Analgesia produced by morphine when acting from the liquor space

A Concise Review of Concepts in Opioid Pharmacology up to the Discovery of Endogenous Opioids

This brief review covers concepts in opioid pharmacology that were promoted during the period leading up to the establishment of the International Narcotics Research Conference (INRC) in the early 1970s and the discovery of endogenous opioid peptides in 1975. The founders of INRC, meeting together during the International Union of Pharmacology meeting in Basel in 1969, recognized that the time was ripe for the creation of an international society that would provide a venue for the discussion of research across disciplines in this rapidly expanding area of science. The emphasis here is on studies leading to the demonstration that specific receptors for morphine-like analgesics exist, the search for endogenous ligands for these receptors, and early attempts to elucidate the mechanisms underlying opiate drug tolerance, dependence, and addiction. SIGNIFICANCE STATEMENT: Research on opioids in the 20th century was driven by the search for nonaddicting analgesics. This review discusses the development of the "analgesic" receptor concept, the demonstration that such receptors existed, and the search for an endogenous ligand. Conceptual models were proposed to explain tolerance to the actions of opiate drugs and the development of dependence and addiction. This review explains these models and indicates how they foreshadowed more recent discoveries on the acute and chronic actions of opiate drugs.

Lipo-endomorphin-1 derivatives with systemic activity against neuropathic pain without producing constipation

To enhance the drug-like properties of the endogenous opioid peptide endomorphin-1 (1 = Tyr-Pro-Trp-Phe-NH(2)), the N-terminus of the peptide was modified with 2-aminodecanoic acid, resulting in compound 3. Tyr in compound 1 was replaced with 2,6-dimethyltyrosine yielding compound 2. Derivative 2 was also substituted with 2-aminodecanoic acid producing compound, 4. Lipoamino acid-modified derivatives showed improved metabolic stability and membrane permeability while maintaining high μ-opioid (MOP) receptor binding affinity and acting as a potent agonist. In vivo studies showed dose-dependent antinociceptive activity following intravenous (i.v.) administration of compounds 3 and 4 in a chronic constriction injury (CCI)-rat model of neuropathic pain with ED(50) values of 1.22 (± 0.93) and 0.99 (± 0.89) µmol/kg, respectively. Pre-treatment of animals with naloxone hydrochloride significantly attenuated the anti-neuropathic effects of compound 3, confirming the key role of opioid receptors in mediating antinociception. In contrast to morphine, no significant constipation was produced following i.v. administration of compound 3 at 16 µmol/kg. Furthermore, following chronic administration of equi-potent doses of compound 3 and morphine to rats, there was less antinociceptive tolerance for compound 3 compared with morphine.

The effect of an osmotic stimulus on the release of neurohypophysial hormones in the cat: preferential release of vasopressin with a possible involvement of the area postrema

Abstract In cats anaesthetized with intravenous chloralose, the injection of 0.05 to 0.4 ml 1.54 M NaCl solution (hypertonic saline, HS) into a lateral cerebral ventricle caused a large release of vasopressin. The concentration of vasopressin greatly exceeded that of oxytocin in the same samples of plasma. Vasopressin was also released when HS was injected into the fourth ventricle and into the cisterna magna from which there is no access in the cat to the ventricles, but it was less effective by these routes than when injected into a lateral ventricle in the same cat. This suggests a possible action of HS on circumventricular organs related to the third ventricle but also indicates an additional site of action reached from the subarachnoid space which would give access to the ventral and dorsal surfaces of the brainstem. Vasopressin was not released on topical application of HS to the 'nicotine sensitive area' on the ventral surface of the brainstem where nicotine acts to release vasopressin without oxytocin. Vasopressin, however, was released without detectable oxytocin on topical appliction of HS to the dorsal surface of the brainstem either outside the fourth ventricle or to the floor of the ventricle at its distal extremity, in the region of the obex. A possible site where HS acts to cause a preferential release of vasopressin on injection into a lateral ventricle is the area postrema, a circumventricular organ which impinges on the walls of the fourth ventricle at the obex. Preferential release of vasopressin might then be mediated by a selective neural input, possibly through the nucleus of the tractus solitarius, from osmoreceptors in the area postrema to the vasopressin-secreting cells in the supraoptic and paraventricular nuclei.

