Solubilization and Characterization of a [3H]Hemicholinium-3 Binding Site in Rat Brain

A sodium-dependent high-affinity [3H]-hemicholinium-3 ([3H]HCh-3) binding site was solubilized from rat striatal synaptic plasma membranes by 0.2% deoxycholate. Deoxycholate solubilization of the [3H]HCh-3 binding site was dependent upon both detergent concentration and ionic strength of the solubilization medium. Specific [3H]HCh-3 binding to the solubilized preparation was both sodium- and chloride-dependent and saturable, exhibiting an affinity of 14.2 nM and a capacity (Bmax) of 695 fmol/mg protein. Choline and other analogs inhibited specific [3H]HCh-3 binding to the solubilized preparation in a concentration-dependent manner with the similar rank order of potency observed in crude synaptic membranes. Treatments known to disrupt both protein and lipid moieties resulted in diminished specific [3H]HCh-3 binding. These results suggest that the characteristics of the solubilized [3H]HCh-3 binding site are similar to those of the membrane-bound site.     

High-Affinity Choline Transport Sites: Use of [3H]Hemicholinium-3 as a Quantitative Marker

Abstract: High-affinity choline transport (HAChT), the rate-limiting and regulatory step in acetylcholine (ACh) synthesis, is selectively localized to cholinergic neurons. Hemicholinium-3 (HC3), a potent and selective inhibitor of HAChT, has been used as a specific radioligand to quantify HAChT sites in membrane binding and autoradiographic studies. Because both HAChT velocity and [³H]HC3 binding change as in vivo activity of cholinergic neurons is altered, these markers are also useful measures of cholinergic neuronal activity. Evidence that [³H]HC3 is a specific ligand for HAChT sites on cholinergic terminals is reviewed. The ion requirements of HAChT and [³H]HC3 binding indicate that sodium and chloride are required for recognition of both choline and [³H]HC3. A common recognition site is also indicated by the close correspondence of the potency of HC3 and choline analogues for inhibiting both HAChT and [³H]HC3 binding. The parallel regional distributions of both markers in adult brain, during development and after specific lesions, all indicate specific cholinergic localization. The close association of HAChT and [³H]HC3 binding sites is also supported by parallel regulatory changes occurring after in vivo drug treatments and in vitro depolarization. Overall, the data indicate a close association between HAChT and [³H]HC3 binding and are consistent with the sites being identical. Methodologic considerations in using [³H]HC³ as a ligand and considerations in interpretation of results are also discussed.     

High-Affinity [3H]Choline Accumulation in Cultured Human Skin Fibroblasts

[3H]Choline can be transported across cell membranes by high-affinity (KT less than 5 microM) and low-affinity (KT much greater than 5 microM) systems. High-affinity choline accumulation (HACA) has been demonstrated in synaptosomes made from cholinergic brain regions such as the hippocampus and caudate-putamen. In cell culture, HACA has been demonstrated in glia and avian telencephalon, dissociated spinal cord, and muscle fibroblasts. We examined [3H]choline accumulation in a single normal human fibroblast line cultured from skin biopsy. [3H]Choline accumulation was temperature-dependent and linear with incubation time up to 6 min at 0.125 microM-choline. The apparent KT for [3H]choline was 5 microM, which is similar to that observed in avian fibroblasts. Isoosmotic replacement of Na+ with either Li+ (144 mM) or sucrose (288 mM) severely reduced [3H]choline accumulation (by 70-90%). Pre-incubation with ouabain (100 microM), sodium orthovanadate (100 microM), or 2,4-dinitrophenol (100 microM), or replacement of Ca2+ by Mg2+ had little or no effect on subsequent [3H]choline accumulation. [3H]Choline accumulation was inhibited by hemicholinium-3 (HC-3); after pre-incubation in HC-3 at 37 degrees C for 10 min, the IC50 (at 0.125 microM-choline) was 5.6 microM. The HC-3 sensitivity, Na+ dependence, and low KT suggest that human skin fibroblasts have a high-affinity transport system for choline.     

