Neuroscience Letters, 120 (1990) 55-57 Elsevier ScientificPublishers Ireland Ltd.
55
NSL 07319
Responses of cortical cerebral blood flow produced by stimulation of cervical sympathetic trunks are well maintained in aged rats A. H e r v o n e n * , A. K i m u r a , A. S a t o a n d Y. S a t o * * Department of Physiology, Tokyo Metropolitan Institute of Gerontology, Tokyo (Japan)
(Received2 August 1990;Accepted 6 August 1990) Key words: Aging;Cervicalsympathetictrunk; Cortical cerebral blood flow;Vasodilatation; Vasoconstriction
Responses of cortical cerebral blood flow (CBF) in the parietal cortex followingelectricalstimulation of cervicalsympathetictrunks (CSTs) were compared in healthy adult (4-6 months old) and healthy aged (28-32 months old) Fischer-344rats anesthetizedwith urethane. The cortical CBF was continuously monitored using laser Doppler fiowmetry(LDF). The electrical stimulation of CSTs with supramaximum intensity caused the frequency-dependentinitial-increaseand late-decreaseresponses of cortical CBF in both adult and aged rats. There were no significantdifferences in magnitude and time course of the responsesof the cortical CBF with the comparable frequenciesof the electricalstimulation of the CSTs between the adult and the aged rats. These evidencessuggest that the neural control of the cortical CBF by the sympatheticnerves is quite well maintained in aged rats.
Recently, we demonstrated that electrical stimulation of cervical sympathetic trunks (CSTs) at supramaximum intensity with lower frequency (1-10 Hz) produces an increase in the local cortical cerebral blood flow (CBF) during stimulation and a slight decrease after the end of stimulation, while stimulation of the same nerves at higher frequency (20-30 Hz) produces a short-term increase followed by a dominant long-lasting decrease in CBF in adult anesthetized rats [11]. The initial-increase and late-decrease responses of cortical CBF induced by CSTs stimulation have been shown to be produced by the activation o f the r - a n d ~-adrenergic receptors, respectively, and both of the increase- and decrease-responses were considered to be due to vasodilatation and vasoconstriction of cortical microvasculatures, respectively [11]. The CBF has a tendency to decline in aged people [2, 3], and it seems important to determine whether or not the sympathetic neural regulation of the cerebral blood vessels decline with age. The present experiment was undertaken to compare the responses o f cortical CBF to CSTs stimulation in adult and aged rats by continuously monitoring the cor-
*Visiting professor and present address: Section of Gerontology, Universityof Tampere, Box 607, SF-33101Tampere, Finland. **Present address: Division of Physiology,Tsukuba Collegeof Technology,4-3 Amakubo, Tsukuba 305,Japan. Correspondence: A. Sato, Department of Physiology,Tokyo Metropolitan Institute of Gerontology, 35-2 Sakaecho, Itabashiku, Tokyo 173, Japan. 0304-3940/90/$03.50 © 1990ElsevierScientificPublishers Ireland Ltd.
