Prevention of chronic anthracycline cardiotoxicity in the adult Fischer 344 rat by dexrazoxane and effects on iron metabolism.

PURPOSE: Anthracyclines, such as doxorubicin and daunorubicin, continue to be widely used in the treatment of cancer, although they share the adverse effect of chronic, cumulative dose-related cardiotoxicity. The only approved treatment in prevention of anthracycline cardiotoxicity is dexrazoxane, a putative iron chelator. Previous in vitro studies have shown that disorders of iron metabolism, including altered IRP1-IRE binding, may be an important mechanism of anthracycline cardiotoxicity. METHODS: This study examined the role of IRP1-IRE binding ex vivo in a chronic model of daunorubicin cardiotoxicity in the Fischer 344 rat and whether dexrazoxane could prevent any daunorubicin-induced changes in IRP1 binding. Young adult (5-6 months) Fischer 344 rats received daunorubicin (2.5 mg/kg iv once per week for 6 weeks) with and without pretreatment with dexrazoxane (50 mg/kg ip). Other groups received saline (controls) or dexrazoxane alone. Rats were killed either 4 h or 2 weeks after the last dose of daunorubicin to assess IRP1-IRE binding. RESULTS: Contractility (dF/dt) of atrial tissue, obtained from rats 2 weeks after the last dose of daunorubicin, was significantly reduced in daunorubicin-treated compared to control rats. Dexrazoxane pretreatment protected against the daunorubicin-induced decrease in atrial dF/dt. However, left ventricular IRP1/IRE binding was not affected by daunorubicin treatment either 4 h or 2 weeks after the last dose of daunorubicin. CONCLUSIONS: IRP1 binding may not be altered in the rat model of chronic anthracycline cardiotoxicity.

Obesity-inducing lesions of the central nervous system alter leptin uptake by the blood-brain barrier.

Leptin regulates body adiposity by decreasing feeding and increasing thermogenesis. Obese humans and some obese rodents are resistant to peripherally administered leptin, suggesting a defect in the transport of leptin across the blood-brain barrier (BBB). Defective transport of exogenous leptin occurs in some models of obesity, but in other models transport is normal. This shows that factors other than obesity are associated with impairment of leptin transport across the BBB. In order to further investigate these factors, we determined leptin transport in rats made obese by lesioning of the ventromedial hypothalamus (VMH), paraventricular nucleus (PVN), or posterodorsal amygdala (PDA). These regions all contain leptin receptors and lesions there induce obesity and hyperleptinemia and alter the levels of many feeding hormones which might participate in leptin transporter regulation. We measured the uptake of radioactively labeled leptin by the BBB by multiple-time regression analysis which divides uptake into a reversible phase (Vi, e.g., receptor/transporter binding to the brain endothelial cell) and an irreversible phase (Ki, complete transport across the BBB). Leptin uptake was not affected in rats with VMH lesions. No significant change occurred in the entry rate (Ki) for any group, although Ki declined by over 35% in rats with PVN lesions. Decreased uptake was observed in rats with PVN lesions and with PDA lesions. This was primarily due to a reduced Vi (about 21% for the PDA). This decreased uptake is most likely explained by decreased binding of leptin to the brain endothelial cell, which could be because of decreased binding by either receptors or transporters. This suggests that some of the feeding hormones controlled by the PVN and PDA may participate in regulating leptin uptake by the BBB.

Randomized, double-blind, placebo-controlled trial of coenzyme Q10 in isolated systolic hypertension.

BACKGROUND: Increasing numbers of the adult population are using alternative or complementary health resources in the treatment of chronic medical conditions. Systemic hypertension affects more than 50 million adults and is one of the most common risk factors for cardiovascular morbidity and mortality. This study evaluates the antihypertensive effectiveness of oral coenzyme Q10 (CoQ), an over-the-counter nutritional supplement, in a cohort of 46 men and 37 women with isolated systolic hypertension. METHODS: We conducted a 12-week randomized, double-blind, placebo-controlled trial with twice daily administration of 60 mg of oral CoQ and determination of plasma CoQ levels before and after the 12 weeks of treatment. RESULTS: The mean reduction in systolic blood pressure of the CoQ-treated group was 17.8 +/- 7.3 mm Hg (mean +/- SEM). None of the patients exhibited orthostatic blood pressure changes. CONCLUSIONS: Our results suggest CoQ may be safely offered to hypertensive patients as an alternative treatment option.

