Adult-born dentate granule cell excitability depends on the interaction of neuron age, ontogenetic age and experience.

Early during their maturation, adult-born dentate granule cells (aDGCs) are particularly excitable, but eventually develop the electrophysiologically quiet properties of mature cells. However, the stability versus plasticity of this quiet state across time and experience remains unresolved. By birthdating two populations of aDGCs across different animal ages, we found for 10-month-old rats the expected reduction in excitability across cells aged 4-12 weeks, as determined by Egr1 immunoreactivity. Unexpectedly, cells 35 weeks old (after genesis at an animal age of 2 months) were as excitable as 4-week-old cells, in the dorsal hippocampus. This high level of excitability at maturity was specific for cells born in animals 2 months of age, as cells born later in life did not show this effect. Importantly, excitability states were not fixed once maturity was gained, but were enhanced by enriched environment exposure or LTP induction, indicating that any maturational decrease in excitability can be compensated by experience. These data reveal the importance of the animal's age for aDGC excitability, and emphasize their prolonged capability for plasticity during adulthood.

Changes in brain amine levels associated with the morphological and behavioural development of the worker honeybee.

Changes in biogenic amine levels associated with the morphological and behavioural development of the worker honeybee are examined. A significant increase in amine levels in the head of the honeybee is associated with transition from the larval to pupal stage. Adult emergence is also accompanied by a significant increase in 5-HT levels in the brain, but no significant change in brain dopamine (DA) levels. NADA (N-acetyldopamine) levels increase during larval and pupal development, but in contrast to both DA and 5-HT, drop significantly during the transition from pupa to adult. Levels of DA in the brain of nectar and pollen forager bees, presumed to be among the oldest adults sampled, were found to be significantly higher than in nurses, undertakers or food storers. These results suggest that an age-dependent change in amine levels occurs in the brain of the worker bee. In the optic lobes, levels of DA and 5-HT were found to be significantly higher in pollen forager bees than in all other behavioural groups. Significant differences in amine levels in the optic lobes of nectar foragers and pollen foragers indicate that some differences in amine levels occur independent of worker age. The functional significance of differences in brain amine levels and whether or not biogenic amines play a direct role in the control of honeybee behaviour has yet to be established.

Differences between rats and mice in MDMA (methylenedioxymethylamphetamine) neurotoxicity.

In both rats and mice a single large dose of methylenedioxymethylamphetamine (MDMA; 25 mg/kg i.p.) caused a fall 3 h after injection in the content of 5-hydroxytryptamine (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) in cortex, a fall in noradrenaline in hippocampus and cerebellum, and a rise in dopamine (DA) but fall in dihydroxyphenylacetic acid (DOPAC) in striatum. These effects were transient, levels being essentially back to normal by 24 h after injection. Repeated large doses (3 x 25 mg/kg in 24 h) of MDMA caused a large long-lasting fall in the content of 5-HT and 5-HIAA in cortex in rats but had only a slight effect in mice. Increasing the dose to 3 x 50 mg/kg in mice produced a large long-lasting fall in striatal DA. The analogue MDEA(3,4-methylenedioxyethylamphetamine) caused a similar slight fall in 5-HT but in contrast to MDMA caused a slight rise in DA content in mice. The nature and degree of neurotoxicity with methylenedioxyamphetamines appears to be drug and species-specific.

Comparative effects of ethanol and other depressant drugs on membrane order in rat synaptosomes using ESR spectroscopy.

The effects of ethanol, t-butanol and pentobarbitone on the membrane order of rat synaptosomal membranes have been compared using 3 spin-label probes, 5-doxyl-stearic acid which reports from a lipid site near the membrane surface, 16-doxyl-stearic acid which reports from a deeper lipid site, and maleimide-TEMPO which covalently binds to membrane protein. The sensitivity of the membrane proteins to a fluidizing effect of ethanol was increased by lowering the concentration of protein-binding probe. Significant decreases in membrane order were observed at anaesthetic concentrations of ethanol and t-butanol with all three probes; pentobarbitone produced a similar effect but only at very high concentrations. Pentobarbitone caused a marked change in high-field peak shape of the 16-doxyl-stearic acid spectra at anaesthetic concentrations; this effect was seen slightly with t-butanol and trichlorethanol but not with ethanol. These studies indicate that the membrane sites of action of ethanol and pentobarbitone as shown by ESR probes are different.

The use of 2,2,6,6-tetramethyl-4-maleimido-piperidin-1-oxyl in electron paramagnetic resonance spin label studies of drug interactions with erythrocyte membranes.

The electron spin resonance (EPR) spectrum of erythrocyte membranes labeled with 2,2,6,6-tetra-methyl-4-maleimido-piperidin-1-oxyl (MAL-6) indicates both weakly and strongly immobilized labeling sites. Changes in the ratio of the membrane's low field spectral peaks (W/S) have often been used to monitor drug-erythrocyte interactions. We have investigated a number of experimental factors that may influence this ratio even in the absence of drug. Instrumental settings on the EPR spectrometer had no obvious effect. As the weight ratio of label/protein decreased, the W/S ratio increased. Similarly an increase in labeling time and temperature lead to an increase in the ratio. The ratio also increased with time after labeling; this change was most marked in samples kept at 37 degrees C, but was insignificant in samples kept at 4 degrees C. Increasing the viscosity of the sample with low-molecular-weight substances such as sucrose or glycerol, but not with those with high molecular weight such as dextran and PVP, caused a reduction in the ratio. Increasing the pH and changing the buffer concentrations also lead to a small increase in W/S. These results suggest that it is very important that all of these factors be kept constant and at some optimal value if reliable and consistent results are to be obtained using this method to monitor drug-erythrocyte interactions.

ESR measurements on the effects of ethanol on the lipid and protein conformation in biological membranes.

The effects of ethanol on the conformation of proteins or fluidity of lipids was studied in human erythrocyte ghosts and rat brain synaptosomal membranes. A maleimide nitroxide probe (MAL-6) was coupled to membrane protein, or 5-doxylstearic (5NS) was dissolved in the membrane lipid, and the electron spin resonance (ESR) spectra were recorded in the presence of increasing ethanol concentrations and at different temperatures from 4 degrees to 37 degrees. The lipid probe at all temperatures studied showed an increase in fluidity in the membrane lipid when ethanol was added in concentration over 20 mM. The protein probe however required a high concentration of ethanol (200 mM) to produce an increase in the rigidity of the protein conformation of 4 degrees in both erythrocyte and synaptosomal membranes. A decrease in protein rigidity was observed at high ethanol concentrations at 37 degrees only in the erythrocytes. Thus an effect of ethanol on the conformation of membrane protein was observed using MAL-6 only at high ethanol concentrations and depended on the membrane used and the temperature of measurement.