Lorne M. Mendell - US grants

[unreadable] DESCRIPTION (provided by applicant): Neurotrophin molecules have been used by numerous investigators to encourage regeneration of damaged spinal axons in models of spinal cord injury. However, these molecules also have short and long term effects on the strength of synaptic connections in the brain. This action is important to understand in the context of encouraging recovery of function using neurotrophins since many neurons and their axons passing through the injury zone survive the contusion which is the most common form of spinal cord injury. Neurotrophins might help strengthen these useful connections, but on the other hand they could strengthen connections responsible for damaging side effects, e.g., pain. We are proposing to continue studies of neurotrophin action on synaptic transmission focusing on determining mechanisms underlying acute (short term) and chronic (long term) effects of neurotrophins on synaptic efficacy. We use the motoneuron and its synaptic inputs as our test system because of the availability of 2 synaptic inputs that can be studied in the same cell. These connections mature at different times, and this has been very helpful in demonstrating in neonatal rats that the acute effects of neurotrophins on AMPA/kainate transmission has a strong dependence on the availability of functional NMDA receptors on the motoneuron associated with that synaptic input. We will determine whether the long lasting chronic effect is simply an extension of the acute effect or involves a different mechanism. We will also extend these studies to adults where NMDA receptors in motoneurons exhibit a substantial decline in function compared to neonates; do neurotrophins still elicit short and long term effects on synaptic transmission in adults? We will determine directly whether NMDA receptors participate directly in the maturation of AMPA/kainate receptor- mediated responses in the motoneuron. Finally, we will follow up on some recent work indicating NMDA receptor mobility in motoneurons of neonates that declines as the rat matures. We are interested in exploring whether NMDA receptor mobility is an important requirement for plasticity of synaptic connections on motoneurons. These studies have application to synaptic transmission, synaptic development and synaptic plasticity. They are expected to contribute to continued efforts to promote recovery from spinal cord injury. [unreadable] [unreadable] [unreadable]

The long term objective of this research is to develop a model of spinal cord plasticity using nerve growth factor. This neuronotrophin appears to act selectively on nociceptive afferents that are responsible for signalling damage to the skin. The experiments will make use of electrophysiological, anatomical and behavioral methods to investigate various aspects of this plasticity in rats. Several hypotheses will be investigated, including a) the possibility that nociceptive afferents require NCF in a postnatal critical period and that deprivation from NGF results in conversion of these afferents to low threshold mechanoreceptors; b) that such conversion results in changes in spinal projection of these afferents; c) that behavioral hyperalgesia observed in NGF-treated adults results from changes in spinal projection systems, particularly those involving certain amino acid (NMDA) receptors; and d) that reinnervation of nociceptors in adults requires NCF. Experimental design involves examining the effects of NGF and its antibody on sensory neurons (identified physiologically or anatomically), on identified spinal neurons and circuits, and on certain behavioral responses to natural and electrical stimulation. Animals treated with NGF or anti-NGF will be compared to each other and to age-matched untreated controls. The health- related significance of these experiments derives from the need to have further basic information concerning the capacity of the adult nervous system to modify its connectivity after injury, and to document the ability of naturally-occurring biological agents such as NGF to regulate this plasticity. Furthermore, NGF is a candidate to promote regeneration of certain neuronal types after injury, and it is necessary to ascertain its functional effects on the mammalian spinal cord in vivo.

The long term objective of this research is to develop a model of spinal cord plasticity using nerve growth factor. This neuronotrophin appears to act selectively on nociceptive afferents that are responsible for signalling damage to the skin. The experiments will make use of electrophysiological, anatomical and behavioural methods to investigate various aspects of this plasticity in rats. Several hypotheses will be investigated, including a) the possibility that nociceptive afferents require NGF in a postnatal critical period and that deprivation from NGF results in conversion of these afferents to low threshold mechanoreceptor; b) that such conversion results in changes in spinal projection of these afferents; c) that behavioural hyperalgesia observed in NGF-treated adults results from changes in spinal projection systems, particularly those involving certain amino acid (NMDA) receptors; and d) that reinnervation of nociceptors in adults requires NGF. Experimental design involves examining the effects of NGF and its antibody on sensory neurons (identified physiologically or anatomically), on identified spinal neurons and circuits, and on certain behavioural responses to natural and electrical stimulation. Animals treated with NGF or anti-NGF will be compared to each other and to age-matched untreated controls. The health-related significance of these experiments derives from the need to have further basic information concerning the capacity of the adult nervous system to modify its connectivity after injury, and to document the ability of naturally-occurring biological agents such as NGF to regulate this plasticity. Furthermore, NGF is a candidate to promote regeneration of certain neuronal types after injury, and it is necessary to ascertain its functional effects on the mammalian spinal cord in vivo.

Although a role for NGF in inflammatory pain is now well accepted, the mechanism by which this neurotrophin sensitizes nociceptive neurons in the periphery is still not well understood. In part this problem arises from the multiplicity of cell types, both neural and non-neural, in the skin that expresses trkA, the high affinity receptor for NGF. The possibility that NGF affects nociceptors indirectly via trkA-expressing mast cells also obtained some experimental support. However, there is some evidence that NGF can bypass mast cells to sensitize nociceptors to noxious heat. Our recent preliminary data using NGF to acutely condition, capsaicin evoked currents in isolated DRG neurons indicate that NGF may elicit direct effects on DRG neurons indicate that NGF may elicit direct effects on DRG neurons. Capsaicin can be viewed as a surrogate for noxious heat since the latter activates the capsaicin receptor VR1. It therefore follows that NGF may sensitive the response to noxious heat directly. Further study of this action is proposed using patch clamp methodology to examine a number of hypotheses: a) Neurotrophins have a direct effect on the response of nociceptive neurons to nociceptive stimuli via the action of their corresponding trk receptor; b) NGF has the same sensitizing action on the capsaicin and noxious heat responses of both the somata of small DRG cells and the terminals of polymodal nociceptors; c) Cells supplying inflamed skin or skin with genetically altered levels of neurotrophin expression respond differently to acutely applied neurotrophins. Completion of these experiments will provide further understanding of the physiology and physiology of the sensitization of nociceptors to noxious heat by NGF.