Functional change in the 5-HT presynaptic receptor in spinal cord of aged rats.

The present study assessed the functional integrity of the serotonin (5-HT) presynaptic receptor in the spinal cord of aged (18-20 months) rats. Previous research determined that exogenous 5-HT and 5-HT1B agonists, in adult (3 months) rats, activated the 5-HT presynaptic receptor resulting in a dose-dependent decrease in 3H-5-HT release from spinal cord slices. Contrastively, exogenous 5-HT and the selective 5-HT1B agonist 1-(m-chlorophenyl) piperazine HCl (mCPP) produced a dose-dependent increase in depolarization-evoked 3H-5-HT release from spinal cord of aged rats. The selective 5-HT1A agonist 8-hydroxy-2(di-n-propylamino) tetralin (8-OHDPAT) increased 3H-5-HT release from aged rat spinal cord slices similar to results previously obtained from adult rat spinal cord slices. The inability of 5-HT and mCPP to inhibit 3H-5-HT release from spinal cord of aged rats indicates a functional change in the 5-HT presynaptic receptor in these animals. Possible mechanisms mediating this functional change are discussed.