Chronic hyperglycemic diabetes in the rat is associated with a selective impairment of cerebral vasodilatory responses.

Diabetes has been reported to impair vasodilatory responses in the peripheral vascular tissue. However, little is known about vasodilatory function in the diabetic brain. We therefore studied, in the N2O-sedated, paralyzed, and artificially ventilated rat, the effects of chronic hyperglycemic diabetes on the cerebral blood flow (CBF) responses to 3 acutely imposed vasodilatory stimuli: hypoglycemia (HG) (plasma glucose = 1.6-1.9 mumol ml-1), hypoxia (HX) (PaO2 = 35-38 mm Hg), or hypercarbia HC) (PaCO2 = 75-78 mm Hg). In addition, we evaluated the somatosensory evoked potential (SSEP) and plasma catecholamine changes in rats exposed to acute glycemic reductions. Diabetes was induced via streptozotocin (STZ, 60 mg kg-1 i.p.). All results in diabetic rats were compared to those obtained in age-matched nondiabetic controls. The animals were studied at 6-8 weeks (HG experiments) or 4-6 months (HG, HX, and HC experiments) post-STZ. Values for CBF were obtained for the cortex (CX), subcortex (SC), brainstem (BS), and cerebellum (CE) employing radiolabeled microspheres. Up to three CBF determinations were made in each animal. In 6-8 week diabetics vs. controls, CBF increased to a lesser value in the CX, SC, and BS (p less than 0.05). Thus, in the diabetics, going from chronic hyperglycemia to acute hypoglycemia, CBF values (in ml 100 g-1 min-1 +/- SD) increased (p less than 0.05) from 89 +/- 22 to 221 +/- 57 in the CX, from 82 +/- 21 to 160 +/- 52 in the SC, and from 79 +/- 34 to 237 +/- 125 in the BS. In controls, going from normoglycemia to acute hypoglycemia, the CBF changes (p less than 0.05) were 128 +/- 27 to 350 +/- 219 (CX), 117 +/- 11 to 358 +/- 206 (SC), and 130 +/- 29 to 452 +/- 254 (BS). CBF changes and absolute values in the CE were similar in the two groups. At 4-6 months post-STZ, a complete loss of the hypoglycemic CBF response was found in the CX, SC, and CE. In the BS, a CBF response to hypoglycemia was seen in the diabetic rats, with the CBF increasing from 114 +/- 28 (hyperglycemia) to 270 +/- 204 ml 100 g-1 min-1 (p less than 0.05), compared to a change from 147 +/- 36 (normoglycemia) to 455 +/- 299 ml 100 g-1 min-1 (p less than 0.05) in the control group.(ABSTRACT TRUNCATED AT 250 WORDS)

Nitric oxide synthesis and regional cerebral blood flow responses to hypercapnia and hypoxia in the rat.

The role of nitric oxide (NO) synthesis in the cerebral hyperemic responses to hypercapnia and hypoxia was investigated in anesthetized rats. Regional CBF (rCBF) measurements were obtained in the cortex (CX), subcortex (SC), brainstem (BS), and cerebellum (CE) using radiolabeled microspheres. The rCBF responses to either hypercapnia (PaCO2 = 70-80 mm Hg) or hypoxia (PaO2 = 40-45 mm Hg) were compared in rat groups studied in the presence and absence of NO synthase inhibition induced via the intravenous infusion of nitro-L-arginine methyl ester (L-NAME, 3 mg kg-1 min-1). Administration of L-NAME under normocapnic/normoxic conditions produced a 40-60% reduction in baseline rCBF values, indicating the presence of a NO "tone" in the cerebral vasculature. Infusion of L-NAME resulted in a substantial attenuation, in all regions measured, of the rCBF increases that normally accompany hypercapnia. In comparing saline-infused to L-NAME-infused rats, the percentage increases in rCBF (from normocapnic baseline values) were 351% versus 166% (CX), 446% versus 199% (SC), 443% versus 206% (BS), and 483% versus 174% (CE), respectively. The rCBF changes from baseline (delta rCBF in ml 100 g-1 min-1) were 488 versus 57 (CX), 570 versus 60 (SC), 434 versus 72 (BS), and 393 versus 45 (CE), respectively. These differences were all statistically significant (p < 0.05). During hypoxia, when compared to rats not given L-NAME, inhibition of NO synthase activity resulted in significantly greater (p < 0.05) percentage increases in rCBF (from normoxic baseline values) in most regions.(ABSTRACT TRUNCATED AT 250 WORDS)