Sarcoplasmic reticulum calcium release is stimulated and inhibited by daunorubicin and daunorubicinol.

Cardiac effects of anthracyclines or their metabolites may include both the stimulation and inhibition of Ca(2+) release from sarcoplasmic reticulum. In this study, the ability of daunorubicin and its primary metabolite, daunorubicinol, to stimulate and inhibit Ca(2+) release from canine sarcoplasmic reticulum (SR) vesicles was investigated. It was observed that both daunorubicin and daunorubicinol were several fold more potent at inhibiting than they were at stimulating SR Ca(2+) release. Respective IC50 inhibition of daunorubicin and daunorubicinol for caffeine-induced calcium release was 1.2 and 0.6 microM, and for spontaneous Ca(2+) release was 3 and 1 microM. EC50's for daunorubicin- and daunorubicinol-induced calcium release were 30 and 15 microM, respectively. Inhibition of either spontaneous or caffeine-induced SR Ca(2+) release was inversely related to the amount of Ca(2+) loaded into the SR before exposure to daunorubicin or daunorubicinol. The free-radical scavenger dithiothreitol did not attenuate the ability of anthracyclines to inhibit SR Ca(2+) release. A nonquinone daunorubicin derivative, 5-iminodaunorubicin, was less potent than daunorubicin at inhibiting caffeine-induced Ca(2+) release. These data suggest anthracyclines and their metabolites may produce cardiotoxicity through free-radical independent, concentration-dependent effects on SR Ca(2+) release. These effects involve either inhibition or stimulation of SR Ca(2+) release and are partly dependent upon the presence of the quinone moiety.

Daunorubicin cardiotoxicity: evidence for the importance of the quinone moiety in a free-radical-independent mechanism.

Anthracyclines, such as daunorubicin (Daun), and other quinone-containing compounds can stimulate the formation of toxic free radicals. The present study tests the hypothesis that the quinone moiety of Daun, by increasing free-radical production, disrupts sarcoplasmic reticulum (SR) function and thereby inhibits myocardial contractility in vitro. We compared Daun with its quinone-deficient analogue, 5-iminodaunorubicin (5-ID), using experimental interventions to produce various contractile states that depend on SR function. At concentrations of Daun or 5-ID that did not alter contractility (dF/dt) of steady-state contractions (1 Hz) in electrically paced atria isolated from adult rabbits, only Daun significantly attenuated the positive inotropic effects on dF/dt of increased rest intervals (PRP; post-rest potentiation) or increased stimulation frequencies. Attenuation was to 98+/-6% at 1 Hz, and 73+/-8 and 67+/-8% for 30 and 60 sec PRP, respectively, and 73+/-3 and 63 +/-3% at 2 and 3 Hz, respectively, for 88 microM Daun (P<0.05, vs pre-drug baseline values, mean +/- SEM). These effects of Daun were similar to those of caffeine (2 mM), an agent well known to deplete cardiac SR calcium. We also examined the effect of Daun in isolated neonatal rabbit atria, which lack mature, functional SR; Daun did not alter the force-frequency relationship or PRP contractions. Additional studies in Ca(2+)-loaded SR microsomes indicated that both Daun and 5-ID opened Ca(2+) release channels, with Daun being 20-fold more potent than 5-ID in this respect. Neither anthracycline, however, induced free-radical formation in SR preparations (assayed via nicking of supercoiled DNA) prior to stimulating Ca(2+) release. Thus, our results indicate that Daun impairs myocardial contractility in vitro by selectively interfering with SR function; the quinone moiety of Daun appears to mediate this cardiotoxic effect, acting through a mechanism that does not involve free radicals.

Prevention by dexrazoxane of down-regulation of ryanodine receptor gene expression in anthracycline cardiomyopathy in the rat.

