Localization of opioid receptor antagonist [3H]-LY255582 binding sites in mouse brain: comparison with the distribution of mu, delta and kappa binding sites

Agonist stimulation of opioid receptors increases feeding in rodents, while opioid antagonists inhibit food intake. The pan-opioid antagonist, LY255582, produces a sustained reduction in food intake and body weight in rodent models of obesity. However, the specific receptor subtype(s) responsible for this activity is unknown. To better characterize the pharmacology of LY255582, we examined the binding of a radiolabeled version of the molecule, [(3)H]-LY255582, in mouse brain using autoradiography. In mouse brain homogenates, the K(d) and B(max) for [(3)H]-LY255582 were 0.156 +/- 0.07 nM and 249 +/- 14 fmol/mg protein, respectively. [(3)H]-LY255582 bound to slide mounted sections of mouse brain with high affinity and low non-specific binding. High levels of binding were seen in areas consistent with the known localization of opioid receptors. These areas included the caudate putamen, nucleus accumbens, claustrum, medial habenula, dorsal endopiriform nucleus, basolateral nucleus of the amygdala, hypothalamus, thalamus and ventral tegmental area. We compared the binding distribution of [(3)H]-LY255582 to the opioid receptor antagonist radioligands [(3)H]-naloxone (mu), [(3)H]-naltrindole (delta) and [(3)H]-norBNI (kappa). The overall distribution of [(3)H]-LY255582 binding sites was similar to that of the other ligands. No specific [(3)H]-LY255582 binding was noted in sections of mu-, delta- and kappa-receptor combinatorial knockout mice. Therefore, it is likely that LY255582 produces its effects on feeding and body weight gain through a combination of mu-, delta- and kappa-receptor activity.

Characterization of neuropeptide Y Y1-like and Y2-like receptor subtypes in the mouse brain

To characterize receptor subtypes in the mouse, we performed autoradiographic localization and pharmacological characterization studies using the selective radiolabeled agonists, [(125)I]-Leu(31), Pro(34)-PYY and [(125)I]-PYY 3-36. The pharmacology of [(125)I]-Leu(31), Pro(34)-PYY and [(125)I]-PYY 3-36 binding to mouse brain homogenates were consistent with Y1-like and Y2-like receptors, respectively. Using receptor autoradiography, high Y1-like binding was observed in the islands of Calleja and dentate gyrus. [(125)I]-PYY 3-36 binding was highest in the hippocampus, lateral septum, stria terminalis of the thalamus, and compacta and lateralis of the substantia nigra. In addition, there are differences in receptor distribution in mouse brain compared to other species that may translate into different functional roles for the NPY receptors within each species.

Pharmacological characterization of (125)I-1229U91 binding to Y1 and Y4 neuropeptide Y/Peptide YY receptors

1229U91 (GW1229 or GR231118) [lle,Glu,Pro,Dpr,Tyr, Arg,Leu,Arg, Tyr-NH(2))2 cyclic (2,4'),(2'4)-diamide] has been reported by several research groups to be a potent antagonist at the Y1 neuropeptide Y (NPY) receptor subtype. However, 1229U91 also displaces (125)I-peptide YY (PYY) with high affinity from the Y4 subtype. Previously, we reported that 1229U91 had full agonist properties for the Y4 receptor. To characterize the pharmacological properties of 1229U91 directly, we had it radioiodinated with the chloromine-T method. (125)I-1229U91 bound to cell lines expressing the human Y1 and Y4 receptors with high affinity. The K(d) and B(max) for (125)I-1229U91 binding to Y1 were 14.9 pM and 1458 fmol/mg protein, respectively. The Y4 receptor bound (125)I-1229U91 with a K(d) of 12.5 pM and a B(max) of 1442 fmol/mg protein. When competing (125)I-1229U91 binding from Y1 and Y4 receptors, a similar rank order of potency was observed: 1229U91 > [Leu(31),Pro(34)]-NPY >/= [Leu(31),Pro(34)]-PYY > PYY >/= NPY > NPY(2-36) > PYY(3-36). Pancreatic polypeptide (PP) potently displaced (125)I-1229U91 from the Y4 receptor, but displayed little affinity for Y1. In autoradiographic studies with rat brain sections, (125)I-1229U91 bound with a distribution similar to that reported for the Y1 receptor when localized with (125)I-[Leu(31),Pro(34)]-PYY. Brain regions exhibiting binding sites for (125)I-PP were not detected with this radioligand. Those include the interpeduncular nucleus and the periventricular nucleus of the hypothalamus. Furthermore, (125)I-labeled rat PP was not displaced from these areas with 10 nM 1229U91. Thus, (125)I-1229U91 is a high affinity Y1 and Y4 radioligand and binds with a distribution in the rat brain consistent with the localization of the Y1 receptor.

