Visual processing in the leech central nervous system

In animals with complex visual systems spatial contrast is enhanced by mutual inhibition between retinal neurones monitoring different fields. By analogy, if animals like leeches and some caterpillars, that have several simple (non-image-forming) eyes aimed in different directions, are capable of rudimentary form detection, one might predict mutual antagonism between eyes monitoring different fields. In support of this prediction, I report here a paired interneurone in the central nervous system (CNS) of the leech which is stimulated by eyes on one side of the animal and inhibited by eyes on the other. There are striking parallels between these neurones and other integrating neurones, in particular those processing bilateral auditory input in crickets, suggesting that the visual system of the leech may be representative of a general class of sensory processing systems.     

Sensory neurones recognise defined pathways in Drosophila central nervous system

An analysis of the central projection of various sensory neurones in the homoeotic mutant bithorax postbithorax, and in flies where an adult nerve had been experimentally misrouted, reveals that neurones are able to develop a normal projection even if they enter the central nervous system at an unusual place.     

Different factors from the central nervous system and periphery regulate the survival of sensory neurones

Work on nerve growth factor has established that the survival of developing vertebrate neurones depends on the supply of a neurotrophic factor from their target field. The discovery of several new neurotrophic factors has raised the possibility that neurones which innervate multiple target fields require several different neurotrophic factors for survival. Here we show that two distinct neurotrophic factors, one in the central nervous system (CNS) and the other in skeletal muscle, promote the survival of proprioceptive neurones in culture. At saturating concentrations, either factor alone supported most neurones and there was no additional survival in the presence of both factors, but at subsaturating concentrations the combined effect was additive. The neurotrophic activity of each factor was greatest during the period of natural neuronal death. Our results demonstrate that each cultured proprioceptive neurone responds to two distinct neurotrophic factors present in its respective central and peripheral target fields, and suggest that these factors cooperate in regulating survival during development.     

ATP receptor-mediated synaptic currents in the central nervous system.

Until now, the only well documented, fast excitatory neurotransmitter in the brain has been glutamate. Although there is evidence for adenosine 5'-triphosphate (ATP) acting as a transmitter in the peripheral nervous system, suggestions for such a role in the central nervous system have so far not been supported by any direct evidence. Here we report the recording of evoked and miniature synaptic currents in the rat medial habenula. The fast rise time of the currents showed that they were mediated by a ligand-activated ion channel rather than a second messenger system, thus limiting the known transmitter candidates. Evidence was found for the presence on the cells of glutamate, gamma-aminobutyric acid, acetylcholine and ATP receptors, but not for 5-hydroxytryptamine (5HT3) or glycine receptors. The evoked currents were unaffected by blockers of glutamate, gamma-aminobutyric acid or acetylcholine receptors but were blocked by the ATP receptor-blocker, suramin and the desensitizing ATP receptor-agonist alpha,beta-methylene-ATP. Our evidence identifies for the first time synaptic currents in the brain, mediated directly by ATP receptors.     

Regenerating the damaged central nervous system

It is self-evident that the adult mammalian brain and spinal cord do not regenerate after injury, but recent discoveries have forced a reconsideration of this accepted principle. Advances in our understanding of how the brain develops have provided a rough blueprint for how we may bring about regeneration in the damaged brain. Studies in developmental neurobiology, intracellular signalling and neuroimmunology are bringing the regeneration field closer to success. Notwithstanding these advances, clear and indisputable evidence for adult functional regeneration remains to be shown.     

Identified neurones isolated from leech CNS make selective connections in culture.

Neurones cultured in vitro offer distinct advantages for studying how processes grow towards their targets and form synaptic connections. In contrast to the complex events occurring during the development of the nervous system, synapse formation in culture can be analysed in a few neurones at a time and under controlled conditions. We have now dissected out and cultured single identified neurones from the central nervous system (CNS) of the adult leech. Various types of sensory cells, motor cells, and interneurones can be identified in leech ganglia--each with a stereotyped set of properties, including: (1) the electrical characteristics of its membrane, (2) the arborisation of its branches and the morphology of its terminals and (3) the pattern of connections it makes with other identified neurones, skin or muscle. Thus, cultured cells can be compared in detail with their counterparts in situ. We have found that isolated cells survive for several weeks, maintain their membrane properties, sprout and form selective connections.     

Peripheral organs control central neurogenesis in the leech

Interactions between developing nerve centres and peripheral targets are known to affect neuronal survival and thus regulate the adult number of neurons in many systems. Here we provide evidence that peripheral tissues can also influence cell numbers by stimulating the production of neurons. In the leech Hirudo medicinalis, there is a population of several hundred neurons that is found only in the two segmental ganglia that innervate the genitalia and which seem to be added gradually during post-embryonic maturation. By monitoring 5-bromo-2'-deoxyuridine incorporation immunohistochemically, we have now determined that these neurons are actually born late in embryogenesis, well after all other central neurons are born and after efferent and afferent projections are established between these ganglia and the periphery. Ablation of the male genitalia early in embryogenesis, or evulsion of the nerves that connect them to the ganglia, prevent the birth of these neurons. However, they fail to appear ectopically when male genitalia are transplanted to other segments, despite innervation by local ganglia. We conclude that the generation of the late-appearing neurons depends on a highly localized signal produced by the male genitalia, to which only the ganglia that normally innervate these organs have the capacity to respond.     

The myeloid cells of the central nervous system parenchyma

A microglial cell is both a glial cell of the central nervous system and a mononuclear phagocyte, which belongs to the haematopoietic system and is involved in inflammatory and immune responses. As such, microglia face a challenging task. The neurons of the central nervous system cannot divide and be replenished, and therefore need to be protected against pathogens, which is a key role of the immune system, but without collateral damage. In addition, after physical injury, neural cells need restorative support, which is provided by inflammatory responses. Excessive or chronic inflammatory responses can, however, be harmful. How microglia balance these demands, and how their behaviour can be modified to ameliorate disorders of the central nervous system, is becoming clear.     

中枢神经系统内的胰岛素及其受体

众所周知,胰岛素是一个小分子的蛋白质类激素,其主要作用是维持外周的血糖平衡,而对于中枢神经系统内胰岛素的研究则相对较少。大脑中非胰岛素依赖性葡萄糖的摄取,是大家认为脑是胰岛素不敏感器官的主要原因。然而,最近的研究结果显示,大脑的提取物中有高浓度的胰岛素;外周分泌的胰岛素,可以经过特异的转运系统穿过血脑屏障进入中枢神经系统。与大脑中胰岛素的来源、定位和功能的研究相比,中枢神经系统中胰岛素受体的表达也得到了相当高的关注。将从中枢神经系统尤其是脑内的胰岛素及其受体,中枢神经系统内胰岛素的功能,包括繁殖、食物摄入与体重控制、糖代谢调节和记忆等方面进行简要的综述。