Different modulation of flash responses of fast and slow relay cells by aversive electrical brain stimulation

Electrical activity of single neurons of the dorsal lateral geniculate nucleus of rats was recorded from stereotactically fixed animals. The neurons were classified as fast or slow ones, according to their response latency to electrical stimulation of the chiasma opticum. After this, the responses to diffuse flashes, to aversive electrical stimulation of the brain (ESB) and to a combination of both, brain stimulation immediately followed by a flash, were recorded. One week prior to testing, bipolar nichrom electrodes had been implanted into the tectotegmental region for the ESB, and their effectiveness was tested behaviorally. Out of 78 neurons 56 fast and 22 slow ones were found. Fast neurons have a significantly higher background activity than slow neurons. Both classes show a modulation of flash evoked responses by the ESB, although the ESB has nearly no effect on the spontaneous discharge frequency in any of the neurons. The influence on the flash response differs according to the cell class: the class of fast neurons is predominantly facilitated up to 1100 ms after the flash application, whereas the class of slow neurons is inhibited during this period. Later on (up to 2260 ms after flash) fast cells show no clear net effect, and slow cells were still slightly inhibited.