Safety and efficacy of a combination of pethidine and levallorphan for pain relief during labor: An observational study.

AIM: To evaluate the safety, effect on breastfeeding and efficacy of a combination of pethidine and levallorphan (Pethilorfan) for pain relief during labor. METHODS: We compared maternal or neonatal morbidities, suckling difficulties in newborns and breastfeeding rates between 177 women who received 50-200 mg (as pethidine) of Pethilorfan during labor (Pethilorfan group) and 354 women who delivered their infants without analgesic drugs immediately before or after each woman in the Pethilorfan group (control group) from January 1, 2005 to December 31, 2016. We performed univariate and multivariate analyses for comparison between the two groups. We also evaluated the efficacy of Pethilorfan retrospectively. RESULTS: The Pethilorfan group included more women with prolonged and/or operative deliveries than the control group. Nevertheless, no significant differences were seen between the two groups in the rates of Apgar scores less than 7 at 1 or 5 min, composite neonatal morbidities, hyperbilirubinemia or respiratory disturbances. The incidence of suckling difficulties lasting over 24 h and the breastfeeding rates at discharge or after 1 month were also similar. Maternal adverse effects of Pethilorfan were generally mild and transient. The efficacy ratio of Pethilorfan was 83.6%, although its analgesic effect was usually incomplete. CONCLUSION: Pethilorfan can be used safely for labor pain relief without increasing maternal or neonatal morbidities, or impeding breastfeeding, if it is administered at a prudent dosage. Parenteral opioids including Pethilorfan should remain as an option for treating women in labor pain, particularly when epidural analgesia is not readily available or contraindicated.

Opiate agonists and antagonists discriminated by receptor binding in brain.

Receptor binding of opiate agonists and antagonists can be differentiated in vivo and in vitro. Administration of either rapidly elevates stereospecific [(3)H]dihydromorphine binding to mouse brain extracts by 40 to 100 percent, but antagonists are 10 to 1000 times more potent than agonists; as little as 0.02 milligram of naloxone per kilogram of body weight significantly enhances opiate receptor binding. Sodium enhances antagonist binding in vitro but decreases agonist binding, a qualitative difference that may be relevant to the divergent pharmacological properties of opiate agonists and antagonists.

Targeting sigma receptor-binding benzamides as in vivo diagnostic and therapeutic agents for human prostate tumors.

Sigma receptors are known to be expressed in a variety of human tumor cells, including breast, neural, and melanoma tumors. A very high density (1.0-1.5 million receptors/cell) of sigma receptors was also reported in a human androgen-dependent prostate tumor cell line (LNCaP). In this study, we show that a very high density of sigma receptors is also expressed in an androgen-independent human prostate tumor cell line (DU-145). Pharmacological binding studies using the sigma-1-selective ligand [3H](+)-pentazocine showed a high-affinity binding (Kd = 5.80 nM, Bmax = 1800 fmol/mg protein). Similarly, binding studies with [3H]1,3-di-o-tolylguanidine in the presence of dextrallorphan also showed a high-affinity binding (Kd = 15.71 nM, Bmax = 1930 fmol/mg protein). Radioiodinated benzamide N-[2-(1'-piperidinyl)ethyl]-3-[125I]iodo-4-methoxybenzamide ([125I]PIMBA) was also shown to bind DU-145 cells in a dose-dependent manner. Three different radioiodinated benzamides, [125I]PIMBA, 4-[125I]iodo-N-[2-(1'-piperidinyl)ethyl]benzamide, and 2-[125I]-N-(N-benzylpiperidin-4-yl)-2-iodobenzamide, were screened for their potential to image human prostate tumors in nude mice bearing human prostate cells (DU-145) xenografts. All three compounds showed a fast clearance from the blood pool and a high uptake and retention in the tumor. Therapeutic potential of nonradioactive PIMBA was studied using in vitro colonogenic assays. A dose-dependent inhibition of cell colony formation was found in two different human prostate cells. These results demonstrate the potential use of sigma receptor binding ligands in non-invasive diagnostic imaging of prostate cancer and its treatment.

