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Aging and the blood-brain barrier: Changes in the carrier-mediated transport of peptides in rats
Neuroscience Letters, 61 (1985) 171-175
171
Elsevier Scientific Publishers Ireland Ltd.
NSL 03594
AGING A N D T H E B L O O D - B R A I N BARRIER: C H A N G E S IN T H E C A R R I E R - M E D I A T E D T R A N S P O R T OF P E P T I D E S IN RATS
W I L L I A M A. BANKS* and ABBA J. K A S T I N
Veterans Administration Medical Center and Tulane University School o f Medicine, New Orleans, LA 70146 (U.S.A.) (Received June 9th, 1985; Revised version received and accepted July 29th, 1985)
Key words." aging - blood-brain barrier - peptide - neuropeptide - opioid - transport - central nervous system - dementia - rat
Age-related changes in the brain's saturable, specific, carrier-mediated transport system for the small, N-tyrosinated peptides Tyr-MIF-I (Tyr-Pro-Leu-Gly-NH2) and methionine-enkephalin (Met-Enk) were studied in Fischer 344 rats aged 4 and 26 months. These studies showed statistically significant differences between the two age groups for both the Tmax(transport maximum) [3.22 + 0.013 nmol/min/g (young rats, mean + S.E.M.) vs 2.41 _ 0.009 nmol/min/g (aged rats)] and Ts0 (the a m o u n t required to achieve 50~o of that maximum) [84.9+ h0 nmol/g (young) vs 65.1 +0.60 nmol/g (aged)]. The Ts0:Tmax ratio was nearly equal for the two groups: 26.4 (young) vs 26.9 (aged), consistent with the uncompetitive type of inhibition indicative of alterations in the substrate-varrier complex. In addition, blood concentrations of T y r - M I F - l like immunoactivity were nearly doubled in aged rats (3.24+0.373 vs 1.67+0.0904 pM/ml), while blood concentrations of Met-Enk-like immunoactivity and brain concentrations of immunoactive Tyr-MIF-I and Met-Enk showed no statistically significant difference between age groups. Thus, a cartier-mediated system responsible for the transport of peptides across the blood-brain barrier undergoes changes with aging.
The blood-brain barrier (BBB) is important in maintaining the internal environment of the central nervous system (CNS) by regulating the entry of substances into the CNS. It accomplishes this by a variety of mechanisms such as a high degree of impenetrability and a number of carrier-mediated transport systems. How these systems change in disease, with aging and in age-related pathologies such as dementias is poorly studied, although it is clear that changes do occur in BBB morphology with aging [6, 8, 10, 12]. The question of BBB regulation of peptide penetration is particularly intriguing. Behavioral effects of peripherally administered peptides have raised the possibility that peptides cross the BBB in therapeutically significant amounts [13, 16]. Furthermore, since peptides are produced in peripheral organs and circulate in the blood [18], such passage could also be important in brain-soma interactions. It has been *Author for correspondence at: Research Service, Veterans Administration Medical Center - New Orleans, 1601 Perdido Street, New Orleans, LA 70146, U.S.A. 0304-3940/85/$ 03.30 © 1985 Elsevier Scientific Publishers Ireland Ltd.