tical CBF using laser Doppler flowmetry (LDF) under the anesthetized condition. The experiments were performed on 6 healthy adult (4-6 months old) and 6 healthy aged (28-32 months old) Fischer-344 male rats anesthetized with urethane (initial dose 1.1 g/kg, i.p., and supplemented later during experiment by further i.v. administration of 0.2-0.3 g/kg). Each animal's trachea was cannulated and respiration was maintained with a respirator (Harvard pump 683, U.S.A.) after immobilization with gallamine triethiodide (initial dose 20 mg/kg i.v., and supplemented during experiment when necessary). The end-tidal CO2 levels, monitored by means of a gas monitor (1H26, NEC Sanei, Japan), were adjusted at around 4.0% by changing the frequency and volume of the respirator. The rectal temperature was maintained between 37.0 and 38.0°C with a DC heating pad and an infrared lamp. Systemic arterial blood pressure was recorded continuously through a polyethylene catheter inserted in a femoral artery, and systolic blood pressure was maintained above 100 mmHg by intravenous administration of Ficoll solution (Pharmacia Fine Chemicals AB, Sweden) through the femoral vein when necessary. The animals were placed in a prone position and their heads were fixed in a stereotaxic head-holder instrument (SR-5, Narishige, Japan). CSTs were bilaterally isolated, and both nerves were cut at the level of the neck. The peripheral cut end of both nerves was placed on bipolar stimulating electrodes. The dissected nerves were kept in a liquid paraffin pool to prevent them from drying. The nerves were electri-
56
cally stimulated by square pulse waves. Electrical stimulation with parameters of 0.5 ms duration, 10 V intensity, and varying frequencies was delivered to CSTs for a period of 1 min. The skull, which was approximately 20 mm 2 in area, was opened, and the dura mater was kept intact. The recording probe (diameter of 0.8 mm) of the laser Doppler flowmeter (ALF2100, Advance, Japan) was placed on a surface of the dura mater in a parietal cortex. The probe was carefully positioned so as to avoid visible pial vessels under a binocular microscope. With this system, cortical CBF was continuously and quantitatively measured. The area for measurement of CBF by this instrument was about ! mm below the probe. It has been demonstrated that there was a significant correlation between the two values of local cortical CBF measured by LDF and the hydrogen clearance method [11]. CBF expressed by electrical signals in mV from the LDF was continuously recorded on a polygraph (NihonKohden, Japan). The CBF response was expressed as the percentage of the basal CBF averaged for 30 s just prior to the onset of stimulation. The statistical significance of the difference of two responses in the adult and aged rats was tested by Student's t-test. Cortical CBF under the resting condition without CSTs stimulation was not significantly different in both adult and aged rats (379 ___55 mV in the adult vs 322 + 38 mV in the aged, mean + S.E.M.). This result is in agreement with the previous finding that regional CBF in
A
adult
B
frontal lobe under the resting condition is not significantly different in 12, 24 and 34 months old conscious Fischer-344 rats when measured using the [14C]iodoantipyrine method [9]. In addition, we have confirmed, using the [14C]iodoantipyrine method, that cortical CBF in healthy adult rats anesthetized with halothane (1.0%) was identical to that in aged rats (102+ 8 ml/100 g/rain in 6 adult Wistar male rats of 3-5 months old and 112_+ 15 ml/100 g/min in 6 aged Wister male rats of 2425 months old, unpublished data). Fig. 1 summarizes cortical CBF responses produced by electrical stimulation of bilateral CSTs with the supramaximum intensity and various frequencies as indicated. Fig. IA and B show sample records from one adult (A) and one aged (B) rat, and Fig. 1C is a summary of responses presented in percentages from 6 adult and 6 aged rats. The results for the adult rats have been published [11]. Electrical stimulation of CSTs with supramaximum intensity produced a frequency-dependent response both in the adult and aged rats. Stimulation with lower frequency between 1 and 10 Hz produced an increase in CBF during stimulation and a slight decrease for about 30 s after the end of stimulation. Stimulation with higher frequency at 20 and 30 Hz produced a shortterm initial-increase, which was followed by a dominant long-lasting late-decrease in the cortical CBF. Electrical stimulation of CSTs produced little change in systemic blood pressure: in some cases, however, a slight increase in blood pressure of about 10 mmHg in both adult and
aged
C ' ~
'Hz]
/
2
10 ~
]
20 ~
mV
30 Hz
"~-,-
1300 =260
lOV, lmin 0
30
60
90 120s
Fig. 1. Cortical CBF responses produced by electrical stimulations of bilateral CSTs for I min (shown by horizontal bars) at a 10 V intensity with a 0.5 ms pulse duration at varying frequencies from 0.5 to 30 Hz. A, B: example recordings of CBF responses obtained from one adult (A) and from one aged (B) rats. C: summarized responses of CBF for 6 adult (closed circles) and 6 aged (open circles) rats. The time course of the CBF response was calculated every 5 s and expressed as the percentage of the basal CBF for 30 s prior to the onset of stimulation. The dashed vertical lines and solid underbars indicate the time at which the stimulation was delivered to bilateral CSTs. Each point and vertical bar shows the mean + S.E.M. (data in adult rats from our previous report [I 1]).
