Postnatal changes in the overall postsynaptic currents evoked in CA1 pyramidal neurons of the rat hippocampus

Developmental changes in structural and functional properties of hippocampal AMPARs parallels the emergence of deliberative spatial navigation in juvenile rats

The neural mechanisms that support the late postnatal development of spatial navigation are currently unknown. We investigated this in rats and found that an increase in the duration of AMPAR-mediated synaptic responses in the hippocampus was related to the emergence of spatial navigation. More specifically, spontaneous alternation rate, a behavioral indicator of hippocampal integrity, increased at the end of the third postnatal week in association with increases in AMPAR response duration at SC-CA1 synapses and synaptically driven postsynaptic discharge of CA1 pyramidal neurons. Pharmacological prolongation of glutamatergic synaptic transmission in juveniles increased the spontaneous alternation rate and CA1 postsynaptic discharge and reduced the threshold for the induction of activity-dependent synaptic plasticity at SC-CA1 synapses. A decrease in GluA1 and increases in GluA3 subunit and transmembrane AMPAR regulatory protein (TARP) expression at the end of the third postnatal week provide a molecular explanation for the increase in AMPAR response duration and reduced efficacy of AMPAR modulators with increasing age. A shift in the composition of AMPARs and increased association with AMPAR protein complex accessory proteins at the end of the third postnatal week likely "turns on" the hippocampus by increasing AMPAR response duration and postsynaptic excitability and reducing the threshold for activity-dependent synaptic potentiation.

Late postnatal maturation of excitatory synaptic transmission permits adult-like expression of hippocampal-dependent behaviors

Sensorimotor systems in altricial animals mature incrementally during early postnatal development, with complex cognitive abilities developing late. Of prominence are cognitive processes that depend on an intact hippocampus, such as contextual-configural learning, allocentric and idiocentric navigation, and certain forms of trace conditioning. The mechanisms that regulate the delayed maturation of the hippocampus are not well understood. However, there is support for the idea that these behaviors come "on line" with the final maturation of excitatory synaptic transmission. First, by providing a timeline for the first behavioral expression of various forms of learning and memory, this study illustrates the late maturation of hippocampal-dependent cognitive abilities. Then, functional development of the hippocampus is reviewed to establish the temporal relationship between maturation of excitatory synaptic transmission and the behavioral evidence of adult-like hippocampal processing. These data suggest that, in rats, mechanisms necessary for the expression of adult-like synaptic plasticity become available at around 2 postnatal weeks of age. However, presynaptic plasticity mechanisms, likely necessary for refinement of the hippocampal network, predominate and impede information processing until the third postnatal week.

S-Methylcysteine may be a Causal Factor in Monohalomethane Neurotoxicity

S-methylcysteine (SMC) is formed after exposure to monohalomethanes in rodents as well as in humans. The present study was performed to study whether SMC, directly or indirectly, contributes to the well-known neurotoxicity of monohalomethanes. We have investigated the effects of acute exposure to SMC by means of electrophysiolocal measurements in freshly prepared hippocampal slices and dissociated hippocampal neurons in culture. For longer-term exposures (24 h) we have used organotypic cultures (2 weeks in culture), taking electrophysiologic recordings and assessing membrane integrity with propidium iodide (PI) fluorescence. We found that only high concentrations of SMC (10(-2) M; exposure time 30 min) in freshly isolated slices of adult rats reduce synaptically evoked population spikes in the CA1 region. This effect was at least partially reversible. In organotypic cultures, at 5 x 10(-5) M after 24 h of exposure, SMC compromises membrane integrity as revealed by PI fluorescence, only in the dentate gyrus, spreading to pyramidal cell layers at 50 x 10(-4) M. At 5 x 10(-6) and 2 x 10(-5) M, under the same experimental conditions, no changes were seen with the PI method, but we recorded increased population spike amplitudes, repetitive discharges and frequency potentiation (at a stimulus repetition rate of 0.05 Hz). Using whole-cell patch clamp in hippocampal dissociated neurons we have found that SMC (applied for approximately 1s) reduces GABA-induced currents ( IC(50) = 4.4 x 10(-4) M) without having an effect of its own, acting like a competitive antagonist at GABA(A) receptors. Our findings are in line with the view that the ability of monohalomethanes to induce the formation of SMC is an important factor for their neurotoxicity, provided that SMC is allowed to act at least for several hours. The effects exerted by SMC seem to be due, at least in part, to its interaction with GABA receptors.