Introduction

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Dopamine is a major neurotransmitter in brain that is released at synapses located mainly in the striatum, a number of areas of the cerebral cortex, and cortical and subcortical structures of the limbic system. Dopamine or dopamine agonists have been shown in vitro and in vivo to have vasoactive effects on blood vessels and to produce vasoconstriction or vasodilatation, depending on the vascular bed and dose. Vasodilatation is seen in renal, mesenteric, coronary, and cerebral blood vessels, and because it is inhibited by dopaminergic antagonists, it is probably mediated by specific dopamine receptors (Goldberg, 1972, 1975; von Essen and Roos, 1974; Toda, 1976; Boullin et al., 1977; Edvinsson et al., 1977, 1978; McCulloch and Harper, 1977). Dopamine has vasoconstrictor effects in most vascular beds, including that of the brain, but because the vasoconstriction is blocked not by dopamine receptor antagonists but rather by -adrenergic and serotonin receptor antagonists, it is believed to be mediated by -adrenergic and serotonin receptors (Goldberg, 1975; Boullin et al., 1977; Edvinsson et al., 1977, 1978; von Essen et al., 1980; Lacombe and MacKenzie, 1997).

Dopamine receptor subtypes were recently localized by immunoreactivity in the smooth muscle of pial vessels (Amenta et al., 2000), and innervation of the intraparenchymal microvasculature by dopaminergic axons was demonstrated by light and electron microscopic immunocytochemistry in dopamine-rich regions of the frontal cortex (Krimer et al., 1998). Furthermore, perivascular application of dopamine in cortical brain slices in vitro was found to produce concentration-dependent vasoconstriction of the microvessels (Krimer et al., 1998). These observations led to the hypothesis that dopaminergic innervation of cortical blood vessels might play an important role in the regulation of cerebral blood flow (CBF) and in the mechanism of the enhancement of CBF by neuronal functional activation (Krimer et al., 1998). To examine this possibility directly, we have measured local CBF in vivo in unanesthetized rats and determined the effects of dopamine receptor blockade on baseline CBF and on its enhancement in four stations of the whisker-to-barrel cortex sensory pathway evoked by vibrissal stimulation.

	Materials and Methods

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Chemicals

4-Iodo[N-methyl-¹⁴C]antipyrine ([¹⁴C]antipyrine; [¹⁴C]IAP) (specific activity, 54 mCi/mmol) was purchased from PerkinElmer Life Sciences (Boston, MA). Haloperidol was purchased from Sigma-Aldrich (St. Louis, MO).

Animals

Normal, adult, male Sprague-Dawley rats (330-450g) were purchased from Charles River Laboratories (Wilmington, MA) and maintained in a climate-controlled room on a normal 12-h light/dark cycle, with food and water available ad libitum. Four groups of rats were studied, two treated with different doses of haloperidol and two corresponding control groups. The group that received the lower dose of haloperidol was infused intravenously over a 5-min period with 0.1 mg/kg haloperidol dissolved in physiological saline and adjusted to pH 4.0; a second group that served as controls for this lower dose was administered an equivalent infusion of the saline solution alone. The third was similarly infused but with 1.0 mg/kg haloperidol dissolved in physiological saline that had been adjusted to pH 2.8, and the fourth group, which served as controls for the group with the higher dose, was infused with comparable amounts of physiological saline at the same pH. The lower pH was required to dissolve the higher concentration of haloperidol.

Procedures

All procedures performed on animals were in strict accordance with the National Institutes of Health Guide for the Care and Use of Laboratory Animals and approved by the local Animal Care and Use Committee.

Preparation of Animals. The rats were anesthetized with halothane (5% for induction and 1.0-1.5% for maintenance) in 70% N₂O/30% O₂. Polyethylene catheters (PE 50; Clay-Adams, Parsippany, NJ) were inserted into the left femoral vein and both femoral arteries. The venous catheter was used for injection of drugs and infusion of the [¹⁴C]iodoantipyrine. One arterial catheter was used for continuous monitoring of mean arterial blood pressure, whereas the other was used for sampling of arterial blood. The length of the sampling catheter was fixed at precisely 16 cm to facilitate correction for the effects of catheter delay and dead-space washout. After insertion of the catheters, the surgical wounds were sutured and treated with 5% lidocaine ointment. After completion of the surgical procedure, a loose-fitting plaster cast was applied to the pelvic area and taped to a lead brick to prevent locomotion. Because the whisker-to-barrel cortex pathway was to be functionally activated unilaterally by vibrissal stimulation only on the left side, the whiskers on the right side of the face were cut close to the skin to minimize the possibility of spurious stimulation of vibrissae on the unstimulated control side. Unilateral stroking of whiskers in rats has been shown to produce selective unilateral increases in local CBF in at least four stations of the pathway, i.e., the ipsilateral spinal and principal trigeminal nuclei and the contralateral ventral posteromedial (VPM) nucleus of the thalamus and barrel region of the sensory cortex (Ginsberg et al., 1987; Adachi et al., 1994).

