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J Appl Physiol 98: 1792-1798, 2005. First published December 30, 2004; doi:10.1152/japplphysiol.00690.2004
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Effects of capsaicin and nitric oxide synthase inhibitor on increase in cerebral blood flow induced by sensory and parasympathetic nerve stimulation in the rat

Kazuhide Ayajiki, Hideyuki Fujioka, Kazuya Shinozaki, and Tomio Okamura

Department of Pharmacology, Shiga University of Medical Science, Seta, Otsu, Japan

Submitted 2 July 2004 ; accepted in final form 22 December 2004


   ABSTRACT
TOP
 ABSTRACT
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Effects of electrical stimulation of the nerve bundles including sensory and parasympathetic nerves innervating cerebral arteries on cerebral blood flow (CBF) and mean arterial blood pressure (MABP) were investigated with a laser-Doppler flowmeter and a blood pressure monitoring system in anesthetized rats pretreated with and without capsaicin. The electrode was hooked on the nerve bundles including the distal nasociliary nerve from trigeminal nerve and parasympathetic nerve fibers from sphenopalatine ganglion. In control rats, the nerve stimulation for 30 s increased CBF in the ipsilateral side and MABP. Hexamethonium attenuated the increase in CBF and abolished that in MABP. Under treatment with hexamethonium, NG-nitro-L-arginine (L-NNA, 1 mg/kg) significantly attenuated the stimulation-induced increase in CBF, which was restored by the addition of L-arginine. Although the dose of L-NNA was raised up to 10 mg/kg, the stimulation-induced increase in CBF was not further inhibited and was never abolished. In capsaicin-pretreated rats, magnitudes of the stimulation-induced increases in CBF and MABP were lower than those in control rats. Hexamethonium attenuated the increase in CBF and abolished that in MABP. Under treatment with hexamethonium, L-NNA abolished the stimulation-induced increase in CBF in capsaicin-pretreated rats. In conclusion, nitric oxide released from parasympathetic nerves and neuropeptide(s) released antidromically from sensory nerves may be responsible for the increase in CBF in the rat. The afferent impulses by nerve stimulation may stimulate the trigeminal nerve and lead to the rapid increase in MABP, which partly contributes to the increase in CBF.

arterial blood pressure; electrical nerve stimulation

NEUROGENIC CONTROL of cerebral blood flow (CBF) and cerebrovascular tone has long been investigated. We demonstrated that isolated dog and monkey cerebral arteries relax in response to perivascular nerve stimulation with electrical pulses, and the relaxations were abolished by treatment with nitric oxide (NO) synthase inhibitors and restored by L-arginine (25). Similar findings have been reported in other mammals, including humans (26), pigs (13), cows (3), sheep (14), and rats (11). Histochemical studies have demonstrated the presence of perivascular nerve fibers containing NO synthase immunoreactivity (16). These findings suggest that cerebral arteries are innervated by nitrergic nerves in which NO acts as a neurotransmitter (31). We also reported that electrical stimulation of sphenopalatine ganglion in the anesthetized dog (29) and monkey (30) induces vasodilatation of cerebral arteries, which is abolished by NO synthase inhibitors. The stimulation did not affect systemic arterial blood pressure (29, 30).

In the rat, sensory nerve fibers of the nasociliary nerve, a branch of the trigeminal nerve, together with parasympathetic nerve fibers derived from the sphenopalatine ganglion run into the intracranial space through the ethmoidal foramen as tightly bound nerve bundles, and then both nerves distribute to the cerebral arteries (15, 23). Selective stimulation of either parasympathetic efferent nerve or sensory nerve has been reported to increase CBF without elevating systemic blood pressure, but these findings were obtained in rats preceding surgery to denervate the other type of nerves and another surgery to cut the proximal nasociliary nerve just after branching from the ophthalmic nerve to exclude the sensory transmission to the central nervous system (22, 23).