Structure-activity relationship of a series of diaminoalkyl substituted benzimidazole as neuropeptide Y Y1 receptor antagonists

A series of benzimidazoles (4) was synthesized and evaluated in vitro as potent and selective NPY Y1 receptor antagonists. Substitution of the piperidine nitrogen of 4 with appropriate R groups resulted in compounds with more than 80-fold higher affinity at the Y receptor compared to the parent compound 5 (R = H). The most potent benzimidazole in this series was 21 (Ki = 0.052 nM).

Synthesis and evaluation of a series of novel 2-[(4-chlorophenoxy)methyl]benzimidazoles as selective neuropeptide Y Y1 receptor antagonists

A series of novel benzimidazoles (BI) derived from the indole 2 was synthesized and evaluated as selective neuropeptide Y (NPY) Y1 receptor antagonists with the aim of developing antiobesity drugs. In our SAR approach, the (4-chlorophenoxy)methyl group at C-2 was kept constant and a series of BIs substituted with various piperidinylalkyl groups at N-1 was synthesized to identify the optimal spacing and orientation of the piperidine ring nitrogen relative to the benzimidazole. The 3-(3-piperidinyl)propyl in 33 was found to maximize affinity for the Y1 receptor. Because of the critical importance of Arg33 and Arg35 of NPY binding to the Y1 receptor, the incorporation of an additional aminoalkyl functionality to the structure of 33 was explored. Methyl substitution was used to probe where substitution on the aromatic ring was best tolerated. In this fashion, the C-4 was chosen for the substitution of the second aminoalkyl functionality. Synthesis of such compounds with a phenoxy tether using the 4-hydroxybenzimidazole 11 was pursued because of their relative ease of synthesis. Functionalization of the hydroxy group of 45 with a series of piperidinylalkyl groups provided the dibasic benzimidazoles 55-62. Among them, BI 56 demonstrated a Ki of 0.0017 microM, which was 400-fold more potent than 33. To evaluate if there was a stereoselective effect on affinity for these BIs, the four constituent stereoisomers (69-72) of the BI 60 were prepared using the S- and R-isomers of bromide 17. Antagonist activity of these BIs was confirmed by measuring the ability of selected compounds to reverse NPY-induced forskolin-stimulated cyclic AMP. The high selectivity of several BI antagonists for the Y1 versus Y2, Y4, and Y5 receptors was also shown.

[125I]Leu31, Pro34-PYY is a high affinity radioligand for rat PP1/Y4 and Y1 receptors: evidence for heterogeneity in pancreatic polypeptide receptors

Cloned receptors for the PP-fold peptides are subdivided into Y1, Y2, PP1/Y4, Y5 and Y6. NPY and PYY have similar affinity for Y1, Y2, Y5 and Y6 receptors while PP has highest affinity for PP1. Pro34-substituted analogs of NPY and PYY have selectivity for Y1 and Y1-like receptors over Y2 receptors. In the present study, we found the putative Y1-selective radioligand, [125I]Leu31, Pro34-PYY, also binds with high affinity to the rat PP1 receptor in cell lines expressing the receptor. However, in rat brain sections, [125I]Leu31, Pro34-PYY does not appear to bind to the interpeduncular nucleus, a brain region containing a high density of [125I]-bPP binding sites. Therefore, it appears there is additional heterogeneity in receptors recognizing PP.

Differential distribution of neuropeptide Y Y1 and Y2 receptors in rat and guinea-pig brains

