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 GABA_A receptors but the contribution of metabotropic GABA_B receptors (GABA_B‐Rs) was not studied yet. Here, we indicate that CSF‐cNs express functional GABA_B‐Rs that inhibit postsynaptic calcium channels but fail to activate inhibitory potassium channel of the kir3‐type. We further show that GABA_B‐Rs localize 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 GABA_B‐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‐cNs activity was shown to be regulated by inhibitory synaptic inputs involving ionotropic GABA_A receptors, but the contribution of the G‐protein coupled GABA_B receptors was not studied yet. Here, we used a combination of immunofluorescence, electrophysiology and calcium imaging to investigate the expression and function of GABA_B‐Rs in CSF‐cNs of the mouse brainstem. We found that CSF‐cNs express GABA_B‐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 (Ca_V 2.2) channels and GABA_B‐Rs recruit Gβγ subunits to inhibit Ca_v channels activity induced by membrane voltage steps or under physiological conditions by action potentials. Moreover, electrical stimulation evoked in CSF‐cNs GABAergic inhibitory (IPSCs) but also glutamatergic excitatory (EPSCs) synaptic currents, 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 GABA_B‐Rs in medullar CSF‐cNs as well as presynaptic GABA_B auto‐ and heteroreceptors regulating the release of GABA and glutamate. Remarkably, postsynaptic GABA_B‐Rs associate with Ca_V but not GIRK channels indicating that GABA_B‐Rs function as a calcium signalling modulator without GIRK‐dependent inhibition in CSF‐cNs.

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