Effect in cat locus coeruleus lesions on the response of cerebral blood flow and cardiac output to altered paCO2

Brain Research, 365 (1986) 278-288 Elsevicr

278 BRE 11446

Effect in Cat of Locus Coeruleus Lesions on the Response of Cerebral Blood Flow and Cardiac Output to Altered PaCO2 S.V. RAMANA REDDY*, TONY L. YAKSH, ROBERT E. ANDERSON and THORALF M. SUNDT, Jr.

Laboratory of Neurosurgical Research and Cerebrovascular Research Center, Mayo Clinic, Rochester, MN 55905 (U. S.A. ) (Accepted June 18th, 1985)

Key words: locus coeruleus - - cardiac output - - cerebral blood flow - - propranolol - - hypercapnia

In pentobarbital-anesthetized cats, over arterial PaCO2 values of 20-60 mm Hg, cerebral blood flow (CBF, Xenon) and cardiac output (CO, thermal dilution) show positively inflected curves with slopes significantlygreater than zero. To examine the role of the locus coeruleus (LC) in these responses, bilateral stereotactic thermo-coagulation lesions of the LC were made. The effect of lesions confirmed to involve the LC bilaterally (n = 10), were compared with the effects of misdirected lesions placed in the cerebellum and lateral to the LC (n = 10) and sham lesions (n = 10). Ten days after the lesioning procedure, the animals were re-anesthetized with pentobarbital and paCO2 response curves were determined for CBF and CO prior to and following intravenous administration of proprano1ol (1 mg/kg, i.v.). The results obtained with the sham-operated animals and the animals with lesions outside of the LC were indistinguishable. Bilateral LC lesions had no significant effect on normocapnic CBF as compared to control animals. They did, however, significantly reduce the slope of the CBF paCO2 response curve. Propranolol produced a significant reduction in CBF in lesioned and non-lesioned animals measured at all levels of pCO 2 and did not alter the slope of the pCO2 response curve for any group as compared to predrug values. Bilateral lesions of the LC did not significantlyalter either normocapnic CO or the slope of the CO-p~CO2 relationship, but did reduce the elevation in mean arterial blood pressure otherwise observed during hypercarbia. Measurement of norepinephrine levels in cortex indicate a close correlation between the ability of the lesion to reduce norepinephrine content and produce the observed physiological effects. The results of these experiments suggest that the hypercapnic response of CBF, but not CO to arterial paCO2 is modulated by systems which traverse the dorsal brainstem. The role of the locus coeruteus-catecholamine cell bodies in this effect, however, must be considered speculative until further transmitter-selective interventions are carried out. INTRODUCTION The close relationship between arterial pCO 2 and the cerebral vasculature is well k n o w n 23,36,39. Although increased H ÷ ion concentration can evoke dilation in cerebral vessels 24,26,49, the concept of a brainstem center mediating or modulating the cerebral vasodilatation induced by elevated paCO2 is suggested by serveral experimental observations. Early studies with large brainstem lesions suggested that brain structures r e l e v a n t , t o a pCO2-cerebral blood flow (CBF) coupling were located at the level of the ports 6,42. The locus coeruleus (LC), a vascularized structure lying in the floor of the 4th ventricle gives rise to norepinephrine-containing fibers which project diffusely throughout the telencephalon 1,33. Hypercapnia results in a rapid increase in the discharge

rate of LC neurons which is not abolished by deafferentation of peripheral chemoreceptors 11. Axonal terminals deriving from the catecholamine neurons of the LC, are believed to terminate on the small intraparenchymal cerebral arteries or arterioles38.46. These observations led to the consideration of whether the LC system might play a role in the apparent coupling between arterial pCO2 and CBF, and to the query of what happens to C B F and its response to changes in paCO2 when this system is destroyed. Earlier work from this laboratory demonstrated in 3 cats that bilateral stereotaxic lesions of this nucleus decreased the hypercapnic response. Subsequent studies by others in rats using intra-cerebrally injected 6 - O H D A and/or electrolytic lesions reported either no change in the CO 2 response 7 or an increasO o. Because of the limited n u m b e r of animals we sought to

