Neuroscience Letters, 118 (1990) 4 1 ~ 4 Elsevier Scientific Publishers Ireland Ltd.
41
NSL 07177
Visualization of histamine H1 receptors in dog brain by positron emission tomography K a z u h i k o Y a n a i 1, T a k e h i k o W a t a n a b e I , J u n H a t a z a w a 2, M a s a t o s h i I t o h 2, K a z u o N u n o k i 1, K e n t a r o H a t a n o 2, R e n I w a t a 2, Kiichi Ishiwata 2, T a t s u o I d o 2 a n d Taiju M a t s u z a w a 3 ~Department of Pharmacology, School of Medicine, 2Cyclotron and Radioisotope Center and 3Department of Radiology and Nuclear Medicine, Research Institutefor Tuberculosis and Cancer, Tohoku University, Sendal (Japan) (Received 17 April 1990; Revised version received 11 June 1990; Accepted 12 June 1990)
Key words: Histamine; Histamine H~ receptor; Positron emission tomography; In vivo study; Dog; [t~C]Pyrilamine; [~C]Doxepin Histamine Hj receptors were visualized in the living dog brain using [HC]pyrilamine or [~C]doxepin by positron emission tomography (PET). The regional distribution of these carbon-I 1 labeled compounds in the brain corresponded well with that of the histamine Ht receptors separately determined by in vitro binding assay. The radioactivity in the brain was reduced by treatment with triprolidine (1 mg/kg), a histamine H~ antagonist. The results of our study indicate that it is feasible to visualize histamine H~ receptors in human brain using these HC-labeled compounds and PET.
Histamine is widely distributed in mammalian brain and is found in both neuronal and non-neuronal components [7, 10]. Its physiological functions as a neurotransmitter or neuromodulator are mediated through histamine H1, H2, and H3 receptors (for reviews see refs. 7, 8). Pyrilamine (or mepyramine) and doxepin are known to be potent histamine antagonists that bind to histamine H1 receptors [4, 5]. The recent development of llC-labeled pyrilamine and doxepin enables us to visualize histamine H~ receptors in vivo in the living human brain by positron emission tomography [3, 11]. This paper describes the visualization of histamine HI receptors in the dog brain in vivo as a pre-clinical step. [llC]Pyrilamine and [llC]doxepin were synthesized by reacting [lIC]methyliodide with the desmethylated precursors of pyrilamine and doxepin, respectively [3, 11]. The experiments were performed in 3 beagle dogs (weighing 18-20 kg) anesthetized by intravenous injection of pentobarbital (25 mg/kg). The animals were placed on their side on a PET scanner (PT 931, CTI [9]). The dogs were carefully positioned in a special headholder using a laser beam so as to obtain planes parallel to Orbito-Inion line (Fig. 1D). We utilized the OrbitoInion line in our dog study instead of Orbito-Meatal line (OM line) because obstruction of airways did not occur in this position during PET study. A Ge-68/Ga-68 transCorrespondence: K. Yanai, Department of Pharmacology I, Tohoku University School of Medicine, Seiryou machi 2-1 Aoba-Ku, Sendai 980, Japan. 0304-3940/90/$ 03.50 :C~ 1990 Elsevier Scientific Publishers Ireland Ltd.
