Cocaine potentiates the blood pressure and cerebral blood flow response to norepinephrine in rats

European Journal of Pharmacology, 249 (1993) 287-292

287

© 1993 Elsevier Science Publishers B.V. All rights reserved 0014-2999/93/$06.00

EJP 53381

Cocaine potentiates the blood pressure and cerebral blood flow response to norepinephrine in rats J u d i t h K. M u i r a n d E a r l F. Ellis * Department of Pharmacology and Toxicolog); Medical College of Virginia, Virginia Commonwealth University; Box 6t3, MCV Station, Richmond, VA 23298, USA

Received4 August 1993, accepted 25 August 1993

Acute drug-induced hypertension is known to have adverse consequences on the cerebral vasculature. Cocaine abuse has been reported to be associated with an increased frequency of hemorrhagic or ischemic stroke. The purpose of this study was to determine whether cocaine alters the blood pressure or cerebral blood flow response to exogenous norepinephrine. A craniectomy was made over the parietal cortex in rats and cortical blood flow changes were measured using laser-Doppler flowmetry. Ten minutes after cocaine (1 mg/kg, i.v.) or saline, increasing doses of norepinephrine (0.01-10 ~g/kg, i.v.) were given by bolus injection and changes in blood pressure and flow were monitored. Cocaine produced a transient 27 + 5% increase in blood pressure and a 38 ± 9% increase in blood flow. Cocaine significantly potentiated the blood pressure and cerebral blood flow responses produced by submaximal pressor doses of norepinephrine (0.01-0.6/zg/kg, i.v.). In summary, cocaine causes a rapid, transient increase in blood pressure and cortical blood flow and potentiates the magnitude and duration of the pressure and flow response to norepinephrine. Repetitive blood pressure elevations in cocaine abusers is one of the proposed mechanisms leading to damage of cerebral vessels. These results may be relevant to an increased frequency of cerebrovascular accidents in cocaine-abusing individuals. Cocaine; Cerebral microcirculation; Laser-Doppler; Stroke; (Rat)

I. Introduction

Cocaine is a known cardiac stimulant, increasing blood pressure and altering heart rate in humans and experimental animals (Ritchie and Greene, 1985; Foltin and Fischman, 1992). T h e sympathomimetic properties of cocaine are due to the drug's ability to increase sympathetic outflow and to block monoamine reuptake by presynaptic nerve terminals. Thus the actions of monoamines, such as norepinephrine, may be prolonged in the presence of cocaine since neuronal reuptake is the primary mechanism involved in terminating the actions of this catecholamine (Jacobs et al., 1989; Levine and Welch, 1988; Mangiardi et al., 1988; VanDette and Cornish, 1989; Ritchie and Greene, 1985). Although cocaine causes a variety of central and peripheral consequences, research has primarily focused on the mechanisms of its abuse potential. Recent reports have associated cocaine use with cerebrovascular accidents, including hemorrhagic and ischemic stroke (Brust and Richter, 1977; Lichtenfeld et al., 1984;

* Corresponding author. Tel. (804) 786-8399, fax (804) 371-7519.

Levine et al., 1987; Klonoff et al., 1989; Mangiardi et al., 1988). Many of these reports have hypothesized that cocaine may affect the cerebral circulation due to repetitive hypertensive episodes, both drug-induced and stress-induced. A variety of experimental models have shown that cocaine can potentiate the blood pressure response to neurally released and exogenously administered norepinephrine (Jain et al., 1990; Levy and Blattberg, 1978; Masuda et al., 1980; Zanger and Enero, 1974), however, few studies have examined the pressor response of cocaine on the cerebral circulation. T h e purpose of the present study was to determine how acute, systemic cocaine administration affects the cerebral circulation and whether it may enhance the blood pressure and cerebral blood flow response produced by exogenous norepinephrine.