172
suggested that alteration of BBB permeability to behaviorally active, endogenously produced substances such as peptides could lead to CNS dysfunction [2]. For example, an excess of opiate activity has been incriminated in dementia [15, 22]. In the current study, we looked at the effect of aging on the kinetics of the saturable, carriermediated transport system for small, N-tyrosinated peptides [3]. Fischer 344 rats that were either 4 or 26 months old were anesthetized with i.p. pentobarbital (65 mg/kg) and administered 250,000 cpm of ['25I]N-Tyr-MIF-I into the left lateral ventricle (i.c.v.) by the method of Noble et al. [19]. N-Tyr-MIF-I was synthesized by solid-phase methods, iodinated by the chloramine-T method, and purified on a column of Sephadex G-15. The [~25I]N-Tyr-MIF-1 had a specific activity of about 100 mCi/mol. Varying amounts of unlabeled Tyr-MIF-1 (0, 0.1, 1.0, 10, 100 and 1000 nmol/rat) were injected simultaneously with the ['25I]N-Tyr-MIF-I. Ten minutes after the i.c.v, injection, the rats were decapitated and whole brain (except for the pineal and pituitary) were transferred to pre-weighed borosilicate tubes, weighed, counted in a gamma counter, and the cpm/g of brain computed. The mean cpm/g brain (b) for each concentration of Tyr-MIF-1 for young or aged rats were plotted against concentration in nmol/g of peptide injected i.c.v. (c) and the asymptotic value (a) estimated. Transport rate (T) in nmol/min/g was then determined by the equation: T=
c(a - h)/at
where t is time of injection before decapitation (10 min). Kinetic parameters were determined and compared using the A L L F I T program (Biomedical Computing Technology Information Center, Vanderbilt Medical Center, Nashville, TN, U.S.A.). For rats that were injected with the zero dose of Tyr-MIF-1, brain and blood samples were collected into tubes in an ice-water bath. The tubes for blood contained 0.05 ml of ethylenediaminetetraacetic acid (EDTA) and were centrifuged at 4'C at 2000 g for 10 min and the plasma stored at - 7 0 ' C until time of assay. Whole brains were homogenized in 20 ml of 0.1 M acetic acid in distilled water with 4'!,',, aprotinin (Mobay Chemical Co., New York, NY) for 20 s using a polytron (Brinkmann Instruments, Westbury, NY) set at 4.5, centrifuged for 1 h at 5000 g at 4'~C, and aliquots of the supernatant lyophilized. Brain and plasma samples were then assayed for MetEnk (Immuno Nuclear, Stillwater, MN) and Tyr-MIF-1 [14]. Values for young and aged rats were compared with analysis of variance (ANOVA) and are reported as means + S.E.M. A small but statistically significant difference occurred for brain weight between young (1.78_+0.021 g, n = 3 8 ) and aged (1.91_+0.024 g, n = 3 7 ) rats, F.,74=16.3. P<0.01. The relationship between T and the concentration of Tyr-MIF-I injected i.c.v. (c) is shown in Fig. 1: young: T = 3.22 - 356/( 111 + c '-°6) aged: T = 2.42 - 802/(332 + c I~'*) The transport maximum (Tm~,x)for young rats was 3.22 + 0.0133 nmol/min/g and for aged rats was 2.41 _+0.0088 nmol/min/g, and the concentration needed for 50°/,; trans-
173 3.0
Young
Aged
? .E E C
20 io
ii!f I
100
250
500
nmoles/g Fig. I. Transport rate in nmol/min/g of brain versus amount injected in nmol/g of brain in young (solid circles) and aged (open circles) rats. Inset shows inverse of Tmaxand T~0 plotted.
port (Ts0) was 84.9_ 1.0 nmol/g for young rats and 65.1 _ 0.60 nmol/g for aged rats. Analysis with shared parameters in the A L L F I T program showed that the two equations were significantly different with statistically reliable differences in the Tmax, /'50 and slope values, but not in the minimum (zero dose) values. The Ts0:Tmaxratio was essentially the same for young (26.4) and aged (26.9) rats, indicating that aging is associated with an uncompetitive type of inhibition suggesting an alteration in the substrate-carrier complex. Plasma concentrations of Tyr-MIF-l-like immunoactivity were much higher in aged rats (3.24___0.373 pmol/ml) than in young rats (1.67 + 0.0904 pmol/ml), El,17 = 17.5, P < 0.01. Plasma concentrations of immunoactive Met-Enk and brain concentrations of immunoactive Tyr-MIF-1 and Met-Enk did not differ significantly between young and aged rats (Table I). Using these brain concentrations as values for c, we found approximations for brain-to-blood transport in young rats to be 12.8 pmol/min/g and for aged rats to TABLE I I M M U N O A C T I V E CONCENTRATIONS OF Tyr-MIF-1- A N D Met-Enk IN PLASMA (pmol/ml) A N D IN BRAIN (pmol/g) IN A G E D ( n = 9 ) A N D Y O U N G ( n = 10) RATS M e a n s _ S.E.M. *P<0.01.
