Diverse Neuronal Populations Mediate Local Circuit Excitation in Area CA3
of Developing Hippocampus.
Smith, Karen L., Donald H. Szarowski, James N. Turner, and John W. Swann.
The Cain Foundation Laboratories, Department of Pediatrics, and Division of
Neuroscience, Baylor College of Medicine, One Baylor Plaza, Houston, TX 77030,
Wadsworth Center for Laboratories & Research, New York State Department of
Health, Department of Biomedical Sciences, School of Public Health, The
University of Albany, P.O. Box 509, Empire State Plaza, Albany, NY 12201-
0509.
APStracts 2:0097N, 1995.
SUMMARY AND CONCLUSIONS
1. Studies were undertaken to better understand why the developing hippocampus
has a marked capacity to generate prolonged synchronized discharges when
exposed to GABAA receptor antagonists. 2. Excitatory synaptic interactions
were studied in small microdissected segments of the hippocampal area CA3.
Slices were obtained from 10- to 16-day-old rats. Application of the GABAA
receptor antagonist, penicillin, produced prolonged synchronized discharges in
minislices that were very similar, if not identical, to those recorded in
intact slices. The size of minislices was systematically varied. Greater than
90% of those that measured more than 600 [mu]m along the cell body layer
produced prolonged synchronized discharges, while most minislices measuring
300 [mu]m produced only brief interictal spikes. 3. Action potentials in the
majority (75%: 158/254) of cells impaled with microelectrodes were able to
entrain the entire CA3 population. They were also able to increase (on average
26%) the frequency of spontaneous population discharges. The population
discharges were followed by a refractory period that lasted 5 to 60 sec,
during which single cells were unable to initiate a population discharge. 4.
The majority (87%) of neurons with intrinsic burst properties were found to
entrain the CA3 population. The electrophysiologic characteristics of these
cells were reminiscent of recordings obtained from more mature rats. Action
potentials were quite prolonged and demonstrated a secondary shoulder or hump
on the down-slope of the spike. 5. When bursting cells were filled with
lucifer yellow and imaged during recording sessions by videomicroscopy and
later using confocal microscopy, they showed the anatomical features of CA3
hippocampal pyramidal cells. Confocal microscopy permitted detailed
characterization of individual neurons and showed substantial variation in
cellular microanatomy. 6. Another class of cells found to entrain the CA3
population but did not demonstrate intrinsic bursts were termed regular firing
cells. These cells possessed many of the anatomical and physiological features
of bursting cells with the exception of burst firing. They were rarely
encountered in intracellular recordings. 7. The third physiological class of
cells were termed fast spiking cells. These had action potentials that were
shorter in duration than the other two cell types. They were distinct in the
rapid rate of spike repolarization. They demonstrated modest degrees of spike
frequency adaptation and fired repeatedly and at relatively high frequencies.
Compared to reports on fast spiking cells in mature hippocampus and neocortex,
action potentials appear to be slower and repetitive discharging of a lower
frequency. 8. Anatomically, fast spiking cells were classified as
"simple" pyramidal cells or multipolar stellate cells. They had
anatomical features of hippocampal interneurons, yet both cell types were able
to entrain the CA3 population. 9. Paired intracellular recordings showed 25%
of the cells that entrained the CA3 population produced epsps in nearby cells.
Indeed in many instances the onset of synchronized discharging was preceded by
epsps recorded in the second cell of a pair. 10. Of bursting cells, 25%
produced epsps in nearby cells. Polysynaptic epsps, produced by 2-4 action
potentials, were large (2.0 +/- 0.3 mV), with time to onset of 11.1 +/- 3.5
msec and time to peak amplitude of 30.1 +/- 9.9 msec. Their probability of
occurrence was surprisingly high, on average 0.82. 11. Dye-coupling was
observed in 65% of cases following intracellular injection of lucifer yellow.
As many as six neurons were recovered after a single injection. Dye-coupling
was evident not only in confocal images but during recording sessions where
cells were viewed with fluorescent videomicroscopy. In this regard, during
dual recordings, four percent of cell pairs (6/149) were found to be
electrotonically-coupled. 12. 73% of fast spiking cells studied entrained the
CA3 population. 13% produced epsps in nearby pyramidal cells. Compared to
pyramidal cells, more presynaptic action potentials were required of fast
spiking cells to either entrain the CA3 population or generate a polysynaptic
epsp. Time to epsp onset was longer (40-70 msec) than that recorded from
bursting cells. 13. Results suggest that the recurrent excitatory networks in
area CA3 are very well developed early in postnatal life. In addition to the
pyramidal cell to pyramidal cell connections a novel form of local circuit
excitation may exist that is mediated by local circuit interneurons. Local
circuit excitation likely contributes importantly to hippocampal
hyperexcitability during this critical period in development.
Received 21 June 1994; accepted in final form 31 March 1995.
APS Manuscript Number J366-4.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 25 April 1995.