* Present address: Department of Neurologic Surgery, McLaren General Hospital, Flint, MI, U.S.A. Correspondence: T.L.Yaksh, Mayo Clinic, Rochester, MN 55905, U.S.A. 0006-8993/86/$03.50© 1986 Elsevier Science Publishers B.V. (Biomedical Division)

279 re-examine this issue. In addition, activity in corticopetal catecholamine systems may alter forebrain cAMP25 and increase C M R O 2 by a fl-adrenergic receptor 16. As the magnitude of the CBF response to hypercapnia has been reported to be diminished as cerebral CMRO 2 falls 13, a loss of response to CO2 in the LC-lesioned animal might reflect these changes in cerebral metabolism produced by LC lesions 16. Thus, propranolol produces a reduction in the CMRO 2 and in the CO 2 responsiveness of CBF 32. We therefore sought to determine whether comparable reductions would occur in LC-lesioned animals. MATERIALS AND METHODS

Surgical preparation of locus coeruleus lesions Mongrel cats of either sex weighing from 2.3 to 4.6 kg were anesthetized with ketamine (30 mg/kg, i.m.). The head was placed in a stereotaxic frame; the skull exposed by a midline incision. Burr holes were made on either side of the midline as determined by stereotaxic coordinates based on the Berman 4 atlas. The lesion coordinates were: 2 mm posterior to the interaural plane, 2 mm below the zero plane and 2 mm to each side of the midline (P2, H - 2 , L + 2). The electrode was passed at a 45 ° angle from the horizontal to avoid bony tentorium. Lesions were produced using a stainless-steel electrode (0.5 mm diameter, insulated by epoxylate except for 0.5 mm at the tip) coupled to a Grass LM4 lesion maker (Grass Medical Instruments, Quincy, MA). A current of 1.5 mA for 20 s was used to produce the lesion as preliminary studies revealed that this produced a 1.5 mm lesion at the tip of the electrode. The dura in the burr hole sites was incised with the sharp edge of a needle. The electrode was then carefully advanced through the cerebellum to the required depth and the lesions created by passing the current through the electrode. All procedures were carried out using sterile precautions. After the lesioning procedure the electrode was removed and the skin was closed with silk sutures. All the cats were given 600,000 units of Bicillin intramuscularly. Cats were then returned to their cages and allowed normal access to food and water. Ten of the cats were submitted to sham lesions. These cats underwent an identical procedure of advancing and removing the electrode, but no current was passed.

Animal preparation for CBF and CO measurements Ten days after the lesioning procedure, each cat was anesthetized with intraperitoneal sodium pentobarbital (30 mg/kg). Arterial and venous catheters were introduced into the right femoral artery and vein for blood pressure and blood gas measurements, and administration of drugs. A polyethylene catheter (PE-50) was passed into the right lingual artery and advanced into the carotid artery. A temperature probe was placed in the rectum and core temperature was maintained at normothermia with a warming blanket. The animal was paralyzed with 0.5 mg of pancuronium bromide and mechanically ventilated with a mixture of air, oxygen and carbon dioxide. The arterial pO2, pCO2, and pH were determined before each measurement of CO and CBF. To measure cardiac output (CO), a flow directed thermal dilution catheter (7 French) was passed under fluoroscopic control through a femoral vein into the fight atrium and then carefully guided into the pulmonary artery. Iced 5% dextrose (3 cc) in water was injected through the thermal dilution catheter each time a CO determination was made. CO was determined using a cardiac output computer (instrumentation Laboratory, Lexington, MA). At least two measurements were done each time. If there was a difference of over 10% between these two measurements, a third measurement was obtained and mean of the two closest determinations considered as the CO. To measure CBF, the skin, fascia, and temporalis muscles, were excised bilaterally from the skull to the level of zygomatic arch. This assured that the 133Xe CBF measurements were free of contamination from muscle circulation. CBF was measured from the washout curve of a 0.3 cc bolus of 133Xe (approximately 500/~Ci) injected into the right lingual artery catheter. The sodium iodide detector was placed perpendicular to the skull over the right temporo-parietal region. The CBF in 100 g of brain tissue/min was calculated by the initial slope technique. The collimation data for the probes used in this study have been described elsewhere3,14. Pulse height analysis was employed to minimize Compton scatter. Blood loss during the preparation of the animal was less than 5 cc.