The distribution of neuropeptide Y Y1 and Y2 receptors has been extensively studied in the rat using selective masking of [125I]peptide YY binding by Y1- and Y2-selective peptides. In the present study, we characterized the binding of the subtype-selective radioligands [125I](Leu31-Pro34)-peptide YY and [125I]peptide YY3-36 to rat and guinea-pig brains and identified differences in Y1 and Y2 distribution. [125I](Leu31Pro34)-peptide YY and [125I]peptide YY3-36 bound to single sites in rat and guinea-pig brains with pharmacologies consistent with the Y1 and Y2 receptors, respectively. Autoradiographic studies were conducted using adjacent sections from rat and guinea-pig brains. Compared to the rat, the distribution of both Y1 and Y2 receptors was markedly different in the guinea-pig. For example, a high level of Y1 binding was detected within the thalamus of the rat while, in the guinea-pig, very little Y1 binding was observed in this region. Y1 binding was very low in the rat hippocampus, while the guinea-pig hippocampus contained high levels of Y1 binding. High levels of both Y1 and Y2 binding were observed in the guinea-pig cerebellum while, in the rat, only low levels of Y2 binding were visible. Therefore, the guinea-pig brain exhibits dramatic differences in the pattern of Y1 and Y2 receptors when compared to the rat. It is likely that Y1 and Y2 receptors perform different roles in the central nervous system of guinea-pigs and rats.

The neuropeptide Y Y1 receptor selective radioligand, [125I][Leu31,Pro34]peptide YY, is also a high affinity radioligand for human pancreatic polypeptide 1 receptors

A number of receptors for the pancreatic polypeptide-fold peptides are proposed based on findings from pharmacology and molecular biology studies. Neuropeptide Y and peptide YY have similar affinity for neuropeptide Y Y1 and neuropeptide Y Y2 while pancreatic polypeptide has highest affinity for pancreatic polypeptide 1. Pro34-substituted analogs of neuropeptide Y and peptide YY have selectivity for neuropeptide Y Y1 over neuropeptide Y Y2 receptors. In the present study, we found that one such 'neuropeptide Y Y1-selective' radioligand, [125I][Leu31,Pro34]peptide YY, also binds with high affinity to the pancreatic polypeptide 1 receptor. Therefore, caution needs to be exercised when using Pro34-analogs to define the neuropeptide Y Y1 receptor in vivo and using tissue preparations.

Pharmacological characterization of neuropeptide Y-(2-36) binding to neuropeptide Y Y1 and Y2 receptors

Neuropeptide Y-(2-36) has been reported by several research groups to be a more potent orexigenic agent than intact neuropeptide Y. Therefore, it has been proposed that a novel 'Y1 variant' may modulate ingestive behavior. To define the receptor subtype involved in neuropeptide Y-stimulated feeding behavior, we evaluated the binding properties of neuropeptide Y-(2-36) and [125I]neuropeptide Y-(2-36) in established neuropeptide Y1 and Y2 containing cell lines and tissues. Neuropeptide Y-(2-36) displaced [125I]peptide YY binding to SK-N-MC cells (neuropeptide Y Y1 receptors) with a Ki of 3.69 nmol and SK-N-BE(2) cells (neuropeptide Y Y2 receptors) with a Ki of 3.08 nmol. Neuropeptide Y-(2-36) also displaced [125I]peptide YY binding to rat cerebral cortex, hippocampus and olfactory bulb with similar affinities. To examine the brain distribution of [125I]peptide YY, [125I]neuropeptide Y and [125I]neuropeptide Y-(2-36), adjacent sections were labeled and the binding sites detected by autoradiography. A similar distribution of binding was observed for each radioligand in all regions examined. Therefore, neuropeptide Y-(2-36) binds non-selectively to neuropeptide Y Y1 and neuropeptide Y Y2 receptors, but with lower affinity than neuropeptide Y and peptide YY. The increased potency and selectivity seen with neuropeptide Y-(2-36) in feeding studies cannot be explained on the basis of a unique in vitro pharmacology.

Expression cloning of a human brain neuropeptide Y Y2 receptor

The 36-amino acid peptide, neuropeptide Y (NPY), is a member of a peptide family that includes the endocrine peptides, peptide YY (PYY), and pancreatic polypeptide (PP). NPY receptors have been broadly subdivided into postsynaptic Y1 receptors and presynaptic Y2 receptors based on the preference of Pro34-substituted analogues for the Y1 receptors and carboxyl-terminal fragments for the Y2. A Y1 receptor has been cloned, and this receptor appears to mediate several effects of NPY, including vasoconstriction and anxiolysis in animal models. We report the cloning of a human brain Y2 receptor from a human brain library. Pools of clones were transiently expressed in COS-1 cells, and 125I-PYY binding pools were identified by autoradiography. After a single positive pool was detected in the original screening, a single clone was isolated by four rounds of sequential enrichment. The clone encoded a 381-amino acid protein of the heptahelix (seven TM) type. Amino acid identity of this receptor with the Y1 receptor was 31% overall with 40% identity in the TM regions. Comparison with the human PP1 receptor indicated 33% overall amino acid identity with 42% identity in the TM regions. Pharmacologically, the receptor exhibited high affinity for NPY, PYY, and carboxyl-terminal fragments of NPY and PYY. In addition, Pro34-substituted analogues had very low affinity. With the use of Northern blot analysis, high levels of Y2 mRNA were detected in a variety of brain regions with little expression in peripheral tissues. Thus, the receptor protein has the pharmacological properties and distribution of the human Y2 receptor.