The central site of the sympatho-inhibitory action of 5-hydroxytryptamine in the cat

The aim of this study was to determine the site in the CNS at which 5-hydroxytryptamine (5-HT) inhibits efferent sympathetic nerve activity in the cat. 5-Hydroxytryptamine (3 and 10 micrograms/kg), given into the lateral cerebral ventricle, produced immediate non dose related increases in mean blood pressure (MBP), heart rate (HR) and renal nerve activity (RNA). Larger doses (30 and 100 micrograms/kg i.c.v.) produced gradual decreases in blood pressure, heart rate and renal nerve activity, which did not occur when access of the drug to the fourth ventricle was prevented. Administration of 5-HT (10 and 30 micrograms/kg) into the fourth ventricle produced only decreases in blood pressure, heart rate and renal nerve activity after 15-40 min, which were accompanied by decreases in cardiac output and renal vascular resistance, but little or no change in total peripheral resistance. Application of 5-HT onto the ventral surface of the medulla, into the subarachnoid space at various levels along the spinal cord or into various parts of the nucleus tractus solitarius produced no effect on blood pressure heart rate or renal nerve activity. However, application of a cotton wool pledget soaked in a 5-HT solution (3 mg/ml) over the entire obex/NTS region produced immediate decreases in blood pressure, heart rate and renal nerve activity. These studies suggest that the sympatho-inhibitory effect of 5-HT is due to an action at a site near the caudal end of the dorsal surface of the medulla.

Intraventricular morphine for control of pain in terminal cancer patients

Satisfactory control of intractable pain has been achieved in 17 terminal cancer patients by injecting small doses of morphine into the lateral cerebral ventricle via an Ommaya reservoir. Pain relief together with a favorable behavioral response was obtained without interference with other sensory modalities, noticeable physical changes, or side effects annoying or severe enough for the patient to discontinue therapy. Eleven patients developed tolerance, but this phenomenon does not require withdrawal of treatment. Chronic intraventricular morphine administration can be safely performed on an outpatient basis, and results in control of midline, bilateral, and diffuse pain associated with orofacial and disseminated cancer. However, this experience is preliminary and further clinical trials are needed to determine the place of this method of therapy in the management of chronic pain.

Role of the nucleus raphe magnus in opiate analgesia as studied by the microinjection technique in the rat

The analgesic effects of morphine (5 microgram, 0.2 microliter) microinjected into the nucleus raphé magnus (NRM) and the surrounding reticular formation of the rat were tested using vocalization after electric shock to the tail as the test for analgesia. Only sites in the NRM produced powerful analgesic effects, strongest analgesia being equivalent to 3 mg/kg i.v. morphine. The analgesia produced by the microinjection was reversed by systemic naloxone. Pretreatment with systemic cinanserin, a blocker of serotonergic receptors, led to a pronounced diminution of the analgesic effects of the morphine. The effects of microinjections of naloxone (5 microgram 0.2 microliter) were studied for their effect on analgesia produced by systemic morphine. The analgesia following 3 mg/kg i.v. morphine was diminished by the microinjection of naloxone but the naloxone almost completely reversed the analgesic effects of 1.5 mg/kg i.v. morphine. These results further substantiate the role of the NRM in analgesic mechanisms.

The effect of morphine and some other narcotic analgesics on brain tryptophan concentrations

An acute dose of morphine increased brain tryptophan in mice. This effect was not prevented by naloxone nor was it produced by other narcotic analgesics. Dextrorphan, but not levorphanol, had a similar effect to morphine. A large dose of tryptophan had no effect on the antinociceptive action of morphine in mice. Morphine increased brain tryptophan in rats. This effect was prevented by naloxone. A large dose of tryptophan antagonised the antinociceptive action of morphine in the rat.

C-fragment of lipotropin--an endogenous potent analgesic peptide

1 A series of peptides derived from porcine lipotropin was examined for analgesic and other morphine-like properties on infusion into the cannulated third ventricle of cats.2 Lipotropin (LPH 1-91) itself produced no analgesia or other morphine-like effects when infused in a dose of 150 mug.3 C-fragment (LPH 61-91) produced strong long-lasting analgesia when infused in a dose of 10 or 20 mug; on a molar basis the potency was between 90 and 180 times that of morphine. The following morphine-like effects were also produced: shivering leading to fever, vasodilatation of the pinnae, mydriasis, opening of the palpebral fissures, tachypnoea with bouts of panting, vocalization, hyperexcitability, restlessness and catalepsy. All the effects, including analgesia, were abolished by an intraperitoneal injection of naloxone (1 mg/kg).4 Hyperglycaemia, another central effect produced by morphine, was obtained with C-fragment infused in a dose of 60 mug.5 On intravenous injection, C-fragment produced analgesia with a dose of about 200 mug/kg. Administered by this route, C-fragment was again more potent than morphine.6 C'-fragment (LPH 61-87), LPH 61-78 and LPH 61-69, either had no analgesic effect or produced weak short-lasting analgesia when infused in doses up to 100 mug.7 Methionine enkephalin (LPH 61-65) either produced very weak short-lasting analgesia or had no analgesic effect when infused in doses of between 30 and 400 mug.8N-methyl methionine enkephalin amide in which both termini of methionine enkephalin were protected against degradation by exopeptidases produced long-lasting analgesia when infused in doses of 150 to 180 mug; its analgesic potency was approximately 100 times less than that of C-fragment. Blocking only one terminus of methionine enkephalin did not appear to endow the peptide with analgesic properties. The N-methyl pentapeptide amide produced other morphine-like effects of which the most striking was catalepsy. All the effects were abolished by intraperitoneal naloxone (1 mg/kg).