57
aged rats was observed (not shown in Fig. 1). The maximum initial-increased responses in CBF with stimulus frequency of 20 Hz reached 113 + 2 % and 117 + 3 ~ of the resting cortical CBF 15 s after the start of stimulation in both adult and aged rats, respectively. The maximum late-decreased responses reached 87 + 2% (in the adult) and 85 + 5 % (in the aged) about 30-50 s after the cessation of stimulation at 30 Hz. There were no significant differences in magnitude and time course of the responses of the cortical CBF with the comparable frequencies of the electrical stimulation of the CSTs between the healthy adult and the aged rats. Responsiveness of cardiovascular function to catecholamine has been suggested generally to decrease with age. For example, the contractile effect of norepinephrine on the aorta is much diminished in aged rats than in adult rats [12]. Further, there is a significant attenuation of ~agonist (phenylephrine)-induced pressor responses in the aged group [7]. The maximum contractile response of isolated cardiac muscle to fl-adrenergic stimulation is much diminished in senescent rats than in the adult [4, 8]. On the contrary, it was noted that a part of a neurotransmitter synthesis and its release in the superior cervical ganglion could be enhanced in aged rats, because activity of tyrosine hydroxylase was significantly higher in aged rats than in adult ones [10]. The present results demonstrate that the vasodilative and vasoconstrictive effect of stimulation of the CST with various stimulus frequencies on cerebral cortical blood vessels are well maintained in aged rats just as in adult rats. The results appear to indicate that the total function of neuromuscular transmission including a release of norepinephrine from the postganglionic sympathetic nerve terminals, and responsiveness of both flreceptors for vasodilatations and the ~-receptors for vasoconstrictions of cerebral blood vessels, is well maintained in the aged rats. It has been reported that the cholinergic fibers originating in the nucleus basalis of Meynert play an important role for vasodilatation of the cortical CBF in rats" [1, 5], and also that this cholinergic vasodilative mechanism was proved to be well maintained in aged rats 2728 months old [6]. These results are'in accord with the present finding of the well-maintained responses of the cortical CBF by electrical stimulation of CSTs in aged rats. These evidences suggest that the neural controls of the cortical CBF by the sympathetic nerves and intracra-
nial cholinergic nerves are quite well maintained in aged rats. The authors are grateful for the technical assistance by Miss Harumi Hotta. This work was entrusted by the Science and Technology Agency, using the Special Coordination Funds for Promoting Science and Technology, and also supported by a research grant from the Ministry of Health and Welfare, Japan.
