Faculty Bibliography

Specific chemical lesion of the rat inferior olive by intraperitoneal administration of 3-acetylpyridine prevents recuperation from motor abnormalities generated by unilateral labyrinthine lesion. Moreover, in animals that have recuperated from the balyrinthine lesion, 3-acetylpyridine produces a reversal of the symptoms within 2 hours of administration. These results indicate that the integrity of the olivo-cerebellar system is necessary for the acquisition and retention of this form of motor learning, but that the cerebellum itself is not the seat of such learning.

Aequorin, a protein that emits light in the presence of calcium, was injected in the presynaptic terminal of the squid giant synapse. This injection was preceded by intracellular tetraethylammonium administration, which prolonged the duration of the presynaptic action potential. After this procedure light emission was evoked by single presynaptic spikes capable of releasing synaptic transmitter. In a second set of experiments, presynaptic tetraethylammonium injection was followed by the administration of tetrodotoxin extracellularly, which abolished the presynaptic action potential. Under these conditions artificial depolarization of the presynaptic terminal triggered the release of synaptic transmitter, in a graded manner. However, as previously reported by other authors, membrane potential steps to an internal positive value of approximately plus 90 mV (the suppression potential) produced a blockage of transmitter release for the duration of the imposed potential. Synaptic transmission recurred, nevertheless, as the current injection was terminated. A similar set of experiments, performed after the intracellular injection of aequorin in the presynaptic fiber, demonstrated that the aequorin light response was evoked by membrane potential steps capable of releasing synaptic transmitter. If the membrane potential was made positive to the 'suppression' level, no light response was evoked but the light emission appeared, as did transmitter release, at the end of the current pulse. These experiments demonstrate that release of transmitter is directly correlated with intracellular calcium concentration and that the suppression potential is compatible with the existence of a calcium equilibrium potential at the presynaptic terminal.

A brief sketch of the historic development of the concept of neuronal integration is given. Some of the properties of dendritic spikes in chromatolyzed motoneurons, alligator Purkinje cells, and fish oculomotor neurons are reviewed, as well as the various ways in which the generation of dendritic spikes has altered our basic concept of neuronal integration. The principles of dendritic inhibition are elaborated, both as a 'tonic modulatory action' and as a 'functional amputating system' for dendrites that generate spikes.