Identification of Sodium-Dependent, High-Affinity Choline Uptake Sites in Rat Brain with [3H]Hemicholinium-3

[3H]Hemicholinium-3 (HC-3) was used to label sodium-dependent, high-affinity choline uptake sites in regions of rat brain. Autoradiography revealed a high density of [3H]HC-3 binding sites in brain regions with a high density of cholinergic terminals, such as the interpeduncular nucleus, caudate-putamen, and olfactory tubercle. This distribution of [3H]HC-3 binding sites was in close agreement with the amounts of choline acetyltransferase in specific nuclei and subregions of rat brain. Destruction of presynaptic cholinergic projections in the cerebral cortex and the basal ganglia by injection of excitotoxins reduced [3H]HC-3 binding by 40-50%. These data indicate that sodium-dependent [3H]HC-3 binding sites are related to the choline transport system present in cholinergic neurons.     

Characteristics of [3H]Hemicholinium-3 Binding to Rat Striatal Membranes: Evidence for Negative Cooperative Site-Site Interactions

The characteristics of [3H]hemicholinium-3 ([3H]HC-3) interactions with rat striatal membranes were investigated. Under the described assay conditions, [3H]-HC-3 binds with a saturable population of membrane binding sites having the following regional distribution: striatum much greater than hippocampus greater than or equal to cerebral cortex greater than cerebellum. The specific binding of [3H]HC-3 showed an obligatory requirement for NaCl; other halide salts of sodium or KCl failed to substitute for NaCl. The Scatchard transformation of saturation isotherm data generated a curvilinear plot with high- and low-affinity components of binding. The dissociation of [3H]HC-3 at infinite dilution was also multiexponential. The dissociation could, however, be accelerated if unlabeled HC-3 was included in the diluting buffer, and this increase in dissociation appeared to be dependent on the concentrations of unlabeled HC-3 used, with the maximal increase demonstrable at 100 nM. The dissociation was also dependent on the fractional saturation of binding sites with labeled HC-3, such that, at higher fractional saturation of binding sites, the overall dissociation was faster and the difference in the dissociation observed between "dilution only" and "dilution + unlabeled HC-3" was reduced. This occupancy-dependent change in dissociation could also be influenced by temperature and pH. Based on the results of these kinetic studies, the steady-state [3H]HC-3 binding data were analyzed for a homogeneous population of binding sites undergoing site-site interactions of the negative cooperative type. Such an analysis yielded a KD of 9.3 nM for the high-affinity state and a KD of 22.8 nM for the low-affinity state of binding sites, with a Bmax of 434 fmol/mg of protein. Competitive binding studies showed that unlabeled HC-3 was most potent in displacing [3H]HC-3, followed by choline. Other drugs known to have little influence on the synaptosomal sodium-dependent high-affinity choline uptake system (SDHACU) had no significant effect on [3H]HC-3 binding sites. Similarities in ionic dependencies, regional distributions, and pharmacological selectivities of [3H]HC-3 binding with synaptosomal SDHACU suggest that [3H]HC-3 selectively labels SDHACU sites located on presynaptic cholinergic neurons in rat CNS. We suggest that the two affinity states of [3H]HC-3 binding sites represent the different "functional" states of the SDHACU system. The binding of HC-3 (or choline) with the high-affinity state of the binding sites induces negative cooperative site-site interactions among the binding sites, resulting in the formation of a low-affinity binding state.(ABSTRACT TRUNCATED AT 400 WORDS)     

[3H]GBR 12935 Binding to Dopamine Uptake Sites in the Human Brain

Binding of 1-[2-(diphenylmethoxy)ethyl]-4-(3-phenylpropyl)piperazine ([3H]GBR 12935) was studied in membrane preparations of several human brain regions. In putamen, the substituted piperazine derivates cis- and trans-flupenthixol displaced 90% of the total [3H]GBR 12935 binding. Computer-assisted analysis of the competition curves revealed a high-affinity site (30%; KiH = 54 nM) and a low-affinity site (60%; KiL = 4.5 microM). The dopamine uptake blockers mazindol and nomifensine only displaced 30% of the total [3H]GBR 12935 binding in a monophasic way. Binding of [3H]GBR 12935 to the dopamine uptake sites, i.e., that displaced by dopamine uptake blockers, corresponded to part of the binding having low affinity for flupenthixol and was only detected in putamen, nucleus caudatus, nucleus accumbens, and substantia nigra. Even after masking the high-affinity binding site for flupenthixol by including 1 microM cis-flupenthixol in the binding assays, no dopamine uptake sites could be detected in globus pallidus, amygdala, thalamus, hippocampus, and cerebral cortex. Binding of [3H]GBR 12935 to dopamine uptake sites was lost in the nucleus caudatus ipsilateral to ventral midbrain infarctions, confirming their location on nigrostriatal nerve endings. Gross unilateral lesions of the striato- and pallidonigral pathways did not affect the number of dopamine uptake sites in the ipsilateral substantia nigra, suggesting that they may reside on the soma or dendrites of nigral neurons.     