Anthracyclines can cause cumulative dose-related cardiotoxicity characterized by changes in Ca(2+) metabolism, including dysfunction of the sacroplasmic reticulum (SR) and decreased expression of Ca(2+)-handling proteins, such as the ryanodine receptor (RyR2). In this study, we examined the effect of dexrazoxane (ICRF-187), an iron chelator which prevents anthracycline cardiotoxicity, on RyR2 gene expression in rats treated chronically with daunorubicin. Daunorubicin (2.5 mg kg(-1) i.v. weekly for 6 weeks) produced cardiotoxicity as demonstrated by histopathologic changes. The ryanodine receptor/glyceraldehyde phosphate dehydrogenase (GAPDH) mRNA ratio was decreased by 38+/-3% (P<0.02) compared to values in control rats. Dexrazoxane pre-treatment (50 mg kg(-1); 1 h prior to each daunorubicin injection) prevented the decrease in RyR2/GAPDH mRNA ratio and histopathologic lesions in daunorubicin-treated rats. This is the first report that a protective agent such as dexrazoxane can ameliorate the decreased expression of a specific gene involved in anthracycline-induced cardiotoxicity.

Effects of cyclopentyladenosine on isoproterenol response in adult and senescent cardiac tissue from Fischer 344 rats.

To characterize age-related changes in beta-adrenergic responsiveness and to test the hypothesis that an increase in the effects of adenosine contribute to impaired beta-adrenergic responsiveness, Fischer 344 rat right atria (RA), left atria (LA), and left ventricular trabeculae carnae were exposed to the beta-receptor agonist isoproterenol (ISO), followed by four doses of the selective adenosine A(1) receptor agonist cyclopentyladenosine (CPA). Spontaneous contractile rates of adult RA were inhibited more than senescent RA by CPA. Contractility (+dF/dt) of adult LA was reduced more than senescent LA by CPA. Left trabeculae carnae tissue responded weakly to CPA, but senescent tissue was less responsive than adult tissue. Senescent atrial A(1) receptor density was 56% greater than in adult tissue, whereas the density in senescent ventricles was 39% lower than in adult tissue. No significant difference in antagonist affinities (K(d)) of A(1) receptor was observed between adult and senescent atria. In addition, agonist competition curves indicated a significant increase in senescent atrial and a decrease in senescent ventricular tissue in the affinity of agonist for high-affinity A(1) receptors with no difference in dissociation constant (K(i)). No significant age-related differences in atrial or ventricular tissues occurred in either the antagonist affinity (K(d)) or density (B(max)) of the beta-adrenergic receptors. CPA was found to inhibit ISO-stimulated adenylate cyclase activity more in senescent than in adult atrial and ventricular membrane preparations. We conclude that age-related differences in functional response to ISO and CPA, A(1) receptor density, and ISO-stimulated adenylate cyclase activity differ in atrial and ventricular myocardium.

Dissociation of analgesic and rewarding effects of endomorphin-1 in rats.

The mu-receptor is the primary mediator of the effects of morphine and the endogenous opiates, endomorphin-1 and endomorphin-2. Here we demonstrate a dissociation of the analgesic and rewarding effects of endomorphin-1 in rats. Tail-flick results revealed that endomorphin-1 produced significant analgesic effects within 10-min after injection. However, it failed to show reward properties in the standard 45- min conditioned place preference (CPP) paradigm or in an abbreviated 10-min pairing which paralleled the time frame of the tail-flick findings. Morphine induced both analgesia and reward. Endomorphin-1 therefore is the first mu opiate shown to produce potent analgesia in the absence of reward behavior, and thus may have significant clinical potential.

Tolerance and morphine-induced cross-tolerance are not shown to Tyr-W-MIF-1 analgesia.

Tolerance and cross-tolerance between Tyr-W-MIF-1, a mixed micro-agonist/antagonist, and morphine were examined. Opiate dependence also was examined. Rats were pretreated with Tyr-W-MIF-1, morphine, or saline for 4 days. On day 5, the animals were tested for Tyr-W-MIF-1 analgesia, morphine analgesia, or naloxone-precipitated withdrawal. Tyr-W-MIF-1- and morphine-pretreated animals showed similar levels of dependence. Animals pretreated with Tyr-W-MIF-1 failed to express tolerance to Tyr-W-MIF-1 analgesia but did display cross-tolerance to morphine analgesia. Animals pretreated with morphine displayed tolerance to morphine analgesia but did not express cross-tolerance to Tyr-W-MIF-1 analgesia. Therefore, tolerance and morphine-induced cross-tolerance were not expressed to Tyr-W-MIF-1 analgesia.

Tyr-W-MIF-1-induced conditioned place preference.

