Autoradiographic localization of serotonin receptor subtypes in cat visual cortex: transient regional, laminar, and columnar distributions during postnatal development
Postnatal changes in the distribution of 5-HT receptor subtypes in the visual cortex of cats were assessed both qualitatively and quantitatively using in vitro autoradiographic methods. The 5-HT 1A, 1C, 2, and 3 receptor subtypes and the 5-HT uptake (5-HTUp) site were visualized with 3H-8-hydroxy-2(di-n-propyl-amino)tetralin, 3H- mesulergine, (2,5-dimethoxy-4-125I-iodophenyl)-2-aminopropane, 3H- BRL43694, and 3H-cyanoimipramine, respectively. Although specific labeling of 5-HT3 receptors was not detected in the cat visual cortex at any age, each of the remaining 5-HT receptor subtypes exhibited unique temporal, regional, and laminar patterns of expression in visual cortical areas 17, 18, and 19 and lateral suprasylvian cortex (LS). 5- HT1A receptors were the earliest to demonstrate visual cortex-specific changes in expression. They exhibited peak levels of expression in all visual cortical areas, predominantly in supra- and infragranular layers, between postnatal day 10 (PD10) and PD30. Their levels in all areas declined progressively with increasing age. 5-HT1c receptors demonstrated their highest levels of expression in the deeper half of layer IV, but only in area 17, between PD40 and PD75. The most striking feature of their distribution throughout this period was that, in layer IV and extending into layer III, the 5-HT1c receptors were concentrated in columns that were 400 microns wide and had a center-to-center spacing of about 900 microns. This transient pattern of expression was not present beyond PD90. 5-HT2 receptors were most densely expressed in layer IV between PD40 and PD120, but they displayed a distinctly different distribution pattern. The densest binding of 5-HT2 receptors was limited to the upper half of layer IV and found in areas 17, 18, and LS. The distribution of layer 5-HT2 receptors along the dense band in layer IV of area 17 was discontinuous, exhibiting patches that were found in the same vertical columns as were the 5-HT1c receptors. Intermediate binding levels for the 5-HT2 receptors were found through layers I-III, the remainder of layer IV, and the subcortical white matter. The levels of 5-HT uptake sites increased gradually to reach adult levels by PD40, but with a distribution pattern that was basically homogeneous, both across cortical regions and across laminae.(ABSTRACT TRUNCATED AT 400 WORDS)