Hannele Lahtinen

Pattern of neuronal death in the rat hippocampus after status epilepticus. Relationship to calcium binding protein content and ischemic vulnerability

The pattern of hippocampal cell death has been studied following hippocampal seizure activity and status epilepticus induced by 110-min stimulation of the perforant pathway in awake rats. The order of vulnerability of principal cells in the different hippocampal subfields--as determined by silver impregnation--was found to be very similar to the pattern found in ischemia; i.e., dentate hilus greater than CA1, subiculum greater than CA3c greater than CA3a,b greater than dentate granule cells. The hilar somatostatin-containing cells were the most vulnerable cell type, whereas all other subpopulations of nonprincipal neurons--visualized by immunocytochemistry for the calcium binding proteins parvalbumin and calbindin--were remarkably resistant. Pyramidal cells in the CA3 region containing neither of the examined calcium binding proteins were more resistant to overexcitation than CA1 pyramidal cells, most of which do contain calbindin. This indicates that no simple relationship exists between vulnerability in status epilepticus and neuronal calcium binding protein content, and that local and/or systemic hypoxia during status epilepticus may be responsible for the ischemic pattern of cell death.

D-cycloserine, a modulator of the N-methyl-D-aspartate receptor, improves spatial learning in rats treated with muscarinic antagonist.

The results of the present study indicate that D-cycloserine, a partial agonist at the glycine binding site, which is a positive modulation site of the N-methyl-D-aspartate receptor, could improve dose dependently the acquisition of a water maze task in rats treated with scopolamine, a muscarinic antagonist. The low to moderate doses of D-cycloserine which improved learning did not affect swimming speed in scopolamine-treated rats. The higher dose did not improve learning, but it increased swimming speed in scopolamine-treated rats. These results show a dissociation between impaired acquisition and abnormal behavioral activity in scopolamine-treated rats in the water maze task. Furthermore, the present results suggest that D-cycloserine can act as a cognitive enhancer at the appropriate doses.

Biochemical and Morphological Changes in the Rat Hippocampus Following Transection of the Fimbria-Fornix

According to electrophysiological studies, the subcortically denervated hippocampus has been suggested as a model for limbic epilepsy. We investigated a) whether fimbrial lesioning leads to any biochemical or morphological changes in the rat hippocampus, b) if these changes give any explanation to the previously indicated hyperexcitability, and c) if the changes are in line with the findings in other experimental models and human epilepsy. The fimbria-fornix transection was done by aspiration. Four months later, spontaneous EEG activities were recorded, and the hippocampal formation was processed for histology. In addition, a separate group of lesioned rats was used for hippocampal amino acid analysis. Hyperexcitable functioning of the hippocampus was seen as frequent and rhythmic spiking activity in 25% of the fimbria-fornix-lesioned rats, although the rest of them had spikes occasionally. The amino acids analysis revealed a notable decrease in the concentration of GABA but no significant changes in the amount of excitatory amino acids. This suggests impaired GABAergic functioning but does not exclude possible abnormalities in the release of both excitatory and inhibitory amino acids. The number of somatostatin-immunoreactive (SOM-IR) neurons, a subpopulation of GABAergic neurons, was decreased in all the areas of the hippocampus (CA3 > CA1 > hilus), but this was statistically significant only in the CA3 area. Interestingly, it is the region from which interictal spiking activity in the subcortically denervated rat presumable originates. Immunostaining for synaptophysin showed a dense band of granules in the inner molecular layer of the dentate gyrus, indicating probable synaptic reorganization of associational afferents.

Selegiline treatment after transient global ischemia in gerbils enhances the survival of CA1 pyramidal cells in the hippocampus.