1 Biesold, D., Inanami, O., Sato, A. and Sato, Y., Stimulation of the nucleus basalis of Meynert increases cerebral cortical blood flow in rats, Neurosci. Lett., 98 (1989) 39-44. 2 Davis, S.M., Ackerman, R.H., Correia, J.A., Alpert, N.M., Chang, J., Buonanno, F., Kelley, R.E., Rosner, B. and Taveras, J.M., Cerebral blood flow and cerebrovascular CO2 reactivity in stroke-age normal controls, Neurology, 33 (1983) 391-399. 3 Frackowiak, R.S.J., Lenzi, G.-L., Jones, T. and Heather, J.D., Quantitative measurement of regional cerebral blood flow and oxygen metabolism in man using 150 and positron emission tomography: Theory, procedure, and normal values, J. Comput. Assist. Tomogr., 4 (1980) 727 736. 4 Guarnieri, T., Filburn, C.R., Zitnik, G., Roth, G.S. and Lakatta, E.G., Contractile and biochemical correlates offl-adrenergic stimulation of the aged heart, Am. J. Physiol., 239 (1980) H501-508. 5 Kurosawa, M., Sato, A. and Sato, Y., Stimulation of the nucleus basalis of Meynert increases acetylcholine release in the cerebral cortex in rats, Neurosci. Lett., 98 (1989) 45-50. 6 Kurosawa, M., Sato, A. and Sato, Y., Well-maintained responses of acetylcholine release and blood flow in the cerebral cortex to focal electrical stimulation of the nucleus basalis of Meynert in aged rats, Neurosci. Lett., 100 (1989) 198-202. 7 Kurosawa, M., Sato, A., Sato, Y. and Suzuki, H., Undiminished reflex responses of adrenal sympathetic nerve activity to stimulation of baroreceptors and cutaneous mechanoreceptors in aged rats, Neurosci. Lett., 77 (1987) 193-198. 8 Lakatta, E.G., Gerstenblith, G., Angell, C.S., Shock, N.W. and Weisfeldt, M.L., Diminished inotropic response of aged myocardium to catecholamines, Circ. Res., 36 (1975) 262-269. 9 0 h a t a , M., Sundaram, U., Fredericks, W.R., London, E.D. and Rapoport, S.I., Regional cerebral blood flow during development and ageing of the rat brain, Brain, 104 (1981) 319-332. 10 Partanen, M., Waller, S.B., London, E.D. and Hervonen, A., Indices of neurotransmitter synthesis and release in aging sympathetic nervous system, Neurobiol. Aging, 6 (1985) 227-232. 11 Saeki, Y., Sato, A., Sato, Y. and Trzebski, A., Effects of stimulation of cervical sympathetic trunks with various frequencies on the local cortical cerebral blood flow measured by laser Doppler flowmetry in the rat, Jpn. J. Physiol., 40 (1990) 15-32. 12 Tuttle, R.S., Age-related changes in the sensitivity of rat aortic strips to norepinephrine and associated chemical and structural alterations, J. Gerontol., 21 (1966) 510-516.
55
NSL 07319
Responses of cortical cerebral blood flow produced by stimulation of cervical sympathetic trunks are well maintained in aged rats A. H e r v o n e n * , A. K i m u r a , A. S a t o a n d Y. S a t o * * Department of Physiology, Tokyo Metropolitan Institute of Gerontology, Tokyo (Japan)
(Received2 August 1990;Accepted 6 August 1990) Key words: Aging;Cervicalsympathetictrunk; Cortical cerebral blood flow;Vasodilatation; Vasoconstriction
Responses of cortical cerebral blood flow (CBF) in the parietal cortex followingelectricalstimulation of cervicalsympathetictrunks (CSTs) were compared in healthy adult (4-6 months old) and healthy aged (28-32 months old) Fischer-344rats anesthetizedwith urethane. The cortical CBF was continuously monitored using laser Doppler fiowmetry(LDF). The electrical stimulation of CSTs with supramaximum intensity caused the frequency-dependentinitial-increaseand late-decreaseresponses of cortical CBF in both adult and aged rats. There were no significantdifferences in magnitude and time course of the responsesof the cortical CBF with the comparable frequenciesof the electricalstimulation of the CSTs between the adult and the aged rats. These evidencessuggest that the neural control of the cortical CBF by the sympatheticnerves is quite well maintained in aged rats.
