Date of Award
Doctor of Philosophy
The first sensory synapse is the site where sensory afferent fibers make synaptic connections with second order neurons. Somatic and craniofacial afferents terminate in spinal cord dorsal horn (SDH) and caudal spinal trigeminal nucleus (CSTN). Neurotransmitter release from first order nerve terminals regulates ascending sensory transmission. Several lines of evidence indicate that plasticity in the spinal cord dorsal horn underlies secondary hyperalgesia. The sensory receptors, Transient Receptor Potential (TRP) channels, are expressed not only at peripheral terminals, but also at the central terminals of sensory neurons. While the role of these channels at the periphery is detecting environmental stimuli, their function at central terminals is not fully understood. Furthermore, TRP channel expression has been shown in CNS nuclei like hippocampus that are not tightly linked to somatosensation. In this study, I first determined the functionality of TRP channels at the first sensory synapse and hippocampus using pharmacological activators. I then determined if putatively endogenous TRP channel activators modulate synaptic transmission at the first sensory synapse. Lastly, I determined if recordings that respond to capsaicin demonstrate synaptic plasticity in either hippocampus or spinal cord, in an attempt to attribute synaptic plasticity mechanisms to TRPV1 activity at glutamatergic terminals. I have used slice patch-clamp technique to record miniature, spontaneous and evoked currents in lamina II neurons of spinal cord dorsal horn, CSTN and hippocampus. In lamina II neurons of SDH and CSTN, capsaicin, a TRPV1 agonist, robustly increased the frequency of mEPSCs and sEPSCs in a dose dependant manner. Although capsaicin increased m/sEPSC frequency, eEPSC amplitude, which reflects synchronous action potential propagation at glutamatergic terminals, was markedly depressed by capsaicin. Our studies indicate capsaicin inhibits action potential dependant transmission at central terminals. Resiniferatoxin (RTX) is a TRPV1 agonist that displays higher potency (>100 fold) compared to capsaicin, and deactivation with this agonist is minimal. RTX also depressed eEPSC amplitude in lamina II neurons of SDH and CSTN; unexpectedly, RTX increased m/sEPSC frequency to lesser extent compared to capsaicin. The TRPA1 agonist, N-methyl maleimide (NMM), increased s/mEPSC frequency in lamina II neurons; however, NMM did not depress eEPSC amplitude like capsaicin and RTX. It is possible that inhibition of nerve terminal firing is a unique property of TRPV1 agonists compared to other noxious chemicals. To justify a physiological relevance for nociceptive TRP channel expression at the first sensory synapse, I studied the effect of endogenous TRP channel agonists on synaptic transmission at the first sensory synapse. Anandamide (AEA) is an agonist of CB1/CB2 and TRPV1 receptors; it is less potent at TRPV1 receptors than capsaicin. AEA increased sEPSC frequency in 70% of neurons, whereas the remainder of neurons showed a decrease in sEPSC frequency. Unlike capsaicin and RTX, anandamide did not dramatically depress eEPSC amplitude. Methyl glyoxal (MG) is a putative TRPA1 agonist produced during conditions of hyperglycemia. MG increased the frequency of sEPSCs in SDH lamina II neurons. I next used high frequency synaptic stimulation (HFS-100 Hz, 1s) to model synaptic activity during pain transmission. HFS induced a modest increase in sEPSC frequency and minimally changed eEPSC amplitude; patches that showed HFS modulation also responded to capsaicin. In studying the role of TRP channels in modulating synaptic transmission at central synapses, I finally performed experiments in hippocampus with 2 objectives; 1) to determine extent of capsaicin responsiveness as an indicator of TRPV1 functionality, and 2) to evaluate synaptic plasticity in response to HFS. Capsaicin effect on sEPSC frequency in CA1 and CA3 neurons was minimal in comparison to its effect in dorsal horn neurons. HFS at schaffer collateral region caused LTP in CA1 neurons that was more pronounced than for spinal cord. In conclusion, TRP channels are expressed at central terminals of nociceptors where they modulate glutamatergic transmission. Studying their role at the first sensory synapse enhances our understanding of nociceptive transmission, and this study suggests this receptor for a target for intervening in pathological pain transmission at the level of spinal cord.
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