Binding Sites for [3H]Glutamate and [3H]Aspartate in Human Cerebellum

Abstract The binding of [³H]aspartate and [³H]glutamate to membranes prepared from frozen human cerebellar cortex was studied. The binding sites differed in their relative proportions, their inhibition by amino acids and analogues, and by the effects of cations. A proportion (about 30%) of [³H]glutamate binding was to sites similar to those labelled by [³H]aspartate. An additional component of [³H]gluta-mate binding (about 50%) was displaced by quisqualate and aL-amino-3-hydroxy-5-methylisoxazole-4-propionic acid, and may represent a “quisqualate-preferring” receptor. Neither N-methyl-d-aspartic acid-sensitive nor dl-2-amino-4-phosphonobutyric acid-sensitive [³H]glutamate binding was detected.     

Regulation of Rat Brain Synaptosomal [3H]Hemicholinium-3 Binding and [3H]Choline Transport Sites Following Exposure to Choline Mustard Aziridinium Ion

Abstract: Choline uptake by cholinergic nerve terminals is increased by depolarization; the literature suggests that this results from either the appearance of occult transporters or the increased activity of existing ones. The present experiments attempt to clarify the mechanism by which choline transport is regulated by testing if the preexposure of synaptosomes to choline mustard aziridinium ion prevents the stimulation-induced appearance of hemicholinium-3 binding sites and/or choline transport activity. Choline mustard inhibited irreversibly most of the “ground-state” (basal) high-affinity choline transport but only 50% of “ground-state” hemicholinium-3 binding sites. Exposure of both striatal and hippocampal synaptosomes to the mustard, before stimulation, inhibited K⁺-stimulated increases in choline transport and of [³H]hemicholinium-3 binding. We conclude that the mechanism by which choline transport is regulated involves the increased activity of a pool of transport sites that are occluded to hemicholinium-3 but are available to choline mustard aziridinium ion, and presumably to choline, before stimulation. However, the concentration of mustard needed to inhibit the stimulation-induced increase of [³H]hemicholinium-3 binding and choline transport was lower for striatal synaptosomes than for hippocampal synaptosomes. In the absence of extracellular Ca²⁺ or presence of high Mg²⁺ levels, the choline mustard did not prevent the appearance of extra striatal hemicholinium-3 binding sites. Also, high Mg²⁺ levels removed the ability of the mustard to inhibit K⁺-stimulated increases of either [³H]hemicholinium-3 binding or choline transport by hippocampal synaptosomes. In contrast, the preexposure of hippocampal synaptosomes to the mustard in the presence of a calcium ionophore (A23187) reduced the concentration of inhibitor needed to prevent the activation of [³H]hemicholinium-3 binding and choline uptake. Thus, we conclude that the ability of the choline mustard to alkylate the pool of choline transporters that are activated by stimulation appears dependent on the entry of extracellular Ca²⁺.     

Binding of [3H]Hemicholinium-3 to the High-Affinity Choline Transporter in Electric Organ Synaptosomal Membranes

Sodium-dependent binding of [3H]hemicholinium-3 was observed to be 10-fold higher with presynaptic membranes from the electric organ than with electroplaque membranes and this binding site copurified with synaptosomal membranes. The KD for specific [3H]hemicholinium-3 binding was found to be 31 +/- 4 nM and the Bmax, 5.0 +/- 0.2 pmol/mg protein; a Ki of 16 nM was estimated for hemicholinium-3 as a competitive inhibitor of high-affinity choline transport in electric organ synaptosomes. Choline and choline analogues were equally potent as inhibitors of [3H]choline uptake and [3H]hemicholinium-3 binding. Tubocurarine and oxotremorine also inhibited uptake and binding, but carbachol was without effect in both tests. These findings suggest that [3H]hemicholinium binds to the high-affinity choline transporter present at the cholinergic nerve terminal membrane. A comparison of maximal velocities for choline transport and the maximal number of hemicholinium-3 binding sites indicated that the high-affinity choline transporter has an apparent turnover number of about 3s-1 at 20 degrees C under resting conditions. The high transport rates observed in electric organ synaptosomes are likely due to the high density of high-affinity choline transporters in this tissue, estimated on the basis of [3H]hemicholinium-3 binding to be of the order of 100/micron2 of synaptosomal membrane.     

