Ps. Buckmaster et Pa. Schwartzkroin, INTERNEURONS AND INHIBITION IN THE DENTATE GYRUS OF THE RAT IN-VIVO, The Journal of neuroscience, 15(1), 1995, pp. 774-789
Inhibitory cells are critically involved in shaping normal hippocampal
function and are thought to be important elements in the development
of hippocampal pathologies. However, there is relatively little inform
ation about the extent and pattern of axonal arborization of hippocamp
al interneurons and, therefore, about the sphere of influence of these
cells. What we do know about these cells is based largely on in vitro
slice studies, in which interneuronal interactions may be severely at
tenuated. The present study was carried out to provide a more realisti
c picture of interneuron influence. Intracellular recordings were obta
ined from dentate interneurons in the intact brain of anesthetized rat
s, and cells were intracellularly labeled with biocytin. The axonal ar
bors of two classes of dentate interneurons were traced through the hi
ppocampus; each was found to extend long distances (up to half of the
total septotemporal length of the hippocampus) perpendicular to the hi
ppocampal lamellae and to target preferential strata. These results su
ggest that dentate interneurons have far-reaching effects on target ce
lls in distant hippocampal lamellae. One implication of this finding i
s that dentate neurons should receive more inhibitory synaptic drive i
n vivo than in slice preparations, in which many inhibitory axon colla
terals are amputated. Synaptic responses to perforant path stimulation
were examined in granule cells, messy cells, and CA3 pyramidal cells
in vivo, for comparison with previously published results from hippoca
mpal slice studies. In vivo, all cell types showed excitatory synaptic
responses that were brief and limited by robust IPSPs that were large
r in amplitude and conductance than responses to comparable stimuli re
corded in vitro. This difference could not be explained by a change in
the intrinsic physiological properties of the cells in the slice prep
aration, because those parameters were similar in vivo and in vitro. W
e conclude that dentate gyrus inhibitory interneurons can affect the e
xcitability of neurons in distant areas of the hippocampus, and that t
hese distant influences cannot be appreciated in conventional in vitro
preparations.