Introduction
In vivo or in vitro preexposure of brain cells to the sublethal stimuli increases their tolerance to the subsequent lethal insult suggesting existence of the endogenous neuroprotective mechanisms 1 . Exploration of the phenomenon of preconditioning also revealed the existence of indirect neuroprotection: increased tolerance to damaging insult of brain cells not directly affected by sublethal preconditioning stimulus 2 . Findings suggest the existence of system mechanisms of neuroprotection which activate in response to noxious stimuli and render brain cells tolerant to damaging insult. We demonstrated that tolerance to the damaging stimuli can be increased by stimulation of specific brain areas such as cerebellar fastigial nucleus (FN), subthalamic vasodilator area (SVA), or periaqueductal grey (PAG) 3 . This phenomenon was termed neurogenic neuroprotection. Cellular mechanisms, such as opening of ATP-sensitive potassium channels; increased mitochondrial tolerance to depolarization and apoptogenic stimuli, seem to be common for neurogenic neuroprotection and preconditioned neuroprotection 3 . These facts raise the possibility that FN participates in the mechanisms of ischemic preconditioning. We tested this hypothesis by analyzing the efficiency of global ischemic preconditioning in rats in which FN neurons were lesioned by excitotoxin.
Methods
In anesthetized artificially ventilated Sprague Dawley rats (220–250 g) global ischemia (GI) was triggered by four-vessel occlusion (5 min for preconditioning and 20 minutes for damaging insult). Blood gases and body temperature were maintained at normal levels. Arterial pressure, CBF and EEG were monitored. Seven days after the insult brains were histologically processed for quantitative evaluation of hippocampal CA1 neurons loss. FN or midline thalamic (control) neurons were lesioned three days before GI by microinjection of ibotenic acid with subsequent histological verification.
Results
GI for 20 min decreased number of hippocampal neurons by 34±1.2% (n=6, p<0.05). Ischemic preconditioning (5 min GI) three days before the insult completely reversed GI-induced cell loss. Excitotoxic lesion of FN neurons three days before the preconditioning blocked salvaging effect of the latter: cell loss of 36±0.13% (n=5, p<0.01 compared to excitotoxic lesion of midline thalamus, which did not affect the salvaging effect of ischemic preconditioning). Lesion of FN neurons also aggravated cell loss induced by 20 min GI alone: cell loss of 59% (n=5, p<0.01).
Conclusions
Neuroprotective effect of ischemic preconditioning can be reversed by lesion of FN neurons. Moreover, lesions of FN neurons aggravate hippocampal cells damage by global ischemia alone. Integrity of neurons of the cerebellar fastigial nucleus is necessary for the full expression of the neuroprotective effect of ischemic preconditioning. Findings suggest that FN may play physiological role in the mechanisms of ischemic preconditioning. FN may be a part of the intrinsic integrative neuroprotective mechanisms in the brain, which through activation of the cellular defensive mechanisms play an important role in the neuroprotection in vivo. These mechanisms may be involved in the indirect cross-neuroprotection phenomenon.