mission scan was used to correct for auto-attenuation. Four to 6 mCi (148-222 MBq, 0.5-1.0 nmol/kg b.wt.) of the llC-labeled compounds (spec. act.) 200-500 mCi/ /tmol, 7.4-18.5 GBq//tmol at the time of use) were intravenously injected at time=0. The radioactivity in the brain was monitored over 60 min. Arterial blood samples were collected during the scanning time through a transcutaneous femoral catheter. The radioactivity of the plasma samples was measured in an NaI scintillation counter, and the percentage of ~lC-labeled ligand in the total I1C-radioactivity was analyzed by high-performance liquid chromatography (HPLC) [12]. In 4 experiments, a large amount of triprolidine, a histamine Ht receptor antagonist (1 mg/kg), was co-injected with the labeled compounds (pretreatment experiments, n = 2), or injected 30 min after the administration of the carbon- 11 labeled ligands (displacement experiments, n = 2) in order to disclose the specific binding sites. A regional variation was observed in the distribution of [llC]pyrilamine and [llC]doxepin as illustrated in Figs. 1 and 2. High concentrations of radioactivity were observed in the cerebral cortex and olfactory area, and low concentrations in the basal ganglia, hippocampus, and cerebellum. This distribution was similarly observed both in the studies using [llC]pyrilamine and [llC]doxepin, though the images obtained by [llC]doxepin were more contrasted than those of [I 1C]pyrilamine" The kinetics of [llC]pyrilamine distribution (Fig. 2A) revealed that cerebral [llC]pyrilamine activity reached a maximum 10-20 min after bolus injection, and that after a
42 TABLE 1 SPECIFIC [3H]PYRILAMINE BINDING TO MEMBRANE OF DOG BRAIN In vitro binding experiments were performed by a modification of Chang et al. [2]. In brief, tissues were homogenized in a Polytron in 30 vols. of ice-cold Na+-K ÷ phosphate buffer (50 mM, pH 7.5) and homogenate was centrifuged twice at 50,000 g for 20 min. Incubation of membrane fractions with [3H]pyrilamine (Amersham, spec. act. 28 mCi//Lmol) was carried out at 37"C for 10 min in the presence and absence of 2 #M triprolidine. To determine the K~values for doxepin and pyrilamine at 37°C, [3H]pyrilamine at 5 nM was incubated with the tissue and various concentrations of drugs.
-2
0
Inion
0
2
4
6
8
cm
Orbita
Region
Bma~ (pmol/g)
Region
Bma~ (pmol/g)
Frontal cortex Temporal cortex Occipital cortex Olfactory bulb Hypothalamus
14.4 13.3 13.1 11.6 8.88
Hippocampus Striatum Thalamus Cerebellum
7.56 6.95 6.70 2.41
plateau, a gradual decrease occurred until the end o f scanning. [11C]Doxepi n also showed a m a x i m u m uptake within 15 min, but the distribution then remained constant t h r o u g h o u t the rest o f the P E T study (Fig. 2B). W h e n 15 n m o l / k g o f cold pyrilamine was simultaneously injected with [13C]pyrilamine ' the brain uptake o f [llC]pyrilamine was reduced (data n o t shown). Displacement experiments with 1 m g / k g o f triprolidine showed a gradual decrease o f the cerebral activity soon after the administration o f cold antagonist (Fig. 3). A slow dissociation in vivo was also observed as previously reported [6, 12]. The brain radioactivity of ]lC-labeted pyrilamine was reduced by treatment with triprolidine, and the brain kinetics showed a rapid clearance. The [llC]pyrilamine concentrations in different regions o f the brain were almost the same in pretreatment experiments. Similarly, the specific distribution o f [~~C]doxepin disappeared after treatment with a large a m o u n t o f triprolidine (1 mg/kg) (data not shown). In spite o f the small brain size (approx. 7 x 6 cm) and p o o r spatial resolutions (8.5 mm), specific binding o f these c o m p o u n d s could be disclosed by the treatment with a large a m o u n t o f triprolidine. It is well k n o w n that considerable species differences occur in the affinity and the regional distribution o f histamine H1 receptors [2]. The distribution o f histamine H~ receptors in d o g brain has not been reported in the literature. Therefore, specific binding o f [3H]pyrilamine to
2cm
Fig. 1. PET images obtained with [uC]pyrilamine (A) and [uC]doxepin (B) at 4045 min after the injection. The planes have a transaxial spatial resolution of 8.5 mm at full width half maximum. The brain radioactivity is mainly localized in frontal, temporal, and occipital cortex. C: trans-axial view of the dog brain which represents the level
of the PET images (7 mm thickness in axis). D: a sagittal view of dog brain indicating the seven 7-mm-thick slices parallel to Orbito-Inion line. The 7 slices covered the whole dog brain. The solid line represents the slice of PET images listed in A and B. The cerebellum is included on the lower planes.
