Opioid supraspinal analgesic synergy between the amygdala and periaqueductal gray in rats

Analgesia can be elicited following microinjections of morphine, μ-selective agonists and β-endorphin into the amygdala. These analgesic responses are mediated by opioid synapses in the periaqueductal gray (PAG) since general (naltrexone), μ (β-funaltrexamine) and δ₂ (naltrindole isothiocyanate) opioid antagonists administered into the PAG significantly reduce both morphine and β-endorphin analgesia elicited from the amygdala. Supraspinal multiplicative opiate analgesic interactions have been observed between the PAG and rostroventromedial medulla (RVM), the PAG and locus coeruleus (LC), and the RVM and LC. The present study further examined the relationship between the amygdala and PAG in analgesic responsiveness by determining whether multiplicative analgesic interactions occur following paired administration of subthreshold doses of morphine into both structures, β-endorphin into both structures, morphine into one structure and β-endorphin into the other structure, or morphine and β-endorphin into one structure. Co-administration of subthreshold doses of morphine into both the amygdala and PAG results in a profound synergistic interaction on the jump test, but not the tail-flick test. Co-administration of subthreshold doses of β-endorphin into both structures also results in a profound test-specific synergistic interaction. In both cases, the magnitude of the interaction was similar regardless of the site receiving the fixed dose of the opioid, and the site receiving the variable dose of the opioid. Co-administration of β-endorphin (1 μg) into the amygdala and morphine (1 μg) into the PAG produced a potent interaction, but co-administration of morphine (1 μg) into the amygdala and β-endorphin (1 μg) into the PAG failed to produce interactive effects. Finally, co-administration of morphine (1 μg) and β-endorphin (1 μg) into either the amygdala alone or the PAG alone failed to produce an interaction, indicating the importance of regional opioid activation. These data are discussed in terms of the test-specificity of nociceptive processing in the amygdala, in terms of the multiple modulatory mechanisms mediating β-endorphin analgesia in the PAG, and in terms of whether the interactions are either mediated by anatomical connections between the amygdala and PAG or by mechanisms initiated by these two sites converging at another site or sites.     

Medullary μ and δ opioid receptors modulate mesencephalic morphine analgesia in rats

Supraspinal opioid analgesia is mediated in part by connections between the midbrain periaqueductal gray (PAG) and rostral ventral medulla (RVM) which includes the nuclei raphe magnus and reticularis gigantocellularis. Serotonergic 5HT₂ and 5HT₃ receptor subtypes appear to participate in this pathway since general and selective serotonergic antagonists microinjected into the RVM significantly reduced morphine analgesia elicited from the PAG. Since both an enkephalinergic pathway between the PAG and RVM and intrinsic enkephalinergic cells in the RVM exist, the present study evaluated the abilities of general (naltrexone), μ-selective (β-funaltrexamine: B-FNA) andδ₂-selective (naltrindole) opioid receptor subtype antagonists microinjected into the RVM to alter morphine (2.5 μg) analgesia elicited from the PAG as measured by the tail-flick and jump tests. Mesencephalic morphine analgesia was significantly reduced after pretreatment in the RVM with naltrexone (1–10 μg), B-FNA (0.5–5 μg) or naltrindole (0.5–5 μg). Naltrexone in the RVM failed to alter basal nociceptive thresholds and none of the opioid antagonists were effective in reducing mesencephalic morphine analgesia when they were microinjected into placements lateral or dorsal to the RVM. These data indicate that μ andδ₂ opioid receptors in the RVM modulate the transmission of opioid pain-inhibitory signals from the PAG.     

