Detecting spatial memory deficits beyond blindness in tg2576 Alzheimer mice.

The retinal degeneration Pde6b(rd1) (rd) mutation can be a major pitfall in behavioral studies using tg2576 mice bred on a B6:SJL genetic background, 1 of the most widely used models of Alzheimer's disease. After a pilot study in wild type mice, performance of 8- and 16-month-old tg2576 mice were assessed in several behavioral tasks with the challenge of selecting 1 or more task(s) showing robust memory deficits on this genetic background. Water maze acquisition was impossible in rd homozygotes, whereas Y-maze alternation, object recognition, and olfactory discrimination were unaffected by both the transgene and the rd mutation. Spatial memory retention of 8- and 16-month-old tg2576 mice, however, was dramatically affected independently of the rd mutation when mice had to recognize a spatial configuration of objects or to perform the Barnes maze. Thus, the latter tasks appear extremely useful to evaluate spatial memory deficits and to test cognitive therapies in tg2576 mice and other mouse models bred on a background susceptible to visual impairment.

Spatial reference- (not working- or procedural-) memory performance of aged rats in the water maze predicts the magnitude of sulpiride-induced facilitation of acetylcholine release by striatal slices.

Cluster analysis of water-maze reference-memory performance distinguished subpopulations of young adult (3-5 months), aged (25-27 months) unimpaired (AU) and aged impaired (AI) rats. Working-memory performances of AU and AI rats were close to normal (though young and aged rats differed in exploration strategies). All aged rats showed impaired procedural-memory. Electrically evoked release of tritium was assessed in striatal slices (preloaded with [(3)H]choline) in the presence of oxotremorine, physostigmine, atropine+physostigmine, quinpirole, nomifensine or sulpiride. Aged rats exhibited reduced accumulation of [(3)H]choline (-30%) and weaker transmitter release. Drug effects (highest concentration) were reductions of release by 44% (oxotremorine), 72% (physostigmine), 84% (quinpirole) and 65% (nomifensine) regardless of age. Sulpiride and atropine+physostigmine facilitated the release more efficiently in young rats versus aged rats. The sulpiride-induced facilitation was weaker in AI rats versus AU rats; it significantly correlated with reference-memory performance. The results confirm age-related alterations of cholinergic and dopaminergic striatal functions, and point to the possibility that alterations in the D(2)-mediated dopaminergic regulation of these functions contribute to age-related reference-memory deficits.

Distinct molecular targets for the central respiratory and cardiac actions of the general anesthetics etomidate and propofol.

General anesthetics are among the most widely used and important therapeutic agents. The molecular targets mediating different endpoints of the anesthetic state in vivo are currently largely unknown. The analysis of mice carrying point mutations in neurotransmitter receptor subunits is a powerful tool to assess the contribution of the respective receptor subtype to the pharmacological actions of clinically used general anesthetics. We examined the involvement of beta3-containing GABA(A) receptors in the respiratory, cardiovascular, hypothermic, and sedative actions of etomidate and propofol using beta3(N265M) knock-in mice carrying etomidate- and propofol-insensitive beta3-containing GABA(A) receptors. Although the respiratory depressant action of etomidate and propofol, as determined by blood gas analysis, was almost absent in beta3(N265M) mice, the cardiac depressant and hypothermic effects, as determined by radiotelemetry, and the sedative effect, as determined by decrease of motor activity, were still present. Taken together with previous findings, our results show that both immobilization and respiratory depression are mediated by beta3-containing GABA(A) receptors, hypnosis by both beta3- and beta2-containing GABA(A) receptors, while the hypothermic, cardiac depressant, and sedative actions are largely independent of beta3-containing GABA(A) receptors.

Cognitive deficits in aged rats correlate with levels of L-arginine, not with nNOS expression or 3,4-DAP-evoked transmitter release in the frontoparietal cortex.

Aging is associated with altered neurotransmitter function in the brain. In this study, we measured release parameters for acetylcholine (ACh), norepinephrine and serotonin in the frontoparietal cortex of young and aged rats. We also determined cortical amino acid concentrations and nitric oxide (NO) synthase function. Prior to sacrifice, the rats had been tested for Morris water-maze performance. In aged, compared with young rats, we observed a reduction in both uptake of choline and acetylcholine release. Serotonin release and L-arginine concentrations (a precursor of NO) showed an aging-related increase; however, L-citrulline/L-arginine ratios were decreased in aged rats. Moreover, while most age-related changes in transmitter release or neurochemical markers were not related to the learning performance, L-arginine concentrations were positively correlated to cognitive deficits. NO synthase concentrations were not affected by aging. It is suggested that events related to L-arginine-to-L-citrulline/NO metabolism in the frontoparietal cortex may take part in age-related cognitive deficits.