43 3
3
•
~3
zl
'
~,
v
,
e
I
~
A
,J
7
v
,
'
= '
2b
'
4'o
'
6b
'
Time after injection Imln)
2b
'
,b
'
g
Time after injection (mln)
Fig. 2. Time courses of change in total radioactivity in dog brain after bolus injection of [ILC]pyrilamine (A) and [ttC]doxepin (B). Frontal cortex (O); temporal cortex (A); occipital cortex (11); striatum ([B); cerebellum (V). Results are expressed as normalized radioactivity per tissue volume (percentage of the injected radioactivity per I00 ml tissue volume). Data shown are obtained in the same dog by two different experiments and are representative of three [HC]pyrilamine and two [] LC]doxepin experiments.
membranes of dog brain in vitro was assayed in order to demonstrate that the distributions of the two ligands obtained by PET reflect histamine Hi receptors. As listed in Table I, the highest levels of binding were observed in cerebral cortex, with values in the hippocampus, corpus striatum, and thalamus approximately 50% of this level, while the binding in the cerebellum was lowest. Thus, regional distribution of specifically bound [3H]pyrilamine in the dog brain agreed with the distribution of the two nlC-labeled compounds. This distribution was almost the same as that in human and monkey brain [1, 2]. The Kd value determined at 37°C was 5.9_+ 1.0 nM in the dog brain, 4 times higher than the Kd in guinea pig cerebel-
lum (1.5 nM), while Bma x values for the two species are the same (14.4 pmol/g and 15.7 pmol/g) [14]. The Ki value for doxepin was 2.8 +0.6 nM at 37°C, about a quarter of that for pyrilamine (11.7-+ 1.1 nM), which is consistent with longer retention and clearer images of [1iC]doxepin. All these results indicate that l nC-labeled pyrilamine and doxepin meet criteria for in vivo binding, and can be used to image histamine H1 receptors in vivo by positron emission tomography. Studying histamine H~ receptors by PET opens a new field of research on the physiological and pathological significance of histaminergic neuronal system in humans.
3
Triprolidine
--.....
Contro,
Pretreatm ~l:cement
~-,-e ....... • ...........• .............. o
'
2b
Plasma '
4'o
'
60
Time alter injection (men) Fig. 3. Pretreatment and displacement experiments of [ulC]pyrilamine distribution. Animals were injected with either saline (control, Lq---U) or, 1 mg/kg of triprolidine (pretreatment, V - - V ) 5 min before administration of [HC]pyrilamine. In displacement experiment, cold triprolidine (1 mg/kg) was injected 30 min after the ['C]ligand administration (displacement, I 1 - - 1 1 ) . The cerebral radioactivity was determined in frontal, temporal, occipital cortex using elliptical regions of interest (ROIs) positioned on the continuous PET images. Values of regions of interest in the cortex were averaged and plotted. The total ["C]radioactivity ( 0 - - - 0 ) and [lIC]pyrilamine radioactivity (O ............O) corrected for radioactive metabolites in the plasma were plotted in the figure. Data shown here are obtained in the same dog by 3 different experiments and are representative of two similar experiments.
