Rebekah Loy

Sprouting of sympathetic axons in the hippocampal formation: Conditions necessary to elicit ingrowth

Abstract Sympathetic, noradrenaline (NA)-containing axons, which do not normally innervate the parenchyma of the hippocampal formation, grow into this structure following lesions of the fimbria. This anomalous innervation apparently arises by collateral sprouting from the normal sympathetic plexus on arteries within the subarachnoid space along the medial surface of the hemisphere. These NA axons travel orthogonal to the septotemporal axis within the dentate hilus and stratum oriens of CA3, and parallel to the septotemporal axis within the stratum lucidum of CA3. Regardless of the type of lesion (electrolytic, aspiration, knife cut) the septohippocampal input must be transected to elicit the ingrowth. No ingrowth follows selective lesions of the commissural or entorhinal cortical inputs or of the vasculature and cortex overlying the septal pole of the hippocampal formation, including central NA collaterals in the fasciculus cinguli. Discrete lesions of portions of the fimbria permit localized sympathetic ingrowth into the septal (medial fimbria) or temporal (lateral fimbria) ends of the structure. The pattern of anomalous innervation is neither extended by additional commissural or entorhinal cortical deafferentation, nor apparent in all areas deafferented by the septal lesion. Thus, the ingrowth is not regulated solely by the presence of degenerating terminal fields, and although damage to the septal afferent appears necessary, it is not sufficient to permit sympathetic axon and terminal proliferation within the entire hippocampal formation.

Hormonal regulation of axonal sprouting in the hippocampus

Removal of septal fibers to the rat hippocampal formation is followed by an ingrowth of sympathetic axons into the deafferented regions. We have shown previously that the pattern of sprouting is more restricted in males than in females when the lesions are made in mature animals, but is almost the same if lesions are made during the early postnatal period. In the present study, the relationship of circulating sex hormones to the sprouting response was investigated by comparing the extent of sympathetic axon ingrowth following fimbrial lesions in intact or gonadectomized adult male and female rats. The effects of manipulating sex steroids during development was examined by comparing sympathetic axonal sprouting after fimbrial lesions in rats which were castrated (male) or treated with testosterone (females and castrated males) on postnatal day 2. We find that (1) gonadectomy of either adult female or male rats does not affect the sprouting response, but (2) neonatal castration of male rats permits sprouting in a pattern similar to normal females, and neonatal testosterone treatment of females or castrated males results in the more limited sprouting response characteristic of normal males. These results indicate that the sex-related differential response to fimbrial lesions may be determined by developmental differences in endogenous steroid levels.

Ontogeny of the noradrenergic innervation of the rat hippocampal formation.

SummaryThe noradrenergic (NA) innervation of the rat hippocampal formation arrives embryonically into a structure in which cytogenesis and cell migration are still active processes. At embryonic day 18 (E18) the first fluorescent axons appear in the septal end of CA3 at the boundary of the marginal zone and cortical plate, the future stratum lucidum. By birth axons invade the subiculum and also course along the septo-temporal axis in a longitudinal associational system in stratum moleculare of CA3. The innervation of the area dentata increases significantly by postnatal day four (P4). The innervation pattern throughout the dentate and Ammons horn is fairly complete by P10. High affinity uptake of 3H-NA also matures embryonically and correlates postnatally with the extent of innervation estimated by fluorescence histochemistry. The levels of endogenous NA develop more slowly, showing only 60–80% of older adult values by P48.Compared to the maturation of other hippocampal afferents, the NA innervation is extremely precocious. It is localized in areas which could allow it to have significant trophic functions during early stages of histogenesis. In addition, its presence in the rapidly developing structure may contribute to its eventual distribution in a relatively less organized terminal pattern than that of the later-arriving entorhinal, commissural and septal afferents.

Development of afferent lamination in Ammon's horn of the rat.

SummaryThe establishment of afferent lamination is apparently a prenatal event in Ammons horn of the rat. Segregation between the entorhinal cortical and commissural/associational axon systems and their mutual respect of a common boudary are already apparent autoradiographically by the day of birth. Additional Fink-Heimer analysis suggests several maturational gradients once lamination has been established. First, the perforant path to CA3 appears to mature slightly earlier than the temporoammonic tract to CA1 at a septal level; the former also shows a septal to temporal progression in maturation. Second, the commissural and associational systems also appear to mature earlier in CA3 than in CA1. This is particularly evident for the commissural system, which progressively matures from the point of entry at CA3b simultaneously towards CA3c and CA1. The commissural system also shows a septal to temporal maturational gradient, which likely reflects the course of the growing axons. In addition to the transverse and longitudinal gradients, a third, intralaminar, gradient is suggested in the present Fink-Heimer material, in which maturation proceeds disto-proximally (1) from entorhinal to commissural and (2) within the stratum radiatum beginning at the juncture of the commissural/associational and entorhinal cortical terminal fields in CA3. This intriguing result supports an hypothesis that lamination is first established by formation of the boundary between the commissural/associational and entorhinal cortical systems, then is maintained with future dendritic growth as each respective system expands within its own territory.

