Glutamate uptake by squid nerve fiber sheaths

The squid giant nerve exhibits neuron-Schwann cell interactions that appear to involve glutamate as a mediator; however, there is no information available about the possible fate of the released glutamate. In this study, it is demonstrated that the periaxonal sheaths of the extrasynaptic regions of squid giant nerves (where the glial cells are located) possess the capacity to transport glutamate. In whole intact nerves incubated with low-glutamate concentrations for long periods of time, the majority of the glutamate incorporated into the tissue was found in the sheaths. Axoplasm-free sheaths incubated for long periods of time with low concentrations of glutamate were able to accumulate this amino acid against a large apparent concentration gradient. Sheath glutamate uptake occurred in a sodium-dependent fashion over a wide concentration range and displayed both high- and low-affinity components. Glutamate uptake at concentrations below the Km of the high affinity component was independent of homoexchange and displayed a specificity that is similar to that described for high-affinity glutamate transport in mammalian brain. It is proposed that the sheath transport systems may be involved in the regulation of glutamate levels in the intercellular clefts of the nerve fiber, as part of the glutamatergic neuron-glial signaling mechanisms in the squid giant nerve fiber.