44 The a u t h o r s t h a n k Mr. Y. Ishikawa, Mr. S. N a m e kawa, Mr. S. Seo, a n d Mr. S. W a t a n u k i for their technical assistance. This work was s u p p o r t e d in part by Grant-in-Aid
for
Specifically
Promoted
Research
63065004 from the M i n i s t r y of E d u c a t i o n , Science a n d C u l t u r e of Japan. 1 Bielkiewicz,B. and Cook, D.A., The binding of [3H]mepyramine binding to histamine H~ receptors in monkey brain, Can. J. Physiol. Pharmacol., 63 (1985) 756-759. 2 Chang, R.S.L., Tran, V.T. and Snyder, S.H., Heterogeneity of histamine H~ receptors: Species variations in [3H]pyrilaminebinding of brain membranes, J. Neurochem., 32 (1979) 1653-1663. 3 Dannals, R.F., Yanai, K., Wilson, A.A., Ravert, H.T., Frost, J.J., Scheffel, U. and Wagner, H.N., Jr., Synthesis of radiotracers for studying histamine H~ receptors: carbon-I 1 labeled doxepin and pyrilamine, J. Labeled Compd. Radiopharm., 26 (1989) 213-214. 4 Hill, S., Young, J.M. and Marrian, D.H., Specific binding of [3H]mepyramine to histamine HL receptors in intestinal smooth muscle, Nature, 270 (1977) 361-363. 5 Kanba, S. and Richelson, E., Histamine H~ receptors in human brain labelled with [3H]doxepin,Brain Res., 304 (1984) 1-7. 6 Perry, D.C., Mullis, K.B., Oie, S. and Sadee, W., Opiate antagonist
receptor binding in vivo: evidence for a new receptor binding model, Brain Res., 199 (1980) 49-61. 7 Prell, G.D. and Green, M., Histamine as a neuroregulator, Annu. Rev. Neurosci., 9 (1986) 209-254. 8 Schwartz, J.-C., Arrang, J.-M. and Garberg, M., Three classes of histamine receptors in brain, Trends Pharmacol., 7 (1986) 24-28. 9 Spinks, T.J., Guzzardi, R. and Bellina, C.R., Performance characteristics of a whole body positron tomograph. J. Nucl. Med., 29 (1988) 1833-1841. 10 Watanabe, T., Taguti, Y., Shiosaka, S., Tanaka, J., Kubota, H., Terano, J., Kubota, H., Terano, Y., Tohyama, M. and Wada, H., Distribution of the histaminergic neuron system in the central nervous system of rats: a fluorescentimmunohistochemicalanalysis with histidine decarboxylase as a marker, Brain Res., 295 (1984) 13 25. 11 Yanai, K., Dannals, R.F., Wilson, A.A., Ravert, H.T~, Scheffel, U., Tanada, S. and Wagner, H.N. Jr., (N-Methyl-[lIClpyrilamine,a radiotracer for histamine H~ receptors: Radiochemical synthesis and biodistribution study in mice, Nucl. Med. Biol., 15 (1988) 605610. 12 Yanai, K., Yagi, N., Watanabe, T., Itoh, M., lshiwata, K., Ido, T. and Matsuzawa, T., Specificbinding of [3H]pyrilamineto histamine H1 receptors in guinea pig brain in vivo: determination of binding parameters by a kinetic four-compartment model, J. Neurochem., in press.
41
NSL 07177
Visualization of histamine H1 receptors in dog brain by positron emission tomography K a z u h i k o Y a n a i 1, T a k e h i k o W a t a n a b e I , J u n H a t a z a w a 2, M a s a t o s h i I t o h 2, K a z u o N u n o k i 1, K e n t a r o H a t a n o 2, R e n I w a t a 2, Kiichi Ishiwata 2, T a t s u o I d o 2 a n d Taiju M a t s u z a w a 3 ~Department of Pharmacology, School of Medicine, 2Cyclotron and Radioisotope Center and 3Department of Radiology and Nuclear Medicine, Research Institutefor Tuberculosis and Cancer, Tohoku University, Sendal (Japan) (Received 17 April 1990; Revised version received 11 June 1990; Accepted 12 June 1990)
Key words: Histamine; Histamine H~ receptor; Positron emission tomography; In vivo study; Dog; [t~C]Pyrilamine; [~C]Doxepin Histamine Hj receptors were visualized in the living dog brain using [HC]pyrilamine or [~C]doxepin by positron emission tomography (PET). The regional distribution of these carbon-I 1 labeled compounds in the brain corresponded well with that of the histamine Ht receptors separately determined by in vitro binding assay. The radioactivity in the brain was reduced by treatment with triprolidine (1 mg/kg), a histamine H~ antagonist. The results of our study indicate that it is feasible to visualize histamine H~ receptors in human brain using these HC-labeled compounds and PET.
