Preservation of autoregulatory cerebral vasodilator responses to hypotension after inhibition of nitric oxide synthesis

Effects of inhibition of nitric oxide (NO) synthesis on the cerebrovascular autoregulatory vasodilator response to hypotension were studied in conscious rats. Cerebral blood flow (CBF) was determined with [¹⁴C]iodoantipyrine in a saline-treated control group and in three groups following inhibition of NO synthase activity by twice daily intraperitoneal injections of 50 mg/kg ofN^G-nitro-l-arginine methyl ester (l-NAME) for four days. In the saline-control group (n = 8) and in thel-NAME-treated Group (a) (n = 8) CBF was determined while systemic mean arterial blood pressure (MABP) remained at its resting level (means ± S.D., 128±6 and 151±11 mmHg, respectively). In the other groups CBF was determined after MABP was reduced by blood withdrawal to 118±9 and 88±8 mmHg in Groups (b) (n = 8) and (c) (n = 8), respectively. Despite the elevated MABP, global CBF was significantly lower inl-NAME-treated Group (a) than in the saline-controls (P < 0.005), indicating cerebral vasoconstriction striction resulting from inhibition of NO synthesis. Global CBF was not significantly reduced further in the two groups with hypotension. Local CBF in the hypotensive rats showed no significant reductions below values inl-NAME-treated control rats (Group (a)) in 31 of 32 brain structures; the only exception was in the auditory cortex of the severely hypotensive rats (Group (c)). The autoregulatory mechanism for cerebral vasodilatation to compensate for reduced arterial blood pressure is maintained following inhibition of NO synthesis.     

Role of nitric oxide in regulation of cerebral microvascular tone and autoregulation of cerebral blood flow in cats

The role of nitric oxide in the regulation of cerebrocortical microvascular tone and autoregulation of cerebral blood flow (CBF) was examined in 24 anesthetized cats. The local cerebral blood volume (CBV), mean transit time of blood (MTT), and CBF in the cortex were measured by our photoelectric method. CBV represents the cumulative dimensions of the cerebral microvessels. Intravenous injection of 0.35–0.7 mg/kg/minN^G-monomethyl-l-arginine (l-NMMA), an inhibitor of nitric oxide synthesis, significantly increased mean arterial blood pressure (MABP; 8.4–14.1%,P < 0.01), decreased CBV (15.2–28.7%,P < 0.01), and decreased CBF (20.0–29.8%,P < 0.01) in a dose-related manner. The changes in MABP, CBV, and CBF elicited byl-NMMA were inhibited (P < 0.05) by simultaneous infusion of 35 mg/kg/minl-arginine. Autoregulation of CBF was examined during controlled hypotension of −30 to −40 mmHg (artificial bleeding) and recovery of blood pressure (reinfusion of blood). Although CBF remained constant with blood pressure changes in the control state (ΔCBF/ΔMABP of 0.037±0.155 with hypotension), CBF became dependent on blood pressure changes (ΔCBF/ΔMABP of 0.478±0.135, P < 0.05) during infusion of 0.35 mg/kg/minl-NMMA. It is concluded that nitric oxide participates in both the regulation of basal tone of cerebral microvessels and the autoregulation of CBF.     

Is the acetazolamide test valid for quantitative assessment of maximal cerebral autoregulatory vasodilation? An experimental study