In the present study, effects of electrical stimulation of the nerve bundles, consisting of sensory and parasympathetic nerves innervating the cerebral artery, on CBF were investigated in anesthetized rats. The effects of the stimulation on systemic blood pressure were also examined because electrical stimulation of the trigeminal nerve is known to increase the arterial blood pressure via sensory transmission to the brain (20), and CBF increases when systemic arterial blood pressure rapidly increases. To study the antidromic transmission of the sensory nerves, experiments were also performed in the rats pretreated with capsaicin, which depletes peptide neurotransmitters of sensory nerves.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
The Animal Care and Use Committee at Shiga University of Medical Science approved the use of rats along with the experimental protocols in this study.

Animals.   A total of 28 adult male Wistar rats (Japan SLC, Hamamatsu, Japan), weighing 300–400 g, were used and divided into three groups. Seven rats were subcutaneously injected with 125 mg/kg of capsaicin dissolved in 80% physiological saline, 10% ethanol, and 10% Tween 80 in 2 days to deplete neuropeptides in the sensory nerves (10). The solvent was injected subcutaneously into eight control rats. Thirteen rats were used for another series of experiments without pretreatment of capsaicin and the solvent.

Antinociceptive test.   Ten days after systemic treatment with capsaicin or the solvent, 5 ml of capsaicin (1 mg/ml) were applied to the right cornea of the seven capsaicin-pretreated and eight solvent-treated rats. The application to the cornea is known to induce protective scratching/wiping movements that lasted for ~1 min (9). The numbers of wiping movements counted for 2 min after application of capsaicin to the cornea were significantly decreased in the capsaicin-pretreated rats (n = 7, 8.0 ± 2.1 times) compared with those in the control rats (n = 8, 98.0 ± 4.0 times; P < 0.001, unpaired t-test).

Anesthesia, presurgical treatments, and arterial blood gas.   After the antinociceptive test, all rats were anesthetized with intraperitoneal injection of carbamic acid ethyl ester (urethane, 1.2 g/kg), and stable anesthetic conditions were maintained by additional injections of urethane as needed. Arterial systolic and diastolic pressures were monitored with a pressure transducer (MP5100, Baxter, Tokyo, Japan) and amplifier (AP641G, Nihon Kohden, Tokyo, Japan) via a catheter inserted into the right femoral artery. The heart rate was monitored by a heart rate counter (AT601G, Nihon Kohden). The femoral vein was catheterized for administration of the drugs. Rats were intratracheally intubated and fixed and then connected to a respirator (SN-480–7, Shinano Seisakusho, Tokyo, Japan). Atropine (1 mg/kg) was intravenously injected to avoid the parasympathetic reflex, and pentazocine (10 mg/kg) was subcutaneously injected to abolish the pain, which is resistant to urethane anesthesia. Then pancronium (1 mg/kg) was intravenously injected to suppress the spontaneous muscle movement, and stable conditions were maintained by additional injections of pancronium as needed.

When the steady state of the rats were obtained, the values of pH, PO2, and PCO2 in the arterial blood sample obtained from the femoral artery were measured by a blood-gas analyzer (280 Blood Gas System, Ciba Corning Diagnostics, Medfield, MA). The values of pH, PO2, and PCO2 in the eight control rats were 7.44 ± 0.01, 99.6 ± 5.0 Torr, and 40.7 ± 1.4 Torr, respectively. The values of pH, PO2, and PCO2 in the arterial blood sample did not significantly differ between control rats and capsaicin-pretreated rats (unpaired t-test).

