Anatomical and functional study of lateral superior colliculus thalamic projections in predatory behavior in rats

Abstract

A recent study, examining the expression of Fos protein, suggest the key neural systems mobilized during predatory behavior of rats hunting cockroaches. Of particular interest, it was found that predatory hunting induce a distinct activation of the lateral region of the intermediate layer of the superior colliculus (SCl), which does not appear to be particularly mobilized in other behavioral situations, such as after nocturnal peak of food ingestion. In order to understand the potential roles of this collicular region in the context of predatory hunting, it was analyzed the hunting performance of the animals before and after iontophoretic NMDA lesions bilaterally placed into SCl. Notably, collicular lesions did not interfere with the motivation to pursue the roaches, however, they did induce severe deficits related to motor praxia, i.e, lesioned animals handled the preys very awkwardly and were unable to capture and hold them efficiently. In addition, lesioned animals could not properly orient themselves toward fast moving roaches. Next the conncetions of SCl was examined. The efferent projections of the SCl which were analyzed by using the Phaseolus vulgaris leucoagglutinin method, suggest that SCl appear to influence the motor output via two main pathways: a descending putative involved in coordenating eyes, orofacial and forelimb orienting movements toward the moving preys; and ascending pathways, which is in a position to modulate motor responses by influencing thalamic regions also targeted by the basal ganglia and cerebellum. Experiments with the retrograde tracer Fluoro-Gold showed that the SCl, likewise structures related to motor control, receives information related to somatosensory sensibility from the whiskers and orofacial region. Finally, anterograde findings also revealed direct projection from SCl to the compacta part of substantia nigra (SNc), and it showed that SCl supply a direct innervation to dopaminergic cells in the midbrain, and also eletrophysiological relations. Therefore, the SCl may as well convey multimodal sensory information to dopaminergic cells, exerting a critical role in controlling attentional mechanisms and motor strategies shifts. McHaffie et. al. proposed that phylogenetic old closed-loop connections exist between the basal ganglia and brain stem sensory related structures like superior colliculus, however the full extent to which they actually represent functionally segregated parallel closed-loop remains to be determined. So, in attept to better understand the role of superior colliculus, we intend to verify functional aspects of connections between superior colliculus and basal ganglia during predation. Manipulations effects of different components of collicular loops over behavioral reactions elicited by superior colliculus stimulation/activation might provide important information about functional organization of those loops. We suggest that thalamus is an element through which the superior colliculus exerts this effect, once thalamus provide the main excitatory inputs to striatum, and is active during roaches predation. To this investigation animals will be prepared with chronic guide-cannulae providing access to the thalamic nuclei that provide the main excitatory inputs to striatum. Stimulation of SCl by the presence of preys and innactivation of thalamic regions target by SCl by injecting mucimol would have a corresponding greater effect over predation. In order to define thalamic sites to receive mucimol we will inject PHA-L in the SCl and find out whether anterograde fibers merge cells expressing Fos during predation. We believe the inactivation of thalamic areas elicited by SCl activation would result in great effects over predation, though we would be testing the hypothesis that phasic desinhibition mediated by a sub-cortical closed-loop of basal ganglia exerts a significant role over behaviors evoked by stimulation of superior colliculus in the presence of preys.