Immunohistochemical localization of calbindin-D28K in telencephalic regions of microencephalic rats

Zinc supplementation in rats impairs hippocampal-dependent memory consolidation and dampens post-traumatic recollection of stressful event

Zinc is a trace element important for synaptic plasticity, learning and memory. Zinc deficiency, both during pregnancy and after birth, impairs cognitive performance and, in addition to memory deficits, also results in alterations of attention, activity, neuropsychological behavior and motor development. The effects of zinc supplementation on cognition, particularly in the adult, are less clear. We demonstrate here in adult rats, that 4 week-long zinc supplementation given by drinking water, and approximately doubling normal daily intake, strongly impairs consolidation of hippocampal-dependent memory, tested through contextual fear conditioning and inhibitory avoidance. Furthermore, the same treatment started after memory consolidation of training for the same behavioral tests, substantially dampens the recall of the stressful event occurred 4 weeks before. A molecular correlate of the amnesic effect of zinc supplementation is represented by a dysregulated function of GSK-3ß in the hippocampus, a kinase that participates in memory processes. The possible relevance of these data for humans, in particular regarding post-traumatic stress disorders, is discussed in view of future investigation.

Postnatal neurogenesis in the dentate gyrus of the guinea pig

In all species examined, the dentate gyrus develops over an extended period that begins during gestation and continues up to adulthood. The aim of this study was to investigate the pattern of postnatal cell production in the dentate gyrus of the guinea pig, a rodent whose brain development has features more closely resembling the human condition than the most commonly used rodents (rat and mouse). Animals of different postnatal (P) ages received one or multiple injections of bromodeoxyuridine (BrdU), and the number of labeled cells in the dentate gyrus was counted after time intervals of 24 h or longer. The total granule cell number and the volume of the granule cell layer were evaluated in Nissl-stained brain sections from P1 and P30 animals. P1-P5 animals were treated with MK-801 to analyze the effect of NMDA receptor blockade on cell proliferation. Cell production occurred at a high rate (9,000-13,000 labeled cells 24 h after one injection) from P1 to P20, with a peak at 3-6 days of age, and then slowly declined from P20 to P30. The production of new cells continued in adult animals, although at a much-reduced rate (400 cells 24 h after one injection). About 20% of the labeled cells survived after a 17-day period and most (60%) of these cells had a neuronal phenotype. The total number of granule cells increased over the first postnatal month; in 30-day-old animals, it was 20% greater than in 1-day-old animals. Administration of MK-801 to P1-P5 animals caused an increase in cell proliferation restricted to the dorsal dentate gyrus. The present data show that, although the guinea pig dentate gyrus develops largely before birth, the production of new neurons continues at a high rate during the first postnatal month, leading to a considerable increase in cell number. This developmental pattern, resembling the human and nonhuman primate condition, may make the guinea pig a useful rodent model in developmental studies on dentate gyrus neurogenesis.

Nitric oxide negatively regulates proliferation and promotes neuronal differentiation through N-Myc downregulation

Nitric oxide (NO) has been found to act as an important negative regulator of cell proliferation in several systems. We report here that NO negatively regulates proliferation of neuronal cell precursors and promotes their differentiation by downregulating the oncogene N-Myc. We have studied this regulatory function of NO in neuroblastoma cell lines (SK-N-BE) and in primary cerebellar granule cell cultures. In a neuronal NO synthase (nNOS) overexpressing neuroblastoma cell line exposed to the differentiative action of retinoic acid, NO slowed down proliferation and accelerated differentiation towards a neuronal phenotype. This effect was accompanied by a parallel decrease of N-Myc expression. Similar results could be obtained in parental SK-N-BE cells by providing an exogenous source of NO. Pharmacological controls demonstrated that NO's regulatory actions on cell proliferation and N-Myc expression were mediated by cGMP as an intermediate messenger. Furthermore, NO was found to modulate the transcriptional activity of N-Myc gene promoter by acting on the E2F regulatory region, possibly through the control of Rb phosphorylation state, that we found to be negatively regulated by NO. In cerebellar granule cell cultures, NOS inhibition increased the division rate of neuronal precursors, in parallel with augmented N-Myc expression. Because a high N-Myc expression level is essential for neuroblastoma progression as well as for proliferation of neuronal precursors, its negative regulation by NO highlights a novel physiopathological function of this important messenger molecule.