Effects of hypophysectomy and dexamethasone treatment on plasma β-endorphin and pain threshold during pregnancy

During pregnancy, rats and humans show an increase in pain threshold that is mediated by an endorphin system. In order to determine whether plasma β-endorphin and/or other factors of pituitary origin are involved in pregnancy-induced analgesia in the rat, the effects of hypophysectomy (day 12 of pregnancy) or pharmacological suppresson of pituitary function via dexamethasone administration (day 14–21 of pregnancy) were investigated. Hypophysectomy did not affect either the magnitude of the increase or the pattern of change in pain threshold despite the resulting decrease in stress-induced plasma β-endorphin concentrations. However, the observed effect of the surgical and/or postsurgical procedure on pain threshold confounded unequivocal interpretation of these results. Pharmacological suppression of pituitary function with dexamethasone (2 μg/ml), a non-invasive procedure, also produced a significant decrease in resting plasma β-endorphin levels. As was observed for surgical removal of the pituitary gland, this treatment did not produce a significant alteration in the magnitude of the increase in jump threshold. Furthermore, no correlation was found between plasma β-endorphin concentrations and jump threshold values on day 21 of pregnancy. These results indicate that the pituitary gland does not play an essential role in the maintenance of opioid analgesia during pregnancy. It is suggested that pregnancy-induced analgesia depends on central rather than peripheral opioid systems.     

Opioid-induced release of neurotensin in the periaqueductal gray matter of freely moving rats

The midbrain periaqueductal gray matter (PAG) is an important region for endogenous pain suppression. Nerve terminals containing opioid peptides and neurotensin (NT), as well as high densities of opioid- and NT-receptors, have been demonstrated in the ventromedial PAG. Local administration of opioids or NT in this region induces antinociception in experimental animals. In the present microdialysis study, the effect of opioids on the release of NT in the ventromedial PAG was investigated. Perfusion of the microdialysis probe with 10 μM morphine induced a significant increase (P<0.05; n=5) of the extracellular level of NT-like immunoreactivity (NT-LI), while perfusion with a 10-fold higher concentration of morphine had no significant effect on the NT-LI release in the PAG. Also perfusion of the dialysis probe with the μ-opioid receptor-specific agonist [

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-Ala²-N-Me-Phe⁴-Gly⁵-ol]-enkephaline (DAGO) (1 or 100 μM) induced a significant (P<0.05; n=7–9) increase of the NT-LI level. The increase in NT-LI release in response to 1 μM DAGO was both calcium-dependent and naloxone-reversible. Since opioid agonists generally inhibit neuronal activity, an indirect mechanism, involving inhibition of tonically active inhibitory neurons, e.g. γ-aminobutyric acid (GABA) neurons, could be of importance for the opioid induced release of NT. However, local administration in the PAG of the GABA_A antagonist bicuculline (0.1–10 μM) or the GABA_A agonist muscimol (1–100 μM) had no significant effect on the extracellular NT-LI level in the PAG, suggesting that GABAergic mechanisms are not involved in the opioid-induced release of NT-LI. In conclusion, the present data provide in vivo evidence that μ-opioid receptors mediate stimulation of neurotensin release in the PAG.     

Neurochemical studies on the mesolimbic circuitry of antinociception

Previous studies using the technique of microinjection into brain nuclei indicated that the periaqueductal gray (PAG), nucleus accumbens, habenula and amygdala play an essential role in pain modulation and that these nuclei possibly act through a ‘mesolimbic neural loop‘ to exert an analgesic effect, in which Met-enkephalin (MEK) and β-endorphin (β-EP) have been implicated as the two major opioid peptides involved in antinociception. In the present study performed in rabbits, intracranial microinjection was supplemented with push-pull perfusion and radioimmunoassay to determine whether the release of enkephalins (ENK) and β-EP was increased in these nuclei when the putative neural circuit was activated by morphine administered into one of the nuclei. The results showed: (1) microinjection of morphine into the PAG increased the release of ENK and β-EP in the N. accumbens, and vice versa; (2) microinjection of morphine into the N. accumbens increased the release of ENK and β-EP in the amygdala, and vice versa; (3) morphine microinjected into the PAG caused an increase in the release of ENK and β-EP in the amygdala and vice versa, although the release of ENK in PAG was statistically not significant. These results indicate that PAG, N. accumbens and amygdala are connected in a network served by a positive feedback circuitry.     

