Regeneration of the GABAergic septohippocampal projection in vitro

Semaphorin 3C is not required for the establishment and target specificity of the GABAergic septohippocampal pathway in vitro

The septohippocampal (SH) pathway comprises cholinergic and GABAergic fibers. Whereas the former establish synaptic contacts with all types of hippocampal neurons, the latter form complex baskets specifically on interneurons. The GABAergic SH function is associated with the control of hippocampal synchronous networks. Little is known about the mechanisms involved in the formation of the GABAergic SH pathway. Semaphorin (Sema) 3C is expressed in most hippocampal interneurons targeted by these axons. To ascertain whether Sema 3C influences the formation of the SH pathway, we analyzed the development of this connection in Sema 3C-deficient mice. As these animals die at birth, we developed an in vitro organotypic co-culture model reproducing the postnatal development of the SH pathway. In these SH co-cultures, the GABAergic SH pathway developed with target specificity similar to that present in vivo. SH axons formed incipient baskets on several types of hippocampal interneurons at 7 days in vitro, which increased their complexity by 18-25 days in vitro. These SH fibers formed symmetric synaptic contacts on GABAergic interneurons. This synaptic specificity was not influenced by the absence of entorhinal afferents. Finally, the absence of Sema 3C in target neurons or its blockage by neuropilin-1 and -2 ectodomains in slice co-cultures did not lead to major changes in either the target specificity of the GABAergic SH pathway or its density of innervation. We conclude that the formation and synaptic specificity of the GABAergic SH pathway relies on robust molecular mechanisms, independent of Sema 3C, that are retained in our in vitro co-culture model.

Effects of fimbria-fornix transection on calpain and choline acetyl transferase activities in the septohippocampal pathway

The ability of fimbria-fornix bilateral axotomy to elicit calpain and caspase-3 activation in the rat septohippocampal pathway was determined using antibodies that selectively recognize either calpain- or caspase-cleaved products of the cytoskeletal protein alphaII-spectrin. Radioenzymatically determined choline acetyl transferase (ChAT) activity was elevated in the septum at day 5, but reduced in the dorsal hippocampus at days 3, 5 and 7, after axotomy. Prominent accumulation of calpain-, but not caspase-3-, cleaved spectrin proteolytic fragments was observed in both the septum and dorsal hippocampus 1-7 days after axotomy. ChAT-positive neuronal cell bodies in the septum also displayed calpain-cleaved spectrin indicating that calpain activation occurred in cholinergic septal neurons as a consequence of transection of the septohippocampal pathway. Calpain-cleaved alphaII-spectrin immunoreactivity was observed in cholinergic fibers coursing through the fimbria-fornix, but not in pyramidal neurons of the dorsal hippocampus, suggesting that degenerating cholinergic nerve terminals were the source of calpain activity in the dorsal hippocampus following axotomy. Accumulation of calpain-cleaved spectrin proteolytic fragments in the dorsal hippocampus and septum at day 5 after axotomy was reduced by i.c.v. administration of two calpain inhibitors. Calpain inhibition partially reduced the elevation of ChAT activity in the septum produced by transection but failed to decrease the loss of ChAT activity in the dorsal hippocampus following axotomy. These findings suggest that calpain activation contributes to the cholinergic cell body response and hippocampal axonal cytoskeletal degradation produced by transection of the septohippocampal pathway.

Axon regeneration in organotypic slice cultures from the mammalian auditory system is topographic and functional

In vitro models have frequently been employed to investigate the specificity of the formation of axonal projections during both development and regeneration. Such studies demonstrated pathway, target, and laminar specificity, yet they did not tackle the problem of topography. Here, we addressed the issue of regeneration of spatial specificity at the topographic level by lesioning a precisely organized projection from the auditory system of neonatal rats in organotypic slice culture and by analyzing regeneration capacity. Lesioning had no effect on the survival of axotomized neurons or the structure of the auditory nuclei. Anterograde and retrograde biocytin tracing demonstrated that the projection regenerated topographically at the supracellular level. Whole-cell patch-clamp recordings revealed that the regenerated projection was functional. Topographic regeneration was not impaired by blocking spike activity with tetrodotoxin or glycinergic transmission with strychnine. However, if lesioning was performed after the slices had been incubated for 1 week, regeneration capacity was lost despite good survival of neurons. The loss of the regeneration capacity in vitro occurs at a developmental stage that corresponds to the age when the capacity for axonal reorganization is lost in vivo. We conclude that the developmental processes occurring in vivo and in vitro are comparable in this system, which is why we think that essential aspects of the loss of regeneration capacity may be addressed with our culture model in the future.

