Different Subcellular Localization of Muscarinic and Serotonin (S₂) Receptors in Human, Dog, and Rat Brain

Abstract

Cortex from rat, dog, and human brain was submitted to subcellular fractionation using an analytical approach consisting of a two-step procedure. First, fractions were obtained by differential centrifugation and were analyzed for their content of serotonin S2 and muscarinic receptors, serotonin uptake, and marker enzymes. Second, the cytoplasmic extracts were subfractionated by equilibration in sucrose density gradient. In human brain, serotonin and muscarinic receptors were found associated mostly with mitochondrial fractions which contain synaptosomes, whereas in rat brain they were concentrated mainly in the microsomal fractions. Density gradient centrifugation confirmed a more marked synaptosomal localization of receptors in human than in rat brain, the dog displaying an intermediate profile. In human brain, indeed, more receptor sites were found to be associated with the second peak characterized in electron microscopy by the largest number of nerve terminals. In addition, synaptosomes from human brain are denser than those from rat brain and some marker enzymes reveal different subcellular distribution in the three species. These data indicate that more receptors are of synaptosomal nature in human brain than in other species and this finding is compatible with a larger amount of synaptic contacts in human brain.

Subcellular distribution of 5-hydroxytryptamine2A andN-methyl-D-aspartate receptors within single neurons in rat motor and limbic striatum

“…The present ultrastructural localization of 5HT 2A R immunoreactivity mainly to cytoplasmic organelles in somata and medium-sized dendrites of striatal neurons confirms and extends results of previous studies showing enrichment of the receptor protein in cytoplasmic fractions of brain tissue (Laubeya et al, 1986). The prominent 5HT 2A R labeling of Golgi networks, SER, and other vesicular organelles that are known to play a role in intracellular synthesis and/or trafficking of receptors (Boudin et al, 1998;Trimmer, 1999) suggests that most 5HT 2A R are largely nonaccessible to extracellular serotonin and may undergo activity-dependent mobilization to the plasma membrane, as has been shown for certain opioid receptors (Shuster et al, 1999).…”

Effects of muscarinic receptor agonists and antagonists on rat brain serotonergic activity

“…The muscarinic receptor sub-classification, proposed for the functional postsynaptic cholinergic receptors, has now been extended to muscarinic autoreceptors regulating neuronal acetylcholine release (Raiteri etal., 1984;Marchi and Raiteri, 1985;Palacios etal., 1991), and to muscarinic heteroreceptors modulating the release of other neurotransmitters (Raiteri etal., 1984;Bhattacharya and Sen, 1992). The subcellular localization of muscarinic and 5-HT receptors in the rat brain appears to be similar, with a bimodal distribution comprising of a major localization in the microsomal fraction and a second localization in the heavy mitochondrial fraction (Luabeya et al, 1986). A functional inter-relationship between muscarinic and 5-HT receptors has been conclusively demonstrated in the rat brain (Hortnagl et al, 1987).…”

Development and Characterization of Monoclonal Antibodies Specific to the Serotonin 5-HT2AReceptor

“…We believe that the cytoplasmic staining for 5-HT 2A R seen in the Schwann cells (Figure 3) and in the pyramidal cells (Figure 6) may be due to a higher level of intracellular synthesis of the receptor protein in the rough ER, to accumulation in Golgi for secondary processing, or to internalization of the receptor in the cytoplasm. Previous studies using differential centrifu-gation followed by radioligand binding of different subcellular fractions have suggested the localization of 5-HT 2A receptors in cytoplasmic and mitochondrial extracts (Laubeya et al 1986). This cytoplasmic pattern of staining is also observed for various other neurotransmitter receptors, including GluR 1 , AMPA glutamate and mGluR 1 receptors (Molnár et al 1993;Kharazia et al 1996;Petralia et al 1997).…”