The role of endothelium and nitric oxide in rat pial arteriolar dilatory responses to CO2 in vivo.

Using a closed cranial window system and intravital microscopy/videometry, we studied the rat pial arteriolar (30-60 microns) responses to CO2 before and following a light/dye (L/D) endothelial injury or topical application of the nitric oxide synthase (NOS) inhibitor, nitro-L-arginine (L-NA) or its inactive form, D-NA. L/D treatment consisted of intravenous injection of sodium fluorescein and the illumination (for 90 s) of arteriolar discrete segments on the cortical surface with light from a mercury lamp. Functional changes in pial arteriolar endothelium were characterized by evaluating responses to topical application of acetylcholine (Ach, 5 x 10(-4) M) and to intravenous (i.v.) oxotremorine (OXO, a stable blood-brain barrier permeant muscarinic agonist, 1 microgram kg-1 min-1). After the L/D injury, dilation to Ach was absent whereas dilations to the NO donor, S-nitrosoacetyl-penicillamine (SNAP, 10(-5) M) and to CO2 (5%) were unchanged (PaCO2 = 70 mm Hg). Loss of Ach response but intact SNAP response confirmed functional endothelial injury and intact smooth-muscle function. The global endothelium-dependent vasodilation induced by i.v. OXO was markedly attenuated when expanding the L/D injury field from 300 microns to 6 mm in diameter. However, the global vasodilation induced by inhalation of CO2 was still unaffected by this increase in the area of light exposure. This provides evidence that the expanded exposure was capable of impairing global vasodilation resulting from endothelium-dependent stimuli but not from inhalation of CO2. The intact CO2 response despite an endothelial dysfunction suggests that the reported NO dependence of hypercapnia-induced cerebral hyperemia in rats cannot be attributed to an endothelial NO source.(ABSTRACT TRUNCATED AT 250 WORDS)

Regional blood-brain glucose transfer and glucose utilization in chronically hyperglycemic, diabetic rats following acute glycemic normalization.

Regional rates of brain glucose utilization (rCMRglc) and glucose influx (rJin), along with regional brain tissue glucose concentrations, were measured in chronically hyperglycemic diabetic (CHD) rats following acute glycemic normalization. These results were compared to those obtained in nondiabetic normoglycemic controls. The diabetic rats were evaluated at 6-8 weeks following i.p. streptozotocin injection. All rats were N2O (70%) sedated, paralyzed, and artificially ventilated for study. Acutely normoglycemic (plasma glucose = 8.5 mumol/ml), demonstrated significantly higher (p less than 0.05) rCMRglc and rJin values in 8 of the 11 regions analyzed. Tissue/plasma glucose concentration ratios were significantly greater than control in 9 of 11 regions. Prior to acute glycemic normalization, rCMRglc values in CHD rats were either unchanged or moderately lower than control. These findings indicate that no blood-brain barrier glucose transport repression is present in CHD rats. In fact, the results suggest an increased transport capacity. The increased rCMRglc observed in the acutely normalized CHD rats may be a manifestation of the "hypoglycemic symptoms" observed in chronically hyperglycemic patients following acute glycemic reductions to the normal range. The present results imply that these symptoms are not related to the presence of a relative cerebral glucopenia, as others have suggested.