Tyr-MIF-1 (plasma) Met-Enk (plasma) Tyr-MIF-I (brain) Met-Enk (brain)
Young
Aged
1.67 ___ 0.0904 0.265+ 0.0163 1.77 __+ 0.234 150 + 22.5
3.24 0.251 2.13 118
__+ 0.373* + 0.0166 + 0.277 +10.9
174
be 4.34 pmol/min/g. If one assumes that transport of N-Tyr-MIF-1 into the brain is based entirely on membrane permeation, then transport out of the brain greatly exceeds permeation in, even if the maximal extraction value of I is used. Thus, the net movement of peptide across the BBB under physiologic conditions is from brain to blood. It is clear, therefore, that age-related alterations occur in this saturable, carriermediated transport system for small, N-tyrosinated peptides. It is unclear if the uncompetitive type of inhibition is due to the accumulation of an endogenous substance or an alteration in the carrier system. Previous studies have established that the endothelial cells largely responsible for the BBB undergo profound morphologic changes with aging [6, 8, 10, 12], although the restrictive properties of the BBB remain intact [21, 23, 24]. However, the lipophilic properties of the BBB that govern direct membrane permeation and the carrier-mediated systems remain virtually unstudied in aging. Although it appears that most peptides cross the BBB by direct membrane permeation [4, 5], some are transported by a carrier-mediated system [3]. This system seems highly specific for Tyr-MIF-I (which has anti-opiate properties [17]) and Met-Enk (an opiate peptide). Changes in this system with aging could be involved in the varied roles that opiates may play in aging [20], including the theoretical considerations that have incriminated excess opiate activity in dementia [15, 22]. The implications of the alteration in Tyr-MIF-1 plasma concentrations with aging is unclear. The slight but statistically non-significant increase in Tyr-MIF-I brain concentrations is unusual since concentrations of most brain peptides are either unchanged or decreased with aging [1, 7, 9, 1 i]. The tendency towards an increase could be explained by the impairment in transport out of this tetrapeptide with antiopiate properties or as a compensatory response to a relative excess of opiate activity. Concentrations of peptides in the periphery and CNS are usually thought to be a result of the balance between production and degradation. The presence of a transport system adds a third variable in the determination of peptide concentrations. This raises the possibility of a communication loop between the CNS and peripheral organs. It is not clear how such communication would affect peptide concentrations or organ function, nor is it certain what role compensatory mechanisms or agerelated changes in production and degradation plays. Until these points are clarified, it will be difficult to determine what effects age-related changes in peptide transport have on CNS and peripheral concentrations or what role peptide transport plays in the regulation of CNS and peripheral function. An alteration in one carrier-mediated transport system raises the possibility of other changes in BBB function with aging. Such changes could be involved in the CNS dysfunctions found in aging and in age-related diseases. The authors thank Jeanne Proffitt and Seth Strauss for their technical assistance and Dr. James Zadina and Dr. William Murphy for their advice on statistics and kinetics. Supported by the VA and ONR.