Alteration of neuronal nitric oxide synthase activity and expression in the cerebellum and the forebrain of microencephalic rats

Microencephalic rats were obtained through gestational (for the forebrain) or neonatal (for the cerebellum) administration of the DNA-alkylating agent methylazoxymethanol acetate (MAM), which selectively kills dividing cells during neurogenesis. In the microencephalic cerebellum the specific activity of calcium-dependent nitric oxide synthase (NOS) was decreased by 35-40% at 12, 28 and 70 days of age. Other neurochemical markers not related to granule cells (the neuronal population selectively compromised by neonatal MAM treatment), choline acetyltransferase (ChAT) and glutamate decarboxylase (GAD) were not decreased, but actually increased when determined as specific activity. In agreement with the decreased catalytic activity measured in the tube, the expression of neuronal NOS protein was attenuated as judged from immunohistochemistry and Western blotting. In the microencephalic forebrain, the specific calcium-dependent NOS activity measured in homogenates of the whole hemisphere was significantly increased as compared to normal animals. Accordingly, immunohistochemistry for neuronal NOS, as well as NADPH-diaphorase histochemistry revealed an apparent increase in the density of strongly reactive neurons in the underdeveloped cortex and striatum of microencephalic rats. The results reported here demonstrate that permanent alterations of neuronal NOS activity and expression occur when the development of the brain and its neuronal circuits are severely compromised. Furthermore, the permanent downregulation of neuronal NOS in the cerebellum of microencephalic rats may be exploited for the study of the role of NO in mechanisms of synaptic plasticity such as long term depression (LTD).

Ischemic and excitotoxic damage to brain slices from normal and microencephalic rats

Brain slices (olfactory cortex, fronto-parietal cortex and hippocampus) taken from normal or microencephalic rats, obtained by gestational administration of the DNA-alkylating agent methylazoxymethanol acetate (MAM), were subjected to in vitro simulated ischemia or exposed to glutamate (5 mM) or kainate (1 mM). All these neurotoxic insults resulted in decreased viability of the slices, as quantitatively assessed by decrease in the rate of protein synthesis. Hippocampal slices subjected to ischemia and olfactory cortex slices exposed to glutamate or kainate were significantly less sensitive to the neurotoxic insult in microencephalic rats than in controls. The increased efflux of neurotransmitter amino acids (glutamate, aspartate and GABA) in the medium from slices subjected to ischemia or exposed to kainate, showed no significant differences among microencephalic and control rats. The present results suggest that the decreased excitotoxic sensitivity of microencephalic rats is, at least in part, related to intrinsic structural and/or functional alterations of some brain regions which undergo decrease in size as a consequence of the gestational treatment.

Absence of excitotoxic neuropathology in microencephalic rats after systemic kainic acid administration

We have s.c. injected with kainic acid (12 mg/kg) normal adult rats as well as rats rendered microencephalic by selectively timed administration of the DNA alkylating agent methylazoxymethanol acetate (MAM) to the mother during pregnancy. Histological examination of the brains revealed that normal animals underwent neurodegeneration in brain regions sensitive to kainic acid excitotoxicity, such as the olfactory cortex and the hippocampus, while no damage was apparent in the same regions of microencephalic rats. Evaluation of the neurotoxic outcome consequent to the excitotoxic stimulation, was quantitatively performed by measuring the levels of appropriate neurochemical markers 15 days after kainic acid injection. In normal animals, this resulted in significant decrease (up to 60% in the olfactory cortex and 30% in the hippocampus) of markers related to glutamatergic and GABAergic neurons, whereas in MAM-treated rats the same markers were not significantly affected, thus demonstrating a substantial protection against the excitotoxic insult in the microencephalic condition.