The role of neuronal nitric oxide synthase in regulation of cerebral blood flow in normocapnia and hypercapnia in rats.

The nitric oxide synthase (NOS) inhibitors, nitro-L-arginine, its methyl ester, and N-monomethyl-L-arginine, have been shown to attenuate resting CBF and hypercapnia-induced cerebrovasodilation. Those agents nonspecifically inhibit the endothelial and neuronal NOS (eNOS and nNOS). In the present study, we used a novel nNOS inhibitor, 7-nitroindazole (7-NI) to examine the role of nNOS in CBF during normocapnia and hypercapnia in fentanyl/N2O-anesthetized rats. CBF was monitored using laser-Doppler flowmetry. Administration of 7-NI (80 mg kg-1 i.p.) reduced cortical brain NOS activity by 57%, the resting CBF by 19-27%, and the CBF response to hypercapnia by 60%. The 60% reduction was similar in magnitude to the CBF reductions observed in previous studies in which nonspecific NOS inhibitors were used. In the present study, 7-NI did not increase the MABP. Furthermore, the CBF response to oxotremorine, a blood-brain barrier permeant muscarinic agonist that induces cerebrovasodilation via endothelium-derived NO, was unaffected by 7-NI. These results confirmed that 7-NI does not influence eNOS; they also indicated that the effects of 7-NI on the resting CBF and on the CBF response to hypercapnia in this study were solely related to its inhibitory action on nNOS. The results further suggest that the NO synthesized by the action of nNOS participates in regulation of basal CBF and is the major, if not the only, category of NO contributing to the hypercapnic CBF response.

Maintenance of cerebral blood flow and metabolism during pharmacological hypotension in aged hypertensive rats.

Cerebral blood flow (CBF) and cerebral oxygen metabolism (CMRO2) were measured in aged (24 month) spontaneously hypertensive rats (SHR) during sodium nitroprusside (SNP) and nitroglycerin induced hypotension. Both CBF and CMRO2 were decreased in SHR during hypotension induced with SNP. Significant decrements in CMRO2 were observed in aged SHR during even moderate hypotension (80-90 torr). Cerebral autoregulatory responses during nitroglycerin infusion in aged SHR were similar to SNP treated WKY and CMRO2 was maintained at control levels under all hypotensive test conditions. These results indicate that aged SHR are susceptible to cerebral ischemia during SNP induced hypotension, probably due to the combined effects of aging and hypertension on the cerebral vasculature. NTG moderated the decreases in CBF and CMRO2 seen during hypotensive challenges and may decrease the risk of stroke during hypotensive anesthesia.

Cerebrovascular and cerebral metabolic responses of aged rats to changes in arterial PCO2.

Cerebrovascular and cerebral metabolic responses to changes in arterial PCO2 were tested in young (4 month) and aged (24 month) Sprague-Dawley rats. Rats were anesthetized with 70% nitrous oxide and 30% oxygen, paralyzed with tubocurare and artificially ventilated. Cerebral blood flow (CBF) was measured with radioactive microspheres and cerebral oxygen metabolism (CMRO2) was analyzed from arterial and sagittal sinus oxygen content differences. CBF increased in both young and aged rats with increasing arterial PCO2. Aged rats had significantly depressed cerebrovascular reactivity to changes in CO2 compared to young rats (p less than 0.05). CMRO2 was not significantly different between young and aged rats and did not significantly change with changes in arterial PCO2.

Plasma and myocardial catecholamine levels in young and aged rats during halothane anesthesia.