2. Materials and methods

Experiments were conducted in 17 male SpragueDawley rats (270-375 g) anesthetized with sodium pentobarbital (60 m g / k g , i.p.). After tracheostomy, the

288 rats were ventilated with room air using a rodent ventilator (Edco Scientific, Chapel Hill, NC). The endexpiratory CO 2 was continuously monitored with a Transverse Medical CO 2 analyzer (Dynatech ElectroOptics, San Luis Obispo, CA) and was maintained at a level ~ 32 mmHg throughout each experiment by adjusting the respirator rate and volume. Mean arterial blood pressure was measured by a Sorenson pressure transducer connected to a cannula inserted into the left femoral artery. The right femoral artery and vein were cannulated for withdrawal of arterial blood samples (135/xl) and infusion of drugs, respectively. Arterial blood gases and blood pH were analyzed with a pH-blood gas analyzer (Instrumentation Laboratory, Lexington, MA) to insure normal ventilation. The rectal temperature of all animals was maintained at 37°C with a heating pad. The animals were placed in a stereotaxic holder, a midline scalp incision made and the bone surface cleaned of all tissue. A 4 mm i.d. craniectomy was carefully made over the left cortical hemisphere. Care was taken not to damage the dura mater and the brain surface. Immediately adjacent to the craniectomy site, a 1 mm diameter stainless steel screw (No. 0-80 × 3 / 1 6 " , Small Parts Inc., Miami, FL) was turned into the skull such that it held firmly in place, but did not fully penetrate the skull. The purpose of this screw was to provide an anchor for the dental acrylic which holds the laser-Doppler flow probe apparatus in place (see fig. 1). Next a plastic Luer-Loc syringe needle was modified by removing the needle portion and the remaining plastic hub was then reduced to the same diameter as the craniectomy by means of a hand-held pencil sharpener. The modified hub was gently inserted into the craniectomy, such that no pressure was applied to the dura, and secured in place with cyanoacrylate glue. Next, dental acrylate was placed around the modified needle hub and the stainless steel screw, producing a secure, sealed system. A 0.84 mm diameter laser-Doppler probe was held 1-2 mm from the cortical surface by the use of a two-holed Teflon plug, which was placed into the modified hub (see fig.

1). The second hole allowed for the release of pressure when the probe was inserted into the plug. The Teflon plug was rotated until the laser probe was over an area of the microcirculation with control flow values (relative units) that were consistent for all experiments. Also, care was taken to assure that the probe was not placed over any large, visible dural vessels. The probe was not disturbed throughout the remainder of the experiment. As in our previous studies, a Laser Flo model BPM 403A (TSI Inc, St. Paul, MN), was used to monitor cortical cerebral blood flow (Haberl et al., 1989a,b). The theoretical and technical principles of measuring cerebral microvascular blood flow by laser-Doppler flowmetry have been previously described in detail (Haberl et al., 1989a). Also Shepherd et al. (1987) have reported extensively on the characteristics and reliability of the particular instrument we have utilized. Briefly, light (760-800 nm wavelength) from a low power laser is conducted to the brain surface by a flexible fiber-optic light guide. The light strikes the brain surface and penetrates to a depth of approximately 1 mm 3. Some of the light is Doppler-shifted in frequency due to interactions of the light with moving blood cells in the cerebral microcirculation. Some of the light, both Doppler-shifted and non-Doppler shifted, is then reflected back toward the probe and conducted through fiber optics back to the signal-processing unit. The instrument detects changes in both velocity and the number of moving ceils in the microcirculation. The cell volume and velocity signals are electronically multiplied to produce a flow signal. The study was designed to determine if acute cocaine pretreatment (1 m g / k g , i.v.) would alter the blood pressure and cerebral blood flow response to bolus injections of increasing doses of norepinephrine. Two groups of rats were pretreated with either cocaine (n = 9) or saline (n = 8). Cocaine hydrochloride was supplied by the National Institute of Drug Abuse. Ten minutes after either pretreatment norepinephrine bitartrate (Sigma Chemical Company, St. Louis, MO) was administered every 5 min in increasing doses be-

-*-Iaserprobeo L b ~ ~'-c°rtex~+ dura ~*needle hub Fig. 1. Laser-Doppler technique to measure cortical blood flow.

