Immobilization stress sensitizes rat dorsal horn neurons having input from the low back

Abstract

Background: Stress is known to promote several forms of muscle pain including non-specific low back pain. However, the question if stress alone activates nociceptive central neurons has not been studied systematically. Here, we investigated the influence of repeated immobilization stress on dorsal horn neurons and behaviour in the rat. Methods: The stress consisted of immobilization in a narrow tube for 1 h on 12 days. Single dorsal horn neurons were recorded with microelectrodes introduced into the spinal segment L2. In this segment, about 14% of the neurons responded to mechanical stimulation of the subcutaneous soft tissues of the low back in na€ ıve rats. The neurons often behaved like wide dynamic range cells in that they had a low mechanical threshold and showed graded responses to noxious stimuli. Results: The stress-induced changes in neuronal response behaviour were (1) appearance of new receptive fields in the deep tissues of the hindlimb, (2) increased input from deep soft tissues, but unchanged input from the skin and (3) significant increase in resting activity. Surprisingly, the pressure-pain threshold of the low back remained unchanged, although dorsal horn neurons were sensitized. In the open field test, the rats showed signs of increased anxiety. Conclusions: This study shows that stress alone is sufficient to sensitize dorsal horn neurons. The data may explain the enhanced pain low back patients report when they are under stress. The increased resting discharge may lead to spontaneous pain.

Short‐term swimming exercise attenuates the sensitization of dorsal horn neurons in rats with NGF‐induced low back pain

“…The finding that in contrast to the deep input, the input from the skin was not affected by the swimming exercise, repeats own results obtained in previous experiments (Hoheisel et al., ). Simultaneously, the finding supports our assumption that the cutaneous afferents have very rigid and effective synaptic connections with dorsal horn neurons.…”

“… The animals did not show any signs of distress (struggling, fighting and vocalisation) when they were put into the water‐filled cylinder. The marked hyperalgesia measured as PPT 2 days after the first NGF injection and after two bouts of swimming demonstrates the absence of strong stress‐induced analgesia. The proportion of neurons exhibiting new input from deep tissues decreased in our swimming experiments while – in contrast – the proportion of neurons increased in a previous study in which the effects of immobilisation stress was studied (Hoheisel et al., ). Likewise, the proportion of neurons having resting activity and the mean frequency of the resting activity decreased after swimming while in the study on rats deliberately stressed by immobilisation the proportion significantly increased (Hoheisel et al., ). This speaks strongly against a stress‐induced analgesia in the present study. …”

“…The receptive fields (RFs) of the dorsal horn neurons recorded from were identified with mechanical stimuli applied to low back structures (muscle, fascia and skin) of the left hindlimb, hip, lateral abdominal wall and tail. The search for receptive fields was carried out following a strict protocol in both experimental groups: The responsiveness of each neuron was tested by stimulating the body regions in a fixed order (Hoheisel et al, ; Zhang et al., ) starting with the toes followed by the metatarsus, heel, lower leg, knee, thigh, base of the tail, low back and lateral abdomen. As innocuous stimuli, touch with an artist's brush and moderate pressure with a blunt probe were used (stimulus force: 1 N, blunt tip area 3.14 mm 2 ); as noxious stimuli, pinching with a sharpened watchmaker's forceps (skin, thoracolumbar fascia) or noxious pressure (stimulus force: 10 N) with the blunt probe (muscle).…”

Adverse Childhood Experiences and the Consequences on Neurobiological, Psychosocial, and Somatic Conditions Across the Lifespan

“…More precisely, early life social stressors such as emotional abuse are associated with enhanced temporal summation of pain and enhanced touch sensitivity ( 109 ). Recent animal experiments indicate that stress itself is able to sensitize nociceptive neurons in the spinal cord ( 113 – 115 ) and that altered neuronal responses are accompanied by comparable changes in pain-related behavior. Schneider et al ( 115 ) for example have developed a new animal model on social rejection that mimicks ACE and found that social rejection produces long-lasting effects in pain sensitivity and social behavior that persist into adulthood ( 115 , 116 ).…”

Pain, Hyperalgesia and Stress

“…Thus, these dorsal horn neurons behave as if sensitized during deep somatic inflammation. Interestingly, the rate of resting activity in the dorsal horn neurons of chronically stressed rats was considerably higher than in rats with chronically sensitized muscle nociceptors generated by complete Freund′s adjuvant or nerve growth factor (see references in Hoheisel et al., ). Surprisingly, the pain pressure threshold of the low back did not change in the chronically stressed rats, yet they showed signs of increased anxiety in the open field test.…”