Plasma and heart tissue catecholamines were measured in young (4-month) and aged (28-month) Sprague Dawley rats under unanesthetized conditions and following the induction of halothane anesthesia. Arterial blood pressure, blood gases, plasma and heart tissue norepinephrine, epinephrine and dopamine concentrations were measured in separate groups of unanesthetized and halothane anesthetized rats. Young and aged rats were tested under equal anesthetic levels and blood halothane concentrations were measured using gas chromatography. Aged rats required significantly less halothane to maintain anesthesia compared to young animals (7.25 +/- 1.43 vs. 14.91 +/- 0.93 mg/dL, p less than 0.05). Cardiovascular parameters were similar in young vs. aged rats under unanesthetized conditions, but blood pressure decreased significantly more in aged than in young rats during halothane anesthesia (43% vs. 17%, p less than 0.05). Heart tissue catecholamines were not different between young and aged and did not change during halothane anesthesia. Plasma norepinephrine concentrations were consistently elevated in aged vs. young rats under both unanesthetized and anesthetized test conditions, but there was no significant change in these levels from the unanesthetized to the anesthetized state. Results suggest that increased catecholamine levels in aged animals may be necessary to maintain a normal cardiovascular state under unanesthetized conditions and that catecholamines do not increase during the hypotensive state produced by halothane anesthesia in spite of marked hypotension.

The interaction between benzodiazepine antagonists and barbiturate-induced cerebrovascular and cerebral metabolic depression.

It has been reported that pentobarbital facilities binding to benzodiazepine receptors binding at anesthetic concentrations and that this action may play a role in the anesthetic potency of this barbiturate. The interaction between pentobarbital and benzodiazepine receptors was tested with Ro 15-1788 which is reported to be a pure benzodiazepine antagonist and 3-hydroxymethyl-beta-carboline (3-HMC), an antagonist which has inverse activity alone. Cerebral blood flow (CBF) and cerebral oxygen consumption (CMRO2) were measured in rats after injections of pentobarbital with and without the antagonists. Pentobarbital produced dose-dependent decreases in cerebral blood flow and cerebral oxygen consumption at 15 and 30 mg/kg. The antagonist Ro 15-1788 (10 mg/kg) stimulated cerebral blood flow and cerebral oxygen consumption alone but did not alter the cerebral depression produced by pentobarbital. The cerebral metabolic stimulation produced by Ro 15-1788 was unexpected since the drug is reported to be a pure antagonist without agonistic activity, but the lack of effect on pentobarbital-induced cerebral depression is consistent with other reports. 3-Hydroxymethyl-beta-carboline at 10 mg/kg did not stimulate cerebral blood flow and cerebral oxygen consumption but significantly antagonized the decrease in cerebral oxygen consumption produced by 15 mg/kg pentobarbital. 3-Hydroxymethyl-beta-carboline had no significant effect on decreases in cerebral blood flow and cerebral oxygen consumption produced by phenobarbital, a barbiturate which is reported not to alter binding to benzodiazepine receptors. The ability of 3-HMC to antagonize the effects of pentobarbital would be consistent with an action of both drugs at the benzodiazepine receptor but not by altering binding to an endogenous receptor.

Occurrence of potentially detrimental temperature alterations in hospitalized patients at risk for brain injury.

OBJECTIVE: To ascertain the incidence and timing of fever in patients at risk for temperature modulation of brain injury resulting from ischemia or trauma. DESIGN: We retrospectively reviewed the medical records of patients admitted between January 1991 and December 1994. MATERIAL AND METHODS: We investigated three groups of hospitalized patients considered at risk for ongoing brain injury resulting from a prior cerebral insult: successful resuscitation from out-of-hospital cardiac arrest (CA), subarachnoid hemorrhage (SAH), or traumatic closed-head injury (CHI). Forty patients per condition were randomly selected from those who survived for more than 24 hours after hospital admission. RESULTS: During the initial 72 hours of hospitalization, temperature increases to 38 degrees C or more (that is, temperatures previously reported to worsen neurologic outcome after brain injury) were noted in 83% of patients with CA, 70% of those with SAH, and 68% of those with CHI. Within the cohort of febrile patients, 18 to 44% of all temperature measurements were 38 degrees C or higher, and the febrile episodes occurred randomly throughout the study interval. Fewer than one-eighth of the febrile patients received drugs possessing antipyretic properties (such as aspirin or acetaminophen) in a dose appropriate to treat fever. No other method of temperature control (for example, physical means) was used in any patient. The fractions of patients who were dismissed from the hospital with permanent neurologic injury were as follows: CA, 20%; SAH, 45%; and CHI, 43%. CONCLUSION: In these hospitalized patients at risk for ongoing brain injury, the incidence of temperature increases within the range reported to worsen neurologic outcome (elevations of 1.0 degree C or more) was very high. The characterization of these potentially injurious, randomly occurring, and traditionally undertreated temperature increases may have implications for the design of future protocols aimed at providing cerebral protection.

