Tilmann Ott

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.

Low frequency perforant path stimulation as a conditioned stimulus demonstrates correlations between long-term synaptic potentiation and learning

Stimulation of the perforant path with impulse trains of 15 cps and 670 msec duration was used as a conditioned stimulus in a two-way shuttle box avoidance on rats. Field potentials in the dentate area evoked by test stimuli were measured after the training sessions until the 7th day. Foot-shock and unconditioned escape elicited only a transient slight depression of the population spike amplitude (P) and increased also slightly the slope function (SF) of the population EPSP of the evoked test potentials. The control stimulation of the perforant path without pairing with foot-shock as in conditioning did only slightly increase SF of test potentials, but produced a strong transient inhibition followed by a long lasting moderate depression of P. After conditioning, all animals exhibited the same initial inhibition of P as shown in control stimulation of the perforant path. However during the following 4 hours, good learners with a relearning index greater than 30% developed a significant potentiation of P lasting until the second training session 24 hours later, which resulted in a further enhancement. SF of the evoked test potentials increased in good learners with a similar time course after conditioning but without initial depression. After 7 days P showed still enhanced but non-significant values. Poor learners with a relearning index less than 10% did not develop a potentiation of P after conditioning and initial inhibition, but a long-term depression. Also SF of test potentials decreased in poor learners during 4 hours after conditioning and returned almost to baseline until the following day. After 7 days, P and SF did not differ from baseline. The analysis of the observed synaptic changes by E-S curves demonstrated the post-tetanic LTP seems to differ in some ways from post-conditioning LTP in good learners. The latter exhibits a clear tendency of a right shift contrary to the left shift commonly occurring after tetanization. Furthermore poor learners do not only fail to produce long-term potentiation, but fail to show a change in the opposite direction with a left shift of the E-S curves. The observed correlation of LTP in the conditioning pathway with the learning ability suggests an involvement of LTP at least in the acquisition and early retention of this learned behavior. The results do however not finally clarify the role of LTP in long-term retention.

Some effects of RNA precursors on development and maintenance of long-term memory: Hippocampal and cortical pre- and post-training application of RNA precursors

The effect of topically applied RNA precursors on both acquisition and extinction of brightness discrimination was studied. Hippocampal injection of uridine monophosphate (UMP) 30 min prior to onset of training caused a delay in extinction of the learned reaction. The same effect was observed when UMP was injected 1 min after completion of training, whereas hippocampal application of UMP 60 min after completion of training, and injection of UMP into the frontal cortex 30 min before onset of training did not show any influence on the extinction. However, the acquisition capability was not influenced by any of the types of treatment used.From the present findings and previous results, the authors have come to the conclusion that the injection of pyrimidine nucleotides during the training and within the first hour after completion of training would have a positive influence on consolidation, thus improving the long-term storage. The exogenous supply of pyrimidine nucleotides seems to stimulate quantitative and/or qualitative modifications in RNA synthesis, this having effects on subsequent protein synthesis.Furthermore, it is concluded from the findings that the hippocampus is of special importance in the consolidation process.

The effect of intrahippocampally applied anisomycin on the retention of brightness discrimination in rats.

Intrahippocampal application of the protein synthesis inhibitor anisomycin impaired the retention performance of a brightness discrimination task in rats. A proactive effect of anisomycin could be excluded. The impairment of retention was not due to disturbing effects on locomotor activity. The same dosage of anisomycin caused a reduction in leucine incorporation into hippocampal proteins to approximately 6–9%. The present findings suggest that consolidation processes acting during and for a few hours after training may be impaired by inhibition of protein synthesis in the hippocampus.

Influence of post-training intrahippocampally applied oxotremorine on the consolidation of a brightness discrimination.

The posttraining intrahippocampal injection of oxotremorine revealed an improvement of the retention performance in a brightness discrimination task. The oxotremorine effect seemed to be dependent on distinct variables of training and was restricted to rats exhibiting a good acquisition performance. Scopolamine impaired the retention performance of animals with few training errors. The role of hippocampal cholinergic synapses for consolidation was discussed.

Die Wirkung von Orotsäure auf die durch elektrokonvulsiven Schock ausgelöste retrograde Amnesie

The influence of the RNA precursor orotic acid on the retrograde amnesia caused by ECS was investigated in rats using an optical discrimination test. ECS, given two hours or less after training, led to significant retrograde amnesia in control animals if tested 24 hours later. The same effect was found in animals treated with a single dose of 100 mg/kg orotic acid i.p., given 10 min before training.Contrary to these findings, no amnesia could be produced in rats pretreated with a daily dose of 100 mg/kg orotic acid i.p. over four days. The prolongation of extinction, typical after orotic acid treatment, was also seen in these animals.An explanation of these findings may be that repeated administration of orotic acid leads to an improved consolidation of memory, possibly via some changes in the central nucleotide and/or RNA metabolism.ZusammenfassungAn einer Methode der optischen Diskriminierung wurde die Wirkung einer ein-, bzw. viermaligen Injektion des RNS-Präkursors Orotsäure auf die durch ECS erzeugte retrograde Amnesie untersucht. Bei Kontrolltieren führt der 2 Std nach Trainingsende gegebene ECS zu einer im Retentionstest nach 24 Std nachweisbaren signifikanten retrograden Amnesie. Bei Ratten, die 4 Tage lang je 100 mg/kg Orotsäure erhalten hatten, läßt sich keine retrograde Amnesie auslösen. Auch die extinktionsverzögernde Wirkung der Orotsäure bleibt unter diesen Bedingungen erhalten. Die einmalige Injektion von Orotsäure beeinflußt die Wirkung des ECS nicht. Aus den Befunden wird geschlossen, daß die viermalige Injektion von Orotsäure zu quantitativen und/oder qualitativen Veränderungen des Nucleotid-Pools bzw. der RNS des ZNS führt, wodurch die Konsolidierung des Langzeitgedächtnisses begünstigt wird.

