Suprathreshold excitation of frog tectal neurons by short spike trains of single retinal ganglion cell

It has been established that coincident inputs from multiple presynaptic axons are required to achieve a suprathreshold level of excitation for the most of central neurons. The present study, however, was designed to determine whether a train of spikes of an individual retinal ganglion cell (that is, input from a single presynaptic axon) targeting a frog tectum layer F could evoke suprathreshold excitation of tectal neurons. The lungs of immobilized frog were artificially ventilated during experiments. An individual ganglion cell was electrically stimulated in the retina through a multi-channel electrode. Responses evoked in the tectum by the stimulation were recorded extracellularly from a terminal arborization of the retinotectal fiber using the carbon-fiber microelectrode. Negative and negative-positive spikes (referred to as first type population responses) and polyphasic spikes followed by excitatory synaptic potentials (referred to as second type population responses) were observed in the recordings of retinotectal activity. Usually, the population responses have ensued after the frequency facilitated first and/or second testing individual retinotectal synaptic potential and disappeared in a threshold manner with a reduction of retinotectal transmission by an application of kynurenic acid. These observations have suggested that the population responses were a consequence of a suprathreshold excitation of tectal neurons and, therefore, could serve as the sign for such an excitation. Recordings have also demonstrated that sources of the first type population responses (likely, the hillocks of axons or somas of postsynaptic neurons) lie deeper than the optic fiber layer F of the tectum, whereas sources of the second type population responses (likely, axon terminal arborizations of these postsynaptic neurons) are scattered throughout the optic fiber layers. The findings have suggested: 1) a short train of action potentials of an individual retinal ganglion cell (likely darkness, also known as 5th, detector) can excite tectal neurons to suprathreshold level; 2) tectal and perhaps, nucleus isthmi neurons that make up recurrent connection circuits to the optic fiber layers of the tectum are also activated; 3) a suprathreshold level for an individual retinotectal input is achieved primarily due to the frequency facilitation of synaptic potentials; and 4) an artificial ventilation of the lungs of immobilized frog favors the eliciting of a suprathreshold excit