[Binding of a tritiated enkephalinergic analog (FK 33-824) to a mitochondrial fraction of rat brain]. / Liaison d'un analogue enképhalinergique [FK 33-824] tritié à une fraction mitochondriale de cerveau de rat.

FK-33-824 (Try-D-Ala-Gly-MePhe-Met(O)ol) is a potent enkephalin analog which has been tritium labelled with a high specific radioactivity (41 Ci/mmole). The labelled drug exhibits specific and saturable binding to rat brain crude mitochondrial fraction. Specific binding is inhibited by low concentrations of morphine, levallorphan and beta-endorphin, suggesting that FK 33-824 [3H] binds preferentially to mu opiate sites. Binding studies at equilibrium and kinetics of formation and dissociation of the labelled ligand-receptor complex indicate that FK 33-824 [3H] binds to two classes of specific sites. Their affinities are distinguishable at 0 degree (KD = 1.3 and 5.8 nM) and very close to each other at 37 degree (KD = 1.9 nM).

The effects of analgesics on quickly-learned conditioned suppression in mice.

Mice were found to exhibit a marked suppression of motor activity when placed into the same experimental chamber in which they had previously received electrical shocks. This suppression was found to be a stable response when mice, 6-8 weeks of age, received shocks of frequencies of 1/10 or 1/30 Hz, and their motility was examined 24 hr after the delivery of the shock in the same experimental chamber. Control experiments indicated that the mice exhibited similar degrees of locomotor activity, compared to non-shocked mice, that were placed into a different experimental chamber than from the one in which they had previously received electrical shocks. Thus, the suppression of motor activity appeared to be a conditioned emotional response to the environment previously associated with delivery of shock. Morphine, codeine, and pethidine, but not pentazocine and aminopyrine, attenuated the conditioned suppression in a dose-related manner in the shocked mice. The effect of morphine was completely antagonized by pretreatment with naloxone or levallorphan. Thus, it is possible that the opiate-induced reduction of conditioned suppression is mediated by opiate receptor sites.

Antagonism of opiate mydriasis in mice.

1 Morphine-induced mydriasis in mice is antagonized by nalorphine, levallorphan and naloxone in a dose-dependent manner. 2 The relative potency of the three agents is 10:56: 134 respectively, thus being in accordance with other tests of narcotic antagonism. Naloxone has the shortest duration of action. 3 When injected into naive animals, nalorphine (but not levallorphan or naloxone) produces a slight mydriasis. 4 Measurement of the diameter of the pupil in mice seems to be a precise, simple and rapid test for studying narcotic antagonist as well as agonist action and has several advantages over standard methods used for this purpose.

Backward walking and circling: behavioural responses induced by drug treatments which cause simultaneous release of catecholamines and 5-hydroxytryptamine.

1 The roles of catecholamine and 5-hydroxytryptamine (5-HT) release in mediating backward walking and circling were studied in rats. 2 These behaviours occurred in animals given 15 mg/kg intraperitoneally of (+)-amphetamine (which predominantly releases catecholamines) or either p-chloroamphetamine or fenfluramine (which predominantly release 5-HT). They also occurred when smaller doses of (+)-amphetamine (5 mg/kg) and either p-chloroamphetamine (2--5 mg/kg) or fenfluramine (5 mg/kg) were given together. 3 Characteristic dopamine-dependent behaviours (rearing, licking, gnawing) resulting from (+)-amphetamine injection were greatly reduced by p-chloroamphetamine or fenfluramine. 4 Characteristic 5-HT-dependent behaviours (wet dog shake, hind limb abduction) resulting from injection of either p-chloroamphetamine or fenfluramine were unaffected by (+)-amphetamine. 5 Fragmentary backward walking and circling resulting from levallorphan injection (50 mg/kg s.c.) were decreased by (+)-amphetamine at low dosage. 6 Results in general strengthen previous evidence that backward walking and circling are mediated by simultaneous dopamine and 5-HT release. 7 The possible relevance of the above findings to hallucinogenic activity, amphetamine psychosis, schizophrenia and abnormal movements due to L-DOPA treatment is discussed.

