M.H. Millan

Spinal cord dynorphin may modulate nociception via a ℵ-opioid receptor in chronic arthritic rats

Abstract Inoculation of rats with Mycobacterium butyricum produced an arthritis of the limbs which revealed an enhanced sensitivity to noxious mechanical pressure (hyperalgesia). Arthritic rats displayed a pronounced rise in immunoreactive dynorphin in lumbo-sacral spinal cord which correlated both with the intensity and time-course of this hyperalgesia. MR-2266, a relatively preferential antagonist at the ℵ-opioid receptor (at which dynorphin is considered to act) potentiated this hyperalgesia. In contrast, MR 2267 (its inactive stereoisomer) was ineffective. Further, naloxone (a weak ℵ-antagonist), and ICI 154,129 (a preferential σ-antagonist) were, in each case, inactive. The data demonstrate a pronounced response of spinal dynorphin to chronic arthritic pain in the rat. In addition, they raise the possibility of a function of spinal DYN, via a ℵ-receptor, in the modulation of chronic arthritic pain.

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)-beta-endorphin (beta-EP) in the midbrain PAG. This depletion was selective in that: animals placed in the chamber and not stimulated revealed neither an analgesia nor an alteration in levels of ir-beta-EP. No change in levels of ir-beta-EP was detectable in other brain regions. Both stimulated rats and rats placed in the chamber and not stimulated revealed a rise in circulating ir-beta-EP: the magnitude of this rise did not, however, differ between these groups. 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 beta-EP therein. Together with our finding that destruction of PAG-localized beta-EP neurones to block stimulation-analgesia, the data suggest that an activation of intrinsic pools of beta-EP underlies stimulation-produced analgesia elicited from the PAG in the rat.

Vasopressin and oxytocin in the rat spinal cord: distribution and origins in comparison to [Met]enkephalin, dynorphin and related opioids and their irresponsiveness to stimuli modulating neurohypophyseal secretion.

Immunoreactive-vasopressin, -oxytocin, -dynorphin, -dynorphin-(1-8), -alpha-neo-endorphin and -[Met]enkephalin were, in each case, present in greater concentrations in dorsal as compared to ventral, and lumbo-sacral as compared to cervico-thoracic, spinal cord. These differences were significantly more pronounced for vasopressin and oxytocin than for the other peptides. Lesions of the hypothalamic paraventricular nucleus depleted levels of immunoreactive-vasopressin and -oxytocin throughout the cord whereas levels of the opioid peptides therein were unaffected. In contrast, destruction of either the supraoptic or suprachiasmatic nucleus failed to change the content of immunoreactive-vasopressin, -oxytocin or any of the opioid peptides in the cord. Dehydration for 3 days depressed levels of immunoreactive-vasopressin, -oxytocin and -dynorphin in the neurointermediate lobe of the pituitary. In distinction, the levels of these were not modified in the spinal cord. Further, treatment with the synthetic corticosteroid, dexamethasone, elevated levels of immunoreactive-vasopressin, -oxytocin and -dynorphin in the neurointermediate pituitary whereas these were unaffected in the spinal cord. It is concluded that vasopressin and oxytocin in the spinal cord are predominantly derived from the paraventricular nucleus, localized in dorsal lumbo-sacral regions of the cord and insensitive to endocrinological manipulations. These pools may, thus, be modulated differently from their counterparts in the neurohypophysis and have a differing role, possibly in the control of the primary processing, autonomic or motor junctions. Further, there is no evidence from these or our prior studies for a close interrelationship of spinal cord vasopressin with dynorphin-related peptides (or oxytocin with [Met]enkephalin), likewise in contrast to the neurohypophysis.(ABSTRACT TRUNCATED AT 250 WORDS)

Vasopressin and oxytocin in the rat spinal cord: analysis of their role in the control of nociception

Lesions of the hypothalamic paraventricular nucleus depleted immunoreactive (ir)-vasopressin (VP) and -oxytocin (OT) from rat spinal cord but failed to modify nociceptive thresholds. Further, intrathecal introduction of VP and OT into the cord failed either to influence nociceptive thresholds or to modify the antinociceptive action of morphine. However, doses of VP as low as 20 ng caused, in contrast to OT, a hind-limb muscular flaccidity and respiratory disturbances. Rats suffering from chronic arthritis did not, finally, reveal any alterations in levels or ir-VP or ir-OT in the spinal cord. Is is concluded that spinal pools of VP and OT are derived from the paraventricular nucleus and do not play a major role in nociceptive processes in the spinal cord. A role of spinal VP in motor and, possibly autonomic control is, nevertheless, indicated.

