Circuit-specific dendritic development in the piriform cortex

Abstract

Dendritic geometry is largely determined during postnatal development and has a substantial impact on neural function. In sensory processing, postnatal development of the dendritic tree is affected by two dominant circuit motifs, ascending sensory feedforward inputs and descending and local recurrent connections. Two subtypes of layer 2 neurons in the three-layered anterior piriform cortex, layer 2a and layer 2b neurons, display a clear vertical segregation of these two circuit motifs. Here, we combined electrophysiology, detailed morphometry and Ca 2+ imaging -both of neuronal networks as well as of subcellular structures-in acute mouse brain slices and modeling. This allowed us to compare the functional implications of distinct circuit-specific postnatal dendritic growth patterns in these two neuronal subtypes. We observed that determination of branching complexity, dendritic length increases and pruning occurred in distinct growth phases. Layer 2a and layer 2b neurons displayed growth phase specific developmental differences between their apical and basal dendritic trees. This was reflected by compartment-specific differences in Ca 2+ signaling. The morphological and functional developmental pattern differences between layer 2a and layer 2b neurons dendrites provide further evidence that they constitute two functionally distinct streams of olfactory information processing. Materials and Methods Slice preparationAcute brain slices were prepared from C57Bl6N mice except for population Ca 2+ imaging experiments with GCaMP (Fig. 4), where Ai95-NexCre mice were used. In experiments for Figs. 1-6, the horizontal slicing orientation was chosen because it best preserves rostrocaudal association fibers (Demir et al. 2001). For dendritic spike measurements in Fig. 7, we used coronal slices. All procedures were in accordance with the national and institutional guidelines and approved by the local health authority and the local ethics committee (Landesamt für Gesundheit und Soziales, Berlin; animal license number T100/03). For electrophysiological characterization and morphological reconstruction, acute brain slices were prepared at 4 age intervals: p1-2, p6-8, p12-14 and p30-40. For measurements of NMDA-spikes, coronal slices were prepared at p14-21. Brains from p30-40 mice and from mice used for dendritic spike measurements were prepared in ice-cold artificial cerebrospinal fluid (ACSF; pH 7.4) containing (in mM): 87 NaCl, 26 NaHCO3, 10 Glucose, 2.5 KCl, 3 MgCl2, 1.25 NaH2PO4, 0.5 CaCl2 and 50 sucrose. Slices were cut at 400 µm thickness, and incubated at 35°C for 30 min. The slices were then transferred to standard ACSF containing (in mM): 119 NaCl, 26 NaHCO3, 10 Glucose, 2.5 KCl, 2.5 CaCl2, 1.3 MgCl2, and 1 NaH2PO4. Slices from other age groups were cut in ice-cold standard ACSF and incubated for 30 min in standard ACSF at 35°C. The slices were then stored in standard ACSF at room temperature in a submerged chamber for 0.5-6 h before being transferred to the recording chamber. For dendritic spike measurements in Fig. ...