BACKGROUND AND PURPOSE: The cerebral vasodilating effect of acetazolamide (ACZ) injection has been used as an index of the autoregulatory vasodilation (or cerebral perfusion reserve). The question of whether the ACZ test assesses the maximal autoregulatory vasodilating capacity is not definitely resolved. The effects of ACZ injection on this reserve at a dose producing maximal vasodilation have never been evaluated and may help to resolve this problem. METHODS: The effect of ACZ injection on cerebral blood flow (CBF) autoregulation was tested in anesthetized rats. A pilot experiment evaluated the dose-effect relationship of injected ACZ, cumulative doses (n=4, group 1), and independent bolus doses (n=6, group 2). CBF was estimated by laser-Doppler flowmetry, and cerebrovascular resistance (CVR) was calculated from mean arterial blood pressure (MABP) and from CBF (expressed as a percentage of baseline CBF). A bolus of ACZ of 21 mg/kg produced the maximal cerebral vasodilation that could be obtained by ACZ administration. In the main experiment, MABP was lowered from 110 to 20 mm Hg by stepwise bleeding in 3 groups of 6 animals treated 10 minutes before bleeding by injection of saline (group 3), 7 mg/kg ACZ (group 4), or 21 mg/kg ACZ (group 5). RESULTS: The CVR-MABP relationship was linear in all groups, indicating that CBF autoregulation was still effective after ACZ administration. CONCLUSIONS: These results indicate that maximal ACZ-induced cerebral vasodilation is not quantitatively equivalent to maximal autoregulatory vasodilating capacity in anesthetized rats.     

Rapid autoregulation of cerebral blood flow: a laser-Doppler flowmetry study.

The mechanisms underlying autoregulation of CBF were studied in 19 rabbits using laser-Doppler flowmetry. A cranial plexiglas window was chronically inserted in the skull with dental cement under general anesthesia. The animals then were reanesthetized 5-7 days later and subjected to aortic bleeding while CBF was measured with the probe placed on the window. In the first set of experiments, MABP was decreased (from 90 to 30 mm Hg) and was maintained constant for 1 min. During the first seconds, CBF followed the steep decrease of MABP. Then, CBF increased and reached a plateau within 3-13 s, depending on the severity of hypotension. Hyperemia occurred when blood was restored, and the CBF recovered from this posthypotensive hyperemia with a rapid phase (within 2 s) and a slow phase (total recovery within 1 min). The lower limit of autoregulation was found to be 40 mm Hg. An increase in CBF due to papaverine showed that vasodilation was not maximal below this limit. In the second set of experiments, the rabbits were subjected to four episodes of hypotension at 40 mm Hg each but of different durations (from 2-3 to 60 s). The posthypotensive hyperemia was not influenced by the duration of hypotension, but the time of the total recovery phase increased with the duration of hypotension. We conclude that there exist rapid adaptive mechanisms leading to autoregulation and that the vasodilation is not dependent upon the duration of hypotension.     

Local cerebral blood flow response to locally infused 2-chloroadenosine during hypotension in piglets

Brain interstitial adenosine increases during hypotension in piglets. If adenosine is to participate in the regulation of neonatal cerebral blood flow (CBF) during hypotension, it must retain its vasodilatory action under that condition. To examine this issue, we studied the effects of locally infused 2-chloroadenosine (2-CADO), a stable adenosine analog, on local CBF in the piglet frontal cortex during normotension and graded hemorrhagic hypotension. We used the modified brain microdialysis/hydrogen clearance technique to simultaneously infuse 2-CADO into the frontal cortex and measure local CBF from the same area. When 2-CADO from 10(-8) M to 10(-3) M was infused under control conditions (n = 7), CBF increased 61% at 10(-5) M, 167% at 10(-4) M, and 210% at 10(-3) M. In hypotension experiments, local infusion of 10(-5) M 2-CADO (n = 8) caused significant increases in CBF (P less than 0.05) under control conditions (MABP = 65 mmHg) and at hypotensive blood pressures of 55 mmHg and 44 mmHg, respectively. At a blood pressure of 33 mmHg, however, infusion of the analog failed to increase CBF. Local infusion of 10(-3) M 2-CADO also produced a similar change in CBF during graded hypotension. These results indicate that 2-CADO dilates intracerebral vessels during normotension, and mild and moderate hypotension, and support the hypothesis that endogenous adenosine mediates autoregulatory adjustments of CBF during hypotension in newborn piglets.     

