My project has been involved in analysing a class of interneuron that expresses the calcium‐binding protein parvalbumin (PV). In my thesis, I will describe the application of a method that involves the local injection of Creactivated recombinant adeno-associated viruses (AAVs) into a transgenic mouse line that expresses Cre recombinase in PV positive cells. This will drive the expression of a transgene specifically in PV positive cells, at a specific brain region. In the first part of my project, I used this method to introduce the molecular trans-synaptic tracer proteins wheat germ agglutinin and tetanus toxin heavy chain specifically to PV positive neurons to visualize their postand pre‐synaptic connections, respectively. What I found is that while our technique of combining Cre-activated AAVs in transgenic mice has allowed specific labelling of neurons in a brain region and cell type specific manner, we could not definitively identify trans-synaptically traced neurons. In the second part of my project I have used these novel AAV‐based techniques in mice to introduce tetanus toxin light chain (TeLC) to PV neurons in the dentate gyrus. This has been previously used to functionally remove PV neurons from the CA1 of the hippocampus. This protein inhibits neurotransmitter release by cleaving the vesicle docking protein, VAMP2. The DG has been implicated in the separation of sensory inputs (pattern separation) which increases the resolution of the encoded memory and thereby assists in the accurate recall. The lateral inhibition of excitatory activity in the DG is believed to aid accurate encoding. Using our AAV method, I found that PV positive interneurons are required for spatial working and reference memory. Using a new behavioural assay that I developed, I could also show that these neurons are needed to enhance the resolution of spatial information. However, I also discovered that long term expression of TeLC could result in neuronal cell death. I have therefore demonstrated that local injection of Cre recombinase activated AAVs allows for a quick, versatile method of genetic manipulation, provided long term expression (greater than 2 months) is not required.