Peripheral sympatho-inhibitory cardiovascular effects of opioid peptides in anaesthetized rabbits.

1. Opioid agonists influence isolated cardiovascular tissues from rabbits, as well as the cardiovascular system of pithed rabbits, through presynaptic receptors on postganglionic sympathetic nerve fibres. The present experiments were carried out in order to study effects which result from activation of these receptors in anaesthetized rabbits. 2. In pithed rabbits with electrically stimulated sympathetic outflow, infusion of [D-Ala2-D-Leu5]-enkephalin (DADLE) 10 micrograms kg-1 min-1 and dynorphin-(1-13) (dynorphin) 1 microgram kg-1 min-1 decreased the plasma noradrenaline concentration, mean arterial pressure (MAP) and heart rate. The effects of dynorphin and, less completely, those of DADLE were antagonized by the peripherally selective opioid antagonists N-methyl naloxone bromide (NMN) 1.3 mg kg-1 and N-methyl levallorphan methanesulphonate (NML) 1-3 mg kg-1. 3. In pentobarbitone-anaesthetized rabbits, DADLE 3-30 micrograms kg-1 min-1 and dynorphin 0.3-3 micrograms kg-1 min-1 decreased the plasma noradrenaline concentration and MAP. The highest dose of dynorphin also decreased heart rate, whereas DADLE 10 micrograms kg-1 min-1 caused slight cardioacceleration. The effects of DADLE but not those of dynorphin decreased upon repeated administration. 4. The effects of dynorphin 10 micrograms kg-1 min-1 were abolished or greatly attenuated by NMN 1.3 mg kg-1 and NML 3 mg kg-1. In contrast, the antagonists reduced only slightly the blood pressure-lowering effect of DADLE 10 micrograms kg-1 min-1 and did not reduce significantly the effects of DADLE on the plasma noradrenaline level and heart rate. 5. It was concluded that systemically administered dynorphin produces sympatho-inhibition and an ensuing fall in blood pressure by an action at peripheral receptors, in all probability presynaptic Kappa-receptors on postganglionic sympathetic nerve fibres. The effects of DADLE are more complex and may involve both central and peripheral components.

Opposite central and peripheral control by endogenous opioids of intestinal motility in fed rats.

1. The effects of the inhibitors of endopeptidase EC 24.11, thiorphan and phosphoramidon administered i.c.v. (40 micrograms kg-1) i.p. (400 micrograms kg-1), or orally (400 micrograms kg-1), on intestinal motor activity in fed rats was compared to the effects of similar doses of the angiotensin converting enzyme inhibitor, captopril and the synthetic enkephalin analogue [D-Ala2 Met5] enkephalinamide (Dalamide). Drugs were administered alone or after pretreatment with naloxone or N-methyl levallorphan (300 micrograms kg-1, i.p.) given 10 min prior to gavage with a standard meal. 2. In control conditions, in the duodenum, the disruption of the migrating myoelectric complex (MMC) by gavage with a standard meal lasted between 105.6 and 119.1 min. This duration was significantly decreased by thiorphan (60.3 +/- 15.0 min), phosphoramidon (67.9 +/- 7.3 min), captopril (26.3 +/- 10.2 min) and Dalamide (42.4 +/- 9.6 min), administered i.c.v. 3. In contrast, after the i.p. administration of thiorphan, phosphoramidon and Dalamide the delay in the return of the MMC pattern was increased. Such an effect was also seen after the oral administration of phosphoramidon or Dalamide. Neither i.p. nor oral captopril administration altered the duration of postprandial pattern. 4. A prior treatment with naloxone i.p. (300 micrograms kg-1) that had no effect per se, antagonized the effect produced by i.c.v. administration of thiorphan, phosphoramidon or Dalamide, but failed to reverse the effect of captopril. In contrast, i.p. administration of N-methyl levallorphan (300pgkg-1) did not affect the response induced by central administration of thiorphan, phosphoramidon, captopril or Dalamide, but was able to prevent that of thiorphan, phosphoramidon or Dalamide when they were administered i.p. or orally. 5. These data strongly support the hypothesis of a dual control by endogenous opioids of intestinal motility in the rat: a central component that favours, and a peripheral control that delays the occurrence of the MMC profile in fed rats.