Unexpected high density of neuropeptide Y Y1 receptors in the guinea pig spleen

Receptors for neuropeptide Y (NPY) and peptide YY (PYY) have been extensively characterized in the brain. Less is known about NPY receptor subtypes in the spleen, though it is well established that NPY produces vascular contraction in this tissue. In the present study, we found an unusually high density of Y1 receptors in the guinea pig spleen. These receptors are localized to the red pulp and exhibit a pharmacology that is consistent with the Y1 receptor. On the other hand, only very low densities for Y2 receptors were observed. Therefore, the guinea pig spleen may be a ideal tissue for further study of the role of Y1 receptors in cardiovascular and immune function.

[3H]-quinelorane binds to D2 and D3 dopamine receptors in the rat brain

Quinelorane is a BCD partial ergoline with potent dopaminergic effects in vitro and in vivo. Partial ergoline compounds of this series consist of the B-, C- and D-rings of the four ring ergoline skeleton. Many of the pharmacological effects of quinelorane are believed to be due to stimulation of the D2 subtype of the dopamine receptor. Recently, a D3 dopamine receptor was identified that is insensitive to guanine nucleotides and exhibits an unusual distribution in the brain. When this receptor is expressed in Chinese hamster ovary cells, quinelorane has higher affinity for the D3 receptor than the D2 receptor. To further define the pharmacology of quinelorane, we have synthesized [3H]-quinelorane and examined its binding to sections of rat brain in vitro. [3H]-quinelorane bound with high affinity (KD = 1.8 nM) and exhibited very low nonspecific binding. D2 selective antagonists, such as (+)butaclamol and spiperone, were potent inhibitors of binding while the D1 antagonist SCH23390 was significantly less potent. A majority of the binding was inhibited in a concentration-dependent manner by guanylyl-imidodiphosphate with a maximal inhibition at concentrations of 1 microM and greater. Autoradiographic studies were performed in the presence and absence of 10 microM Gpp(NH)p. Binding in D2 containing regions, such as the caudate-putamen, was completely inhibited by guanylyl-imidodiphosphate although binding in D3 containing areas, such as the islands of Calleja, was unaffected. Therefore, [3H]-quinelorane is an excellent agonist radioligand for the localization of D2 and D3 receptors in rat brain.

Comparison of (R)-[3H]tomoxetine and (R/S)-[3H]nisoxetine binding in rat brain

(R)-[3H]Tomoxetine is a radioligand that binds to the norepinephrine (NE) uptake site with high affinity but also binds to a second, lower-affinity site. The goal of the present study was to identify the nature of this low-affinity site by comparing the binding properties of (R)-[3H]tomoxetine with those of (R/S)-[3H]nisoxetine, a highly selective ligand for the NE uptake site. In homogenate binding studies, both radioligands bound to the NE uptake site with high affinity, whereas (R)-[3H]tomoxetine also bound to a second, lower-affinity site. The autoradiographic distribution of binding sites for both radioligands is consistent with the known distribution of NE-containing neurons. However, low levels of (R)-[3H]tomoxetine binding were seen in the caudate-putamen, globus pallidus, olfactory tubercle, and zona reticulata of the substantia nigra, where (R/S)-[3H]nisoxetine binding was almost absent. In homogenates of the caudate-putamen, the NE uptake inhibitors desipramine and (R)-nisoxetine and the serotonin (5-HT) uptake inhibitor citalopram produced biphasic displacement curves. Autoradiographic studies using 10 nM (R)-nisoxetine to mask the binding of (R)-[3H]tomoxetine to the NE uptake site produced autoradiograms that were similar to those produced by [3H]citalopram. Therefore, (R)-[3H]tomoxetine binds to the NE uptake site with high affinity and the 5-HT uptake site with somewhat lower affinity.

