Abstracts

Rationale: Semilunar granule cells (SGCs), novel excitatory dentate projection neurons, are proposed to play a vital role in granule cell feedback inhibition and are distinguished from granule cells (GCs) by their dendritic arbor and intrinsic physiology. While SGCs have been identified and studied in 14-30 postnatal day rats, little is known about their presence, properties and developmental plasticity in neonatal and adult animals. The paucity of information on SGC development poses a significant impediment to examining their role in disease. This study was conducted to determine the developmental plasticity of SGC inhibition and to delineate the structural and functional changes in SGCs and GCs during development. Methods: Whole cell patch clamp recordings were obtained for putative GCs and SGCs in hippocampal slices from rats at three different postnatal (P) time points: neonatal (P9-13), adolescent (P28-42) and adult (P>120). Recorded cells were filled with biocytin, processed to recover morphology, reconstructed and analyzed from 20X confocal image stacks using Neurolucida.   Results:  Morphometic analysis of reconstructed SGCs identified that the total cell surface area (SGC: Neonatal=8504+/-1084 µm²; Adolescent=20207.25+/- 2279 µm²; Adult=13051+/- 2620 µm² by Two way ANOVA, F(3,40)=7.743, p < 0.001) and dendritic length increased from neonatal to adolescent period and and remained elevated in adults. Consistent with increase in surface area, membrane capacitance of both GCs and SGCs increased from neonatal to adolescent rats and remained high in adults. While SGCs input resistance in neonates and adolescents was significantly lower then in GCs, SGC input resistance increased with age while GC input resistance decreased. Thus, in adults, SGC input resistance was higher than that of GCs.   Interestingly, tonic GABA current amplitudes peaked during adolescence in both cell types (SGC: Neonatal=2.4±0.8pA; Adolescent=17.1±1.7pA, Adult=4.1±1.3pA, GC: Neonatal=3.6±2.1pA; Adolescent=10.2±1.6pA; Adult=5.4±1.4pA) but decrease back to neonatal levels in adults (Effect of age: F(2,47)=26.9, p < 0.001, by Two way ANOVA). Spontaneous inhibitory postsynaptic currents (sIPSCs) frequency increased between neonatal and adolescent rats in both SGCs and GCs. However, sIPSC frequency declined back to neonatal levels in adults in SGCs while it was  maintained at adolescent levels in GCs.  Conclusions: Our data demonstrate that anatomical development plays an important contribution to developmental changes of synaptic and extrasynaptic inhibition in both SGCs and GCs. Increase in SGC input resistance between adolescent and adult rats occurs simultaneously with a decrease in SGC tonic GABA currents suggesting a role for inhibition in regulating developmental changes in SGC input resistance. Together, the distinct age-related morphological and physiological features of SGCs may contribute to differential engagement and modulation of SGCs during dentate information processing at diffent developmental stages. Funding: This work is supported by  NJCBIR 11-3223-BIR-E-O to Akshay Gupta and NIH/NINDS NS069861 to Viji Santhakumar.