Pentylenetetrazol kindling changes the ability to induce potentiation phenomena in the hippocampal CA1 region.

The present study describes changes of response enhancement of hippocampal field potentials in slices of kindled rats using different methods to induce long-lasting potentiation. Eight-week-old male Wistar rats were subjected to pentylenetetrazol (PTZ) kindling induced by intraperitoneal injection of 45 mg/kg once every 48 h until the occurrence of seizure stages 4-5. Eight to 12 days after the last kindling session, transverse hippocampus slices were prepared and maintained in an artificial medium. Evoked-field potentials were recorded in the CA1 region upon stimulation of the Schaffer collaterals. Potentiation was induced: 1. By moderate tetanic stimulation of the Schaffer collaterals, 2. by changing the perfusion medium to 0-magnesium for 30 min, and 3. by changing the medium to 4 mM Ca2+ for 7 min. In slices from kindled rats, long-term potentiation (LTP) after tetanic stimulation and increase of the evoked potential by 0-magnesium were significantly enhanced in comparison to slices from sham-kindled rats. However, Ca(2+)-induced LTP could not be induced in slices from kindled rats. The results support the assumption that PTZ kindling also induces lasting changes in the responsiveness of hippocampal structures, expressed as an enhanced ability to induce potentiation. An alteration of N-methyl-D-aspartate (NMDA) receptor-coupled processes can be assumed. The inability to induce Ca(2+)-induced LTP points to more complex effects of PTZ, perhaps also on nonNMDA coupled ionic channels.

N-methyl-D-aspartate receptor activation is required for the induction of both early and late phases of long-term potentiation in rat hippocampal slices.

The possible involvement of N-methyl-D-aspartate (NMDA) receptors in mechanisms enabling the maintenance of long-term potentiation (LTP) was investigated in rat hippocampal slices. The action of the specific NMDA receptor antagonists (-)-2-amino-7-phosphonoheptanoic acid (D-APH) and 2-amino-5-phosphonovaleric acid (DL-APV) as well as of the inactive isomer L-APH was tested on orthodromic population excitatory postsynaptic potential (EPSP) and population spike (PS) responses recorded extracellularly from CA1 pyramidal cells. If the active D-isomer of APH (10 microM) or DL-APV (50 microM), but not if L-APH was present during tetanization, both EPSP and spike potentiation were markedly reduced or even blocked for the whole recording period (8 h after tetanization). It is concluded that the NMDA receptor component expressed during tetanization is a necessary step not only for initiation but also for subsequent mechanisms enabling late phases of synaptic LTP. Some remaining potentiation of the population spike may be related to a second, NMDA-independent mechanism.

Alterations in calmodulin content in fractions of rat hippocampal slices during tetanic- and calcium-induced long-term potentiation.

The content of cytosolic and membrane-bound calmodulin was radioimmunologically determined in fractions of rat hippocampal slices 5 min to 7 hours after long-term potentiation (LTP) had been induced by tetanization or exposure of slices to 4 mM Ca++. In light of concepts presuming multistage dynamics in LTP development as reflecting different cellular mechanisms, similar patterns of calmodulin alterations were observed with both models: The alterations in calmodulin content occurred during the early phase(s) of LTP development and continued for two and one hours during tetanic- and calcium-induced LTP, respectively. Thus, 5-30 min after LTP elicitation, membrane-bound calmodulin increased while cytosolic calmodulin diminished and, inversely, 30 min later an increase in cytosolic and decrease in membrane-bound calmodulin were observed. Consequently, the present results indicate that calmodulin was involved in the early phases(s) of LTP development in terms of a two-step translocation sequence. Hence, calmodulin translocation within both intracellular compartments may reflect the involvement of Ca++-calmodulin-dependent intraneuronal metabolic processes which might induce and/or temporarily maintain neuronal functional changes occurring immediately after repeated or intense stimulation of synaptic functions.

Phorbol ester-induced hippocampal long-term potentiation is counteracted by inhibitors of protein kinase C.

