Neurochemical features of enkephalinergic neurons in the mouse trigeminal subnucleus caudalis

Enkephalinergic Circuit Involved in Nociceptive Modulation in the Spinal Dorsal Horn

Enkephalin (ENK) has been implicated in pain modulation within the spinal dorsal horn (SDH). Revealing the mechanisms underlying ENK analgesia entails the anatomical and functional knowledge of spinal ENK-ergic circuits. Herein, we combined morphological and electrophysiological studies to unravel local ENK-ergic circuitry within the SDH. First, the distribution pattern of spinal ENK-ergic neurons was observed in adult preproenkephalin (PPE)-GFP knock-in mice. Next, the retrograde tracer tetramethylrhodamine (TMR) or horseradish peroxidase (HRP) was injected into the parabrachial nucleus (PBN) in PPE-GFP mice. Immunofluorescent staining showed I-isolectin B4 (IB4) labeled non-peptidergic afferents were in close apposition to TMR-labeled PBN-projecting neurons within lamina I as well as PPE-immunoreactivity (-ir) neurons within lamina II. Some TMR-labeled neurons were simultaneously in close association with both IB4 and PPE-ir terminals. Synaptic connections of these components were further confirmed by electron microscopy. Finally, TMR was injected into the PBN in adult C57BL/6 mice. Whole-cell patch recordings showed that δ-opioid receptor (DOR) agonist, [D-Pen^2,5]-enkephalin (DPDPE, 1 µM), significantly reduced the frequency of miniature excitatory postsynaptic current (mEPSC) and decreased the activity of TMR-labeled neurons. In conclusion, spinal ENKergic neurons receive direct excitatory inputs from primary afferents, which might be directly recruited to release ENK under the condition of noxious stimuli; ENK could inhibit the glutamatergic transmission towards projecting neurons via presynaptic and postsynaptic DORs. These morphological and functional evidence may explain the mechanisms underlying the analgesic effects exerted by ENK within the SDH.

Neuronal calcium signaling in chronic pain

Acute physiological pain, the unpleasant sensory response to a noxious stimulus, is essential for animals and humans to avoid potential injury. Pathological pain that persists after the original insult or injury has subsided, however, not only results in individual suffering but also imposes a significant cost on society. Improving treatments for long-lasting pathological pain requires a comprehensive understanding of the biological mechanisms underlying pain perception and the development of pain chronicity. In this review, we aim to highlight some of the major findings related to the involvement of neuronal calcium signaling in the processes that mediate chronic pain.

Endogenous opiates and behavior: 2011

This paper is the thirty-fourth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2011 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 (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration (Section 16); and immunological responses (Section 17).

Participation of calbindin-D28K in nociception: results from calbindin-D28K knockout mice

Since calbindin-D(28K) (CB-D(28K))-positive neurons have been related to nociceptive sensory processing, we have hypothesized that altered CB-D(28K) expression could alter nociceptive transmission. We have used +/+ and -/- knockout (KO) mice for CB-D(28k) in different behavioral models of pain and sensory responses at the caudalis subdivision of the trigeminal spinal nucleus in order to understand how this protein may participate in nociception. Behavioral responses to formalin injection in the hind paw or at the whisker pad or in the hind paw glutamate or i.p. acetic acid tests showed an increase of the pain threshold in CB-D(28k) -/- mice. KO mice showed a diminution of the inhibitory activity at Sp5C nucleus and a marked reduction of GABA content. Sp5C neurons from CB-D(28k) -/- mice did not change their spontaneous activity or tactile response after formalin injection in the whisker pad. In contrast, Sp5C neurons increased their spontaneous firing rate and tactile response after formalin injection in their receptive field in CB-D(28k) +/+ mice. The results of this study demonstrate the active role played by CB-D(28k) in nociceptive sensory transmission. The lack of this calcium binding protein, associated to deficient GABAergic neurotransmission, translates into dysfunction of sensory processing of nociceptive stimuli.