Selegiline (L-deprenyl) has shown neuroprotective effects in a variety of degenerative processes. The present experiments were designed to test whether post-ischemia administered selegiline would alleviate delayed neuronal death of the gerbil hippocampal pyramidal cells following transient global ischemia. Common carotid arteries were occluded for 5 min. Saline or selegiline, 0.25 mg/kg s.c., was administered 2 h after the ischemia followed by a daily injection for either 3 or 7 days. After decapitation, the delayed death of the hippocampal CA1 pyramidal cells was assessed using Nissl-stained sections. In situ hybridization was used to reveal the expression of hsp70 mRNA 1, 3 or 7 days after the ischemia. Animals treated with selegiline for 7 days showed significantly lower damage score (scale 0-3: 0, normal; 1, < 10% of the neurons damaged; 2, 10-50% damaged; 3, > 50% damaged) compared to the saline-treated animals 1.73 +/- 0.18 and 2.41 +/- 0.16 (mean +/- S.E.M., P = 0.0133), respectively. A similar trend was found in animals after the 3-day treatment: 1.68 +/- 0.32 vs. 2.06 +/- 0.25 (P > 0.5). The expression of hsp70 mRNA in the CA1 pyramidal cell layer was strong still 3 days after the ischemic insult but vanished by 7 days. Densitometric measurements using 14C-plastic standards showed that the intensity of the CA1a hsp70 signal on the 3rd day correlated negatively to the cell-damage score (r = -0.72, P < 0.001), suggesting that hsp70 does not serve as a quantitative marker for CA1 neuronal injury in this model. Instead, the hsp70 expression was associated with improved neuronal survival lasting often longer in selegiline-treated animals (P > 0.5). The results show that a low dose of selegiline can alleviate the delayed hippocampal neuronal death in gerbils when administered 2 h after an ischemic insult.

The effects of selective alpha-2 adrenergic agents on the performance of rats in a 5-choice serial reaction time task

The present study investigated how the systemic administration of alpha-2 adrenergic agents that modulate the function of the noradrenergic system in brain, affect rousal and sustained attention. Food-deprivated rats were trained to detect and respond to brief flashes of light presented randomly in one of five spatially diverse locations. The effects of single-dose administrations of dexmedetomidine, an alpha-2 agonist, and atipamezole, an alpha-2 antagonists, on the choice accuracy, errors of omissions, speed of responding, and collection of the reward could be assessed in this task. Dexmedetomidine increased the amount of omissions, speed of response, and decreased the number of premature responses, although it did not markedly lengthen response latencies and food collection latency. Atipamezole increased the number of premature responses. Neither dexmedetomidine nor atipamezole had any effect on choice accuracy of rats in this task. The results suggest that dexmedetomidine decreased behavioral activity and arousal of rats, whereas atipamezole had mild stimulant effect on behavior.

Tetrahydroaminoacridine improves passive avoidance retention defects induced by aging and medial septal lesion but not by fimbria-fornix lesion

The present study examines whether tetrahydroaminoacridine (THA) can improve the deterioration in passive avoidance (PA) retention performance induced by medial septal (MS) and fimbria-fornix (FF) lesions in young rats or by aging. Retention of young MS-lesioned rats was improved by pretraining injection of THA at 3 mg/kg, but not by THA at 1 mg/kg or by either of the posttraining doses of THA (1 and 3 mg/kg). Pretraining injections of THA at 1 or 3 mg/kg had no effect on the PA retention performance of FF-lesioned rats. Age-induced PA failure was alleviated by pretraining administration of THA at 1 and 3 mg/kg. Posttraining injections of THA (1 or 3 mg/kg) had no effect on PA retention performance of aged rats. These results demonstrate that 1) THA may improve hippocampal cholinergic denervation-induced functional deficits and 2) some of the age-related PA deficits may be due to a cholinergic deficit and can be reversed with THA.