Cerebral extracellular lactate concentration and blood flow during chemical stimulation of the nucleus tractus solitarii in anesthetized rats

The extracellular lactate concentration and blood flow in the cerebral cortex of urethane-anesthetized, paralyzed and artificially ventilated rats were monitored continuously and simultaneously using an enzyme electrode and a laser Doppler flowmeter (LDF), respectively, during chemical stimulation of the nucleus tractus solitarii (NTS) by microinjection of -glutamate (1.7 nmol 50 nl). Chemical stimulation of the NTS significantly decreased the arterial blood pressure (ABP) from 85 ± 17 to 68 ± 14 mmHg, heart rate from 418 ± 13 to 402 ± 19 beats · min⁻¹ and cerebral blood flow (CBF) by 17.9 ± 6.2% (P < 0.001). However, chemical stimulation of the NTS significantly increased the lactate concentration by 58.9 ± 17.3 μM (P < 0.001). Barostat maneuver, which held systemic ABP constant during chemical stimulation of the NTS attenuated the responses in CBF and lactate concentration by 30 and 27%, respectively. The onset of the increase in lactate concentration was delayed about 19 s after that of the CBF decrease. Circulatory lactate produced no significant change in the cerebral extracellular lactate concentration. These results indicate that chemical stimulation of the NTS induces an increase in extracellular lactate concentration in the cerebral cortex through a decrease in CBF via cerebral vasoconstriction.     

Cardiovascular effects elicited by central administration of physostigmine via M2 muscarinic receptors in conscious cats

The cardiovascular effects of an intracerebroventricular (i.c.v.) injection of physostigmine were studied using conscious cats. Physostigmine (5–25 μg: 5 μl) caused a dose-dependent increase in mean arterial pressure (MAP) and heart rate (HR). The highest dose (25 μg) increased MAP and HR by 32 ± 3 mmHg and 45 ± 5 beats/min, respectively (n = 5). Pre-administration of the muscarinic receptor antagonist, atropine (25 μg; i.c.v.) blocked the effects of physostigmine (25 μg; i.c.v.). Also, the pre-administration of the M₂ muscarinic antagonist, methoctramine (25 μg; i.c.v.), antagonized the cardiovascular effects of physostigmine without altering the baseline variables. However, the M₁ muscarinic antagonist, pirenzepine (100 μg; i.c.v.) did not alter baseline MAP or HR, and also failed to inhibit the cardiovascular responses to physostigmine. Similarly, the M₃ muscarinic blocker, 4-diphenyl-acetoxy-N-methylpiperidine methiodide (50 μg; i.c.v.), neither changed baseline cardiovascular variables nor blocked the effects of physostigmine. When the same cats were anesthetized with intravenous injection of sodium pentobarbital (25–30 mg/kg), physostigmine (25 μg; i.c.v.) evoked a decrease in MAP and HR of 13 ± 6 mmHg and 15 ± 6 bpm, respectively (n = 5). These results demonstrate that the increases in MAP and HR to the i.c.v. administration of physostigmine in conscious cats arepossibly mediated through stimulation of central M₂ muscarinic receptors. In addition, anesthesia reverses the effects elicited by the central administration of physostigmine to a decrease in MAP and HR.     

Comparison of the myogenic response in rat cerebral arteries of different calibers

The myogenic response, the characteristic of blood vessels to contract with increasing pressure, was studied at three different locations along the middle cerebral artery (MCA) vascular tree. We hypothesized that smaller caliber vessels would have a more pronounced myogenic response at lower pressures than larger diameter arteries, corresponding to pressures normally experienced in vivo. Cerebral vessels (MCAs, branches of the MCA, and penetrating arterioles) were isolated from male rats, cannulated with glass micropipettes, and pressurized. Changes in diameter were measured as the transmural pressure was increased from 20–100 mmHg. The MCAs, which had a resting diameter of 202±10 μm (n=9) at 50 mmHg, showed its greatest myogenic response between 60–100 mmHg (8±2% constriction, n=9, p<0.001). The penetrating arterioles [58±4 μm (n=8) at 50 mmHg], on the other hand, showed its greatest myogenic response between 20–60 mmHg (10±4% constriction, n=8, p<0.05). Branches of the MCA [118±14 μm (n=8) at 50 mmHg] showed a slight constriction over the entire pressure range (5±9% constriction between 20–100 mmHg, p=ns). Our results suggest that the myogenic response appears to be best developed in the range of pressures found during physiological conditions for a given vessel in the MCA territory. This characteristic is fundamental in the overall control of cerebrovascular resistance.     