280

Measurement of cortical norepinephrine Immediately upon termine'!on of the experiment with the animal under deep anesthesia, the cranium was exposed bilaterally and small portions of cortex were removed, blotted and frozen on dry ice. Care was taken to include only gray matter and to ensure that the samples were taken from bilaterally symmetrical sites under the region where blood flow was monitored. The frozen tissue was stored at -30 °C until assayed spectrofluorometrically by the method of Kariya and Aprison 2~ as modified by Tyce and Owen 4~. We are grateful to Dr. Gertrude Tyce for carrying out these assays.

least-squares analysis. This was done for each animal in a given group (sham lesions, brainstem lesions not in the LC, and animals with lesions in the LC) Where appropriate, the slopes of pairs of CO, response curves were compared using either a t-test for independent samples (e.g. sham vs lesioned) or paired (e.g. before and after drug treatment). Data were also analyzed according to a modification of the 3way analysis of variance model with repeated measures described by Winer52. Comparisons between specific cells where the A N O V A was statistically significant was carried out with a Newman-Keuls analysis using the appropriate error term from the ANOVA.

Experimental protocol Each animal was prepared for CO and CBF determinations as described above and mechanically ventilated using a Harvard ventilator (Harvard Apparatus Company, Millis, MA). To determine the CO 2 response relationship, the paCO2 was varied by altering the amount of CO2 in the inspired gas from around 35 mm Hg (normocapnia for cats) to 20 mm Hg, then to 60 mm Hg, and finally to 35 mm Hg. At each paCO 2 level, the true paCO 2, CO and CBF were determined. A 10°min interval was allowed to elapse between each subsequent measurement. Propranolol hydrochloride (1 mg/kg/ml, i.v.) was then slowly injected intravenously and measurements of CBF and CO recorded again at normocapnia, hypocapnia, hypercapnia, and normocapnia.

RESULTS Forty-four cats were used in this investigation. The first 3 cats were used to establisli stereotaxic coordinates and standardize the methodology. Nine of the remaining cats died following ablation of the LC or prior to completion of the second stage of the study: 1 died of bleeding from the burr hole sites at the time of lesioning; 2 were found to have died during the night

Histologic examination After completion of the experiment and removal of the cortical samples, the brainstem was removed and fixed in formalin. After fixation, a slice of brainstem at the level of inferior coUiculus consisting of the lower part of midbrain and upper pons was embedded in paraffin and sectioned at 20/~m intervals. The sections were stained with Luxol Fast blue and examined microscopically to determine the extent and location of the lesions. Assessment of lesion was made without knowledge of either experimental results in effect on levels of cortical norepinephrine.

Statistics The C B F - C O 2 and C O - C O 2 responses in each animal before and after propranolol were observed to be well fitted by a linear regression calculated with a

Fig. i. Histologicalsection of brainstem at the level of the inferior colliculus, depicting bilaterally placed lesions in the region. of the locus coeruleus ( × 12).

281 the study. Ten of these cats belonged to the sham lesion group and 22 to the LC ablation group. Fig. 1 presents a histological section from an animal with bilateral lesions in the LC.

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Fig. 2. Diagrammatic representation of the section of the brainstem depicting the location of accurately placed lesions (A). The dotted areas indicate the location of lesions and the number represents the number of cats with lesions in that location; and (B) inaccurately placed lesions. The dotted areas indicate the location of lesions and the number represents the number of cats with lesions in that location.

in their cages after surgery due to aspiration and asphyxia, 5 died of cardiac arrhythmias during cardiac output measurements, and 1 died of cardiac arrhythmias and hypotension after the i.v. injection of propranolol. Two animals were excluded because of technical problems with equipment during blood flow measurements. Thirty-two cats were thus available to complete

