581
S-20-5
Imaging Human
of Neurotransmitter Brain by Positron
Receptors Emission
K. YANAI, 1 T.WATANABE,1
in the Living
Tomography
M.ITOH,2
and
(PET)
T.IDo2
'Department of Pharmacology I , Tohoku University School of Medicine, 2-1 Seiryomachi, Aoba-ku, Sendai 980 (Japan) 2Cyclotron and Radioisotope Center , Tohoku University, Aoba Aaza-Aramaki, Aobaku, Sendai 980 (Japan) I.
INTRODUCTION Various brain imaging techniques such as X-ray computerized tomography and magnetic resonance imaging have become available in the past decade. Positron emission tomography (PET) is a new and unique approach to identify neurotransmitter receptors in the living human brain [1]. Dopamine Di and D2, serotonin 5HT2, muscarinic cholinergic, opiate, benzodiazepine, histamine Hi receptors, and monoamine oxidases (A and B) have been targets of PET studies in the living human brain [1, 2]. We have successfully imaged histamine H, receptors using_ ujpyrilamine (or mepyramine), an antihistaminic, or_ujdoxepine, a tricyclic antidepressant with a potent antihistaminic action, by PET [3-5]. The present article gives an outline of concepts to visualize neurotransmitter receptors in the living human brain by PET on the basis of our original works.
II.
THE BASIC
CONCEPTS
TO VISUALIZE
NEUROTRANSMITTER
RECEPTORS
BY PET
With the development of receptor binding techniques in vitro, the in vivo tracer distribution experiments were employed using regional dissection study or autoradiographic study of ligand binding (Fig. 1). PET measurements of drug binding sites are not based on the in vitro binding techniques, but on the in vivo receptor-ligand binding. The selection of a ligand suited to in vivo receptor binding experiments is dependent not only on factors considered in in vitro studies, but also on additional aspects including biodistribution, penetration through blood-brain barrier, drug metabolism, etc. [1]. The pharmacological properties of in vivo binding are essentially the same as those of the in vitro binding to brain homogenate. But discrepancy is known to be present between in vivo and in vitro bindings [6]. It has been reported that association and dissociation of binding of spiperone, diprenorphine, or pyrilamine are slower in vivo than in vitro. There are several possible explanations for slower association and dissociation kinetics in vivo: (1)Receptor internalization would occur in postsynaptic membranes and (2)diffusion barrier would exist in microenvironmnet of synapses. These characteristic features in vivo should taken into consideration when we assess data obtained by PET procedures.
III.
RADIOCHEMISTRY
The choice of a ligand for in vivo binding in PET protocols must consider constraints imposed by available radiochemical synthetic routes because of short half-life of positron-emitters (11C, 20.38 min; 19F, 109.8 min). Rapid synthetic routes yielding radioligands with high specific activity and high radiochemical yield are required. The syntheses of many [11C] labeled receptor ligands from [11C]-methyl iodide (Fig. 2) are very useful and convenient [7]. We have also developed several PET tracers for imaging histamine Hi-receptors using this synthetic route. TABLE 1 lists ligands used in the literatures of PET studies.
Symposium:
582
TABLE 1 Neurotransmitter
All
ligands
Fig.
1
receptors
listed
(Left).
An
rat
after
injection
of
a
aging
plate
system 5HT2
labeled
of
brain.
The
radioligand Fuji
Amine
PET
in
in
Setoperone.
vivo
receptor
horizontal
Photo
visualized
in
humans
except
binding
image
(250ƒÊCi,
(BAS3000, receptors
by
with 11C
autoradiogram
to
serotonin
visualized
were
methylspiperone
20, Biogenic
190 Film, the
of
a
pmol)
was
using
a
Tokyo, jJapan). striatum
of
brain
[11C]-Nobtained
45
im-
Dopamine
and
min
high-sensitive
frontal
D2
cortex,
and respec-
tively.
Fig.
2
labeled
An
automated
receptor-binding
thesis
system
product tem
(Right).
is is
closed
of purified with
synthesis ligands
[11C]methyliodide by leaded
system from (small
HPLC
(•¨). doors.
for
routine
prepartion
[11C]-methyliodide
For
yellow the
protection
[7]. box,
*). of
of An
The radiation
[11C]
automated crude dose,
synreaction the
sys-
K YANA
et
al.