Based on the evidence that Tyr-Pro-Trp-Gly NH2 (Tyr-W-MIF-1) produced dose-dependent, mu-opiate agonistic/antagonistic effects, we investigated whether Tyr-W-MIF- exhibited similar properties in the conditioned place preference (CPP) test. To examine the opiate agonistic effects on CPP, rats were conditioned with alternating ICV injections of saline and Tyr-W-MIF-1 (0 or 200 microg). This procedure resulted in Tyr-W-MIF-1-induced CPP. To examine the opiate antagonistic properties of low doses of Tyr-W-MIF-1, morphine-induced CPP was challenged with Tyr-W-MIF-1 (0, 25, 50, or 100 microg). Morphine-induced CPP was not affected by Tyr-W-MIF-1 at these doses. These findings show that in the CPP test Tyr-W-MIF-1 produced opiate agonistic effects at the high dose and was without opiate antagonistic properties at lower doses.

Upregulation of tumor necrosis factor alpha transport across the blood-brain barrier after acute compressive spinal cord injury.

Tumor necrosis factor alpha (TNF) is a cytokine that is involved in the inflammatory process after CNS injury and is implicated in neuroregeneration. A saturable transport system for TNF located at the blood-brain barrier (BBB) is responsible for the limited entry of TNF from blood to the CNS in normal mice. After partial disruption of the BBB by compression of the lumbar spinal cord, permeability to TNF was increased not only in the lumbar spinal cord but also in brain and distal spinal cord segments, where the BBB remained intact. The increase in the entry of TNF to the CNS followed a biphasic temporal pattern, with a first peak immediately after injury and a second peak starting on day 3; these changes lasted longer than the mere disruption of the BBB. The increased entry of TNF was abolished by addition of excess unlabeled TNF, showing that the transport system for TNF remained saturable after spinal cord injury (SCI) and providing evidence that the enhanced entry of TNF could not be explained by diffusion or leakage. This study adds strong support for our concept that the saturable transport system for TNF across the BBB can be upregulated in the diseased state, and it suggests that the BBB is actively involved in the modulation of the processes of degeneration and regeneration after SCI.

Adenosine A1 receptor-dependent and -independent effects of the allosteric enhancer PD 81,723.

The 2-amino-3-benzoylthiophene PD 81,723 has been shown to exhibit allosteric enhancement of adenosine A1 receptor binding and function. The aim of this study was to clarify the mechanism of this effect using membranes purified from rat brain and Chinese hamster ovary (CHO)-A1 cells that stably express the rat adenosine A1 receptor as well as intact CHO-A1 and nontransfected CHO cells. In membranes containing 100 microM magnesium, (2-amino-4, 5-dimethyl-3-thienyl)-[3-(trifluoromethyl)phenyl]methanone (PD 81, 723) significantly increased the affinity of the adenosine A1 receptor agonist, cyclopentyladenosine, for the low-affinity receptor without affecting high-affinity binding or Bmax. In intact cells, PD 81,723 inhibited basal adenylyl cyclase (AC) activity as well as forskolin-, cholera toxin-, and pertussis toxin-stimulated AC activity in CHO-A1 and CHO cells. Basal AC activity was inhibited 49% in CHO and 82% in CHO-A1 cells by 30 microM PD 81,723. In CHO-A1 cells, half-maximal inhibition of forskolin-stimulated AC occurred at 5 microM PD 81,723 compared to 10 microM in CHO cells. Cholera toxin-stimulated AC was reduced 90% in both CHO and CHO-A1 cells by 30 microM PD 81,723. At the same concentration of PD 81,723, pertussis toxin-stimulated AC activity was reduced 86% (CHO-A1) and 77% (CHO). [3H]forskolin was displaced from purified rat liver AC by PD 81,723 with an IC50 of 96 microM. These results demonstrate that two mechanisms appear to contribute to the observed effects of PD 81, 723. One mechanism is allosteric enhancement of adenosine A1 receptor function. Results from transfected and nontransfected cells suggest that PD 81,723 also inhibits AC directly by binding to the catalytic unit at or near the forskolin-binding site.

Endogenous opiates: 1998.

This paper is the twenty-first installment of our annual review of research concerning the opiate system. It summarizes papers published during 1998 that studied the behavioral effects of the opiate peptides and antagonists, excluding the purely analgesic effects, although stress-induced analgesia is included. The specific topics covered this year include stress; tolerance and dependence; eating and drinking; alcohol; gastrointestinal, renal, and hepatic function; mental illness and mood; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurologic disorders; electrical-related activity; general activity and locomotion; sex, pregnancy, and development; immunologic responses; and other behaviors.

