INCREASED EXPRESSION OF SEROTONIN TRANSPORTER MESSENGER-RNA IN RAPHE NEURONS OF THE AGED RAT

The action of serotonin (5-hydroxytryptamine; 5HT) in the nervous system is terminated by reuptake of the neurotransmitter into the presynaptic nerve terminal via a plasma membrane-bound transporter protein. Recently, cDNAs encoding serotonin transporter protein were cloned. Using in situ hybridization we have studied the expression of serotonin transporter mRNA in the bulbospinal 5HT system of adult (2-3 months) and aged (30 months) rats. In adult rats, serotonin transporter mRNA labelled neurons were detected in the nucleus raphe obscurus, pallidus and magnus. In all aged rats, the optical density of the serotonin transporter mRNA labelling of individual cell profiles was increased by 20-30% (P < 0.001) when compared with adult rats. Five of the six aged rats studied had various degrees of hindlimb motor dysfunctions. In rats with the most pronounced signs of hindlimb motor dysfunction the number of labelled neurons was increased by 25-75% (P < 0.01) when compared with young adult rats. It is concluded that an increased serotonin transporter gene expression is present in the bulbospinal 5HT system of aged animals.