583
IV.
VISULAIZATION OF NEUROTRANSMITTER RECEPTORS BY PET In 1983, Wagner and his coworkers [2] used [11C]-N-methylspiperone which, like spiperone, has a high affinity for central dopamine D2 and serotonin 5HT2 receptors, to visulalize these receptors in the striatum and frontal cortex, respectively (Fig. 1). Their pioneering experiments in humans were subsequently further developed by their groups and other groups all over the world using receptor-binding drugs labeled by 11C, 18F and 76Br. More than 15 receptors and enzymes can be visualized and measured by PET in the living human brains (TABLE 1). We have successfully labeled histamine Hi-receptors in human brains using [11C]-pyrilamine (or mepyramine) or [11C]-doxepin by PET (Fig. 3). The concentration of these [11C]-labeled ligands was markedly lower in the cerebellum than in the cerbral cortex and showed the heterogeneity. The radioactivity was highest in the frontal, temporal, and parietal cortices, and the thalamus. The primary visual cortex showed less radioactivity than other cortical structures. The distribution of [11C]-1abeled ligands is essentially consistent with those of histamine Hi-receptors in autopsied human brains measured by in vitro binding with [3H]-pyrilamine as a marker. In the blocking study, the radioactivity in all brain regions became less than that observed in the corresponding images in the control study.
V. PET
AGE-DEPENDENT DISCREPANCY AND IN VITRO BINDING ASSAY Age-related
decrease
in
the
IN RECEPTOR in
vivo
DENSITIES
binding
of
IN specific
HUMAN BRAINS ligands
MEASURED to
Fig. 3. Visualization of histamine Hi receptors in the living human healthy young volunteers. A: [11C]doxepine study, B: Blocking studies antihistaminic d-chlorpheniramine (5 mg). C: [11C]pyrilamine study.
BY
dopaminer-
brain with
of the
Symposium (12)
584
Biogenic
Amine
gic, serotonergic, muscarinic, and histaminergic receptors have been demonstrated using PET. The magnitude of the decrease in histamine H1 receptor binding observed in our PET study is consistent with that observed in PET studies on D1 and D2, and muscarinic cholinergic receptors. In data obtained in autopsied human brains using in vitro assay, however, age-related decrease in the binding is reported to be quite small in dopaminergic [8], muscarinic, and histaminergic receptors [5]. There are several possible explanations for larger effect observed by PET. One possible explanation is that data obtained using in vitro binding techniques, unlike PET, necessitate the use of postmortem material and are inevitably subject to alterations due to tissue autolysis resulting in an underestimation of receptor number. Secondly, PET measurements are based on the receptor-ligand binding in vivo. As mentioned before, the mechanism for receptor-ligand binding in vivo is not exactly the same as that in vitro. If the microenvironment of receptors is changed with age, the in vivo binding parameters will be affected in spite of preservation of receptor number.
SUMMARY More cal and obtained the 1 autopsied
than
30
psychiatric by PET gand-receptor human
findings
revealed
receptors
or
subtypes
disorders using should be carefully brain by
binding samples PET
technique
in are
will
be
investigated
PET in evaluated,
the
vivo. important
Extensive to
to
substantiate
in
near future. because the assess its
various
However, PET method
studies the
neurologiis
the dada based on
on animals significance
of
and the
validity.
ACKNOWLEDGEMENTS
The work was supported by Grants-in-Aid from the Minsitry of Education, Science and Culture (#01770119, #02857028, #63065004), Minsitry of Health (#2-10, #3-5), Yamanouchi Foundation for Research on Metabolic Disorders, and Terumo Life Science Foundation.
