Plasticity of excitatory synaptic transmission in kitten visual cortex depends on voltage-dependent Ca2+ channels but not on NMDA receptors

Spatiotemporal properties of the action potential propagation in the mouse visual cortical slice analyzed by calcium imaging

The calcium ion (Ca(2+)) is an important messenger for signal transduction, and the intracellular Ca(2+) concentration ([Ca(2+)](i)) changes in response to an excitation of the cell. To reveal the spatiotemporal properties of the propagation of an excitatory signal with action potentials in the primary visual cortical circuit, we conducted a Ca(2+) imaging study on slices of the mouse visual cortex. Electrical stimulation of layer 4 evoked [Ca(2+)](i) transients around the stimulus electrode. Subsequently, the high [Ca(2+)](i) region mainly propagated perpendicular to the cortical layer (vertical propagation), with horizontal propagation being restricted. When the excitatory synaptic transmission was blocked, only weak and concentric [Ca(2+)](i) transients were observed. When the action potential was blocked, the [Ca(2+)](i) transients disappeared almost completely. These results suggested that the action potential contributed to the induction of the [Ca(2+)](i) transients, and that excitatory synaptic connections were involved in the propagation of the high [Ca(2+)](i) region in the primary visual cortical circuit. To elucidate the involvement of inhibitory synaptic connections in signal propagation in the primary visual cortex, the GABA(A) receptor inhibitor bicuculline was applied. In this case, the evoked signal propagated from layer 4 to the entire field of view, and the prolonged [Ca(2+)](i) transients were observed compared with the control condition. Our results suggest that excitatory neurons are widely connected to each other over the entire primary visual cortex with recurrent synapses, and inhibitory neurons play a fundamental role in the organization of functional sub-networks by restricting the propagation of excitation signals.

Columnar distribution of serotonin-dependent plasticity within kitten striate cortex

Recent studies have identified the potential for an important role for serotonin (5-HT) receptors in the developmental plasticity of the kitten visual cortex. 5-HT(2C) receptors are transiently expressed in a patchy fashion in the visual cortex of kittens between 30-80 days of age complementary to patches demarcated by cytochrome oxidase staining. 5-HT, operating via 5-HT(2C) receptors, increases cortical synaptic plasticity as assessed both in brain slices and in vivo. Herein, we report that bath application of 5-HT substantially increases the probability of long-term potentiation within 5-HT(2C) receptor-rich zones of cortex, but this effect is not observed in the 5-HT(2C) receptor-poor zones. Instead, in these zones, 5-HT application increases the probability of long-term depression. These location-specific effects of 5-HT may promote the formation of compartment-specific cortical responses.

Layer-specific NO dependence of long-term potentiation and biased NO release in layer V in the rat auditory cortex

1. We investigated the role of nitric oxide (NO) in the induction of long-term potentiation (LTP) in slices prepared from the rat auditory cortex. 2. Tetanic stimulation of layer IV elicited LTP of field potentials in layer II-III (LTPII-III) and in layer V (LTPV). The magnitude of LTPII-III measured at 30 min after tetanic stimulation was 171 +/- 9% (n = 15, mean +/- s.e.m.) of the control measured before tetanic stimulation, while that of LTPV was 138 +/- 3% (n = 17). 3. NO synthase (NOS) inhibitors had no apparent effect on LTPII-III, but LTPV was significantly suppressed (P < 0.001). This suppression of LTPV was significantly antagonized by a NO donor (P < 0.001) or a cGMP analogue (P < 0.001). 4. Small non-pyramidal neurones in the auditory cortex were stained with an anti-neuronal NOS antibody. More neurones were stained with the antibody in the deeper cortical layers. 5. We measured neocortical NO release with electrochemical NO probes. Layer IV stimulation elicited significantly more NO release in layer V than in layer II-III (P < 0.001). The amplitude of the increase in NO concentration elicited by stimulation at 20 Hz for 5 s was 380 +/- 14 pM (n = 55) in layer V and 55 +/- 8 pM (n = 5) in layer II-III. 6. NO release in layer V was partially but significantly suppressed by non-NMDA (P < 0.002) or NMDA (P < 0.002) receptor antagonists. Simultaneous application of the antagonists of the two types blocked NO release almost completely. 7. These results clearly indicate the NO dependence of the induction of LTPV, and the greater NO release in the deeper layer of the rat auditory cortex.

Aging differentially alters forms of long-term potentiation in rat hippocampal area CA1

Long-term potentiation (LTP) of the Schaffer collateral/commissural inputs to CA1 in the hippocampus was shown to consist of N-methyl-D-aspartate receptor (NMDAR) and voltage-dependent calcium channel (VDCC) dependent forms. In this study, the relative contributions of these two forms of LTP in in vitro hippocampal slices from young (2 mo) and old (24 mo) Fischer 344 rats were examined. Excitatory postsynaptic potentials (EPSP) were recorded extracellularly from stratum radiatum before and after a tetanic stimulus consisting of four 200-Hz, 0.5-s trains given 5 s apart. Under control conditions, a compound LTP consisting of both forms was induced and was similar, in both time course and magnitude, in young and old animals. NMDAR-dependent LTP (nmdaLTP), isolated by the application of 10 microM nifedipine (a voltage-dependent calcium channel blocker), was significantly reduced in magnitude in aged animals. The VDCC dependent form (vdccLTP), isolated by the application of 50 microM D,L-2-amino-5-phosphonvalerate (APV), was significantly larger in aged animals. Although both LTP forms reached stable values 40-60 min posttetanus in young animals, in aged animals vdccLTP increased and nmdaLTP decreased during this time. In both young and old animals, the sum of the two isolated LTP forms approximated the magnitude of the compound LTP, and application of APV and nifedipine or genestein (a tyrosine kinase inhibitor) together blocked potentiation. These results suggest that aging causes a shift in synaptic plasticity from NMDAR-dependent mechanisms to VDCC-dependent mechanisms. The data are consistent with previous findings of increased L-type calcium current and decreased NMDAR number in aged CA1 cells and may help explain age-related deficits in learning and memory.

Serotonin facilitates synaptic plasticity in kitten visual cortex: an in vitro study

We have addressed the role of serotonin-2C (5-HT2C) receptors in the development and maintenance of synaptic plasticity in the kitten visual cortex. In visual cortical slices, taken from 40- to 80-day-old kittens, bath application of serotonin markedly facilitated the induction of both long-term depression (LTD) and long-term potentiation (LTP). Field potential responses to white matter stimulation were recorded from layer IV after a regime of low frequency stimulation (LFS; 1 Hz, 15 min), which reliably induced LTP or LTD in younger kittens (less than 30 days of age). At 40-80 days, this protocol almost never induced LTD or LTP in layer IV. However, in 50% of the visual cortical slices studied in 40-80-day-old kittens, LTD or LTP was induced, if serotonin (1 or 10 microM) was co-applied with LFS. No such serotonin facilitation of long-term plasticity was ever detected in > 120-day-old animals, indicating that serotonin facilitates synaptic plasticity within a defined period of visual cortical development. Serotonergic 5-HT2C receptors are likely to contribute to the synaptic plasticity observed in layer IV, since mesulergine, an antagonist of the 5-HT2C receptor, completely blocked synaptic modifications induced by the combination of low frequency stimulation and serotonin application.