On the Respective Roles of Nitric Oxide and Carbon Monoxide in Long-Term Potentiation in the Hippocampus

NO, CO and H2S: A Trinacrium of Bioactive Gases in the Brain

Oxygen and carbon dioxide are time honored gases that have direct bearing on almost all life forms, but over the past thirty years, and in large part due to the Nobel Prize Award in Medicine for the elucidation of nitric oxide (NO) as a bioactive gas, the research and medical communities now recognize other gases as critical for survival. In addition to NO, hydrogen sulfide (H₂S) and carbon monoxide (CO) have emerged as a triumvirate or Trinacrium of gases with analogous importance and that serve important homeostatic functions. Perhaps, one of the most intriguing aspects of these gases is the functional interaction between them, which is intimately linked by the enzyme systems that produce them. Despite the need to better understand NO, H₂S and CO biology, the notion that these are environmental pollutants remains ever present. For this reason, incorporating the concept of hormesis becomes imperative and must be included in discussions when considering developing new therapeutics that involve these gases. While there is now an enormous literature base for each of these gasotransmitters, we provide here an overview of their respective physiologic roles in the brain.

Two sides to long-term potentiation: a view towards reconciliation

Almost since the discovery of long-term potentiation (LTP) in the hippocampus, its locus of expression has been debated. Throughout the years, convincing evidence has accumulated to suggest that LTP can be supported either presynaptically, by an increase in transmitter release, or postsynaptically, by an increase in α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor number. However, whereas postsynaptic enhancement appears to be consistently obtained across studies following LTP induction, presynaptic enhancement is not as reliably observed. Such discrepancies, along with the failure to convincingly identify a retrograde messenger required for presynaptic change, have led to the general view that LTP is mainly supported postsynaptically, and certainly, research within the field for the past decade has been heavily focused on the postsynaptic locus. Here, we argue that LTP can be expressed at either synaptic locus, but that pre- and postsynaptic forms of LTP are dissociable phenomena mediated by distinct mechanistic processes, which are sensitive to different patterns of neuronal activity. This view of LTP helps to reconcile discrepancies across the literature and may put to rest a decades-long debate.

Correlation between hippocampal levels of neural, epithelial and inducible NOS and spatial learning skills in rats

In the present study, to better understand the role of different nitric oxide synthase (NOS) isoforms in hippocampus-dependent forms of learning, we examined the expression of neural, endothelial, and inducible NOS in the hippocampus of young-adult rats classified as "poor" and "good" learners on the basis of their performance in the partially baited 12-arm radial maze. Taking into consideration strain-dependent differences in learning skills and NOS expression, experiments were performed on two different lines of laboratory rats: the inbred Wistar (W) and the outcrossed Wistar/Spraque-Dawley (W/S) line. The hippocampal levels of NOS proteins were assessed by Western Blotting. In the present study, genetically more homogenous W rats showed a slower rate of learning compared to the genetically less homogenous outcrossed W/S rats. The deficient performance in the W rat group compared to outcrossed W/S rats, and in "poor" learners of both groups compared to "good" learners was due to a higher percentage of reference memory errors. The overall NOS levels were significantly higher in W group compared to outcrossed W/S rats. In both rat lines, the rate of learning positively correlated with hippocampal levels of nNOS and negatively correlated with iNOS levels. Hippocampal eNOS levels correlated negatively with animals' performance but only in the W rats. These results suggest that all 3 NOS isoforms are implemented but play different roles in neural signaling.

The Protective Effect of Black Ginseng Against Transient Focal Ischemia-induced Neuronal Damage in Rats

Black ginseng (BG) has been widely used as herbal treatment for improving physiological function. In order to investigate the neuroprotective action of this herbal medicine, we examined the influence of BG on the learning and memory of rats using the Morris water maze, and we studied the effects of BG on the central cholinergic system and neural nitric oxide synthesis in the hippocampus of rats with neuronal and cognitive impairment. After middle cerebral artery occlusion was applied for 2h, the rats were administered BG (100 or 400 mgkg(-1), p.o.) daily for 2 weeks, followed by training and performance of the Morris water maze test. The rats with ischemic insults showed impaired learning and memory on the tasks. Treatment with BG produced improvement in the escape latency to find the platform. Further, the BG groups showed a reduced loss of cholinergic immunoreactivity and nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d)-positive neurons in the hippocampus compared to that of the ISC group. These results demonstrated that BG has a protective effect against ischemia-induced neuronal and cognitive impairment. Our results suggest that BG might be useful for the treatment of vascular dementia.

