The distribution of septal projections to the hippocampus of the rat

Limbic system interrelations: functional division among hippocampal-septal connections

Neuronal activity was recorded simultaneously from hippocampus and medical or lateral septum during classical conditioning of the rabbit nictitating membrane response. Although similarities exist between hippocampal and lateral septal patterns of activity, medial septal unit discharges indicate a different role during learning.

The role of the septal nuclei in the release of acetyl-choline from the rabbit cerebral cortex and dorsal hippocampus and the effect of atropine

Acetylcholine (ACh) was collected from the alvear surface of the dorsal hippocampus and cerebral cortex in chloralose-urethane anaesthetized or unanaesthetized rabbits. With anaesthesia, the resting release of ACh from the hippocampus was greater than that from the cortex. Wthout anaesthesia, the resting release from both areas was much higher and very similar. The addition of atropine sulphate (1 microgram/ml) to the collecting fluid or the administration of Artane (2 mg/kg i.v.) increased resting ACh release from both the hippocampus and cortex to similar output levels. Atropine also increased ACh release due to stimulation of the medial septum (MS) or mesencephalic reticular formation (MRF). Removal of the septum abolished the effect of atropine on resting ACh release and on release evoked by MRF stimulation from both the hippocampus and cortex. The data indicate that the septum is an essential pathway for cholinergic fibres ascending to the cerebral cortex and hippocampus. They also demonstrate that the septal cholinergic fibres must be intact and active for atropine to increase ACh release from their terminals.

Histochemical evidence of altered development of cholinergic fibers in the rat dentate gyrus following lesions. II. Effects of partial entorhinal and simultaneous multiple lesions

It has been concluded previously that the septohippocampal fibers which project to the rat dentate gyrus extend or branch in the denervated area of the molecular layer following a complete ipsilateral entorhinal lesion. The septohippocampal fibers thus appear to replace some of the perforant fibers which degenerate as a result of the lesion. The reactive fibers eventually become localized to a much smaller and more superficial area after lesions of immature rats than after lesions made in adulthood. To determine whether this difference in the response results from a selective reaction to loss of the lateral perforant path in the immature rat, various portions of the entorhinal cortex were removed at the age of 11 days, and the cholinergic septohippocampal fibers were visualized by acetylcholinesterase histochemistry. An alternative possibility, that the difference between immature and adult rats is attributable to an interaction with other reactive afferents, was tested by removing other sources of input (the contralateral entorhinal cortex, contralateral hippocampal formation or both) along with the ipsilateral entorhinal cortex at the age of 11 days and then demonstrating the septohippocampal fibers histochemically. Lesions of the lateral part of the ipsilateral entorhinal cortex (source of the lateral perforant path) at 11 days of age evoked a septohippocampal reaction along the outer edge of the molecular layer, where the lateral perforant path fibers normally terminate. This result matched that produced by a complete entorhinal lesion. Lesions of the medial entorhinal cortex evoked no obvious reaction. In contrast, the septohippocampal fibers in adult rats proliferated in the denervated area of the molecular layer after lesions of either part of the entorhinal cortex. Combining lesions of other sources of innervation to the dentate gyrus with an ipsilateral entorhinal lesion at 11 days of age did not alter the response of septohippocampal fibers, as determined histochemically. Neither did the septohippocampal fibers react to removal of commissural afferents alone. The response at any age was unaffected by prior or subsequent removal of the contralateral entorhinal cortex. These results indicate that in immature rats the septohippocampal fibers respond only to loss of the lateral perforant path, but these same fibers can later react to loss of any part of the perforant path. They are regarded as support for the hypothesis that the reactive septohippocampal fibers preferentially interact with dendritic growth cones. Our results do not support explanations based on a hypothetical attraction between septohippocampal and crossed perforant path fibers (which react in the same area) or on competition with commissural fibers (which reinnervate an adjacent area). We suggest further that proximity to the degenerating elements does not in itself determine the pattern of reinnervation after lesions of the central nervous system.