REFERENCES
[1] Frost, J.J., and Wagner, H. N. Jr. (1990): Quantitative Imaging, Neuroreceptors, Neurotransmitters and Enzymes. Raven Press, New York. [2] Wagner, H. N. Jr., Burns, H. D., Dannals, R. F., Wong, D. F., Langstrom, B., Duefer, T.,Frost, J. J., Ravert, H.T., and Links, J.M. (1983): Imaging dopamine receptors in the human brain by positron tomography. Science, 221, 1264-1266. [3] Yanai, K., Watanabe, T., Hatazawa, J., Itoh, M., Nunoki, K., Hatano, K., Iwata, R. Ishiwata, K., Ido, T., and Matsuzawa, T. (1990): Visualization of histamine H1 receptors in dog brain by positron emission tomography. Neurosci. Lett., 118, 41-44. [4] Watanabe, T., and Yanai, K. (1991): Most recent advances in the histaminergic neuron sytem, in Histaminergic Neurons: Morphology and Function, ed. by Watanabe, T., and Wada, H., CRCPress, Boca Raton, pp.403-405. [5] Yanai, K., Watanabe, T., Hatazawa, J., Itoh, M., Yokoyama, H., Takahashi, T., Ishiwata, K., Iwata, R., Ido, T. and Matsuzawa, T. 0990: Visualization of histamine H1 receptors in human brain by positron emission tomography: Comparison with the in vitro binding. J. Cereb. Blood Flow Metab., 11, s871. [6] Yanai, K., Yagi, N., Watanabe, T., Itoh, M., Ishiwata, K., Ido, T., and Matsuzawa, T. (1990): Specific binding of [3H]pyrilamine to histamine H1 receptors in guinea-pig brain in vivo: Determination of binding parameters by a kinetic four-compartment model. J. Neurochem., 55, 409-420. [7] Iwata, R., Hatano, K., Yanai, K., Takahashi, T., Ishiwata, K., and Ido, T. (1991): A semi-automated synthesis system for routine prepartion of [11C]-YM-09151-2 and [11C]pyrilamine from [11C]methyliodide. Appl. Radiat. Isot., 42, 201-205. [8] Seeman, P., Bzowej, J. E., Guan, H.-C., Bergeron, P., Becker, L. e., Reynolds, G. P., Bird, E.D., Riederer, P., Jellinger, K., Watanabe, S., and Tourtellottte, W.W. (1987): Human brain dopamine receptors in children and aging adults. Synapse, 1 399-404. ,
S-20-5
Imaging Human
of Neurotransmitter Brain by Positron
Receptors Emission
K. YANAI, 1 T.WATANABE,1
in the Living
Tomography
M.ITOH,2
and
(PET)
T.IDo2
'Department of Pharmacology I , Tohoku University School of Medicine, 2-1 Seiryomachi, Aoba-ku, Sendai 980 (Japan) 2Cyclotron and Radioisotope Center , Tohoku University, Aoba Aaza-Aramaki, Aobaku, Sendai 980 (Japan) I.
INTRODUCTION Various brain imaging techniques such as X-ray computerized tomography and magnetic resonance imaging have become available in the past decade. Positron emission tomography (PET) is a new and unique approach to identify neurotransmitter receptors in the living human brain [1]. Dopamine Di and D2, serotonin 5HT2, muscarinic cholinergic, opiate, benzodiazepine, histamine Hi receptors, and monoamine oxidases (A and B) have been targets of PET studies in the living human brain [1, 2]. We have successfully imaged histamine H, receptors using_ ujpyrilamine (or mepyramine), an antihistaminic, or_ujdoxepine, a tricyclic antidepressant with a potent antihistaminic action, by PET [3-5]. The present article gives an outline of concepts to visualize neurotransmitter receptors in the living human brain by PET on the basis of our original works.
II.
THE BASIC
CONCEPTS
TO VISUALIZE
NEUROTRANSMITTER
RECEPTORS
BY PET
With the development of receptor binding techniques in vitro, the in vivo tracer distribution experiments were employed using regional dissection study or autoradiographic study of ligand binding (Fig. 1). PET measurements of drug binding sites are not based on the in vitro binding techniques, but on the in vivo receptor-ligand binding. The selection of a ligand suited to in vivo receptor binding experiments is dependent not only on factors considered in in vitro studies, but also on additional aspects including biodistribution, penetration through blood-brain barrier, drug metabolism, etc. [1]. The pharmacological properties of in vivo binding are essentially the same as those of the in vitro binding to brain homogenate. But discrepancy is known to be present between in vivo and in vitro bindings [6]. It has been reported that association and dissociation of binding of spiperone, diprenorphine, or pyrilamine are slower in vivo than in vitro. There are several possible explanations for slower association and dissociation kinetics in vivo: (1)Receptor internalization would occur in postsynaptic membranes and (2)diffusion barrier would exist in microenvironmnet of synapses. These characteristic features in vivo should taken into consideration when we assess data obtained by PET procedures.
