Effects of medial septal stimulation & its blockade with atropine on hippocampal rhythmical slow activity in free-moving rabbits.

The influence of the medial septal nucleus (MSN) on hippocampal rhythmical slow activity (RSA) was investigated in hunger motivated free-moving rabbits. Single unit activity of 29 CA3 dorsal hippocampal neurones was studied under two paradigms viz., electrical stimulation (3v/1msec/5Hz) and chemical blockade of atropine-sensitive medial septal cells (dose of 5 micrograms in 5 microliters). The analysis of interspike interval sample histograms recorded with Apple II, along with single unit neuronal spikes recorded on polygraph showed bimodal pattern with two peaks; one at bin 10-20 (high frequency) and second at bin 100-300 (theta frequency). On stimulation of MSN, there was total suppression of bin 100-300 with peaks at bin 1-20 and at bin greater than 1000 (very low frequency). Blockade of cholinergic receptors at MSN showed reduction of bin 100-300 and peak at bin 700- greater than 1000; this was followed by gradual decrease of spike's amplitude and thereafter its total abolition. The results indicate that the septal region is not a pacemaker per se in the genesis of hippocampal RSA (theta rhythm), but serves more in the nature of a relay station for a number of inputs and that its influence on the hippocampal electrical activity depends on the integrity of cholinergic inputs, be it from hippocampus proper or from mid-brain reticular formation.

Significance of CA3 region of hippocampus in memory function.

In mobile unanaesthetised adult rabbits, spontaneous single unit activities of CA3 neurons of Hippocampus showed a specific change in firing pattern in response to conditioned stimuli (CS+). This pattern could be correlated with changes in the general behaviour of these animals. Major deafferntation of the CA3 region did not alter the conditioned response, though neuronal firing pattern and behavioural response changed to that of the pre-conditioned state after antidromic stimulation of CA3 region through fimbria. The importance of CA3 region in memory retention process has been indicated in these experiments.