Opioid Peptide Family Research Articles

A model of chronic pain in the rat: high-resolution neuroanatomical approach identifies alterations in multiple opioid systems in the periaqueductal grey

Inoculation of the tail base of rats with Mycobacterium butyricum led to an arthritic swelling and inflammation of the limbs which displayed a hyperalgesia to noxious pressure: these effects peaked at 3 weeks postinoculation. In vitro autoradiography of coronal sections of rat brain was used for a parallel determination of binding to μ-, δ-and κ-opioid binding sites. In only two regions, the dorsomedial and dorsolateral parts of the periqueductal grey (PAG), was a significant change seen: this comprised an increase in binding to κ-sites, whereas μ- and δ-sites therein were unaffected. This region was analysed for opioid peptides derived from each of the three opioid peptide families known. While no change was seen in levels of immunoreactive (ir)-dynorphin 1–17 A (DYN) and ir-Met-enkephalin, a decrease was detected in those of ir-β-endorphin (β-EP): this change was restricted to the PAG. These data demonstrate a highly localized and selective influence of chronic arthritic pain upon multiple opioid systems in the PAG of the rat, a structure playing a key role in the control of pain and in the expression of the antinociceptive actions of opioids. The data suggest a possible significance of PAG pools of β-EP and κ-receptors in the response to and modulation of chronic pain.

Circadian rhythm of feeding induced changes in hypothalamic Met-enkephalin concentrations

Accumulating evidence suggests that opioid peptides play an important role in the hunger component of the control of food intake. The enkephalins, one of the opioid peptide families, stimulate feeding when injected into specific hypothalamic areas and endogenous concentrations change with the fed/fasted condition of rats and sheep and with phase of circadian cycle. To demonstrate a possible circadian rhythm in feeding-induced changes in Met-enkephalin (MEK), 54 male rats initially weighing 255±3 g were adapted to a 12-hr fast during the light (light-fasted) or dark (dark-fasted) phase of the circadian cycle, then sacrificed before (non-fed) or after (fed) being allowed to eat a meal. In non-fed compared with fed rats, MEK concentrations were higher in the paraventricular nucleus (PVN, 170 vs. 109 pg/mg tissue, p<0.05) and ventromedial hypothalamus (VMH, 209 vs. 161 pg/mg tissue, p<0.05) in the dark (light-fasted) but not light (dark-fasted), even though rats ate a larger meal in the light (8.6 vs. 5.0 g, p<0.01). In rats fed the same amount of food in the light (dark-fasted) as ad lib fed rats in the dark (light-fasted), MEK concentrations did not differ in the PVN or VMH, suggesting that circadian rhythm is more important than meal size. Rats gavaged with an amount of milk equal in calories to dark ad lib-fed rats (light-fasted) had MEK concentrations not different from light-fasted non-fed rats (216 vs. 209 pg/mg tissue, NS) suggesting that feeding behavior, pregastric stimuli and/or form of diet is important for influencing MEK concentrations. These results demonstrate that feeding-induced MEK concentration changes in areas previously shown to be important in the control of food intake are circadian in nature and are larger in the dark when most feeding occurs in rats.

Evolution of Proenkephalin and Prodynorphin

SYNOPSIS. Since the discovery of the enkephalins in 1975, three separate families of opioid peptides have been identified, proopiomelanocortin, proenkephalin, and prodynorphin. The lattertwo have multiple copies of the enkephalin sequence in larger enkephalin containing polypeptides (ECPs). The amino acid and nucleotide sequences of these two proteins from human as well as other species have been determined. A comparison of each of the proteins between species indicates a number of highly conserved regions in not only the bioactive peptides but at other locations as well. A number of common features between the two proteins suggested a common ancestral protein. Using computeraided metric analysis, it is possible to align their DNA segments so thatthe cysteine residues at the amino end are aligned, as well as the enkephalin sequences at the carboxyl end. The exact alignment of the central regions is currently being studied. Metric analysis thus provides strong evidence that proenkephalin and prodynorphin diverged from a single ancestral protein.

Opiate activity of peptides derived from the three opioid peptide families on the rat vas deferens

The inhibitory activity of opioid peptides derived from pro-opiomelanocortin (POMC), pro-enkephalin A and pro-enkephalin B (= pro-dynorphin) on the electrically evoked twitch of the rat vas deferens (RVD) was evaluated. The POMC-derived β-endorphin exhibits the greatest potency on this preparation. In addition, all peptides derived from pro-enkephalin A show full agonistic activity with BAM-22P and peptide E as the most potent peptides. In contrast, the majority of peptides derived from pro-enkephalin B (= pro-dynorphin) were essentially inactive on this tissue. Moreover, no antagonistic properties of these peptides were demonstrable in this preparation; thus the opioid receptors present in the RVD (putative epsilon receptors) might not possess any particular affinity for the pro-enkephalin B derived peptides.