Surgical procedures and physiological recordings.   A sagittal scalp incision was made near the right upper orbit. Under observation with the microscope (OM-5, Takagi Seiko, Nagano, Japan), intraorbital structures were retracted laterally, and nerve bundles, including nasociliary nerve and parasympathetic nerve fibers from the sphenopalatine ganglion, were separated. A fine, bipolar, concentric stimulating electrode was put under the nerve bundles, which allowed the nerves to be held gently. Anatomical information is illustrated in Fig. 1. At the stimulated portion of the nerve bundles, it is difficult to separate both nerves. The nasociliary nerve contained in this portion is not a proximal but a distal part of the nerve. The electrode was connected to the electronic stimulator (SEN-1101, Nihon Kohden). The nerve bundles were stimulated by 1.0-ms electrical square pulses of 10 V at frequencies of 5, 10, and 20 Hz for a period of 30 s every 3–5 min. To measure CBF of the cortex, a 2.0-cm midline incision was made on the skull to expose the right parietal bones. A burr hole of 2.0-mm diameter was made with a dental drill over the part of the cortex that is irrigated by the right middle cerebral artery, 3 mm lateral from the sagittal suture and 4 mm caudal from the coronal suture. The dura mater was carefully incised to expose the cortical surface of the brain. CBF of the right parietal cortex was continuously monitored by a laser-Doppler flowmeter system (ALF2100, Advance) based on a procedure previously reported (10, 15, 23). The probe of the flow meter (diameter 2.0 mm), carried on an electrode manipulator, was positioned 0.2–0.3 mm above the cortical surface. Flow values are expressed in arbitrary units (perfusion units). The body temperature was kept between 37.0 and 37.5°C with a heating blanket. The CBF, heart rate, and systolic and diastolic blood pressures were simultaneously recorded (RTA-1200, Nihon Kohden). In a separate series of experiments with seven rats, the burr holes were made on both right and left parietal bones to measure the CBF in the right and left cortex, respectively. The nerve stimulation was applied on the right nerve bundles, and the increases in CBF in the right (stimulated side) and left (nonstimulated side) were compared. Other procedures were the same as mentioned above.



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  		Fig. 1. Schematic presentation of electrical stimulation of the nerve bundles including the distal nasociliary nerve and the parasympathetic nerve fibers from sphenopalatine ganglion that enter the ethmoidal foramen of the rat.

 
Experimental protocols.   After stabilization of the responses to the nerve stimulation at 5, 10, and 20 Hz, hexamethonium, a ganglion blockade (10 mg/kg plus a continuous infusion of 0.5 mg·kg–1·min–1), was applied intravenously to the rats pretreated with and without capsaicin. Injection of hexamethonium significantly decreased mean arterial blood pressure (MABP) transiently (from 81.3 ± 2.7 to 70.6 ± 3.1 mmHg, P < 0.001, n = 15, paired t-test). Five to 15 min after the injection of hexamethonium, MABP almost returned to the level before injection of the drug. Even then, hexamethonium successfully abolished the reflex bradycardia in response to norepinephrine. These observations and determination of the effective dose of hexamethonium were made in the preliminary study with nonatropinized rats. In the present study with atropinized rats, the effects of nerve stimulation on CBF, heart rate, and MABP were evaluated 30 min after injection of hexamethonium. Then, NG-nitro-L-arginine (L-NNA; 1 mg/kg) was injected intravenously, and the effect of L-NNA on the responses to nerve stimulation was evaluated 30 min after the injection. After such responses were obtained, L-arginine (100 mg/kg) was injected intravenously. Thirty minutes after injection of L-arginine, the responses to nerve stimulation were obtained. After the responses to nerve stimulation under treatment with L-NNA (1 mg/kg) and L-arginine (100 mg/kg) were obtained, L-NNA (10 mg/kg) was injected intravenously. More than 60 min after the treatment with L-NNA (10 mg/kg) to avoid the remaining effect of L-arginine, the responses to nerve stimulation were obtained.

In a different series of experiments, CBF of the right and left parietal cortex and MABP in response to the unilateral nerve stimulation were measured to compare the MABP-CBF relationship in the stimulated and nonstimulated sides in seven rats. The experiments were performed in the absence of hexamethonium, and the order of CBF measurement (right or left) was randomized. The percent increase in CBF induced by nerve stimulation relative to that before the stimulation is presented. In the conditions without nerve stimulation, effects of the drugs on CBF, heart rate, and MABP are also presented.

In addition, the relationship between increase in MABP induced by rapid intravenous injection of norepinephrine (1 µg/kg) and increase in CBF was examined in six rats.