Recently, we demonstrated that electrical stimulation of cervical sympathetic trunks (CSTs) at supramaximum intensity with lower frequency (1-10 Hz) produces an increase in the local cortical cerebral blood flow (CBF) during stimulation and a slight decrease after the end of stimulation, while stimulation of the same nerves at higher frequency (20-30 Hz) produces a short-term increase followed by a dominant long-lasting decrease in CBF in adult anesthetized rats [11]. The initial-increase and late-decrease responses of cortical CBF induced by CSTs stimulation have been shown to be produced by the activation o f the r - a n d ~-adrenergic receptors, respectively, and both of the increase- and decrease-responses were considered to be due to vasodilatation and vasoconstriction of cortical microvasculatures, respectively [11]. The CBF has a tendency to decline in aged people [2, 3], and it seems important to determine whether or not the sympathetic neural regulation of the cerebral blood vessels decline with age. The present experiment was undertaken to compare the responses o f cortical CBF to CSTs stimulation in adult and aged rats by continuously monitoring the cor-
*Visiting professor and present address: Section of Gerontology, Universityof Tampere, Box 607, SF-33101Tampere, Finland. **Present address: Division of Physiology,Tsukuba Collegeof Technology,4-3 Amakubo, Tsukuba 305,Japan. Correspondence: A. Sato, Department of Physiology,Tokyo Metropolitan Institute of Gerontology, 35-2 Sakaecho, Itabashiku, Tokyo 173, Japan. 0304-3940/90/$03.50 © 1990ElsevierScientificPublishers Ireland Ltd.
tical CBF using laser Doppler flowmetry (LDF) under the anesthetized condition. The experiments were performed on 6 healthy adult (4-6 months old) and 6 healthy aged (28-32 months old) Fischer-344 male rats anesthetized with urethane (initial dose 1.1 g/kg, i.p., and supplemented later during experiment by further i.v. administration of 0.2-0.3 g/kg). Each animal's trachea was cannulated and respiration was maintained with a respirator (Harvard pump 683, U.S.A.) after immobilization with gallamine triethiodide (initial dose 20 mg/kg i.v., and supplemented during experiment when necessary). The end-tidal CO2 levels, monitored by means of a gas monitor (1H26, NEC Sanei, Japan), were adjusted at around 4.0% by changing the frequency and volume of the respirator. The rectal temperature was maintained between 37.0 and 38.0°C with a DC heating pad and an infrared lamp. Systemic arterial blood pressure was recorded continuously through a polyethylene catheter inserted in a femoral artery, and systolic blood pressure was maintained above 100 mmHg by intravenous administration of Ficoll solution (Pharmacia Fine Chemicals AB, Sweden) through the femoral vein when necessary. The animals were placed in a prone position and their heads were fixed in a stereotaxic head-holder instrument (SR-5, Narishige, Japan). CSTs were bilaterally isolated, and both nerves were cut at the level of the neck. The peripheral cut end of both nerves was placed on bipolar stimulating electrodes. The dissected nerves were kept in a liquid paraffin pool to prevent them from drying. The nerves were electri-
56
cally stimulated by square pulse waves. Electrical stimulation with parameters of 0.5 ms duration, 10 V intensity, and varying frequencies was delivered to CSTs for a period of 1 min. The skull, which was approximately 20 mm 2 in area, was opened, and the dura mater was kept intact. The recording probe (diameter of 0.8 mm) of the laser Doppler flowmeter (ALF2100, Advance, Japan) was placed on a surface of the dura mater in a parietal cortex. The probe was carefully positioned so as to avoid visible pial vessels under a binocular microscope. With this system, cortical CBF was continuously and quantitatively measured. The area for measurement of CBF by this instrument was about ! mm below the probe. It has been demonstrated that there was a significant correlation between the two values of local cortical CBF measured by LDF and the hydrogen clearance method [11]. CBF expressed by electrical signals in mV from the LDF was continuously recorded on a polygraph (NihonKohden, Japan). The CBF response was expressed as the percentage of the basal CBF averaged for 30 s just prior to the onset of stimulation. The statistical significance of the difference of two responses in the adult and aged rats was tested by Student's t-test. Cortical CBF under the resting condition without CSTs stimulation was not significantly different in both adult and aged rats (379 ___55 mV in the adult vs 322 + 38 mV in the aged, mean + S.E.M.). This result is in agreement with the previous finding that regional CBF in
A
adult
B
