Hippocampal interneurons co-express transcripts encoding the α7 nicotinic receptor subunit and the cannabinoid receptor 1

The notion of functional interactions between the α7 nicotinic acetylcholine (α7 nACh) and the cannabinoid systems is emerging from recent in vitro and in vivo studies. Both the α7 nACh receptor and the cannabinoid receptor 1 (CB1) are highly expressed in the hippocampus. To begin addressing possible anatomical interactions between the α7 nACh and the cannabinoid systems in the rat hippocampus, we investigated the distribution of neurons expressing α7 nACh mRNA in relation to those containing CB1 mRNA. By in situ hybridization we found that the α7 nACh mRNA is diffusely expressed in principal neurons and is highly expressed in a subset of interneurons. We observed that the pattern of distribution of hippocampal interneurons co-expressing transcripts encoding α7 nACh and glutamate decarboxylase (GAD; synthesizing enzyme of GABA) closely resembles the one displayed by interneurons expressing CB1 mRNA. By double in situ hybridization we established that the majority of hippocampal interneurons expressing α7 nACh mRNA have high levels of CB1 mRNA. As CB1 interneurons contain cholecystokinin (CCK), we investigated the degree of cellular co-expression of α7 nACh mRNA and CCK, and found that the cellular co-existence of α7 nACh and CCK varies within the different layers of the hippocampus.In summary, we established that most of the hippocampal α7 nACh expressing interneurons are endowed with CB1 mRNA. We found that these α7 nACh/CB1 interneurons are the major subpopulation of hippocampal interneurons expressing CB1 mRNA. The α7 nACh expressing interneurons represent half of the detected population of CCK containing neurons in the hippocampus. Since it is well established that the vast majority of hippocampal interneurons expressing CB1 mRNA have 5-HT type 3 (5-HT3) receptors, we conclude that these hippocampal α7 nACh/5HT3/CB1/CCK interneurons correspond to those previously postulated to relay inputs from diverse cortical and subcortical regions about emotional, motivational, and physiological states.