Unmyelinated primary afferent fiber stimulation depletes dynorphin A (1-8) immunoreactivity in rat ventral horn

Dorsal horn neurons firing at high frequency, but not primary afferents, release opioid peptides that produce micro-opioid receptor internalization in the rat spinal cord

To determine what neural pathways trigger opioid release in the dorsal horn, we stimulated the dorsal root, the dorsal horn, or the dorsolateral funiculus (DLF) in spinal cord slices while superfusing them with peptidase inhibitors to prevent opioid degradation. Internalization of mu-opioid receptors (MOR) and neurokinin 1 receptors (NK1R) was measured to assess opioid and neurokinin release, respectively. Dorsal root stimulation at low, high, or mixed frequencies produced abundant NK1R internalization but no MOR internalization, indicating that primary afferents do not release opioids. Moreover, capsaicin and NMDA also failed to produce MOR internalization. In contrast, dorsal horn stimulation elicited MOR internalization that increased with the frequency, being negligible at <10 Hz and maximal at 500 Hz. The internalization was abolished by the MOR antagonist D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH2 (CTAP), in the presence of low Ca2+ and by the Na+ channel blocker lidocaine, confirming that it was caused by opioid release and neuronal firing. DLF stimulation in "oblique" slices (encompassing the DLF and the dorsal horn of T11-L4) produced MOR internalization, but only in areas near the stimulation site. Moreover, cutting oblique slices across the dorsal horn (but not across the DLF) eliminated MOR internalization in areas distal to the cut, indicating that it was produced by signals traveling in the dorsal horn and not via the DLF. These findings demonstrate that some dorsal horn neurons release opioids when they fire at high frequencies, perhaps by integrating signals from the rostral ventromedial medulla, primary afferents, and other areas of the spinal cord.

Central changes in nociceptin dynorphin B and Met-enkephalin-Arg-Phe in different models of nociception

The newly identified neuropeptide nociceptin/orphanin FQ (NOC) was measured in different rat brain areas related to the descending anti-nociceptive pathways and compared to two opioid peptides, dynorphin B (DYN B) and Met-enkephalinArgPhe (MEAP). Two experimental models of chronic nociception, one neurogenic and one inflammatory, used in this study, reveal how different pathological conditions may influence these endogenous systems. Nerve injury is induced by ligation of the sciatic nerve and inflammation by a carrageenan injection in the gluteal muscle, 2 weeks prior to decapitation. Selected brain areas were dissected out and frozen. NOC-, DYN B- and MEAP-like immunoreactivity (LI) is determined by radioimmunoassay. Nerve injury increased the NOC-LI levels in the cortex cinguli, DYN B-LI levels in the dorsal and the ventral part of the spinal cord, whereas a decrease in the MEAP-LI levels is seen in the dorsal part of the periaqueductal grey (PAG). After inflammation, the NOC-LI levels increased in cortex cinguli, hypothalamus and in the dorsal spinal cord, whereas DYN B-LI levels increased in the dorsal part of the PAG. A general increase in MEAP-LI levels is found after inflammation in all analyzed brain areas except in hippocampus. In conclusion, increased levels of NOC-LI were found in cortex cinguli in both treatment groups and in hypothalamus and spinal cord following carrageenan treatment. The changes in the NOC-LI concentrations were not parallelled by changes in DYN B-LI and MEAP-LI, suggesting that NOC and opioid peptides elicit different reactions in the systems of nociception/antinociception.

Enkephalinergic and dynorphinergic neurons in the spinal cord and dorsal root ganglia of the polyarthritic rat - in vivo release and cDNA hybridization studies

Complex and contradictory data have been reported regarding the changes in spinal opioidergic systems associated with chronic inflammatory pain in the rat. In an attempt to solve these discrepancies, the in vivo release of met-enkephalin and dynorphin and the expression of the corresponding propeptide genes were investigated at the spinal level in arthritic rats and paired controls. A dramatic increase in the concentration of prodynorphin mRNA (+300-550%) and a less pronounced elevation of that of dynorphin-like material (+40-50%) were found in the dorsal part of cervical and lumbar segments of the spinal cord in rats rendered arthritic by an intradermal injection of Freund's adjuvant four weeks prior to these measurements. In addition, the spinal release of dynorphin-like material (assessed through an intrathecal perfusion procedure in halothane-anaesthetized animals) was approximately twice as high in arthritic rats as in controls. In spite of significant elevations in the levels of both met-enkephalin (+30-70%) and proenkephalin A mRNA (+40-50%) in the dorsal part of cervical and lumbar segments, the spinal release of met-enkephalin-like material was decreased (-50%) in arthritic rats as compared to paired controls. Proenkephalin A mRNA (but not prodynorphin mRNA) could be measured in dorsal root ganglia, and its levels were dramatically reduced in ganglia at the lumbar segments in arthritic rats. Such parallel reductions in the spinal release of met-enkephalin-like material and the levels of proenkephalin A mRNA in dorsal root ganglia of arthritic rats support the idea that the activity of primary afferent enkephalinergic fibres decreases markedly during chronic inflammatory pain.

