It has long been acknowledged that inhibitory interneurons play a crucial role in regulating the input-output functions of principal cells in the hippocampus. The investigations we conducted focus on one specific population of interneurons, expressing the protein parvalbumin. The thesis describes three different studies, aimed to characterize anatomical and functional aspects of parvalbumin positive interneurons in the mouse hippocampal formation. The first study examines long-range projections of these neurons from CA1 and subiculum to distant regions of the brain, finding potential targets mainly in septal, thalamic and hypothalamic areas. The second study investigates the role of parvalbumin-positive interneurons of the dentate gyrus in spatial memory. Behavioural experiments with radial arm and Morris water maze have been carried out to understand how these GABAergic interneurons regulate information flow during reference and working memory. Finally, a third study describes basic anatomical features of parvalbuminpositive dendritic spines in the dentate gyrus. Their characteristics have been widely studied in principal neurons but little is known about spines in interneurons. Here I show a peculiar distribution of spines on apical dendrites of these cells, with a predominant localization within the inner third of the molecular layer. All studies utilized a combination of transgenic Cre-expressing mice and Creactivatable AAVs. For the first and third study AAV-based neuronal labeling was applied to visualize neurons, including their projections and their spines, respectively, through expression of fluorescent proteins. For the second study instead two genetic tools have been used in order to disrupt the neurotransmission from parvalbuminpositive interneurons and examine the effects on behavioral task performance.