Cerebral blood flow regulation in neonatal rabbits is altered by chronic cocaine administration

Maternal cocaine abuse has several deleterious effects in the newborn, including perinatal asphyxia, hypoxia, and hypercapnia. We hypothesized that chronic cocaine exposure during development may alter cerebral blood flow (CBF) regulation. We studied 16 neonatal rabbits that had received cocaine (20 mg/kg, i.p. b.i.d.) or saline since birth. Changes in CBF were measured by laser doppler flowmetry before (baseline), and during hypercapnia (FiCO₂=7.5%), hypoxia (FiO₂=12%), and asphyxia (apnea for 1 min). During hypercapnia, CBF increased less in cocaine than in control animals (28±3% vs. 69±10%, P<0.05). During hypoxia, CBF increased similarly in both groups. During reventilation after asphyxia, CBF increased more in cocaine than in control animals (391±52% vs. 225±43%, P<0.05). Chronic cocaine exposure during brain development appears to alter CBF regulation to hypercapnia and asphyxia, which may put the drug exposed newborn at risk for neurologic injury around birth.     

Morphine blocks the bradycardia associated with severe hemorrhage in the anesthetized rat

Progressive hemorrhage in the absence of tissue injury produces a biphasic response: an initial tachycardia, vasoconstriction and maintenance of arterial blood pressure by the baroreflex, followed by bradycardia, vasodilatation and hypotension due to the activation of a second ‘depressor’ reflex. The present study has investigated the effect of morphine (a μ-opioid receptor agonist) on the cardiac chronotropic response to a progressive hemorrhage at 2% total estimated blood volume (BV) min⁻¹ in the anesthetized rat. In control rats (20 μl saline intracerebroventricularly, i.c.v.) heart period initially decreased significantly (P < 0.05) by a maximum of 5.4 ± 0.8 ms from a baseline of 147.3 ± 2.2 ms after a blood loss of 8.3 ± 1.5% BV, and then increased significantly by a maximum of 43.0 ± 5.5 ms above the baseline after the loss of 34.5 ± 1.6% BV. Blood pressure was initially maintained and then fell during the hemorrhage. The increase in heart period was prevented by treatment with morphine (10 μg i.c.v.), and the fall in blood pressure delayed significantly. These effects of morphine were prevented by pretreatment with naloxone (20 μg i.c.v.). Intravenous (i.v.) administration of morphine (10 μg) had no effect on the response to hemorrhage. However, a clinically relevant dose of 0.5 mg · kg⁻¹ morphine (i.v.) abolished the bradycardia and delayed the fall in blood pressure associated with hemorrhage. These results indicate that morphine, acting on central nervous opioid receptors, can abolish the bradycardia and delay the hypotension associated with progressive hemorrhage, a pattern of response reminiscent of the effects of musculo-skeletal injury on the response to blood loss.     

Consecutive in vivo measurement of nitric oxide in transient forebrain ischemic rat under normothermia and hypothermia

The effects of hypothermia on production of nitric oxide (NO) in ischemic brain were investigated by using in vivo microdialysis. Male Wistar rats were randomly divided into three groups; saline-treated normothermic group (37°C, n=6), 30 mg/kg N-nitro- -arginine methyl ester( -NAME)-treated normothermic group (n=6), and saline-treated hypothermic group (30°C, n=6). Transient forebrain ischemia was produced by bilateral common carotid artery occlusion combined with hypotension (MABP=50 mmHg). Saline-treated normothermic animals resulted in a reduction of LCBF to 9% of baseline. Saline-treated hypothermic rats revealed the similar changes of LCBF. In contrast, -NAME administration reduced the basal CBF to 85% of saline-treated group and to 8% after ischemia. NO products were decreased during ischemia and transiently increased after reperfusion in saline-treated groups. However, the increase of NO products after reperfusion was less significant in the hypothermia. -NAME-treated group showed a constant reduction of NO production during ischemia and after reperfusion.     