Statistics.   The results shown in the text, tables, and figures are expressed as mean values ± SE. Statistical analyses were made using the paired and unpaired Student's t-tests for two groups and Tukey's test after one-way ANOVA for three and more groups. Dunnett's test after ANOVA for repeated measurements was used to determine the significance of changes during multiple time-dependent observations.

Drugs used.   Drugs used were L-NNA (Peptide Institute, Osaka, Japan), L-arginine, hexamethonium bromide, capsaicin (Nacalai Tesque, Kyoto, Japan), atropine sulfate (Tanabe, Osaka, Japan), carbamic acid ethyl ester (urethane; Tokyo Kasei Kogyo, Tokyo), pancronium bromide, norepinephrine (Sankyo, Tokyo, Japan), and pentazocine (Yamanouchi Pharmaceutical, Tokyo, Japan).


   RESULTS
TOP
 ABSTRACT
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Effects of pretreated drugs on CBF, heart rate, and MABP in the resting condition.   Injection of hexamethonium produced a significant decrease in CBF and heart rate, but it did not significantly change MABP in the resting condition just before the nerve stimulation was applied (Table 1). Under treatment with hexamethonium, the injection of L-NNA at 1 mg/kg did not significantly change CBF and heart rate, but it significantly increased MABP (Table 1). The additional injection of L-arginine (100 mg/kg) did not significantly affect CBF, heart rate, and MABP compared with those treated with hexamethonium alone (Table 1). The injection of L-NNA at 10 mg/kg significantly increased MABP and decreased heart rate, but it did not change CBF compared with those treated with hexamethonium alone (Table 1).


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  		Table 1. Effects of hexamethonium, L-NNA, and L-arginine on the basal CBF, HR, and MABP in anesthetized rats under treatment with atropine, pentazocine, and pancronium

 
Effects of electrical nerve stimulation on CBF, heart rate, and MABP.   In the control rats, the nerve stimulation induced a marked increase in CBF together with MABP in a frequency-dependent manner (Fig. 2). The values of systolic arterial blood pressure during the nerve stimulations at 5, 10, and 20 Hz were 118.8 ± 2.9, 129.4 ± 4.0, and 132.4 ± 4.6 mmHg, respectively. The heart rate was not significantly changed at all (Fig. 2). The stimulation-induced increase in CBF was significantly attenuated by the treatment with hexamethonium (Fig. 3, left), and the stimulation-induced increase in MABP was almost abolished by the treatment (Fig. 3, right). Injection of L-NNA at 1 mg/kg significantly inhibited the stimulation-induced increase in CBF, and the additional injection of L-arginine (100 mg/kg) reversed the inhibition in the presence of hexamethonium (Fig. 4). However, L-NNA at 10 mg/kg showed the similar inhibition at 1 mg/kg and did not abolish the increase in CBF caused by the nerve stimulation (Fig. 4).



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  		Fig. 2. Typical tracings of changes in cerebral blood flow (CBF; top), heart rate (HR; middle), and blood pressure (BP; bottom) caused by electrical stimulation (5, 10, and 20 Hz) of the nerve bundles in an anesthetized rat. PU, perfusion units that express CBF in arbitrary units. bpm, beats/min.

 


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  		Fig. 3. Frequency-related increase in CBF and mean arterial blood pressure (MABP) for electrical stimulation (5, 10, and 20 Hz) of the nerve bundles in anesthetized rats (control rat) as affected by hexamethonium (10 mg/kg iv). Ordinate on left indicates the percent increase in CBF relative to that before the electrical stimulation. Ordinate on right indicates absolute values in MABP (mmHg) before (none) and during the stimulation. Significantly different from the control, **P < 0.01 (paired t-test). Numbers in parentheses indicate the number of rats. Vertical bars represent SE.

