Colocalization of neurotransmitter transporters on the plasma membrane of the same nerve terminal may reflect cotransmission.

There is increasing evidence for the neuronal coexistence of classical transmitters. Implications in favor of cotransmission have often been represented by the identification, in the same neuron, of the putative cotransmitters, their synthetic enzymes and/or their vesicular transporters. In contrast, coexpression of neurotransmitter transporters on the plasma membrane of the same nerve terminal, although a potentially important indication for cotransmission, has received poor attention. We here used preparations of isolated nerve endings to functionally identify transporters coexpressed on the plasma membrane of the same terminal, in order to verify if such transporter coexpression indeed exists in neuronal systems in which cotransmission has already been established or reasonably suspected through other technical approaches. We could observe that functional transporters for glycine and glutamate are coexpressed on nerve terminals in the cerebellum; transporters for dopamine and GABA coexist on striatal terminals; transporters for glycine and GABA, previously found to coexist as cotransmission markers on nerve terminals of spinal cord and cerebellum, are not coexpressed in neocortex and hippocampus, where cotransmission has not been proposed to occur; transporters for GABA, glycine and glutamate are colocalized on nerve terminals of the spinal cord. Confocal microscopy experiments were performed to substantiate functional data, highlighting the presence of the co-existing transporters under study on MAP-2 positive synaptosomes. It is concluded that investigating the colocalization of functional neurotransmitter transporters on the plasma membrane of nerve terminals can provide useful information on the possibility of cotransmission.