Endogenous opioids, circadian rhythms, nutrient deprivation, eating and drinking.

Immunoreactive (ir) beta-endorphin (b-END) and dynorphin (DYN) in rat brain and pituitary were measured after food and water deprivation and from brains taken during either day or night. In other rats, eating and drinking were measured following lesions in the arcuate n. Ir-DYN levels are higher in hypothalamus and lower in pituitary at night. Deprivation, particularly water deprivation, increases hypothalamic, day-time ir-DYN. Water deprivation decreases pituitary levels of ir-DYN. Arcuate-lesions, depleting both ir-b-END and ir-DYN, do not modify total daily intake of water or food but does modify circadian rhythmicity of eating and drinking. These data support the conclusion that b-END and DYN are involved in maintaining day-night patterns of eating and drinking.

Depletion of central β-endorphin blocks midbrain stimulation produced analgesia in the freely-moving rat

The present study examines the role of central beta-endorphin in the generation of stimulation-induced analgesia from the ventral midbrain periaqueductal gray of freely-moving rats. Electrical stimulation of the ventral midbrain periaqueductal gray led to an antinociception against noxious heat which gradually subsided post-stimulation over a period of about 15 min. Locomotor effects (ipsilateral rotation) were also seen which were not correlated in intensity with the analgesia and which disappeared immediately with termination of stimulation. There was no indication of any aversive effects. Application of the opioid antagonist, naloxone, 10 min pre-stimulation, strongly attenuated the antinociception without changing basal thresholds. It did not influence the locomotor changes. Bilateral, radiofrequency lesions of the mediobasal arcuate hypothalamus greatly depleted immunoreactive beta-endorphin from brain tissues without affecting its levels in plasma. Lesioned rats showed a pronounced reduction of stimulation-produced antinociception in the absence of any change in basal thresholds; the locomotor effects of stimulation were not influenced. The degree of depletion of immunoreactive-beta-endorphin significantly correlated with the degree of attenuation of antinociception. These data suggest: stimulation of the ventral midbrain periaqueductal gray leads both to an antinociception and locomotor effects in freely-moving rats: these can be clearly dissociated from each other; the antinociception (but not locomotor effects) are mediated by an endogenous opioid sensitive to blockade by naloxone; and central beta-endorphin may be the endogenous opioid mediating stimulation-produced antinociception from the ventral midbrain periaqueductal gray in the rat.

The influence of selective adeno- and neurointermedio-hypophysectomy upon plasma and brain levels of ß-endorphin and their response to stress in rats

Selective ablation of the anterior lobe (AL) of the pituitary led to a fall in basal plasma levels of beta-endorphin immunoreactivity (beta-EI) at 3 and 20 weeks post-surgery (p.s.). Further, the stress-evoked rise in circulating levels of beta-EI was abolished. This operation did, however, severely deplete the beta-EI content of the neurointermediate lobe (NIL). Removal of the NIL did not, in contrast, decrease the beta-EI content of the AL but depressed basal plasma levels of beta-EI at 3 weeks p.s. and attenuated, but did not abolish, the increase in these elicited by stress at both 3 and 20 weeks p.s. In rats not possessing a NIL, a secretion of beta-EI into plasma can thus occur. The possibility that NIL pools of beta-EI contribute to circulating levels of beta-EI is discussed. Removal of the AL depressed the beta-EI content of the hypothalamus and periventricular tissue at 3 and 20 weeks p.s. The Met-enkephalin-immunoreactivity (ME-I) content of the hypothalamus was, in contrast, unaffected. These animals still responded to stress at 20 weeks p.s. with a significant fall in hypothalamic levels of beta-EI. Extirpation of the NIL did not, in contrast, change brain levels of either beta-EI or ME-I. The presence of the AL, but not the NIL, is thus essential for the maintenance of usual levels of beta-EI and ME-I in the brain.

The Hypothalamic Paraventricular Nucleus: Relationship to Brain and Pituitary Pools of Vasopressin and Oxytocin as Compared to Dynorphin, β-Endorphin and Related Opioid Peptides in the Rat