Nitric oxide contributes to learning and memory deficits observed in hypothyroid rats during neonatal and juvenile growth

INTRODUCTION

Severe cognitive impairment follows thyroid hormone deficiency during the neonatal period. The role of nitric oxide (NO) in learning and memory has been widely investigated.

METHODS

This study aimed to investigate the effect of hypothyroidism during neonatal and juvenile periods on NO metabolites in the hippocampi of rats and on learning and memory. Animals were divided into two groups and treated for 60 days from the first day of lactation. The control group received regular water, whereas animals in a separate group were given water supplemented with 0.03% methimazole to induce hypothyroidism. Male offspring were selected and tested in the Morris water maze. Samples of blood were collected to measure the metabolites of NO, NO2, NO3 and thyroxine. The animals were then sacrificed, and their hippocampi were removed to measure the tissue concentrations of NO2 and NO3.

DISCUSSION

Compared to the control group's offspring, serum thyroxine levels in the methimazole group's offspring were significantly lower (P<0.01). In addition, the swim distance and time latency were significantly higher in the methimazole group (P<0.001), and the time spent by this group in the target quadrant (Q1) during the probe trial was significantly lower (P<0.001). There was no significant difference in the plasma levels of NO metabolites between the two groups; however, significantly higher NO metabolite levels in the hippocampi of the methimazole group were observed compared to controls (P<0.05).

CONCLUSION

These results suggest that the increased NO level in the hippocampus may play a role in the learning and memory deficits observed in childhood hypothyroidism; however, the precise underlying mechanism(s) remains to be elucidated.

Ryanodine receptors contribute to cGMP-induced late-phase LTP and CREB phosphorylation in the hippocampus

We previously found that the nitric oxide (NO)-cGMP-cGMP-dependent protein kinase (PKG) signaling pathway acts in parallel with the cAMP-cAMP-dependent protein kinase (PKA) pathway to produce protein and RNA synthesis-dependent late-phase long-term potentiation (L-LTP) and cAMP response element-binding protein (CREB) phosphorylation in the CA1 region of mouse hippocampus. We have now investigated the possible involvement of a downstream target of PKG, ryanodine receptors. L-LTP can be induced by either multiple-train tetanization, NO or 8-Br-cGMP paired with one-train tetanization, or the cAMP activator forskolin, and all three types of potentiation are accompanied by an increase in phospho-CREB immunofluorescence in the CA1 cell body area. Both the potentiation and the increase in phospho-CREB immunofluorescence induced by multiple-train tetanization or 8-Br-cGMP paired with one-train tetanization are reduced by prolonged perfusion with ryanodine, which blocks Ca(2+) release from ryanodine-sensitive Ca(2+) stores. By contrast, neither the potentiation nor the increase in immunofluorescence induced by forskolin are reduced by depletion of ryanodine and inositol-1,4,5-triphosphate (IP3)-sensitive Ca(2+) stores. These results suggest that NO, cGMP, and PKG cause release of Ca(2+) from ryanodine-sensitive stores, which in turn causes phosphorylation of CREB in parallel with PKA during the induction of L-LTP.

Specific spatial learning deficits become severe with age in beta -amyloid precursor protein transgenic mice that harbor diffuse beta -amyloid deposits but do not form plaques

Memory impairment progressing to dementia is the main clinical symptom of Alzheimer's disease (AD). AD is characterized histologically by the presence of beta-amyloid (Abeta) plaques and neurofibrillary tangles in specific brain regions. Although Abeta derived from the Abeta precursor protein (beta-APP) is believed to play a central etiological role in AD, it is not clear whether soluble and/or fibrillar forms are responsible for the memory deficit. We have generated and previously described mice expressing human wild-type beta-APP(751) isoform in neurons. These transgenic mice recapitulate early histopathological features of AD and form Abeta deposits but no plaques. Here we describe a specific and progressive learning and memory impairment in these animals. In the Morris water maze, a spatial memory task sensitive to hippocampal damage, one pedigree already showed significant differences in acquisition in 3-month-old mice that increased in severity with age and were expressed clearly in 6-month- and 2-year-old animals. The second transgenic pedigree displayed a milder impairment with a later age of onset. Performance deficits significantly decreased during the 6 days of training in young but not in aged transgenic animals. Both pedigrees of the transgenic mice differed from wild-type mice by less expressed increase of escape latencies after the platform position had been changed in the reversal experiment and by failure to prefer the goal quadrant in probe trials. Both pedigrees performed at wild-type level in a number of other tests (open field exploration and passive and active place avoidance). The results suggest that plaque formation is not a necessary condition for the neuronal beta-APP(751) transgene-induced memory impairment, which may be caused by beta-APP overexpression, isoform misexpression, or elevated soluble Abeta.