ACTH1–24 blocks opiate - induced analgesia in the rat

Samples of 1 μl containing 15 μg of morphine sulfate with or without 1 μg of synthetic ACTH were injected into the fourth ventricle of rats. The inclusion of ACTH eliminated the analgesic effect of morphine as evaluated by the tail-flick test, both in restrained and in unrestrained, lightly sedated animals. The same result was obtained when β-endorphin was used to bring about analgesia. Since the effect of the peptides was shown to be mediated by central actions alone, the results are discussed in light of the brain ACTH/β-endorphin system.     

Effects of intrathecal ACTH on opiate A algesia in the rat

The effects of administration of opiates and ACTH onto the lumbar spinal cord of rats were investigated. ACTH, morphine and β-endorphin were administered to the lumbar spinal cord via chronically implanted spinal catheters. Effects on nociceptive thresholds using the tail-flick test were observed. Intrathecal morphine and β-endorphin caused analgesia whereas effects of intrathecal ACTH were variable. About half the animals showed small increases in tail-flick latency with intrathecal ACTH, the other half showing either decreases in tail-flick latency or no effect. In contrast, intrathecal ACTH consistently reversed the analgesia caused by either intrathecal morphine or β-endorphin. The results are discussed in terms of interactions between ACTH and opiates.     

Morphine-induced place preference in the CXBK mouse: characteristics of μ opioid receptor subtypes

The role of μ opioid receptor subtypes, μ₁ and μ₂, in morphine-conditioned place preference was examined using ddY and μ₁ opioid receptor-deficient CXBK mice. In ddY mice, the μ receptor agonist morphine caused a dose-related preference for the drug-associated place, but the κ agonist U-50,488H produced a dose-related place aversion. These results demonstrated that the mouse is available for place preference conditioning using opioids. Under this condition, the influence of pretreatment with selective μ₁ opioid receptor antagonist naloxonazine morphine-induced place preference was investigated in ddY mice. Although pretreatment with the selective μ₁ antagonist naloxonazine (35 mg/kg, s.c.) did not modify the morphine-induced place preference, pretreatment with the selective μ antagonist β-funaltrexamine (β-FNA 10 mg/kg, s.c.) eliminated the appetitive effect of morphine. Furthermore, morphine (1–5 mg/kg, s.c.) produced a dose-related preference for the drug-associated place in CXBK mice. These findings suggest that the morphine-induced conditioned place preference may be mediated by naloxonazine-insensitive sites (μ₂ opioid receptors). In addition, chronic infusion of the dopamine D₁ antagonist SCH23390 (1.0 mg/kg/day) during the conditioning sessions eliminated the morphine-induced place preference in CXBK mice. Similarly, morphine combined with naloxonazine failed to produce the place preference in ddY mice chronically treated with SCH23390. The blocking effect of SCH23390 on the morphine-conditioned place preference suggests that μ₂ receptors may regulate the dopaminergic system, especially dopamine D₁ receptors, and are also involved in the reinforcing effects of morphine.     

μ-, δ-, κ- and ε-Opioid receptor modulation of the hypothalamic-pituitary-adrenocortical (HPA) axis: subchronic tolerance studies of endogenous opioid peptides

