Connections between EM2-containing terminals and GABA/μ-opioid receptor co-expressing neurons in the rat spinal trigeminal caudal nucleus

Morphological investigations of endomorphin-2 and spinoparabrachial projection neurons in the spinal dorsal horn of the rat

It has been proved that endomorphin-2 (EM2) produced obvious analgesic effects in the spinal dorsal horn (SDH), which existed in our human bodies with remarkable affinity and selectivity for the μ-opioid receptor (MOR). Our previous study has demonstrated that EM2 made synapses with the spinoparabrachial projection neurons (PNs) in the SDH and inhibited their activities by reducing presynaptic glutamate release. However, the morphological features of EM2 and the spinoparabrachial PNs in the SDH have not been completely investigated. Here, we examined the morphological features of EM2 and the spinoparabrachial PNs by using triple fluorescence and electron microscopic immunohistochemistry. EM2-immunoreactive (-ir) afferents directly contacted with the spinoparabrachial PNs in lamina I of the SDH. Immunoelectron microscopy (IEM) were used to confirm that these contacts were synaptic connections. It was also observed that EM2-ir axon terminals contacting with spinoparabrachial PNs in lamina I contained MOR, substance P (SP) and vesicular glutamate transporter 2 (VGLUT2). In lamina II, MOR-ir neurons were observed to receive direct contacts from EM2-ir varicosities. The synaptic connections among EM2, MOR, SP, VGLUT2, and the spinoparabrachial PNs were also confirmed by IEM. In sum, our results supply morphological evidences for the analgesic effects of EM2 on the spinoparabrachial PNs in the SDH.

Effect of endomorphin-2 on orofacial pain induced by orthodontic tooth movement in rats

Endomorphin-2 demonstrates potent antinociceptive effects in various pain models. The objectives of the present study were to explore the role of endomorphin-2 in the modulation of orofacial pain induced by orthodontic tooth movement in rats. An orthodontic pain model was established in male Sprague-Dawley rats by ligating coiled springs to mimic orthodontic force (40 g). On days 0, 1, 3, 5, 7, and 14 following orthodontic tooth movement, bite force was recorded as a surrogate measure of orthodontic pain. Ipsilateral trigeminal ganglia, trigeminal nucleus caudalis, and periodontal tissues were harvested for immunostaining. Endomorphin-2, endomorphin-2 + naloxone (a non-selective opioid receptor antagonist), naloxone, and saline were injected into trigeminal ganglia and periodontal tissues to explore the role of endomorphin-2 on orthodontic pain. The results showed that following orthodontic tooth movement, endomorphin-2 expression levels in trigeminal ganglia were elevated on days 1, 3, 5, and 7. Orthodontic pain levels were increased on days 1, 3, and 5. The administration of endomorphin-2 into both trigeminal ganglia and periodontal tissues alleviated orthodontic pain. Moreover, the effects of endomorphin-2 could be blocked by naloxone completely in trigeminal ganglia but only partially in periodontal tissues. Therefore, endomorphin-2 plays an important role in the modulation of orthodontic pain both centrally and peripherally, probably through different pathways.

Endogenous opiates and behavior: 2014

This paper is the thirty-seventh consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2014 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (endogenous opioids and receptors), and the roles of these opioid peptides and receptors in pain and analgesia (pain and analgesia); stress and social status (human studies); tolerance and dependence (opioid mediation of other analgesic responses); learning and memory (stress and social status); eating and drinking (stress-induced analgesia); alcohol and drugs of abuse (emotional responses in opioid-mediated behaviors); sexual activity and hormones, pregnancy, development and endocrinology (opioid involvement in stress response regulation); mental illness and mood (tolerance and dependence); seizures and neurologic disorders (learning and memory); electrical-related activity and neurophysiology (opiates and conditioned place preferences (CPP)); general activity and locomotion (eating and drinking); gastrointestinal, renal and hepatic functions (alcohol and drugs of abuse); cardiovascular responses (opiates and ethanol); respiration and thermoregulation (opiates and THC); and immunological responses (opiates and stimulants). This paper is the thirty-seventh consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2014 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (endogenous opioids and receptors), and the roles of these opioid peptides and receptors in pain and analgesia (pain and analgesia); stress and social status (human studies); tolerance and dependence (opioid mediation of other analgesic responses); learning and memory (stress and social status); eating and drinking (stress-induced analgesia); alcohol and drugs of abuse (emotional responses in opioid-mediated behaviors); sexual activity and hormones, pregnancy, development and endocrinology (opioid involvement in stress response regulation); mental illness and mood (tolerance and dependence); seizures and neurologic disorders (learning and memory); electrical-related activity and neurophysiology (opiates and conditioned place preferences (CPP)); general activity and locomotion (eating and drinking); gastrointestinal, renal and hepatic functions (alcohol and drugs of abuse); cardiovascular responses (opiates and ethanol); respiration and thermoregulation (opiates and THC); and immunological responses (opiates and stimulants).