Location o f lesions Of the 22 cats in the lesion group, 10 had lesions accurately placed in the region of the LC bilaterally, 2 had a unilateral locus lesion; the second lesion in these cats could not be detected, presumably because the electrode tip on the opposite side could have been located in the fourth ventricle at the time of lesioning, and 10 had lesions in other areas of brainstem or more frequently, the cerebellar vermis. Schematic representation of the distribution of the lesions are presented in Fig. 2. Following placement of the lesions, some of the animals appeared to sleep more often in the initial one or two postoperative days, but subsequently all of the animals were able to feed, drink, and ambulate normally with no signs of motor dysfunction. None of the animals showed evidence of infection at the burr hole sites. Though levels of norepinephrine in cortex were not assayed in all animals, cats with bilateral lesions of the LC, characteristically depicted by the histology in Fig. 1, showed a significant bilateral reduction in cortical norepinephrine levels (see Table I). Animals with lesions in the cerebellum or dorsal mesencephalon showed no significant changes as compared to sham-operated controls. Data for 4 cats, representative of the effects produced by the several lesions (sham, inaccurate, unilateral and bilateral LC lesions) on CBF, CO and TABLE I

Levels of norpinephrine in left and right cerebral cortex of central accurate bilateral locus coeruleus lesioned and misplaced brainstem/cerebellar lesioned cats Inaccurately lesioned: placement of lesions are as typically shown by Fig. 2A. Accurate placements: placement of lesions are as typically shown by Fig. 2B.

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0.171_+0.032 0.168+0.021 0.038_+0.002*

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282 SHAM (Cat no. 4 6 )

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Fig. 3. Responses of cerebral blood flow (ml/100 g/min), cardiac output (liters/kg of b. wt.) and mean arterial blood pressure (mm Hg) to alteration of arterial carbon dioxide tension (mm Hg) in cats with sham (O----~), inaccurate (~ ~), unilateral locus (x x) or bilateral locus (A A) lesions. Data from a single representative cat from each group are depicted. At 50 min each cat received i.v. propranolol (1 mg/kg).

M A B P are presented in Fig. 3. As shown in these representative animals and as will be discussed below, bilateral lesions of the LC, resulting in a bilateral reduction in cortical norepinephrine, produce a significant reduction in the slope of the relationship between p~CO 2 and CBF, MABP, but not CO. Propranolol produced a general reduction in CBF and CO during all states of paCO2. The group data depicting the group CO 2 response curves for CBF, CO and M A B P are given in Figs. 4, 5 and 6, respectively.

Cerebral blood flow In the control group, sequential measurement of

blood flow during altered paCO/indicated a significant effect on CBF as measured by Xenon clearance (F = 5.16, df = 3,81, P < 0.01). Fig. 4 presents the group data for the CBF measured as a function of arterial paCO2. The CBF-paCO2 response function was readily fitted by a least-squares linear regression and displayed a positive slope significantly greater than zero (see Table II). As in other reports from this laboratory9, x4, repetition of the CO2 response sequences in the absence of other experimental manipulation results in no significant differences with replication in the same animal over the time course (approximately 6 0 - 9 0 rain) of

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Fig. 4.Curves present the mean + S.E.M. of the cerebral blood flow as measured by Xenon washout at paCO:s ranging from 20 to 70 mm Hg in animals with bilateral locus coeruleus lesions (LC lesion) or controls (shams), before and after propranoiol (1 mg/kg, i.v.). Data from non-operated controls are not presented for clarity, but did not differ from those of the sham-operated animal.

these experiments (see Table II control and control + saline). In lesioned animals, analysis of variance revealed a statistically significant interaction of the lesion with paCO2 (F = 6.13; df = 2,81; P < 0.01). Examination of the cell values with Newman-Keuls indicated that there were no significant differences in measured CBF among the groups (LC lesions, inaccurate lesions, sham-operated) during hypocapnia and normocapnia but the CBF responses to hypercapnia were significantly attenuated in those animals determined to have bilateral LC destruction (P < 0.05). Animals with misdirected lesions were no different from sham-lesioned animals (P > 0.10). As shown in Fig. 4, bilateral LC lesioned animals displayed a highly significant reduction in the slope of CBF-paCO 2 response curve. As summarized in Table II, sham-operated animals and animals with lesions lying outside of the LC displayed slopes which were around 3 times those observed in animals with bilateral LC lesions. Analysis of the action of propranolol indicated a

