The dorsodorsal division (DD) of the teleost telencephalon has been implicated in memory processes similar to those associated with the mammalian hippocampus. The network connectivity and neural activity underlying this involvement have remained unclear. This thesis attempts to elucidate both. Attempts have been made to record the neural activity of DD neurons, but none have succeeded in correlating the recorded firing with any meaningful stimuli. In this thesis, I present single-unit electrophysiological recordings of DD neurons that reveal persistent activity in the form of up-states which are evoked by two modalities of naturalistic sensory stimuli – visual and electrosensory. The anatomy of DD was a little better understood than the neural activity. Recent anatomical work has shown that DD is strongly inter-connected with the cortical-like dorsolateral division (DL) of the pallium, re-inforcing its similarity to mammalian hippocampal structures. This same work has also revealed much of DD’s extrinsic connectivity. It was not, however, of a resolution fine enough to disambiguate the connections of the various DD subregions, nor to clarify the existence and structure of intrinsic DD connectivity. In this thesis, I further elucidate the connectivity of DD, by isolating its subregions. This was done by means of very small and precise neurotracer injections. These injections revealed strong recurrent connectivity within individual DD subregions, multiple pathways between DD and DL, and striking similarities between the connectivities of DL and DD and those of the mammalian dentate gyrus and CA3 respectively. From the results of these investigations I propose a model of homology between the teleost DD-DL loop and the putative pattern separation and completion networks contained in the mammalian cortico-hippocampal circuitry, as well as a role for the observed persistent activity in DD within this model. I further propose the dorsal teleost telencephalon as an excellent model system for the further study of the network mechanisms of pattern separation and completion.

Under anaesthesia or in deep sleep, different parts of the brain have a distinctive slow oscillatory activity, characterised by states of high membrane potential and intensive spiking activity, the Up-states; followed by hyperpolarisation and quiescence, the Down-states. This activity has been previously described in vitro and in vivo in the cortex and the striatum, across several species. Here, we look into it, during anaesthesia, in the mouse brain. Using whole-cell patch-clamp recordings of cortical cells, it was possible to compare different signal processing methods used to extract the Up-and- Down states in extracellular recordings of the cortex. Our results show that the method based on the Multi-Unit Activity (&gt; 200Hz) have better ac- curacy than High-Gamma Range (20 100Hz) or wavelet decomposition (&lt; 2Hz band). After establishing the most robust method, this was used to compare the intracellular recordings of striatal cells to different parts of the cortex. The results obtained here support a functional connection between the dorsolateral striatal neurons and the ipsilateral barrel field. They also support a functional connection between dorsomedial striatal cells and the primary visual cortex. The analysis of delay between recordings allowed to establish temporal relationships between the contralateral barrel field, the ipsilateral barrel field, and the dorsolateral striatum; and between the ipsilateral barrel field, the ipsilateral primary visual field and the dorsomedial striatum. / <p>External Advisor: Dr. Ramon Reig, from Karolinska Institutet</p>