Synaptic plasticity in epileptogenesis: cellular mechanisms underlying long-lasting synaptic modifications that require new gene expression

Phrenic motoneuron structural plasticity across models of diaphragm muscle paralysis

Structural plasticity in motoneurons may be influenced by activation history and motoneuron-muscle fiber interactions. The goal of this study was to examine the morphological adaptations of phrenic motoneurons following imposed motoneuron inactivity while controlling for diaphragm muscle inactivity. Well-characterized rat models were used including unilateral C2 spinal hemisection (SH; ipsilateral phrenic motoneurons and diaphragm muscle are inactive) and tetrodotoxin phrenic nerve blockade (TTX; ipsilateral diaphragm muscle is paralyzed while phrenic motoneuron activity is preserved). We hypothesized that inactivity of phrenic motoneurons would result in a decrease in motoneuron size, consistent with a homeostatic increase in excitability. Phrenic motoneurons were retrogradely labeled by ipsilateral diaphragm muscle injection of fluorescent dextrans or cholera toxin subunit B. Following 2 weeks of diaphragm muscle paralysis, morphological parameters of labeled ipsilateral phrenic motoneurons were assessed quantitatively using fluorescence confocal microscopy. Compared to controls, phrenic motoneuron somal volumes and surface areas decreased with SH, but increased with TTX. Total phrenic motoneuron surface area was unchanged by SH, but increased with TTX. Dendritic surface area was estimated from primary dendrite diameter using a power equation obtained from three-dimensional reconstructed phrenic motoneurons. Estimated dendritic surface area was not significantly different between control and SH, but increased with TTX. Similarly, TTX significantly increased total phrenic motoneuron surface area. These results suggest that ipsilateral phrenic motoneuron morphological adaptations are consistent with a normalization of motoneuron excitability following prolonged alterations in motoneuron activity. Phrenic motoneuron structural plasticity is likely more dependent on motoneuron activity (or descending input) than muscle fiber activity.

Differential Activation of Calpain-1 and Calpain-2 following Kainate-Induced Seizure Activity in Rats and Mice

Systemic injection of kainate produces repetitive seizure activity in both rats and mice. It also results in short-term synaptic modifications as well as delayed neurodegeneration. The signaling cascades involved in both short-term and delayed responses are not clearly defined. The calcium-dependent protease calpain is activated in various brain structures following systemic kainate injection, although the precise involvement of the two major brain calpain isoforms, calpain-1 and calpain-2, remains to be defined. It has recently been reported that calpain-1 and calpain-2 play opposite roles in NMDA receptor-mediated neuroprotection or neurodegeneration, with calpain-1 being neuroprotective and calpain-2 being neurodegenerative. In the present study, we determined the activation pattern of calpain-1 and calpain-2 by analyzing changes in levels of different calpain substrates, including spectrin, drebrin, and PTEN (phosphatase and tensin homolog; a specific calpain-2 substrate) in both rats, and wild-type and calpain-1 knock-out mice. The results indicate that, while calpain-2 is rapidly activated in pyramidal cells throughout CA1 and CA3, rapid calpain-1 activation is restricted to parvalbumin-positive and to a lesser extent CCK-positive, but not somatostatin-positive, interneurons. In addition, calpain-1 knock-out mice exhibit increased long-term neurodegeneration in CA1, reinforcing the notion that calpain-1 activation is neuroprotective.

Expression of RGS2, RGS4 and RGS7 in the developing postnatal brain

The abundant expression of RGS (regulator of G-protein signalling) proteins in neurons, together with their modulatory function on G-protein-dependent neurotransmission, provides the basis for cellular adaptation to sensory inputs. To identify the molecular mechanism involved in the sensory experience-induced neural development, we performed a systematic survey of the localization of mRNAs encoding three subtypes of the RGSs (RGS2, RGS4 and RGS7) in developing rat brains by in situ hybridization through postnatal day 2 (P2), P10 and P18 to adult. The most dramatic changes of expression patterns were observed in the discrete neuronal cell layers of the cerebral neocortex (for RGS2 and 4), the hippocampus (for RGS2, 4 and 7), the thalamus (for RGS4) and the cerebellum (for RGS2 and 7). In the neocortex, RGS2 mRNA was enriched in the superficial cortical plate at P2, in contrast to RGS4, which was enriched in more mature neurons of the deeper layer V and VI. In the hippocampus, the neuronal cell layer-specific expression pattern of RGS2 developed from P2 to P18. RGS4 expression was temporarily confined to the CA pyramidal cell layer and not detectable in the dentate gyrus at P10 and P18. Similarly, a high level of expression of RGS7 was observed in the CA area, but not in the dentate gyrus at P2 and P10. In the cerebellum, the maturation of laminar expression patterns for the three RGSs correlated with neuronal maturation and synaptogenesis at P18. The most characteristic temporal pattern among the three RGSs was observed for RGS4 mRNA, which was highly enriched in the thalamocortical regions. The peaks of RGS4 expression were seen in the following regions with distinct onset and duration: the neocortex (from P2 onward), the hippocampus (P10 and P18) and the thalamus (from P18 onward). The divergent temporal and spatial expression of RGS subtypes and their dynamic control in the cortex, the hippocampus and the thalamus suggest that the RGS family could play multiple distinct roles in experience-dependent brain development.