Colocalization of immunoreactive proenkephalin and prodynorphin products in medullary neurons of the rat

This study addressed the possible coexistence of products of the proenkephalin and prodynorphin opioid peptide precursors in single neurons of the central nervous system of the rat. Antisera directed against met-enkephalin-arg-gly-leu and against Dyn B were used in immunohistochemical preparations of sections through the rat medulla. Examination of serial three micron frozen sections stained alternately with the two different antisera revealed that the majority of labelled neurons stain with only one of the two antisera. In specific area, however, immunoreactive m-enk and Dyn B could be detected in the same neuron. This was particularly true of the caudal ventrolateral nucleus of the solitary tract, where the two peptides were colocalized in most neurons. Other areas where the two peptides coexist include the midline raphe and the nucleus reticularis paragigantocellularis. These data provide the first evidence for colocalization of different opioid peptide families in single CNS neurons.

Kappa- and delta-opioid receptor agonists differentially inhibit striatal dopamine and acetylcholine release.

At least three different families of endogenous opioid peptides, the enkephalins, endorphins and dynorphins, are present in the mammalian central nervous system (CNS). Immunocytochemical studies have demonstrated their localization in neurones, which supports the view that these peptides may have a role as neurotransmitter or neuromodulators. However, the target cells and cellular processes acted upon by the opioid peptides are still largely unknown. One possible function of neuropeptides, including the opioid peptides, may be presynaptic modulation of neurotransmission in certain neuronal pathways, for example, by inhibition or promotion of neurotransmitter release from the nerve terminals. Here we report that dynorphin and some benzomorphans potently and selectively inhibit the release of (radiolabelled) dopamine from slices of rat corpus striatum, by activating kappa-opioid receptors. In contrast, [Leu5]enkephalin and [D-Ala2, D-Leu5]enkephalin selectively inhibit acetylcholine release by activating delta-opioid receptors.

Distribution of different endogenous opioid peptide families in the rat periaqueductal grey (PAG)

Distribution of different endogenous opioid peptide families in the rat periaqueductal grey (PAG)

Cardiovascular effects of endogenous opiate systems.

Opiates are among the oldest pharmacological substances known to man; their analgesic, euphoric, and addictive effects have been traditional focal points for opiate research. Without doubt, the cardiovascular effects of opiates have also been apparent to the user or abuser of opiate substances for several centuries. Persons seeking analgesic, euphoric, or antidiarrheal actions from opiate alkaloids have probably noted dizziness upon sudden st,anding due to the orthostatic hypotension these substances produce. His­ torically, however, scientific study on the cardiovascular responses to ad­ ministered opiates has lagged behind other areas of opiate research. This is understandable as, relative to doses required for their analgesic actions in humans, the cardiovascular effects of opiates are less noticeable and, indeed, undesirable. Early studies established that cardiovascular responses to morphine var­ ied among species; both autonomic and histamine-releasing properties con­ tributed to their hypotensive and bradycardic actions (1-5). Generally speaking, morphine was shown to produce prominent effects upon brain­ stem and hypothalamic centers that resulted in increased parasympathetic and decreased sympathetic tone (3-6). These effects upon autonomic out­ flow caused a depression of both heart rate and blood pressure. A resurgence of interest in opiate-cardiovascular interactions followed the discovery of endogenous opiate systems (for review see 7). A family of opioid peptides were found to be located at sites suggesting an autonomic action (8); also, opiate receptors were shown to be densely distributed in the brainstem and hypothalamus in close proximity to cardiovascular centers as well as endogenous opiate pathways (9, to). These anatomical findings

K receptor activities of the three opioid peptide families

A large number of opioid peptides derived from the three established precursors, pro-opiomelanocortin (POMC), proenkephalin A (pro-enk A), and pro-enkephalin B (pro-enk B) were tested for their ability to inhibit electrically induced contractions of the isolated rabbit vas deferens, a preparation sensitive to k but not to μ opioid ligands. In the presence of enzyme inhibitors, all peptides exhibit roughly similar potencies, but the shorter peptides display lower potencies in the absence of enzyme inhibitors. This suggests that the loss in activity is due to their enzymatic degradation. It is concluded that the pro-enk B gene codes peptides with high k-receptor activities, whereas peptides produced by the POMC gene are devoid of k-receptor activity and the peptides, coded by the pro-enk A gene exhibit weak to moderate k-receptor activity.