Extrinsic projections from area CA1 of the rat hippocampus: olfactory, cortical, subcortical, and bilateral hippocampal formation projections.

Hippocampal area CA1 provides the major cortical output of the hippocampus, but only its projections to the subiculum and lateral septal nucleus are well characterized. The present study reexamines these extrinsic projections by using anterograde and retrograde tracing techniques. Injections of the anterograde tracer Phaseolus vulgaris leucoagglutinin (PHA-L) in the septal one-third of CA1 label axons and terminals in subicular, postsubicular, retrosplenial, perirhinal, and entorhinal cortices, lateral septal nucleus, and diagonal band of Broca. The septal CA1 injections also label terminal fields in contralateral CA1, and in contralateral subicular, postsubicular, perirhinal, and entorhinal cortices. Injections into the splenial one-third of CA1 label axons and terminals in subiculum, postsubiculum, ventral area infraradiata, and lateral septal nucleus, but they do not label axons and terminals on the contralateral side of the brain. Injections in the temporal one-third of CA1 label axons and terminals in subicular, parasubicular, entorhinal, and infraradiata cortices, anterior olfactory nucleus, olfactory bulb, lateral septal nucleus, nucleus accumbens, amygdala, and hypothalamus. The temporal CA1 injections label no axons on the contralateral side of the brain. These data demonstrate that CA1 has more widespread projections than previously appreciated, and they provide the first clear evidence that CA1 projects to the contralateral cortex and to the ipsilateral olfactory bulb, amygdala, and hypothalamus. The results also demonstrate a heterogeneity in the efferent projections originating in different septotemporal levels of CA1.