Behavioural, electrophysiological and histopathological changes following sustained stimulation of the perforant pathway input to the hippocampus: Effect of the NMDA receptor antagonist, CGP 39551

Sustained stimulation of the perforant path has been shown to damage the CA1 area and impair spatial memory in rats. The pattern of cell death is similar in human epileptics, who also exhibit memory deficits. In this study we demonstrate that the learning/memory impairment in water maze test and the development of interictal spikes that also followed stimulation-induced damage were antagonized by CGP 39551. Pretreatment with this NMDA receptor antagonist also slightly diminished somatostatin cell loss in the hilus but not CA1 pyramidal cell damage. These results indicate that the impairment of spatial learning/memory seems to be dependent not only on the degree of cell degeneration in the CA1 subfield of the hippocampus but also on the frequency of interictal spikes, at least in this model of epilepsy.

Spatial learning and noradrenaline content in the brain and periphery of young and aged rats.

The present experiment studied whether a dysfunction of the noradrenergic neurons is related to spatial learning impairment by investigating the levels of noradrenaline in the brain and periphery as well as the acquisition of water maze task in saline-pretreated young rats, in noradrenergic neurotoxin (DSP-4)-pretreated young rats and in saline-pretreated aged rats. Aged rats, which had an increased escape latency onto the hidden platform, revealed a decreased noradrenaline content in the heart, but not in the hippocampus, striatum, or hypothalamus, whereas DSP-4-pretreated rats had decreased noradrenaline content in the brain; the acquisition of water maze task was not impaired. These results suggest that the peripheral noradrenergic system can show age-related changes different from those in the central noradrenergic system, and they failed to provide support for the hypothesis that decreased activity of the central noradrenergic nerves is related to impairment in the acquisition of the water maze task.

Acute effects of γ-vinyl GABA on the GABAergic system in rats as studied by microdialysis

The acute effects of the irreversible γ-aminobutyric acid (GABA) transaminase inhibitor, γ-vinyl GABA (Vigabatrin), were studied in the central nervous system of the rat. GABA concentrations were monitored in the hippocampus by implantation of microdialysis probes. Two doses of γ-vinyl GABA (1.6 and 8.0 mM) were administered via the probes and were found to cause a transient increase in the basal GABA outflow (10-fold) during the period of drug administration. In addition, γ-vinyl GABA pretreatment (1.6 mM) seemed to decrease K+-evoked GABA release (P < 0.05). The immediate increase of GABA outflow after γ-vinyl GABA administration may be the result of direct blockade of GABA uptake sites, a finding which further indicates that the action of GABA transaminase inhibitors may be mediated partly through GABA uptake inhibition.

d-Cycloserine, a partial agonist at the glycine site, enhances the excitability of dentate granule cells in vivo in rats

The present study investigated the dose-dependent effects of d-cycloserine, a partial agonist at the glycine modulatory site associated with the NMDA receptor complex, on the hippocampal field potentials of dentate granule cells in awake, freely moving rats. Five sequential field potentials were recorded from the dentate gyrus of the dorsal hippocampus, by stimulating the perforant path in the entorhinal cortex at 30-s intervals. The slope of the population excitatory postsynaptic potential (e.p.s.p.) and the amplitude of the population spike of these field potentials were analysed and averaged with a computer. The effects of d-cycloserine (1.0, 3.0, 9.0 mg/kg) were recorded 40 min and 24 h after the i.p. injection. Although the slope of the population e.p.s.p. showed no significant change after the administration of d-cycloserine, the high doses produced a substantial increase in the amplitude of the population spike. This increase was observed 40 min but not 24 h after the injection. These findings indicate that d-cycloserine does not change the synaptic input from the perforant path to the granule cells but dose dependently enhances the excitability of the hippocampal dentate granule cells. In addition, the data give further support to the suggestion that in the brain area where NMDA receptor density is relatively high, the glycine site of the NMDA receptor may not be fully saturated by endogenous glycine in normal in vivo conditions. This suggests that there is a possibility for pharmacological modulation of NMDA receptor-mediated synaptic events by exogenous glycine or glycine analogues.