289 TABLE 1 Baseline physiological parameters for control and cocaine-treated rats. Values are the m e a n s _+S.E.M. Control parameters

Group

100,

|

80

.~

60

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i

Weight (g) M e a n arterial blood pressure (ram Hg) PaCO 2(mmHg) PaO z Arterial p H

Saline (n = 8)

Cocaine ( n = 9)

.o

347

327

O.

±7

i

± 12

40-

0

102 ±4 33 +1 112 ±3 7.47±0.01

97 ± 33 ± 110 ± 7.46±

4 1 3 0.01

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,

.

,

10 *~

10 °

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10 2

Norepinephrine (pg/kg)

tween 0.01-10/zg/kg by bolus i.v. injection. The dose range utilized in the current protocol has previously been shown by our laboratory to produce mild to marked elevations in both blood pressure and cortical blood flow (Zhang and Ellis, 1991), The vehicle of all drugs was normal saline and each was mixed to a total volume of 0.2 ml. The laser-Doppler-measured flow was recorded at the time of maximal change in blood pressure. All data are expressed as means +_ S,E.M, and analysis was by repeat measure ANOVA followed by Student-Newman-Keuls comparison test. A P-value equal to or less than 0.05 was considered significant.

3. Results

Table 1 shows that the baseline physiological parameters for the control and cocaine-treated groups were similar. Fig, 2 shows how cocaine alters blood pressure and blood flow during the 10 min period before norepinephrine was administered. Cocaine caused a transient 27 + 5% increase in blood pressure and a 38 ± 9% increase in flow within 5-10 s after administration.

o]_

Fig. 3. Effect of cocaine on the m e a n arterial blood pressure response to norepinephrine. Values are the means_+ S.E.M. for control ( n , n = 8) and cocaine-treated ( B , n = 9) rats. P < 0.05, repeat m e a s u r e A N O V A (0.01-1 ~ g / k g doses), * P < 0.05, -t p < 0.06, * P < 0.08, Newrnan-Keuls comparison test.

After 10 min these parameters had returned to control levels. Statistical analysis showed that the blood pressure and flow were not different between the cocaine and saline-treated groups before the start of the norepinephrine dose-response study. Fig. 3 shows how cocaine altered the maximal blood pressure response to increasing doses of exogenous norepinephrine, It is clearly seen that the blood pressure response to norepinephrine (0.01-0.6 /~g/kg) is enhanced after cocaine pretreatment. A similar effect was seen for the blood flow response to norepinephrine after cocaine pretreatment (fig. 4). The rats which received cocaine showed an enhanced maximum response to exogenous norepinephrine which was also significant up to the 0.6 ~ g / k g dose. It should be emphasized that the bolus injections of norepinephrine used in this experimental paradigm caused transient, not steady state, elevations in blood pressure, thus we

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.

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Fig. 2. Time course of cocaine's effect on blood pressure and blood flow values are the m e a n s ± S . E . M , for rats given 1 m g / k g , i.v. cocaine (n = 9); B , m e a n arterial blood pressure; D, cortical blood flow.

%

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10 o

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.........

