Elimination of granular cells after intrahippocampal colchicine injection causing disappearance of evoked potentials and deterioration of development of the conditioned avoidance reflex in rats.

The aim of our work was to study the involvement, in the process of learning, of synapses between the perforant path and granular cells of the hippocampal dentate fascia for formation of a conditioned reflex (CR) of active avoidance elaborated in response to electrostimulation of the perforant path. To achieve this we used the property of the neurotoxin colchicine at a certain concentration of eliminating selectively the fascia cells. One month after colchicine injection we observed destruction of granular cells, disappearance of the dentate fascia focally evoked potentials, and considerable deterioration in the elaboration of active avoidance CR. Administration of saline solution in control experiments did not bring about such changes. The results obtained show the important role of the entorhinal input into the hippocampus in the learning model used.

[Switching off of granule cells after intrahippocampal injection of colchicine eliciting the disappearance of evoked potentials and a deterioration in the development of a conditioned avoidance reflex in rats]. / Vykliuchenie granuliarnykh kletok posle vnutrigippokampal'noi in''ektsii kolkhitsina, vyzyvaiushchei ischeznovenie vyzvannykh potentsialov i ukhudshenie vyrabotki uslovnogo refleksa izbeganiia u krys.

The aim of the work was to study the involvement in the process of learning of synapses between the perforant path and granular cells of the hippocampal dentate fascia for formation of conditioned reflex (CR) of active avoidance elaborated in response to electrostimulation of the perforant path. For solving this task, the property was used of the neurotoxin colchicine in a definite concentration to abolish selectively the fascia cells. One month after colchicine injection, destruction of granular cells was observed, disappearance of the dentate fascia focal evoked potentials and considerable deterioration of elaboration of active avoidance CRs. Administration of saline solution in control experiments did not elicit such changes. The obtained results show the decisive role of entorhinal input to the hippocampus in the used learning model.

Monosynaptic activation of the hippocampus as a conditioned stimulus: behavioral effects.

The aim of the present study with rats was to show that electrical stimulation of a monosynaptic pathway, whose high plastic potency is well-known, can serve as a conditioned stimulus (CS) in a learning paradigm. Using chronic rats, stimulation of the perforant pathway, which activates the entorhinal cortex input to the dentate gyrus of the hippocampus, was used as the CS in a footshock motivated two-way avoidance task (shuttle box). Among the different stimulation parameters tested, only trains of at least 15 Hz were shown to be effective as a CS, whereas the application of single impulses with a frequency of 1,7 Hz did not result in the establishment of conditioned behavior. Using the 15 Hz-paradigm, in "good learners," the development of reliable conditioned responses started at the end of the first training session (40 trials) and become fairly stable during a further training session, given on the following day. The application of stimulation trains with 100 Hz as a CS led to the development of a high rate of conditioned responses, however, there was also a high level of intertrial reactions. Considering the critical importance of the frequency parameters of the perforant pathway stimulation for the development of conditioned behavior the possible involvement of hippocampal long-term potentiation in behavioral plasticity is discussed. Additionally, it is concluded that this new learning paradigm offers the advantage of concurrent analysis of plastic processes both at the behavioral level and at the level of the synaptic population.