Retinal axons are led out of the eye by netrin-1, an attractive guidance cue which is secreted at the optic nerve head. In the optic pathway, however, netrin-1 is expressed in areas that exclude ...retinal axon growth. This suggests that axons may change in their responsiveness to netrin-1 as they advance along the pathway. Indeed, in our 'whole-pathway' preparation in Xenopus, a gradual change from attraction to repulsion occurred as retinal axons emerged from progressively distal points along the pathway. We also found that axons that were aged in culture without pathway experience underwent a similar change, which correlated with a decline in cyclic AMP (cAMP) and netrin-1 receptor expression. Cyclic AMP elevators and adenosine A2b receptor agonists rejuvenated the behavior of old growth cones, causing them to regain attraction to netrin-1, whereas antagonists caused young growth cones to be repelled. These findings show that netrin-1 responsiveness is developmentally regulated and suggest that intrinsic changes that lower cAMP levels underlie this regulation.
The failure of axon regeneration in the injured adult CNS has been ascribed to axon growth inhibitory molecules expressed by the resident glial cell populations, especially oligodendrocytes. Unlike ...their adult counterparts, however, early embryonic neurons are able to send lengthy axons through myelinated fiber tracts when transplanted into the adult brain. One explanation is that they have yet to express receptors for factors that inhibit the growth of older neurons. To test this possibility, we have used the cryoculture technique to study the regeneration of rat central and peripheral neurons, over a developmental period that encompasses the stages before, during, and after target contact, when cultured on either unmyelinated (neonatal) or myelinated (adult) optic nerve tissue sections. Early embryonic (days 14-15) retinal ganglion cells extended neurites on neonatal optic nerve, but few grew on adult optic nerve. In the case of early embryonic dorsal root ganglion neurons, however, neurite outgrowth on either neonatal optic nerve or on adult optic nerve was extensive. This response declined sharply with age. In contrast, neurite outgrowth by dorsal root ganglion neurons on laminin substrata remained relatively constant (> 80% extended neurites) over the same period. This suggests that (a) inhibition of neurite outgrowth within the optic nerve is mediated not only by oligodendrocytes, but also by molecules expressed prior to the onset of myelination; (b) neurons acquire receptors for these inhibitors only late in embryonic development; (c) differences exist between developing central and peripheral neurons in the response to myelin-associated axon-growth inhibitors.
A decline in developing neuronal cAMP levels appears to render mammalian axons susceptible to growth inhibitory factors in the damaged CNS. cAMP elevation enhances axon regeneration, but the cellular ...mechanisms involved have yet to be fully elucidated. Epac has been identified as a signaling protein that can be activated by cAMP independently of PKA, but little is known of its expression or role in the nervous system. We report that Epac expression is developmentally regulated in the rat nervous system, and that activation of Epac promotes DRG neurite outgrowth and is as effective as cAMP elevation in promoting neurite regeneration on spinal cord tissue. Additionally, siRNA mediated knockdown of Epac reduces DRG neurite outgrowth, prevents the increased growth promoted by cAMP elevation and also diminishes the ability of embryonic neurons to grow processes on spinal cord tissue. Furthermore, we show that asymmetric activation of Epac promotes attractive growth cone turning in a similar manner to cAMP activation. We propose that Epac plays a role in mediating cAMP-dependent axon growth and guidance, and may provide an important target for inducing axon regeneration
in vivo.
Growing axons are guided by both diffusible and substrate-bound factors. Growth cones of retinal neurons exhibit chemoattractive turning towards the diffusible factor netrin-1 in vitro and are guided ...into the optic nerve head (ONH) by localized netrin-1 (ref. 5). Here we report that, in Xenopus, laminin-1 from the extracellular matrix (ECM), converts netrin-mediated attraction into repulsion. A soluble peptide fragment of laminin-1 (YIGSR) mimics this laminin-induced conversion. Low levels of cyclic AMP in growth cones also lead to the conversion of netrin-induced attraction into repulsion, and we show that the amount of cAMP decreases in the presence of laminin-1 or YIGSR, suggesting a possible mechanism for laminin's effect. At the netrin-1-rich ONH, where axons turn sharply to leave the eye, laminin-1 is confined to the retinal surface. Repulsion from the region in which laminin and netrin are coexpressed may help to drive axons into the region where only netrin is present, providing a mechanism for their escape from the retinal surface. Consistent with this idea, YIGSR peptides applied to the developing retina cause axons to be misdirected at the ONH. These findings indicate that ECM molecules not only promote axon outgrowth, but also modify the behaviour of growth cones in response to diffusible guidance cues.