Effects of opiates and opiate antagonists on the Straub tail reaction in mice.

1. Subcutaneous injections of opiates produced the Straub tail reaction in mice. The potencies of the opiates in mice were consistent with previous estimates of the analgesic potencies in animals and in man.2. The potencies of sixteen antagonists in counteracting the reaction were consistent with those previously obtained with the rat tail-flick test.3. The (-) isomers of four benzomorphan derivatives were much more potent in counteracting the reaction than their (+) isomers and about twice as potent as their racemates. The activity of the isomers seemed to follow Pfeiffer's rule: the lower the effective dose of a drug, the greater the difference in the pharmacological effects of the optical isomers. One of the trans isomers acted like an opiate, while its cis isomer acted like an antagonist.4. Naloxone and nalorphine fulfilled conventional criteria for competitive antagonism, whereas atropine and the (-) and the (+) isomers of pentazocine and of cyclazocine did not do so.5. The Straub tail test seems to be useful for studying structure-activity relations among opiates and opiate antagonists.

Effects of narcotic analgesics and antagonists on the in vivo release of acetylcholine from the cerebral cortex of the cat.

1. In cats under light allobarbitone anaesthesia, the effects of intravenous injections of narcotic and non-narcotic analgesics, of a general depressant, and of narcotic antagonists were investigated on the spontaneous release of acetylcholine (ACh) from the surface of the sensorimotor cortex.2. The narcotic analgesics morphine (0.1, 1.0 and 5 mg/kg), meperidine (1.0 and 2.0 mg/kg), methadone (1.0 mg/kg) and codeine (5.0 and 10.0 mg/kg) greatly reduced ACh release.3. The non-narcotic analgesics pentazocine (1.0 and 2.0 mg/kg) and propoxyphene (5.0 and 10.0 mg/kg) as well as the depressant chlorpromazine (0.25, 0.5 and 1.0 mg/kg) also greatly reduced ACh release.4. Two of the three narcotic antagonists examined, levallorphan (0.1, 1.0 and 5 mg/kg) and nalorphine (1.0 mg/kg) had the property of reducing ACh release. They were thus partial agonists. With levallorphan the greatest reduction occurred with the smallest dose injected and the effect was regularly obtained, whereas with nalorphine a reduction was obtained in some experiments only. The third, naloxone, was a specific narcotic antagonist and did not reduce the ACh release in any dose (0.01, 0.1, 0.5 and 1.0 mg/kg) examined. In a dose of 1.0 mg/kg it actually produced a small increase in Ach release.5. Naloxone (0.1-1.0 mg/kg) restored the reduction in ACh release produced by the narcotic analgesics and by the partial agonist levallorphan. It partially restored the reduction produced by the non-narcotic analgesics and by nalorphine, but had no effect on the reduction produced by chlorpromazine.6. The relevance of these results with regard to analgesia and to the narcotic abstinence syndrome is discussed.

Effects of narcotic analgesics and their antagonists on the rabbit isolated heart and its adrenergic nerves.