frontal lobe under the resting condition is not significantly different in 12, 24 and 34 months old conscious Fischer-344 rats when measured using the [14C]iodoantipyrine method [9]. In addition, we have confirmed, using the [14C]iodoantipyrine method, that cortical CBF in healthy adult rats anesthetized with halothane (1.0%) was identical to that in aged rats (102+ 8 ml/100 g/rain in 6 adult Wistar male rats of 3-5 months old and 112_+ 15 ml/100 g/min in 6 aged Wister male rats of 2425 months old, unpublished data). Fig. 1 summarizes cortical CBF responses produced by electrical stimulation of bilateral CSTs with the supramaximum intensity and various frequencies as indicated. Fig. IA and B show sample records from one adult (A) and one aged (B) rat, and Fig. 1C is a summary of responses presented in percentages from 6 adult and 6 aged rats. The results for the adult rats have been published [11]. Electrical stimulation of CSTs with supramaximum intensity produced a frequency-dependent response both in the adult and aged rats. Stimulation with lower frequency between 1 and 10 Hz produced an increase in CBF during stimulation and a slight decrease for about 30 s after the end of stimulation. Stimulation with higher frequency at 20 and 30 Hz produced a shortterm initial-increase, which was followed by a dominant long-lasting late-decrease in the cortical CBF. Electrical stimulation of CSTs produced little change in systemic blood pressure: in some cases, however, a slight increase in blood pressure of about 10 mmHg in both adult and
aged
C ' ~
'Hz]
/
2
10 ~
]
20 ~
mV
30 Hz
"~-,-
1300 =260
lOV, lmin 0
30
60
90 120s
Fig. 1. Cortical CBF responses produced by electrical stimulations of bilateral CSTs for I min (shown by horizontal bars) at a 10 V intensity with a 0.5 ms pulse duration at varying frequencies from 0.5 to 30 Hz. A, B: example recordings of CBF responses obtained from one adult (A) and from one aged (B) rats. C: summarized responses of CBF for 6 adult (closed circles) and 6 aged (open circles) rats. The time course of the CBF response was calculated every 5 s and expressed as the percentage of the basal CBF for 30 s prior to the onset of stimulation. The dashed vertical lines and solid underbars indicate the time at which the stimulation was delivered to bilateral CSTs. Each point and vertical bar shows the mean + S.E.M. (data in adult rats from our previous report [I 1]).
57
aged rats was observed (not shown in Fig. 1). The maximum initial-increased responses in CBF with stimulus frequency of 20 Hz reached 113 + 2 % and 117 + 3 ~ of the resting cortical CBF 15 s after the start of stimulation in both adult and aged rats, respectively. The maximum late-decreased responses reached 87 + 2% (in the adult) and 85 + 5 % (in the aged) about 30-50 s after the cessation of stimulation at 30 Hz. There were no significant differences in magnitude and time course of the responses of the cortical CBF with the comparable frequencies of the electrical stimulation of the CSTs between the healthy adult and the aged rats. Responsiveness of cardiovascular function to catecholamine has been suggested generally to decrease with age. For example, the contractile effect of norepinephrine on the aorta is much diminished in aged rats than in adult rats [12]. Further, there is a significant attenuation of ~agonist (phenylephrine)-induced pressor responses in the aged group [7]. The maximum contractile response of isolated cardiac muscle to fl-adrenergic stimulation is much diminished in senescent rats than in the adult [4, 8]. On the contrary, it was noted that a part of a neurotransmitter synthesis and its release in the superior cervical ganglion could be enhanced in aged rats, because activity of tyrosine hydroxylase was significantly higher in aged rats than in adult ones [10]. The present results demonstrate that the vasodilative and vasoconstrictive effect of stimulation of the CST with various stimulus frequencies on cerebral cortical blood vessels are well maintained in aged rats just as in adult rats. The results appear to indicate that the total function of neuromuscular transmission including a release of norepinephrine from the postganglionic sympathetic nerve terminals, and responsiveness of both flreceptors for vasodilatations and the ~-receptors for vasoconstrictions of cerebral blood vessels, is well maintained in the aged rats. It has been reported that the cholinergic fibers originating in the nucleus basalis of Meynert play an important role for vasodilatation of the cortical CBF in rats" [1, 5], and also that this cholinergic vasodilative mechanism was proved to be well maintained in aged rats 2728 months old [6]. These results are'in accord with the present finding of the well-maintained responses of the cortical CBF by electrical stimulation of CSTs in aged rats. These evidences suggest that the neural controls of the cortical CBF by the sympathetic nerves and intracra-
nial cholinergic nerves are quite well maintained in aged rats. The authors are grateful for the technical assistance by Miss Harumi Hotta. This work was entrusted by the Science and Technology Agency, using the Special Coordination Funds for Promoting Science and Technology, and also supported by a research grant from the Ministry of Health and Welfare, Japan.