While prison is recognised as a setting for infectious disease transmission among drug users, little is known about psychological and situational factors influencing high-risk behaviours, knowledge ...vital to prison-based interventions. Qualitative interview and focus group data were collected from staff and prisoners in six Scottish prisons. A general view was that prison heroin use had increased, but injecting and sharing remained a covert and minority behaviour. "Anti-injecting culture" among staff and most prisoners emerged as an important factor, though not linked by prisoners to an "anti-drug culture". Of individual and social risk factors identified, only the desire to inject in prison for maximum effect was unique to prison injectors and sharers. This decision-based behaviour requires further theory-focussed research. Given these findings, introducing needle exchanges into Scottish prisons could undermine their low drug injection rates. Enabling injecting, albeit within a public health framework, conflicts with the major prison objective of rehabilitation.
Objectives. This longitudinal study focused on 126 long‐term heroin users who had never been in specialist treatment for use of any drug. The primary aim of the study was to assess whether this ...‘hidden’ population resembled heroin users identified with drug treatment agencies, or alternatively, to test whether heroin could indeed be used in a controlled, non‐intrusive fashion for an extended period of time.
Design and methods. Recruitment was achieved through chain‐referred purposive sampling methods, and data were collected through two semi‐structured interviews. 67% of participants were re‐recruited for follow‐up.
Results. Participants had levels of occupational status and educational achievement comparable to that in the general UK population, and considerably higher than typically found in heroin research. At the conclusion of the study, six participants had entered treatment. While there was evidence of intensive risky patterns of drug use among the sample, there was equal evidence for planned, controlled patterns of use. Some drug‐related negative health and social outcomes had occurred on a lifetime basis, but ongoing problems were rare, and heroin was not a significant predictor in either context. In contrast to typical samples of heroin users, high levels of negative health and social outcomes did not appear to be inevitable within this sample. Frequency of heroin use was predicted by attributional items, indicating the importance of psychological factors in drug use and addiction.
Conclusions. Drug research should more fully incorporate previously hidden populations to more fully inform theory and practice. The pharmacological properties of specific substances should not be assumed to inevitably lead to addictive and destructive patterns of drug use.
In the immune system, mCD24, the mouse homolog of the human glycosyl phosphatidylinositol-anchored glycoprotein CD24, may play a role in cell adhesion. In the nervous system, the function of mCD24 ...has not been determined, but its transient expression by neurons suggests that it may be involved in axon growth in development. Here we show that retinal ganglion cells (RGCs) and dorsal root ganglion (DRG) neurons express mCD24 in the developing but not adult mouse in vivo and in DRG neurons of the injured adult peripheral nervous system (PNS). In vitro, mCD24 was expressed by immature neurons and by a subpopulation of adult DRG neurons. To analyze the possible function of mCD24 in the nervous system, we prepared rat C6 glioma cells stably transfected or retrovirally infected with mCD24 cDNA. The cells did not exhibit changes in their adhesive properties or cell division rate after transfection or infection. When mCD24-expressing cells were used as monolayer substrates for culturing RGCs and DRG neurons, neurite outgrowth was inhibited, depending on neuronal age and on the relative levels of mCD24 in the monolayer. This inhibition, however, was not dependent on the expression of mCD24 by the neurons themselves, because DRG neurons of a mouse deleted of the mCD24 gene showed the same response. These results show that mCD24 interacts in a heterophilic manner with a developmentally regulated molecule expressed by neurons, and they suggest that in vivo, mCD24 may inhibit the further extension or collateral branching of axons in late embryonic development.
In
Xenopus tadpoles, all retinal ganglion cells (RGCs) send axons contralaterally across the optic chiasm. At metamorphosis, a subpopulation of EphB-expressing RGCs in the ventrotemporal retina begin ...to project ipsilaterally. However, when these metamorphic RGCs are grafted into embryos, they project contralaterally, suggesting that the embryonic chiasm lacks signals that guide axons ipsilaterally. Ephrin-B is expressed discretely at the chiasm of metamorphic but not premetamorphic
Xenopus. When expressed prematurely in the embryonic chiasm, ephrin-B causes precocious ipsilateral projections from the EphB-expressing RGCs. Ephrin-B is also found in the chiasm of mammals, which have ipsilateral projections, but not in the chiasm of fish and birds, which do not. These results suggest that ephrin-B/EphB interactions play a key role in the sorting of axons at the vertebrate chiasm.
The central proposition of this article is that if people are thoughtful, well-prepared and aware of the means and best environments for using a particular drug, then the risks associated with the ...use of a particular drug - any drug - can be minimal. The types of drugs discussed in this context focus on those assumed to be the most 'addictive' - heroin and cocaine - to those less well-known but potentially more hazardous to use without prior knowledge and preparation - such as Yage and Fly Agaric. This proposition is discussed in the context of different definitions of relatively non-problematic patterns of drug use, specifically: controlled, recreational and unobtrusive. It is concluded that while the effect of taking a particular drug is a primary motivation to the user, the role of set and setting are of fundamental importance in ensuring that the effects of that drug are as intended for and expected by the user.
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