 


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  		Fig. 4. Frequency-related increase in CBF for electrical stimulation (5, 10, and 20 Hz) of the nerve bundles in anesthetized rats (control rat) under treatment with hexamethonium (10 mg/kg iv) as affected by 1 mg/kg of NG-nitro-L-arginine (L-NA1, iv) and additional treatment with L-arginine (L-NA1 + L-Arg, 100 mg/kg iv) and as affected by 10 mg/kg of L-NNA (L-NA10, iv). Ordinate indicates the percent increase in CBF relative to that before the electrical stimulation. Significantly different from control, *P < 0.05 (Tukey's test after 1-way ANOVA). Significantly different from L-NA1, {dagger}P < 0.05. Numbers in parentheses indicate the number of rats. Vertical bars represent SE.

 
In the capsaicin-pretreated rats, the nerve stimulation induced an apparent increase in CBF (Fig. 5, left) and MABP (Fig. 5, right) in a frequency-dependent manner. The values of systolic arterial blood pressure during the nerve stimulation at 5, 10, and 20 Hz were 110.4 ± 3.5, 113.3 ± 3.0, and 114.3 ± 3.2 mmHg (n = 7), respectively. The increase in either CBF or MABP in the capsaicin-pretreated rats was significantly lower than that in the control rats (Figs. 3 and 5). The stimulation-induced increase in CBF was moderately attenuated by treatment with hexamethonium (Fig. 5, left), whereas the increase in MABP was abolished by hexamethonium (Fig. 5, right). Injection of L-NNA (1 mg/kg) significantly inhibited the stimulation-induced increase in CBF, and an additional injection of L-arginine (100 mg/kg) reversed the inhibition (Fig. 6). L-NNA (10 mg/kg) abolished the stimulation-induced increase in CBF (Fig. 6).



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  		Fig. 5. Frequency-related increase in CBF and MABP for electrical stimulation (5, 10, and 20 Hz) of the nerve bundles of anesthetized capsaicin-pretreated rats as affected by hexamethonium (10 mg/kg iv). Ordinate on left indicates the percent increase in CBF relative to that before the electrical stimulation. Ordinate on right indicates absolute values in MABP (mmHg) before (none) and during the stimulation. Significantly different from the control, *P < 0.05 and **P < 0.01 (paired t-test). Numbers in parentheses indicate the number of rats. Vertical bars represent SE.

 


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  		Fig. 6. Frequency-related increase in CBF for electrical stimulation (5, 10, and 20 Hz) of the nerve bundles of anesthetized capsaicin-pretreated rats under treatment with hexamethonium (10 mg/kg iv) as affected by 1 mg/kg of L-NNA (L-NA1 iv) and additional treatment with L-arginine (l-NA1 + L-Arg, 100 mg/kg iv) and as affected by 10 mg/kg of L-NNA (L-NA10 iv). Ordinate indicates the percent increase in CBF relative to that before the electrical stimulation. Significantly different from control, **P < 0.01 (Tukey's test after 1-way ANOVA). Significantly different from L-NA1, {dagger}P < 0.05 and {dagger}{dagger}P < 0.01. Numbers in parentheses indicate the number of rats. Vertical bars represent SE.

 
In the rats without treatment of hexamethonium, increase in CBF induced by electrical nerve stimulation of the right nerve bundles was significantly lower in the left parietal cortex compared with that in the right (Table 2). The values of increase in MABP during the nerve stimulations at 5, 10, and 20 Hz were 5.6 ± 0.7, 14.5 ± 1.1, and 15.5 ± 1.9 mmHg, respectively. These values did not significantly differ between the cases measuring CBF in the right and left parietal cortex.


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  		Table 2. Comparison of increase in CBF of the right and left parietal cortex induced by electrical nerve stimulation of the right nerve bundles consisting of distal part of the nasociliary nerve and the postganglionic nerve fibers from sphenopalatine ganglion in the rats

 
Intravenous injection of norepinephrine (1 µg/kg) induced increase in MABP together with percent increase in CBF within 60 s. The values of increased MABP at 15, 30, and 60 s after the injection were 12.3 ± 1.5, 9.7 ± 1.6, and 4.7 ± 1.6 mmHg, respectively. The values of percent increase in CBF at 15, 30, and 60 s after the injection were 6.7 ± 3.5, 8.2 ± 5.1, and 5.9 ± 3.6, respectively.


