Neurobiology

David D. Potter, Ph.D.

Research Professor of Neurobiology

For many years I have been interested in synaptic functions. The culture of dis- sociated neurons offers advantages in studying these functions, including enhanced control of the cellular and fluid environment of the neurons, good visibility, and easy access for electrical recording. For twenty years this laboratory, in collaboration with Professor Furshpan's laboratory, has used a microculture procedure in which neurons of particular type or types are grown on small islands 1mm or less in diameter. Growing nerve processes are confined to this small footprint and consequently form a high density of synaptic endings on the defined target neurons. This increases the probability that synaptic functions, including weak functions, will be detected by recording from the neurons individually. If only a single neuron is present, it forms a high density of synapses on itself, and there is no ambiguity about which neuron is responsible for the observed effects.

With this procedure it was possible to demonstrate conclusively that neurons can release more than one transmitter, to study the diversity of transmitter repertoires of individual nerve cells and to study plasticity of transmitter functions in the culture environment.

A recent interest in this laboratory is the synaptic repertoires of microcultured neurons of the striatum, a part of the brain involved in controlling movement and in motivated behaviors (Parkinsonism and Huntington's disease involve lesions to the striatum). A current interest, in collaboration with Professor Fischbach, is the actions of nicotine on microcultured nerve cells. A second current interest is collaboration with Professor Furshpan on the project.
Simultaneous electrical recordings (with whole-cell patch electrodes) from two neurons in a chronically blocked hippocampal culture (below). The upper pair of records chronicle the effects, over a period of 80 minutes, of exposing the culture to different synaptic blocking agents. The lower three pairs of records show selected portions (1, 2, & 3) of the experiment on greatly expanded time bases. Each of these records shows two degrees of expansion; the first part is expanded about 45-fold, the later part about 450-fold (the 1 sec calibration bar applies to the second part of each record). Initially, when the neurons were exposed to the broad spectrum glutamate-receptor blocker, kynurenate (KYN), seizure activity was absent. When only the NMDA class of glutamate receptors was blocked by APV, seizure activity, consisting of characteristic paroxysmal depolarization shifts (PDSs) appeared synchronously in the two neurons. Then, in the absence of any blocking agents (normal perfusion medium = PM), the PDSs became more prolonged and less frequent than in APV (compare records 1 and 2) and, in addition, one of the neurons developed a slowly rising sustained depolarization which is also characteristic of brain seizures. Note that a second application of APV reversed the sustained depolarization, while the PDSs remained. When the non-NMDA receptors were blocked with CNQX, the PDSs were still produced (but they became less frequent and more prolonged). A sustained depolarization slowly developed in CNQX. Our conclusions from such experiments is that glutamate receptors are essential for generating seizure activity. Synchronous PDSs (population events) can be generated by means of either NMDA or non-NMDA glutamate receptors, although their time courses depend on which receptors are operative. The generation of the slow sustained depolarizations (which may be important for neuronal death) depends particularly on activation of NMDA receptors.