Ayahiko Nishigori

Long-term potentiation and N-methyl-D-aspartate receptors in the visual cortex of young rats.

1. Long‐term potentiation (LTP) of synaptic transmission following tetanic stimulation of the white matter was studied by recording extracellular field potentials and intracellular synaptic potentials from layer II/III of visual cortical slices from young rats ranging in age from 21 to 40 days. 2. Single shocks applied to the white matter at 0.1 Hz, used as test stimuli, elicited field potentials that consisted of primary and secondary components. The removal of Ca2+ ions from the perfusate allowed identification of the secondary component as originating postsynaptically and the primary one as reflecting a mixture of antidromic and postsynaptic potentials. 3. Tetanic stimulation at 5 Hz for 60 s was delivered to the white matter and field potentials were observed for 20 min to 9 h after the tetanus. LTP was defined as being present when the response displayed more than a 20% increase in amplitude of the Ca2+‐sensitive components 20 min after the tetanus. LTP was induced in twelve of twenty‐three slices tested, and this potentiation lasted throughout the period of observation. The average magnitude of potentiation was 147.8 +/‐ 28.4% of the control value for the twelve slices. 4. Administration of D,L‐2‐amino‐5‐phosphonovalerate (APV), an antagonist selective for N‐methyl‐D‐aspartate (NMDA)‐preferring receptors, slightly reduced the amplitudes of Ca2+‐sensitive components of the field potentials. The average magnitude of reduction was 80.2 +/‐ 15.3% of the pre‐drug control values. In the presence of APV, LTP was induced in only one slice of twelve tested. 5. Stable intracellular recordings were obtained from twenty‐three cells from layer II/III. Excitatory postsynaptic potentials (EPSPs) evoked by white matter stimulation had mean onset and peak latencies of 4.1 and 11.3 ms, respectively. In some cells these fast EPSPs were followed by another slow EPSP with a much longer latency and higher amplitude. Administration of APV revealed further that the fast EPSPs consisted of two components, i.e. early and late components. 6. Tetanization of the white matter induced long‐lasting enhancement of EPSPs in eight of twelve cells tested. In five of these eight cells, fast EPSPs were enhanced in amplitude and in the remaining three cells, slow EPSPs appeared de novo after the tetanus. 7. APV reduced the amplitudes of the fast EPSPs and abolished the slow EPSPs if present. The average magnitude of reduction for the fast EPSPs was 65.6 +/‐ 15.1% and this reduction was due mainly to an elimination of the late component.(ABSTRACT TRUNCATED AT 400 WORDS)

Long-term depression but not potentiation is induced in Ca(2+)-chelated visual cortex neurons.

An entry of Ca2+ into postsynaptic sites may play a role in the induction of long-term potentiation (LTP) of synaptic transmission in the visual cortex. To test this hypothesis, a Ca(2+)-chelator was injected into layer II/III neurons of sliced visual cortex obtained from young rats, and excitatory postsynaptic potentials (EPSPs) of these cells to test stimulation of the white matter were observed before and after tetanic stimulation of the same site. To confirm the effectiveness of the tetanus, field potentials reflecting the activities of many cells were recorded with another extracellular electrode. The chelator injection led to long-term depression (LTD) of EPSPs following tetanic stimuli which simultaneously induced LTP of field potentials derived from unchelated cells in most of the slices tested. This suggests that a low concentration of post-synaptic, free Ca2+, when associated with tetanic inputs, may lead to LTD while a rise of Ca2+ may lead to LTP.

Actions of excitatory amino acid antagonists on synaptic potentials of layer II/III neurons of the cat's visual cortex

