Developmental profile and synaptic origin of early network oscillations in the CA1 region of rat neonatal hippocampus

Abstract

1. By applying fura_2-based fluorometric calcium imaging to neonatal rat hippocampal slices we identified a developmentally regulated spontaneous neuronal activity in the CA1 region of the hippocampus. The activity consisted of bursts of intracellular Ca¥ transients recurring synchronously at a slow rate of 0·4-2 min¢ in the entire population of pyramidal neurones and interneurones. 2. These early network oscillations (ENOs) were present during a restricted period of postnatal development. Thus, they were not detected at the day of birth (P0), at P1-P4 they consisted of bursts of large (up to 1·5 ìÒ) Ca¥ transients, gradually transforming into regularly occurring, smaller Ca¥ transients during the subsequent week. Beyond P15-P16 no ENOs were detected. 3. The ENOs were blocked by tetrodotoxin (TTX) and by a reduction in temperature from 33-35°C to 20-22°C. By combining fluorometric imaging with whole-cell current-clamp recordings, we found that each ENO-related Ca¥ transient was associated with a highfrequency (up to 100 Hz) train of action potentials riding on a depolarizing wave. 4. Recordings in the voltage-clamp mode revealed barrages of synaptic currents that were strictly correlated with the ENO-associated Ca¥ transients in neighbouring pyramidal neurones. Perfusing the cells with an intracellular solution that allowed for a discrimination between GABAA and glutamate receptor-mediated currents showed that these barrages of synaptic currents were predominantly of GABAergic origin. 5. The ENOs were totally blocked by the GABAA receptor antagonist bicuculline and they were also substantially reduced by the glutamatergic antagonists d,l-2-amino-5-phosphonovaleric acid (d,l-APV) and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). 6. Synaptic stimulation and application of the GABAA receptor agonist muscimol mimicked the spontaneous Ca¥ transients in pyramidal neurones. The efficacy of muscimol in evoking Ca¥ transients decreased during development in parallel with the gradual disappearance of the ENOs. 7. The developmental decrease in the amplitude of ENO-associated Ca¥ transients occurred in parallel with the transformation of the excitatory synaptic transmission in the hippocampus from the immature GABAergic to the mature glutamatergic form. Thus, at the beginning of the first postnatal week single-shock synaptic stimulation produced Ca¥ transients that were completely blocked by bicuculline. At the end of the second postnatal week the same type of evoked synaptic stimulation produced a Ca¥ transient that was little affected by bicuculline but was abolished by the combined application of d,l-APV and CNQX. 8. These results demonstrate the presence of periodic and spontaneous Ca¥ transients in the majority of pyramidal cells and interneurones of the neonatal CA1 hippocampal network. These ENOs exhibit a highly region-specific developmental profile and may control the activity-dependent wiring of the synaptic connectivity during early postnatal development.

Neuronal Activity Regulates Correlated Network Properties of Spontaneous Calcium Transients in AstrocytesIn Situ

“…We conclude that activation of ionotropic glutamate receptors is not necessary for the generation of spontaneous astrocytic activity in situ, but that this activation is a major regulator of their spatiotemporal coordination into correlated networks. Moreover, most of the blocking agents only slightly altered the pattern of spontaneous [Ca 2ϩ ] i oscillations in neurons (data not shown), whereas APV treatment reduced the number of active neurons by 36.8%, in agreement with previous reports (Ben-Ari et al, 1997, Garaschuk et al, 1998.…”

“…4 E, G). Postnatal hippocampal neurons displayed frequent synchronous events, which corroborated previous studies (Garaschuk et al, 1998, Ben-Ari, 2001). The highly synchronous patterns of hippocampal neuronal network activity were easily observed in raster plots (Fig.…”

“…In our experiments, visual examination of the movies recorded in the neocortex and hippocampus showed synchronous patterns of activity in neuronal populations (Fig. 4), which have been described previously (Ben-Ari et al, 1997; Garaschuk et al, 1998). Interestingly, spatiotemporal correlations of Ca 2ϩ signaling between GFAP/GFP-positive astrocytes were also observed.…”

“…Similarly, in the thalamus in situ, Parri et al (2001) described spontaneous synchronous correlations among up to five neighboring astrocytes. The highly synchronous activation characteristic of postnatal hippocampal neurons (Ben-Ari et al, 1997;Garaschuk et al, 1998;Ben-Ari, 2001) might account for these differences in network complexity. However, we favor the view that it is the combined transgenic/imaging approach used in this study, designed to screen activity in large numbers of astrocytes, that has allowed the detection of these complex correlated networks.…”

Caracterização de conexinas neuronais no desenvolvimento pós-natal do hipocampo de ratos.

“…Os GDPs são propagados através de rede de interações entre neurônios, que abrange virtualmente todo hipocampo (região CA1 e CA3 e hilo) durante a primeira e segunda semana do desenvolvimento pós-natal de roedores (P0-P10). Esse padrão de atividade espontânea apresenta um perfil marcado por importantes mudanças na concentração transiente de cálcio, que ocorre durante os primeiros dias após o nascimento, seguido por um aumento da frequência dos eventos de cálcio com uma gradual diminuição de amplitude (Leinekugel et al, 1997, Strata et al, 1997, Garaschuk et al, 1998 Em roedores, esse padrão de atividade espontânea envolve células piramidais e interneurônios GABAérgicos situados em CA3 e CA1, assim como interneurônios presentes no hilo. Este evento é mais propício de ocorrer em CA3, afetando outras áreas através da comunicação neuronal que envolve a ativação de receptores GABA A e NMDA.…”