Protein kinase C suppresses receptor-mediated pial arteriolar relaxation in the diabetic rat.

Cerebral vasodilatory responses are selectively impaired in chronically hyperglycemic, diabetic rats. In this study, we tested the hypothesis that chronic hyperglycemia-induced protein kinase C (PKC) activation can account for the suppression of 2 separate receptor-mediated vascular relaxation processes: (1) endothelium-derived nitric oxide (NO) release, and (2) NO-independent beta-adrenergic receptor (beta-AR) activation. The in vivo reactivity of pial arterioles was evaluated in anesthetized rats (streptozotocin-treated diabetics and controls) using a closed cranial window and intravital microscopy. Compared with controls, diabetic rats showed a substantial attenuation or loss of the arteriolar relaxation response accompanying suffusion of the receptor-linked, NO-dependent agonists, acetylcholine (Ach) and adenosine diphosphate (ADP), and the beta-AR-agonist, isoproterenol (ISO). The vasodilatation induced by the direct NO donor, sodium nitroprusside (SNP), was the same in both groups. In the presence of the PKC inhibitor, staurosporine (STAURO), the Ach, ADP, and ISO responses were, largely restored and the SNP response was unaffected. STAURO produced no changes in Ach, ADP, ISO, or SNP responses in non-diabetic rats. These results suggest that PKC activation in chronically hyperglycemic, diabetic rats suppresses receptor-dependent NO release and desensitizes beta-ARs.

The role of nitric oxide in modulating brain activity and blood flow during seizure.

The role played by nitric oxide (NO) in modulating seizure activity and cerebral blood flow (CBF) during seizures was investigated in rats. Seizures were induced with bicuculline (a GABA antagonist, 1.2 mg kg-1, i.v.). Each animal was subjected to an initial bicuculline-induced seizure followed by treatment with either L-nitroarginine (L-NA, a NO synthase inhibitor) or its less active enantiomer D-NA as a 50 mg kg-1 bolus followed by an infusion of 1 mg kg-1 min-1. The animals then received a second bicuculline treatment. Seizure duration was monitored using EEG and CBF was measured with laser-Doppler. There was no difference in seizure duration before or after D-NA administration. Seizure duration doubled from (6 +/- 1 to 12 +/- 2 min p < 0.05) following inhibition of NO synthase with L-NA. The increase in CBF that accompanied the seizure activity paralleled the seizure duration. Our data support the concept that (1) NO acts as an endogenous anticonvulsant, with seizure duration doubling when NO synthase is acutely inhibited, and (2) that NO is not the messenger that couples CBF to metabolism during bicuculline-induced seizures.

Effect of sympathetic blockade on central prostaglandin E2-induced hyperthermia.

The mechanisms by which intracerebroventricular (i.v.t.) prostaglandin E2 (PGE2) produce hyperthermia in the rat were investigated. I.v.t. PGE2 produced dose-related increases in blood pressure, heart rate and rectal temperature which were significant with a dose of 0.5 ng. Oxygen consumption also increased and remained above baseline over an hour with 50 and 500 ng PGE2 doses. Ganglionic blockade with hexamethonium (20 mg/kg) attenuated the blood pressure and heart rate response to PGE2 but metabolic rate and rectal temperature increases were unchanged. Propranolol (2 mg/kg i.v.) decreased the heart rate response to PGE2 but had no significant effect on blood pressure, metabolic rate and rectal temperature responses. These results suggest a similar sensitivity of central receptors for mediating cardiovascular and metabolic rate/temperature increases but suggest that the mechanisms mediating these effects are separate.