Monitoring of Physiological Variables. Following the surgical preparation, several physiological variables that could influence CBF were monitored before and during the experimentally induced conditions. Mean arterial blood pressure was measured with a blood pressure analyzer (model 300; Digi-Med, Louisville, KY) that had been calibrated with an air-damped mercury manometer. Arterial blood pCO₂, pO₂, and pH were measured with a blood-gas analyzer (model 288 Blood Gas System; Ciba-Corning Diagnostics Corp., Medfield, MA). Arterial plasma glucose concentration was determined in a Beckman Glucose Analyzer 2 (Beckman Coulter, Inc., Fullerton, CA).

Determination of Cerebral Blood Flow. Measurement of local CBF was initiated at least 3 h after recovery from the anesthesia, 30 min after the infusion of the 0.1-mg/kg dose of haloperidol or its vehicle, and 4 h after the infusion of the 1.0-mg/kg dose or its vehicle. The longer time following the higher dose or its vehicle was required for recovery from the transient acidosis caused by the lower pH of the solution. Local CBF was determined by the quantitative autoradiographic [¹⁴C]iodoantipyrine method (Sakurada et al., 1978) with the following modification; the [¹⁴C]IAP (40 µCi in 0.8 ml of physiological saline) was administered by a programmed intravenous infusion that produced a nearly linear rise in arterial tracer concentration throughout the period of measurement of CBF (Adachi et al., 1994). Throughout the approximately 1-min period of infusion, timed arterial blood samples were collected on weighed filter paper discs, which were assayed later for their [¹⁴C]IAP concentrations by liquid scintillation counting, as previously described (Sakurada et al., 1978). At a precisely recorded time, approximately 1 min after onset of the infusion of [¹⁴C]IAP, the rat was decapitated, and the brain was rapidly removed, frozen in isopentane maintained at 40^o to 50°C with dry ice, and cut into 20-µm sections in a cryostat at 22°C. The frozen brain sections were thaw-mounted on glass cover-slips, immediately dried on a hot plate at about 60°C, and autoradiographed together with calibrated [¹⁴C]methylmethacrylate standards on Kodak EMC-1 X-ray film (Eastman Kodak, Rochester, NY). Local tissue concentrations of [¹⁴C]IAP were determined by densitometric analysis of the autoradiograms. Rates of local CBF were calculated from the local tissue concentrations and the time course of the arterial [¹⁴C]IAP concentration by means of the operational equation of the method (Sakurada et al., 1978) and the computer program developed by G. Mies (Max Planck Institut für Neurologische Forschung, Köln, Germany) for use with the NIH Image program (W. Rasband; National Institute of Mental Health, Bethesda, MD) and a Macintosh computer (Apple Computer, Cupertino, CA). Corrections for delay and dispersion in the arterial catheter sampling system were incorporated in the computation of blood flow, as previously described (Freygang and Sokoloff, 1958), although the magnitude of these corrections was reduced to almost negligible levels by adjusting the blood flow through the arterial sampling system to approximately 60 dead-space volumes/min.

Statistical Analyses

Values of local CBF in the structures of the whisker-to-barrel cortex pathway of the stimulated and unstimulated sides of the brain were statistically compared by paired t tests. The effects of haloperidol on physiological variables, absolute values of local CBF, and the logarithm of the percent stimulation of CBF by functional activation were evaluated for statistical significance by unpaired t tests. If statistically significant effects (p < 0.05) were found by the t tests, Bonferroni corrections for multiple comparisons were applied where appropriate.

	Results

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Behavior. All of the haloperidol-treated rats exhibited dose-dependent signs of catalepsy. The degree of catalepsy was evaluated according to the scale described by Pizzolato et al. (1984), which ranges from 0 (no effect) to 4 (completely cataleptic) on the basis of the following features: presence and intensity of reduced spontaneous movements of head and forelimbs; hypertonic-akinetic posture with kyphotic trunk, extended head, broad-based support, and rigid up-turned tail; resistance to imposed horizontal displacement of forequarters; and ptosis. Rats given the low dose of haloperidol fell in the 1 to 2 range, and those given the high dose fell within the 3 to 4 range. The behavioral effects appeared shortly after haloperidol administration and persisted throughout the measurement of CBF.