Histochemical evidence of altered development of cholinergic fibers in the rat dentate gyrus following lesions. I. Time course after complete unilateral entorhinal lesion at various ages

The entorhinal cortex of rats was removed at various times during development, and the reaction of the cholinergic septohippocampal input to the dentate gyrus was examined by use of acetylcholinesterase histochemistry. When the ipsilateral entorhinal cortex is completely removed, the outer 70-75% of the molecular layer of the dentate gyrus is almost completely denervated. After such a lesion at 5 to 33 days of age, the acetylcholinesterase staining initially intensified throughout the denervated area, indicating that the septohippocampal fibers branched or elongated. This reaction could be detected within one day after a lesion at 11 days of age and within three or five days after lesions at earlier or later times. Whereas the initial response of the septohippocampal fibers was independent of the age at which the lesion was made, their final localization depended on the developmental state of the animal. After lesions at the age of 5 or 11 days, the reactive septohippocampal fibers became restricted to the outer one-sixth to one-third of the molecular layer within two days after appearance of their initial reaction. A similar concentration of reactive fibers was demonstrable after lesions at 16, 18 or 21 days of age, but some reaction persisted in the middle third of the molecular layer. Finally, after lesions at 26 or 33 days of age the proliferating cholinergic fibers ultimately were uniformly distributed throughout the outer 60% of the molecular layer. These results suggest that septohippocampal fibers initially extend or sprout throughout the denervated area to replace the lost perforant path fibers. However, the reactive fiber population becomes restricted to the outer edge of the molecular layer if the entorhinal lesion is made before the period of cholinergic synaptogenesis and concentrates in this same zone if it is made while cholinergic synapses are forming. We suggest that either the proliferative reaction continues in the outer part of the molecular layer and subsides in other parts of the denervated area or septohippocampal fibers move outward through the molecular layer to assume a more superficial location. After entorhinal lesions at 16 days of age or later the pale-staining zone (containing fibers that originate in hippocampus regio inferior) immediately deep to the denervated area widened. If the lesion was made earlier, this zone never developed at most septotemporal levels of the dentate gyrus. These results are probably related to the extension of regio inferior fibers into the denervated area.

Development of afferent lamination in the fascia dentata of the rat

In the present study we examine the development of afferent lamination in the fascia dentata of the postnatal rat, as a first step in determining possible mechanisms controlling synaptic specificity in this system. This analysis is based on degeneration-induced argyrophilia as well as autoradiographic labeling of the entorhinal and commissural/associational afferents. Both methods show that in spite of the immaturity of the neonatal fascia dentata, these afferent systems have already established territorial relationships by 4 days of age which persist into adulthood. At 4 days, the entorhinal projection is restricted approximately to the outer 45 mum of the 80 mum wide molecular layer. The commissural/associational projection occupies appoximately the inner 35 mum of the molecular layer. At older ages the commissural/associational zone increases in width very slowly relative to the entorhinal zone. We also discuss these results in relation to potential mechanisms of afferent development and dendritic differentiation.

Efferent connections of the septal area in the rat: an analysis utilizing retrograde and anterograde transport methods

Experiments were performed by either retrograde (horseradish peroxidase histochemistry) or anterograde ([3H]leucine radioautography) transport methods to determine the efferent connections of the septum in the rat. It was observed that the dorsal septum projects to the lateral preoptic area, lateral hypothalamus, periventricular hypothalamus and midline thalamus. Fibers from the ventral half of the septum project topographically to the hippocampal formation, thalamus, hypothalamus and midbrain. Specifically, neurons located along the midline in the vertical limb of the diagonal band project through the dorsal fornix to all CA fields of the dorsal hippo campus and adjacent subicular cortex. Other fibers from this region project through the stria medullaris to the medial habenular nucleus and anteromedial nuclhe pars posterior of the medial mammillary nucleus. Cells located immediately lateral to the midline in the vertical limb of the diagonal band project through the medial part of the fimbria to all CA fields of the posterior hippocampus and adjacent subicular cortex. Other fibers which originate from this region project through the stria medullaris to both the medial and lateral habenular nuclei and the paratenial nucleus of the thalamus, and through the medial forebrain bundle to the pars posterior of the medial mammillary nucleus. Cells located in the intermediolateral septum project through the lateral part of the fimbria to all CA fields of the ventral hippocampus and adjacent subicular and entorhinal cortices. These cells also send fibers through the stria medullaris to the lateral habenular nucleus and mediodorsal thalamic nucleus. Other axons arising from these cells descend through the medial forebrain bundle to terminate in a region dorsal to the interpeduncular nucleus. Fibers from the most lateral part of the ventrl septum (i.e., bed nucleus of the anterior commissure) project through the stria terminalis to the ventral subiculum. In addition, cells located in the horizontal limb of the diagonal band project massively to the pars posterior of the medial mammillary nucleus and the ventral tegmental area, and amygdala.