Autoradiographic comparison of [125I]LSD-labeled 5-HT2A receptor distribution in rat and guinea pig brain

Although the density and distribution of 5-HT2A (5-hydroxytryptamine-2A) receptors is well established for rat brain, the 5-HT2A receptor distribution and density in guinea pig brain has not been extensively studied. In the present in vitro study, we have utilized 125I-lysergic acid diethylamide ([125I]LSD) to quantify and compare 5-HT2A receptor density in coronal sections of rat and guinea pig brain. Spiperone (1 microM) and sulpiride (1 microM) were used to displace [125I]LSD binding from 5-HT2A and D2 binding sites, respectively. Ligand binding was quantified by computer-aided image analysis densitometry (MCID). Similar to the rat, areas of highest specific 5-HT2A receptor binding (fmol/mg protein) in guinea pig brain included the claustrum and Layer 4 of the cerebral cortex. Significant binding was also found in remaining neocortical layers, islands of Calleja, caudate putamen, olfactory bulb, nucleus accumbens, and choroid plexus. While the rat brain exhibited a high level of specific binding in the tenia tecta and mammillary nuclei, little binding was observed in these regions in the guinea pig. In both rat and guinea pig, low specific binding was found in amygdaloid, thalamic, or cerebellar areas. These studies indicate a general similarity between 5-HT2A binding site distribution and relative density in guinea pig and rat brain but point to a few brain regions where significant differences exist.

Localization of neuropeptide Y immunoreactivity and [125I]peptide YY binding sites in the human pituitary

High levels of neuropeptide Y (NPY) are found in the hypothalamus, median eminence, pituitary portal blood, and the pituitary in a number of species. Neuropeptide Y may influence the synthesis and secretion of a variety of hormones by interacting with specific receptors in the hypothalamus and/or the pituitary. To further define the function of NPY in the pituitary, we have examined the distribution of NPY immunoreactivity and NPY receptors in sections of human pituitary using immunohistochemical and autoradiographic techniques. Neuropeptide Y-immunoreactive varicose axons were seen throughout the neural lobe. A moderate number of NPY-immunoreactive cells were found in the anterior lobe. A very high level of [125I]PYY binding was seen in the neural lobe with low levels in the anterior lobe. The binding in the neural lobe was inhibited by NPY(13-36) at a Ki of 5.3 nM and [Leu31-Pro34]NPY at a Ki of 390 nM, indicating the receptor was the Y2 subtype. Therefore, neuronally released NPY may modulate human neural lobe function through a Y2 receptor.

Synthesis and evaluation of [125I]-(S)-iodozacopride, a high affinity radioligand for 5HT3 receptors

We have developed a high specific activity radioiodinated ligand for the biochemical evaluation and autoradiographic localization of 5HT3 receptors in the brain. [125I]-(S)-iodozacopride was synthesized by radioiodination of deschloro-(S)-zacopride using chloramine-T, and the product was purified by HPLC. The equilibrium kinetics and pharmacology of the binding of this radioligand were studied in homogenates of rat cerebral cortex, while the distribution of binding was examined by quantitative autoradiography. [125I]-(S)-iodozacopride bound to a single, saturable, specific binding site (Kd = 192 +/- 9 pM, Bmax = 1.2 +/- 0.2 fmol/mg protein). The binding had the pharmacological properties of a 5HT3 receptor, being potently inhibited by a variety of 5HT3 agonists and antagonists including (S)-zacopride (Ki = 0.032 nM), Quipazine (Ki = 0.45 nM), LY278584 (Ki = 0.5 nM), (1-m-chlorophenyl)-biguanide (Ki = 0.6 nM) and ICS 205-930 (Ki = 1.0 nM). Autoradiographic studies were undertaken by incubating sections with 400 pM [125I]-(S)-iodozacopride and exposing them to film for 3-7 days to obtain suitable autoradiograms. Specific binding of [125I]-(S)-iodozacopride was found at various amounts in a variety of brain regions. The highest levels of binding were found in the brainstem, principally the nucleus of the solitary tract with somewhat lower levels in the area postrema, substantia gelatinosa of the trigeminal nucleus and dorsal motor nucleus of the vagus. In the rat forebrain, moderate levels of specific binding were found in the glomerular layer of the olfactory bulb, anterior olfactory nucleus and various subnuclei of the amygdala. Lower levels of binding were seen in the superficial laminae of the parietal cerebral cortex and diffusely distributed throughout the hippocampal formation. In conclusion, [125I]-(S)-iodozacopride binds to a receptor site with the pharmacological properties and distribution that is consistent with the 5HT3 receptor. [125I]-(S)-iodozacopride represents a significant improvement in autoradiographic studies of the 5HT3 receptor by reducing the required exposure time for producing autoradiograms from the 3-6 months required for [3H]-labeled ligands to 3-7 days.

