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Brain Research 885 (2000) 295–297

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Short communication

7-Nitroindazole reduces cerebral blood flow following chronic nitric

oxide synthase inhibition

a a b ,

_*

Paul A.T. Kelly , Isobel M. Ritchie , Douglas E. McBean

a

Department of Clinical Neurosciences, University of Edinburgh, Western General Hospital, Edinburgh EH4 2XU, UK

b

Faculty of Health Sciences, Dietetics and Nutrition, Queen Margaret University College, Clerwood Terrace, Edinburgh EH12 8TS, UK

Accepted 29 August 2000

Abstract

Blood flow and glucose utilization were measured in rat brain after chronic L-NAME treatment followed by acute 7-nitroindazole.

Following chronicL-NAME, blood flow was not significantly different from control. Treatment with acute 7-nitroindazole reduced blood

flow to the same extent in both chronic saline andL-NAME groups. Glucose utilization was unaffected. These results suggest that residual

NOS activity in brain is sufficient to provide tonic, NO-dependent cerebrovascular dilator tone.  2000 Elsevier Science B.V. All rights reserved.

Theme: Other systems of the CNS

Topic: Brain metabolism and blood flow

Keywords: Nitric oxide; Local cerebral blood flow; ChronicL-NAME; Acute 7-nitroindazole

G

Acute treatment with N -substituted arginine analogues produced. The purpose of this study was to examine the

inhibits nitric oxide synthase (NOS) and produces wide- effects of the relatively selective nNOS inhibitor

7-nitro-spread vasoconstriction [3,5]. This is manifest as both an indazole (7-NI) [13] upon lCBF and glucose utilization

increase in peripheral arterial blood pressure and a de- (lCMRglu) in rats previously treated chronically with L

-crease in local cerebral blood flow (lCBF) [11,12]. Re- NAME. If the recovery of cerebrovascular dilator tone

peated daily injection with these arginine analogues, following chronicL-NAME is an eNOS phenomenon acute

inhibits nitric oxide synthase (NOS) activity in the brain 7-NI should have no effect, but if the blood vessels of the

by 95% or more [1,2], but we have previously shown that brain have become more sensitive to NOS of whatever

over several days of such treatment with nitro-L-arginine source, 7-NI should produce a reduction in lCBF.

methyl ester (L-NAME), lCBF returns towards normal Rats were injected with L-NAME (75 mg / kg i.p.; n₅

control levels whilst mean arterial blood pressure remains 20) or saline (n520) once daily for 10 days. We have

elevated [8]. Moreover, despite the almost total inhibition previously shown that this treatment results in a reduction

of NOS activity, a further acute challenge with L-NAME in cerebral NOS activity of at least 95% [1,9]. Fifteen

causes a reduction in lCBF. One possible explanation for hours after the final L-NAME injection, the rats were

this apparently paradoxical response is that measurements treated with a single acute dose of either 7-NI (25 mg / kg of NOS activity in the brain reflect predominantly neuronal i.p.; n520) or oil (n520), and lCBF or lCMRglu were

NOS (nNOS) and cerebrovascular endothelial NOS measured from brain tissue samples as described by us

(eNOS) reacts differently to chronic L-NAME treatment. previously [10]. Data (presented as mean₆S.D.) were

Alternatively, cerebral blood vessels may become more analysed using Dunnett’s t-test to allow comparisons for

sensitive to the 5% of residual NO which may still be each measurement between the chronic saline / acute oil

treated control group and the three drug treatment groups. Acceptable levels of significance were set at P,0.05.

*Corresponding author. Tel.:144-131-317-3622; fax: 1

44-131-317-In rats treated chronically withL-NAME, mean arterial

3528.

E-mail address: dmcbean@qmuc.ac.uk (D.E. McBean). blood pressure (MABP; mmHg) increased significantly to

296 P.A.T. Kelly et al. / Brain Research 885 (2000) 295 –297

172610 (cf. 13867 in controls). Acute treatment with 7-NI had no significant effect on MABP in rats treated

chronically with either saline (130612) or L-NAME

(172618). There were no significant differences in any of the other measured physiological variables ( pCO , pO ,2 2

pH, rectal temperature, plasma glucose) between control and drug-treated groups.

In keeping with our previously published data, mean lCBF in the chronic L-NAME group was not significantly

different from control although small decreases were

apparent (between 25% in hippocampus and 212% in

frontal cortex) (Table 1). Acute 7-NI, following chronic saline treatment, resulted in significant decreases in lCBF

from control ranging from 227% in hippocampus to

237% in frontal cortex. In rats treated chronically with

L-NAME, acute 7-NI injection produced significant

de-creases in lCBF (228% in hippocampus and 233% in Fig. 1. The relationship between mean lCMRglu and mean lCBF in the five brain areas measured for each of the four treatment regimes. The

frontal cortex) which were remarkably similar, or even

best-fitting straight lines through each set of data points are illustrated.

identical to those observed in the chronic saline group.

