Passive avoidance response distribution by post-training substantia nigra functional tetrodotoxin inactivation in the rat.

The tetrodotoxin (TTX) functional ablation technique was employed to assess the temporal coordinates of rat's substantia nigra (SN) in memory processing. TTX (10 ng in 1 microliter saline) was stereotaxically administered to rats under general ketamine anesthesia, either bilaterally or unilaterally. TTX was injected in different groups of rats respectively 0.25, 6, 24, and 48 hours after passive avoidance acquisition testing. Rats always underwent retrieval testing 48 hours later, after full recovery from TTX effects. The results show that: i) unilateral TTX blockade significantly impairs PAR only up to 0.25 h and not 6 h after acquisition testing, and ii) bilateral TTX blockade dramatically disrupts passive avoidance responding up to 24 but not 48 hours after acquisition testing. The results indicate a much more important SN role in memory processing than was previously assessed. The experimental evidence is discussed both in relation to previous TTX functional ablation findings (amygdala, parabrachial nuclei, nucleus basalis magnocellularis) and in relation to SN anatomical and functional connections with other subcortical structures.