Cardiac output As shown in Fig. 5, CO was significantly altered during changes in arterial paCO2 (F = 3.16; df = 3,81; P < 0.05) with the least-squares linear regression displaying a positive slope significantly greater than zero (Table II). Analysis of the main effects associated with the lesions revealed no difference (F = 1.37; df = 2,81; P > 0.10) in slope, though CO tended to be uniformly lower at any given paCO2, this difference was also not statistically significant. Propranolol administration was associated with a marked effect on CO (F = 5.68; df = 1,27; P < 0.05). The analysis of the interaction between propranolol treatment and lesion (F = 1.16; df = 1,27; P > 0.10) or propranolol and pCO 2 (F = 1.52; df = 3,81; P > 0.10) was not statistically significant indicating that drug treatment resulted in a uniform reduction in CO, regardless of the lesion. Inspection of the slope of the CO-pCO 2 response curves suggests the propranolol treatment may result in a reduction in slope in all groups but these differences were not statistically significant (see Table II). Mean arterial blood pressure As with the other measures, MABP showed a significant variation in response to pCO 2 (F = 3.62; df = 3,81; P < 0.05). In control animals over the range of PaCO2 values examined, there was a linear relationship between paCO 2 and MABP with a positively inflected slope statistically greater than zero (see Fig. 6 and Table II). A significant interaction was observed between CO2 and lesion treatments (F = 4.16; df = 2,27; P < 0.05). The resting MABP at normo- and hypocarbia in LC-lesioned animals were significantly greater than those measures obtained in control or sham-lesioned animals (P < 0.05). Pressures meas-

284 TABLE II Effects of locus coeruleus lesions and propranolol on the slope of the pCO2-CBF , P~CO2-CO , P.CO2-MABP response curves Results derived from those data presented in Figs. 4-6. Description of groups are as presented in text. n = number of animals in group. r = correlation coefficients of linear regression. Treatment groups Control

Control + saline

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Control + propranolol

Sham

10

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LC lesion

10

LC lesion + propranolol 10

1.51 +0.31 0.75

1.33 +0.28 0.91

1.66 +0.21 0.94

1.29 +0.38 0.79

1.59 +0.32 0.81

1.39 +0.30 0.88

0.57 +0.23 0.87

0.64 +_0.19 0.96

0.31 +0.06 0.91

0.26 +0.09 0.92

0.34 +0.08 0.95

0.25 +0.11 0.90

0.31 +0.12 0.89

0.21 +0.13 0.89

0.41 +0.08 0.96

0.21 _4:,0.10 0.96

0.72 +0.18 0.84

0.69 +0.20 0.81

0.64 +0.25 0.80

0.20 +0.19 0.83

0.71 +0.20 0.82

0.28 +0.09 0.92

0.13 +0.03 0.96

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ured during hypercarbia did not differ (P > 0.10) between any control or drug-treated group (P > 0.20). As shown in Fig. 6, LC-lesioned animals showed no t--

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Fig. 5. Curves present the mean + S.E.M. of the cardiac output (ml/kg/min) at paCO2 values ranging from around 20 to 70 mm Hg, in animals with bilateral locus coeruleus lesions or control (sham) before and after the administration of propranolol (1 mg/kg, i.v0. Data from unoperated control animals are not presented for clarity, but do not differ from those of the shamoperated controls.

Fig. 6. Curves present the mean + S.E.M. of arterial blood pressure (ram Hg), measured in the presence of paCO2 values ranging from around 20 to 70 mm Hg, in cats with bilateral locus coeruleus lesions or in control (sham) animals before and after the administration of propranolol (1 mg/kg, Lv.). Data from unoperated controls are not presented, but do not differ from those of the sham-operated animals.

