Hippocampal GABAergic interneurons are responsible for controlling the output and
efficacy of synaptic input of large principal cell populations and, thereby, determine
the oscillatory discharge patterns and synaptic plasticity in the hippocampus. Single
interneurons are able to prevent repetitive firing of postsynaptic pyramidal cells
(R. Miles, K. Toth, A.I. Gulyas, N. Hajos, and T.F. Freund. Neuron, 16: 815-823, 1996),
whereas on occasion a single pyramidal cell may be able to activate an interneuron
under in vitro conditions (A.I. Gulyas, R. Miles, A. Sik, K. Toth, N. Tamamaki, and
T.F. Freund. Nature (London), 366: 683-687, 1993). Inhibition is therefore extremely
powerful. Transient suppression of interneuronal activity allows the precise timing
and synchronization of inhibitory postsynaptic potentials arriving at principal cells.
A rhythmic suppression or modulation of interneuron discharge may be brought about
by input from at least two major sources: (i) from other local interneurons or (ii)
from subcortical centers. Of the possible local sources, in the present review particular
attention will be paid to GABAergic neurons specialized to innervate other interneurons.
Subcortical pathways known to modulate specific inhibitory functions in the hippocampus,
i.e., the GABAergic and cholinergic septohippocampal and the serotonergic raphe hippocampal
pathways, will also be reviewed. Roles of these control mechanisms may include the
generation of theta and higher frequency oscillations, and the selective removal of
inhibition from the termination zone of specific excitatory afferents, thereby increasing
their efficacy and (or) plasticity.