Axotomy-induced c-JUN expression in young medial septal neurons is regulated by nerve growth factor

In the present study we investigated the axotomy-induced expression of the proto-oncogene c-jun in young rat medial septal neurons and its regulation by nerve growth factor. First, medial septal neurons were retrogradely labelled by Fast Blue injection into the hippocampus at postnatal day 1 (P1). Rats of different developmental ages (P6, P9, P14, P21, P28 and P42) were then subjected to bilateral fimbria-fornix transection resulting in the axotomy of septohippocampal projection neurons. After the lesion, c-JUN immunoreactivity was observed in the nuclei of axotomized medial septal neurons of all stages examined, suggesting that c-JUN induction is an age-independent feature of axotomized medial septal neurons. Double immunolabelling for choline acetyltransferase and c-JUN or parvalbumin and c-JUN, respectively, revealed that both cholinergic and GABAergic septohippocampal projection neurons express c-JUN after axotomy. In addition, a co-localization of immunostaining for c-JUN and the neuropeptide galanin was found after lesion, as both proteins were induced in the same medial septal neurons following fimbria-fornix transection. Next, the regulation of c-JUN expression in axotomized medial septal neurons was studied in organotypic cultures of the medial septum. Axotomized medial septal neurons in culture did not express c-JUN in contrast to the in vivo situation. With the concept that nerve growth factor suppresses c-JUN expression, slice cultures of the medial septum were treated with antibodies against nerve growth factor. This treatment caused a dose-dependent increase in c-JUN-positive cells in these slice cultures. Simultaneous addition of nerve growth factor and antibodies against nerve growth factor resulted in the reversal of this effect. These data suggest an age-independent induction of c-JUN in axotomized medial septal neurons and its regulation by nerve growth factor.

Role of NGF in axotomy-induced c-Jun expression in medial septal cholinergic neurons

The extent of neuronal regeneration after axotomy largely depends on the survival capacity of the injured cell. It has been shown for a long time that nerve fiber transection results in retrograde changes in the parent neuronal cell body, and that these changes may eventually lead to neuronal degeneration. At present, little is known about the sequence of events initiated in a nerve cell body by the transection of its axonal process. In this report, we will focus on an interaction of nerve growth factor (NGF) with the transcription factor c-Jun in intact and axotomized septohippocampal projection neurons.

Parvalbumin immunoreactive neurons in the rat septal complex have substantial glial coverage and receive few direct contacts from catecholaminergic terminals

Our previous studies have demonstrated that septohippocampal neurons in the rat septal complex have substantial glial coverage and have a number of synaptic associations with catecholaminergic terminals. While similar ultrastructural characteristics are observed for septal cholinergic neurons, the morphology and synaptic relations of catecholaminergic terminals with septal GABAergic neurons is largely unknown. Since the GABAergic septohippocampal neurons colocalize the calcium-binding protein, parvalbumin (PVA), the present study examined the ultrastructural relations of PVA neurons with catecholaminergic terminals in the septal complex. Single sections were dually labeled with antibodies to PVA and either tyrosine hydroxylase (TH) or dopamine-beta-hydroxylase (DBH). By light microscopy, processes with TH- and DBH- (TH/DBH) immunoreactivity were near PVA-labeled neurons. By electron microscopy, PVA-labeled perikarya had an average diameter of 14.9+/-6 microm and were ovoid or elongated. PVA-labeled perikarya (n = 124) had a large amount of astrocytic coverage (75+/-14%) and a low amount of terminal coverage (15+/-12%). PVA-labeled perikarya and dendrites mostly were contacted by terminals lacking immunoreactivity for either PVA or TH/DBH (82% of 1,663). Of the TH/DBH terminals or axons near PVA somata and dendrites, few (3% of 1,663) directly contacted them while the majority abutted adjacent glial or neuronal profiles. Some TH/DBH- and PVA-labeled terminals contacted the same dendrites; a few of these contained immunoreactivity for PVA. The results demonstrate that PVA-containing GABAergic septal neurons, like cholinergic neurons, are mostly surrounded by astrocytes and have very little terminal coverage. However, in contrast to cholinergic neurons, PVA-containing neurons are contacted primarily by non-catecholaminergic terminals suggesting that any functional interactions would be indirect. These findings further support the functional diversity of subpopulations of septohippocampal neurons.

Correlated axonal sprouting and dendritic spine formation during kainate-induced neuronal morphogenesis in the dentate gyrus of adult mice

Several examples of structural plasticity in the adult brain have been provided in the hippocampus, among which the most striking concerns axonal remodeling of the dentate gyrus granule cells. We have recently demonstrated that a single injection of kainic acid into the dorsal hippocampus of adult mice triggers a conspicuous morphogenetic response of granule cells. Cellular labeling with biocytin 1, 2, and 4 weeks after injection of kainate revealed a progressive increase in dendritic thickness and length (up to 2.5-times), combined with an increase in the number of dendritic spines. This correlation resulted in the conservation of total spine density. No modifications of the dendritic arborization pattern were noted. In addition to dendritic changes, the number of axonal profiles observed within the hypertrophied granular layer and the inner part of the molecular layer appeared dramatically increased. Timm staining and anterograde labeling of two of the main extra-hippocampal afferent systems (i.e., septal, entorhinal) evidenced sprouting of mossy fibers and of septal afferents. Entorhinal fibers were not obviously modified. As revealed by calretinin-immunohistochemistry, commissural afferents also responded by an extensive sprouting. In addition, increases of dendritic spine number and dendritic length were noticeably greater in portions of dendrites that receive mossy fiber collaterals and septal and hypothalamic afferents, than in the external portion which receives entorhinal afferents. Although qualitative, this correlation suggests a relationship between kainate-induced structural plasticity of mature granule cells and the specific capacities of afferent systems to elaborate axon collaterals.