Pharmacological and Regional Characterization of [3H]LY278584 Binding Sites in Human Brain

Abstract: Binding of [³H]LY278584, which has been previously shown to label 5-hydroxytryptamine₃ (5-HT₃) receptors in rat cortex, was studied in human brain. Saturation experiments revealed a homogeneous population of saturable binding sites in amygdala ( K _D= 3.08 ± 0.67 n M, B _max= 11.86 ± 1.87 fmol/mg of protein) as well as in hippocampus, caudate, and putamen. Specific binding was also high in nucleus accumbens and entorhinal cortex. Specific binding was negligible in neocortical areas. Kinetic studies conducted in human hippocampus revealed a K _on of 0.025 ± 0.009 n M ⁻¹ min⁻¹ and a K _off of 0.010 ± 0.002 min⁻¹. The kinetics of [³H]LY278584 binding were similar in the caudate. Pharmacological characterization of [³H]LY278584 specific binding in caudate and amygdala indicated the compound was binding to 5-HT₃ receptors. We conclude that 5-HT₃ receptors labeled by [³H]LY278584 are present in both limbic and striatal areas in human brain, suggesting that 5-HT₃ receptor antagonists may be able to influence the dopamine system in humans, similarly to their effects in rodent studies.     

Distribution of Specific High-Affinity Binding Sites for [3H]Imipramine in Human Brain

Abstract: [³H]Imipramine binds with high affinity to membranes from different regions of the human brain. The highest density of binding sites was observed in the hypothalamus and substantia nigra and the lowest density in the white matter and cerebellum. As found in rat brain, tricyclic antidepressant drugs are potent inhibitors of [³H]imipramine binding. Atypical antidepressants are, however, much weaker at inhibiting the specific binding. The [³H]imipramine binding site in human cortex is apparently identical to the site already described in the rat brain and in human platelets.     

Studies on Muscarinic Binding Sites in Human Brain Identified with [3H]Pirenzepine

Pirenzepine, a potent antimuscarinic agent with apparent selectivity for a subtype (M1) of muscarinic receptors, was used in tritiated form to characterize its binding to human brain tissue. Specific [3H]pirenzepine binding showed rapid association and dissociation. From kinetic and competitive binding experiments, its KD was 5.5 nM and 9 nM, respectively. Regional distribution of [3H]pirenzepine binding determined in parallel with [3H]quinuclidinyl benzilate binding, a nonselective muscarinic antagonist, indicated a significant correlation for the maximum number of binding sites for the two radioligands in 13 brain regions, with the highest amount of binding for each in the putamen and the least in the cerebellum. Binding for [3H]pirenzepine averaged 57% of that for [3H]quinuclidinyl benzilate, with a range of 20% (cerebellum) to 77% (frontal cortex). Most antidepressants and neuroleptics tested had affinities for [3H]pirenzepine binding sites that were not significantly different from their previously reported values obtained with the use of [3H]quinuclidinyl benzilate.     

Affinity Labelling and Identification of the High-Affinity Choline Carrier from Synaptic Membranes of Torpedo Electromotor Nerve Terminals with [3H]Choline Mustard

The physiological mechanisms regulating activity of the sodium-dependent, high-affinity choline transporter and the molecular events in the translocation process remain unclear; the protein has not been purified or characterized biochemically. In the present study, [3H]choline mustard aziridinium ion [( 3H]ChM Az), a nitrogen mustard analogue of choline, bound irreversibly to presynaptic plasma membranes from Torpedo electric organ in a hemicholinium-sensitive, and sodium-, time-, and temperature-dependent manner. Specific binding of this ligand was greatest when it was incubated with membranes in the presence of sodium at 30 degrees C. Separation of the 3H-labelled membrane proteins by sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealed that most of the radiolabel was associated with a polypeptide of apparent molecular mass of approximately 42,000 daltons; labelling of this species was abolished in membranes incubated with ligand in the presence of HC-3. Two other 3H-labelled polypeptides were detected, with apparent molecular masses of approximately 58,000 and 90,000 daltons; radiolabelling of the former was also HC-3 sensitive. [3H]ChM Az may be a useful affinity ligand in the purification of the choline carrier from cholinergic neurons.     