   DISCUSSION
TOP
 ABSTRACT
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
The present study revealed that electrical stimulation of the nerve bundles consisting of nasociliary nerve and parasympathetic nerve fibers from sphenopalatine ganglion, which enter the ethmoidal foramen, increased the ipsilateral CBF in the parietal cortical area together with an elevation of MABP in anesthetized rats. Under treatment with hexamethonium, which abolished the MABP elevation, the stimulation-induced increase in CBF was partially inhibited by treatment with intravenous administration of L-NNA at 1 mg/kg and was restored by L-arginine, indicating that the nerve stimulation-induced increase of the CBF unrelated to the MABP elevation is partly due to NO liberated from postganglionic parasympathetic nerves as seen in the previous studies (15). Although the dose of L-NNA was raised up to 10 mg/kg, the stimulation-induced increase in CBF was not further inhibited and was not abolished. The presence of NO synthase in the nerve fibers innervating rat cerebral arteries originated from the ganglion has been reported (16). Although presence of immunoreactive choline acetyltransferase in the parasympathetic nerves from sphenopalatine ganglion innervating cerebral arteries has also been reported (24), cholinergic function cannot be involved in this response, because the rats were treated with atropine in the present study, and scopolamine has been reported to fail to affect the stimulation-induced increase in CBF of rats (23).

Capsaicin has been demonstrated to deplete substance P (6) and calcitonin gene-related peptide (CGRP) (19) in the nerves innervating rodent cerebral arteries. In the present study, pretreatment with capsaicin significantly reduced the response to nociceptive stimuli, suggesting that the function of the sensory nerve was strongly suppressed. In the capsaicin-pretreated rats, both increases in CBF and MABP caused by the nerve stimulation were smaller compared with those in the control rats. Therefore, the function of sensory nerves sensitive to capsaicin contributes to the increases in CBF and MABP. Treatment with hexamethonium abolished the increase in MABP and partially inhibited the increase in CBF caused by the nerve stimulation in capsaicin-pretreated rats. The increase of CBF in the presence of hexamethonium was almost abolished by treatment with L-NNA and was restored by L-arginine. Therefore, in the capsaicin-pretreated rats treated with hexamethonium, the increase in CBF induced by the nerve stimulation is due solely to NO liberated from parasympathetic nerves originated from sphenopalatine ganglion. This vasodilator function of nitrergic nerves originating from the ganglion has been reported by measuring CBF in the rat (15) and by measuring cerebroarterial diameters in the dog (29) and monkey (30). Furthermore, unilateral denervation of parasympathetic nerves from the ganglion produced instant contraction of the ipsilateral cerebral artery (29) and has exacerbated the infarction caused by ipsilateral occlusion of the middle cerebral artery in rats (12), indicating the tonic vasodilator control by nitrergic nerve in the cerebral artery and its protective role against cerebral infarction. Electrical stimulation of the parasympathetic nerves cannot be considered to directly affect the brain function because the afferent impulses, if any, of the nerves never reach the preganglionic nerves.

Efferent impulses to the cerebroarterial wall and afferent impulses to central nervous system via the trigeminal nerve may be caused by electrical stimulation of the nasociliary nerve. Increase in MABP caused by the afferent impulses, which may affect CBF, was abolished by hexamethonium. Thus the possible effect of afferent impulses on increase in CBF may be negligible in control rats treated with hexamethonium. Furthermore, the present study demonstrated that the nerve stimulation-induced increase in CBF was abolished by pretreatment with capsaicin in the rats treated with hexamethonium and L-NNA. Therefore, under treatment with hexamethonium, the nerve stimulation-induced increase in CBF resistant to L-NNA in control rats may be caused by neuropeptides released antidromically from the branches of nasociliary nerves innervating cerebral arteries. Suzuki et al. (21) reported that the branches from nasociliary nerves innervating cerebral arteries contain substance P and CGRP. On the other hand, isolated rat intracerebral arterioles did not respond to substance P, but to CGRP, with relaxation, and the CGRP-induced relaxation was suppressed by treatment with CGRP-(8–37), the receptor antagonist of CGRP (7). Therefore, CGRP can be vasodilating transmitters released from the sensory nerves in the rat. Recently, it has been reported that transcorneal stimulation of trigeminal nerve endings induces cerebrovasodilation and the increase in CBF, suggesting that the stimulation may improve the cerebral ischemic conditions such as cerebrovasospasm (2). Thus the nasociliary nerve may also play a protective role against cerebral ischemia as postulated in the nitrergic nerve from the sphenopalatine ganglion (12).

