In a recent study published in Cell, researchers studied the social relevance of G protein-coupled receptor (GPCR) Mrgprb4-lineage touch neurons in sexual touch and receptivity.
The sensation of a pleasurable touch is initiated by mechanical, electric signals that are transduced by neurons located in dermatological tissues, and the sensation is critical for sexual activities. Despite the centrality of rewarding social touch in routine lives, the cutaneous neurons detecting sexual touch and shaping perception in brain cells are not well-characterized. Previous studies have reported that C mechanical receptors that innervate hairy skin, labeled among Mrgprb4Cre mice, respond to strokes and generate conditioned place preferences, indicating that Mrgprb4Cre activation could be rewarding.
About the study
In the present study, researchers investigated whether sensory-type neurons among Mrgprb4Cre mice are critical for promoting rewarding social touch involving mesolimbic reward pathways.
The team used an integrated murine genomics analysis, slice electrophysiology analysis, circuit tracing analysis, behavior exemplars, and cranial imaging in vivo for elucidating dermatological-cranial circuits that mediate rewards of sexual touch. ChR2 (channelrhodopsin) was used for focal stimulation of the Mrgprb4-lineage neuronal cells in vivo using the Mrgprb4Cre driver. Mrgprb4Cre mice and RosaChR2/ChR2 murine animals have crossed, and in-situ hybridization and immunofluorescence analyses were performed to characterize ChR2-eYFP protein expression among Mrgprd-positive, Mrgprc11-positive, and Mrgpra3-positive DRG (dorsal root ganglion) neurons.
The team evaluated the mechanical sensitivities of the Mrgprb4-lineage touch neurons based on complete-cell patch-clamp method recordings from DRG neurons that were acutely dissociated. They investigated whether light-induced and synapse-driven currents could be induced to neurons of second order, which were located in the spinal cord’s dorsal horn. The Gpr83-green fluorescent protein (GFP) line murine animals were crossed with Mrgprb4Cre and RosaChR2/ChR2 murine animals. Mrgprb4-lineage touch neuron terminals were optogenetically activated, and recordings from the Gpr83-GFP spinoparabrachial neurons were obtained.
The experiments were repeated using MrgprdCre-ERT2; RosaChR2/ChR2 mice. Next, the team investigated whether optogenetic-activated Mrgprb4-lineage touch neurons could evoke behavioral stereotypes among freely behaving female mice using high-speed videography to capture behavior at high temporal and spatial resolution. Further, to interrogate the role of the Mrgprb4-lineage touch neurons in natural behavior, the murine animals were ablated with DTA (diphtheria toxin subunit A).
To investigate whether females require Mrgprb4-lineage touch neurons for sexual receptivity, LQ (lordosis quotient) assays were performed. Female mice were ovariectomized and treated with progesterone and estradiol to model behavioral estrus. The GPR83-eGFP animals were injected with pAAV.CAG.LSL.tdTomato, CRE-DOG (Cre recombinase dependent on GFP), and BFP (blue fluorescent protein) retrograde viruses and fiber photometry analysis were performed.
Mrgprb4-lineage sensory-type touch neuron activation was rewarding and induced lordotic posture. The neurons were essential for sexual receptivity and engaged dopaminergic neurons during social interactions. Despite social isolation, optogenetic stimulation of the Mrgprb4-lineage touch neurons via the back region’s skin was adequate for inducing conditioned place preferences and striking dorsiflexion resembling the lordotic copulatory posture.
Different populations of dopamine neurons in the ventral tegmental area were associated with anogenital sniffing vs. mounting, indicative of heterogeneous dopamine responses for rewarding social touch. The posture was not observed upon optogenetic activation of Mrgprd+ neurons or Mrgpra3+ neurons and could not be regulated by progesterone and estradiol. Female Mrgprb4Cre and RosaChR2/ChR2 murine animals were preferentially stimulated by blue-color light, indicating that focal Mrgprb4-lineage touch neuronal activation in the back region’s skin was inherently rewarding.
Mrgprb4-lineage touch neurons initiated a skin-to-brain circuit encoding the rewarding quality of pleasurable touch. Without Mrgprb4-lineage touch neurons, the female murine animals did not find male mounts rewarding, and sexual-type receptivity was replaced by aggression with a concomitant reduction in the release of dopamine from NAc (nucleus accumbens). Mrgprb4-positive terminals provided innervation to all hairy regions of skin examined, including the back, underbelly, and anogenital regions of both genders, except the internal wall of the vagina.
Synaptic connectivity was observed between Mrgprb4-lineage touch neurons and GPR83+ projection neurons of the spinal cord. Mrgprb4Cre; RosaChR2/ChR2 homozygotes showed monosynaptic and/or polysynaptic inputs, and glutamatergic synaptic transmission was observed between the Mrgprb4-lineage touch neurons and second-order neurons. The findings showed that photo-stimulation for 1.0 millisecond of the Mrgprb4-lineage touch neurons evoked action potential among 43.0% of cells.
Blockade of inhibitory neuro-transmission using gamma-aminobutyric acid and antagonists of glycine receptors such as strychnine and bicuculline increased responses among GFP+ spinoparabrachial neurons to the Mrgprb4-lineage touch neuron photo-stimulation were observed. Injecting the Cre-dependent virus into Mrgprb4Cre mice and pulsing orange ChRmine laser light to their shaven back skin did not induce lordosis. However, touch-like responses such as turning and orienting toward the light beam were observed.
Control female mice showed enhanced sexual receptivity, whereas Mrgprb4Cre; RosaDTA females’ sexual receptivity plummeted in the second pairing, reducing further in the third pairing, indicating that sexual touch becomes negatively reinforcing in the absence of Mrgprb4-lineage touch neurons. Blue fluorescent protein-positive (ventral tegmental area-projecting) cell bodies in the lateral parabrachial nucleus showed visible overlapping with SPB GPR83+ terminals.
Overall, the study findings showed that cutaneous Mrgprb4-lineage touch neurons are essential for sexual receptivity and dopamine release that makes the social touch pleasurable.