Role of nitric oxide, adenosine, N-methyl-D-aspartate receptors, and neuronal activation in hypoxia-induced pial arteriolar dilation in rats.

In this study, we tested the hypothesis that nitric oxide (NO) and adenosine (ADO) are the principal mediators of severe hypoxia-induced vasodilation. In addition, we examined whether activation of N-methyl-D-aspartate (NMDA) receptors and/or perivascular nerves plays a role. A closed cranial window and intravital microscopy system was used to monitor diameter changes in pial arterioles (approximately 40 microns) in anesthetized rats. The relative contributions of ADO, NMDA, NO, and neuronal activation to hypoxic cerebrovasodilation were assessed using the blockers 8-sulfophenyltheophylline (8-SPT), MK-801, nitro-L-arginine methylester (L-NAME), and tetrodotoxin (TTX). Two experimental series were studied. In the first, we tested the effects of NOS inhibition, via topical L-NAME (1 mM), on moderate (PaO2 approximately 46 mmHg) then severe (PaO2 approximately 34 mmHg) hypoxia-induced dilation. To confirm that L-NAME was affecting specifically NO-dependent responses, we also examined, in each experiment, the vasodilatory responses to topical applications of NOS-dependent (adenosine diphosphate (ADP); acetylcholine (ACh)) and -independent (sodium nitroprusside (SNP)) agents, in the presence of L-NAME or, in controls, the presence of D-NAME or no added analogue. In the second series, topical suffusions of ADP, ADO, and NMDA were sequentially applied, followed by 5 min exposure to severe hypoxia (PaO2 approximately 32 mmHg). Following return to normoxia, a suffusion of either 8-SPT (10 microM), MK-801 (10 microM), TTX (1 microM), or 8-SPT+MK-801 was initiated (or, in controls, application of a drug-free suffusate was maintained), and the above sequence repeated. In control, TTX, and 8-SPT+MK-801 experiments, baseline conditions were then restored and hypercapnia (PaCO2 = 70-85 mmHg) was imposed. In the series 1 control groups, moderate and severe hypoxia elicited approximately 20% and 35-40% increases in diameter, respectively. L-NAME attenuated ADP- and ACh-induced dilations, did not alter the arteriolar responses to SNP or moderate hypoxia, but prevented further dilation upon imposition of severe hypoxia. This suggested that 45-50% of the severe hypoxia response was NO-dependent. In series 2, 8-SPT blocked the adenosine response and reduced severe hypoxia-induced dilation by 46%. MK-801 predictably blocked NMDA-induced relaxation and reduced the hypoxic response by 42%. When combined, 8-SPT and MK-801 affected hypoxic vasodilation additively. After TTX, the ADP and ADO responses were normal, but NMDA and hypoxia responses were completely blocked. Hypercapnia-induced dilation was unaffected by TTX or 8-SPT+MK-801. The results imply that severe hypoxia-induced release of NO and ADO, and the accompanying pial arteriolar dilation, are wholly dependent on the capacity to generate action potentials in perivascular nerves. The similarity of the L-NAME and MK-801 effects on hypoxic cerebrovasodilation suggests that the NO-dependency, to a large degree, derives from NMDA receptor activation.

The influence of aging and hypertension on cerebral autoregulation.

Cerebral autoregulation was tested here in 3- and 18-month-old spontaneously hypertensive rats (SHR) and Wistar Kyoto controls (WKY). Cerebral blood flow (CBF) was measured in each rat using radioactive micropheres under unanesthetized control conditions and after hypotension induced by ganglionic blockade. Under control conditions there was no significant difference in CBF between SHR and WKY. During drug induced hypotension CBF decreased significantly in all except 3 month WKY. CBF was decreased significantly more in 18 month SHR after ganglionic blockade compared to other test groups. These results indicate an influence of both age and hypertension in altering the ability of the cerebrovasculature to autoregulate.

