Comparison of Methods Used to Investigate Coalescence in Emulsions.

We present a study of moderately stable dilute emulsions. These emulsions are models for water contaminated by traces of oil encountered in many water treatment situations. The purification of water and the elimination of oil rely on the emulsion stability. Despite actively being studied, the topic of emulsion stability is still far from being fully understood. In particular, it is still unclear whether experimental methods accessing different length scales lead to the same conclusions. In the study presented in this paper, we have used different methods to characterize the emulsions, such as centrifugation and simple bottle tests, as well as investigations of the collision of single macroscopic oil drops at an oil-water interface. We studied different emulsions containing added polymer or surfactant. In the case of added polymer, centrifugation and single drop experiments led to opposite trends in stability when the polymer concentration is varied. In the case of added surfactant, both centrifugation and single drop experiments show a maximum stability when the surfactant concentration is increased, whereas bottle tests show a monotonous increase in stability. We propose tentative interpretations of these unexpected observations. The apparent contradictions are due to the fact that different methods require different drop sizes or different drop concentrations. The puzzling decrease in emulsion stability at a higher surfactant concentration observed with some methods, however, remains unclear. This coalescence study illustrates the fact that different results can be obtained when different experimental methods are used. It is therefore advisable not to rely on a single method, especially in the case of emulsions of limited stability for reasons explained in the paper.

Nxnl2 splicing results in dual functions in neuronal cell survival and maintenance of cell integrity.

The rod-derived cone viability factors, RdCVF and RdCVF2, have potential therapeutical interests for the treatment of inherited photoreceptor degenerations. In the mouse lacking Nxnl2, the gene encoding RdCVF2, the progressive decline of the visual performance of the cones in parallel with their degeneration, arises due to the loss of trophic support from RdCVF2. In contrary, the progressive loss of rod visual function of the Nxnl2-/- mouse results from a decrease in outer segment length, mediated by a cell autonomous mechanism involving the putative thioredoxin protein RdCVF2L, the second spliced product of the Nxnl2 gene. This novel signaling mechanism extends to olfaction as shown by the progressive impairment of olfaction in aged Nxnl2-/- mice and the protection of olfactory neurons by RdCVF2. This study shows that Nxnl2 is a bi-functional gene involved in the maintenance of both the function and the viability of sensory neurons.

Converging action of alcohol consumption and cannabinoid receptor activation on adult hippocampal neurogenesis.

Alcoholism is characterized by successive periods of abstinence and relapse, resulting from long-lasting changes in various circuits of the central nervous system. Accumulating evidence points to the endocannabinoid system as one of the most relevant biochemical systems mediating alcohol addiction. The endocannabinoid system regulates adult neurogenesis, a form of long-lasting adult plasticity that occurs in a few areas of the brain, including the dentate gyrus. Because exposure to psychotropic drugs regulates adult neurogenesis, it is possible that neurogenesis might be implicated in the pathophysiology, and hence treatment, of neurobiological illnesses related to drugs of abuse. Here, we investigated the sensitivity of adult hippocampal neurogenesis to alcohol and the cannabinoid receptor agonist WIN 55,212-2 (WIN). Specifically, we analysed the potential link between alcohol relapse, cannabinoid receptor activation, and adult neurogenesis. Adult rats were exposed to subchronic alcohol binge intoxication and received the cannabinoid receptor agonist WIN. Another group of rats were subjected to an alcohol operant self-administration task. Half of these latter animals had continuous access to alcohol, while the other half were subjected to alcohol deprivation, with or without WIN administration. WIN treatment, when administered during alcohol deprivation, resulted in the greatest increase in alcohol consumption during relapse. Together, forced alcohol binge intoxication and WIN administration dramatically reduced hippocampal neurogenesis. Furthermore, adult neurogenesis inversely correlated with voluntary consumption of alcohol. These findings suggest that adult hippocampal neurogenesis is a key factor involved in drug abuse and that it may provide a new strategy for the treatment of alcohol addiction and dependence.

Somatostatin contributes to in vivo gamma oscillation modulation and odor discrimination in the olfactory bulb.