Effects of serotonin and carbachol on glial and neuronal rubidium uptake in leech CNS

Effects of serotonin (5-HT) and carbachol on Rb uptake (used as a K marker) in leech neuron and glia were studied by electron probe microanalysis (EPMA). Hirudo medicinalis ganglia were perfused 60 s in 4 mM Rb substituted normal leech Ringer's with and without 5-HT (dosage range 5–500 μM) or carbachol (range 10–1000 μM), quench frozen, cryosectioned, and subjected to EPMA to determine elemental mass fractions and cell water content. Both 5-HT and carbachol altered leech neuron and glial cell elemental distribution and water content. In glial cells, a dose-dependent increase in Rb uptake was observed following 5-HT (control: 26 ± 2 μM; 5 μM: 47± 4; 50 μM: 62 ± 4; 500μM: 82±11 mmol/kg dry wt. ± S.E.M.) and carbachol (10 μM: 35±3; 100 μM: 52±3; 1000 μM: 68±3 mmol/kg dry w wt. ± S.E.M.). In neurons, 5-HT and carbachol had small effects. 5-HT decreased glial and neuronal cell water content. Carbachol decreased neuronal (but not glial) water content by approximately the same amount (mean decrease 9%) regardless of dose. Both 5-HT and carbachol affected glial cell K-accumulating properties, providing evidence that certain neurotransmitters may modulate invertebrate glial cells' K clearance function.     

Cardiovascular effects of central administration of clonidine in conscious cats

The effects on arterial blood pressure and heart rate after an intracerebroventricular (i.c.v.) administration of clonidine were investigated using conscious normotensive cats. Injection of clonidine (5–10 μg; 5 μl; i.c.v.) elicited a decrease in mean arterial pressure (MAP) and heart rate (HR) in a dose-dependent manner. The highest dose of 10 μg of clonidine decreased MAP and HR by 39 ± 3 mmHg and 74 ± 5 b.p.m., respectively (n = 7). Pretreatment with yohimbine, the α₂-adrenoceptor antagonist (8 μg; 5 μl; i.c.v.) blocked the cardiovascular responses to a subsequent i.c.v. injection of 10 μg clonidine (n = 7). Furthermore, preadministration of cimetidine (100 μg; 5 μl; i.c.v.), the H₂ histamine receptor antagonist with imidazoline receptor activating properties, prevented the decreases in MAP and HR to a subsequent i.c.v. injection of 10 μg clonidine (n = 7). By contrast, pretreatment with the specific I₁ imidazoline receptor blocker, efaroxan (100–500 μg; 5 μl; i.c.v.), failed to inhibit the cardiovascular effects of an i.c.v. administration of 10 μg clonidine (n = 7). These results suggest that the effects of centrally administered clonidine on MAP and HR are probably not mediated through activation of the I₁ subtype of imidazoline receptors in conscious cats. However, the cardiovascular effects elicited by i.c.v. administration of clonidine appear to result from stimulation of central α₂-adrenergic or the H₂ histaminergic-like receptors.     

Stimulatory effects of centrally injected kainate and N-methyl- -aspartate on gastric acid secretion in anesthetized rats