Physiological Variables. There were no statistically significant differences in arterial blood pCO₂, pO₂, pH, arterial plasma glucose concentration, and mean arterial blood pressure between either of the haloperidol-treated groups and their corresponding controls (Table 1). Arterial blood pO₂ levels and arterial plasma glucose concentrations were higher in the rats receiving 1.0 mg/kg haloperidol than in those receiving 0.1 mg/kg, but these differences probably resulted from the lower pH of the vehicle for the higher dose because these differences were also present and statistically significant in the vehicle-treated controls (p < 0.02 and p < 0.002, respectively) (Table 1).

Effects of Haloperidol on Local CBF in Representative Cerebral Structures Unrelated to the Whisker-to-Barrel Cortex Sensory Pathway. The 0.1-mg/kg dose of haloperidol had no statistically significant effects on CBF in any of the structures outside the whisker-to-barrel cortex pathway examined. By standard t tests, the 1.0-mg/kg dose statistically significantly lowered CBF in the frontal (p < 0.05) and visual (p < 0.05) cortices, hippocampus (p < 0.05), dentate gyrus (p < 0.05), inferior olive (p < 0.01), and cerebellar cortex (p<0.05), and increased CBF in the lateral habenula (p < 0.01), but all of these changes lost statistical significance after Bonferroni corrections for multiple comparisons (Table 2).

Effects of Haloperidol on CBF Responses to Vibrissal Stimulation. The effects of haloperidol on baseline levels of local CBF were determined not only from its effects in structures outside the whisker-to-barrel cortex sensory pathway (Table 2) but also from its effects on CBF in the unstimulated side of the structures within the pathway. The effects of vibrissal stimulation on local CBF were determined by side-to-side comparisons between the stimulated and unstimulated sides in the structures of the whisker-to-barrel cortex pathway. As in the structures outside the pathway, the 0.1-mg/kg dose of haloperidol had no statistically significant effects on baseline CBF in the unstimulated side. Neither did it have any effect on the magnitude of the enhancement of CBF by vibrissal stimulation. Of the four structures of the pathway examined, the 1.0-mg/kg dose lowered baseline CBF statistically significantly (p < 0.01) only in the VPM thalamic nucleus by standard t tests and not after Bonferroni correction for multiple comparisons. The 1.0-mg/kg dose did, however, statistically significantly enhance the percent increases in CBF due to the vibrissal stimulation in the spinal trigeminal nucleus (p < 0.006) and VPM thalamic nucleus (p < 0.004) but not in the principal sensory trigeminal nucleus and barrel cortex, and these changes retained statistical significance even after correction for multiple comparisons (p < 0.05) (Figs. 1 and 2).

View larger version (34K): [in this window] [in a new window]   Fig. 1.   Effects of administration of 0.1 or 1.0 mg/kg haloperidol on baseline rates of local CBF and on increases in CBF induced by unilateral whisker stroking in four structures of the whisker-to-barrel cortex sensory pathway. Values are means ± S.E. obtained in the number of rats indicated. Vibrissal stimulation induced statistically significant side-to-side differences in each of the four structures in all conditions (paired t tests, p < 0.004). Baseline CBF was unaffected by haloperidol except in the VPM thalamic nucleus in which it was statistically and significantly reduced by 1.0 mg/kg haloperidol by grouped t test (, p < 0.01) but statistically insignificant after Bonferroni correction for multiple comparisons. The numbers above the solid bars are the means of the individual percent increases in CBF due to stimulation. Statistical analyses of the effects of the haloperidol treatments on the percent increases in CBF by vibrissal stimulation were carried out by comparisons of the logarithms of the percent side-to-side differences between the haloperidol-treated and corresponding vehicle-treated rats (*, p < 0.006 and **, p < 0.004 by grouped t tests and p < 0.05 after Bonferroni correction for multiple comparisons). , unstimulated side; , stimulated side.

View larger version (64K): [in this window] [in a new window]   Fig. 2.   Color-coded quantitative autoradiograms of brain showing effects of haloperidol on local baseline and functionally activated CBF in brain sections at the level of the spinal and principal trigeminal nuclei, VPM thalamic nucleus, sensory cortex, and frontal cortex. The rate of CBF is encoded in the color scale. The left side of the brain is on the left. Functional activation was achieved by unilateral stimulation of the vibrissae on the left side of the face.