Binding of [3H]Neurotensin in Human Brain: Properties and Distribution

The binding of [3H]neurotensin to membranes from human brain at 0 degrees C was specific, saturable, and reversible. In the frontal cortex, the equilibrium dissociation constant (KD) for [3H]neurotensin determined from the ratio of rate constants (k-1/k1), saturation isotherms, and inhibition binding experiments was 0.80, 2.0, and 2.0 nM, respectively, and the maximum number of binding sites (Bmax) from the saturation isotherms and the competitive binding experiments was 2.4 and 2.2 pmol/g of tissue, respectively. Hill coefficients for binding were equal to 1, indicating the presence of single, noncooperative binding sites. Inhibition of specific binding of [3H]-neurotensin by several analogs of neurotensin showed that [Gln4]neurotensin and neurotensin(8-13) had the highest affinities for these binding sites in human frontal cortex, with each analog being approximately 13-fold more potent than neurotensin. In addition, these data showed that the carboxy-terminal portion of neurotensin played an important part in the binding of this neuropeptide in human brain, a result described for other species. Regional distribution of binding sites was different from that reported for animal brains. Of the 33 different regions investigated, the uncus and substantia nigra showed the highest specific binding of [3H]neurotensin, whereas such areas as the pineal body, medulla, and corpus callosum had few binding sites.     

[3H] GBR-12935 Binding to Human Cerebral Cortex Is Not to Dopamine Uptake Sites

Abstract: The binding of the dopamine uptake inhibitor [³H] GBR-12935 to 16 regions of the human brain was investigated in competition experiments with increasing concentrations of GBR-12909, mazindol, and dopamine. The methodology used included a relatively high tissue concentration (8 mg/ml) and addition of 5 m M KCI in the assay buffer. GBR-12909 inhibited 80–90% of the binding in most regions, whereas dopamine only inhibited the binding in the striatum. Mazindol inhibited only part of the cortical binding at concentrations of >1 μ M , whereas the inhibition in the caudate and the putamen also contained a high-affinity component representing the dopamine uptake site. It is concluded that the [³H] GBR-12935 binding sensitive to GBR-12909 cannot be regarded as specific binding to the dopamine uptake site because the displaceable binding most likely is not related to the dopamine uptake site.     

[3H]Nitrobenzylthioinosine Binding as a Probe for the Study of Adenosine Uptake Sites in Brain

Abstract: The binding of the potent adenosine uptake inhibitor [³H]nitrobenzylthioinosine ([³H]NBI) to brain membrane fractions was investigated. Reversible, saturable, specific, high-affinity binding was demonstrated in both rat and human brain. The K_d in both was 0.15 nM with B_max values of 140–200 fmol/mg protein. Linear Scatchard plots were routinely obtained, indicating a homogeneous population of binding sites in brain. The highest density of binding sites was found in the caudate and hypothalamus in both species. The binding site was heat labile and trypsin sensitive. Binding was also decreased by incubation of the membranes in 0.05% Triton X-100 and by treatment with dithiothreitol and iodoacetamide. Of the numerous salt and metal ions tested, only copper and zinc had significant effects on [³H]NBI binding. The inhibitory potencies of copper and zinc were IC₅₀= 160 μM and 6 mM, respectively. Subcellular distribution studies revealed a high percentage of the [³H]NBI binding sites on synaptosomes, indicating that these sites were present in the synaptic region. A study of the tissue distribution of the [³H]NBI sites revealed very high densities of binding in erythrocyte, lung, and testis, with much lower binding densities in brain, kidney, liver, muscle, and heart. The binding affinity in the former group was approximately 1.5 nM, whereas that in the latter group was 0.15 nM, suggesting two types of binding sites. The pharmacologic profile of [³H]NBI binding was consistent with its function as the adenosine transport site, distinct from the adenosine receptor, since thiopurines were very potent inhibitors of binding whereas adenosine receptor ligands, such as cyclohexyladenosine and 2-chloroadenosine, were three to four orders of magnitude less potent. [³H]NBI binding in brain should provide a useful probe for the study of adenosine transport in the brain.     