Neuropeptides are systematically encountered in local interneurons, but their functional contribution in neural networks is poorly documented. In the mouse main olfactory bulb (MOB), somatostatin is mainly concentrated in local GABAergic interneurons restricted to the external plexiform layer (EPL). Immunohistochemical experiments revealed that the sst2 receptor, the major somatostatin receptor subtype in the telencephalon, is expressed by mitral cells, the MOB principal cells. As odor-activated mitral cells synchronize and generate gamma oscillations of the local field potentials, we investigated whether pharmacological manipulations of sst2 receptors could influence these oscillations in freely behaving mice. In wild-type, but not in sst2 knock-out mice, gamma oscillation power decreased lastingly after intrabulbar injection of an sst2-selective antagonist (BIM-23627), while sst2-selective agonists (octreotide and L-779976) durably increased it. Sst2-mediated oscillation changes were correlated with modifications of the dendrodendritic synaptic transmission between mitral and granule cells. Finally, bilateral injections of BIM-23627 and octreotide respectively decreased and increased odor discrimination performances. Together, these results suggest that endogenous somatostatin, presumably released from EPL interneurons, affects gamma oscillations through the dendrodendritic reciprocal synapse and contributes to olfactory processing. This provides the first direct correlation between synaptic, oscillatory, and perceptual effects induced by an intrinsic neuromodulator.

Turnover of newborn olfactory bulb neurons optimizes olfaction.

Postdevelopmental neurogenesis occurs in the olfactory bulb (OB), to which new interneurons are continuously recruited. However, only a subset of the adult-generated interneurons survives, as many undergo programmed cell death. As part of homeostatic processes, the removal of new neurons is required alongside the addition of new ones, to ensure a stable neuron number. In addition to a critical role in tissue maintenance, it is still unclear whether this neuronal elimination affects the functioning of adult circuits. Using focal drug delivery restricted to the OB, we investigated the significance of programmed cell death in the adult OB circuits. Cell death was effectively blocked by the broad-spectrum caspase inhibitor benzyloxycarbonyl-Val-Ala-Asp(OMe)-fluoromethylketone (zVAD). The zVAD effect differed with newborn interneuron location, either in the superficial (periglomerular cells) or in the deep (granule cells) OB layers. Furthermore, whereas sensory experience potentiated the effect of zVAD on the survival of new granule cells, it had no additional effect on the survival of new periglomerular cells. Thus, distinct mechanisms control the survival/elimination decision of newborn interneuron subtypes. However, zVAD had no effect on the olfactory sensory neurons projecting to the bulb. Remarkably, psychophysical analyzes revealed that a normal rate of new neuron elimination was essential for optimal odorant exploration and discrimination. This study highlights the importance of cell elimination for adjusting olfactory performance. We conclude that adult-generated OB interneurons are continually turned over, rather than simply added, and the precise balance between new and mature interneurons, set through active selection/elimination processes, is essential for optimizing olfaction.

Centrifugal drive onto local inhibitory interneurons of the olfactory bulb.

The olfactory bulb is known to receive signals from sensory neurons and to convey them to higher processing centers. However, in addition to relaying sensory information to the cortex, the olfactory bulb is actively involved in sensory information processing. Hence, olfactory sensory inputs generate a reproducible spatial pattern of restricted activation in the glomerular layer that is subsequently transformed into highly distributed patterns by lateral interactions between output relay neurons and diverse types of local interneurons. Odor representation is thus highly dynamic and temporally orchestrated, right from the first central relay of the olfactory system. This major function of the olfactory bulb is subject to extensive local and extrinsic synaptic influences. The external (or centrifugal) inputs include the dense innervations preferentially targeting the granule cells of the olfactory bulb. The continuous arrival of newly generated neurons in the olfactory bulb of adults provides another source of plasticity influencing the olfactory circuitry. This review deals with the neuromodulation of granule cell activity and of the continuous recruitment of these cells throughout life.

Learning and survival of newly generated neurons: when time matters.

New interneurons are continually added to the olfactory bulb (OB), the first central relay for processing olfactory information, throughout life. It remains unknown how these adult-generated interneurons integrate into preexisting networks or die. We used immunohistochemical approaches to quantify adult neurogenesis in mice subjected to olfactory training. We identified a critical period in the life of an adult-generated OB interneuron, during which learning triggers distinct consequences. Using a discrimination learning task performed at various times after the birth of new interneurons, we found that olfactory training could increase, decrease, or have no effect on the number of surviving newly generated neurons. Cell survival and elimination depend on both the age of the cell and its location within the granule cell layer. This study provides new insight into the contribution of the newly generated interneurons to OB function. It demonstrates that neuronal elimination is an active process, rather than a simple consequence of nonuse.