	Discussion

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There is considerable evidence of dopaminergic innervation and effects of dopamine on blood vessels, including those in the brain. Some of these effects appear to be mediated by specific dopamine receptors because they can be blocked by dopamine antagonists (Goldberg, 1972, 1975; von Essen and Roos, 1974; Toda, 1976; Boullin et al., 1977; Edvinsson et al., 1977, 1978; McCulloch and Harper, 1977; Krimer et al., 1998). On the basis of these observations, it was hypothesized that dopaminergic mechanisms might be involved in the normal regulation of CBF and, more particularly, in its enhancement by neuronal functional activation (Krimer et al., 1998). In the present studies, which were undertaken to test this hypothesis, the effects of blockade of dopamine receptors on the increases in local CBF evoked by functional activation of a sensory pathway were examined. In view of the uncertainty about which of the several dopamine receptors might be involved in such mechanisms, the dopamine antagonist haloperidol was chosen because of its relatively low receptor specificity. To compensate for quantitative differences in the affinity of haloperidol for the various receptors, low and high doses of haloperidol were used to cover the range of doses previously shown to alter glucose utilization in cerebral structures known to be components of dopaminergic pathways and to be rich in dopaminergic receptors (McCulloch et al., 1980, 1982)

The low dose of haloperidol (0.1 mg/kg) had no statistically significant effects on baseline CBF in any of the 30 structures examined; nor did it alter the percent increases in CBF in the four structures of the whisker-to-barrel cortex pathway evoked by vibrissal stimulation. The high dose of haloperidol (1.0 mg/kg) did alter baseline CBF statistically significantly in the VPM thalamic nucleus and in several structures of the brain outside the whisker-to-barrel cortex pathway (i.e., reduced baseline CBF in the frontal and visual cortices, hippocampus, dentate gyrus, inferior olive, and cerebellar cortex, and increased CBF in the lateral habenula), but these changes lost statistical significance after Bonferroni corrections for multiple comparisons. Bonferroni statistics, however, are notoriously conservative and prone to type II errors, which might possibly have obscured a tendency for small selective local effects on CBF by haloperidol. For example, in the six structures outside the whisker-to-barrel cortex pathway in which the uncorrected t tests showed statistically significant lowering of CBF by the high dose of haloperidol, all but the inferior olive had been reported to exhibit significant reductions in local glucose utilization after administration of the same dose of haloperidol; glucose utilization in the VPM thalamic nucleus was not reported but found to be reduced in other parts of the thalamus (McCulloch et al., 1980, 1982; Pizzolato et al., 1984, 1985). There were a few discrepancies. For example, haloperidol was reported to reduce glucose utilization in the parietal cortex, medial geniculate, and hypothalamus (McCulloch et al., 1980, 1982; Pizzolato et al., 1984, 1985), but even by the uncorrected t tests, it had no statistically significant effects on CBF in these structures. Also, the lateral habenula and the nucleus accumbens are two of the very few reported to show increased glucose utilization in response to 1.0 mg/kg haloperidol (McCulloch et al., 1980, 1982; Pizzolato et al., 1984, 1985), and both showed increases in CBF in the present study, although the lateral habenula was the only structure in which the uncorrected t test showed haloperidol to cause a statistically significant increase in CBF. This tendency for corresponding changes in both CBF and glucose metabolism in a number of structures suggests that haloperidol might have some effects on local CBF, but they are likely to be secondary to changes in energy metabolism rather than to inhibition of direct vasoactive actions of dopamine at the level of dopamine receptors on the cerebral blood vessels. Such an interpretation is consistent with that of McCulloch and Harper (1977), who found hemispheric CBF and oxygen consumption to rise and then decline in parallel following apomorphine administration and both effects to be blocked by the neuroleptic pimozide; they also concluded that the blood flow changes were secondary to the effects of apomorphine on energy metabolism.

A major purpose of the present study was to evaluate the possibility that the augmentation of CBF by neuronal functional activation was mediated by vasoactive actions of dopamine. As previously observed (Ginsberg et al., 1987; Adachi et al., 1994), unilateral vibrissal stimulation markedly raised blood flow in all stations of the pathway on the stimulated side, i.e., the spinal and principal trigeminal nuclei in the brain stem, the VPM nucleus of the thalamus, and the barrel region of the sensory cortex. Blockade of dopamine receptors with doses of haloperidol sufficient to produce profound behavioral effects and to reduce blood flow in some regions of the brain failed to abolish or even to reduce the enhancement of blood flow by functional activation in any of the stations of the pathway. In fact, in the spinal trigeminal nucleus and VPM thalamic nucleus, the high dose of haloperidol statistically significantly increased rather than decreased the percent enhancement of CBF due to vibrissal stimulation. These results, therefore, fail to support a role for dopaminergic mechanisms in the functional activation of local CBF, at least not in the whisker-to-barrel cortex pathway, a representative sensory pathway often used to study the circulatory and metabolic responses to neuronal functional activation.