The purpose of this proposed research is to more fully characterize the processes which underlie the postsynaptic response at lobster excitatory neuromuscular junctions. The information gained from this research will advance our knowledge of the mechanisms which underlie synaptic transmission at an excitatory synapse where an amino acid (glutamate) functions as the neurotransmitter. Since glutamate and aspartate may be major excitatory neurotransmitters in the CNS of vertebrates the results of these studies may provide a clearer understanding of the actions of these amino acids in the vertebrate CNS and should aid the interpretation of data obtained on vertebrate preparations. In our proposed studies the postsynpatic receptors will be activated neurally (through evoked transmitter release) and by iontophoretically applying glutamate, and glutamate plus aspartate simultaneously. The postsynaptic response will be determined by measuring changes in the membrane potential and membrane conductance. There are 4 major objectives of this proposed research: 1. To more fully characterize the interaction of glutamate and aspartate with the excitatory postsynaptic receptors. This will involve the generation and analysis of dose-response curves and a determination of the reversal potential for glutamate alone and glutamate in combination with aspartate. In these experiments various analogs of glutamate and aspartate will be superfused onto the muscle fibers and tested for possible specific blocking effects on the action of glutamate and aspartate. 2. To determine if certain ions (e.g., Na ion and Ca ions) are required for receptor activation to occur, and to establish which ions participate in carrying the current which generates the exictatory postsynaptic response. 3. To characterize receptor desensitization and resensitization. In these experiments the time constants for desensitization and resensitization of the postsynaptic receptors will be determined as a function of temperature and the external concentrations of Ca ions, MG ions and Na ion. 4. To provide information on the relative roles of diffusion and transmitter uptake in the transmitter inactivation process.