BILATERAL ORGANIZATION OF PARALLEL AND SERIAL PATHWAYS IN THE DENTATE GYRUS
DEMONSTRATED BY CURRENT SOURCE DENSITY ANALYSIS IN THE RAT.
Golarai, Golijeh, Thomas P. Sutula.
The Neuroscience Training Program, and the Departments of Neurology,
Anatomy, University of Wisconsin, Madison, WI, 53792, USA.
APStracts 2:0253N, 1995.
SUMMARY AND CONCLUSIONS
1- Membrane currents evoked in the dentate gyrus (DG) by stimulation of
afferent pathways from the entorhinal cortex (EC) and contralateral DG were
examined by current-source density (CSD) analysis in urethane anesthetized
rats. Stimulation of each afferent pathway evoked membrane currents with
distinct spatial and temporal organization in the DG. CSD and anatomical
analysis revealed that afferent input from the EC activated the DG bilaterally
through parallel and serial pathways. The analysis provided a detailed
description of the location, timing, and relative amplitude of evoked
monosynaptic and multisynaptic currents in the DG. 2- Orthodromic stimulation
of the perforant path (PP) evoked a large ipsilateral excitatory postsynaptic
current (iEPSC) at a latency of 2.5-4 ms in the middle and outer stratum
moleculare (STM) of the DG, and a population spike that was generated by an
excitatory inward current at a latency of 5-9 ms in the stratum granulosum
(STG). 3- A variable, low amplitude ipsilateral inward current followed the
population spike at a latency of 13-16 ms in the inner STM, which is the site
of synaptic terminals of the associational and commissural pathways arising
from neurons in the hilus of the DG. Orthodromic stimulation of the
commissural pathway from the hilus of the contralateral DG also evoked an
inward current in the inner STM at a latency of 2-6 ms, which was 40-60% of
the amplitude of the monosynaptic iEPSC. 4- Orthodromic stimulation of the EC
evoked a low amplitude contralateral excitatory postsynaptic current (cEPSC)
at a latency of 3-6 ms in the outer and middle STM of the contralateral DG,
that was generated by monosynaptic transmission in the sparse crossed pathway
from the EC. In contrast to the variable, low amplitude inward current in the
inner STM that followed the iEPSC, the cEPSC was consistently followed by a
large inward current at a latency of 8-14 ms in the inner STM of the
contralateral DG. 5- The large inward current in the inner STM of the
contralateral DG was abolished by transection of the PP ipsilateral and
rostral to the stimulating electrode, but was unaffected by transection of the
PP at a similar location in the hemisphere contralateral to the stimulating
electrode. These observations suggested that this current was most likely
generated by a multisynaptic pathway involving the ipsilateral PP and the
commissural pathway arising from the hilus of the DG. 6- In conclusion,
efferents from the EC activated the DG bilaterally by parallel and serial
pathways. Parallel monosynaptic pathways from the EC activated homologous
distal segments of granule cell dendrites bilaterally, by a massive input to
the ipsilateral DG and a sparse input to the contralateral DG. Sequential
propagation from the EC through a multisynaptic pathway that included the
commissural pathway from the hilus of the DG bilaterally activated the
proximal segments of granule cell dendrites at longer latencies. The amplitude
of the multisynaptic currents generated by serial pathways in the proximal
dendrites varied inversely with the amplitudes of the monosynaptic currents
evoked by parallel pathways in the distal dendrites. These observations may
have implications for synaptic integration, associative properties, and
information processing in the DG.
Received 3 February 1995; accepted in final form 7 August 1995.
APS Manuscript Number J71-5.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 24 August 1995.