Reduction of stress-induced analgesia following ethanol exposure in mice.

In the present study, we examined the effects of ethanol treatment on the subsequent expression of opioid and nonopioid forms of swim stress-induced analgesia (SSIA). In Experiment 1, mice were injected with ethanol (2.5 g/kg, i.p.) or an equal volume of saline once a day for two days. Animals received no treatment on day 3. On day 4, the animals were tested for opioid (3-min swim in water maintained at 32 degrees C) or nonopioid (3-min swim in water maintained at 20 degrees C) SSIA in the hotplate test (52 degrees C). Mice pretreated with ethanol injections showed a decrease in nonopioid SSIA, but not in opioid SSIA. In Experiment 2, mice were given an ethanol solution (10%) or tap water to drink for 15 days. On day 16, all animals were given tap water to drink. On day 17, the animals were tested for opioid or nonopioid SSIA. Neither form of SSIA was modified in mice that drank the ethanol solution. These results show that ethanol pretreatment can modify nonopioid endogenous analgesic responses in mice. Further, the route of administration influences the effects of ethanol pretreatment on SSIA.

Tolerance to ethanol analgesia is not accompanied by cross-tolerance to morphine analgesia in rats.

In the present study, we examined the development of environment-independent and environment-dependent tolerance to ethanol-induced analgesia (EIA) and cross-tolerance with morphine-induced analgesia (MIA). To examine the development of environment-independent tolerance, male Long-Evans rats were given increasing amounts of ethanol (5 days each of 5% (v/v), 10% (v/v), and 15% (v/v)) added to their drinking water over a 15-day period. A control group was given plain tap water to drink. On day 16, all rats were given plain tap water to drink. On day 17, the animals were tested for EIA (2.5 g/kg, I.P.) or MIA (10 mg/kg, I.P.) in the hot plate test. To examine the development of environment-dependent tolerance, animals were injected with ethanol (2.5 g/kg, I.P.) or an equal volume of saline once a day for 2 days. On day 3, the animals received no treatment. On day 4, the animals were tested for either EIA (2.5 g/kg, I.P.) or MIA (10 mg/kg, I.P.) in the hot plate test. It was found that rats pretreated with ethanol (both self-administration and I.P. injections) displayed tolerance to EIA, which was not accompanied by cross-tolerance to MIA.

Endogenous opiates: 1997.

This paper is the twentieth installment of our annual review of research concerning the opiate system. It summarizes papers published during 1997 that studied the behavioral effects of the opiate peptides and antagonists, excluding the purely analgesic effects, although stress-induced analgesia is included. The specific topics covered this year include stress; tolerance and dependence; eating and drinking; alcohol; gastrointestinal, renal, and hepatic function; mental illness and mood; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurologic disorders; electrical-related activity; general activity and locomotion; sex, pregnancy, and development; immunologic responses; and other behaviors.

Contractile failure in chronic doxorubicin-induced cardiomyopathy.

The mechanisms for the progression of the anthracycline-induced cardiomyopathy to contractile failure has not been defined. In vitro, doxorubicin (DOX) appears to modify calcium-mediated excitation-contraction coupling, which depresses cardiac contractility. This study characterizes the onset of contractile failure associated with the development of DOX-induced cardiomyopathy. Rabbits were treated with DOX (1 mg/kg i.v. twice weekly, 12-18 doses; DOX-treated group) and compared with a pair-fed Control group infuse with saline vehicle. The severity of the cardiomyopathy was determined by numerically-scored histopathology. Myocardial contractility was determined in thin fiber bundles from right ventricular (RV) papillary muscles and left atria that were removed and mounted on a force transducer in oxygenated Krebs-bicarbonate buffer (pH=7.4 at 30 degrees C) to record the amplitude (DT) and maximum rate (+dT/dt ) of isometric tension. Myofibrillar and calcium loading properties were determined by the calcium and caffeine-activated tension responses respectively in chemically-permeabilized fibers. With the onset of the cardiomyopathy (score <2) DT at low frequency (0.5 Hz) was depressed (0.61+/-0.01 mN/mg; n=14) compared to Control (0.93+/-0.09 mN/mg; n=15). Contractility at higher rates (1 Hz) was not different in this DOX-treated and Control groups. Maximum calcium and caffeine-activated force and the pCa to half-maximum force of permeabilized fibers were comparable in DOX-treated and Control groups. The loss of contractility of the DOX-treated group was related to reduction in sarcoplasmic reticulum calcium release channel density, as determined by Bmax for 3H-ryanodine binding in cardiac microsomal membrane fraction. Post-rest potentiation of contractility, as well as frequency-dependent (0.25-1.5 Hz) and post-extrasystolic potentiation of contractility were preserved in the DOX-treated group. In vitro, DOX depressed post-rest potentiation of contractility. Thus, the onset of contractile failure of the DOX-induced cardiomyopathy is characterized by effects consistent with disordered calcium-mediated excitation-contraction coupling and these effects are qualitatively different than in vitro effects of DOX.