,

102

Norepinephrine (l~g/kg)

Fig. 4. Effect of cocaine on the cerebral blood norepinephrine. Values a r e t h e m e a n s + S.E.M. for and cocaine-treated (11, n = 9 ) rats. P < 0 . 0 1 , A N O V A (0.01-1 / ~ g / k g doses), * P < 0.05, t p Keuls comparison test.

flow response to control ( 13, n = 8) repeat m e a s u r e < 0.06, Newman-

290 can not comment on the autoregulatory capacity of the cortical microcirculation. In a recent study measuring cortical blood flow with laser-Doppler flowmetry, steady-state hypotension caused an immediate reduction of cortical blood flow followed by autoregulation of blood flow to near baseline (Florence and Seylaz, 1992). Similarly, we would expect cortical blood flow to increase at the time of maximal blood pressure change, thus the elevation of flow measured by laser-Doppler flowmetry would reflect transient pressor-induced changes in brain blood flow. Although the effect of norepinephrine to elevate blood pressure and flow in both groups was transient, the duration of elevated blood pressure and flow responses was approximately 1 min greater in the cocaine-treated group (data not shown). Both blood pressure and blood flow returned to approximately each group's respective control before the next dose of norepinephrine was administered 5 min later. The change in cerebral blood flow was not due to an increase in pCO2, as the end-expiratory CO2 never changed significantly upon drug administration. Statistical analysis showed that the blood pressure and flow were not different between the cocaine- and saline-treated groups before administration of each individual dose of norepinephrine. Figs. 3 and 4 also show that at the higher doses of norepinephrine, a convergence of the dose-response curves for mean arterial blood pressure and cerebral blood flow in the saline- and cocaine-treated groups occurs, signifying that a similar maximal effect for each dose will occur regardless of pretreatment. To address the possibility that the convergence at the higher doses of norepinephrine was due to the passage of time and cocaine metabolism, an additional group of rats were prepared in a similar manner, but only the three highest doses of norepinephrine were administered 10 min following cocaine or saline. In this second study, the blood pressure and cerebral blood flow response to the higher doses of norepinephrine was not different between groups, indicating that cocaine metabolism was not a significant factor in this or the full norepinephrine dose-response study (n = 16, data not shown).

4. Discussion

The design employed in our experiment utilizes bolus injections of drugs which cause transient changes in blood pressure. This, in turn, causes a transient increase in cerebral blood flow which typically lasts no longer than 1-2 min before returning to baseline. The elevation in cortical blood flow due transient hypertension can be accurately detected by laser-Doppler flowmetry because of the technique's continuous nature. Due to the non-steady state responses we can not

comment on how these drugs, especially cocaine, may or may not be altering the autoregulatory capacity of the cerebral vessels. To our knowledge, no study has examined whether cocaine alters autoregulation. Cocaine abuse has been reported to be associated with hemorrhagic and ischemic stroke and it has been speculated that the pressor response to cocaine affects the cerebral circulation. Studies indicate that barbiturate anesthesia blunts the blood pressure response of cocaine, as compared to studies performed in conscious animals (Pitts et al., 1987; Kiritsy-Roy et al., 1990). Laser-Doppler flowmetry is sensitive to movement, thus anesthesia is required in our protocol since the drugs would cause increased motor behavior. In our current study, cocaine caused a transient elevation in blood pressure from 97 + 4 to 124 _+ 9 mmHg (27 _+ 5%). This blood pressure change would not be expected to damage cerebral vessels and allow passage of exogenous norepinephrine across the blood-brain barrier and have secondary effects. Cocaine also caused a transient elevation of cortical blood flow. Cocaine-induced elevations in cerebral blood flow and local glucose utilization have also been demonstrated recently in conscious rats, even after blood pressure had returned to baseline (Sharkey et al., 1991; Stein and Fuller, 1992). Since the blood flow response in our study tends to be short lived, anesthesia may also influence blood flow, possibly by decreased neuronal firing and lowering of local glucose utilization. Drug-induced hypertension is not the only possible influence of intravenous cocaine on the cerebral circulation, since cocaine can freely cross the blood-brain barrier and may have a direct effect on cerebral vessels. Topical application of cocaine on pial arterioles using the cranial window technique has yielded conflicting results. Drug application under cranial windows circumvents alterations in flow due to transient changes in blood pressure. In these studies, cocaine caused vasoconstriction in the newborn piglet (Kurth et al., 1993), but produced vasodilation in cats (Dohi et al., 1990). These different results may reflect species and age differences. The topical doses used in these two studies ranged from 10 -8 to 10 -4 M. The systemic doses which we used would not be expected to create as high a local concentration. How chronic cocaine use affects the cerebral circulation is unknown, however, one study in human cocaine abusers reported reduced cerebral blood flow in the prefrontal cortex (Volkow et al., 1988). This loss was still evident after ten days of cocaine abstinence, indicating possible damage to the cerebral vessels. The investigators speculated that the reduced cerebral blood flow may be due to vasospasm or hemorrhagic episodes caused by the drug itself on vessels or alternatively, damaging vessels as a result of repetitive blood pressure elevation.