175 1 Banks, W.A. and Kastin, A.J., Permeability of the blood-brain barrier to neuropeptides: the case for penetration, Psychoneuroendocrinology, in press. 2 Banks, W.A. and Kastin, A.J., Aluminium increases permeability of the blood-brain barrier to labelled DSIP and/~-endorphin: possible implications for senile and dialysis dementia, Lancet, ii (1983) 1227 1229. 3 Banks, W.A. and Kastin, A.J., A brain-to-blood carrier-mediated transport system for small, N-tyrosinated peptides, Pharmacol. Biochem. Behav., 21 (1984) 943 946. 4 Banks, W.A. and Kastin, A.J., Physicochemical factors as predictors of blood-brain barrier penetrance of iodinated peptides, Clin. Res., 32 (1984) 873A. 5 Banks, W.A., Kastin, A.J. and Coy, D.H., Evidence that ~25I-N-Tyr-delta sleep-inducing peptide crosses the blood-brain barrier by a non-competitive mechanism, Brain Res., 301 (1984) 201-207. 6 Bar, T., Morphometric evaluation of capillaries in different laminae of rat cerebral cortex by automatic image analysis: changes during development and aging, Adv. Neurol., 20 (1978) 1 9. 7 Barnea, A., Cho, G. and Porter, J.C., A reduction in the concentration of immunoreactive corticotropin, melanotropin and lipotropin in the brain of the aging rat, Brain Res., 232 (1982) 345-353. 8 Burns, E.M., Buschmann, M.B.T., Kruckeberg, T.W., Gaetano, P.K. and Meyer, J.M., Blood-brain barrier, aging, brain blood flow, and sleep, Adv. Neurol., 30 (1981) 301-306. 9 Dupont, A., Savard, P., Merand, Y., Labrie, F. and Boissier, J.R., Age-related changes in central nervous system enkephalins and substance P, Life Sci., 29 (1981) 2317 2322. 10 Hicks, P., Rolsten, C., Brizzee, D. and Samorajski, T., Age-related changes in rat brain capillaries, Neurobiol. Aging, 4 (1983) 69-75. 11 Hoffman, G.E. and Sladek, J.R., Age-related changes in dopamine, LHRH, and somatostatin in the rat hypothalamus, Neurobiol. Aging, 1 (1980) 27-37. 12 Hunziker, O., Abdel'A1, S., Schulz, U. and Schweizer, A., Architecture of cerebral capillaries in aged human subjects with hypertension, Adv. Neurol., 20 (1978) 471477. 13 Kastin, A.J., Olson, R.D., Schally, A.V. and Coy, D.H., CNS effects of peripherally administered brain peptides, Life Sci., 25 (1979) 401~414. 14 Kastin, A.J., Lawrence, S.P. and Coy, D.H., Radioimmunoassay of MIF-1/Tyr-MIF-l-like material in rat pineal, Pharmacol. Biochem. Behav., 13 (1981) 901-905. 15 Kastin, A.J., Olson, G.A., Sandman, C.A. and Ehrensing, R.H., Possible role of peptides in senile dementia. In T. Crook and S. Gershon (Eds.), Strategies for the Development of an Effective Treatment for Senile Dementia, Mark Powley Assoc., New Canaan, CT, 1981, pp. 139-152. 16 Kastin, A.J., Banks, W.A., Zadina, J.E. and Graf, M., Brain peptides: the dangers of constricted nomenclatures, Life Sci., 32 (1983) 295-301. 17 Kastin, A.J., Stephens, E., Ehrensing, R.H. and Fischman, A.J., Tyr-MIF-I acts as an opiate antagonist in the tail-flick test, Pharmacol. Biochem. Behav., 21 (1984) 937-941. 18 Krieger, D.T. and Martin, J.B., Brain peptides, N.Engl. J. Med., 304 (1981) 876-885; 944-951. 19 Noble, P., Wurtman, R.J. and Axelrod, J., A simple and rapid method for injection of3H norepinephrine into the lateral ventricle of the rat brain, Life Sci., 6 (1967) 281-291. 20 Olson, G.A., Olson, R.D. and Kastin, A.J., Endogenous opiates: 1983, Peptides, 5 (1984) 975-992. 21 Rapoport, S.I., Ohno, K. and Pettigrew, K.D., Blood-brain barrier permeability in senescent rats, J. Gerontol., 34 (1979) 162-169. 22 Roberts, E., A speculative consideration on the neurobiology and treatment of senile dementia. In T. Crook and S. Gershon (Eds.), Strategies for the Development of an Effective Treatment for Senile Dementia, Mark Powley Assoc., New Canaan, CT, 1981, pp. 247-320. 23 Rudick, R.A. and Buell, S.J., Integrity of blood-brain barrier to peroxidase in senescent mice, Neurobiol. Aging, 4 (1983) 283-287. 24 Sankar, R., Blossom, E., Clemons, K. and Charles, P., Age-associated changes in the effects of amphetamine on the blood-brain barrier of rats, Neurobiol. Aging, 4 (1983) 65-68.