Localization of rat brain binding sites for [3H]tomoxetine, an enantiomerically pure ligand for norepinephrine reuptake sites

The distribution of binding sites for the potent inhibitor of norepinephrine (NE) reuptake, [3H]tomoxetine, was examined in rat brain using quantitative autoradiography. Scatchard analysis of [3H]tomoxetine-binding to slide-mounted sections of rat forebrain indicated that the ligand bound to two sites, a high-affinity site with a Kd of 0.29 nM and a lower-affinity site with a Kd of 16 nM. Pharmacological characterization of this high-affinity site was consistent with labelling a NE-uptake site in brain. Autoradiographic localization of the binding sites for [3H]tomoxetine was performed at a ligand concentration of 1 nM representing the distribution of high-affinity sites. The radioligand bound with a distribution of binding sites that was consistent with the known distribution of NE-containing neurons. The highest levels of binding were seen in regions, such as the locus coeruleus, bed nucleus of the stria terminalis, anterior ventral nucleus of the thalamus and the paraventricular nucleus of the hypothalamus. Low levels were seen in regions such as the caudate-putamen, ventral tegmental area and zona reticulata of the substantia nigra, where NE-containing neurons have been reported to be low. Binding to all these sites was inhibited by 1 microM desipramine which produced autoradiograms with a uniform nonspecific binding. These results indicate that low concentrations of [3H]tomoxetine can be used to localize and characterize NE-binding sites. Further study will be necessary to determine the nature of the low-affinity binding site.

Comparison of the distribution of binding sites for the potassium channel ligands [125I]apamin, [125I]charybdotoxin and [125I]iodoglyburide in the rat brain

Potassium channels represent a diverse and promising target for drug development. Pharmacological subtypes of K channels have begun to emerge based on the development of both organic molecules and peptide toxins which possess subtype selectivity. In order to evaluate the neuroanatomical distribution of these subtypes we have utilized the ligands [125I]apamin, [125I]charybdotoxin and [125I]iodoglyburide in an autoradiographic study of rat brain. In the rat brain, these ligands have selectivity for the low conductance Ca(2+)-activated, voltage-gated K channels and ATP-sensitive K channels respectively. The distribution of binding sites for these three ligands were distinctly different. [125I]Apamin binding was highest in various thalamic and hippocampal structures, while only low to moderate levels of [125I]charybdotoxin binding were seen in these regions. In contrast, very high levels of [125I]charbydotoxin were seen in white matter regions such as the lateral olfactory tract and fasciculus retroflexus. High levels of [125I]charybdotoxin binding were also seen in gray matter-containing regions such as the zona incerta, medial geniculate and superior colliculus, where low to moderate [125I]apamin binding was found. [125I]Iodoglyburide presented a more uniform binding with the highest levels in the globus pallidus, islands of Calleja, anteroventral nucleus of the thalamus and zonas reticulata of the substantia nigra. These results indicate that subtypes of K channels have very different distributions in the brain. As such, the results imply differing CNS actions for potential modulators of K channel subtypes.

[Leu31-Pro34] neuropeptide Y identifies a subtype of 125I-labeled peptide YY binding sites in the rat brain

Subtypes of the neuropeptide Y (NPY) receptor in the rat brain were identified by the use of the selective Y-1 analog, [Leu34-Pro34] NPY. In rat brain homogenate binding studies, [Leu31-Pro34] NPY was found to produce a partial inhibition of 100 pM 125I-labeled peptide YY (PYY) binding with a plateau at 50-1000 nM [Leu31-Pro34] NPY resulting in a 70% inhibition of binding. The C-terminal fragment NPY 13-36, a putative Y-2 agonist, exhibited very little selectivity in rat brain homogenates. Scatchard analysis of 125I-labeled PYY binding to rat brain homogenate yielded biphasic plots with Kd values of 40 and 610 pM. Inclusion of 100 nM [Leu31-Pro34] NPY was found to eliminate the low affinity component of 125I-labeled PYY binding leaving a single, high affinity binding site with a Kd of 68 pM. In autoradiographic studies, displacement curves indicated that [Leu31-Pro34] NPY completely inhibited binding in the cerebral cortex with little effect on the binding in the hypothalamus. On the other hand NPY 13-36 inhibited binding in the hypothalamus at low concentrations but required higher concentrations to inhibit binding in the cerebral cortex. Other brain regions such as the hippocampus, appeared to contain both subtypes. Subsequent to these studies, a quantitative autoradiographic map was conducted using 50-100 pM 125I-labeled PYY in the presence and absence of [Leu31-Pro34] NPY which produced a selective displacement of binding in certain distinct brain regions. These areas included the cerebral cortex, certain thalamic nuclei and brainstem while ligand binding was retained in other brain regions including the zona lateralis of the substantia nigra, lateral septum, nucleus of the solitary tract and the hippocampus. Numerous brain regions appeared to contain both receptor subtypes. Therefore, the Y-1 and Y-2 receptor subtypes exhibited a somewhat distinct distribution in the brain. In addition, 125I-labeled PYY appears to label the Y-2 receptor with relatively higher affinity when compared to the Y-1 receptor.

