Figure 3. DCS does not affect CHI-induced activation of astrocytes and microglia at the CA1 area. Sixteen days after CHI, mice were sacri- ficed, and their brains processed for immuno- histochemistry as described in the methods. Microglia, stained with lectin (A, B) and acti- vated astrocytes, stained with GFAP (C, D) are shown at the CA1 area of the hippocampus of the injured hemisphere in DCS (A, C) and vehicle (B, D) treated animals. No difference could be detected in the number or distribution of either cellular type between the two groups of animals. Scale bar 100 m.
mice), whereas no significant preservation of the syn- aptophysin immunoreactivity was detected (Fig. 4Ah). To further support our immunohistochemical findings quantitatively, we performed Western blot analysis on hippocampal homogenates prepared from 16 d post CHI mice and determined the levels of BDNF using specific antibodies. As shown in Fig. 4B, DCS restored the decrease in protein levels of BDNF found in vehicle-treated mice, almost to the levels found in sham untreated mice (P0.05 vs. vehicle treated mice). Taken together, these results suggest that DCS restored CHI-induced reduction of BDNF but did not affect the reduction of synaptophysin, and are in line with our electrophysiological data.
The results of the present study show, for the first time to our knowledge, that long-term potentiation (LTP) of synaptic transmission in the hippocampus, which is markedly impaired after CHI can be rescued by DCS, a coagonist of the NMDAR, when given as a single dose even as late as 24 h post-injury. The effect of DCS involves post-synaptic modifications since DCS did not affect the impairment of synaptic release reflected by paired pulse depression seen in CHI mice. The im- proved LTP was associated with recovery of cognitive function, pointing to the role of declined LTP in impaired cognition after CHI. In addition, DCS treat- ment significantly facilitated the recovery of motor functions and restored the decreased levels of the neurotrophic factor BDNF.
The present study is based on two observations, made in our previous report (20), namely: 1) A progressive decrease in the functional NMDAR density occurs between 1 and 24 h post-injury and lasts for at least a week. 2) Administration of the NMDAR agonist NMDA to mice, 1 and 2 d after injury, significantly improves
general neurological and cognitive function up to 2 wk. These observations led us to challenge the dogma that hyperactivation of the NMDAR underlies cognitive and neurological impairments after head injury and to propose that stimulation, rather than inhibition of NMDARs in the subacute post-injury phase, will be beneficial to head-injured patients. The present results, thus, not only corroborate our findings on the benefit of NMDAR activation as a treatment, they also point to a possible clinical application of a drug, currently used as “cognitive enhancer” in patients with schizophrenia or Alzheimer’s diseases (30, 32). The current findings fully agree with those of Temple and Hamm (1996), who showed that daily injections of DCS during 15 d post-injury ameliorated TBI-associated cognitive defi- cits (26).
Earlier reports demonstrated that TBI results in a chronic inability of the CA1 hippocampal synapses to maintain synaptic plasticity (15–19). Similarly, the present results show that mice, failing to perform in the ORT, also display impaired LTP when recorded in hippocampal slices isolated 14–16 d after injury. Inter- estingly, a single dose of DCS, at 24 h after injury, a time of maximal down-regulation of the NMDAR, was suffi- cient to activate the receptors such as to ameliorate the cognitive function (in vivo) as well as the LTP (in vitro), assessed 2 wk later. It is well established that the induction of hippocampal long-term potentiation (LTP), the leading cellular model of learning and memory, largely depends on NMDAR activation (14). Therefore, it is reasonable to assume that the recovery in LTP following DCS injection reflects the improve- ment in the functionality of NMDARs and can be thought as the underlying molecular basis for the improved hippocampal-related cognitive function. As was previously reported using a fluid percussion injury model (45), TBI resulted in impairment in synaptic release machinery measured by PPR 14 d following
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