Neuromodulation within a spinal locomotor network : Role of metabotropic glutamate receptor subtypes

Abstract: The metabotropic glutamate receptors, mGluRs, are G-protein coupled receptors. They consist of eight cloned subtypes, which are divided into three groups depending on the amino acid sequence similarity, pharmacology and their signal pathways. In the lamprey spinal cord, group I mGluRs are located postsynaptically, while group II and III are presynaptic and depress synaptic transmission. The goal of this thesis has been to elucidate the mechanisms by which the two subtypes of group I mGluRs, i.e. mGluR1 and mGluR5, modulate the firing properties of single neurons, the synaptic interactions and the overall activity of the spinal locomotor network in the lamprey. mGluR1 activation by endogenously released glutamate increases the frequency of the locomotor rhythm induced by NMDA in the isolated lamprey spinal cord preparation. This increase in the frequency is the result of a number of cellular and molecular mechanisms that have been studied in detail. Firstly, mGIuR1 potentiates the NMDA-induced current and modulates NMDAinduced TTX-resistant membrane potential oscillations known to occur during locomotion. Mathematical simulations of the interaction between mGluR1 and NMDA receptors reproduce the modulation of the NMDA-induced oscillations and the increase in the locomotor frequency. Secondly, mGluR1 activation depolarizes the membrane potential of neurons and consequently induces repetitive firing. These effects are due to an inhibition of a leak current responsible for setting the resting membrane potential. Interestingly, mGluR1 activates different signaling pathways to modulate NMDA current and leak conductance. Both effects require activation of Gproteins. The mGluR1-mediated inhibition of leak current requires PLC activation and release of Ca2+ from internal stores, as well as tyrosine kinase activation. The potentiation of NMDA current is not, however, dependent on an increase in intracellular Ca2+ or on tyrosine kinases. Thirdly, activation of mGluR1 receptors gives rise to a synthesis and release of endocannabinoids from postsynaptic neurons. The released endocannabinoids act as retrograde messengers which bind to presynaptic receptors and reduce glycinergic synaptic transmission. The reduced inhibitory transmission will result in an increase in the locomotor frequency. Hence, mGluR1 activation triggers the release of endocannabinoids which thus contribute to the mGlur1 mediated modulation of the locomotor network operation. Finally, endogenous activation of mGluR5 during locomotion decreases the burst frequency and produces long-lasting oscillations of the intracellular Ca2+ concentration. These oscillations are mediated through PLC and Ca2+ release from internal stores. Furthermore, they are also dependent on Ca2+ influx through L-type Ca2+ channels but do not involve PKC activation. Thus, mGluR5 seems to modulate the locomotor frequency via mechanisms involving oscillations of intracellular Ca 2+ concentration. In conclusion, the two group I mGluRs subtypes, mGluR1 and mGluR5, use separate signaling pathways and mediate opposite effects on locomotor activity. While the modulatory effects of mGluR5 seems to involve Ca2+ oscillations, those of rnGluR1 depend on different cellular and synaptic mechanisms which act in a synergistic manner to regulate the locomotor frequency.