Postnatal Development Of Postsynaptic Currents And Dendritic Arborization In The Rat Hippocampal CA1 Pyramidal Neurons

Excitatory and inhibitory pathways in the hippocampus continue to develop after birth. The integration of excitatory and inhibitory input signals plays an important role in the excitability of hippocampal CA1 neurons, which determines the output signal level from the hippocampus. Both the fast excitatory postsyanptic postentials (fEPSPs) and fast inhibitory postsynaptic potentials (flPSPs) elicited by the electrical stimulation of inputs to the CA1 neurons temporally overlap in the neuron. So far few studies have examined the ontogeny of the overall synaptic responses in rat hippocampal CA1 neurons during postnatal development A systematical study of the evoked synaptic responses during development is useful for understanding the developmental integration of neural signals on the CA1 pyramidal neurons. The hippocampal CA1 pyramidal neurons also continue to grow morphologically after birth. Morphometric studies using Golgi-Cox techniques have shown that the CA1 pyramidal neurons rapidly maturate during the first postnatal month. However, the advance of intracellular labeling techniques enables us not only to visualize more complete dendritic arbor for qualitative analysis, but also to examine the relation between changes in the dendritic arborization and the evoked fast postsynaptic curents-3-(fPSCs) in the same neurons during the postnatal developmentThe aim of this study was to systematically examine the postnatal changes in the configuration of fPSCs evoked by the focal stimulation of the stratum radiatum of the CA1 region, and the relationship between the dendritic arborization and evoked fPSCs in the rat hippocampal CA1 pyramidal neurons using whole-cell blind patch recording technique combined with biocytin intracellular labeling during the postnatal development (postnatal day 2-70, P2-P70).The focal stimulation of the stratum radiatum in the CA1 region elicited fPSCs in 80% of the neurons P6-7, 90% of P9-10 and 100% of ^Pll. In neurons P6-7, the fPSCs were exclusively inward and had multiple (on average 5.6) peaks. The fPSCs increased in the amplitude with the growth of dendritic arborization but decreased in the number of peaks. Distinct outward fPSC following the inward fPSC emerged in neurons ^Pll and was abolished by bicuculline (50 uM). Bicuculline increased the amplitude and duration of the initial inward fPSC in all age groups and characteristically recruited the polysynaptic second component of fEPSCs in neurons P11-P21. No spontaneous periodic inward current was detected in any age groups after blocking GABAA receptors. The concomitant application of DL-2-amino-5-phosphonopentanoic acid (APS, 100 jiM) with bicuculline did not eliminate the polysynaptic second component, but the second component was only elicited in slices in which the CA3 region was kept intact. Moreover, the bicuculline- and AP5-resistant second component was due to the burst activity of CA3 pyramidal neurons, which were antidromically excited by the stimulation of the Schaffer collaterals. The results on the generation of the polysynaptic second component suggest that the recurrent excitatory inputs are therefore better developed in the CAS region than in the CA1 region of the rats Pll-21.The morphology of the recorded CA1 pyramidal neurons was obtained by biocytin-4-intracellular staining methods. The results demonstrated that the most rapid growth of the dendrites occurred during the second postnatal week and the apical dendrites more rapidly matured in branching and lengthening than the basal dendrites did. Neurons P2 to P5 were diverse in both growth of the apical dendrite and generation of the evoked fPSCs, and the total length and number of branches of apical dendrites were significantly longer or larger in neurons producing fPSCs than in neurons producing no fPSCs. The rise time and half-decay time of the fast excitatory postsynaptic currents (fEPSCs), which were evoked in neurons ^P6 in the presence of bicuculline (50 uM) alone or in combination with APS(lOOuM), became slow with the postnata...