Autoradiographic localization of [125I]charybdotoxin binding sites in rat brain

Charybdotoxin, a 37 amino acid peptide isolated from scorpion venom, is a potent inhibitor of potassium channel function. [125I]charybdotoxin was originally believed to be a selective ligand for the Ca(2+)-sensitive channel in many tissues, but it appears to bind only to a voltage-sensitive potassium channel in brain. We found high densities of [125I]charybdotoxin binding in lateral olfactory tract, interpeduncular nucleus and a variety of mesencephalic nuclei. Moderate levels were found in the cerebral cortex, medial thalamus, hypothalamus and selected thalamic nuclei. These results indicate that [125I]charybdotoxin identifies a potassium channel or channels with a unique distribution in the brain.

Autoradiographic localization of [3H]quinpirole binding to dopamine D2 and D3 receptors in rat brain

A radiolabelled form of the dopamine agonist, quinpirole (LY17155), has been evaluated as a ligand for dopamine receptors in the rat brain. Quinpirole has been reported to be a selective D2 dopamine agonist; however, a recent report has indicated that it may have high affinity for a novel dopamine binding site which has been termed D3. In rat brain sections, [3H]quinpirole binding exhibited a distribution similar to that described for dopamine D2 receptors using either agonist or antagonist labelling. High densities of binding could be found in caudate-putamen, nucleus accumbens, olfactory tubercle and islands of Calleja. When the labelling was done in the presence of 10 microM guanylyl-5'-imidodiphosphate to convert the dopamine D2 receptor to a 'low affinity agonist conformation', binding was inhibited in most brain regions with the notable exception of the islands of Calleja which retained most of the [3H]quinpirole binding. The guanine nucleotide insensitivity of this binding and distribution of this site indicates that [3H]quinpirole is binding to dopamine D3 receptors in this region of the brain. Therefore, these results indicate that [3H]quinpirole labels a high affinity agonist conformation of dopamine D2 receptors as well as dopamine D3 receptors in rat brain. In addition, this study provides the first detection the dopamine D3 receptor protein in the brain.

Localization of 5-HT3 receptors in the rat brain using [3H]LY278584

5-HT3 receptors have been localized in the rat brain using the selective antagonist ligand [3H]LY278584. The binding of this ligand to slide mounted tissue sections was characterized by a Kd value of 1.5 nM and a Bmax value of 110 fmol/mg tissue dry weight. The specific binding was displaced by 5-HT or a number of 5-HT3 antagonist compounds. High densities of 5-HT3 receptors were detected in the nucleus of the solitary tract, dorsal motor nucleus of the vagus and area postrema. Moderate levels of binding were found in the glomerular layer of the olfactory bulb, substantia gelatinosa of the trigeminal nucleus and spinal cord and various nuclei of the amygdala. Low levels of binding were found in the superficial laminae of the cerebral cortex and relatively evenly distributed in the hippocampus. These results indicate that [3H]LY278584 is a useful ligand to study 5-HT3 receptors by quantitative autoradiography.