In opiate-naive rats, the endogenous opioid peptides, β-endorphin, dynorphin(1–13) and Met---Enk---Arg---Phe (MEAP) and the synthetic enkephalin analogue -Ala²- -Leu⁵-Enk (DADLE) potently stimulated plasma corticosterone in a dose-dependent, naloxone-reversible manner. To characterize their in vivo affinities, the effects of these peptides on plasma corticosterone release were tested in rats made tolerant to morphine, U50488H, DADLE/morphine or β-endorphin. These cross-tolerance studies showed that dynorphin and MEAP exerted their action on plasma corticosterone release at κ-opioid receptors. The action of DADLE occurred at δ-opioid receptors, while the action of β-endorphin occurred principally at another receptor site. These results indicate that there is independent modulation of the hypothalamic-pituitary-adrenal axis by endogenous opioid peptides at μ-, δ- and κ-opioid receptors. In addition, there may be modulation by β-endorphin at a separate site that we suggest could be a central ε-receptor site. This cross-tolerance paradigm, using a neuroendocrine model, provides in vivo evidence for the action of centrally active endogenous opioid peptides at multiple and independent opioid receptors.     

Social conflict-induced changes in nociception and β-endorphin-like immunoreactivity in pituitary and discrete brain areas of C57BL/6 and DBA/2 mice

The present study characterizes the time course of social conflict analgesia and its reversibility by opioid antagonist drugs in the C57BL/6 and DBA/2 inbred strains of mice and examines the relationship between alterations in brain and pituitary levels of β-endorphin-like immunoreactivity (β-ELIR) and the antinociception elicited by social stress. Data revealed statistically strain differences in regard to β-ELIR in control animals. The pituitary content of β-ELIR was higher in DBA/2, while the values in the periaqueductal grey (PAG) and in the amygdala were higher in C57BL/6 mice. No interstrain differences were found in the hypothalamus. Exposure to 50 attack bites resulted in a 6-fold higher analgesia in DBA/2 mice and in a strain-dependent fall of ELIR in pituitary (27%) and PAG (23%). PAG but not pituitary β-ELIR levels in C57BL/6 mice correlated positively with the increase in tail-flick latency after attack. Mere confrontation with a non-aggressive opponent failed to induce analgesia and was associated in C57BL/6 mice with a significant reduction in the β-ELIR content of both the pituitary and the PAG. The data are discussed in terms of genotype-dependent sensitivity of the β-endorphin system to stress and its relation to analgesia.     

Activation of periaqueductal grey pools of β-endorphin by analgetic electrical stimulation in freely moving rats

Electrical stimulation of the ventral midbrain periaqueductal grey (PAG) elicited an antinociception (analgesia) in freely moving rats. Stimulated animals displayed a pronounced decrease in levels of immunoreactive (ir)-β-endorphin (β-EP) in the midbrain PAG. This depletion was selective in that: (1) animals placed in the chamber and not stimulated revealed neither an analgesia nor an alteration in levels of ir-β-EP. (2) No change in levels of ir-β-EP was detectable in other brain regions. (3) Both stimulated rats and rats placed in the chamber and not stimulated revealed a rise in circulating ir-β-RP: the magnitude of this rise did not, however, differ between these groups. (4) Levels of ir-Met-enkephalin, ir-Leu-enkephalin and ir-dynorphin A were modified neither in the PAG nor in other CNS tissues. The data demonstrate that electrical stimulation of the midbrain PAG selectively influences (presumably activates) pools of β-EP therein. Together with our finding that destruction of PAG-localized β-EP neurones to block stimulation-analgesia¹⁴. the data suggest that an activation of intrinsic pools of β-EP underlies stimulation-produced analgesia elicited from the PAG in the rat.     

Differential modulation of angiotensin II and hypertonic saline-induced drinking by opioid receptor subtype antagonists in rats