1 Biesold, D., Inanami, O., Sato, A. and Sato, Y., Stimulation of the nucleus basalis of Meynert increases cerebral cortical blood flow in rats, Neurosci. Lett., 98 (1989) 39-44. 2 Davis, S.M., Ackerman, R.H., Correia, J.A., Alpert, N.M., Chang, J., Buonanno, F., Kelley, R.E., Rosner, B. and Taveras, J.M., Cerebral blood flow and cerebrovascular CO2 reactivity in stroke-age normal controls, Neurology, 33 (1983) 391-399. 3 Frackowiak, R.S.J., Lenzi, G.-L., Jones, T. and Heather, J.D., Quantitative measurement of regional cerebral blood flow and oxygen metabolism in man using 150 and positron emission tomography: Theory, procedure, and normal values, J. Comput. Assist. Tomogr., 4 (1980) 727 736. 4 Guarnieri, T., Filburn, C.R., Zitnik, G., Roth, G.S. and Lakatta, E.G., Contractile and biochemical correlates offl-adrenergic stimulation of the aged heart, Am. J. Physiol., 239 (1980) H501-508. 5 Kurosawa, M., Sato, A. and Sato, Y., Stimulation of the nucleus basalis of Meynert increases acetylcholine release in the cerebral cortex in rats, Neurosci. Lett., 98 (1989) 45-50. 6 Kurosawa, M., Sato, A. and Sato, Y., Well-maintained responses of acetylcholine release and blood flow in the cerebral cortex to focal electrical stimulation of the nucleus basalis of Meynert in aged rats, Neurosci. Lett., 100 (1989) 198-202. 7 Kurosawa, M., Sato, A., Sato, Y. and Suzuki, H., Undiminished reflex responses of adrenal sympathetic nerve activity to stimulation of baroreceptors and cutaneous mechanoreceptors in aged rats, Neurosci. Lett., 77 (1987) 193-198. 8 Lakatta, E.G., Gerstenblith, G., Angell, C.S., Shock, N.W. and Weisfeldt, M.L., Diminished inotropic response of aged myocardium to catecholamines, Circ. Res., 36 (1975) 262-269. 9 0 h a t a , M., Sundaram, U., Fredericks, W.R., London, E.D. and Rapoport, S.I., Regional cerebral blood flow during development and ageing of the rat brain, Brain, 104 (1981) 319-332. 10 Partanen, M., Waller, S.B., London, E.D. and Hervonen, A., Indices of neurotransmitter synthesis and release in aging sympathetic nervous system, Neurobiol. Aging, 6 (1985) 227-232. 11 Saeki, Y., Sato, A., Sato, Y. and Trzebski, A., Effects of stimulation of cervical sympathetic trunks with various frequencies on the local cortical cerebral blood flow measured by laser Doppler flowmetry in the rat, Jpn. J. Physiol., 40 (1990) 15-32. 12 Tuttle, R.S., Age-related changes in the sensitivity of rat aortic strips to norepinephrine and associated chemical and structural alterations, J. Gerontol., 21 (1966) 510-516.