III.
RADIOCHEMISTRY
The choice of a ligand for in vivo binding in PET protocols must consider constraints imposed by available radiochemical synthetic routes because of short half-life of positron-emitters (11C, 20.38 min; 19F, 109.8 min). Rapid synthetic routes yielding radioligands with high specific activity and high radiochemical yield are required. The syntheses of many [11C] labeled receptor ligands from [11C]-methyl iodide (Fig. 2) are very useful and convenient [7]. We have also developed several PET tracers for imaging histamine Hi-receptors using this synthetic route. TABLE 1 lists ligands used in the literatures of PET studies.
Symposium:
582
TABLE 1 Neurotransmitter
All
ligands
Fig.
1
receptors
listed
(Left).
An
rat
after
injection
of
a
aging
plate
system 5HT2
labeled
of
brain.
The
radioligand Fuji
Amine
PET
in
in
Setoperone.
vivo
receptor
horizontal
Photo
visualized
in
humans
except
binding
image
(250ƒÊCi,
(BAS3000, receptors
by
with 11C
autoradiogram
to
serotonin
visualized
were
methylspiperone
20, Biogenic
190 Film, the
of
a
pmol)
was
using
a
Tokyo, jJapan). striatum
of
brain
[11C]-Nobtained
45
im-
Dopamine
and
min
high-sensitive
frontal
D2
cortex,
and respec-
tively.
Fig.
2
labeled
An
automated
receptor-binding
thesis
system
product tem
(Right).
is is
closed
of purified with
synthesis ligands
[11C]methyliodide by leaded
system from (small
HPLC
(•¨). doors.
for
routine
prepartion
[11C]-methyliodide
For
yellow the
protection
[7]. box,
*). of
of An
The radiation
[11C]
automated crude dose,
synreaction the
sys-
K YANA
et
al.
583
IV.
VISULAIZATION OF NEUROTRANSMITTER RECEPTORS BY PET In 1983, Wagner and his coworkers [2] used [11C]-N-methylspiperone which, like spiperone, has a high affinity for central dopamine D2 and serotonin 5HT2 receptors, to visulalize these receptors in the striatum and frontal cortex, respectively (Fig. 1). Their pioneering experiments in humans were subsequently further developed by their groups and other groups all over the world using receptor-binding drugs labeled by 11C, 18F and 76Br. More than 15 receptors and enzymes can be visualized and measured by PET in the living human brains (TABLE 1). We have successfully labeled histamine Hi-receptors in human brains using [11C]-pyrilamine (or mepyramine) or [11C]-doxepin by PET (Fig. 3). The concentration of these [11C]-labeled ligands was markedly lower in the cerebellum than in the cerbral cortex and showed the heterogeneity. The radioactivity was highest in the frontal, temporal, and parietal cortices, and the thalamus. The primary visual cortex showed less radioactivity than other cortical structures. The distribution of [11C]-1abeled ligands is essentially consistent with those of histamine Hi-receptors in autopsied human brains measured by in vitro binding with [3H]-pyrilamine as a marker. In the blocking study, the radioactivity in all brain regions became less than that observed in the corresponding images in the control study.
V. PET
AGE-DEPENDENT DISCREPANCY AND IN VITRO BINDING ASSAY Age-related
decrease
in
the
IN RECEPTOR in
vivo
DENSITIES
binding
of
IN specific
HUMAN BRAINS ligands
MEASURED to
Fig. 3. Visualization of histamine Hi receptors in the living human healthy young volunteers. A: [11C]doxepine study, B: Blocking studies antihistaminic d-chlorpheniramine (5 mg). C: [11C]pyrilamine study.
