A challenge in neuroscience is to understand the mechanisms underlying synapse formation. Most excitatory synapses in the brain are built on spines, which are actin-rich protrusions from dendrites. Spines are a major substrate of brain plasticity, and spine pathologies are observed in various mental illnesses. Here we investigate the role of neurobeachin (Nbea), a multidomain protein previously linked to cases of autism, in synaptogenesis. We show that deletion of Nbea leads to reduced numbers of spinous synapses in cultured neurons from complete knockouts and in cortical tissue from heterozygous mice, accompanied by altered miniature postsynaptic currents. In addition, excitatory synapses terminate mostly at dendritic shafts instead of spine heads in Nbea mutants, and actin becomes less enriched synaptically. As actin and synaptopodin, a spine-associated protein with actin-bundling activity, accumulate ectopically near the Golgi apparatus of mutant neurons, a role emerges for Nbea in trafficking important cargo to pre- and postsynaptic compartments.

Human genetics has identified rare copy number variations and deleterious mutations for all neurexin genes (NRXN1-3) in patients with neurodevelopmental diseases, and electrophysiological recordings in animal brains have shown that Nrxns are important for synaptic transmission. While several mouse models for Nrxn1α inactivation have previously been studied for behavioral changes, very little information is available for other variants. Here, we validate that mice lacking Nrxn2α exhibit behavioral abnormalities, characterized by social interaction deficits and increased anxiety-like behavior, which partially overlap, partially differ from Nrxn1α mutant behaviors. Using patch-clamp recordings in Nrxn2α knockout brains, we observe reduced spontaneous transmitter release at excitatory synapses in the neocortex. We also analyse at this cellular level a novel NRXN2 mouse model that carries a combined deletion of Nrxn2α and Nrxn2β. Electrophysiological analysis of this Nrxn2-mutant mouse shows surprisingly similar defects of excitatory release to Nrxn2α, indicating that the β-variant of Nrxn2 has no strong function in basic transmission at these synapses. Inhibitory transmission as well as synapse densities and ultrastructure remain unchanged in the neocortex of both models. Furthermore, at Nrxn2α and Nrxn2-mutant excitatory synapses we find an altered facilitation and N-methyl-D-aspartate receptor (NMDAR) function because NMDAR-dependent decay time and NMDAR-mediated responses are reduced. As Nrxn can indirectly be linked to NMDAR via neuroligin and PSD-95, the trans-synaptic nature of this complex may help to explain occurrence of presynaptic and postsynaptic effects. Since excitatory/inhibitory imbalances and impairment of NMDAR function are alledged to have a role in autism and schizophrenia, our results support the idea of a related pathomechanism in these disorders.

Neurotransmission at different synapses is highly variable, and cell-adhesion molecules like α-neurexins (α-Nrxn) and their extracellular binding partners determine synapse function. Although α-Nrxn affect transmission at excitatory and inhibitory synapses, the contribution of neurexophilin-1 (Nxph1), an α-Nrxn ligand with restricted expression in subpopulations of inhibitory neurons, is unclear. To reveal its role, we investigated mice that either lack or overexpress Nxph1. We found that genetic deletion of Nxph1 impaired GABA B receptor (GABA B R)-dependent short-term depression of inhibitory synapses in the nucleus reticularis thalami, a region where Nxph1 is normally expressed at high levels. To test the conclusion that Nxph1 supports presynaptic GABA B R, we expressed Nxph1 ectopically at excitatory terminals in the neocortex, which normally do not contain this molecule but can be modulated by GABA B R. We generated Nxph1-GFP transgenic mice under control of the Thy1.2 promoter and observed a reduced short-term facilitation at these excitatory synapses, representing an inverse phenotype to the knockout. Consistently, the diminished facilitation could be reversed by pharmacologically blocking GABA B R with CGP-55845. Moreover, a complete rescue was achieved by additional blocking of postsynaptic GABA A R with intracellular picrotoxin or gabazine, suggesting that Nxph1 is able to recruit or stabilize both presynaptic GABA B R and postsynaptic GABA A R. In support, immunoelectron microscopy validated the localization of ectopic Nxph1 at the synaptic cleft of excitatory synapses in transgenic mice and revealed an enrichment of GABA A R and GABA B R subunits compared with wild-type animals. Thus, our data propose that Nxph1 plays an instructive role in synaptic short-term plasticity and the configuration with GABA receptors. synaptic transmission | thalamus | autism | neuroligin | ultrastructure

The mammalian pretectal nuclear complex (PNC) is a visual and visuomotor control structure which is strongly connected to other subcortical visual structures. This indicates that the PNC also controls subcortical visual information flow during the execution of various oculomotor programs. A prominent, presumably GABAergic, projection from the PNC targets the superficial grey layer of the superior colliculus (SC), which itself is a central structure for visual information processing necessary for the generation of saccadic eye movements. In order to characterize the pretectotectal projection in vitro, we performed whole-cell patch-clamp recordings from SC and PNC neurons in slices obtained from 3-6-week-old pigmented rats. Focal glutamate injections into the PNC and electrical PNC stimulation were used to induce postsynaptic responses in SC neurons. Electrical stimulation of the SC allowed electrophysiological identification of PNC neurons that provide the inhibitory pretectotectal input. Only inhibitory postsynaptic currents could be elicited in SC neurons both by pharmacological and by electrical activation of the ipsilateral PNC. Concomitantly, a small number of PNC neurons could be antidromically activated from the ipsilateral SC. Most SC cells postsynaptic to the prectectal input showed the dendritic morphology of wide-field and narrow-field cells and are therefore regarded as projection neurons. All inhibitory currents evoked by PNC activation could be completely blocked by bath application of the selective GABA(A) receptor antagonist bicuculline. Together these results indicate that SC projection neurons receive a direct inhibitory input from the ipsilateral PNC and that this input is mediated by GABA(A) receptors.

The ventral lateral geniculate nucleus (vLGN), the pretectal nuclear complex (PNC) and the superior colliculus (SC) are structures that all receive retinal input. All three structures are important relay stations of the subcortical visual system. They are strongly connected with each other and involved in circadian and/or visuomotor processes. However, the information transferred along these pathways is unknown and their possible functions are, therefore, not well understood. Here, we characterized multiple pathways between the vLGN, the PNC, and the SC electrophysiologically and anatomically in an in vitro study using acute rat brain slices. Using orthodromic and antidromic electrical stimulation, we first characterized vLGN neurons that receive pretectal input and those that project to the PNC. Morphological reconstructions of cells labeled after patch clamp recordings identified these neurons as geniculo-tectal neurons and as medium-sized multipolar neurons. We identified inhibitory connections in both pathways and we could show that inhibitory postsynaptic currents (IPSCs) evoked from the PNC in vLGN neurons are mediated only by GABAA receptors, while IPSCs evoked in PNC neurons by vLGN stimulation are either mediated by both, GABAA and GABAC receptors or by a GABA receptor with mixed GABAA and GABAC receptor-like pharmacology. Finally, retrograde double labeling experiments with two different fluorescent dextran amines indicated that pretectal neurons which project to the ipsilateral vLGN also project to the ipsilateral SC.