285 significant change in MABP in response to changes in the arterial pCO 2 (P > 0.10), and the slope of the MABP-pCO 2 response function was not different from zero. Propranolol had no effect in the LC lesioned animals on either the resting MABP or slope (see Table II). In contrast, in control and sham-operated animals, propranolol did result in an increase in normo- and hypocarbia pressures comparable to that observed in LC-lesioned animals. Their slopes did not statistically differ (P > 0.10). DISCUSSION Although it is clear that the lowering of the pH of the vascular environment can directly dilate the cerebral vessels, the present experiments suggest that exceedingly discrete lesions of the brainstem in the vicinity of the anatomically identified region of the LC which result in a significant loss of cortical norepinephrine will effect a significant reduction in the slope of the CBF-paCO 2 response curve. Previous studies in rats after 6 - O H D A injections or electrolytic lesions have observed no change in normocarbic flow as assessed with a clearance of either 14C, ethanol or 133Xe7,10. With regard to CO z activity, Edvinsson and colleagues 10 observed an increase in CO 2 reactivity, while Dahlgren et al. 7 observed no change. As in the present experiments, Mendelow et al. 34 observed a reduction in CO2 reactivity during hypercarbia. The present results are also in clear support of previous studies from our laboratory where, in a limited number of animals (3 cats) with accurate bilateral lesions in the locus coeruleus, a significant reduction in the slope of the pCO 2 response curve could be observed. The present results, however, differ from the previous studies in our failure to see a significant increase in normocarbic flow after LC lesions. The reason for this difference is not known, but we consider that the repeatability of the present results in 10 animals warrant their acceptance. Propranolol treatment resulted in a significant reduction in flow at all levels of CO 2, e.g. no change in the slope of the CO 2 response curve in both control and LC-lesioned cats. These results therefore differ from the reports by MacKenzie and colleagues 32 in baboons, in which it was observed that even lower doses of propranolol (0.12/ag/kg) would significantly

attenuate the slope of the paCO2-CBF response function. Although these lesions also produced a concurrent change in the M A B P - p C O 2 response, we do not believe it likely that the changes in the slope of the CBF-paCO 2 response function are due to inadequate perfusion pressure. The systemic blood pressure lay well within the range of pressure autoregulation. Moreover, in previous studies, LC lesions failed to alter the autoregulatory response over the range of pressure observed in these experiments 3. Finally, inspection of the MABP in the present experiments indicated little difference between sham- and LC-lesioned animals during hypercarbia. The mechanism whereby LC lesions and propranolol alter CBF and CO 2 reactivity, are likely complex, but several points should be briefly addressed. Given: (a) the existence on forebrain vessels of fl-receptors which regulate cAMP and dilatelT, TM along with noradrenergic innervation2S,46; and (b) the increase in firing rate of locus neurons in the presence of elevated arterial CO 2 (ref. 11), a direct coupling between LC activity and fl-adrenoreceptor mediated dilation could be postulated. Lesion of the locus coeruleus might thus be mimicked by fl-receptor blockade. Although a plausible hypothesis, we note that propranolol and LC lesions exert very distinguishable effects on forebrain C B F - C O 2 reactivity. Thus, LC lesions result in a significant reduction in the slope of the CO 2 response function with no major change in normocarbic flow. In contrast, propranolol in these experiments, essentially produced a parallel shift in the pCO2-CBF response function in control and LC-lesioned animals with a uniform lowering of CBF measured at all levels of paCO2. These effects were observed even in the absence of the LC. This finding is in general agreement with the results reported in other experiments where propranolol was observed to result in a significant reduction in normocarbic CBFI2, 35. Further, as noted above, Elam and colleagues u, observed an increase in LC activity as a function of p a C O 2 . If the effects of LC-lesions on CBF-CO 2 reactivity were thus simply mediated by a loss of fl-receptor-mediated input, the propranolol effect should only have been observed at the higher levels of CO2, e.g. it should have acted by reducing the slope of the pCO2-CBF response curve. It should be pointed out that MacKenzie and colleagues 32 did