The effects of N-methyl- -aspartate (NMDA), kainate and (±)-α-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA), ionotropic glutamate agonists, on gastric acid secretion were investigated in the continuously perfused stomach of anesthetized rats. The lateral ventricular (LV) injection of kainate (0.01–1 μg) or NMDA (0.3–3 μg) dose-dependently stimulated gastric acid secretion. AMPA (3–10 μg) also stimulated gastric acid secretion but the effect was very weak. Repeated injections of kainate (0.1 μg) or NMDA (1 μg), at least twice, stimulated gastric acid secretion to a similar degree. The effect of kainate (0.1 μg) was blocked by the kainate receptor antagonists, 6-cyano-7-nitroquinoxaline-2,3-dione disodium (3 μg, LV) and -γ-glutamylaminomethanesulfonic acid (30 μg, LV), but not by NMDA receptor antagonists. The effect of NMDA (10 μg) was blocked by (±)-3-(2-carboxypiperazin-4-yl)-1-propylphosphonic acid (10 μg, LV), a competitive NMDA receptor antagonist, and (+)-5-methyl-10,11-dihydro-5H-dibenzocyclo-hepten-5,10-imine hydrogen maleate (10 μg, LV), a non-competitive NMDA receptor antagonist, but not by kainate receptor antagonists. Moreover, the gastric acid secretion stimulated by kainate and NMDA were completely blocked by systemic atropine injection (1 mg/kg, i.v.) and vagotomy. These findings suggest that kainate and NMDA receptor mechanisms are independently involved in the central nervous system to control gastric acid secretion through vagus cholinergic activation.     

The peripheral nociceptive actions of intravenously administered 5-HT in the rat requires dual activation of both 5-HT2 and 5-HT3 receptor subtypes

In 16-week-old Sprague-Dawley rats lightly anesthetized with pentobarbital, 5-HT (3–96 μg/kg, i.v.;n = 6) produced distinct pseudaffective responses and a dose-dependent (slope= 17.2 ± 6.8s/log₁₀dose) inhibition of the tail-flick (TF) reflex (ED₅₀ = 32.6 ± 9.2 μg/kg). In the same rats, a 1:1 combination of α-methyl 5-HT (a 5-HT₂ receptor selective agonist) and 2-methyl 5-HT (a 5-HT₃ receptor selective agonist) (3–192 μg/kg, i.v.), produced the same profile of pseudaffective responses and also resulted in a dose-dependent (slope= 34.0± 7.0s/log₂dose) inhibition of the TF reflex (ED₅₀ = 88.4 ± 20.5 μg/kg). In contrast, administration of α-methyl 5-HT (3–192 μg/kg, i.v.) or 2-methyl 5-HT (3–192 μg/kg, i.v.) alone did not produce any pseudaffective responses or any change in TF latency from baseline. In conscious 16-week-old male Sprague-Dawley rats, administration of 5-HT (48 μg/kg, i.v.;n = 5), or a 1:1 combination of α-methyl 5-HT and 2-methyl 5-HT (total dose= 120 μg/kg, i.v.;mn = 5), resulted in a passive avoidance behavior assessed in a step-down paradigm (slopes= 139.7 ± 58.2and154.9 ± 63.9s/trial, respectively), and the same profile of distinct pseudaffective responses exhibited by the lightly pentobarbital-anesthetized rats. However, administration of either α-methyl 5-HT (96 μg/kg, i.v.;n = 4) or 2-methyl 5-HT (96 μg/kg, i.v.;n = 4), while producing significant 5-HT receptor-mediated cardiovascular responses, produced a learned behavior not different from saline (0.25 ml, i.v.;n = 6) (slopes= 7.6 ± 2.5, 6.3 ± 1.8and7.4 ± 3.6s/trial, respectively). These results are consistent with the hypothesis that the peripheral nociceptive responses to i.v. 5-HT requires dual activation of 5-HT₂ and 5-HT₃ receptor subtypes.     

Genesis of biphasic thermal response to intrapreoptically microinjected clonidine

