Adaptation of the hypothalamic blood flow to chronic nitric oxide synthase blockade

Nitric oxide (NO) plays an essential role in the maintenance of the resting cerebrovascular tone and CBF. During chronic NO synthase (NOS) blockade, however, the CBF is normalized ¹ indicating the activation of compensatory mechanisms. One possibility for the adaptation could be the hyperreactivity of cerebral vessels to NO, since previous studies indicated a remnant 10% cerebral NOS activity during chronic NOS inhibition^{1, 2} accompanied by a recovery of acetylcholine-induced cerebrovascular relaxation ² . Another potential compensatory mechanism could be upregulation of vasodilator prostanoids. We attempted to clarify the contribution of these mechanisms to the adaptation of the hypothalamic blood flow (HBF) to chronic NOS blockade. HBF was measured in anesthetized (urethane 1.3 g/kg) male Wistar rats and ex vivo hypothalamic NOS activity was determined as described previously ¹ . Chronic NOS inhibition was induced by 1 mg/ml NG-nitro-L-arginine methyl ester (L-NAME) administration in the drinking water. Urinary and CSF concentrations of stable metabolites of vasodilator prostaglandins (PGI2, PGE2 and PGD2) were measured by enzyme immunoassay. Semiquantitative RT-PCR analysis of hypothalamic cyclooxygenase mRNA expression was performed as described ³ . One week of L-NAME pretreatment resulted in marked arterial hypertension, but HBF remained unchanged in spite of the significantly reduced hypothalamic NOS activity. Steady state arterial blood gas and acid-base parameters showed no significant differences between control and L-NAME pretreated animals. Reversal of the chronic NOS blockade by intravenous L-arginine infusion (10 mg/kg/min) normalized the blood pressure and at the same time evoked marked hypothalamic hyperemia indicating the presence of a compensatory vasodilator mechanism in the cerebrovascular bed. However, this compensation was independent of the remnant NO production, since intravenous administration of high dose (50 mg/kg) L-NAME after chronic NOS blockade did not influence the HBF. With regard to vasodilatory prostanoids, urinary PGI2 excretion increased significantly during chronic L-NAME treatment. On the other hand, CSF levels of PGI2, PGE2, PGD2 metabolites were similar in control animals and after chronic NOS blockade. Although all three known cyclooxygenase enzymes (COX-1, COX-2 and COX-3) were found to be expressed constitutively in the hypothalamus, none of them showed upregulation during chronic NOS inhibition. General COX blockade by indomethacin (5 mg/kg, iv.) but not specific COX-2 inhibition by diclofenac (10 mg/kg, iv.) decreased the HBF in control rats. Neither of these COX-inhibitors, however, showed an altered response after chronic L-NAME treatment. We conclude that the adaptation of the hypothalamic circulation to the reduction of NO synthesis is independent of the remnant NO production or upregulation of the cerebral vasodilatory prostanoid mechanisms. Constitutive COX-1 or COX-3 activity appears to contribute to the maintenance of the HBF both under physiological conditions and during chronic NOS inhibition.