As was shown previously (Reymann et al. 1988), the protein kinase C (PKC)-inhibitor polymyxin B prevents the maintenance of electrically induced long-term potentiation (LTP) of synaptic transmission to CA1 neurons, indicating that posttranslational phosphorylation processes mediated by PKC are involved in mechanisms underlying this form of synaptic plasticity. To make sure that 1.) the polymyxin B actually acts against PKC activation and 2.) the long-lasting potentiation elicited by phorbol esters (Malenka et al. 1986) is mediated by PKC-activation, we have tested polymyxin B as well as the potent PKC-inhibitor K-252b during phorbol ester-induced LTP. 4-beta-phorbol-12,13-dibutyrate (PDBu) - a known activator of protein kinase C, induces a remarkable potentiation at concentrations as low as 0.5 microM. When 20 microM polymyxin B or 40 nM K-252b was administered to rat hippocampal slices prior to such a weak phorbol ester treatment, this potentiation did not develop with the exception of a small increase in the population spike in spite of polymyxin B-treatment (42% instead of 120% increase at 2 h after PDBu). In contrast, spike potentiation induced by high concentrations of PDBu (10 microM) could not be counteracted by 100 microM polymyxin B. It is concluded that at low concentrations the phorbol ester-induced potentiation is mainly mediated by a selective activation of protein kinase C and that the prevented maintenance of electrically induced LTP by polymyxin B is in fact due to inhibition of this kinase. The spike potentiation developed faster than that of the EPSP raising the possibility that PDBu activates two separate PKC-dependent processes.

The duration of long-term potentiation in the CA1 region of the hippocampal slice preparation.

The duration of long-term potentiation (LTP) of the monosynaptic excitatory Schaffer collateral-commissural input to hippocampal neurons of the CA1 region was examined in the in vitro slice. Relatively stable evoked potentials were obtained under conventional perfusion conditions at least for 10 hours. Tetanic stimulation (100 Hz, 1 sec) increased the population spike (pop-spike) amplitude by about 150% and the slope of the field-EPSP by about 30% over the pre-LTP baseline, whereas the latency and peak latency of the pop-spike decreased. In comparison to control experiments (same number of stimuli at 0.2 Hz) the differences were statistically significant for 2 hr (field-EPSP) and for greater than or equal to 10 hr (pop-spike), respectively. Repeated tetanization (3 X 100 Hz/1 sec), however, substantially prolongs EPSP-LTP (greater than or equal to 10 hr) and doubles the approximated half-life of pop-spike LTP. The threshold current intensity to elicit pop-spike responses decreased after the induction of LTP. Furthermore, the smaller field-EPSP values necessary to evoke near-threshold pop-spikes demonstrate an E-S potentiation (left-shift) at least in the low-intensity range. While the total duration of potentiation of the different parameters has not been determined, all the above mentioned effects could be observed at least 10 hr following the repeated tetanization. It is proposed that the slice preparation is suitable for the investigation of mechanisms of a postulated late phase of LTP if appropriate conditions are used.

Effects of beta-casomorphin on dentate hippocampal field potentials in freely moving rats.

Intracerebroventricular administration of 166 nmoles of the exogenic opioid beta-casomorphin (5) produced a potent and reversible depression of the compound action potential evoked in dentate granule cells by stimulation of the medial perforant path, whereas the extracellularly recorded excitatory postsynaptic potential is left unchanged. This in vivo effect of beta-casomorphin was obviously different from those observed previously in the CA 1 region in hippocampal slice experiments. The results suggest that more than one opioid mechanism determines the granule cell excitability. Some of the possible mechanisms involved in the effects of beta-casomorphins in the hippocampus are briefly discussed.

Effects of orotic acid and pirazetam on cortical bioelectrical activity in rabbits.

In order to obtain further evidence of the pharmacological effects of retention-facilitating substances, the influence of sodium-orotate, methylglucamine orotate, and pirazetam on the bioelectrical activity of central nervous system structures were studied. Sodium orotate showed no influence on the cortical and hippocampal spontaneous EEGs of conscious nonimmobilized rabbits. The amplitude of the first surface-positive wave of the cortical evoked potential that was elicited by stimulation of tooth pulp was, however, significantly increased in long-lasting terms by both pirazetam and the two salts of orotic acid. Similarly, the three substances also influenced the changes in excitability induced by PTP of tooth pulp. The findings suggest that these chemicals facilitate in nonspecific manner responses to externally applied stimuli through central structures. The relationships between the altered stimulus response and the retention-facilitating action of these substances are discussed.