Search

Classical conditioning is a fundamental associative learning process in which repeated pairings of a conditioned stimulus (CS) with an unconditioned stimulus (US) lead to the CS eliciting a conditioned response (CR). Previous research has identified key neural regions involved in processing reward-predicting cues and mediating licking behavior. However, the mechanisms that sustain high conditioned response rates across repeated sessions remain elusive, particularly regarding how the reward expectation is represented on a session-by-session basis. While early learning phases in classical conditioning have been extensively studied, the neural mechanisms that support consistent performance over time remain unclear. In this study, we sought to understand how cortical regions, particularly the posterior parietal cortex (PPC), contribute to maintaining high CR rates across sessions. Using the core complex framework derived from Integrated Information Theory (IIT), we explored the dynamics of neural networks during sessions of high CR performance. Our findings suggest that while traditional functional connectivity (FC) methods struggled to capture the complexity of sustained behavioral engagement, the core complex framework revealed key regions, notably the PPC, that were significantly correlated with enhanced CR sessions. This work suggests the potential role of the PPC in supporting reward expectations and maintaining consistent behavioral responses. By applying the core complex framework to investigate neural substrates of sustained behavior, we provide novel insights into the interaction of cortical networks during classical conditioning, offering promising directions for future research in associative learning and behavior. [ABSTRACT FROM AUTHOR]

G-CaMP is a [Ca.sup.2+] probe based on a single green fluorescent protein (GFP). G-CaMP shows a large fluorescence increase upon [Ca.sup.2+] binding, but its fluorescence is dim and pH sensitive, similar to other single GFP-based probes. Here we report an improved G-CaMP, named G-CaMP1.6, which enables easier detection of intracellular [Ca.sup.2+] signals. G-CaMP1.6 was ~40 times more fluorescent than G-CaMP, mainly due to an increase in quantum yield. Furthermore, compared with G-CaMP, G-CaMP1.6 had not only a lower pH sensitivity but also a higher selectivity for divalent cations having an ionic radius similar to [Ca.sup.2+]. [Ca.sup.2+] sensitivity of G-CaMP1.6 ([K.sub.d] = 146 nM, Hill coefficient = 3.8, [F.sub.max]/[F.sub.min] = 4.9) was slightly shifted toward higher affinity compared with that of G-CaMP. When expressed in mammalian cells, G-CAMP1.6 showed large fluorescence changes with drug applications. Notably, local [Ca.sup.2+] changes in such tiny structures as dendritic spines of neurons were successfully observed with G-CaMP1.6, this being the first observation using a GFP-based probe. Additional mutations in [Ca.sup.2+]-binding sites of G-CaMP1.6 shifted the affinity for [Ca.sup.2+] and reduced the [Ca.sup.2+]-buffering effect. G-CaMP1.6-CaM(E140K), which has a mutation in the [Ca.sup.2+] binding site, is an improved probe with its increased brightness and reduced [Ca.sup.2+]-buffering capacity.

Author(s): Masanori Matsuzaki [1]; Naoki Honkura [1]; Graham C. R. Ellis-Davies [2]; Haruo Kasai (corresponding author) [1] Dendritic spines of pyramidal neurons in the cerebral cortex undergo activity-dependent structural remodelling [...]

Dendritic spines, which receive most of the excitatory synaptic input in the cerebral cortex, are heterogeneous with regard to their structure, stability and function. Spines with large heads are stable, express large numbers of AMPA-type glutamate receptors, and contribute to strong synaptic connections. By contrast, spines with small heads are motile and unstable and contribute to weak or silent synaptic connections. Their structure-stability-function relationships suggest that large and small spines are 'memory spines' and 'learning spines', respectively. Given that turnover of glutamate receptors is rapid, spine structure and the underlying organization of the actin cytoskeleton are likely to be major determinants of fast synaptic transmission and, therefore, are likely to provide a physical basis for memory in cortical neuronal networks. Characterization of supramolecular complexes responsible for synaptic memory and learning is key to the understanding of brain function and disease.