The role of corticosterone in the blockade of tolerance to morphine analgesia by formalin-induced pain in the rat.

We previously reported that morphine fails to produce analgesic tolerance when administered in the presence of formalin-induced pain, which may be related to activity of the hypothalamic-pituitary-adrenal axis. In the present study, we examined whether suppression of corticosterone secretion during pain prevents the blockade of tolerance to morphine analgesia. Male Long-Evans rats were injected with morphine (20 mg/kg) or saline for 4 consecutive days in the presence or absence of formalin-induced pain. To suppress corticosterone activity, some animals were injected daily with the corticosterone synthesis inhibitor, metyrapone (100 mg/kg), 24 h and 30 min before formalin injections. The analgesic effect of a test dose of morphine (10 mg/kg) was then measured in the tail-flick test 24 h after tolerance induction (i.e. day 5). The presence of pain during tolerance induction prevented the development of analgesic tolerance. Furthermore, inhibition of corticosterone synthesis by metyrapone prevented the blockade of tolerance by pain. These results suggest that the blockade of tolerance to morphine analgesia by formalin-induced pain depends on stress-induced corticosterone increases.

Age-related pharmacokinetics of daunorubicin and daunorubicinol following intravenous bolus daunorubicin administration in the rat.

Age-related differences in pharmacokinetics may be important in determining altered anthracycline cardiotoxicity in the senescent rat and also in older humans. This study examined the effect of aging on daunorubicin pharmacokinetics in the Fischer 344 rat. Daunorubicin 7.5 mg/kg was administered i.v. to 6- and 24-month-old male Fischer 344 rats and plasma and tissue sampling was performed over 168 h for assay of daunorubicin and daunorubicinol concentrations by high-performance liquid chromatography. Systemic clearance of daunorubicin was decreased in older compared to younger animals (56 +/- 4 versus 202 +/- 17 ml min-1 kg-1; P < 0.05). In addition, the area under the plasma daunorubicinol concentration/time curve was significantly increased in older rats. In the heart, the area under the concentration/time curve was significantly increased in senescence both in the case of daunorubicin (201 +/- 12 versus 86 +/- 4 micrograms h g-1; P < 0.05) and daunorubicinol (1347 +/- 118 versus 182 +/- 4 micrograms h g-1; P < 0.05). Furthermore, the peak mean concentrations of daunorubicin were increased in older compared to younger rats both in plasma (1078 +/- 82 versus 663 +/- 66 ng ml-1; P < 0.05) and in heart (27 +/- 1 versus 10 +/- 1 micrograms g-1; P < 0.05). This also was true for daunorubicinol in plasma (284 +/- 39 versus 168 +/- 27 ng ml-1; P < 0.05) and in myocardium (8.6 +/- 0.6 versus 2.4 +/- 0.2 micrograms g-1; P < 0.05). Following daunorubicin injection, the ratio of daunorubicinol to daunorubicin concentrations in tissues increased with time, particularly in plasma and heart in senescent rats. Thus, there are significant age-related changes in daunorubicin and daunorubicinol kinetics in the rat that could alter susceptibility to acute systemic toxicity and to chronic cardiotoxicity.

Endogenous opiates: 1996.

This paper is the nineteenth installment of our annual review of research concerning the opiate system. It summarizes papers published during 1996 reporting the behavioral effects of the opiate peptides and antagonists, excluding the purely analgesic effects, although stress-induced analgesia is included. The specific topics covered this year include stress, tolerance and dependence; eating; drinking; gastrointestinal, renal, and hepatic function; mental illness and mood; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; electrical-related activity; general activity and locomotion; sex, pregnancy, and development; immunological responses; and other behaviors.