POSTSYNAPTIC POTENTIALS AND SPIKE VARIATIONS IN FELINE HIPPOCAMPUS DURING POSTNATAL ONTOGENESIS

Intracellular recording from hippocampal neurons of kittens 1 to 18-days-old was carried out to examine the postsynaptic potentials (PSPs) evoked by fornix stimulation at early developmental stages in which axodendritic pathways constitute the major input to hippocampal neurons. Fornix stimulation in neonatal kittens (<3 days old) elicits 60-80 msec duration IPSPs which exhibit summation during repetitive stimulation. Spontaneously recurring IPSPs have similar durations. Fornix-evoked EPSPs are not prominent in young kittens but become more effective in eliciting cell discharges in 2 to 3-week-old animals. The IPSP durations similar to those observed in adult animals (200-300 msec.) were found by the end of the first postnatal week. Repetitive discharges in excess of 20/sec were rarely observed in hippocampal neurons of neonatal kittens. Spike potentials (40-60 mv) of neurons in young kittens exhibit relatively long durations compared with spike discharges in hippocampal neurons of 3-week-old animals. Spike potential variations were commonly observed and may consist of partial responses, prominent delayed depolarizations and all-or-none fast prepotentials. It is concluded that fornix-evoked IPSPs of immature hippocampal pyramidal neurons are effected largely by axodendritic synapses. Postnatal alterations in EPSPs and IPSPs are attributable to progressive development of axosmatic synapses as well as further elaboration of axodendritic synapses. © 1968.