BY
dopaminer-
brain with
of the
Symposium (12)
584
Biogenic
Amine
gic, serotonergic, muscarinic, and histaminergic receptors have been demonstrated using PET. The magnitude of the decrease in histamine H1 receptor binding observed in our PET study is consistent with that observed in PET studies on D1 and D2, and muscarinic cholinergic receptors. In data obtained in autopsied human brains using in vitro assay, however, age-related decrease in the binding is reported to be quite small in dopaminergic [8], muscarinic, and histaminergic receptors [5]. There are several possible explanations for larger effect observed by PET. One possible explanation is that data obtained using in vitro binding techniques, unlike PET, necessitate the use of postmortem material and are inevitably subject to alterations due to tissue autolysis resulting in an underestimation of receptor number. Secondly, PET measurements are based on the receptor-ligand binding in vivo. As mentioned before, the mechanism for receptor-ligand binding in vivo is not exactly the same as that in vitro. If the microenvironment of receptors is changed with age, the in vivo binding parameters will be affected in spite of preservation of receptor number.
SUMMARY More cal and obtained the 1 autopsied
than
30
psychiatric by PET gand-receptor human
findings
revealed
receptors
or
subtypes
disorders using should be carefully brain by
binding samples PET
technique
in are
will
be
investigated
PET in evaluated,
the
vivo. important
Extensive to
to
substantiate
in
near future. because the assess its
various
However, PET method
studies the
neurologiis
the dada based on
on animals significance
of
and the
validity.
ACKNOWLEDGEMENTS
The work was supported by Grants-in-Aid from the Minsitry of Education, Science and Culture (#01770119, #02857028, #63065004), Minsitry of Health (#2-10, #3-5), Yamanouchi Foundation for Research on Metabolic Disorders, and Terumo Life Science Foundation.
REFERENCES
[1] Frost, J.J., and Wagner, H. N. Jr. (1990): Quantitative Imaging, Neuroreceptors, Neurotransmitters and Enzymes. Raven Press, New York. [2] Wagner, H. N. Jr., Burns, H. D., Dannals, R. F., Wong, D. F., Langstrom, B., Duefer, T.,Frost, J. J., Ravert, H.T., and Links, J.M. (1983): Imaging dopamine receptors in the human brain by positron tomography. Science, 221, 1264-1266. [3] Yanai, K., Watanabe, T., Hatazawa, J., Itoh, M., Nunoki, K., Hatano, K., Iwata, R. Ishiwata, K., Ido, T., and Matsuzawa, T. (1990): Visualization of histamine H1 receptors in dog brain by positron emission tomography. Neurosci. Lett., 118, 41-44. [4] Watanabe, T., and Yanai, K. (1991): Most recent advances in the histaminergic neuron sytem, in Histaminergic Neurons: Morphology and Function, ed. by Watanabe, T., and Wada, H., CRCPress, Boca Raton, pp.403-405. [5] Yanai, K., Watanabe, T., Hatazawa, J., Itoh, M., Yokoyama, H., Takahashi, T., Ishiwata, K., Iwata, R., Ido, T. and Matsuzawa, T. 0990: Visualization of histamine H1 receptors in human brain by positron emission tomography: Comparison with the in vitro binding. J. Cereb. Blood Flow Metab., 11, s871. [6] Yanai, K., Yagi, N., Watanabe, T., Itoh, M., Ishiwata, K., Ido, T., and Matsuzawa, T. (1990): Specific binding of [3H]pyrilamine to histamine H1 receptors in guinea-pig brain in vivo: Determination of binding parameters by a kinetic four-compartment model. J. Neurochem., 55, 409-420. [7] Iwata, R., Hatano, K., Yanai, K., Takahashi, T., Ishiwata, K., and Ido, T. (1991): A semi-automated synthesis system for routine prepartion of [11C]-YM-09151-2 and [11C]pyrilamine from [11C]methyliodide. Appl. Radiat. Isot., 42, 201-205. [8] Seeman, P., Bzowej, J. E., Guan, H.-C., Bergeron, P., Becker, L. e., Reynolds, G. P., Bird, E.D., Riederer, P., Jellinger, K., Watanabe, S., and Tourtellottte, W.W. (1987): Human brain dopamine receptors in children and aging adults. Synapse, 1 399-404. ,