171
Elsevier Scientific Publishers Ireland Ltd.
NSL 03594
AGING A N D T H E B L O O D - B R A I N BARRIER: C H A N G E S IN T H E C A R R I E R - M E D I A T E D T R A N S P O R T OF P E P T I D E S IN RATS
W I L L I A M A. BANKS* and ABBA J. K A S T I N
Veterans Administration Medical Center and Tulane University School o f Medicine, New Orleans, LA 70146 (U.S.A.) (Received June 9th, 1985; Revised version received and accepted July 29th, 1985)
Key words." aging - blood-brain barrier - peptide - neuropeptide - opioid - transport - central nervous system - dementia - rat
Age-related changes in the brain's saturable, specific, carrier-mediated transport system for the small, N-tyrosinated peptides Tyr-MIF-I (Tyr-Pro-Leu-Gly-NH2) and methionine-enkephalin (Met-Enk) were studied in Fischer 344 rats aged 4 and 26 months. These studies showed statistically significant differences between the two age groups for both the Tmax(transport maximum) [3.22 + 0.013 nmol/min/g (young rats, mean + S.E.M.) vs 2.41 _ 0.009 nmol/min/g (aged rats)] and Ts0 (the a m o u n t required to achieve 50~o of that maximum) [84.9+ h0 nmol/g (young) vs 65.1 +0.60 nmol/g (aged)]. The Ts0:Tmax ratio was nearly equal for the two groups: 26.4 (young) vs 26.9 (aged), consistent with the uncompetitive type of inhibition indicative of alterations in the substrate-varrier complex. In addition, blood concentrations of T y r - M I F - l like immunoactivity were nearly doubled in aged rats (3.24+0.373 vs 1.67+0.0904 pM/ml), while blood concentrations of Met-Enk-like immunoactivity and brain concentrations of immunoactive Tyr-MIF-I and Met-Enk showed no statistically significant difference between age groups. Thus, a cartier-mediated system responsible for the transport of peptides across the blood-brain barrier undergoes changes with aging.
The blood-brain barrier (BBB) is important in maintaining the internal environment of the central nervous system (CNS) by regulating the entry of substances into the CNS. It accomplishes this by a variety of mechanisms such as a high degree of impenetrability and a number of carrier-mediated transport systems. How these systems change in disease, with aging and in age-related pathologies such as dementias is poorly studied, although it is clear that changes do occur in BBB morphology with aging [6, 8, 10, 12]. The question of BBB regulation of peptide penetration is particularly intriguing. Behavioral effects of peripherally administered peptides have raised the possibility that peptides cross the BBB in therapeutically significant amounts [13, 16]. Furthermore, since peptides are produced in peripheral organs and circulate in the blood [18], such passage could also be important in brain-soma interactions. It has been *Author for correspondence at: Research Service, Veterans Administration Medical Center - New Orleans, 1601 Perdido Street, New Orleans, LA 70146, U.S.A. 0304-3940/85/$ 03.30 © 1985 Elsevier Scientific Publishers Ireland Ltd.
