Parallel ascending spinal pathways for affective touch and pain

The parabrachial-to-amygdala pathway provides aversive information to induce avoidance behavior in mice

The neuronal circuitry for pain signals has been intensively studied for decades. The external lateral parabrachial nucleus (PB) was shown to play a crucial role in nociceptive information processing. Previous work, including ours, has demonstrated that stimulating the neuronal pathway from the PB to the central region of the amygdala (CeA) can substitute for an actual pain signal to drive an associative form of threat/fear memory formation. However, it is still unknown whether activation of the PB-CeA pathway can directly drive avoidance behavior, escape behavior, or only acts as strategic freezing behavior for later memory retrieval. To directly address this issue, we have developed a real-time Y-maze conditioning behavioral paradigm to examine avoidance behavior induced by optogenetic stimulation of the PB-CeA pathway. In this current study, we have demonstrated that the PB-CeA pathway carries aversive information that can directly trigger avoidance behavior and thereby serve as an alarm signal to induce adaptive behaviors for later decision-making.

cGMP signalling in dorsal root ganglia and the spinal cord: Various functions in development and adulthood

Cyclic GMP (cGMP) is a second messenger that regulates numerous physiological and pathophysiological processes. In recent years, more and more studies have uncovered multiple roles of cGMP signalling pathways in the somatosensory system. Accumulating evidence suggests that cGMP regulates different cellular processes from embryonic development through to adulthood. During embryonic development, a cGMP-dependent signalling cascade in the trunk sensory system is essential for axon bifurcation, a specific form of branching of somatosensory axons. In adulthood, various cGMP signalling pathways in distinct cell populations of sensory neurons and dorsal horn neurons in the spinal cord play an important role in the processing of pain and itch. Some of the involved enzymes might serve as a target for future therapies. In this review, we summarise the knowledge regarding cGMP-dependent signalling pathways in dorsal root ganglia and the spinal cord during embryonic development and adulthood, and the potential of targeting these pathways.

The Effects of Ageing on Tactile Function in Humans

Ageing is accompanied by a steady decline in touch sensitivity and acuity. Conversely, pleasant touch, such as experienced during a caress, is even more pleasant in old age. There are many physiological changes that might explain these perceptual changes, but researchers have not yet identified any specific mechanisms. Here, we review both the perceptual and structural changes to the touch system that are associated with ageing. The structural changes include reduced elasticity of the skin in older people, as well as reduced numbers and altered morphology of skin tactile receptors. Effects of ageing on the peripheral and central nervous systems include demyelination, which affects the timing of neural signals, as well as reduced numbers of peripheral nerve fibres. The ageing brain also undergoes complex changes in blood flow, metabolism, plasticity, neurotransmitter function, and, for touch, the body map in primary somatosensory cortex. Although several studies have attempted to find a direct link between perceptual and structural changes, this has proved surprisingly elusive. We also highlight the need for more evidence regarding age-related changes in peripheral nerve function in the hairy skin, as well as the social and emotional aspects of touch.

A spinoparabrachial circuit defined by Tacr1 expression drives pain

Painful stimuli evoke a mixture of sensations, negative emotions and behaviors. These myriad effects are thought to be produced by parallel ascending circuits working in combination. Here, we describe a pathway from spinal cord to brain for ongoing pain. Activation of a subset of spinal neurons expressing Tacr1 evokes a full repertoire of somatotopically directed pain-related behaviors in the absence of noxious input. Tacr1 projection neurons (expressing NKR1) target a tiny cluster of neurons in the superior lateral parabrachial nucleus (PBN-SL). We show that these neurons, which also express Tacr1 (PBN-SLTacr1), are responsive to sustained but not acute noxious stimuli. Activation of PBN-SLTacr1 neurons alone did not trigger pain responses but instead served to dramatically heighten nocifensive behaviors and suppress itch. Remarkably, mice with silenced PBN-SLTacr1 neurons ignored long-lasting noxious stimuli. Together, these data reveal new details about this spinoparabrachial pathway and its key role in the sensation of ongoing pain.

Extended amygdala-parabrachial circuits alter threat assessment and regulate feeding

An animal's evolutionary success depends on the ability to seek and consume foods while avoiding environmental threats. However, how evolutionarily conserved threat detection circuits modulate feeding is unknown. In mammals, feeding and threat assessment are strongly influenced by the parabrachial nucleus (PBN), a structure that responds to threats and inhibits feeding. Here, we report that the PBN receives dense inputs from two discrete neuronal populations in the bed nucleus of the stria terminalis (BNST), an extended amygdala structure that encodes affective information. Using a series of complementary approaches, we identify opposing BNST-PBN circuits that modulate neuropeptide-expressing PBN neurons to control feeding and affective states. These previously unrecognized neural circuits thus serve as potential nodes of neural circuitry critical for the integration of threat information with the intrinsic drive to feed.

Physical Contact Promotes the Development of Emotional Contagion Between Mice

The establishment and maintenance of strong affiliative relationships is fundamental for group cohesion and crucial for overall individual well-being. Empathy is considered a critical process for promoting attachment and the long-term stability of social bonds. However, it is unclear how different modalities of social communication contribute to the development of empathy. Physical contact between individuals, such as gentle touching, is a highly salient form of social communication. Despite mounting evidence that touch may be crucial for promoting social bonds, the role of touch in the development of empathy is currently not well understood. Animal models have become a powerful tool for the experimental manipulation and examination of empathy related behaviors such as emotional contagion. Here, we use the Tube Co-Occupancy Test (TCOT) to promote voluntary physical contact between mice and examine whether social, physical contact promotes emotional contagion of pain between mice. We found that repeated exposure to TCOT promoted the development of emotional contagion between mice. However, preventing physical contact in the TCOT assay also prevented the development of emotional contagion of pain. These results demonstrate that voluntary physical contact is a critical component in the formation of social bonding and emotional contagion in mice.