SummaryActions of excitatory amino acid (EAA) antagonists on the responses of cells in layers II/III and IV of the cats visual cortex to stimulation of layer VI and the underlying white matter were studied in slice preparations. Antagonists used were 2-amino-5-phosphonovalerate (APV), a selective antagonist for the N-methyl-D-aspartate (NMDA) type of EAA receptors, and kynurenate, a broadspectrum antagonist for the three types of EAA receptors. In extracellular recordings it was demonstrated that most of the layer II/III cells were sensitive to APV, while the great majority of the layer IV cells were not, By contrast, kynurenate suppressed the responses completely in both layers. Excitatory post-synaptic potentials (EPSPs) evoked by stimulation of layer VI and the while matter were recorded intracellularly from layer II/III neurons. To determine whether the EPSPs were elicited mono- or polysynaptically, the synaptic delay for each EPSP was calculated from a pair of onset latencies of EPSPs evoked by stimulation of the two sites. Forty-two percent of the layer II/III cells were classified as having monosynaptic EPSPs. In 60% of these monosynaptic cells, the rising slope of the EPSPs was reduced by APV while in the other 40%, it was not. In the former (APV-sensitive cells), subtraction of the APV-sensitive component from the total EPSP indicated that the onset latency of the NMDA receptor-mediated component was roughly equal to that of the non-NMDA component. In the latter (APV-resistant cells), only the slowly-decaying component was in part mediated by NMDA receptors. The conduction velocities of the afferent fibers innervating APV-resistant cells were slower than those of the APV-sensitive cells, suggesting that both types of cells are innervated by different types of afferents. The polysynaptic EPSPs of almost all layer II/III cells were sensitive to APV. The subtraction method indicated that the NMDA component had about the same magnitude as the non-NMDA components. When the slices were superfused by a Mg2+-free solution, the EPSPs were potentiated dramatically, but this potentiation was reduced to the control level during the administration of APV. Similarly, APV-sensitive components were potentiated during the administration of bicuculline, a selective antagonist for gamma-aminobutyric acid receptors of A type. These results suggest that NMDA receptors participate, at varying degrees, in excitatory synaptic transmission at most layer II/III cells in the cats visual cortex, and their actions appear to be regulated by intracortical inhibition.

Contribution of quisqualate/ kainate and NMDA receptors to excitatory synaptic transmission in the rat's visual cortex

Action of antagonists for excitatory amino-acid (EAA) receptors on extracellularly and intracellularly recorded responses of layer II/III cels to electrical stimulation of the underlying white matter were studied in a slice preparation of rats visual cortex. Antagonists used were 2-amino-5-phosphonovalerate (APV) and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), which are selective antagonists for EAA receptors of N-methyl-D-aspartate (NMDA) and quisqualate/kainate (non-NMDA) type, respectively. In extracellular recordings, it was found that responses of almost all of the cells were suppressed by CNQX. In contrast, sensitivity to APV was different between cells with short-and long-latency responses; 81% of the former responses were not suppressed by APV, while about a half of the latter were suppressed. Excitatory postsynaptic potentials (EPSPs) evoked by white-matter stimulation were recorded intracellularly from 42 neurons. Most of polysynaptically elicited EPSPs were sensitive to AVP, whereas the majority of monosynaptic EPSPs, were not. CNQX almost completely suppressed EPSPs irrespective of monosynaptically or polysynaptically evoked, but in some cases slow EPSPs with low amplitude were spared. These CNQX-resistant EPSPs were elicited polysynaptically and had an anomalous voltage dependence, a characteristic of NMDA receptors. It is suggested that non-NMDA receptors contribute dominantly to first-order synaptic transmission while NMDA receptors participate substantially in second-order transmission so as to serve as a booster of outputs from visual cortex.

Input-specific induction of long-term depression in Ca2+-chelated visual cortex neurons

An input-dependent increase in postsynaptic Ca2+ may play a role in long-term potentiation (LTP) of synaptic transmission while no or subthreshold increase in Ca2+ is associated with long-term depression (LTD) in the developing visual cortex. To see whether LTD is induced only at tetanized synapses, a Ca(2+)-chelator was injected into layer 2/3 neurons in cortical slices from young rats, and excitatory postsynaptic potentials (EPSPs) of these cells, after test stimulation of the white matter and layer 1/2, were observed before and after tetanic stimulation of the former site. The chelator injection led to LTD of EPSPs at tetanized synapses, but no changes were seen at non-tetanized synapses. These results suggest that tetanic inputs induce LTD at tetanized synapses when they are associated with no or subtle increase in postsynaptic Ca2+.

