Spinal gastrin releasing peptide receptor expressing interneurons are controlled by local phasic and tonic inhibition

Abstract

Dorsal horn gastrin-releasing peptide receptor (GRPR) neurons have a central role in itch transmission. Itch signaling has been suggested to be controlled by an inhibitory network in the spinal dorsal horn, as increased scratching behavior can be induced by pharmacological disinhibition or ablation of inhibitory interneurons, but the direct influence of the inhibitory tone on the GRPR neurons in the itch pathway have not been explored. Here we have investigated spinal GRPR neurons through in vitro and bioinformatical analysis. Electrophysiological recordings revealed that GRPR neurons receive local spontaneous excitatory inputs transmitted by glutamate and inhibitory inputs by glycine and GABA, which were transmitted either by separate glycinergic and GABAergic synapses or by glycine and GABA co-releasing synapses. Additionally, all GRPR neurons received both glycine- and GABA-induced tonic currents. The findings show a complex inhibitory network, composed of synaptic and tonic currents that gates the excitability of GRPR neurons, which provides direct evidence for the existence of an inhibitory tone controlling spontaneous discharge in an itch-related neuronal network in the spinal cord. Finally, calcium imaging revealed increased levels of neuronal activity in Grpr-Cre neurons upon application of somatostatin, which provides direct in vitro evidence for disinhibition of these dorsal horn interneurons.

The Neurokinin-1 Receptor is Expressed with Gastrin-Releasing Peptide Receptor in Spinal Interneurons and Modulates Itch

“…To determine the appropriate Cre driver for the genetically encoded Ca 2+ indicator GCaMP6s, we performed dual FISH studies. These showed extensive colocalization of the glutamatergic neuronal marker Vglut2 with Tacr1 (99.0% ± 0.6 of Tacr1 neurons) and Grpr (98.5% ± 0.5 of Grpr neurons) in SDH neurons, indicating that these are excitatory neuron populations (Figure 5c-e), in agreement with previous studies [20,25,71]. Therefore, we selectively targeted Vglut2-positive excitatory neurons within the SDH in Ca 2+ imaging experiments.…”

Chemogenetic activation of central gastrin‐releasing peptide‐expressing neurons elicits itch‐related scratching behavior in male and female mice

“…15 As suggested that GRP is released from glutamatergic interneurons in the SDH, different groups of researchers suggested that activation of GRPR + neurons through α-amino-3-hydroxy-5methyl-4-isoxazolepropionic acid receptor (AMPAR) by glutamate is a key component of itch processing in the SDH. [33][34][35] We also recently demonstrated that GRP and glutamate cooperatively activate itch-responsive GRPR + neurons in the SDH under acute and chronic itching conditions. 15 Notably, i.d.…”

A spinal neural circuitry for converting touch to itch sensation

“…Secondly, GABA/glycine are adequate for regulating Tac2 / Ucn3 tdTom neurons through feedforward inhibition via GABAergic/glycinergic neurons 26 . Because mechanical itch travels through GRPR neurons, which are directly subject to GABA/glycine-mediated inhibition of galanin neurons 54 , 56 , 57 , the physiological relevance of inhibitory regulation of Tac2 / Ucn3 tdTom neurons is currently unclear. It is possible that Tac2 / Ucn3 tdTom neurons are kept in a quiescent state under an inhibitory control, which may explain why mechanical itch is rather infrequently experienced.…”