Interaction of age and sex in sympathetic axon ingrowth into the hippocampus following septal afferent damage

SummaryDamage to the medial septal nucleus or to the fimbria/fornix in the adult rat elicits sprouting of vascular sympathetic fibers into the deafferented regions of the hippocampal formation. The following study examines the effects of developmental stage and sex on this sprouting phenomenon using both fluorescence histochemistry and high affinity uptake of noradrenaline. We find that (1) the sprouting, which is reduced in adult and juvenile males relative to females, is equivalent in the two sexes after transections at postnatal day 3, and (2) the period of maximum ingrowth is sexually regulated, occurring near postnatal day 3 in the male and postnatal day 13 in the female.

A delayed sprouting response to partial hippocampal deafferentation: Time course of sympathetic ingrowth following fimbrial lesions

Sympathetic, noradrenaline-containing fibers grow into the hippocampal formation following lesions of the medial septum or fimbria/fornix. Fluorescent histochemical analysis reveals that these fibers begin to arise as collateral sprouts of the normal sympathetic innervation of the internal and external transverse hippocampal arteries at 9 days post-lesion. These initial fibers are oriented orthogonally to the septo-temporal axis of the hippocampal formation. They grow towards the granule cells of the fascia dentata and the CA3 pyramidal cells, where they begin to proliferate at 14 days post-lesion. This process continues until 29 days, resulting in a final distribution of fibers in areas of septal deafferentation: stratum lucidum, the inner one-third of stratum oriens and stratum pyramidale of CA3; and the hilus, the inner one-third of stratum moleculare and stratum granulosum of the fascia dentata. The time course of this sprouting response is relatively late in onset and slow in completion when compared to sprouting responses of intrinsic afferent systems of the hippocampal formation following entorhinal cortical or commissural deafferentation.

Nest-building behavior in two cerebellar mutant mice: Staggerer and weaver

Nest-building behavior was studied in two separate (different genes) but phenotypically similar autosomal recessive neurological (cerebellar) mutants, staggerer and weaver, as well as their wild-type siblings. None of the test animals or their parents had previous experience with the nesting materials used in these experiments. All of the mice, with the exception of the staggerer mutant, exhibited the nest-building behavior. This observation, coupled with other phenotypes expressed in this mutant, suggest the mutation may more generally affect neuroendocrine development and/or regulation rather than be restricted to cerebellar development.

Association of sympathetic axons in denervated hippocampus to intracerebral vasculature

SummaryCombining glyoxylic acid and pontamine sky blue fluorescence methods allows simultaneous visualization of catecholamine fibers and the intra-hippocampal vascular bed. The normal noradrenergic innervation of the hippocampal formation is not closely associated with the vasculature in any region. However, following lesions of the septo-hippocampal fibers, norepinephrine-containing sympathetic fibers course along the penetrating arterioles into the parenchyma of regio inferior and the area dentata. Within the respective cell layers, the anomalous sympathetic axons collateralize freely, thus suggesting a dual relationship with the vasculature and with the neuropil of selective hippocampal zones.

Sprouting of sympathetic fibers in the hippocampus in the absence of major target cell candidates

Noradrenergic fibers of sympathetic origin enter the hippocampal formation following removal of the cholinergic septal afferents. These sympathetic fibers apparently form synaptic contacts with cells in the hippocampus, but the identity of the postsynaptic cell is unknown. As the two major cell types of the hippocampal formation are the pyramidal cells and the dentate granule cells, we sought to determine how their absence would affect the ingrowth and distribution of the sympathetic fibers following septal lesions. Hippocampal pyramidal cells were selectively removed with kainic acid and granule cells were selectively removed with colchicine 4 days before lesions were placed in the medial septal-diagonal band nuclear complex. Animals were sacrificed 4-5 weeks after the septal lesions and the brains prepared for catecholamine fluorescence histochemistry. Sympathetic fibers innervate the hippocampal formation of brains depleted of pyramidal cells or granule cells. Thus, it appears that neither cell type is an obligatory postsynaptic target or is necessary for the ingrowth of sympathetic fibers. The anomalous sympathetic fibers may be contacting interneurons or glia.

REGENERATIVE GROWTH OF SYMPATHETIC FIBERS INTO RAT HIPPOCAMPUS

ABSTRACT Noradrenaline (NA)-containing fibers, originating in the superior cervical ganglion (SCG), grow into the hippocampal formation (HF) of the rat following transection of the anterior end of the structure. These fibers form a plexus of coarse, intensely fluorescent axons with large terminal varicosities. Electron microscopic examination after 5-hydroxydopamine (5-OHDA) injections indicates that in transected animals with locus coeruleus (LC) lesions, small granular vesicles (SGVs) appear within varicosities in the HF neuropil. Biochemical assay indicates a large increase in endogenous HF NA by 15 days after LC lesion and secondary HF transection. The anomalous fibers appear most dense in the infragranular hilus, the stratum granulosum, the inner 1/3 of the dentate molecular layer, and stratum lucidum of CA3.