Cerebral mitochondrial respiration in diabetic and chronically hypoglycemic rats.

The respiratory function of cerebral mitochondria harvested from genetically diabetic (BB/W) and streptozotocin-diabetic rats deprived of insulin for 3-4 weeks was found to be unchanged from control values. Furthermore, insulin-deprived BB/W rats subjected to 30 min of insulin-induced hypoglycemic coma demonstrated a normal mitochondrial respiration following a 60 min period of glucose restitution, a finding consistent with earlier results in non-diabetic rats. However, in rats exposed to 1 week of moderate hypoglycemia (plasma glucose = 3.0 mumol.ml-1), both state 3 respiration and the respiratory control ratio (RCR) were reduced from control. In fact, when the chronic hypoglycemia was imposed following a 3-4 week period of diabetic hyperglycemia, the state 3 rate and RCR were found to be reduced to a greater degree than in chronically hypoglycemic, non-diabetic, previously normoglycemic rats. Finally, when 1 week of moderate hypoglycemia preceded a 30 min period of insulin-induced hypoglycemic coma, a disturbed pattern of mitochondrial respiration (i.e. increased state 4, decreased RCR) was found at 60 min of recovery following coma. These results indicate that chronic increases in glucose (and insulin deprivation) have no effect on cerebral mitochondrial respiratory function, whereas prolonged, albeit moderate, reductions in cerebral glucose supply result in perturbations in mitochondrial respiration. These results demonstrate the importance of an adequate glucose supply for normal mitochondrial activity.

Nitric oxide (NO) is an endogenous anticonvulsant but not a mediator of the increase in cerebral blood flow accompanying bicuculline-induced seizures in rats.

Neurons synthesize NO, which may act as a retrograde messenger, involved in either potentiating or depressing neuronal excitability. NO may also play a role in the cerebral vasodilatory response to increased neuronal activity (i.e., seizures). In this study, two questions were asked: (1) is NO an endogenous anticonvulsant or proconvulsant substance? and (2) is the cerebral blood flow (CBF) increase accompanying bicuculline (BC)-induced seizures mediated by NO? The experiments were performed in 300-400-g Wistar rats anesthetized with 0.6% halothane and 70% N2O/30% O2. CBF was measured using the intracarotid 133Xe clearance method or laser-Doppler flowmetry. EEG activity was recorded. Chronic treatment (4 days) with nitro-L-arginine (L-NA), a potent NO synthase (NOS) inhibitor (400 mg/kg total), suppressed brain NOS by > 97% and prolonged seizure duration from 6 +/- 1 (saline-treated controls) to 12 +/- 2 min. In the L-NA-treated group, the CBF increase was sustained as long as seizure activity remained, indicating that CBF was still tightly coupled to seizure activity. Interestingly, the supposed inactive enantiomer of L-NA, D-NA, also showed an inhibition of brain NOS activity, ranging from 87 to 100%. The duration of seizures in this group (average 8 +/- 2 min) corresponded directly to the magnitude of reduction in NOS activity (r = 0.83, P < 0.05). Specifically, the D-NA results indicated that NOS inhibition had to exceed 95% before any effect on seizure duration could be seen.(ABSTRACT TRUNCATED AT 250 WORDS)

Cerebrovascular and cerebral metabolic effects of flurazepam and a benzodiazepine antagonist, 3-hydroxymethyl-beta-carboline.

