Opioid peptides and primary biliary cirrhosis

Endogenous opioids accumulate in plasma in a rat model of acute cholestasis

To obtain data on the degree to which the opioid system is changed in cholestasis, endogenous opioid activity in plasma of rats with acute cholestasis was determined 5 days after bile duct resection. Total plasma opioid activity was determined using a radioreceptor technique that measured the displacement of the opiate receptor ligand [3H]-DAMGO from lysed synaptosomal fractions of normal rat brain. Plasma total opioid activity was threefold greater in bile duct-resected rats than in sham-operated and unoperated controls (P less than or equal to 0.05). Plasma levels of the individual endogenous opioid, methionine-enkephalin, were determined using a sensitive radioimmunoassay, and the specificity of the assay was confirmed using high-performance liquid chromatography. In cholestatic rats, plasma methionine-enkephalin levels were more than six-fold greater than in sham-operated controls (P less than or equal to 0.001) and more than 17-fold greater than in unoperated controls (P less than or equal to 0.001). However, plasma methionine-enkephalin levels accounted for less than 5% of total plasma opioid activity after bile duct resection. Plasma methionine-enkephalin levels in both cholestatic plasma and plasma from sham-operated animals were stable when incubated in vitro despite the presence of undiminished activity of the major enkephalin-degrading enzymes. Thus, protection of methionine-enkephalin from degradation may be a factor contributing to the elevated plasma levels of methionine-enkephalin found in cholestasis. The magnitude of the increase in plasma endogenous opioid activity in bile duct-resected rats provides support for the hypothesis that endogenous opioids contribute to the pathophysiology of cholestasis.

Methionine enkephalin accumulates in plasma but not in brain or cerebrospinal fluid of rats with acute toxic hepatitis

In order to determine whether acute toxic hepatitis in the rat is associated with an accumulation of methionine enkephalin in plasma and increased blood-to-brain transfer of methionine enkephalin, immunoreactive methionine enkephalin levels were determined by radioimmunoassay in plasma, cerebrospinal fluid and whole brain samples from rats with thioacetamide induced acute toxic hepatitis. Thioacetamide treatment was associated with an 8.7-fold increase in plasma immunoreactive methionine enkephalin levels (P less than or equal to 0.005) 24 h after treatment. However, this marked elevation in plasma immunoreactive methionine enkephalin levels was not associated with an increase in whole brain or cerebrospinal fluid immunoreactive methionine enkephalin levels. These data suggest that increased plasma-to-brain transfer of methionine enkephalin does not occur in this model of acute toxic hepatitis.

Central mu-opioid receptors are down-regulated in a rat model of cholestasis

Ameliorations of the pruritus of cholestasis by opioid antagonists are consistent with this form of pruritus being centrally mediated by the opioid system. To determine whether the central opioid system is altered in cholestasis, the specific binding of a selective mu-opioid receptor ligand, 3H-DAMGO, to mu-opioid receptors was studied in rats with acute cholestasis due to bile duct resection. Using whole brain membranes and subcellular mitochondrial-synaptosomal fractions the density of mu-receptor sites was 30% (p less than 0.01) and 22% (p = 0.03) less in bile-duct-resected rats than in sham-resected rats. Using membranes from individual brain regions specific binding of 3H-DAMGO was reduced by 43-53% in the cerebral cortex, hippocampus and caudate nucleus of bile-duct-resected rats. Thus mu-opioid receptors in the brain are down-regulated in a classical model of cholestasis. This alteration of the central opioid system could be a consequence of increased exposure of opioid receptors to endogenous opioids in cholestasis and may reflect an important mechanism in the pathogenesis of the pruritus of cholestasis.

Pruritus: a practical approach

Pruritus is usually caused by a primary disorder of the skin, but can also be caused by a systemic disease (Table 1). Some dermatologic conditions that cause pruritus can be inconspicuous or nonspecific (Table 2), while others are usually apparent on physical examination (Table 3). Classification of pruritus as localized (Fig. 1) vs. generalized (Fig. 3) can be helpful in arriving at a correct diagnosis. The history and physical examination are the most important diagnostic tools, though laboratory testing for systemic disease may be necessary. In refractory cases, one should consider occult systemic disease (such as malignancy), psychiatric disease (especially depression), and HIV infection. Subsequent referral to a dermatologist may be indicted. When treatment of the underlying cause of pruritus is not possible, antihistamines and topical agents (menthol, phenol, and/or pramoxine) can be helpful.

A controlled trial of naloxone infusions for the pruritus of chronic cholestasis

To test the hypothesis that opioid agonist activity contributes to the pruritus of cholestasis, a placebo-controlled single-blinded trial of naloxone, an opioid antagonist, was conducted in eight patients with primary biliary cirrhosis. After discontinuation of all conventional antipruritic medications, one or two continuous (24-hour) IV infusions of naloxone (0.2 micrograms.kg-1.min-1) and placebo solution were administered consecutively in an order that was not predetermined. Pruritus was assessed subjectively by means of four hourly recordings of a visual analogue score. In addition, objective measurements of scratching activity that were independent of gross body movements were continuously recorded using an apparatus specifically designed to measure the frequencies associated with this activity. No side effects associated with naloxone infusions were observed. Only scratching activity data obtained for the same periods of day and night during both naloxone and placebo infusions were compared. Naloxone infusions were consistently associated with a decrease in values of the scratching activity index. In addition, in 50% of the patients the infusions were associated with a decrease in visual analogue score. The mean decrease in scratching activity ranged from 29% to 96% (mean, 50%; P less than 0.001). These findings imply that increased opioid agonist activity contributes to scratching activity in cholestatic patients.

Opioid needs of terminal care patients: variations with age and primary site

The records of 1383, terminal cancer patients have been reviewed to determine whether opioid requirements vary in a consistent pattern with age and with site of primary cancer. Opioid requirements are shown to decrease with age in a regular pattern and from early adult life. Opioid requirements also differ significantly with site of primary cancer and there appears to be a discernible pattern of opioid need for different cancers. Patients with sexually related cancer have the highest opioid requirements, irrespective of sex. When the sexually determined cancers are excluded, the analgesic needs of men and women are not significantly different (P = 0.191).

Plasma leucine enkephalin is increased in liver disease

Plasma methionine enkephalin is increased in liver disease and may contribute to some of the clinical manifestations of hepatic failure. To determine if another 'small' opioid peptide is increased in the plasma of patients with liver disease, leucine enkephalin was measured by radioimmunoassay. Its plasma concentration was raised approximately five-fold in patients with acute liver disease (median 1490 pmol/l, range 830-2420) and three-fold in patients with cirrhosis with ascites (960 pmol/l, 470-2900), compared with disease controls (325 pmol/l, 180-740) and healthy controls (305 pmol/l, 180-560). The increase in plasma leucine enkephalin was proportional to the degree of liver damage, as judged in the patients with acute liver disease by its correlation with the prothrombin time (r = 0.691, p less than 0.01) and alanine aminotransferase (r = 0.502, p less than 0.05), and in the patients with cirrhosis by its negative correlation with the plasma albumin (r = -0.743, p less than 0.001). It is unclear whether the raised plasma leucine enkephalin in liver disease is a consequence of diminished hepatic inactivation, increased secretion from sympathetic nerves and adrenal glands, or both.