291

The current study shows that cocaine can potentiate the magnitude and duration of the blood pressure and cerebral blood flow responses to submaximal pressor doses of exogenous norepinephrine. Figs. 3 and 4 show that the norepinephrine dose-response curves converge at the higher doses indicating the maximal effect of norepinephrine will occur regardless of pretreatment. One possibility to explain this result is that the dose of cocaine has been metabolized due to its relatively short half-life. To address this question, a second group of rats received only the three highest doses of norepinephrine 10 min following administration of cocaine or saline. The blood pressure and cerebral blood flow response to the higher doses of norepinephrine was not different between groups, thus showing that cocaine metabolism was not a significant factor in this or the full norepinephrine dose-response study. Another, more likely, possibility may be similar to that reported by Trendelenburg (Trendelenburg, 1959). His studies were conducted on the nictitating membrane of the spinal cat and tested how cocaine may alter the contraction magnitude produced by electrical stimulation or exogenous norepinephrine. He found that cocaine could potentiate submaximal stimuli, but not those of supramaximal stimulation. Our study has shown that acute cocaine administration causes a rapid, transient increase in both blood pressure and cortical blood flow. Secondly, cocaine potentiates the magnitude and duration of the blood pressure and cerebral blood flow responses to submaximal pressor doses of norepinephrine. Since barbiturate anesthesia blunts the pressor response to cocaine, one would expect the hypertension produced by intravenous cocaine and possibly norepinephrine to be of greater magnitude in conscious experimental animals or human cocaine abusers than the pressor response seen in our study. Cocaine use may lead to increased incidence of stroke due to increased vascular damage by repetitive drug-induced a n d / o r sympathoadrenal elevations in blood pressure. Acute hypertension is known to cause abnormalities in the cerebral circulation. Blood pressure exceeding 160 mmHg may break through autoregulation, leading to abnormal regulation of cerebral blood flow. Breakthrough of autoregulation may also contribute to increased cerebrovascular permeability and brain edema. This increase in permeability may allow vasoactive substances, such as norepjnephrine and serotonin, which are normally excluded to cross the blood-brain barrier and affect cerebral vessels. Norepinephrine and serotonin have been shown to constrict cerebral vessels in isolated preparations or when applied topically (Edvinsson et al., 1993). It has recently been demonstrated that cocaine can potentiate the vasoconstriction due to norepinephrine when both drugs are applied topically in the newborn piglet

(Kurth et al., 1993). In our experiments, the doses of norepinephrine from 0.01 to 0 . 6 / z g / k g did not cause maximal blood pressure to rise above 150-160 mmHg, thus these pressor effects should not have caused vascular damage with subsequent increased passage of exogenous norepinephrine to the brain. Further research needs to be conducted to determine how hypertension-induced damage may be involved in the increased risk of stroke in cocaine abusers.

Acknowledgements Supported by National Institute of Neurological Disorders and Stroke grant 27214 and National Institute of Drug Abuse grants 05274 and 07027. E. Ellis is the recipient of a Jacob Javits Neuroscience Investigator Award. We are grateful to S.A. Holt for her excellent administrative assistance.

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