Accumulating evidence indicate that the neuropeptide urotensin II and urotensin II receptors are expressed in subsets of mammal spinal motoneurons. In fact, a role for the peptide in the regulation of motoneuron function at neuromuscular junction has been suggested, while roles for urotensin II at central synapses in spinal cord have never been addressed. We found that urotensin II receptors were closely associated with cholinergic terminals apposed to a subset of motoneuron and non-motoneuron cell bodies in the ventral horn of the adult mouse cervical spinal cord; urotensin II receptor was also expressed on non-cholinergic nerve terminals. In particular, urotensin II receptor appeared associated with both large cholinergic C-boutons and standard cholinergic terminals contacting some motoneuron perikarya. Cholinergic nerve terminals from mouse cervical spinal cord were equipped with functional presynaptic urotensin II receptors linked to excitation of acetylcholine release. In fact, functional experiments conducted on cervical spinal synaptosomes demonstrated a urotensin II evoked calcium-dependent increase in [(3)H]acetylcholine release pharmacologically verified as consistent with activation of urotensin II receptors. In spinal cord these actions would facilitate cholinergic transmission. These data indicate that, in addition to its role at the neuromuscular junction, urotensin II may control motor function through the modulation of motoneuron activity within the spinal cord.
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