The behavioral effects of bilaterial lesions in the hippocampal formation are not clear-cut. Such lesions have not hindered the formation of "conditioned emotional responses" in the rat (Brady & Hunt, 1955) nor altered the acquisition rate of avoidance responses in the cat (Brady, Schreiner, Geller, & Kling, 1954). Recently, Kimura (1958) reported that lesions in posterior hippocampus produce adverse effects in the learning of a rather unusual avoidance response. Lesions of similar size in anterior hippocampus did not have any noticeable effect. However, Kimura's lesions were relatively small, involving less than half the structure.Allen (1940) reported the effects of nearly complete lesions of the hippocampus of the dog as it learned conditioned olfactory responses. Operated dogs were not inferior to normal animals in acquisition of the response, and in fact there was some suggestion they might even be somewhat superior to unoperated animals. To investigate the effect of near total removal of the hippocampus on the acquisition of an avoidance response, three groups of rats were prepared. In one group the hippocampus was removed under visual exposure, in another cortical lesions were made in the same area damaged during ablation of hippocampus in the first group, and the third group was made up of normal animals. METHOD SubjectsTwenty-eight male rats of the Long-Evans strain serves as 5s. Nine animals had bilateral ablation of the hippocampus, 9 had cortical lesions, and 10 served as 1 The research was supported in part by USPHS Grant M-3202. A preliminary report of this research was presented at the 1960 meetings of the American Psychological Association. unoperated controls. All 5s were between 3 months and 5 months old at time of surgery.

The distribution of vasoactive intestinal polypeptide-like (VIP) immunoreactivity in neurons and processes within the suprachiasmatic nuclei (SCN) of the rat was investigated with light and electron microscopic immunocytochemical techniques. These studies utilized well characterized antisera directed to synthetic vasoactive intestinal polypeptide. Specificity was established by absorption of the antisera with synthetic vasoactive intestinal polypeptide. Neurons and dendrites exhibiting specific VIP immunoreactivity are concentrated in the ventral half of the nucleus, with the greatest concentration of immunoreactive perikarya occurring in the ventral SCN immediately adjacent to and within the optic chiasm (OC). Thin varicose axons containing VIP immunoreactivity are present throughout the SCN. A large number of immunoreactive axons leave the dorsal aspect of the SCN to reach the periventricular hypothalamic nucleus and continue dorsally to form an extensive plexus along the ventral border of the paraventricular hypothalamic nucleus. Other immunoreactive axons project upon the contralateral SCN via the OC. Ultrastructurally, VIP-containing neurons in the SCN are characterized by a spherical to slightly elongated soma and an invaginated nucleus that fills the majority of the cell body. In the soma, peroxidase reaction product is localized on the outer membrane of all cellular organelles. The reaction product of immunoreactive boutons is related primarily to vesicles, and some of these boutons establish axodendritic synaptic contacts in the SCN. The demonstration of VIP-containing neurons in the SCN provides further evidence that this nucleus is composed of a heterogeneous population of neurons which form distinct subfields within it.

Synaptogenesis was studied in the rat suprachiasmatic nucleus (SCN) using quantitative ultrastructural analysis and synapsin I immunohistochemistry. SCN neurons are formed between embryonic days 13 and 17 (E13-E17), and the SCN is a distinct hypothalamic nucleus by E18. At E19 the nucleus is very immature and there are few synapses. Synaptogenesis proceeds slowly until P4 but increases rapidly between P4 and P10. At P10 the number of synapses per unit area is the same as in the adult SCN and all synaptic types present in the adult are evident. However, the SCN continues to increase in volume to the adult with approximately 30% of total synaptic number added between P10 and a young adult age. The appearance of synapsin I immunoreactivity correlates very precisely with the development of synapses in the SCN as shown by ultrastructural analysis between E19 and P6. The pattern of appearance of synapsin I immunoreactivity demonstrates that synaptogenesis in the SCN is significantly delayed in comparison to adjacent hypothalamic nuclei. Synapsin I immunohistochemistry is a reliable marker of synapse formation in the developing SCN. A correlation of these anatomical data with prior functional studies suggests that SCN neurons are born as individual circadian oscillators that undergo a rapid development in the first 10 days after birth to form a functional neural network subserving circadian rhythm generation and regulation.

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