Evoked responses from an in vitro slice preparation of a primary gustatory nucleus: the vagal lobe of goldfish.

The vagal lobe of goldfish (Carassius auratus) is a laminated structure in which primary gustatory afferents terminate in a stereotypical pattern. Because the afferent fibers enter and distribute within the lobe in the transverse plane, the structure appeared suitable for in vitro slice electrophysiology. Slices were cut on a vibratome at 400-800 microns thickness and placed in a fresh water teleost Ringer's solution. Following a recovery period, clear population responses were recorded following electrical stimulation of the incoming fiber bundle. The later two components of this evoked waveform were eliminated by removal of the calcium from the bathing solution indicating the synaptic origin of these potentials. Further, the waveform was highly dependent on the position of the recording electrode, both in terms of laminar and tangential position. Evoked response maxima corresponded to the layers in which the primary afferent fibers terminate. In addition, the maximal evoked response was limited to a tangential distance of approximately 100 microns. The spatial restriction of the evoked waveform therefore corresponds well with the known anatomical organization of the primary gustatory afferent fibers. The evoked waveforms are sensitive to stimulus repetition rate, being facilitated by stimulus trains less than 50 ms in duration and fatigued by stimulus repetitions as slow as 1 Hz. This in vitro preparation should provide a means for investigating the physiological and pharmacological properties of primary gustatory fibers and nuclei.