COMPARATIVE, IN-VITRO, STUDIES OF HIPPOCAMPAL TISSUE FROM HOMING AND NON-HOMING PIGEON

The purpose of this research was to characterize morphologically and e lectrophysiologically tissue slices obtained from the hippocampus of h oming and non-homing pigeons. When hippocampal slices from the brain o f homing and non-homing pigeons are observed under the dissecting micr oscope, diffuse fiber paths can be seen. These fiber pathways appeared to be identical with the medial fiber tract (VM) previously described histologically in the hippocampus of homing pigeon. Visualization of these tracts in living slices allowed placement of stimulating and rec ording electrodes in corresponding locations in these slices in both h oming and non-homing pigeons. Extracellular potentials recorded from V M regions of the brains of both homing and non-homing pigeons were sen sitive to CNQX indicating that glutamate may be a neurotransmitter in this area of pigeon hippocampus. These potentials could undergo long-t erm potentiation (LTP) following high frequency stimulation. This LTP was blocked by NMDA receptor antagonist APV in the hippocampus of homi ng pigeon, but was APV-resistant in the hippocampus of non-homing pige on. Extracellular potentials from the hippocampus of homing pigeons we re increased in amplitude when slices were perfused with Mg2+-free Rin ger, while potentials recorded from hippocampal slices from non-homing pigeons were unaffected by Mg2+-free solutions. Intracellular recordi ngs from the hippocampal slices of homing pigeons revealed that about half the cells demonstrated excitatory synaptic potentials evoked by e xtracellular stimulation. The EPSP was sometimes large enough to trigg er an action potential. Neurons filled with the fluorescent dye, Lucif er Yellow,in the hippocampus of homing pigeons showed multipolar struc ture. The response of these cells to extracellular stimulation provide s the activity responsible for the extracellular potentials which can undergo LTP.