-46619 (0.5, 1, 2 micro g) produced a dose- and time-dependent increase in arterial pressure and reversed the
hypotension of this condition.
Hemorrhage caused small increases in extracellular hypothalamic
acetylcholine and
choline levels. Intracerebroventricular administration of
U-46619 (1 micro g) further increased the levels of extracellular
acetylcholine and
choline by 57% and 41%, respectively. Pretreatment with
SQ-29548 (8 mug; i.c.v.), a selective TxA2 receptor antagonist, completely abrogated the effects of subsequent injection of
U-46619 (1 mug; i.c.v.) on arterial pressure and extracellular
acetylcholine and
choline levels. Pretreatment with
mecamylamine (50 micro g; i.c.v.), a
cholinergic nonselective
nicotinic receptor antagonist, attenuated the pressor effect of
U-46619 (1 micro g, i.c.v.) in hemorrhaged rats whereas pretreatment with
atropine (10 micro g; i.c.v.), a
cholinergic nonselective
muscarinic receptor antagonist, had no effect. Interestingly, pretreatment of rats with
methyllycaconitine (10 micro g; i.c.v.) or
alpha-bungarotoxin (10 micro g; i.c.v.), selective antagonists of alpha-7 subtype
nicotinic acetylcholine receptors (alpha7nAChRs), partially abolished the pressor effect of
U-46619 (1 micro g; i.c.v.) in the hypotensive condition. Pretreatment with a combination of
mecamylamine plus
methyllycaconitine or
mecamylamine plus
alpha-bungarotoxin attenuated the reversal effect of
U-46619, but only to the same extent as pretreatment with either antagonist alone. In conclusion, i.c.v. administration of
U-46619 restores arterial pressure and increases posterior hypothalamic
acetylcholine and
choline levels by activating central TxA2 receptors in hemorrhaged hypotensive rats. The activation of central nicotinic
cholinergic receptors, predominantly alpha7nAChRs, partially acts as a mediator in the pressor responses to i.c.v. injection of
U-46619 under these conditions.