Sensory integration in the hippocampal formation of pre- and post-weanling rats

‘Place cells’ are hippocampal pyramidal neurons which fire only when an animal visits a particular location in an environment (‘place fields’). Their location-specific firing is supported by configurations of multi-modal sensory cues. Almost all work on place cells focused on the properties of these cells in adult rats. Recent work (Langston et al., 2010; Scott et al., 2011; Wills et al., 2010) however could identify place cells in the hippocampus of very young rat pups (ca. 2 weeks old), and furthermore show that these cells undergo a strong functional maturation in terms of their location-specific firing properties. In this thesis, we investigated when the configural integration of sensory information first emerges during the postnatal development of the hippocampus, by probing the response of place cells to manipulations of sensory cues in a familiar environment in pre- (aged 2-3 weeks) and post-weanling (aged 3-4 weeks) rat pups as well as in adult controls. These included changing certain parts of the local olfactory/tactile cues as well as removing visual cues. Recordings were also undertaken in a completely novel environment. These experiments will further our understanding about how the brain’s system for the representation of space develops and in particular will shed light on the question whether place cells in young rat pups are driven by single cues or already have configural properties as in adult rats. The results described in this thesis are compatible with the view that place cell responses recorded in very young rat pups already integrate multimodal cues, as in adults, suggesting that hippocampal spatial responses are inherently configural. However, some evidence points out to a stronger influence of local non-visual intra-maze cues early in development, while the influence of vision seems to increase across development. This could be due to the concurrent maturation of the sensory systems. In a novel environment, animals of all ages form a novel spatial representation, showing that even animals as old as 2 weeks can already distinguish between two distinct environments