Striatal cholinergic interneurons (CIN) are pivotal for the regulation of the striatal network. Acetylcholine (ACh) released by CIN is centrally involved in reward behavior as well as locomotor or cognitive functions. Recently, BAC transgenic mice expressing channelrhodopsin-2 (ChR2) protein under the control of the choline acetyltransferase (ChAT) promoter (ChAT-ChR2) and displaying almost 50 extra copies of the VAChT gene were used to dissect cholinergic circuit connectivity and function using optogenetic approaches. These mice display over-expression of the vesicular acetylcholine transporter (VAChT) and increased cholinergic tone. Consequently, ChAT-ChR2 mice are a valuable model to investigate hypercholinergic phenotypes. Previous experiments established that ChAT-ChR2 mice display an increased sensitivity to amphetamine induced-locomotor activity and stereotypes. In the present report, we analyzed the impact of VAChT over-expression in the striatum of ChAT-ChR2 mice. ChAT-ChR2 mice displayed increased locomotor sensitization in response to low dose of cocaine. In addition, we observed a dramatic remodeling of the morphology of CIN in ChAT-ChR2 transgenic mice. VAChT immunolabeling was markedly enhanced in the soma and terminal of CIN from ChAT-ChR2 mice as previously shown (Crittenden et al. 2014). Interestingly, the number of cholinergic varicosities was markedly reduced (-87%) whereas their size was significantly increased (+177%). Moreover, VAChT over-expression dramatically modified its trafficking along the somatodendritic and axonal arbor. These findings demonstrate that ChAT-ChR2 mice present major alterations of CIN neuronal morphology and increased behavioral sensitization to cocaine, supporting the notion that the increased levels of VAChT observed in these mice make them fundamentally different from wild-type mice.
The lateral superior olive (LSO), a nucleus in the auditory brainstem, computes interaural intensity differences for sound localization by comparing converging excitatory and inhibitory inputs that carry tonotopically matched information from the two ears. Tonotopic refinement in the inhibitory projection pathway from the medial nucleus of the trapezoid body (MNTB) is known to be established during the first postnatal week in rats. During this period, immature MNTB terminals in the LSO contain vesicular transporters for both inhibitory and excitatory amino acids and release glutamate. The primary Ca(2+) sensors for vesicular release in the CNS are understood to be synaptotagmins, and in adult auditory brainstem synaptotagmin 2 is the predominant synaptotagmin. We asked here whether a different Ca(2+) sensor might be expressed in the immature auditory brainstem. We have found that synaptotagmin 1 is indeed expressed transiently in the immature auditory brainstem, most highly in those areas that receive glutamate-releasing immature inhibitory inputs from the MNTB, and that during the first postnatal week synaptotagmin 1 co-localizes with the vesicular glutamate transporter VGLUT3, a marker of glutamate-releasing immature inhibitory terminals from the MNTB. We suggest that immature MNTB terminals may contain two populations of synaptic vesicles, one expressing the vesicular inhibitory amino acid transporter together with synaptotagmin 2 and another expressing VGLUT3 together with synaptotagmin 1. Because Ca(2+) sensing is an important determinant of release properties for the presynaptic terminal, differential expression of the synaptotagmins might allow the differential release of excitatory and inhibitory neurotransmitters in response to differing patterns of neural activity.