An evaluation of L-glutamate as the transmitter released from optic nerve terminals of the pigeon

The possibility was investigated that L-glutamic acid is the excitatory transmitter released from the optic nerve terminals of the pigeon optic tectum. (1) Superficial layers of the tectum contained high levels of endogenous glutamate and accumulated L-[3H]glutamate by a high affinity uptake process. (2) Subcellular and autoradiographic studies indicated that 10-30% of the exogenously accumulated L-[3H]glutamate was localized within synaptosomes, and that 11-15% of the synaptosomes had been labelled. (3) The glutamate-accumulating synaptosomes sedimented to the same isopycnic density as pinched-off optic nerve terminals. (4) GABA-and noradrenaline-accumulating synaptosomes were also associated with this subcellular population. (5) Retinal ablation did not change endogenous glutamate concentrations or the high affinity uptake of glutamate. The results are discussed in relation to a possible role for L-glutamate as the 'optic nerve transmitter' and in the context of previous evidence implicating glutamate as an excitatory transmitter.