Spinal release of immunoreactive dynorphin A(1-8) with the development of peripheral inflammation in the rat

Microprobes bearing immobilised antibodies to dynorphin A(1-8) were used to study the basal and evoked release of this prodynorphin derived peptide in the spinal cord of urethane anaesthetised normal rats and those with a peripheral inflammation. In the absence of any active peripheral stimulus the antibody microprobes detected immunoreactive (ir)-dynorphin A(1-8) in two areas (lamina I and laminae IV-V) in the dorsal horn of the spinal cord of normal rats. With the development of unilateral ankle inflammation over 3 to 5 days following subcutaneous injections of Freund's complete adjuvant, a basal presence of ir-dynorphin A(1-8) was found in both the dorsal and ventral horn regions of both sides of the spinal cord. Lateral compression of the ankles of the normal animals did not release ir-dynorphin A(1-8) during the period of stimulation, but this neuropeptide was detected in increased amounts in the ventral horn following the stimulus. By contrast, compression of inflamed ankles produced elevated levels of ir-dynorphin A(1-8) during the period of stimulus application at three major sites in the ipsilateral spinal grey matter. The largest peak was in the deep dorsal horn/upper ventral horn (laminae VI-VII), with further sites of significant release in the mid dorsal horn (laminae II-V) and the lower ventral horn. The observation that ir-dynorphin A(1-8) is physiologically released in the ventral and deep dorsal in addition to the superficial dorsal horn of the rat suggests an involvement of dynorphins in several aspects of spinal function.

Spinal dynorphin immunoreactivity increases bilaterally in a neuropathic pain model

Increased spinal levels of dynorphin, an endogenous opioid kappa agonist, are seen in models of both chronic and acute hyperalgesia. This study determined the extent and localization of spinal immunoreactive dynorphin following sciatic cryoneurolysis (SCN), a neuropathic pain model produced by a peripheral nerve freeze lesion. SCN results in behaviors associated with neuropathic pain such as autotomy (the gnawing and scratching of the affected limb), touch-evoked and mechanical allodynia, and spontaneous nociceptive behavior. Following SCN, 4 rats that displayed autotomy and 3 rats that did not were randomly chosen for immunohistochemical staining of dynorphin-like immunoreactivity (DLIR). The area of DLIR above a standardized threshold level was quantified in both dorsal horns of each spinal cord section using a computer-assisted image analyzer to express DLIR in pixels. DLIR was observed both ipsilateral and contralateral to the injured peripheral nerve. In addition, the area of DLIR was significantly greater (P = 0.05) in rats that showed autotomy behavior (mean = 52.6 x 10(3) +/- 25.6) compared to rats with no autotomy (mean = 13.8 x 10(3) +/- 4.78). In sharp contrast to the ipsilateral dynorphin increases observed in other neuropathic pain models, we observe a bilateral increase at 21 days following SCN.

Endogenous opiates: 1991

This paper is the fourteenth installment of our annual review of research concerning the opiate system. It includes papers published during 1991 involving the behavioral, nonanalgesic, effects of the endogenous opiate peptides. The specific topics this year include stress; tolerance and dependence; eating; drinking; gastrointestinal and renal function; mental illness and mood; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; electrical-related activity; general activity and locomotion; sex, pregnancy, and development; immunological responses; and other behaviors.

The effects of A- and C-fiber stimulation on patterns of neuropeptide immunostaining in the rat superficial dorsal horn

The present study determines the effects of sciatic nerve stimulation at intensities that activate A-fibers alone or both A- and C-fibers on immunostaining for substance P (SP), cholecystokinin-octapeptide (CCK-8), galanin (GAL), dynorphin (DYN) and vasoactive intestinal polypeptide (VIP) in the superficial dorsal horn of the rat spinal cord. The goal of this study is to provide a more precise spatial localization of the sites of release or accumulation of these compounds in relation to specific types of stimuli. Following A-fiber stimulation, there was no significant change in immunostaining for any of these compounds. However, A- and C-fiber stimulation resulted in major changes. For SP, CCK-8, GAL and DYN there was a large and significant loss of immunostaining in medial regions of the dorsal horn. This is the area where sciatic nerve primary afferent fibers terminate and the depletion is probably correlated with activity in these fibers. By contrast, VIP immunostaining is increased in the lateral part of the superficial cord, which is outside of the central sciatic afferent fiber terminations. This indicates that the increase is not in the fine sciatic sensory axons that are directly stimulated. As a final point, the fact that C-fiber but not A-fiber stimulation causes marked changes in the immunocytochemical distribution of all these compounds is further evidence, albeit indirect, that they are involved in nociceptive information processing.