Opioid modulation of ingestion includes general opioid antagonism of different forms of water intake, μ₂ receptor modulation of deprivation-induced water intake and δ₂ receptor modulation of saccharin intake. Water intake is stimulated by both central administration of angiotensin II (ANG II) and peripheral administration of a hypertonic saline solution; both responses are reduced by general opioid antagonists. The present study examined whether specific opioid receptor subtype antagonists would selectively alter each form of water intake in rats. Whereas systemic naltrexone (0.1–2.5 mg/kg, s.c.) reduced water intake induced by either peripheral ANGII (500 μg/kg, s.c.) or hypeptonic saline (3 ml/kg, 10%), intracerebroventricular (i.c.v.) naltrexone (1–50 μg) only inhibited central ANGII (20 ng)-induced hyperdipsia. Both forms of drinking were significantly and dose-dependently inhibited by the selective κ antagonist, nor-binaltorphamine (Nor-BNI, 1–20 μg). Whereas both forms of drinking were transiently reduced by the μ-selective antagonist, β-funaltrexamine (β-FNA, 1–20 μg), the μ₁ antagonist, naloxonazine (40 μg) stimulated drinking following hypertonic saline. The δ₁ antagonist, [d-Ala², Leu⁵, Cys⁶]-enkephalin (DALCE, 1–40 μg) significantly reduced drinking following ANGII, but not following hypertonic saline; the δ antagonist, naltrindole failed to exert significant effects. These data indicate that whereas κ opioid binding sites modulate hyperdipsia following hypertonic saline, μ₂, δ₁ and κ opioid binding sites modulate hyperdipsia following ANGII. The μ₁ opioid binding site may normally act to inhibit drinking following hypertonic saline.     

Streptozotocin-induced diabetes selectively alters the potency of analgesia produced by μ-opioid agonists, but not by δ- and κ-opioid agonists

To investigate the possible mechanisms of the alterations in morphine-induced analgesia observed in diabetic mice, we examined the influence of streptozotocin-induced (STZ-induced) diabetes on analgesia mediated by the different opioid receptors. The antinociceptive potency of morphine (10 mg/kg), administered s.c., as determined by both the tail-pinch and the tail-flick test, was significantly reduced in diabetic mice as compared to that in controls. Mice with STZ-induced diabetes had significantly decreased sensitivity to intracerebroventricularly (i.c.v.) administered μ-opioid agonists, such as morphine (10 μg) and [d-Ala², N-Me Phe⁴,Gly-ol⁵]enkephalin (DAMGO, 0.5 μg). However, i.c.v. administration of [d-Pen^2,5]enkephalin (DPDPE, 5 μg), a δ-opioid agonist, and U-50,488H (50 μg), a κ-opioid agonist, produced pronounced antinociception in both control and diabetic mice. Furthermore, there were no significant differences in antinociceptive potency between diabetic and control mice when morphine (1 μg), DAMGO (10 μg), DPDPE (0.5 μg) or U-50,488H (50 μg) was administered intrathecally. In conclusion, mice with STZ-induced diabetes are selectively hyporesponsive to supraspinal μ-opioid receptor-mediated antinociception, but they are normally responsive to activation of δ- and κ-opioid receptors.     

Release of immunoreactive met-enkephalin from the spinal cord by intraventricular β-endorphin but not morphine in anesthetized rats

Effect of β-endorphin and morphine injected intraventricularly on the release of immunoreactive Met-enkephalin, Leu-enkephalin and dynorphin_1–13 from the spinal cord was studied in anesthetized rats. Intraventricular β-endorphin, 16 μg, caused a marked spinal release of immunoreactive Met-enkephalin and to a much lesser extent, of immunoreactive Leu-enkephalin while intraventricular morphine, 40 μg, did not cause any significant release of immunoreactive enkephalins. The release of immunoreactive Met-enkephalin was not blocked by the pretreatment with 5 mg/kg naloxone, i.p. Immunoreactive dynorphin_1–13 was not released by either β-endorphin or morphine. High performance liquid chromatographic analysis indicated that immunoreactive Met-enkephalin released by β-endorphin had a retention time identical to [³H]Met-enkephalin. These findings in conjunction with previous pharmacological studies suggest different modes of pharmacological action for β-endorphin and morphine.     

Serotonin receptor subtype antagonists in the medial ventral medulla inhibit mesencephalic opiate analgesia.

