Whole-cell recordings of tiger salamander ganglion cells were obtained using a superfused retinal slice preparation. Membrane current fluctuations were recorded under voltage-clamp conditions with cells usually held at −70 mV. Current fluctuations at rest (Mg2+ = 1 mM) were reduced by adding d-2-amino-7-phosphonoheptanoate (d-AP7). Resting fluctuations were increased by adding N-methyl-D-aspartate (NMDA) or by removing extracellular Mg2+. These increased fluctuations were blocked by d-AP7. Blocking NMDA receptors under control conditions also reduced a tonic inward current by −1 to −15 pA. Fluctuation analysis of current noise shows that the noise power spectrum measured in the presence of NMDA is similar to that measured under Mg2+-free conditions. We conclude that NMDA receptors are active in cells held at −70 mV even in the presence of 1 mM Mg2+. We believe this activation is due to the presence of endogenous glutamate in the retina. The observations of this study strongly suggest that NMDA receptors contribute to the resting noise and conductance properties of retinal ganglion cells. Our results suggest NMDA receptors are activated by an ambient level of extracellular glutamate whose source has yet to be determined.

Neural activity in the optic tectum was compared with activity in the nucleus isthmi (NI) of both goldfish and sunfish with the aim of understanding how the two brain structures interact to process visual information. The two species yielded very similar results. Superficial tectum responds reliably to visual stimulation with topographically organized receptive fields; deep tectum and NI respond to stimulation throughout the field of the contralateral eye and habituate rapidly. Bursts of large-amplitude spiking in NI occur spontaneously and in response to contralateral visual stimulation. These NI bursts correlate with activity bursts across the tectal lobe on the same side, especially in the deeper layers. NI bursts may also synchronize with spiking activity in deep tectum. Trains of small-amplitude spikes in NI can be elicited by both ipsilateral and contralateral stimulation, but are not reflected in tectal activity. Simultaneous recordings from two sites in one NI were almost identical, suggesting that NI operates as a functional unit, broadcasting the same message across the ipsilateral tectal lobe.

MacDRP is a Macintosh application that calculates Density Recovery Profiles.