The present study examines the relationship of the hypothalamic paraventricular nucleus (PVN) to discrete brain and pituitary pools of immunoreactive (ir) vasopressin (VP), oxytocin (OT) and particular opioid peptides in the rat. Selective, bilateral destruction of the PVN resulted in a parallel depression in levels of ir-VP, ir-OT, ir-dynorphin (DYN), ir-DYN and ir-alpha-neo-endorphin (alpha-NE) in the neurointermediate lobe of the pituitary, whereas in its anterior counter-part no decrease in the content of any of these peptides was seen. In contrast, the content of ir-beta-endorphin (beta-EP) in the neurointermediate lobe was not significantly altered. Further, a rise in levels of ir-beta-EP in the anterior lobe, together with a fall in these in systemic plasma, was found. In the hypothalamus, in distinction to ir-met-enkephalin (ME), a diminution in the content of ir-VP, ir-OT, ir-DYN, ir-DYN, ir-alpha-NE and, in addition, ir-beta-EP was observed. In the septum, midbrain and medulla/pons, however, a selective depression in levels of ir-VP (and, as measured in the medulla/pons, of ir-OT) was seen: the content of ir-DYN, ir-DYN, ir-alpha-NE, ir-beta-EP and ir-ME was unchanged in these tissues. These data indicate that: (1) the PVN is an important contributor to neurointermediate lobe, but not anterior lobe, pools of ir-VP, ir-OT, ir-DYN, ir-DYN and ir-alpha-NE in contrast to ir-beta-EP.(ABSTRACT TRUNCATED AT 250 WORDS)

Contribution of the Supra-Optic Nucleus to Brain and Pituitary Pools of Immunoreactive Vasopressin and Particular Opioid Peptides, and the Interrelationships between These, in the Rat

Discrete, bilateral, radiofrequency destruction of the supra-optic nucleus resulted in a parallel fall in levels of immunoreactive (ir) dynorphin (DYN), ir-alpha-neo-endorphin (alpha-NE) and ir-vasopressin (VP) in the hypothalamus, neurointermediate lobe (NIL) of the pituitary and septum of rats, whereas in the medulla/pons, midbrain and anterior lobe (AL), levels of theses peptides were not significantly changed. The content of ir-beta-endorphin (beta-endorphin (beta-EP) was, in contrast, elevated in the hypothalamus and unchanged in the NIL and AL, while that of ir-met-enkephalin (ME) was modified in neither the hypothalamus nor medulla/pons. As evaluated in lesioned and sham rats, collectively, hypothalamic levels of ir-DYN, ir-alpha-NE and ir-vP were positively correlated with their counterparts in the NIL but not, with the exception of ir-VP, with those in the AL. Ir-beta-E did not show these relationships. Within the NIL and in the hypothalamus, levels of ir-DYN, ir-alpha-NE and ir-VP were positively correlated with each other and independent of those of ir-beta-EP and ir-ME. In the ALK and other brain tissues, however, only those of ir-DYN and ir-alpha-NE were positively and significantly correlated in contrast to ir-VP. Further, across all structures, although the ratio of levels of ir-alpha-NE to ir-DYN was quite constant, that of ir-VP to ir-DYN varied enormously. These data indicate that: (1) the supra-optic nucleus may contribute to NIL, hypothalamic and septal but not to AL, midbrain or medulla/pons pools of ir-VP, ir-DYN and ir-alpha-NE, and that it interacts with those of ir-beta-EP in the hypothalamus. (2) in the NIL and hypothalamus, ir-DYN, ir-alpha-NE and ir-VP are, as compared to ir-beta-EP and ir-ME, closely interrelated. (3) AL pools of DYN and ir-alpha-NE are closely coupled but, in contrast to NIL pools, not related to ir-VP and independent of the hypothalamus. (4) Extrahypothalamic brain pools of ir-DYN and ir-alpha-NE may be not related to ir-VP. The possible existence of ir-DYN and ir-alpha-NE independently or ir-vP in the AL and extrahypothalamic nervous tissue is discussed.

Evidence for a role of the ventro-medial posterior hypothalamus in nociceptive processes in the rat.

Bilateral, radio-frequency destruction of the ventro-medial posterior hypothalamus (VMPH) resulted, as compared to sham-operated and control rats and evaluated in the tail-flick and vocalization tests, in a significant decrease in basal nociceptive threshold on day 4 post-surgery. By day 12, however, no significant difference between sham and lesioned rats was seen. At this time the antinociception elicited by either acute foot-shock or cold-water-immersion stress was profoundly attenuated. The antinociceptive response to various doses of morphine was not, in contrast, diminished. As established by use of radioimmunoassay, these lesions did not significantly alter hypothalamic levels of beta-endorphin, met-enkephalin, dynorphin or alpha-neo-endorphin. They did, however, produce a pronounced and significant fall in the hypothalamic content of substance P. These data are indicative that the VMPH may, via a mechanism not involving endorphins, be of importance in the determination of basal nociceptive threshold and in the generation of stress-, but not morphine-, evoked antinociception. The relationship of these findings to the interconnections of the VMPH, and to the possible significance of substance P and the pituitary in nociceptive processes, is discussed.