Neuroscience Vol. 28, No. 2, pp. 387-392, 1989 Printed in Great Britain 0306-4522/89 $3.00 + 0.00 Pergamon Press plc 0 1989 IBRO LONG-TERM POTENTIATION IN THE HIPPOCAMPUS OF THE ANAESTHETIZED RAT IS NOT ASSOCIATED WITH A SUSTAINED ENHANCED RELEASE OF ENDOGENOUS EXCITATORY AMINO ACIDS L. ANIKSZTEJN,* M. P. ROISIN, R. AMSELLEM and Y. BEN-ARI INSERM U29, Hbpital de Port-Royal, 123 Bld de Port-Royal, 75014 Paris, France Abstract-The relationship between long-term potentiation of synaptic transmission and the release of endogenous glutamate and aspartate has been investigated in the CA1 region of the hippocampus and in the fascia dentata of the anaesthetized rat. A high-frequency train of electrical stimulation of afferent pathways produced a long lasting (r2 h) enhancement of the field excitatory postsynaptic potential in CA1 and of the population spike in the fascia dentata. In both regions, this was not associated with a significant long lasting increase in the release of glutamate and aspartate. It is concluded that the maintenance of long-term potentiation is not associated with a sustained increase in the release of excitatory amino acids. High-frequency trains of electrical stimulation pro- duce a long lasting enhancement of synaptic trans- mission.7 This long-term potentiation (LTP), which has been extensively studied in the hippocampus,M persists for weeks or months in the intact animal.6s8*‘s LTP is considered to be a physiological substrate of learning and memory processesz3**’ and as such has attracted considerable interest in its underlying mech- anisms. The contribution of pre- and postsynaptic pro- cesses to LTP has not been clarified. Using a push-pull technique, Bliss and coworkers5*16 have provided evidence in favour of a presynaptic mech- anism in the fascia dentata, i.e. a sustained enhance- ment of the release of glutamate and aspartate which are thought to mediate synaptic transmission in the perforant pathway. 14,29 However, to observe this en- hancement, Bliss et ~1.~ have compared one group of animals receiving control stimulation (0.033 Hz) to another group which received high-frequency stimu- lation; in fact there was no statistical difference in the release of excitatory amino acids in the same group of animals before and after the train (e.g. Fig. 3).5 In contrast to the above, we found that the long-lasting potentiation induced by a phorbol ester26 or by the mast cell degranulated peptide” is not associated with a sustained enhancement of glutamate and aspartate release’ (see also Aniksztejn et al., unpublished obser- vations). Although the underlying mechanisms of the potentiation produced by a train of electrical stimu- lation and by these agents may be different (see for *To whom correspondence should be addressed. Abbreviations: EPSP, excitatory postsynaptic potential; HPLC, high performance liquid chromatography; LTP, long term potentiation; NMDA, N-methyl-d-aspartate; OPA, O-phthaldialdehyde. example Ref. 17), these observations suggest that the postsynaptic neurons are important in the mainte- nance of the LTP. This is in keeping with several other studies.3~‘9~22~25*27 In the present report, we have reexamined the relationship between electrically induced LTP and the release of endogenous glutamate and aspartate, using each animal as its own control. This work has been presented in brief elsewhere.’ EXPERIMENTAL PROCEDURES Twenty-six adult Wistar rats weighing 20&3OOg were used. They were anaesthetized with urethane (2g/kg) and placed in a stereotaxic frame. A push-pull cannula was introduced in the stratum radiatum of CA1 (A = 4; L = 2.8) or in the dentate gyrus (A = 4; L = 2.3) of one hippo- campus. The position of the cannula was adjusted so as to record the typical field potential evoked by the electrical stimulation of the commissural pathway or the perforant pathway respectively. A monopolar electrode (50 pm o.d.) was used for recording; its length was the same as the inner cannula and it was attached to the outer cannula (0.6 mm o.d.). A twisted bipolar electrode was implanted, either in the hippocampal commissure (A = 6.6, L = 1, H = 7) to stimulate the commissural pathway, or in the entorhinal cortex (A = 0, L = 4.2, H = 6.4) to stimulate the medial perforant pathway. Test stimuli (0.05 ms) were ap- plied at a frequency of 0.033 Hz. Once the push-pull cannula was accurately placed, an oxygenated artificial cerebrospinal fluid was perfused, with the following com- position (in mM): 126 NaCl; 3.5 KCl; 2 CaCl,; 1.3 MgCl,; 1.2 NaH,PO,; 25 NaHCO,; 11 glucose, pH 7.4. The flow rate was lOpl/min. The l&fusion was-continued for a stabilization period of 1 h. At the end of this oeriod. samules were collected at 5-min intervals during 1 h (contrdl per&d) and a high-frequency train (100 Hz for 1 s at the same intensity as during the control simulation) was applied. Samples were collected at 5-min intervals for an additional period of 2 h. The content of endogenous glutamate and aspartate was determined by high-performance liquid chro- matography (HPLC). 387