Literature context: List Biological catalog #7032A6 RRID:AB_2313637; 1:20,000) and GFP (chicken ant
Pain perception is strongly influenced by descending pathways from "higher" brain centers that regulate the activity of spinal circuits. In addition to the extensively studied descending system originating from the medulla, the neocortex provides dense anatomical projections that directly target neurons in the spinal cord and the spinal trigeminal nucleus caudalis (SpVc). Evidence exists that these corticotrigeminal pathways may modulate the processing of nociceptive inputs by SpVc, and regulate pain perception. We demonstrate here, with anatomical and optogenetic methods, and using both rats and mice (of both sexes), that corticotrigeminal axons densely innervate SpVc, where they target and directly activate inhibitory and excitatory neurons. Electrophysiological recordings reveal that stimulation of primary somatosensory cortex potently suppresses SpVc responses to noxious stimuli and produces behavioral hypoalgesia. These findings demonstrate that the corticotrigeminal pathway is a potent modulator of nociception and a potential target for interventions to alleviate chronic pain.SIGNIFICANCE STATEMENT Many chronic pain conditions are resistant to conventional therapy. Promising new approaches to pain management capitalize on the brain's own mechanisms for controlling pain perception. Here we demonstrate that cortical neurons directly innervate the brainstem to drive feedforward inhibition of nociceptive neurons. This corticotrigeminal pathway suppresses the activity of these neurons and produces analgesia. This corticotrigeminal pathway may constitute a therapeutic target for chronic pain.
Literature context: (catalog #703, List Biological; RRID:AB_2313637) at 1:2000 for 2 d at 4Â°C. Afte
The basal forebrain (BF) receives afferents from brainstem ascending pathways, which has been implicated first by Moruzzi and Magoun (1949) to induce forebrain activation and cortical arousal/waking behavior; however, it is very little known about how brainstem inhibitory inputs affect cholinergic functions. In the current study, glycine, a major inhibitory neurotransmitter of brainstem neurons, and gliotransmitter of local glial cells, was tested for potential interaction with BF cholinergic (BFC) neurons in male mice. In the BF, glycine receptor α subunit-immunoreactive (IR) sites were localized in choline acetyltransferase (ChAT)-IR neurons. The effect of glycine on BFC neurons was demonstrated by bicuculline-resistant, strychnine-sensitive spontaneous IPSCs (sIPSCs; 0.81 ± 0.25 × 10-1 Hz) recorded in whole-cell conditions. Potential neuronal as well as glial sources of glycine were indicated in the extracellular space of cholinergic neurons by glycine transporter type 1 (GLYT1)- and GLYT2-IR processes found in apposition to ChAT-IR cells. Ultrastructural analyses identified synapses of GLYT2-positive axon terminals on ChAT-IR neurons, as well as GLYT1-positive astroglial processes, which were localized in the vicinity of synapses of ChAT-IR neurons. The brainstem raphe magnus was determined to be a major source of glycinergic axons traced retrogradely from the BF. Our results indicate a direct effect of glycine on BFC neurons. Furthermore, the presence of high levels of plasma membrane glycine transporters in the vicinity of cholinergic neurons suggests a tight control of extracellular glycine in the BF.SIGNIFICANCE STATEMENT Basal forebrain cholinergic (BFC) neurons receive various activating inputs from specific brainstem areas and channel this information to the cortex via multiple projections. So far, very little is known about inhibitory brainstem afferents to the BF. The current study established glycine as a major regulator of BFC neurons by (1) identifying glycinergic neurons in the brainstem projecting to the BF, (2) showing glycine receptor α subunit-immunoreactive (IR) sites in choline acetyltransferase (ChAT)-IR neurons, (3) demonstrating glycine transporter type 2 (GLYT2)-positive axon terminals synapsing on ChAT-IR neurons, and (4) localizing GLYT1-positive astroglial processes in the vicinity of synapses of ChAT-IR neurons. The effect of glycine on BFC neurons was demonstrated by bicuculline-resistant, strychnine-sensitive spontaneous IPSCs recorded in whole-cell conditions.
In the spinal cord dorsal horn, presynaptic GABA(A) receptors (GABA(A)Rs) in the terminals of nociceptors as well as postsynaptic receptors in spinal neurons regulate the transmission of nociceptive and somatosensory signals from the periphery. GABA(A)Rs are heterogeneous and distinguished functionally and pharmacologically by the type of α subunit variant they contain. This heterogeneity raises the possibility that GABA(A)R subtypes differentially regulate specific pain modalities. Here, we characterized the subcellular distribution of GABA(A)R subtypes in nociceptive circuits by using immunohistochemistry with subunit-specific antibodies combined with markers of primary afferents and dorsal horn neurons. Confocal laser scanning microscopy analysis revealed a distinct, partially overlapping laminar distribution of α1-3 and α5 subunit immunoreactivity in laminae I-V. Likewise, a layer-specific pattern was evident for their distribution among glutamatergic, γ-aminobutyric acid (GABA)ergic, and glycinergic neurons (detected in transgenic mice expressing vesicular glutamate transporter 2-enhanced green fluorescent protein [vGluT2-eGFP], glutamic acid decarboxylase [GAD]67-eGFP, and glycine transporter 2 (GlyT2)-eGFP, respectively). Finally, all four subunits could be detected within primary afferent terminals. C-fibers predominantly contained either α2 or α3 subunit immunoreactivity; terminals from myelinated (Aβ/Aδ) fibers were colabeled in roughly equal proportion with each subunit. The presence of axoaxonic GABAergic synapses was determined by costaining with gephyrin and vesicular inhibitory amino acid transporter to label GABAergic postsynaptic densities and terminals, respectively. Colocalization of the α2 or α3 subunit with these markers was observed in a subset of C-fiber synapses. Furthermore, gephyrin mRNA and protein expression was detected in dorsal root ganglia. Collectively, these results show that differential GABA(A)R distribution in primary afferent terminals and dorsal horn neurons allows for multiple, circuit-specific modes of regulation of nociceptive circuits.