Autoradiographic distribution of 5-HT7 receptors in the human brain using [3H]mesulergine: comparison to other mammalian species

Abstract

1. The main aim of this investigation was to delineate the distribution of the 5-HT(7) receptor in human brain. Autoradiographic studies in guinea-pig and rat brain were also carried out in order to revisit and compare the anatomical distribution of 5-HT(7) receptors in different mammalian species. 2. Binding studies were performed in rat frontal cortex membranes using 10 nm [(3)H]mesulergine in the presence of raclopride (10 microm) and DOI (0.8 microm). Under these conditions, a binding site with pharmacological characteristics consistent with those of the 5-HT(7) receptors was identified (rank order of binding affinity values: 5-CT>5-HT>5-MeOT>mesulergine approximately methiothepin>8-OH-DPAT=spiperone approximately (+)-butaclamol>>imipramine approximately (+/-)-pindolol>>ondansetron approximately clonidine approximately prazosin). 3. The autoradiographic studies revealed that the anatomical distribution of 5-HT(7) receptors throughout the human brain was heterogenous. High densities were found over the caudate and putamen nuclei, the pyramidal layer of the CA2 field of the hippocampus, the centromedial thalamic nucleus, and the dorsal raphe nucleus. The inner layer of the frontal cortex, the dentate gyrus of the hippocampus, the subthalamic nucleus and superior colliculus, among others, presented intermediate concentrations of 5-HT(7) receptors. A similar brain anatomical distribution of 5-HT(7) receptors was observed in all three mammalian species studied. 4. By using [(3)H]mesulergine, we have mapped for the first time the anatomical distribution of 5-HT(7) receptors in the human brain, overcoming the limitations previously found in radiometric studies with other radioligands, and also revisiting the distribution in guinea-pig and rat brain.

Figure 1

Saturation curves of [³H]mesulergine binding in rat frontal cortex membranes in the absence and presence of raclopride and DOI. Data points show specific binding calculated by subtracting nonspecific binding (defined in the presence of 10 μM 5-HT) from total binding. Abscissa: concentration of [³H]mesulergine in nM; ordinate: fmol mg protein⁻¹.

Figure 2

Displacement of 10 nM [³H]mesulergine by selected compounds in rat frontal cortex membranes. Curves are representative of all compounds listed in Table 1, with the exception of ondansetron, prazosin, and clonidine, whose K_i values were higher than 0.1 mM (pK_i1<4). All compounds displaced [³H]mesulergine in the presence of raclopride (1 μM) and DOI (0.8 μM) to the nonspecific level defined with 5-HT (10 μM). Abscissa: percent of specific binding in the absence of the displacer. Ordinate: −logarithm of molar concentrations of the displacing drug. Data points represent the mean±s.e.m. of at least three separate experiments, each performing duplicate determinations. Mean±s.e.m. of pK_i values are listed in Table 2.

Figure 3

Correlation between binding affinity values (pK_i) obtained in recombinant rat or mouse 5-HT₇ receptors and expressed in transfected cells (Plassat et al., 1993; Ruat et al., 1993), and binding affinity values (pK_i or pK_D) obtained in rat frontal cortex membranes. High-affinity pK_i values for drugs defining two-binding sites (see Table 2) were used in the analysis. Ondansetron, clonidine, prazosin, and imipramine were not included in the analysis because their exact pK_i values were either not available from the literature or not accurately calculable. Plotted line represents the least-square linear regression y=1.03x−0.479; coefficient of determination (r²)=0.965; P<0.01.

Figure 4

Visualization of 5-HT₇ receptors in the human brain with [³H]mesulergine. Autoradiograms at four coronal section levels, frontal cortex (a, b), hippocampus (c, d), mesencephalum (e, f), and thalamus (g, h). Binding observed with 10 nM of [³H]mesulergine in the presence of raclopride (1 μM) and DOI (0.8 μM) is depicted in (a, c, e, g). Nonspecific binding obtained in the presence of 10 μM of 5-HT is depicted in (b, d, f, h). Prominent binding remained in thalamic nuclei and mesencephalic nuclei, and moderate levels were detected in the cortical regions, hippocampus, and central gray. Abbreviations: Field 1 of Ammon's horn, CA1; caudate nucleus, Cd; central gray, CG; dentate gyrus, DG; dorsal raphe nucleus, DRN; medial thalamic nucleus, M; substantia nigra, SN; subthalamic nucleus, Sth.

Figure 5

Rostro-caudal distribution of [³H]mesulergine-binding sites in rat brain by receptor autoradiography. Digitized pictures obtained from film autoradiograms at three coronal section levels: anterior thalamus and hypothalamus (a, b); hippocampus and posterior thalamus (c, d); mesencephalum (e, f). Binding observed with 10 nM of [³H]mesulergine in the presence of raclopride (1 μM) and DOI (0.8 μM) is shown in (a, c, e). Nonspecific binding defined with 10 μM of 5-HT is shown in (b, d, f). High levels of binding were found in thalamic and hypothalamic nuclei, moderate levels in the hippocampus and mesencephalum and low levels in the cortex layers. Abbreviations: central (periaqueductal) gray, CG; centromedial nucleus of the thalamus, CM; hippocampus, Hc; interpeduncular nucleus, IP; medial geniculate nucleus, MG; superficial gray layer of the superior colliculus, SuG; ventromedial nucleus of the hypothalamus, VMH. Bar=2 mm.

Figure 6

Rostro-caudal distribution of [³H]mesulergine-binding sites in guinea-pig brain by receptor autoradiography. Digitized pictures obtained from film autoradiograms at seven coronal section levels: olfactory bulb (a, h); anterior basal ganglia (b, i); posterior basal ganglia and septum (c, j); anterior thalamus and hypothalamus (d, k); hippocampus and posterior thalamus (e, l); mesencephalum (f, m); pons (g, n). Binding observed with 10 nM of [³H]mesulergine in the presence of raclopride (1 μM) and DOI (0.8 μM) is shown in (a–g). Nonspecific binding defined with 10 μM of 5-HT is shown in (h–n). High levels of binding were found in thalamic and hypothalamic nuclei, moderate levels in the hippocampus and mesencephalum, and low levels in cortex layers. Abbreviations: Field 1 of Ammon's horn, CA1; caudate putamen, Cpu; lateral septal nucleus, LS; olfactory nerve layer, ON; periaqueductal gray, PAG; medullary raphe nuclei, MRn; ventral postero-thalamic nuclei, VP. Bar=2 mm.