Afferent impulses produced by electrical stimulation applied to sensory nerves may stimulate the trigeminal nerve and lead to the increase in MABP together with the increase in CBF. The neuronal pathway leading to the increase in MABP may include the sensory neurons containing substance P and CGRP, and the neurotransmission may be mediated via nicotinic receptors because the stimulation-induced increase in MABP was markedly inhibited by capsaicin and was abolished by hexamethonium. Stimulation of the trigeminal nerve ganglion induced the increase in MABP, which was abolished by treatment with guanethidine in the anesthetized cat, indicating that the increase in MABP may be due to sympathetic nerve stimulation (18). The pathway from the nerve stimulation to the increase in systemic blood pressure probably involves brain stem reflexes that were activated through connections among the trigeminal afferents, the facial nuclei, and the vasomotor center (1, 8).

Electrical stimulation of sensory nerves rapidly elevated MABP in control rats. Because smaller but significant increase in CBF was observed in the contralateral side during the unilateral nerve stimulation in the rats without treatment with hexamethonium, the increase in CBF through the afferent impulses from sensory nerves may be due to the increase in systemic blood pressure. In the capsaicin-pretreated rats, the nerve stimulation-induced increase in CBF was significantly inhibited by treatment with hexamethonium together with abolishment of increase in MABP. CBF is known to be autoregulated; thus the increase in MABP is not generally considered to directly correlate to the increase in CBF (5). However, when MABP rapidly increased, CBF passively followed MABP and no regulatory change in CBF could be recognized (4). In the present study, intravenous injection of norepinephrine (1 µg/kg) rapidly produced increase in CBF even if the increase in MABP was ~10 mmHg. Therefore, rapid increase in MABP induced by the nerve stimulation within 30 s may elevate CBF.

Injection of hexamethonium significantly decreased CBF without the significant change in MABP in the resting condition. It has been reported that intravenous injection of hexamethonium significantly constricted the cerebral artery in the anesthetized dog under treatment with phentolamine, suggesting that tonic vasodilator innervation may be present in the cerebral arterial wall (27). Therefore, the hexamethonium-induced decrease in CBF independent of MABP may be due to the vasoconstriction of cerebral arteries via inhibition of vasodilator nerve function. Cerebral arteries are reciprocally regulated by vasodilating nitrergic nerves and by vasoconstricting noradrenergic nerves, and tonic control of CBF may be predominantly regulated by vasodilator nerves (31).

The injection of L-NNA increased MABP in the rats under treatment with hexamethonium. It has been reported that the increase in blood pressure induced by intravenous injection of NO synthase inhibitors is not influenced by hexamethonium in rats (17), in contrast to those in dogs (28). Therefore, systemic blood pressure may be continuously suppressed by NO derived from endothelium but not from nitrergic nerves in the resistant arteries of rats.

In conclusion, postganglionic parasympathetic neurons physiologically contribute to the increase in CBF by releasing NO in the rat. Sensory neurons also contribute to the CBF increase by antidromic release of CGRP on stimulation. When nasociliary nerves are stimulated, the afferent impulses through the trigeminal nerve may lead to the rapid increase in systemic blood pressure via a central nervous system and in CBF.


   FOOTNOTES
 

Address for reprint requests and other correspondence: T. Okamura, Dept. of Pharmacology, Shiga Univ. of Medical Science, Seta, Otsu 520-2192, Japan (E-mail: okamura{at}belle.shiga-med.ac.jp)

The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.


   REFERENCES
TOP
 ABSTRACT
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 

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