1. In isolated perfused hearts of rabbits, the effects of morphine, methadone, pethidine, fentanyl, levallorphan and naloxone on heart rate, the spontaneous outflow of noradrenaline, the uptake of infused noradrenaline and the overflow of noradrenaline in response to stimulation of the accelerans nerves were investigated.2. At concentrations of 10-100 muM, methadone, fentanyl, levallorphan and naloxone, but not morphine and pethidine, decreased the heart rate. Only pethidine and levallorphan (100 muM) augmented the spontaneous outflow of noradrenaline.3. With the exception of naloxone, all drugs diminished the neuronal uptake of noradrenaline from the perfusion fluid. Methadone and pethidine (1 muM) were the most effective inhibitors. The inhibitory effect of morphine was not antagonized by naloxone.4. All drugs increased the overflow of noradrenaline evoked by stimulation of the accelerans nerves at 5 Hz. Simultaneously, the positive chronotropic effect of stimulation was usually enhanced. Morphine also augmented the response to stimulation at 2.5 and 10 Hz. The effect of morphine was prevented by pre-infusion of cocaine. The response to stimulation was never depressed.5. It is concluded that the adrenergic nerves of the rabbit heart lack specific morphine receptors which in some other sympathetic nerves mediate an inhibition of the stimulation-induced secretion of noradrenaline. The mechanism of the enhancement of adrenergic neurotransmission by relatively high concentrations of the drugs is discussed.

Release of vasopressin in response to hypoxia and the effect of aminergic and opioid antagonists.

Plasma vasopressin, arterial blood gas tensions, pH, arterial blood pressure, heart rate and respiration were monitored in conscious rats breathing room air or exposed to varying degrees of hypoxia. A similar series of observations was made in a group of anaesthetized rats and in rats treated with alpha- and beta-adrenergic and dopaminergic blocking agents. The effect of two opioid antagonists on the vasopressin response was also noted. Hypoxia produced an increase in circulating vasopressin concentrations in both conscious and anaesthetized rats. In the conscious animals the increase reached statistical significance when the animals were exposed to 12% oxygen in nitrogen, which produced a fall in arterial PaO2 of 44.7 +/- 5.0%. Guanethidine, phentolamine and propranolol all produced a significant fall in the basal concentrations of vasopressin, while guanethidine, phenoxybenzamine and propranolol blocked the increase seen on breathing 12% oxygen in nitrogen. Naloxone and levallorphan also reduced the vasopressin response to hypoxia. Thus it appears that aminergic pathways play a role in the maintenance of circulating concentrations of vasopressin and in the response to hypoxia. Endogenous opioids also appear to be involved in the hypoxic response.

Cross-tolerance and enhanced sensitivity to the response rate-decreasing effects of opioids with varying degrees of efficacy at the mu receptor.

The purpose of the present experiment was to determine whether the effects of opioids with varying degrees of efficacy at the mu receptor are differentially altered in morphine-tolerant pigeons. To this end, dose-effect curves were determined for high, intermediate, and low efficacy mu agonists in pigeons responding under a schedule of food presentation prior to, during, and after exposure to a regimen of chronic morphine administration. In pigeons treated with 56 mg/kg/daily morphine, the dose-effect curves for the rate-decreasing effects of the high-efficacy mu agonists morphine and fentanyl were shifted to the right of their prechronic positions (i.e., tolerance). A small degree of tolerance was also conferred to the intermediate-efficacy mu agonists (-)-pentazocine and (-)-metazocine, but not to nalbuphine or butorphanol. In contrast to the effects obtained with these mu agonists, the chronic morphine regimen shifted the dose-effects curves of the mu antagonist naloxone and the low-efficacy mu agonists nalorphine and levallorphan to the left of their prechronic positions (i.e., enhanced sensitivity). These findings demonstrate that morphine tolerance confers cross-tolerance to other high efficacy mu agonists, enhanced sensitivity to mu antagonists and low efficacy mu agonists, and little or no cross-tolerance to intermediate efficacy mu agonists. Disadvantages of using schedule-controlled responding to examine the effects of intermediate efficacy mu agonists are discussed.

The mu opioid irreversible antagonist beta-funaltrexamine differentiates the discriminative stimulus effects of opioids with high and low efficacy at the mu opioid receptor.

