Changes in muscarinic cholinergic, PCP, GABA_A, D₁, and 5-HT_2A receptor binding, but not in benzodiazepine receptor binding in the brains of aged rats

We used in vitro quantitative autoradiography to investigate changes in neurotransmitter receptor binding, including muscarinic cholinergic, PCP, GABA_A, benzodiazepine, D₁ and 5-HT_2A receptor, in the brains of aged rats, compared with such binding in young rats. Scatchard analysis revealed that the maximal number of binding sites for [³H]quinuclidinyl benzilate (QNB) in the caudate/putamen and accumbens was significantly decreased in aged rats compared with young rats, while its affinity remained unchanged. The specific binding of [³H]N-(1-[2-thienyl]cyclohexyl)3,4-piperidine (TCP) for the ion channels coupled with N-methyl-D-aspartate receptors in the caudate/putamen and hippocampus was significantly decreased in aged rats compared with young rats. The [³H]muscimol binding in aged rats was decreased in all brain regions examined compared with that in young rats, whereas [³H]flunitrazepam binding was not changed in any brain regions. The [³H]SCH23390 binding for dopamine D₁ receptors was significantly increased in the parietal cortex, but decreased in the caudate/putamen and accumbers of aged rats compared with that in young rats. The [³H]ketanserin binding for 5-HT_2A receptors in the cortex and accumbens was significantly decreased in aged rats compared with young rats. These results suggest that uneven changes in receptors for various neurotransmitters throughout the brain may be responsible for the decline of brain function in aged rats.