Induction of self-stimulation behavior derived from a brain region lacking in dopaminergic innervation

Self-stimulation behavior has been studied in rats with stimulating electrodes implanted into the posterior part of the medial entorhinal cortex. With low or moderate stimulus intensities self-stimulation rates up to 1000 per hour have been observed, whereas higher stimulus intensities were less effective. Additionally, it was shown that the monosynaptic activation of the hippocampus by the self-stimulation train seems to be unrelated to the behavioral effects, since with different stimulus intensities the self-stimulation rate varied, whereas the “population spike” of the hippocampal-evoked response was suppressed in any case. Since in the medial entorhinal cortex a dopaminergic innervation has not been found so far, it is concluded that at least this type of self-stimulation behavior seems to be independent of a dopaminergic transmission.

Der Einfluß von 6-Azauridin auf die Begünstigung des Wiedererlernens durch Ribonucleinsäure-Präkursoren

Intraventricular injection of orotic acid (OA) or uridine-5-monophosphate (UMP) 30 min before the training of a conditioned avoidance reaction in rats facilitates the relearning after a retention of 24 h, while the inhibitor of orotidine-5-phosphate decarboxylase, 6-aza-uridine alone is without effect. Combined application of 6-aza-uridine and OA or UMP shows as abolition of the OA effect, but no influence on the UMP-effect. The results are compatible with the assumption that OA does not directly facilitate retention, but only after a transformation into pyrimidine nucleotides which act as precursors in brain RNA synthesis.ZusammenfassungDie intraventrikuläre Injektion von Orotsäure (OS) oder Uridin-5-monophosphat (UMP) 30 min vor dem Training einer bedingten Fluchtreaktion erleichtert das Wiedererlernen im Retentionstest nach 24 Std, während 6-Azauridin allein ohne Wirkung ist. Bei kombinierter Applikation von 6-Azauridin mit OS oder UMP wird die Wirkung von OS weitgehend aufgehoben, während der Effekt von UMP nicht beeinflußt wird. Die Befunde unterstützen die Annahme, daß OS nicht direkt zur Retentionsbegünstigung führt, sondern nach Umwandlung in Pyrimidin-Nucleotide, die als Präkursoren für die RNS-Synthese im Gehirn dienen.

Effects of orotic acid on the development of morphine tolerance.

A marked tolerance to both analgesic and cataleptic effects of 20 mg/kg morphine hydrochloride was developed in rats during treatment with daily doses increasing from 20 to 120mg/kg s.c. within 11 days. Daily administration of 100 mg/kg sodium orotate i. p. 14 days before and during treatment with morphine significantly accelerated the development of tolerance for cataleptic as well as analgesic effects of morphine. The disappearance of the tolerance phenomena was delayed in the orotate-treated animals. No influence of orotate, however, could be observed on the formation of stereotyped behaviour and disappearance of stereotype.The results support the assumption, that morphine tolerance may be an adaptive process in the CNS realized by RNA and protein syntheses in different neuronal cell populations. Treatment with the RNA precursor may improve this process, perhaps by enhancing RNA synthesis.

Suppression of uridine monophosphate-induced improvement in long-term storage by cycloheximide

In previous papers, we have shown tha t t rea tment of rats with erotic acid or other precursors of RNA favors the retention of acquired reactions (Ott and Matthies, 1971, 1972b; Ott, LSssner, and Matthies, 1972a) and have come to the conclusion tha t the long-term storage of learned reactions is facilitated by RNA precursors. Recently we have shown tha t the retention-improving effects of erotic acid depend on its conversion to UMP (Oft and Matthies, 1972b). Direct evidence tha t this effect also requires increased incorporation of UMP into the RNA would be desirable but cannot be furnished by behavior experiments, since inhibitors of transcription such as actinomycin D produce severe toxic side effects (Barondes, 1968). According to our hypothesis, the increased supply of RNA precursors during the formation of long-term memory involves a subsequent quanti tat ive and/or qualitative modification of protein synthesis via an increase in RNA synthesis. Therefore, we assumed tha t the effect of the RNA precursor, UMP, on retention might be abolished by means of a sufficient inhibition of protein synthesis. In order to s tudy this question, we used cycloheximide a t a dose sufficient to cause an 850/0 inhibition of protein synthesis in the brain. The experiments were performed on male