Supraspinal opioid analgesia is mediated in part by connections between the midbrain periaqueductal gray (PAG) and the ventral-medial medulla, including the nucleus raphe magnus (NRM) and nucleus reticularis gigantocellularis (NRGC). A serotonergic synapse appears to participate in this pathway since methysergide microinjected into the NRM-NRGC significantly reduced morphine analgesia elicited from the PAG. The present study evaluated the role of specific serotonin receptor subtypes by pretreating rats with microinjections of either the 5HT2 antagonist, ritanserin or the 5HT3 antagonist, ICS205930, into the NRM-NRGC and examining their effects upon morphine (2.5 micrograms) analgesia elicited from the PAG. Mesencephalic morphine analgesia was significantly reduced following pretreatment with both ritanserin (0.25-2.5 micrograms) on the tail-flick (81%) and jump (65%) tests and ICS205930 (0.25-5 micrograms) on the tail-flick (91%) and jump (63%) tests. Neither ritanserin nor ICS205930 altered basal nociceptive thresholds. Medullary placements ventral or lateral to the NRM/NRGC failed to support these antagonistic effects. These data indicate that ventro-medial medullary 5HT2 and 5HT3 serotonergic receptors modulate the transmission of opioid pain-inhibitory signals from the PAG.     

Effects of β-endorphin and its fragments on inhibitory avoidance behavior in rats

The effects on retrieval of a one-trial learning inhibitory avoidance response of β-endorphin, α-endorphin, and γ-endorphin, given prior to test have been studied in rats. β-Endorphin (β-LPH_61–91) in a relatively low dose (1.5 μg sc. or 50 ng icv.) facilitated inhibitory avoidance behavior, while a higher dose (10 μg sc. or 100 ng icv.) caused bimodal changes (facilitation in 50% of the animals and attenuation in another 40%. Peripheral injection of γ-endorphin attenuated inhibitory avoidance behaviour in a dose-dependent manner. The C-terminus of β-endorphin (β-LPH_78–91) was ineffective. α-Endorphin facilitated inhibitory avoidance behavior in a dose-dependent manner. Naltrexone pretreatment antagonized the bimodal effect of β-endorphin: following pretreatment with the opiate antagonist the low latency component disappeared, but the facilitatory effect of the neuropeptide remained the same.It is suggested that β-endorphin carries more than one bit of behavioral information. Inherent activities either related or unrelated to naltrexone-sensitive opiate receptors as well as biotransformation into α- and γ-endorphin may contribute to the multiple behavioral effects of this neuropeptide.     

Etorphine elicits anomalous excitatory opioid effects on sensory neurons treated with GM1 ganglioside or pertussis toxin in contrast to its potent inhibitory effects on naive or chronic morphine-treated cells

The ultra-potent opioid analgesic, etorphine, elicits naloxone-reversible, dose-dependent inhibitory effects, i.e. shortening of the action potential duration (APD) of naive and chronic morphine-treated sensory dorsal root ganglion (DRG) neurons, even at low (pM-nM) concentrations. In contrast, morphine and most other opioid agonists elicit excitatory effects, i.e. APD prolongation, at these low opioid concentrations, require much higher (ca. 0.1–1 μM) concentrations to shorten the APD of naive neurons, and evoke only excitatory effects on chronic morphine-treated cells even at high > 1–10 wM concentrations. In addition to the potent agonist action of etorphine at μ-, δ- and κ-inhibitory opioid receptors in vivo and on DRG neurons in culture, this opioid has also been shown to be a potentantagonist of excitatory μ-, δ- and κ-receptor functions in naive and chronic morphine-treated DRG neurons. The present study demonstrates that the potent inhibitory APD-shortening effects of etorphine still occur in DRG neurons tested in the presence of a mixture of selective antagonists that blocks all μ-, δ- and κ-opioid receptor-mediated functions, whereas addition of the epsilon (ε)-opioid-receptor antagonist, β-endorphin(1–27) prevents these effects of etorphine. Furthermore, after markedly enhancing excitatory opioid receptor functions in DRG neurons by treatment with GM1 ganglioside or pertussis toxin, etorphine showsexcitatory agonist action onnon-μ-/δ-/κ-opioid receptor functions in these sensory neurons, in contrast to its usual potent antagonist action on μ-, δ- and κ-excitatory receptor functions in naive and even in chronic morphine-treated cells which become supersensitive to the excitatory effects of μ-, δ- and -opioid agonists. This weak excitatory agonist action of etorphine on non-μ-/δ-/κ-opioid receptor functions may account for the tolerance and dependence observed after chronic treatment with extremely high doses of etorphine in vivo.     

