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Purpose: The role of α1b-adrenergic receptor (α1b-AR) in relation with neuronal degeneration, drug addiction, and seizure susceptibility has recently emerged. In particular, mice that overexpress α1b-AR undergo spontaneous epileptic seizures and progressive neuronal loss in a variety of brain areas. Therefore, one should expect that the blockade of α1b-AR leads to anticonvulsant and neuroprotective effects. However, the lack of α1b-AR antagonists does not allow testing of this hypothesis.
Methods: The development of α1b-AR knockout (KO) mice led us to measure seizure susceptibility and neurodegeneration following systemic excitotoxins in these mice.
Results: We found that α1b-AR KO mice are markedly resistant to kainate- and pilocarpine-induced seizures. Moreover, when marked seizure duration and severity are obtained by doubling the dose of chemoconvulsants in α1b-AR KO, neuronal degeneration never occurs.
Conclusions: These data indicate that α1b-AR per se plays a fundamental role in the mechanisms responsible for seizure onset, severity, and duration, whereas the brain damage observed in α1b-AR–overexpressing mice is likely to be a secondary phenomenon. In fact, the absence of α1b-AR confers resistance to neurotoxicity induced by seizures/chemoconvulsants. These data, although confirming a pivotal role of α1b-AR in modulating seizure threshold and neuronal death, offer a novel target, which may be used to develop novel anticonvulsants and neuroprotective agents.
The role played by α1-adrenergic receptors (α1-ARs) in vivo, when challenged using α1-agonists or antagonists often led to conflicting results. This becomes even more confused when considering the effects of α1-agonists and antagonists in epileptic seizures (Weinshenker & Szot, 2002; Giorgi et al., 2004).
In recent years, the genetic modeling of a mouse strain that overexpresses α1b-ARs led to spontaneous seizures and widespread degeneration (Zuscik et al., 2000). Overactivity of α1b-ARs is supposed to trigger neuronal death, which depends upon allosteric interaction between α1b-AR and N-methyl-d-aspartate (NMDA)–glutamate receptors, according to Paladini et al. (2001). Spontaneous seizures observed in this strain of mice may be the consequence of enhanced recruitment of glutamate NMDA receptors induced by α1b-AR overexpression (Paladini et al., 2001).
However, when considering the presence of a diffuse neuronal loss, which is also present early in limbic regions of these mice, it is debatable whether seizure activity occurs as a primary effect of the overexpression of α1b-ARs or is instead the consequence of an extensive neuronal damage.
To solve this question in the present study we used the opposite approach. In fact, we used mice lacking α1b-AR (knockout, KO). These mice were challenged with epileptogenic compounds: If overexpression of α1b-AR is the primary trigger for spontaneous seizures we expect that α1b-AR KO mice are resistant to seizures, compared with the wild-type (WT). Conversely, if spontaneous seizures observed in α1b-AR–overexpressing mice are the consequence of the brain damage, α1b-AR KO mice should not be refractory to seizures. Again, if overexpression of α1b-ARs is detrimental for neuronal survival independently of seizure-triggering effects, we expect an increased resistance to excitotoxicity in α1b-AR KO mice.
We recently demonstrated that mice previously exposed to amphetamines develop seizures as part of a latent brain hyperexcitability caused by limbic sensitization (Giorgi et al., 2005); since α1b-AR KO mice are resistant to sensitization induced by amphetamines (Drouin et al., 2002), we also evaluated whether the absence of α1b-ARs suppresses amphetamine-induced seizure susceptibility.
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These experiments demonstrate that α1b-AR plays a fundamental role in the mechanisms responsible for seizure generation, severity, and duration. Moreover, this receptor plays a crucial role in preventing neuron loss and neurodegeneration (Battaglia et al., 2003). Finally, the presence of α1b-AR is critical in the development of sensitization and long-lasting brain hyperexcitability following repeated administration of MDMA. This latter effect extends to the regulation of seizure threshold, which is lowered following repeated administration of low daily MDMA doses.
Therefore, the present data we collected in α1b-AR KO mice mirror those obtained in mice overexpressing α1b-AR. This allows us to conclude that α1b-AR is critical to promote cell death, seizure activity, and behavioral sensitization. These conclusions are justified even in the absence of specific pharmacologic ligands acting as agonists or antagonists at α1b-AR, since the appropriate use of genetically engineered mice (done by combining results obtained in mice overexpressing or knocked out for the same gene) provide convincing evidence for the role of such fascinating receptors.
Although the overexpression of α1b-AR produces spontaneous seizures (Zuscik et al., 2000; Kunieda et al., 2002), we found that the lack of α1b-AR confers a marked resistance to seizures induced by different chemoconvulsants. Again, although overexpression of α1b-AR produces spontaneous neuronal degeneration, which affects the hippocampal formation (Zuscik et al., 2000), we found that lack of α1b-AR suppresses cell loss and neuronal degeneration in the same brain area. In keeping with the suppression of amphetamine-induced sensitization observed in α1b-AR KO mice by Drouin et al. (2002), here we observed the lack of MDMA-induced sensitization in the same strain of mice. Again, such a lack of sensitization was bound to the absence of long-lasting limbic hyperexcitability, which is otherwise responsible for a lowered seizure threshold in MDMA pretreated mice (Giorgi et al., 2005).
Moreover, the data we obtained offer an answer to the potential linkage between seizure susceptibility and brain damage. In fact, the spontaneous occurrence of neurodegeneration and seizures in mice that overexpress α1b-AR left open the question of whether seizure susceptibility was primarily affected in these mice or it was instead the consequence of the brain damage. Our data indicate that the first hypothesis is correct; this was further confirmed by data obtained by doubling the dose of the excitotoxins. In fact, if overexpression of α1b-ARs was detrimental for neuronal survival independent of the triggering of seizures, we expect an increased resistance to excitotoxins in α1b-AR KO mice, as found in the present work. Incidentally, this is in line with the concept that toxicity induced by kainic acid in mice at the dose of 35 mg/kg is not the consequence of seizure activity but the result of its excitotoxic properties, as recently suggested by Benkovic et al. (2006). Therefore, the role of α1b-AR in seizures and neuronal degeneration is not causally related but it is rather the consequence of two independent mechanisms, which eventually may lead to synergistic detrimental effects.
Therefore, we might hypothesize that a novel antiepileptic therapy, based on the blockade of α1b-AR receptor, is expected to be both neuroprotective and anticonvulsant. Such a therapeutic approach should be particularly effective in protecting from seizure-induced brain damage.