Muscarinic Cholinergic Receptors in Human Foetal Brain: Characterization and Ontogeny of [3H]Quinuclidinyl Benzilate Binding Sites in Frontal Cortex

Twenty-two frontal cortices from normal human foetal brains of gestational ages ranging from 16 to 40 weeks and five postnatal brains ranging from 5 to 50 years were analysed for the ontogeny of muscarinic receptors using [3H]quinuclidinyl benzilate (QNB) as the ligand. QNB binding sites were shown to be stable up to 4 1/2 months of storage at -70 degrees C. QNB binding was characterized in frontal cortices of 28-week-old foetal brains as muscarinic receptors by the following criteria: (1) it was localised mainly in particulate fraction; (2) binding was saturable at a concentration of 1.5 nM; (3) the cholinergic antagonists atropine and scopolamine competed for the binding, with IC50 values of 1 and 0.8 nM, respectively. The agonists oxotremorine, carbachol, and pilocarpine gave IC50 values of 1, 15 and 18 microM, respectively. Nicotinic receptor ligands and noncholinergic drugs could not compete for the binding. Bimolecular association and dissociation rate constants for the reversible binding are 6.23 X 10(8) M-1 X min-1 and 2.0 X 10(-2) X min-1, respectively. The equilibrium dissociation constant is 33 pM. The KD obtained by saturation binding data is 103 pM. Ontogeny of muscarinic receptors showed three distinct phases: In phase I, they appear between 16 and 18 weeks [average concentration 109 fmol/mg protein of total particulate fraction (TPF)] and slowly increase up to 20 weeks (average concentration 147 fmol/mg protein TPF). Phase II is a lag period between 20 and 24 weeks at which time receptor concentration does not change perceptibly (average concentration (67 fmol/mg protein TPF).(ABSTRACT TRUNCATED AT 250 WORDS)     

Characterization of [3H]Guanine Nucleotide Binding Sites in Brain Membranes

[3H]GTP [guanosine triphosphate] and [3H]GMP-PNP [guanosine 5'-(beta, 8-imino)triphosphate, a nonmetabolized analog of GTP] have been utilized as ligands to characterize binding sites of guanine nucleotides to rat brain membranes. Binding of both [3H]GTP and [3H]GMP-PNP is saturable, with respective KD values of 0.76 and 0.42 microM. The number of binding sites for GMP-PNP (4 nmol/g) is three times greater than for GTP (1.5 nmol/g). This discrepancy is caused by rapid degradation of GTP to guanosine by brain membranes, which can be partially prevented by addition of 100 microM-ATP. The binding of [3H]guanine nucleotides is selective, with approximately equipotent inhibition by GTP, GDP, and GMP-PNP (at 0.2--1.0 microM), but no inhibition by other nucleotides at 100 microM concentrations. The bindings sites for guanine nucleotides in brain membranes appear not to be associated with microtubules, since treatments that reduce [3H]colchicine binding by 65% have no effect on [3H]GTP binding. [3H]Guanine nucleotide binding is widely distributed in various organs, with highest levels in liver and brain and lowest levels in skeletal muscle. The characteristics of these binding sites in brain show specificity properties of sites that regulate neurotransmitter receptors and adenylate cyclase.     

[3H]Choline Uptake and Metabolism in Nonsynaptic Regions of a Crustacean Sensory Nerve

Abstract: The posterior stomach nerve (PSN) is a crustacean sensory nerve containing about 60 cholinergic neurons, which are devoid of synaptic interactions. Kinetic analysis shows that the PSN takes up [³H]choline by both low-affinity ( K _m= 163 μM) and high-affinity (Na⁺-dependent) ( K _m= 1 μM) processes. The capacity of the high-affinity system is only about 1% that of the low-affinity system. The high-affinity system is not tightly coupled to acetylcholine (ACh) synthesis, and it appears that both ACh and phosphorylcholine are formed from an intracellular pool of choline, which is fed by both uptake systems. There are differences in the rates of [³H]choline uptake and ³H metabolite accumulation between regions of the PSN that contain neuronal cell bodies and those that do not. These differences may arise from differences in the relative proportion of neuronal to nonneuronal tissue in each nerve region.     

A Thermodynamic Study of 5-[3H]Hydroxytryptamine Binding to Human Cortex Membranes

Kinetic and equilibrium measurements of [3H]-serotonin (5-hydroxytryptamine) binding to human frontal cortex membranes have been made between 4 and 30 degrees C. The effects of spiperone and ascorbate on binding have also been determined. Under the conditions used, binding was saturable and reversible. Affinity constants derived from kinetic and equilibrium data were comparable. Serotonin binding to several sites had substantial enthalpic as well as entropic components.