There is a need in clinical practice for an antagonist which can reverse the sedative action of benzodiazepines. Recently, 3-hydroxymethyl-beta-carboline (3-HMC) has been reported to inhibit the sleep inducing effects of flurazepam. The effects of flurazepam (0.5, 5 and 50 mg/kg) on cerebral blood flow (CBF) and cerebral O2 consumption (CMRO2) were evaluated in rats and the ability of 3-HMC to reverse these changes was determined. Regional CBF was measured with radioactive microspheres and cortical CMRO2 was calculated from sagittal sinus-arterial O2 content differences and cortical CBF. Flurazepam produced dose dependent decreases in CBF and CMRO2 which were significant at 5 and 50 mg/kg. 3-HMC (5 mg/kg) inhibited flurazepam induced changes at the 5 mg/kg dose but had little effect on the CBF and CMRO2 depression produced by 50 mg/kg flurazepam. At a dose of 25 mg/kg, 3-HMC inhibited the effects of both 5 and 50 mg/kg flurazepam. Blood pressure and heart rate were also decreased by flurazepam but these variables were not reversed as effectively by 3-HMC treatment. The results indicate that 3-HMC is an active antagonist of the cerebrovascular and cerebral metabolic depression produced by flurazepam and can stimulate CBF and CMRO2 at high doses when given alone.

Regional cerebral blood flow measurement in rats with radioactive microspheres.

The effect of the method of heart catheterization on the measurement of cerebral blood flow (CBF) with radioactive microspheres was evaluated during various experimental procedures in male Sprague-Dawley rats. Catheters were inserted into the left ventricle via the right carotid or right subclavian artery or directly into the left atrium for microsphere injections. CBF was measured in cerebral cortical and subcortical tissues under control anesthetized (70% N2O, 30% O2), hypoxic or hypercapnic test conditions. Under control conditions, CBF was similar in the right vs the left cerebral hemisphere in subclavian artery and atrial catheterized rats but was greater in the left vs the right cortex in carotid catheterized animals (p less than .05). During hypoxia and hypercapnia CBF increased equally in both cerebral hemispheres in atrial catheterized rats. The increase in CBF was significantly attenuated in the cerebral hemisphere ipsilateral to carotid catheterization during hypoxia and hypercapnia, although the percentage increase in flow was similar in both hemispheres. The results indicate the limitations of measuring regional CBF changes under experimental test conditions in rats with a ligated carotid artery and suggest that atrial catheterization is the method of choice when comparable changes in CBF are desired in both cerebral hemispheres.

A benzodiazepine antagonist inhibits the cerebral metabolic and respiratory depressant effects of fentanyl.

It is reported that benzodiazepines such as diazepam will stimulate the opiate receptor system and that B-carboline drugs, which are benzodiazepine antagonists, may interact with opiate receptors directly. The ability of 3-hydroxymethyl-B-carboline (3-HMC) to antagonize several parameters of fentanyl anesthesia was tested here in rats. Fentanyl (25 and 100 micrograms/kg iv) produced dose dependent depression of cerebral blood flow (CBF), measured by radioactive microspheres, and cerebral oxygen consumption (CMRO2). These effects were significantly inhibited by 10 mg/kg 3-HMC iv. To test for the specificity of this effect, 3-HMC was also given to rats ventilated with inspire concentrations of 2% halothane. Halothane depressed CMRO2 equally in 3-HMC and vehicle treated rats, indicating no significant effect of the benzodiazepine antagonist. Blood pressure was increased in 3-HMC compared to vehicle treated animals during both fentanyl and halothane anesthesia. CBF was increased in 3-HMC vs vehicle treated rats during halothane anesthesia but this could be accounted for by the elevated blood pressure and lack of cerebral autoregulation rather than a direct cerebrovascular effect. 3-HMC decreased the sleep time and respiratory depressant effects of fentanyl but enhanced the analgesic effects of the opiate, as measured by time to respond to a hot plate stimulus. These results indicate that 3-HMC has the ability to specifically antagonize fentanyl anesthesia. These effects may be produced by an action of 3-HMC at the benzodiazepine receptor and/or by an action of the B-carboline at opioid receptors.