Intrapreoptic (IPO) microinjections of various agents cause unavoidable brain tissue injury, often resulting in prostaglandin (PG)-mediated core temperature (T_c) rises. However, IPO microinjection of the α₂-adrenoreceptor agonist clonidine (Clo) generally evokes a T_c fall, seemingly avoiding the influence of injury due to the microinjection procedure per se. To clarify this, we microinjected bilaterally into the preoptic/anterior hypothalamus of conscious guinea pigs various doses of Clo dissolved in pyrogen-free saline (PFS, 1 μl/side). Clo caused biphasic hypo-/hyperthermic responses. The initial hypothermia was dose dependent: no decrease in T_c for 0.1 μg of Clo, −0.4 ± 0.1°C for 0.5 μg, −0.9 ± 0.1°C for 1.5 μg, and −1.2 ± 0.1°C for 5.0 μg. During the hyperthermic phase, T_c increased to a dose-independent level (1.0–1.5°C), remaining there up to 5 h postinjection. PFS microinjected IPO also induced hyperthermia, but without any initial T_c decrease. This T_c rise was delayed by 100 min when the cyclooxygenase inhibitor indomethacin (Indo, 50 μg/μl) was injected. Nontreated animals (time controls) maintained T_c at baseline levels during the whole experiment. The α₂-antagoni rauwolscine (2 μg/side), microinjected IPO 10 min before Clo (0.5 μg/side), abolished the hypothermic without affecting the hyperthermic response phase; Indo (10 mg/kg), injected intramuscularly 20 min after the IPO microinjection of Clo (0.5 μg), significantly attenuated the hyperthermic phase. These results confirm that an artifactitious, PG-mediated T_c rise consequent to nonspecific brain tissue injury contaminates the thermal response to agents (hyper- or hypothermizing) microinjected IPO. The similarity of the thermal responses to Clo to those to norepinephrine provides additional evidence that the authentic (hypothermie) effect of IPO norepinephrine in guinea pigs is mediated via α₂-adrenergic receptors.     

Dilatation of cerebral arterioles in response to N-methyl-d-aspartate: role of CGRP and acetylcholine

The purpose of these experiments was to examine mechanisms by which N-methyl-d-aspartate (NMDA) produces nitric oxide-dependent vasodilatation in brain. Some nitrovasodilators appear to dilate cerebral arterioles, in part, by release of calcitonin gene-related peptide (CGRP) from trigeminal fibers. The first goal of this study was to examine the hypothesis that dilatation of cerebral arterioles in response to NMDA is mediated by activation of receptors for CGRP. Diameters of cerebral arterioles were measured using a closed cranial window in anesthetized rabbits. Topical CGRP (1 and 10 nM) dilated cerebral arterioles by 30 ± 9 (mean±S.E.M.) and 72 ± 9%, respectively, from a control diameter of 94 ± 7 μm. This response was inhibited almost completely by the CGRP antagonist CGRP(8–37) (0.5 μM). NMDA (100 and 300 μM) dilated cerebral arterioles by 14 ± 5and38 ± 7% in the absence and 20 ± 5%and30 ± 6% in the presence, respectively, of CGRP(8–37). Neurons may release acetylcholine in response to activation with NMDA. The second goal of the present study was to examine the hypothesis that dilatation of cerebral arterioles in response to NMDA is mediated by acetylcholine. Topical atropine (2 μg/ml) completely inhibited dilatation of cerebral arterioles in response to acetylcholine, but had no effect on vasodilatation in response to NMDA. Thus, vasodilatation of cerebral arterioles in response to NMDA does not appear to be dependent on activation of receptors for CGRP or acetylcholine.     

Protective effect of nimodipine against ischemic neuronal damage in rat hippocampus without changing postischemic cerebral blood flow

The purpose of the present study was to investigate the neuroprotective action of nimodipine. Furthermore, the influence of nimodipine on postischemic local CBF (LCBF) was examined. Forebrain ischemia of the rat was performed for 10 min by bilateral carotid clamping, administration of trimethaphan, and blood withdrawal to obtain an MABP of 40 mm Hg. LCBF was measured after 10 min of postischemic recirculation by injecting [14C]iodoantipyrine in saline solution. Nimodipine (0.1, 0.3, and 1.0 mg/kg) was suspended in miglyol oil and applied orally 60 min prior to ischemia. Histological evaluation was performed 7 days after ischemia. Hippocampal neuronal damage was determined as the percentage of necrotic neurons. After preischemic application of nimodipine, neuronal damage was significantly reduced in the hippocampal CA1 subfield. Postischemic LCBF was not affected by treatment with nimodipine. These findings show that nimodipine is able to protect neurons against ischemic damage. The neuroprotective effect of nimodipine was not mediated by a postischemic cerebral vasodilation, but by a direct action on the neurons.     

