Organization of single components of defensive behaviors within distinct columns of periaqueductal gray matter of the rat: role of N-methyl-D-aspartic acid glutamate receptors

Abstract

The periaqueductal gray matter (PAG) is functionally organized in longitudinal columns arranged along the aqueduct. Stimulation of lateral and dorsal columns produces a complex set of unconditioned behaviors named the 'defense reaction.' Overt responses in rats comprise a tense immobile display, fully opened eyes (herein named exophthalmus), trotting, galloping, jumping, micturition and defecation. Besides, the PAG is rich in glutamate and respective receptors, including the N-methyl-d-aspartic acid (NMDA) type. Therefore, the present study employed regression analysis to map out electrically and NMDA-induced single components of defensive behaviors produced by stepwise increasing stimulation of PAG. Data confirmed the defensive nature of PAG-evoked responses. Neither the appetitive, nor offensive, mouse-killing or male reproductive behaviors were produced by stimulation of PAG in presence of appropriate targets. Threshold and dose-response logistic analyses largely corroborated the columnar organization of PAG-evoked responses. Thus, whereas the defecation was restricted to PAG lateral column, exophthalmus, micturition and somatic defensive responses were similarly organized in dorsolateral and lateral, but not in the ventrolateral column. Moreover, thresholds of dorsolateral and lateral repertoires were strictly hierarchical, with exophthalmus, immobility, trotting, galloping and jumping appearing in this very order. However, the defensive responses of PAG dorsolateral column required NMDA doses significantly lower than those of lateral PAG. Accordingly, NMDA receptors within the dorsolateral PAG are likely to play a major role in the initiation of PAG-evoked defensive responses. In contrast, the present data do not support the organization of unconditioned defensive behaviors in ventrolateral PAG. The neuroanatomical substrate of each response and the role of PAG and NMDA receptors are discussed in relation to the present data. Further, this is the first report on PAG columnar organization of single components of defensive behaviors.