Activation of the cGMP pathway in dopaminergic structures reduces cocaine-induced EGR-1 expression and locomotor activity.

Nitric oxide (NO) and the C-type natriuretic peptide (CNP) exert their action on brain via the cGMP signaling pathway. NO, by activating soluble guanylyl cyclase, and CNP, by stimulating membrane-bound guanylyl cyclase, cause intracellular increases of cGMP, activating cGMP-dependent protein kinases (PKGs). We show here that injection of CNP into the rat ventral tegmental area strongly reduced cocaine-induced egr-1 expression in the nucleus accumbens in a dose-dependent manner. The effect of CNP was reversed by the previous injection of a selective PKG inhibitor, KT5823. Activation of PKG by 8-bromo-cGMP reduced, like CNP, cocaine-induced gene transcription in dopaminergic structures. To confirm the involvement of PKG, this was overexpressed in either the mesencephalon or the caudate-putamen. Using the polyethyleneimine delivery system, an active protein was expressed by injecting a plasmid vector containing the human PKG-Ialpha cDNA. PKG was overexpressed in dopaminergic and GABAergic neurons when the plasmid was injected in the ventral tegmental area, whereas overexpression was observed in medium spiny GABAergic neurons and in both cholinergic and GABAergic interneurons when the PKG vector was injected into the caudate-putamen. Activation of the overexpressed PKG reduced cocaine-induced egr-1 expression in dopaminergic structures and affected behavior (i.e., locomotor activity). These effects were again reversed by previous injection of the selective PKG inhibitor. The current data suggest that NO and the neuropeptide CNP are potential regulators of cocaine-related effects on behavior.

3,4-DAP-evoked transmitter release in hippocampal slices of aged rats with impaired memory.

Based on a slice superfusion technique, this study investigated the release of acetylcholine, noradrenaline and serotonin in the hippocampus of aged rats (25-27 months) showing no or severe deficits in a spatial reference-memory task (water maze). Young adults (3-5 months) were used as controls. 3,4-diaminopyridine (3,4-DAP), a potassium channel antagonist which increases neuronal excitability, was used to evoke the overflow of the three neurotransmitters. The release of [3H]noradrenaline induced by stimulation of presynaptic nicotinic receptors was also assessed. The experiment compared the accumulation and 3,4-DAP-evoked (or nicotine-evoked) overflow of [3H] in hippocampal slices preincubated with [3H]choline, [3H]noradrenaline, or [3H]serotonin. In aged rats, only the accumulation of [3H]serotonin was reduced significantly (-17%). In percent of tissue-[3H], the 3,4-DAP-evoked overflow of [3H]serotonin was increased (+28%), and that of [3H]acetylcholine was reduced (-23%) in the aged rats. The nicotine-evoked overflow of [3H]noradrenaline was not altered in aged rats. There was a significant correlation of water-maze performance (distance to platform) and evoked overflow of [3H]serotonin. It is concluded that hippocampal cholinergic functions are more altered by aging than noradrenergic or serotonergic ones. Excessive excitability of serotonergic terminals, perhaps in addition to cholinergic dysfunction, might be a crucial factor accounting for age-related cognitive deficits in the present population of rats.

Baseline and 8-OH-DPAT-induced release of acetylcholine in the hippocampus of aged rats with different levels of cognitive dysfunction.

During aging, neurotransmission systems such as the cholinergic and serotonergic ones are altered. Using rats aged 3 or 24-26 months, this study investigated whether the well-described 8-OH-DPAT-induced increase of hippocampal acetylcholine release was altered in aged rats and whether it may vary according to the magnitude of age-related cognitive deficits. Long-Evans female rats aged 24-26 months were classified as good or bad performers on the basis of their reference-memory performance in a Morris water-maze task. Subsequently, the efficiency of 5-HT(1A) receptor agonist 8-OH-DPAT (0.5 mg/kg, s.c.) in triggering hippocampal acetylcholine release was evaluated by in vivo microdialysis and high performance liquid chromatography analysis. Besides a reduced baseline release in aged rats and a correlation between the baseline release and probe-trial performance in all rats, the results demonstrated that 8-OH-DPAT produced a significant increase of hippocampal acetylcholine release (peak value) in all rats, whether aged or young. While significant in bad performers (+56%), this increase did not reach significance in good performers (+32%). The results suggest that (i) some aspects of cognitive alterations related to aging might be linked to the baseline release of acetylcholine in the hippocampus, and (ii) the cholinergic innervation of the hippocampus of aged rats responds almost normally to systemic activation of 5-HT(1A) receptors, and (iii) differential alterations of cholinergic/serotonergic interactions assessed by determination of the 8-OH-DPAT-induced release of acetylcholine in the hippocampus could not be linked with clarity to the cognitive status of aged rats.