GABAB receptors modulate Ca2+ but not G protein-gated inwardly rectifying K+ channels in cerebrospinal-fluid contacting neurones of mouse brainstem.

KEY POINTS: Medullo-spinal CSF contacting neurones (CSF-cNs) located around the central canal are conserved in all vertebrates and suggested to be a novel sensory system intrinsic to the CNS. CSF-cNs receive GABAergic inhibitory synaptic inputs involving ionotropic GABAA receptors, but the contribution of metabotropic GABAB receptors (GABAB -Rs) has not yet been studied. Here, we indicate that CSF-cNs express functional GABAB -Rs that inhibit postsynaptic calcium channels but fail to activate inhibitory potassium channel of the Kir3-type. We further show that GABAB -Rs localise presynaptically on GABAergic and glutamatergic synaptic inputs contacting CSF-cNs, where they inhibit the release of GABA and glutamate. Our data are the first to address the function of GABAB -Rs in CSF-cNs and show that on the presynaptic side they exert a classical synaptic modulation whereas at the postsynaptic level they have an atypical action by modulating calcium signalling without inducing potassium-dependent inhibition. ABSTRACT: Medullo-spinal neurones that contact the cerebrospinal fluid (CSF-cNs) are a population of evolutionary conserved cells located around the central canal. CSF-cN activity has been shown to be regulated by inhibitory synaptic inputs involving ionotropic GABAA receptors, but the contribution of the G-protein coupled GABAB receptors has not yet been studied. Here, we used a combination of immunofluorescence, electrophysiology and calcium imaging to investigate the expression and function of GABAB -Rs in CSF-cNs of the mouse brainstem. We found that CSF-cNs express GABAB -Rs, but their selective activation failed to induce G protein-coupled inwardly rectifying potassium (GIRK) currents. Instead, CSF-cNs express primarily N-type voltage-gated calcium (CaV 2.2) channels, and GABAB -Rs recruit Gβγ subunits to inhibit CaV channel activity induced by membrane voltage steps or under physiological conditions by action potentials. Moreover, using electrical stimulation, we indicate that GABAergic inhibitory (IPSCs) and excitatory glutamatergic (EPSCs) synaptic currents can be evoked in CSF-cNs showing that mammalian CSF-cNs are also under excitatory control by glutamatergic synaptic inputs. We further demonstrate that baclofen reversibly reduced the amplitudes of both IPSCs and EPSCs evoked in CSF-cNs through a presynaptic mechanism of regulation. In summary, these results are the first to demonstrate the existence of functional postsynaptic GABAB -Rs in medullar CSF-cNs, as well as presynaptic GABAB auto- and heteroreceptors regulating the release of GABA and glutamate. Remarkably, postsynaptic GABAB -Rs associate with CaV but not GIRK channels, indicating that GABAB -Rs function as a calcium signalling modulator without GIRK-dependent inhibition in CSF-cNs.