Axonal Sprouting of CA1 Pyramidal Cells in Hyperexcitable Hippocampal Slices of Kainate‐treated Rats

Abstract

CA1 pyramidal cells become hyperexcitable following hippocampal kainate lesions. To examine if axonal sprouting contributes to this epileptiform activity, the local axonal arborization of CA1 pyramidal cells was examined after intracellular labelling with biocytin in hippocampal slices from control rats and in hyperexcitable slices obtained from rats treated with kainate (bilateral intracerebroventricular injections) 2-4 weeks previously. Biocytin-labelled cells with an axon that could be followed from the soma to the alveus were drawn and reconstructed with a camera lucida (15 cells from control slices and 14 cells from hyperexcitable slices). Local axonal arborizations were more extensive in cells of hyperexcitable slices. This increase in axon collaterals was generally seen in the alveus and in stratum oriens, but changes were more prominent in the latter. In stratum oriens, cells from hyperexcitable slices showed a significant increase in mean total axon length (1035 versus 373 mu m in control), in mean number of branching points (6.50 versus 0.67 in control) and in mean number of segment orders per axon (3.07 versus 1.47 in control). Their first-order axon segments were similar in length to those of control cells (236 versus 338 pm in control), but with significantly more branching points (2.86 versus 0.53 in control). Their second-order axon segments were significantly longer (381 versus 63 mu m in control) and also showed more branching points (2.71 versus 0.13 in control). Their third- and fourth-order axon segments were also longer and with more branching points. Under high-power light microscopic examination, biocytin-labelled axonal varicosities in cells of hyperexcitable slices were often seen in close apposition with their own dendrites, presumably making synaptic contact (five of nine cells examined). No such appositions were seen in any of the control cells (seven cells examined). These results indicate that, following kainate lesions, there is sprouting of local axon collaterals of CA1 pyramidal cells in stratum oriens and in the alveus. This local increase in axon collaterals may contribute to the epileptiform activity in the CA1 area by providing recurrent excitation via newly formed synaptic, and perhaps even autaptic, contacts with pyramidal cell dendrites.

Cannabidiol exerts antiepileptic effects by restoring hippocampal interneuron functions in a temporal lobe epilepsy model

“…In contrast, CBD favoured a reduced firing of adapting, CCK interneurons that play a role in network modulation, as well as pyramidal cell firing, evidenced by a decrease in membrane time constant and input resistance in both the KA and Mg 2+ ‐free models. Incidentally, a significant loss of principal pyramidal neurons in both human and rodent epileptic hippocampi has been reported (Nadler et al, 1978; Babb et al, 1989), despite others reporting additional axonal sprouting of pyramidal cells, which will lead to enhancement of excitatory networks between pyramidal neurons, and probably explains the hyperactivity (Perez et al, 1996; Esclapez et al, 1999). Whether the CBD effects in reducing pyramidal cell hyperactivity reported here lead to a perseveration of pyramidal cell structure needs further investigation.…”

Restoration of calbindin after fetal hippocampal CA3 cell grafting into the injured hippocampus in a rat model of temporal lobe epilepsy

“…The synaptic reorganization following disruption of hippocampal circuitry mainly comprises abnormal sprouting of dentate mossy fibers into the dentate inner molecular layer (Shetty and Turner, 1995a,b, 1997, 1999bShetty et al, 2003Shetty et al, , 2005, sprouting of entorhinal axons in the CA1 strata radiatum and lacunosum (Shetty, 2002), and sprouting of axons of CA1 pyramidal neurons (Perez et al, 1996). The reductions in GABA-ergic interneuron numbers following ICV KA-induced injury have been observed in both dentate gyrus and CA1 and CA3 subfields of the hippocampus (Shetty and Turner, 2001), which is consistent with loss of functional inhibition observed in this model (Wheal, 1989;Turner and Wheal, 1991;Chen et al, 1999).…”

“…Thus, though the precise reason for calbindin loss following hippocampal injury or during epilepsy is still unclear, the loss of calbindin in dentate granule cells and CA1 pyramidal cells after the KA-induced CA3-region injury implies the existence of hyperexcitability in both dentate gyrus and CA1 subfield. Indeed, physiological studies report the presence of hyperexcitability in both of these regions following CA3 region injury (Tauck and Nadler, 1985;Turner and Wheal, 1991;Perez et al, 1996). The mechanisms responsible for persistent hyperexcitability in the dentate gyrus and the CA1 subfield of the CA3-lesioned hippocampus likely include an aberrant reorganization of the disrupted circuitry, decreased afferent circuitry leading to interneurons, loss of the physiological efficacy of interneurons, and reductions in the number of GABA synthesizing interneurons (Mathern et al, 1993;Mello et al, 1993;Dudek et al, 1994;Okazaki et al, 1995;Shetty andTurner, 2000, 2001;Wuarin and Dudek, 2001;Buckmaster et al, 2002;Scharfman et al, 2003;Shetty et al, 2005) Therefore, strategies that facilitate appropriate reorganization of the disrupted circuitry may be critical for easing hyperexcitability in the injured hippocampus.…”

Physiological and Morphological Characterization of Dentate Granule Cells in the p35 Knock-Out Mouse Hippocampus: Evidence for an Epileptic Circuit

“…The CA3 region of the hippocampus also exhibits cell dispersion, but we have not yet explored the functional consequences of that structural abnormality with respect to, for example, enhanced recurrent excitation (certainly a characteristic feature of CA3 circuitry) (Swann et al, 1990(Swann et al, , 1992. Although several different laboratories have demonstrated the development of abnormal synaptic reorganization, including recurrent excitation, in other areas of the hippocampus in human MTLE patients (Lehmann et al, 2000) and animal models of epilepsy (Perez et al, 1996;Lehmann et al, 2000Lehmann et al, , 2001Shao and Dudek, 2004), similar widespread abnormalities (e.g., in the CA1 subfield) have not yet been investigated in the p35Ϫ/Ϫ mouse.…”