The lines from the two 7-NI-treated groups are almost identical and

There were no significant differences in lCMRglu

appear superimposed in the figure.

between control and treated groups in any region of the brain analysed in this study. The overall relationship

between mean lCBF and mean lCMRglu was very similar mised, irreversible damage to brain tissues is likely to

in control rats (median ratio51.54), and rats treated occur [7,18]. The oligaemia which is produced by acute

chronically withL-NAME (median ratio₅1.51) (Fig. 1). In L-NAME, in the absence of any parallel reduction in

animals treated acutely with 7-NI following chronic saline, lCMRglu, is therefore likely to be physiologically

unsus-decreased lCBF in the absence of any significant change in tainable in the long-term without evoking pathological

lCMRglu, resulted in a downward re-setting of the flow- consequences.

metabolism relationship (median ratio50.92). An almost This re-establishment of dilator tone to the cerebral

identical response was observed when the animals were blood vessels appears be unique to the cerebral circulation.

subjected to chronic L-NAME treatment prior to acute Moreover, the adaptive dilatation appears to involve NO,

challenge with 7-NI (median ratio50.93) (Fig. 1). despite the fact that in our hands the chronic L-NAME

In the peripheral circulation, chronic treatment with treatment has been shown to reduce cerebral NOS activity

L-NAME and other arginine analogues produces a signifi- by 95% or more [1,9]. Acute 7-NI following chronic

cant and sustained increase in mean arterial blood pressure L-NAME treatment, produces a reduction in lCBF to levels

[4,8,15]. In contrast, reductions in lCBF which are appar- which are similar to those found when the drug is given to ent following a single acute injection of L-NAME [11,12] rats treated chronically with saline. This compares with a

are attenuated following repeated exposure (Table 1). The somewhat attenuated response to acuteL-NAME in

chroni-importance for the survival of the animal of whatever cally treated animals [8], but taken together with that

adaptive mechanism underlies this re-establishment of previous study, these results suggest that the residual NOS

normal flow-metabolism relationships, is immediately evi- activity found in brain homogenates (approximately 5%)

dent. Under most normal physiological conditions, lCBF is may be sufficient to provide tonic, NO-dependent

cerebro-closely coupled to underlying metabolic demands of brain vascular dilator tone in vivo, some of which at least may

tissue [17] and where this relationship is severely compro- be of neuronal origin. However, we cannot say that other

Table 1

a

Effects of chronicL-NAME and acute 7-NI treatments upon local cerebral blood flow

Control Drug treatments

Chronic saline1acute oil Chronic saline1acute 7-NI ChronicL-NAME1acute oil ChronicL-NAME1acute 7-NI

Frontal cortex 12069 7668* 106614 80612*

P.A.T. Kelly et al. / Brain Research 885 (2000) 295 –297 297

[8] P.A.T. Kelly, I.M. Ritchie, F.M. Collins, Cerebrovascular

conse-adaptive mechanisms may not also be exerting an in- _G

quences of repeated exposure to N -nitro-L-arginine methyl ester,

fluence, and nor can we discount the possibility that both

Br. J. Pharmacol. 116 (1995) 2771–2777.

L-NAME and 7-NI may be less selective in their effects

[9] P.A.T. Kelly, I.M. Ritchie, D.E. McBean, 7-Nitroindazole reduces

upon the brain and its circulation than had previously been cerebral blood flow following chronic nitric oxide synthase

inhibi-understood [6,14,16]. tion, J. Cereb. Blood Flow Metab. 19 (Suppl. 1) (1999) S220.

[10] P.A.T. Kelly, I.M. Ritchie, M. Sangra, M.J.A. Cursham, E.M. Dickson, B. Kelly, F.P. Neilson, M.J. Reidy, M.C. Stevens, Hy-peraemia in rat neocortex produced by acute exposure to

methyl-Acknowledgements _{enedioxymethamphetamine, Brain Res. 665 (1994) 315–318.}

[11] P.A.T. Kelly, C.L. Thomas, I.M. Ritchie, G.W. Arbuthnott,

Cerebro-This study was supported by the Wellcome Trust. vascular autoregulation in response to hypertension induced by

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N -nitro-L-arginine methyl ester, Neuroscience 59 (1994) 13–20. [12] I.M. Macrae, D.A. Dawson, J.D. Norrie, J. McCulloch, Inhibition of

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