The role of α2-adrenoceptors of the medullary lateral reticular nucleus in spinal antinociception in rats

We attempted to find out the role of α₂-adrenoceptors of the medullary lateral reticular nucleus (LRN) in antinociception in rats. Spinal antinociception was evaluated using the tail-flick test, and supraspinal antinociception using the hotplate test. Antinociceptive effects were determined following local electric stimulation of the LRN, and following microinjections of medetomidine (an α₂-adrenoceptor agonist; 1–10 μg), atipamezole (an α₂-adrenoceptor antagonist; 20 μg) or lidocaine (4%) into the LRN. The experiments were performed using intact and spinalized Hannover-Wistar rats with a unilateral chronic guide cannula. Electric stimulation of the LRN as well as of the periaqueductal gray produced a significant spinal antinociceptive effect in intact rats. Medetomidine (1–10 μg), when microinjected into the LRN, produced no significant antinociceptive effect in the tail-flick test in intact rats. However, following spinalization, medetomidine in the LRN (10 μg) produced a significant atipamezole-reversible antinociceptive effect in the tail-flick test in the hot-plate test, medetomidine (10 μg) in the LRN produced a significant atipamezole-reversible increase of the paw-lick latency in intact rats. Microinjection of atipamezole (20 μg) or lidocaine alone into the LRN produced no significant effects in the tail-flick test. The results are in line with the previous evidence indicating brat the LRN and the adjacent ventrolateral medulla is involved in descending inhibition of spinal nocifensive responses. However, α₂-adrenoceptors in the LRN do not mediate spinal antinociception but, on the contrary, their activation counteracts antinociception at the spinal cord level. The spinal aninociceptive effect of supraspinally administered medetomidine in spinalized rats can be explained by a spread of the drug (e.g., via circulation) which then directly activates α₂-adrenoceptors at the spinal cord level.     

Effects of β-endorphin2–9 on arginine-8-vasopressin and oxytocin levels in hypothalamic and limbic brain regions

Immunoreactive arginine-8-vasopressin (AVP) and oxytocin (OXT) were measured in rat hypothalamic and limbic brain regions after the intracerebroventricular administration of β-endorphin fragment 2–9 (βE_2–9). The peptide decreased the AVP content of the hippocampus and the OXT levels in the septum and amygdala. The present data favor the view that βE_2–9 interacts with limbic AVP- and OXT-systems.     

Studies on the intrathecal effect of β-endorphin in primate

The intrathecal administration of β-endorphin in the primate through an indwelling spinal catheter, produced a significant elevation in the nociceptive threshold as measured by the discrete trial shock titration task. The time of onset, duration of effect and magnitude of effect were all dose-dependent over a range of 150–750 μg. The effects were antagonized in a dose-dependent fashion by the systemic administration of naloxone. Aside from the elevations in the shock titration threshold produced by intrathecal β-endorphin, no untoward effects on the animal's motor function or behavioral reactivity was noted. Significantly, unlike morphine, intrathecal β-endorphin failed to produce any signs of scratching behavior at the doses used in these experiments. Once daily administration of intrathecal β-endorphin (500 μg) showed a significant progressive decline in the antinociceptive effect over an 8-day period. Animals made tolerant to β-endorphin in this fashion showed a significantly reduced response to an otherwise active dose of intrathecal morphine, indicating evidence for cross tolerance.