The purpose of the present study was to determine the relative intrinsic efficacy of various opioids using the irreversible mu opioid antagonist beta-funaltrexamine (betaFNA). To this end, pigeons were trained to discriminate 3.0 (n=6) or 1.8 (n=1) mg/kg morphine from distilled water in a two-key, food-reinforced, drug discrimination procedure. The mu opioids fentanyl, l-methadone, buprenorphine, butorphanol, nalorphine, nalbuphine and levallorphan, as well as the delta opioid BW373U86, substituted completely for the morphine stimulus. The stimulus effects of morphine were antagonized (i.e., produced a significant increase in the ED50 value) by a 10 mg/kg but not a 5 mg/kg dose of betaFNA. Antagonist effects of betaFNA were observed following a 2-h pretreatment, but not following 26-, 50-, 74-, 98- or 146-h pretreatments. The stimulus effects produced by fentanyl, l-methadone and buprenorphine were not antagonized by doses of betaFNA as high as 20, 10 and 10 mg/kg, respectively. The lowest dose of betaFNA required to antagonize the stimulus effects of butorphanol was 10 mg/kg, whereas the effects of nalorphine, nalbuphine and levallorphan were antagonized by a dose of betaFNA as low as 5 mg/kg. The delta BW373U86 substituted for the morphine stimulus, and this effect was not antagonized by 10 mg/kg betaFNA. The pkB values for naloxone (1.0 mg/kg) against the stimulus effects of fentanyl (6.70) and morphine (6.52) were considerably higher than that for BW373U86 (4.60), indicating further that the morphine-like stimulus effects produced by BW373U86 were not mediated by activity at the mu opioid receptor. These findings indicate that the strategy of irreversible antagonism can be used effectively to differentiate opioids with varying degrees of intrinsic efficacy at the mu opioid receptor in a pigeon drug discrimination procedure. In particular, the ranking of these drugs by relative intrinsic efficacy at the mu opioid receptor is: l-methadone=fentanyl> or =buprenorphine> or =morphine> or =butorphanol>nalorphine=nalbuphine=levallorphan. Additionally, the short-acting effect of betaFNA in the pigeon suggests that the recovery of mu opioid receptor function varies across species.

Some altered responses in rats formerly dependent on morphine.

The investigation examined whether or not physical dependence or other abnormalities were detectable 1--3 months after withdrawal in dependent rats that had been treated with the morphine (amintenance dose of 100 X 2 mg/kg/day, s.c.) for 7 weeks. When narcotic antagonists were administered on the 32nd day after withdrawal, nalorphine caused a dose-dependent increase in spontaneous locomotor activity and a complete inhibition of wet-dog shakes and the writhing syndrome. Naloxone was ineffective. A remarkable increase in spontaneous locomotor activity on the 67th day and a significant increase in body weight on the 69th and 92nd day after withdrawal occurred after an acute injection of morphine (10 mg/kg, s.c.). When morphine (10 mg/kg) was administered for 3 days from the 92nd day after withdrawal, withdrawal from morphine produced a significant decrease in body weight. When morphine (10 mg/kg) was administered for 3 days from the 102nd day after withdrawal, a levallorphan injection caused a significant decrease in spontaneous locomotor activity and an increase in the frequency of the diarrheal syndrome. These abnormal responses, not observed in the naive rats, suggest the remains of some behavioral and biochemical abnormalities 3 months after morphine withdrawal.

The antagonizing effect of aspartic acid on morphine withdrawal and levallorphan-precipitated abstinence syndrome signs and on associated changes in brain levels of free amino acids in the rat.

We have previously demonstrated the antagonizing effect of aspartic acid on some effects of morphine and on the development of physical dependence on, and tolerance to, morphine. In the present study, we have withdrawal from morphine or administration of a morphine antagonist. For this purpose sixty five white rats were given morphine and aspartic acid separately and in combination in a 5% saccharose solution instead of drinking water for 30 days. Some of the dependent rats were then withdrawn and others were injected with levallorphan. Flying, jumping, wet-dog shaking, body weight loss and motor activity were estimated and free amino acid levels in the brain were determined. Aspartic acid was found to prevent or antagonize the behavioural signs and the changes in the free amino acid levels in the brain. The results are discussed in the light of the previous data.

Iontophoretic application of opiates to the locus coeruleus.