Histamine is widely distributed in mammalian brain and is found in both neuronal and non-neuronal components [7, 10]. Its physiological functions as a neurotransmitter or neuromodulator are mediated through histamine H1, H2, and H3 receptors (for reviews see refs. 7, 8). Pyrilamine (or mepyramine) and doxepin are known to be potent histamine antagonists that bind to histamine H1 receptors [4, 5]. The recent development of llC-labeled pyrilamine and doxepin enables us to visualize histamine H~ receptors in vivo in the living human brain by positron emission tomography [3, 11]. This paper describes the visualization of histamine HI receptors in the dog brain in vivo as a pre-clinical step. [llC]Pyrilamine and [llC]doxepin were synthesized by reacting [lIC]methyliodide with the desmethylated precursors of pyrilamine and doxepin, respectively [3, 11]. The experiments were performed in 3 beagle dogs (weighing 18-20 kg) anesthetized by intravenous injection of pentobarbital (25 mg/kg). The animals were placed on their side on a PET scanner (PT 931, CTI [9]). The dogs were carefully positioned in a special headholder using a laser beam so as to obtain planes parallel to Orbito-Inion line (Fig. 1D). We utilized the OrbitoInion line in our dog study instead of Orbito-Meatal line (OM line) because obstruction of airways did not occur in this position during PET study. A Ge-68/Ga-68 transCorrespondence: K. Yanai, Department of Pharmacology I, Tohoku University School of Medicine, Seiryou machi 2-1 Aoba-Ku, Sendai 980, Japan. 0304-3940/90/$ 03.50 :C~ 1990 Elsevier Scientific Publishers Ireland Ltd.
mission scan was used to correct for auto-attenuation. Four to 6 mCi (148-222 MBq, 0.5-1.0 nmol/kg b.wt.) of the llC-labeled compounds (spec. act.) 200-500 mCi/ /tmol, 7.4-18.5 GBq//tmol at the time of use) were intravenously injected at time=0. The radioactivity in the brain was monitored over 60 min. Arterial blood samples were collected during the scanning time through a transcutaneous femoral catheter. The radioactivity of the plasma samples was measured in an NaI scintillation counter, and the percentage of ~lC-labeled ligand in the total I1C-radioactivity was analyzed by high-performance liquid chromatography (HPLC) [12]. In 4 experiments, a large amount of triprolidine, a histamine Ht receptor antagonist (1 mg/kg), was co-injected with the labeled compounds (pretreatment experiments, n = 2), or injected 30 min after the administration of the carbon- 11 labeled ligands (displacement experiments, n = 2) in order to disclose the specific binding sites. A regional variation was observed in the distribution of [llC]pyrilamine and [llC]doxepin as illustrated in Figs. 1 and 2. High concentrations of radioactivity were observed in the cerebral cortex and olfactory area, and low concentrations in the basal ganglia, hippocampus, and cerebellum. This distribution was similarly observed both in the studies using [llC]pyrilamine and [llC]doxepin, though the images obtained by [llC]doxepin were more contrasted than those of [I 1C]pyrilamine" The kinetics of [llC]pyrilamine distribution (Fig. 2A) revealed that cerebral [llC]pyrilamine activity reached a maximum 10-20 min after bolus injection, and that after a
42 TABLE 1 SPECIFIC [3H]PYRILAMINE BINDING TO MEMBRANE OF DOG BRAIN In vitro binding experiments were performed by a modification of Chang et al. [2]. In brief, tissues were homogenized in a Polytron in 30 vols. of ice-cold Na+-K ÷ phosphate buffer (50 mM, pH 7.5) and homogenate was centrifuged twice at 50,000 g for 20 min. Incubation of membrane fractions with [3H]pyrilamine (Amersham, spec. act. 28 mCi//Lmol) was carried out at 37"C for 10 min in the presence and absence of 2 #M triprolidine. To determine the K~values for doxepin and pyrilamine at 37°C, [3H]pyrilamine at 5 nM was incubated with the tissue and various concentrations of drugs.
