Abstracts

Granule cell (GC) neurogenesis increases following seizures, and some newly born GCs migrate to abnormal locations within the hilus. These ectopic GCs (EGCs) display robust evoked potentials in response to perforant path stimulation, comparable in intensity to what is observed with GC layer GCs. However, EGCs which migrate into central portions of the hilus and have dendrites almost exclusively restricted to the hilus display significantly longer latencies, suggesting the involvement of polysynaptic pathways. Mossy cells would seem to be a prime candidate for involvement in these pathways. They normally provide feedforward excitation to GCs through connections in the inner molecular layer (IML), and many survive pilocarpine-induced seizures. Electron microscopic (EM) immunolabeling was therefore used to determine if mossy cell axon terminals synaptically contact hilar EGC dendrites. Pilocarpine (380mg/kg i.p.) was given to adult male Sprague-Dawley rats 30 min after atropine methylbromide (1mg/kg s.c.). Diazepam (5mg/kg i.p.) was given 1 hr after the onset of status epilepticus. Over 1 month later, after spontaneous seizures developed, animals were transcardially-perfused, and hippocampal sections were processed for dual EM immunolabeling. Mossy cell axon terminals were immunoperoxidase-labeled with an antibody to calcitonin gene-related peptide (CGRP) (Peninsula, 1:5000) and EGC postsynaptic processes were immunogold-labeled with an antibody to calbindin (CaBP) (Sigma, 1:200). Controls were treated the same, except that pilocarpine was replaced by saline. Light microscopically, large CGRP-immunoreactive (-IR) cells were seen in the hilus, in both control and experimental tissue. These cells had the appearance and distribution of mossy cells. Diffuse labeling was concentrated in the hilus, and the inner IML, although this labeling appeared to be lighter in experimental tissue. EM analysis revealed numerous CaBP-IR dendrites in the hilus of experimental animals. In addition, CGRP-IR axonal terminals were observed forming synapses with CaBP-IR dendrites. Within the hilus of pilocarpine-treated animals, excitatory axonal processes must reorganize to form functional synaptic connections with EGC dendrites as they develop, since EGCs display robust responses to perforant path stimulation. The fact that CGRP-IR terminals can be seen synaptically-contacting CaBP-IR dendrites strongly suggests that mossy cells participate in the polysynaptic curcuits which support the longer latency potentials that are observed. These types of polysynaptic connections could support recurrent excitation, and thus potentially affect seizure threshold. (Supported by NS 41490)