172
suggested that alteration of BBB permeability to behaviorally active, endogenously produced substances such as peptides could lead to CNS dysfunction [2]. For example, an excess of opiate activity has been incriminated in dementia [15, 22]. In the current study, we looked at the effect of aging on the kinetics of the saturable, carriermediated transport system for small, N-tyrosinated peptides [3]. Fischer 344 rats that were either 4 or 26 months old were anesthetized with i.p. pentobarbital (65 mg/kg) and administered 250,000 cpm of ['25I]N-Tyr-MIF-I into the left lateral ventricle (i.c.v.) by the method of Noble et al. [19]. N-Tyr-MIF-I was synthesized by solid-phase methods, iodinated by the chloramine-T method, and purified on a column of Sephadex G-15. The [~25I]N-Tyr-MIF-1 had a specific activity of about 100 mCi/mol. Varying amounts of unlabeled Tyr-MIF-1 (0, 0.1, 1.0, 10, 100 and 1000 nmol/rat) were injected simultaneously with the ['25I]N-Tyr-MIF-I. Ten minutes after the i.c.v, injection, the rats were decapitated and whole brain (except for the pineal and pituitary) were transferred to pre-weighed borosilicate tubes, weighed, counted in a gamma counter, and the cpm/g of brain computed. The mean cpm/g brain (b) for each concentration of Tyr-MIF-1 for young or aged rats were plotted against concentration in nmol/g of peptide injected i.c.v. (c) and the asymptotic value (a) estimated. Transport rate (T) in nmol/min/g was then determined by the equation: T=
c(a - h)/at
where t is time of injection before decapitation (10 min). Kinetic parameters were determined and compared using the A L L F I T program (Biomedical Computing Technology Information Center, Vanderbilt Medical Center, Nashville, TN, U.S.A.). For rats that were injected with the zero dose of Tyr-MIF-1, brain and blood samples were collected into tubes in an ice-water bath. The tubes for blood contained 0.05 ml of ethylenediaminetetraacetic acid (EDTA) and were centrifuged at 4'C at 2000 g for 10 min and the plasma stored at - 7 0 ' C until time of assay. Whole brains were homogenized in 20 ml of 0.1 M acetic acid in distilled water with 4'!,',, aprotinin (Mobay Chemical Co., New York, NY) for 20 s using a polytron (Brinkmann Instruments, Westbury, NY) set at 4.5, centrifuged for 1 h at 5000 g at 4'~C, and aliquots of the supernatant lyophilized. Brain and plasma samples were then assayed for MetEnk (Immuno Nuclear, Stillwater, MN) and Tyr-MIF-1 [14]. Values for young and aged rats were compared with analysis of variance (ANOVA) and are reported as means + S.E.M. A small but statistically significant difference occurred for brain weight between young (1.78_+0.021 g, n = 3 8 ) and aged (1.91_+0.024 g, n = 3 7 ) rats, F.,74=16.3. P<0.01. The relationship between T and the concentration of Tyr-MIF-I injected i.c.v. (c) is shown in Fig. 1: young: T = 3.22 - 356/( 111 + c '-°6) aged: T = 2.42 - 802/(332 + c I~'*) The transport maximum (Tm~,x)for young rats was 3.22 + 0.0133 nmol/min/g and for aged rats was 2.41 _+0.0088 nmol/min/g, and the concentration needed for 50°/,; trans-
173 3.0
Young
Aged
? .E E C
20 io
ii!f I
100
250
500
nmoles/g Fig. I. Transport rate in nmol/min/g of brain versus amount injected in nmol/g of brain in young (solid circles) and aged (open circles) rats. Inset shows inverse of Tmaxand T~0 plotted.
