The aim of this project is to gain a better understanding of the mechanisms involved in the transmission of sensory information from primary afferent fibers to spinal cord neurons in the mammalian spinal cord. Primary afferent synaptic terminals are believed to use excitatory aminoacids (EAAs) as their principal neurotransmitters, and some of them contain in addition neuropeptides that function as more 'atypical' neuromodulators. Synaptic transmission from different primary afferents may have different consequences, e.g. a nociceptive terminal may induce long-lasting changes in the postsynaptic responses that differ from synaptic transmission from large mechanoreceptive fibers. Moreover, the same primary afferent may evoke different postsynaptic responses on different spinal cord neurons, suggesting the involvement of distinct synaptic mechanisms within a single axonal arborization. Our hypothesis is that these differences are partly related to the postsynaptic complement of EAA-receptors. The present proposal aims to use a multidisciplinary approach to identify the EAA receptors associated with primary afferent terminals from different classes of primary afferents and located in different laminae of the spinal cord. We will also test the presence or absence of neuropeptides within their synaptic terminals. Different classes of primary afferents will be electrophysiologically characterized by either axonal recordings in the spinal cord (for large diameter primary afferents, e.g. cutaneous mechanoreceptors, muscle spindle afferents) or by cell body recordings in the dorsal root ganglia (for small diameter afferents, including pain sensory afferents). Recorded elements will be intracellularly marked with electron-dense and/or fluorescent markers to we can then immunolocalize EAA receptors postsynaptic to the intracellularly labeled terminals using immunoelectron microscopic techniques, and/or assess neuropeptide presence using double fluorescent methods. In addition to providing powerful insights into basic mechanisms of sensory physiology ranging from mechanoreception to pain mechanisms, the data will provide valuable knowledge towards full understanding of sensory disorders like hyperalgesia, and other chronic pain pathologies, whose mechanisms are believed to reside partially in alterations of sensory transmission at the first few spinal cord synapses established between primary afferent fibers and spinal cord neurons. In more general terms, our results will also shed some light on the organization at individual synapses of the bewildering diversity of EAA receptors described in molecular biological studies.