-2
0
Inion
0
2
4
6
8
cm
Orbita
Region
Bma~ (pmol/g)
Region
Bma~ (pmol/g)
Frontal cortex Temporal cortex Occipital cortex Olfactory bulb Hypothalamus
14.4 13.3 13.1 11.6 8.88
Hippocampus Striatum Thalamus Cerebellum
7.56 6.95 6.70 2.41
plateau, a gradual decrease occurred until the end o f scanning. [11C]Doxepi n also showed a m a x i m u m uptake within 15 min, but the distribution then remained constant t h r o u g h o u t the rest o f the P E T study (Fig. 2B). W h e n 15 n m o l / k g o f cold pyrilamine was simultaneously injected with [13C]pyrilamine ' the brain uptake o f [llC]pyrilamine was reduced (data n o t shown). Displacement experiments with 1 m g / k g o f triprolidine showed a gradual decrease o f the cerebral activity soon after the administration o f cold antagonist (Fig. 3). A slow dissociation in vivo was also observed as previously reported [6, 12]. The brain radioactivity of ]lC-labeted pyrilamine was reduced by treatment with triprolidine, and the brain kinetics showed a rapid clearance. The [llC]pyrilamine concentrations in different regions o f the brain were almost the same in pretreatment experiments. Similarly, the specific distribution o f [~~C]doxepin disappeared after treatment with a large a m o u n t o f triprolidine (1 mg/kg) (data not shown). In spite o f the small brain size (approx. 7 x 6 cm) and p o o r spatial resolutions (8.5 mm), specific binding o f these c o m p o u n d s could be disclosed by the treatment with a large a m o u n t o f triprolidine. It is well k n o w n that considerable species differences occur in the affinity and the regional distribution o f histamine H1 receptors [2]. The distribution o f histamine H~ receptors in d o g brain has not been reported in the literature. Therefore, specific binding o f [3H]pyrilamine to
2cm
Fig. 1. PET images obtained with [uC]pyrilamine (A) and [uC]doxepin (B) at 4045 min after the injection. The planes have a transaxial spatial resolution of 8.5 mm at full width half maximum. The brain radioactivity is mainly localized in frontal, temporal, and occipital cortex. C: trans-axial view of the dog brain which represents the level
of the PET images (7 mm thickness in axis). D: a sagittal view of dog brain indicating the seven 7-mm-thick slices parallel to Orbito-Inion line. The 7 slices covered the whole dog brain. The solid line represents the slice of PET images listed in A and B. The cerebellum is included on the lower planes.
43 3
3
•
~3
zl
'
~,
v
,
e
I
~
A
,J
7
v
,
'
= '
2b
'
4'o
'
6b
'
Time after injection Imln)
2b
'
,b
'
g
Time after injection (mln)
Fig. 2. Time courses of change in total radioactivity in dog brain after bolus injection of [ILC]pyrilamine (A) and [ttC]doxepin (B). Frontal cortex (O); temporal cortex (A); occipital cortex (11); striatum ([B); cerebellum (V). Results are expressed as normalized radioactivity per tissue volume (percentage of the injected radioactivity per I00 ml tissue volume). Data shown are obtained in the same dog by two different experiments and are representative of three [HC]pyrilamine and two [] LC]doxepin experiments.
membranes of dog brain in vitro was assayed in order to demonstrate that the distributions of the two ligands obtained by PET reflect histamine Hi receptors. As listed in Table I, the highest levels of binding were observed in cerebral cortex, with values in the hippocampus, corpus striatum, and thalamus approximately 50% of this level, while the binding in the cerebellum was lowest. Thus, regional distribution of specifically bound [3H]pyrilamine in the dog brain agreed with the distribution of the two nlC-labeled compounds. This distribution was almost the same as that in human and monkey brain [1, 2]. The Kd value determined at 37°C was 5.9_+ 1.0 nM in the dog brain, 4 times higher than the Kd in guinea pig cerebel-
lum (1.5 nM), while Bma x values for the two species are the same (14.4 pmol/g and 15.7 pmol/g) [14]. The Ki value for doxepin was 2.8 +0.6 nM at 37°C, about a quarter of that for pyrilamine (11.7-+ 1.1 nM), which is consistent with longer retention and clearer images of [1iC]doxepin. All these results indicate that l nC-labeled pyrilamine and doxepin meet criteria for in vivo binding, and can be used to image histamine H1 receptors in vivo by positron emission tomography. Studying histamine H~ receptors by PET opens a new field of research on the physiological and pathological significance of histaminergic neuronal system in humans.
3
Triprolidine
--.....
