Diminished muscarinic receptor-mediated cerebral blood flow response in streptozotocin-treated rats

nNOS-dependent reactivity of cerebral arterioles in Type 1 diabetes

Our goals were to determine whether Type 1 diabetes (T1D) alters neuronal nitric oxide synthase (nNOS)-dependent reactivity of cerebral arterioles and to identify a potential role for oxidative stress in T1D-induced impairment in nNOS-dependent responses of cerebral arterioles. Rats were injected with vehicle (sodium citrate buffer) or streptozotocin (50 mg/kg IP) to induce T1D. Two to three months later, we measured functional responses of cerebral arterioles to nNOS-dependent (NMDA and kainate) and -independent (nitroglycerin) agonists in nondiabetic and diabetic rats before and during inhibition of oxidative stress using tempol (100 microM). In addition, we measured superoxide anion production under basal conditions, during stimulation with NMDA and kainate, and during treatment with tempol. We found that nNOS-dependent, but -independent, vasodilatation was impaired in diabetic compared to nondiabetic rats. In addition, treatment of the cerebral microcirculation with tempol restored impaired nNOS-dependent vasodilatation in diabetic rats toward that observed in nondiabetic rats. Furthermore, the production of superoxide anion (lucigenin chemiluminescence) was increased in parietal cortical tissue of diabetic rats under basal conditions. Application of NMDA and kainate did not increase superoxide anion production in nondiabetic or diabetic rats. However, tempol decreased basal production of superoxide anion in diabetic rats. Our findings suggest that T1D impairs nNOS-dependent dilatation of cerebral arterioles by a mechanism that appears to be related to the formation of superoxide anion.

Central nervous system complications of diabetes mellitus--a perspective from the blood-brain barrier

A host of diabetes-related changes in the central nervous system (CNS) has been recognized. The underlying causes of these changes are multiple. An important contributor to the changes in the CNS is the blood-brain barrier (BBB). Diabetes is associated with changes in both the barrier and transport functions of the cerebral microvessels. Structural changes in cerebral microvessels may account for some of the observed changes. Additional mechanisms include alterations in hemodynamic variables such as arteriovenous shunting, changes in biophysical properties and biochemical compositions of the endothelial cells including changes in lipid fluidity and composition, and alterations of neurotransmitter activity in the cerebral microvessels, notably altered beta adrenergic neurotransmission. These observations indicate that the CNS is not immune against the microangiopathic complications commonly found in various tissues of diabetic animals.

Cerebrovascular responsiveness to NG-nitro-L-arginine methyl ester in spontaneously diabetic rats

1. There is evidence that endothelial dysfunction is associated with diabetes mellitus. The purpose of the present study was to assess local cerebral blood flow (LCBF) and cerebrovascular responsiveness to the NOS inhibitor NG-nitro-L-arginine methyl ester (L-NAME) in spontaneously diabetic insulin-dependent BioBred (BB) rats. 2. Diabetic rats, and non-diabetic controls, were treated with L-NAME (30 mg kg-1, i.v.) or saline, 20 min prior to the measurement of LCBF by the fully quantitative [14C]-iodoantipyrine autoradiographic technique. 3. There were no significant differences in physiological parameters (blood pH, PCO2, and PO2, rectal temperature, arterial blood pressure, or plasma glucose) between any of the groups of rats, and no difference in either the extent or the temporal characteristics of the hypertensive response to L-NAME between diabetic and non-diabetic rats. 4. In diabetic rats, a global reduction in basal LCBF was observed, although significant reductions (between -20 and -30%) were found in only 5 (mainly subcortical) out of the 13 brain regions measured. Following L-NAME injection, significant reductions in LCBF (between -20 and -40%) were found in the non-diabetic animals. In diabetic animals treated with L-NAME, a significant reduction in LCBF was measured only in the hypothalamus (-33%). 5. The cerebrovascular response to acute L-NAME is attenuated in spontaneously diabetic insulin-dependent BB rats. This would be consistent with the endothelial dysfunction in cerebral vessels, known to be associated with diabetes mellitus and it is possible that a loss of NO-induced dilator tone, amongst other factors, may underlie the observed reductions of basal LCBF in these animals.

Nitric oxide mediates the increase in local cerebral blood flow during focal seizures

The role of nitric oxide (NO) in the increase in local cerebral blood flow (LCBF) elicited by focal cortical epileptic seizures was investigated in anesthetized adult rats. Seizures were induced by topical bicuculline methiodide applied through two cranial windows drilled over homotopic sites of the frontal cortex, and LCBF was measured by quantitative autoradiography by using 4-iodo[N-methyl-14C]antipyrine. Superfusion of an inhibitor of NO synthase, N omega-nitro-L-arginine (NA; 1 mM), for 45 min abolished the increase of LCBF induced by topical bicuculline methiodide (10 mM) [164 +/- 18 ml/100 g per min in the artificial cerebrospinal fluid (aCSF)-superfused side and 104 +/- 12 ml/100 g per ml in the NA-superfused side; P < 0.005]. This effect was reversed by coapplication of an excess of L-arginine substrate (10 mM) (218 +/- 22 ml/100 g per min in the aCSF-superfused side and 183 +/- 31 ml/100 g per min in the NA + L-Arg-superfused side) but not by 10 mM D-arginine, a stereoisomer with poor affinity for NO synthase (193 +/- 17 ml/100 g per min in the aCSF-superfused side and 139 +/- 21 ml/100 g per min in the NA + D-Arg-superfused side; P < 0.005). Superfusion of the guanylyl cyclase inhibitor methylene blue attenuated the LCBF increase elicited by topical bicuculline methiodide by 25% +/- 16% (P < 0.05). The present findings suggest that NO is the mediator of the vasodilation in response to focal epileptic seizures.