Long-term depression is induced in Ca2+/calmodulin kinase-inhibited visual cortex neurons

To elucidate a role of Ca2+/calmodulin-dependent protein kinase II (CaMKII) in induction of long-term potentiation (LTP), KN-62, a selective inhibitor for CaMKII, was injected into layer 2/3 neurons of sliced visual cortex obtained from young rats. Tetanic stimulation (5 Hz, 1 min) applied to the white matter after the KN-62 injection induced long-term depression (LTD) of excitatory postsynaptic potentials (EPSPs) evoked by test stimulation of the white matter in 9 of the 14 cells tested. However, EPSPs evoked by test stimulation of the non-tetanized site were not changed, indicating that the induction of LTD was input-specific. Simultaneously, recorded field potentials which were derived from neurons with intact CaMKII showed LTP. These results suggest that postsynaptic CaMKII plays a role in the induction of LTP/LTD in visual cortex.

A Role of NMDA Receptors and Ca2+ Influx in Synaptic Plasticity in the Developing Visual Cortex

Acidic amino acids, such as glutamate (Glu) and aspartate (Asp), are suggested to be excitatory transmitters in the visual cortex (Clark and Collins, 1976; Baughman and Gilbert, 1981; Tsumoto et al. 1986). Recent studies have demonstrated that synaptic receptors for Glu/Asp can be classified into at least three types, on the basis of their most sensitive agonists, i.e., 1) N-methylD-aspartate (NMDA)-preferring receptors, 2) kainate-preferring receptors and 3) quisqualatepreferring receptors (see Watkins and Evans, 1981 for review). In CAl area of the hippocampus, NMDA receptors have been shown to play a role in a form of synaptic plasticity, i.e., long-term potentiation (LTP) of synaptic efficacy probably by controlling entry of Ca2+ into postsynaptic sites (Collingridge et al., 1983; see Fagg et al., 1986; Bliss and Lynch, 1988; Cotman and Monagham, 1988 for review). In the cat’s visual cortex, Tsumoto and his associates (Tsumoto et al., 1987, 1988; Hagihara et al., 1988) have reported evidence suggesting that receptors of kainate/quisqualate types (non-NMDA receptors) are involved in geniculo- cortical synaptic transmission while NMDA receptors may play a role in synaptic plasticity in the developing visual cortex.

Gene expression of GDNF and NGF receptors in sensory neurons, with reference to change of the expression after peripheral nerve damage

Colocalization of GDNF and NGF receptor genes and change of the gene expression following peripheral axotomy were investigated in rat dorsal root ganglion (DRG) neurons using in sifu hybridization. About 60% and 35% of the lumber DRG neurons expressed mRNAs for c-ret and trkA, proto-oncogenes which encode functional receptors for GDNF and NGF, respectively. Of the DRG neurons, however, only 9% was positive to both genes. Marked enhancement of the gene expression for GDNF receptor-o (GDNFR-o) was observed in DRG neurons after sciatic nerve transection, but expression of c-ret mRNA was hardly changed. The trkA-expressing neurons were decreased in number. These findings suggest that GDNF and NGF act on distinct subgroups in intact DRG neurons, and that receptor genes for these factors are di’ferentially controlled by peripheral axotomy.

1319 BNDF is produced in NGF-responsive sensory neurons of rats

FENGYI LIANG’, EDWARD G. JONES2 Epilepsy is characterized by long-term changes in neuronal excitability. We studied activity-dependent alterations in gene expression for nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3) in focal epilepsy induced by tetanus toxin injected in rat motor cortex. In the first 2-3 days after seizure onset, in situ hybridization histochemistry showed widespread increases in BDNF mRNA levels throughout the injected cortex. At later times, focal upregulation of BDNF mRNA was seen at the injection site (layers II-VI) and in second somatosensory area (layers II-III). A zone of cortex surrounding the focus of BDNF upregulation at the injection site showed decreases in BDNF mRNA. NGF and NT-3 mRNAs were not altered. The present findings indicate that changes in BDNF mRNA levels are seizure-dependent. The hyperactive epileptic focus at the injection site is surrounded by a zone of reduced neuronal activity, probably due to enhanced GABAergic inhibition.