Contro,
Pretreatm ~l:cement
~-,-e ....... • ...........• .............. o
'
2b
Plasma '
4'o
'
60
Time alter injection (men) Fig. 3. Pretreatment and displacement experiments of [ulC]pyrilamine distribution. Animals were injected with either saline (control, Lq---U) or, 1 mg/kg of triprolidine (pretreatment, V - - V ) 5 min before administration of [HC]pyrilamine. In displacement experiment, cold triprolidine (1 mg/kg) was injected 30 min after the ['C]ligand administration (displacement, I 1 - - 1 1 ) . The cerebral radioactivity was determined in frontal, temporal, occipital cortex using elliptical regions of interest (ROIs) positioned on the continuous PET images. Values of regions of interest in the cortex were averaged and plotted. The total ["C]radioactivity ( 0 - - - 0 ) and [lIC]pyrilamine radioactivity (O ............O) corrected for radioactive metabolites in the plasma were plotted in the figure. Data shown here are obtained in the same dog by 3 different experiments and are representative of two similar experiments.
44 The a u t h o r s t h a n k Mr. Y. Ishikawa, Mr. S. N a m e kawa, Mr. S. Seo, a n d Mr. S. W a t a n u k i for their technical assistance. This work was s u p p o r t e d in part by Grant-in-Aid
for
Specifically
Promoted
Research
63065004 from the M i n i s t r y of E d u c a t i o n , Science a n d C u l t u r e of Japan. 1 Bielkiewicz,B. and Cook, D.A., The binding of [3H]mepyramine binding to histamine H~ receptors in monkey brain, Can. J. Physiol. Pharmacol., 63 (1985) 756-759. 2 Chang, R.S.L., Tran, V.T. and Snyder, S.H., Heterogeneity of histamine H~ receptors: Species variations in [3H]pyrilaminebinding of brain membranes, J. Neurochem., 32 (1979) 1653-1663. 3 Dannals, R.F., Yanai, K., Wilson, A.A., Ravert, H.T., Frost, J.J., Scheffel, U. and Wagner, H.N., Jr., Synthesis of radiotracers for studying histamine H~ receptors: carbon-I 1 labeled doxepin and pyrilamine, J. Labeled Compd. Radiopharm., 26 (1989) 213-214. 4 Hill, S., Young, J.M. and Marrian, D.H., Specific binding of [3H]mepyramine to histamine HL receptors in intestinal smooth muscle, Nature, 270 (1977) 361-363. 5 Kanba, S. and Richelson, E., Histamine H~ receptors in human brain labelled with [3H]doxepin,Brain Res., 304 (1984) 1-7. 6 Perry, D.C., Mullis, K.B., Oie, S. and Sadee, W., Opiate antagonist
receptor binding in vivo: evidence for a new receptor binding model, Brain Res., 199 (1980) 49-61. 7 Prell, G.D. and Green, M., Histamine as a neuroregulator, Annu. Rev. Neurosci., 9 (1986) 209-254. 8 Schwartz, J.-C., Arrang, J.-M. and Garberg, M., Three classes of histamine receptors in brain, Trends Pharmacol., 7 (1986) 24-28. 9 Spinks, T.J., Guzzardi, R. and Bellina, C.R., Performance characteristics of a whole body positron tomograph. J. Nucl. Med., 29 (1988) 1833-1841. 10 Watanabe, T., Taguti, Y., Shiosaka, S., Tanaka, J., Kubota, H., Terano, J., Kubota, H., Terano, Y., Tohyama, M. and Wada, H., Distribution of the histaminergic neuron system in the central nervous system of rats: a fluorescentimmunohistochemicalanalysis with histidine decarboxylase as a marker, Brain Res., 295 (1984) 13 25. 11 Yanai, K., Dannals, R.F., Wilson, A.A., Ravert, H.T~, Scheffel, U., Tanada, S. and Wagner, H.N. Jr., (N-Methyl-[lIClpyrilamine,a radiotracer for histamine H~ receptors: Radiochemical synthesis and biodistribution study in mice, Nucl. Med. Biol., 15 (1988) 605610. 12 Yanai, K., Yagi, N., Watanabe, T., Itoh, M., lshiwata, K., Ido, T. and Matsuzawa, T., Specificbinding of [3H]pyrilamineto histamine H1 receptors in guinea pig brain in vivo: determination of binding parameters by a kinetic four-compartment model, J. Neurochem., in press.
