Journal of Neuroscience, Vol 14, 4079-4094, Copyright © 1994 by Society for Neuroscience
Morphological evidence that hypothalamic substance P-containing afferents are capable of filtering the signal flow in the monkey hippocampal formation
C Leranth and R Nitsch
Department of Obstetrics and Gynecology, Yale University, School of Medicine, New Haven, Connecticut 06520.
This study in the African green monkey (Cercopithecus aethiops) was
designed to characterize the neurochemical features of hippocampal
nonpyramidal neurons that are specific synaptic targets of substance P-
containing projective neurons located in the supramammillary nucleus. Our
previous studies provided evidence for an excitatory nature to this
hypothalamo-hippocampal pathway and described the mode of termination of
these afferents on hippocampal principal neurons. The present correlated
light and electron microscopic immunocytochemical analysis, using the
nickel-diaminobenzidine/diaminobenzidine double-labeling technique,
revealed that this hippocampal afferent system establishes multiple,
exclusively asymmetric synapses with three specific subpopulations of
nonpyramidal cells: (1) a small portion of parvalbumin-containing basket
cells located periodically in or adjacent to the granule cell layer of the
dentate gyrus, which therefore inhibit only a subpopulation of granule
cells; (2) some of the calbindin- immunoreactive local circuit neurons
located in the hilar area; and (3) calbindin-positive cells occurring
exclusively in the stratum molecular of the middle portion of the CA3
subfield. Postembedding studies revealed that the aforementioned
calbindin-containing cells are GABAergic inhibitory neurons. Our studies
indicate that hypothalamic afferents can effectively filter the information
flow at different levels of the excitatory signal loop in the monkey
hippocampal formation. Dentate granule cells, which are only stimulated by
hypothalamic afferents, will transfer excitatory signals differently than
those that are controlled by a feedforward inhibitory mechanism initiated
by these fibers. In the CA3 subfield, the signal flow can again be
depressed by those pyramidal neurons that are inhibited by
calbindin-containing cells receiving an excitatory hypothalamic input.