Glutamate-mediated synaptic transmission between neuron B4 and salivary cells of Helisoma trivolvis

In this study, we have further characterized the morphology and physiology of the neuroglandular synapse between the identified buccal neuron, B4, and the salivary gland of Helisoma. We demonstrate that the coupling coefficient between salivary cells within an individual acinus is approximately 1.0. We also demonstrate that synapses within the salivary gland are located near a superficial muscle layer. We examine the effects of glutamate on the salivary gland and on the B4-salivary gland EPSP. L-glutamate produces a transient, rapid onset depolarization of salivary gland cells. The response is mimicked by high concentrations of L-homocysteic acid, but not by NMDA, L-aspartate, D-glutamate or kainate. The response is blocked by the presence of L- or D-glutamate in the bath, but not by CNQX, DNQX, DGG, D-AP5, or L-AP3. The depolarization is primarily dependent on the presence of calcium in the bathing solution. When either L- or D-glutamate is present in the bathing solution, the amplitude of the B4-salivary gland EPSP is reversibly reduced. The similar pharmacological properties of the response of the salivary gland to glutamate and the B4 epsp indicate that L-glutamate is a strong candidate for the fast excitatory neurotransmitter at the Helisoma neuroglandular synapse.