The vast majority of morphine-sensitive single units in the area examined were localized to the locus coeruleus. This corresponds well with the known distribution of the highest densities of opiate receptor sites in this region of the midbrain. The effect of iontophoretically applied morphine was a marked and prolonged depression of spontaneous activity. Levorphanol, an opiate agonist, produced an effect similar to that of morphine while comparable doses of dextrorphan, it's clinically inactive stereoisomer, did not. Naloxone and levallorphan prevented as well as reversed the depression due to application of agonists. While the units were depressed following the application of opiate agonists, the cells were still excited by the neurotransmitter acetylcholine. We conclude that (1) neuronal sensitivity to opiates has a high positive correlation with autoradiographically determined opiate receptor sites, and (2) this sensitivity to opiates is blocked by opiate antagonists and is stereospecific in nature.

Self-administration of D-Ala2-Met-enkephalinamide at hypothalamic self-stimulation sites.

The relationship between the action of enkephalin and the reinforcing action of electrical stimulation in the posterior lateral hypothalamus of the rat was studied with the self-administration approach. Adult male albino rats implanted with a combination cannula and stimulation electrode in hypothalamus were pretested for the reinforcing effects of electrical stimulation. Only subjects that self-stimulated at moderate to high rates were given self-administration tests. The chamber for the self-administration tests was fitted with one lever at each end of a rectangular plexiglass box. In a session, one lever was 'active', the other not. The active lever, if depressed, yielded 20 nl of CSF (artificial cerebrospinal fluid), or CSF in which one of the test substances was dissolved. Each subject was tested repeatedly for the reinforcing effects first of a control solution (CSF), then of D-Ala2-Met-enkephalin (DALA), a long-acting synthetic analogue of enkephalin, then of morphine, and then of opioids mixed with naltrexone or naloxone. Following these tests, the subjects were once again given self-stimulation tests to ascertain the functional integrity of the 'reward' system after the repeated self-administration tests. The results demonstrate that when the test solution was DALA instead of CSF the subjects pressed the active lever at a higher rate than for CSF, and they exhausted the supply more rapidly than for CSF. The rate on the active lever was also significantly higher than on the inactive lever. DALA at 1 and 5 microgram/microliter concentrations proved more reinforcing than at 0.1 microgram/microliter. Naltrexone did not block the reinforcing effect of DALA, whereas naloxone blocked DALA-induced reinforcement. These data extend the report of ventricular methionine-enkephalin positive reinforcement to DALA injected directly into the lateral hypothalamus.

Unit activity of amygdala and hippocampal neurons: effects of morphine and benzodiazepines.

The effects of morphine sulfate and two benzodiazepine derivatives, chlordiazepoxide HCl and diazepam, were evaluated upon single unit activity of the amygdaloid nuclear complex and hippocampal formation in immobilized cats. All surgical procedures were performed under halothane anesthesia and all wound margins were infiltrated with Lidocaine after halothane withdrawal. Single unit activity was recorded extracellularly with platinum-iridium microelectrodes. Chlordiazepoxide HCl, 10.0-20.0 mg/kg i.v., or diazepam, 0.05-0.20 mg/kg i.v., suppressed spontaneous firing rates of the amygdala and the hippocampal neurons. In contrast, the spontaneous firing rates of neurons in these limbic structures were augmented by morphine sulfate, 0.50-2.00 mg/kg i.v. The morphine-induced augmentation of hippocampal neuronal activity was effectively antagonized by naloxone, 0.10-0.20 mg/kg i.v. However, naloxone, 0.20-0.40 mg/kg i.v., only partially suppressed the morphine induced augmentation of amygdala neuronal activity. In a dose-dependent fashion, chlordiazepoxide and diazepam administration prevented or antagonized morphine-induced augmentation of amygdala and hippocampal neuronal activity. Our results suggest that, in the cat, the amygdala and hippocampus may play an important role for morphine-induced behavioral responses. Moreover, our data imply that these two limbic structures may be the sites of tranquillizing actions of diazepam and chlordiazepoxide.