Transient brain ischemia in rabbits: the effect of ω-conopeptide MVIIC on hippocampal excitatory amino acids

Neurologic injury that occurs after ischemia results from a cascade of events involving the release of various endogenous neurotoxins. A portion of the release of excitatory neurotransmitters is calcium dependent and may be attenuated by administration of calcium channel blockers. Using an in vivo model of ischemia, we studied the effects of ω-conopeptide MVIIC, a voltage-sensitive calcium channel blocker, and hypothermia (32°C) on hippocampal glutamate and aspartate release in the peri-ischemic period. Thirty-four New Zealand white rabbits of either sex were anesthetized with halothane, intubated, and mechanically ventilated. Monitored variables included blood gases, mean arterial blood pressure, and the electroencephalogram. Microdialysis catheters were transversely inserted through the anterior portion of the dorsal hippocampus and perfused with artificial cerebrospinal fluid at a rate of 2 μl/min. After stabilization period, animals were randomly assigned to one of the following groups: Control group (n = 8), 10 μM ω-conopeptide MVIIC group (n = 7), 100 μM ω-conopeptide MVIIC group (n = 7), Hypothermia group (n = 6; cranial temperature = 32°C), and ω-conopeptide MVIIC + hypothermia group (n = 6; 100 μM ω-conopeptide MVIIC and cranial temperature 32°C). All the rabbits were subjected to 10 minutes of global cerebral ischemia produced by neck tourniquet inflation combined with hypotension during halothane anesthesia. Conopeptide MVIIC was administered in the artificial cerebrospinal fluid used to perfuse the microdialysis catheter. In control animals, ischemia caused a significant increase in glutamate (9.7 fold) and aspartate (11.3 fold) concentrations. This increase was markedly attenuated (P < 0.05) in all treatment groups (MVIIC 10 μM, 100 μM, hypothemia, and MVIIC + hypothermia). These results demonstrate that ω-conopeptide MVIIC (10 μM and 100 μM) can attenuate ischemia-induced increases of glutamate and aspartate concentrations in the peri-ischemic period. This effect is probably caused by a blockade of presynaptic calcium channels and decreased synaptosomal release of excitatory neurotransmitters.     

Cerebrovascular changes elicited by electrical stimulation of the centromedian-parafascicular complex in rat

Electrical stimulation of the centromedian-parafascicular complex (CM-Pf) in anesthetized (chloralose) and paralyzed (tubocurarine) rats elicites a widespread cerebrovascular dilatation. Regional cerebral blood flow (rCBF) was measured in dissected tissue samples of 10 brain regions (medulla, pons, cerebellum, inferior colliculus, superior colliculus, frontal parietal and occipital cortices, caudate-putamen and corpus callosum) by [¹⁴C]iodoantipyrine method. In unstimulated and sham-operated rats rCBF ranged from 40±3 (ml/100g/min) in corpus callosum to 86±6 (ml/100g/min) in inferior colliculus. During CM-Pf stimulation, rCBF increased significantly P < 0.05, analysis of variance and Scheffe's test) in all cerebral regions bilaterally ranging from +118% in parietal cortex to +38% in cerebellum. Although cerebral vasodilation elicited by CM-Pf stimulation persisted after unilateral transection of the cervical sympathetic trunk, the cortical CBF was significantly reduced (P < 0.05) on the denervated side. Acute adrenalectomy significantly (P < 0.05) decreased elevated rCBF during CM-Pf stimulation in all cortical regions (frontal −36%, parietal −34%, and occipital −27%) and in caudate nucleus (−37%). Thus, excitation of neurons originating in, or fibers passing through the CM-Pf can elicit a powerful cerebral vasodilation. The cerebral vasodilation is modulated by cervical sympathectomy and circulating adrenal hormones. We conclude that CM-Pf elicited vasodilation is at least partly mediated by intrinsic neural pathways.