Quantitative autoradiography of the binding sites for [125I] iodoglyburide, a novel high-affinity ligand for ATP-sensitive potassium channels in rat brain

We have developed a high specific activity ligand for localization of ATP-sensitive potassium channels in the brain. When brain sections were incubated with [125I]iodoglyburide (N-[2-[[[(cyclohexylamino)carbonyl]amino]sulfonyl]ethyl]-5-125I-2- methoxybenzamide), the ligand bound to a single site with a KD of 495 pM and a maximum binding site density of 176 fmol/mg of tissue. Glyburide was the most potent inhibitor of specific [125I]iodoglyburide binding to rat forebrain sections whereas iodoglyburide and glipizide were slightly less potent. The binding was also sensitive to ATP which completely inhibited binding at concentrations of 10 mM. Autoradiographic localization of [125I]iodoglyburide binding indicated a broad distribution of the ATP-sensitive potassium channel in the brain. The highest levels of binding were seen in the globus pallidus and ventral pallidum followed by the septohippocampal nucleus, anterior pituitary, the CA2 and CA3 region of the hippocampus, ventral pallidum, the molecular layer of the cerebellum and substantia nigra zona reticulata. The hilus and dorsal subiculum of the hippocampus, molecular layer of the dentate gyrus, cerebral cortex, lateral olfactory tract nucleus, olfactory tubercle and the zona incerta contained relatively high levels of binding. A lower level of binding (approximately 3- to 4-fold) was found throughout the remainder of the brain. These results indicate that the ATP-sensitive potassium channel has a broad presence in the rat brain and that a few select brain regions are enriched in this subtype of neuronal potassium channels.

Angiotensin II receptor subtypes in rat brain: dithiothreitol inhibits ligand binding to AII-1 and enhances binding to AII-2

Angiotensin II (AII) receptor subtypes have been proposed on the basis of the selectivity of non-peptide AII antagonists. In the present study, the sulfhydryl reducing agent dithiothreitol (DTT) was found to reduce binding to the AII-1 receptor while enhancing binding at the AII-2 site. The neuroanatomical distribution of these effects were consistent with the distribution of AII-1 and AII-2 receptors, respectively. These data indicate that AII receptor subtypes in the brain can be differentiated by both biochemical and pharmacological means.

Autoradiographic localization of subtypes of angiotensin II antagonist binding in the rat brain

The non-peptide angiotensin II receptor compounds DuP 753 and WL 19 were utilized to detect subtypes of [125I]Sar1-Ile8-angiotensin II binding to angiotensin II receptors in the rat brain. In rat forebrain homogenates, DuP 753 and WL 19 produced a partial displacement of [125I]Sar1-Ile8-angiotensin II binding with DuP 753 displacing approximately 65% of the binding and WL 19 displacing approximately 35% of the binding. Using the techniques of quantitative receptor autoradiography, a distinct regional distribution of the subtypes of angiotensin II antagonist bind was detected. The angiotensin II-1 binding site (the receptor subtype preferentially displaced by DuP 753) appeared to predominate in the dipsogenic, cardiovascular and endocrine areas, including the subfornical organ, paraventricular and periventricular nuclei of the hypothalamus, anterior pituitary, dorsal motor nucleus of the vagus, nucleus of the solitary tract and the area postrema. Additional areas that contained predominantly the angiotensin II-1 receptor subtype were the ventral hippocampus, substantia gelatinosa of the trigeminal nucleus, nucleus of the lateral olfactory tract, piriform cortex and median preoptic nucleus. The angiotensin II-2 binding site (displaced by WL 19) was the predominant subtype in the thalamus, inferior olive, lateral septum, subthalamic nucleus, locus coeruleus, medial geniculate and medial amygdala. Several areas of the brain appeared to contain both receptor subtypes, including the superior and inferior colliculi, and the olfactory bulb. The angiotensin II-1 binding site was concentrated in areas of the brain involved in mediating angiotensin II effects on drinking, endocrine status and blood pressure. Localization of angiotensin II-2 sites in the thalamus and areas of the brain which process sensory information suggests a novel modulatory role for angiotensin II at this receptor subtype. These results indicate that DuP 753 and WL 19 are highly selective for angiotensin II binding site subtypes in the brain and that, in general these subtypes are compartmentalized in distinct brain regions. The non-peptide compounds used in these studies should provide excellent tools to discern the functional role of angiotensin II receptor subtypes in the brain.

Non-peptide angiotensin II receptor antagonists discriminate subtypes of 125I-angiotensin II binding sites in the rat brain

We have utilized quantitative autoradiography to define subtypes of 125I-angiotensin II (AII) binding in rat brain. AII-1 binding (displaced by DuP 753) was found in the nucleus of the solitary tract and the hypothalamus, while AII-2 binding (displaced by WL 19) was found in the thalamus and lateral septum. These results indicate that subtypes of the AII receptor are present in the brain and the AII-1 receptor subtype is present in regions consistant with the known actions of angiotensin.