Changes in the distribution of the dentate gyrus associational system following unilateral or bilateral entorhinal lesions in the adult rat

The distribution of the dentate gyrus associational system was analyzed in naive adult rats and in those with either unilateral or bilateral lesions of the entorhinal cortex. Horseradish peroxidase histochemistry was used to trace the origin and course of this intrinsic fiber system. The fibers originated in the CA3-4 pyramidal cell field, apparently medial to the origin of the Schaffer collateral system, and followed a trajectory which was essentially identical to that described for this system by Zimmer36. The associational terminal field occupied the inner 26% of the dentate gyrus molecular layer in normal rats and 35-38% of the normal width of that layer following either ipsilateral or bilateral entorhinal lesion. These measurements are quite similar to those previously obtained on the commissural system terminal field in the normal and partially deafferented dentate gyrus. These results are interpreted to reflect axon sprouting by the associational fibers into the adjacent deafferented dendritic field.

GDEE antagonism of iontophoretic amino acid excitations in the intact hippocampus and in the hippocampal slice preparation

Glutamic acid diethylester (GDEE) reversibly antagonized excitations produced by glutamate and aspartate but not those produced by acetylcholine when applied iontophoretically to rat CA1 hippocampal neurons in penthrane (methoxyfluorane) anesthetized rats and to CA1 neurons in in vitro slice preparations. GDEE did not appear to differentiate between the excitations produced by glutamate aspartate and appeared to be a more potent antagonist than has previously been reported. CA1 cells were remarkably sensitive to acetylcholine; 5-50 nA being sufficient to produce marked amino acid-like excitations, which were unrelated to the pH of the acetylcholine. The nature of the responses to applied substances was virtually identical between the intact animal and the in vitro slice preparation. A description of the in vitro technique is given as an Appendix.

Hippocampal theta rhythm. I. Depth profiles in the curarized rat

Systemic injection of curare changes the depth profile of theta rhythm seen in the hippocampus of the freely moving rat. Under curare, dorsoventral microelectrode advancement reveals the presence of a sudden phase reversal and null occurring at the level of the stratum radiatum of CA1. Further advancement reveals the presence of an amplitude peak in the vicinity of the hippocampal fissure. In addition to the change in depth profile, curare alters the relationship between the amplitudes of the two phasereversed components of the theta rhythm. The change in theta rhythm brought about by curare outlasts the paralytic effect of the drug.

Hippocampal theta rhythm. II. Depth profiles in the freely moving rabbit

Depth profiles of hippocampal theta rhythm were investigated in the freely moving rabbit during three behavioral conditions: REM sleep, voluntary movement, and during sensory stimulation applied to the motionless animal. Profiles were found to be the same in all three conditions. Dorsoventral microelectrode penetration of the dorsal hippocampus revealed an approximately uniform amplitude of theta rhythm in strata oriens and pyramidal of CA1. Further microelectrode advancement revealed a sharp reversal of phase and a coincident null in amplitude in the proximal stratum radiatum. There was also a peak of theta rhythm amplitude which occurred in the molecular layer of the dorsal blade of the dentate gyrus. These data imply that in the rabbit, as in the freely moving rat, there are two generators of theta rhythm in the dorsal hippocampus, one in the dentate gyrus and the other in the overlying CA1 layer. The data also indicate the existence of a species difference in generating systems between the rabbit and the rat.