Nitric oxide and the cerebral circulation

BACKGROUND

Nitric oxide (NO) is a potent vasodilator that was initially described as the mediator of endothelium-dependent relaxation (endothelium-derived relaxing factor, EDRF). It is now known that NO is produced by a variety of other cell types.

SUMMARY OF REVIEW

Endothelium produces NO (EDRF) under basal conditions and in response to a variety of vasoactive stimuli in large cerebral arteries and the cerebral microcirculation. Endothelium-dependent relaxation is impaired in the presence of several pathophysiological conditions. This impairment may contribute to cerebral ischemia or stroke. Activation of glutamate receptors appears to be a major stimulus for production of NO by neurons. Neuronally derived NO may mediate local increases in cerebral blood flow during increases in cerebral metabolism. NO synthase-containing neurons also innervate large cerebral arteries and cerebral arterioles on the brain surface. Activation of parasympathetic fibers that innervate cerebral vessels produces NO-dependent increases in cerebral blood flow. Increases in cerebral blood flow during hypercapnia also appear to be dependent on production of NO. Astrocytes may release some NO constitutively, but astrocytes and microglia can release relatively large quantities of NO after induction of NO synthase in response to endotoxin or some cytokines. Expression of inducible NO synthase, perhaps in response to local production of cytokines, may exert cytotoxic effects in brain during or after ischemia.

CONCLUSIONS

Because endothelium, neurons, and glia can all produce NO in response to some stimuli, the influence of NO on the cerebral circulation appears to be very important. Under normal conditions, constitutively produced NO influences basal cerebral vascular tone and mediates vascular responses to a diverse group of stimuli. The inducible form of NO synthase produces much greater amounts of NO that may be an important mediator of cytotoxicity in brain.

Cerebral blood flow and cerebrovascular reactivity after inhibition of nitric oxide synthesis in conscious goats

1. The role of nitric oxide in the cerebral circulation under basal conditions and after vasodilator stimulation was studied in instrumented, conscious goats, by examining the action of inhibiting endogenous nitric oxide production with NG-nitro-L-arginine methyl ester (L-NAME). 2. In 6 unanaesthetized goats, blood flow to one brain hemisphere (electromagnetically measured), systemic arterial blood pressure and heart rate were continuously recorded. L-NAME (35 mg kg-1 by i.v. bolus) decreased resting cerebral blood flow by 43 +/- 3%, increased mean arterial pressure by 21 +/- 2%, and decreased heart rate by 41 +/- 2%; cerebrovascular resistance increased by 114 +/- 13% (P < 0.01); the immediate addition of i.v. infusion of L-NAME (0.15-0.20 mg kg-1 during 60-80 min) did not significantly modify these effects. Cerebral blood flow recovered at 72 h, arterial pressure and cerebrovascular resistance at 48 h, and heart rate at 6 days after L-NAME treatment. 3. A second treatment with L-NAME scheduled as above reproduced the immediate haemodynamic effects of the first treatment, which (except bradycardia) reversed with L-arginine (200-300 mg kg-1 by i.v. bolus). 4. Acetylcholine (0.01-0.3 micrograms), sodium nitroprusside (3-100 micrograms) and diazoxide (0.3-9 mg), injected into the cerebral circulation of 5 conscious goats, produced dose-dependent increases in cerebral blood flow, and decreases in cerebrovascular resistance; sodium nitroprusside (30 and 100 micrograms) also caused hypotension and tachycardia. 5. The reduction in cerebrovascular resistance from resting levels (in absolute values) to lower doses,but not to the highest dose, of acetylcholine was diminished, to sodium nitroprusside was increased, and to diazoxide was unaffected after L-NAME, compared to control conditions. The effects on cerebrovascular resistance to acetycholine normalized within 24 h and to sodium nitroprusside within 48 h after L-NAME treatment.6. This study provides information about the evolution of the changes in cerebral blood flow and cerebrovascular reactivity after inhibition of endogenous nitric oxide in conscious animals. The results suggest: (a) endogenous nitric oxide is involved in regulation of the cerebral circulation by producing a resting vasodilator tone, (b) the cerebral vasodilatation to acetylcholine is mediated, at least in part, by nitric oxide release, and (c) inhibition of nitric oxide production induces supersensitivity of cerebral vasculature to nitrovasodilators.