port (Ts0) was 84.9_ 1.0 nmol/g for young rats and 65.1 _ 0.60 nmol/g for aged rats. Analysis with shared parameters in the A L L F I T program showed that the two equations were significantly different with statistically reliable differences in the Tmax, /'50 and slope values, but not in the minimum (zero dose) values. The Ts0:Tmaxratio was essentially the same for young (26.4) and aged (26.9) rats, indicating that aging is associated with an uncompetitive type of inhibition suggesting an alteration in the substrate-carrier complex. Plasma concentrations of Tyr-MIF-l-like immunoactivity were much higher in aged rats (3.24___0.373 pmol/ml) than in young rats (1.67 + 0.0904 pmol/ml), El,17 = 17.5, P < 0.01. Plasma concentrations of immunoactive Met-Enk and brain concentrations of immunoactive Tyr-MIF-1 and Met-Enk did not differ significantly between young and aged rats (Table I). Using these brain concentrations as values for c, we found approximations for brain-to-blood transport in young rats to be 12.8 pmol/min/g and for aged rats to TABLE I I M M U N O A C T I V E CONCENTRATIONS OF Tyr-MIF-1- A N D Met-Enk IN PLASMA (pmol/ml) A N D IN BRAIN (pmol/g) IN A G E D ( n = 9 ) A N D Y O U N G ( n = 10) RATS M e a n s _ S.E.M. *P<0.01.
Tyr-MIF-1 (plasma) Met-Enk (plasma) Tyr-MIF-I (brain) Met-Enk (brain)
Young
Aged
1.67 ___ 0.0904 0.265+ 0.0163 1.77 __+ 0.234 150 + 22.5
3.24 0.251 2.13 118
__+ 0.373* + 0.0166 + 0.277 +10.9
174
be 4.34 pmol/min/g. If one assumes that transport of N-Tyr-MIF-1 into the brain is based entirely on membrane permeation, then transport out of the brain greatly exceeds permeation in, even if the maximal extraction value of I is used. Thus, the net movement of peptide across the BBB under physiologic conditions is from brain to blood. It is clear, therefore, that age-related alterations occur in this saturable, carriermediated transport system for small, N-tyrosinated peptides. It is unclear if the uncompetitive type of inhibition is due to the accumulation of an endogenous substance or an alteration in the carrier system. Previous studies have established that the endothelial cells largely responsible for the BBB undergo profound morphologic changes with aging [6, 8, 10, 12], although the restrictive properties of the BBB remain intact [21, 23, 24]. However, the lipophilic properties of the BBB that govern direct membrane permeation and the carrier-mediated systems remain virtually unstudied in aging. Although it appears that most peptides cross the BBB by direct membrane permeation [4, 5], some are transported by a carrier-mediated system [3]. This system seems highly specific for Tyr-MIF-I (which has anti-opiate properties [17]) and Met-Enk (an opiate peptide). Changes in this system with aging could be involved in the varied roles that opiates may play in aging [20], including the theoretical considerations that have incriminated excess opiate activity in dementia [15, 22]. The implications of the alteration in Tyr-MIF-1 plasma concentrations with aging is unclear. The slight but statistically non-significant increase in Tyr-MIF-I brain concentrations is unusual since concentrations of most brain peptides are either unchanged or decreased with aging [1, 7, 9, 1 i]. The tendency towards an increase could be explained by the impairment in transport out of this tetrapeptide with antiopiate properties or as a compensatory response to a relative excess of opiate activity. Concentrations of peptides in the periphery and CNS are usually thought to be a result of the balance between production and degradation. The presence of a transport system adds a third variable in the determination of peptide concentrations. This raises the possibility of a communication loop between the CNS and peripheral organs. It is not clear how such communication would affect peptide concentrations or organ function, nor is it certain what role compensatory mechanisms or agerelated changes in production and degradation plays. Until these points are clarified, it will be difficult to determine what effects age-related changes in peptide transport have on CNS and peripheral concentrations or what role peptide transport plays in the regulation of CNS and peripheral function. An alteration in one carrier-mediated transport system raises the possibility of other changes in BBB function with aging. Such changes could be involved in the CNS dysfunctions found in aging and in age-related diseases. The authors thank Jeanne Proffitt and Seth Strauss for their technical assistance and Dr. James Zadina and Dr. William Murphy for their advice on statistics and kinetics. Supported by the VA and ONR.
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