[Maturation of interhippocampal responses. Report and localization of inhibitory and excitatory synapses in the horn of Ammon of the young rabbit]

Extracellular recording from hippocampal areas of new-born rabbits up to 3 months of age was carried out to examine the responses evoked by stimulation of the contralateral alveus of CA1. In CA1, the extracellular potential fields show a positive wave whose amplitude is maximal at the somata of the pyramidal cells at any age. This positive wave has the same latency as an inhibitory postsynaptic potential induced in the same conditions and presumably represents inhibition at this level. In stratum oriens and at the top of stratum pyramidale, this positive wave is accompanied by ripples (at a frequency of about 200 c/sec) that are produced by basket cells. These ripples exhibit a very small amplitude up to 10 days of age. This positivity decreases in the depth towards the apical dendrite, reverses at the beginning of stratum radiatum and becomes rapidly negative. A second positive wave with ripples could be recorded from the layer of deep pyramidal cell somata belonging to the underlying CA3-CA4, only after 7 days. It is concluded that, in both the new-born and the adult rabbit, inhibitory synaptic action from contralateral stimulation is present on or close to the pyramidal cell somata, whereas excitatory action is located in the apical dendrites near the main branching.

Hippocampal innervation by serotonin neurons of the midbrain raphe in the rat

The organization of the brainstem serotonin neuron projection to the hippocampal formation was analyzed in the rat. This projection arises in the raphe nuclei of the midbrain. Following destruction of the midbrain raphe nuclei, chiefly nucleus centralis superior, there is a 72% decrease in hippocampal serotonin content. Injection of tritiated amino acid into the midbrain raphe nuclei results in transport of tritiated protein to the hippocampal formation and this transport is blocked in animals pretreated by intraventricular administration of 5,6-dihydroxytryptamine (5,6-DHT). Autoradiographic analysis indicates that the transport reaches the hippocampal formation primarily via two major pathways, the cingulum and the fornix. Cingulum fibers terminate predominantly in the dorsal hippocampus whereas the fornix distributes throughout the entire hippocampal formation. Some fibers reach the ventral hippocampus from the entorhinal area. Within the hippocampus there is dense labeling in a restricted lamina of the CA1 stratum lacunosum-moleculare with moderate labeling in stratum radiatum. Stratum oriens is sparsely labeled in CA1 and moderately so in CA2 and CA3. Stratum radiatum and stratum lacunosum-moleculare are moderately densely labeled in CA2 and Ca3. The area dentata is sparsely to moderately labeled in the molecular layer and heavily labeled in a thin lamina of the hilar zone immediately beneath the granule cell layer. The remaining hilar zone is moderately labeled. All of the discrete labeling of the hippocampus and area dentata described above is absent in animals pretreated with 5,6-DHT. These observations indicate that serotonin neurons of the midbrain raphe provide a highly organized innervation of the hippocampal formation in the rat.

Time-dependent changes in commissural field potentials in the dentate gyrus following lesions of the entorhinal cortex in adult rats

Previous neuroanatomical work has shown that lesions of the entorhinal cortex in adult rats cause the commissural projections to spread from their normally restricted locus in the inner molecular layer approximately 40-50 mum into the outer molecular layer (that is, into the zone deafferented by the lesion). In the present study we measured the effects of the entorhinal lesion on the distribution of short-latency potentials elicited by commissural stimulation in the molecular layer. Studies with animals tested at various times after the lesion and with a preparation that permitted recording from the same rat at several post-lesion intervals both indicated that the commissural response spread 100-150 mum towards the deafferented outer molecular layer, while the maximum response spread 50-100 mum. These effects were first detectable by 9 days after the lesion and were fully developed by 15 days post-lesion. These findings suggest that the growth of the commissural system seen after entorhinal lesions results in the rapid formation of functional terminals and are discussed in relationship to the behavioral consequences of brain lesions.