Functional specializations within the tectum defense systems of the rat

Here we review the differential contribution of the periaqueductal gray matter (PAG) and superior colliculus (SC) to the generation of rat defensive behaviors. The results of studies involving sine-wave and rectangular pulse electrical stimulation and chemical (NMDA) stimulation are summarized. Stimulation of SC and PAG produced freezing and flight behaviors along with exophthalmus (fully opened bulged eyes), micturition and defecation. The columnar organization of the PAG was evident in the results obtained. Defecation was elicited primarily by lateral PAG stimulation, while the remaining defensive behaviors were similarly elicited by lateral and dorsolateral PAG stimulation, although with the lowest thresholds in the dorsolateral column. Conversely, the ventrolateral PAG did not appear to participate in unconditioned defensive behaviors, which were only elicited by high intensity stimulation likely to encroach on adjacent regions. In the SC, the most important differences relative to the PAG were the lack of stimulation-evoked jumping in both intermediate and deep layers, and of NMDA-evoked galloping in intermediate layers. Therefore, we conclude that the SC may be only involved in the increased attentiveness (exophthalmus, immobility) and restlessness (trotting) of prey species exposed to the cues of a nearby predator. These responses may be distinct from the full-blown flight reaction that is mediated by the dorsolateral and lateral PAG. However, other evidences suggest the possible influences of stimulation schedule, environment dimensions and rat strain in determining outcomes. Overall our results suggest a dynamically organized representation of defensive behaviors in the midbrain tectum.