2~6 observe a major reduction in the C B F - C O : slope, and. we note that in sham-operated animals in the present experiment, a slight reduction in slope ( 1.681.29, see Table IlL was observed after propranolol, but these values did not statistically differ. In LC-lesioned animals, however, if anything, in the presence of the shift in the pCO2-CBF response curve, propranolol produced a slight elevation in slope (0.59-0.61). We thus conclude that by whatever mechanisms the LC lesions act to alter the CO, response curve, they do not uniquely produce their effects by an action mediated through cerebral fl-receptors. An alternate explanation which has been tendered regarding the effects of LC lesions relates to the observation that CO 2 reactivity of the cerebrovasculature co-vary with global measures of cerebral oxygen consumption and glycolytic metabolism 13,32. Several investigators 1s,16,29 have noted that lesions of the locus coeruleus would significantly impair cerebral glycogenolysis secondary to increased energy demands. As the adrenergic effects on cerebral metabolism are thought to be mediated by fi-receptors, the present results disassociating the effects of LC lesions and propranolol treatment on C B F - C O 2 responsiveness would appear to exclude a common mechanism. Moreover, although forebrain catecholamine projections may indeed modulate forebrain metabolism, it does not follow that such changes are the cause of the decreased CO 2 reactivity but rather, they reflect an epiphenomena. Thus, the correlation between the changes induced by manipulations such as anesthetics on CMRO2 and concurrent changes in CO, reactivity, may likely reflect a general depression by the treatment on all central structures, perhaps including those as the LC which potentially modulate C N S - C O : reactivity. Thus, while decreased cerebral metabolism may accompany locus lesions under certain conditions, it does not appear likely that changes in forebrain metabolism per se had anything to do with the characteristics of the effects of LC lesions on altered CO~ activity. Propranolol treatment results in significant reductions in CMRO 2 and appears to yield no selective effect on the CBF-CO2 response, producing a reduction in flow at all levels. In short, we believe that systems which are located in or near the locus coeruleus are activated during periods of increased arterial p~,CO2 and this activity is

translated into significant increases in cerebral blood flow. These results of bilateral LC lesions are remarkably similar to the effects produced by clonidine. a centrally acting alphae-agonist which suppresses LC activity 44. ST-91 an alpha2-agonist which does not apparently cross the blood-brain barrier has no effect when given peripherally, but produces a significant reduction in the slope of the CBF-paCO 2 response function when given into the 4th ventricle (ref. 20: T.L. Yaksh, I.S. Kanawati and R.E. Anderson, unpublished observations). It should be stressed that these data do not indicate a unique role for norepinephrine in mediating this forebrain CBF response, and do not preclude the likelihood that other neurotransmitter systems in this vicinity may play a role including those of serotonin 31.43, substance p19, enkephalin40 or neurotensin 5a. The failure of a fl-receptor antagonist to attenuate selectively the reactivity of CBF to, CO2 argues against a direct mediation by this receptor system of that effect. Thus, these projecting pathways may locally influence the release of potent vasoactive agent such as adenosine 53 or vasoactive intestinal polypeptide (VIP) 30. Indeed, brainstem stimulation resulting in an increase in pial vessel diameter will increase the release of VIP from cortex and this vasodilating effect is attenuated by VIP-antiseraS0,51.

LC lesions, cardiac output. M A B P It has been observed that the physiological response of cardiac output and mean arterial blood pressure to alterations in arterial CO2 are analogous to the CBF responses under these conditions e3,47. Electrical stimulation of LC-produced pressure responses and an increased heart rate in normotensive rats 22, caval occlusion or reduction in central blood volume increased the firing of LC neurons z.5,45. Spinal cord noradrenergic terminals arise from catecholamine neurons in the ventral portions of the LC as well as other cell groups of the brainstemS.4~. The LC innervates all segments of the spinal cord and its projections are widely distributed to both the dorsal and ventral horns z7.37. The inability of bilateral LC lesions in the present study to alter the CO-CO2 response function, however, suggests lack of involvement of this system or the development of compensatory changes during recovery. Curiously, LC lesions did result in a significant in-

287 crease in the resting blood pressure measured during

also reflect a baroreceptor mediated inhibition in fur-

normo- and hypocarbia, an effect mimicked by r - r e ceptor blockade. As central venous pressure was not measured, we cannot determine systemic vascular

ther pressure rises. ACKNOWLEDGEMENTS

resistance. However, the apparent lack of change in M A B P in the presence of an increased CO leads us to

We would like to thank Dr. D o n n a H a m m o n d for

conclude that systemic vascular resistance during

her early assistance in the stereotaxic work and Ms.

hypo- and normocarbia must have been higher than

A n n Rockafellow for preparing this manuscript. This

normal. The apparent lack of change in M A B P may

word was supported by G r a n t NS 06663B (T.L.Y.).

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