Granule cell hyperexcitability in the early post‐traumatic rat dentate gyrus: the ‘irritable mossy cell’ hypothesis

Abstract

Cytochemical and in vitro whole‐cell patch clamp techniques were used to investigate granule cell hyperexcitability in the dentate gyrus 1 week after fluid percussion head trauma. The percentage decrease in the number of hilar interneurones labelled with either GAD67 or parvalbumin mRNA probes following trauma was not different from the decrease in the total population of hilar cells, indicating no preferential survival of interneurones with respect to the non‐GABAergic hilar cells, i.e. the mossy cells. Dentate granule cells following trauma showed enhanced action potential discharges, and longer‐lasting depolarizations, in response to perforant path stimulation, in the presence of the GABAA receptor antagonist bicuculline. There was no post‐traumatic alteration in the perforant path‐evoked monosynaptic excitatory postsynaptic currents (EPSCs), or in the intrinsic properties of granule cells. However, after trauma, the monosynaptic EPSC was followed by late, polysynaptic EPSCs, which were not present in controls. The late EPSCs in granule cells from fluid percussion‐injured rats were not blocked by the NMDA receptor antagonist 2‐amino‐5‐phosphonovaleric acid (APV), but were eliminated by both the non‐NMDA glutamate receptor antagonist 6‐cyano‐7‐nitroquinoxaline‐2,3‐dione (CNQX) and the AMPA receptor antagonist GYKI 53655. In addition, the late EPSCs were not present in low (0·5 mM) extracellular calcium, and they were also eliminated by the removal of the dentate hilus from the slice. Mossy hilar cells in the traumatic dentate gyrus responded with significantly enhanced, prolonged trains of action potential discharges to perforant path stimulation. These data indicate that surviving mossy cells play a crucial role in the hyperexcitable responses of the post‐traumatic dentate gyrus.

Decrease in Tonic Inhibition Contributes to Increase in Dentate Semilunar Granule Cell Excitability after Brain Injury

“…Although brain injury leads to neuronal damage and enhanced excitability of the dentate gyrus within 1 week after trauma (Lowenstein et al, 1992;Toth et al, 1997), intrinsic excitability of the major glutamatergic neurons remains unchanged (Santhakumar et al, 2000;Howard et al, 2007). However, whether hyperexcitability of a subset of glutamatergic neurons may underlie early postinjury increases in dentate excitability is not known.…”

“…A pendulum was dropped to deliver a brief (20 ms) 2.0 -2.2 atm impact on the intact dura. This resulted in a moderate level of injury that has been shown to cause a highly reproducible pattern of hilar cell loss (Toth et al, 1997;Santhakumar et al, 2000). For sham injury, the animals were anesthetized and attached to the fluid percussion device, but the pendulum was not dropped.…”

“…All procedures were performed under protocols approved by the Institutional Animal Care and Use Committee of the University of Medicine and Dentistry of New Jersey, Newark, New Jersey. Lateral fluid percussion injury (FPI) was performed on young adult (postnatal days 24 -26) male, Wistar rats (Charles River) as described previously (Dixon et al, 1987;Lowenstein et al, 1992;Toth et al, 1997;Santhakumar et al, 2000Santhakumar et al, , 2003. Briefly, the rats were placed in a stereotaxic frame under ketamine-xylazine anesthesia.…”

“…Recordings were obtained using an AxoPatch200B amplifier, filtered at 4 kHz using a Bessel filter, and digitized at 10 kHz with a DigiData 1440A analog-to-digital interface (Molecular Devices). The field responses in the granule cell layer were measured at five predetermined points in each slice (Santhakumar et al, 2000(Santhakumar et al, , 2001, including the tips of the dorsal and the ventral blades, the middle of the dorsal and ventral blades, and the middle of the crest, and the largest response was studied further.…”

“…Granule cell synaptic inhibition undergoes significant changes following brain injury due to loss of hilar interneuronal populations and plasticity of surviving neurons (Toth et al, 1997;Ross and Soltesz, 2000;Santhakumar et al, 2000;Hunt et al, 2011). Moreover, granule cells have persistent "tonic" inhibition mediated by high-affinity, extrasynaptic GABA A receptors (GABA A Rs) activated by ambient levels of GABA (Stell et al, 2003;Mtchedlishvili and Kapur, 2006).…”