Inhibition of behavioural and EEG activation induced by morphine acting on lower brain-stem structures

The neurophysiology of sleep and waking: intracerebral connections, functioning and ascending influences of the medulla oblongata

This paper focuses on the successive historical papers related to medulla oblongata (M.O.) intracerebral connections, its activities and ascending influences regulating sleep waking behavior. The M.O. certainly influences the quantitative and qualitative processes of waking. However, its neurophysiological properties are often concealed by those of the upper-situated brain stem structures. The M.O., particularly the solitary tract nucleus, is involved in sleep-inducing processes. This nucleus seem to act as a deactivating system of the above situated reticular formation, but it also impacts directly on the thalamocortical slow wave and spindle-inducing processes. The M.O. is significantly involved in paradoxical sleep mechanisms. Indeed, the mesopontine executive centers are unable to induce paradoxical sleep without the M.O. Moreover, stimulation of the solitary tract nucleus afferents can induce paradoxical sleep, and the M.O. metabolic functioning is specifically disturbed by paradoxical sleep deprivation. Finally. there seems to be a paradoxical sleep Zeitgeber. Our current knowledge shows that this lowest brain stem level is crucial for sleep waking mechanisms. It will undoubtedly be further highlighted by future electrophysiologial and neurochemical studies.

Opiate microinjections in the locus coeruleus area of the cat enhance slow wave sleep

The effects on sleep/wakefulness states of morphine, morphiceptin (specific mu agonist), DPDPE (delta agonist) and U-50,488H (kappa agonist) microinjections in the Locus coeruleus area (LC) were studied in cats. Morphine (0.8-1.75 nmols in 50 nl of saline) and morphiceptin (1.75 nmols) in LC significantly increased the total time spent in slow wave sleep (SWS) and the mean duration of SWS episodes. Prior naloxone administration blocked the morphine hypnogenic effects. The total time spent in SWS was unaffected by delivery of equimolar doses of DPDPE or U-50,488H in LC; however, the mean duration of the SWS episodes increased significantly after U-50,488H microinjections in LC. Thus, when acting in the LC, opiates have a SWS-enhancing effect and this effect appears to be mediated by mu receptors, although kappa receptors may have a subsidiary action.

Investigating the effects of opioid drugs on electrocortical activity using wavelet transform

Fetal electrocortical activity (ECoG) is characterized by two distinct patterns: HVSA (high voltage, slow activity) and LVFA (low voltage, fast activity). Using the wavelet transform (WT), we recently reported that the frequency characteristics of these two ECoG patterns undergo significant maturational changes prior to birth (Akay et al. 1994a). We now report that fetal ECoG can also be significantly affected by pharmacological agents. In this paper, we compared the effects of two opioid drugs (morphine and [D-Pen2, D-Pen5]-enkephalin, DPDPE) on fetal ECoG, using the chronically instrumented fetal lamb model. Morphine was infused intravenously (i.v.) at 2.5 mg/h, while DPDPE was infused into the lateral cerebroventricle (i.c.v.) at 30 micrograms/h. The ECoG was analyzed using WT. We performed multi-resolution decomposition for four sets of parameters D2j where -1 < j < -4. The four series WTs represent the detail signal bandwidths: (1) 16-32 Hz, (2) 8-16 Hz, (3) 4-8 Hz, (4) 2-4 Hz. The data were subjected to statistical analysis using the Kolmogorov-Smirnov (KS) test. Both morphine and DPDPE resulted in a significant increase in power in the first wavelet band, while power was reduced in the second, third and fourth wavelet bands. In addition, both drugs resulted in a disruption of the normal cyclic pattern between the two ECoG patterns. There was a difference in the time course of action between morphine and DPDPE. This is the first occasion in which continuous ECoG has been subjected to rigorous statistical analysis. The results suggest that the WT-KS method is most suitable for quantitating changes in the ECoG induced by pharmacological agents.

Positive feedback action of pituitary beta-endorphin on acupuncture analgesia afferent pathway

Potentials in the final sector of the afferent pathway from the acupuncture point (AP) were enhanced by intraperitoneal 0.5 mg/kg morphine without changing the threshold of AP stimulation and greatly decreased by hypophysectomy. The decreased potentials were restored to the control level by morphine (0.5 mg/kg, IP). Potentials evoked in the final sector of the afferent pathway from the nonacupuncture point (NAP) by NAP stimulation after lesion of the analgesia inhibitory system were greatly enhanced by corticotropin (ACTH) (0.25 mg/kg, IP) and greatly decreased by hypophysectomy. Diminished potentials were restored to the control level by ACTH (0.25 mg/kg, IP). Both morphine (0.5 mg/kg, IP) and ACTH (0.25 mg/kg, IP) produced analgesia, but morphine did not affect acupuncture analgesia (AA) and ACTH did not affect nonacupuncture point stimulation-produced analgesia (NAA). All analgesia, that due to 0.5 mg/kg morphine or 0.25 mg/kg ACTH, AA, and NAA were abolished by hypophysectomy. The abolished AA and NAA were restored by 0.5 mg/kg morphine and 0.25 mg/kg ACTH, respectively. Hence, beta-E and ACTH liberated from the pituitary gland by stimulation of an AP and NAP may act as positive feedback on the AA and NAA afferent pathways, respectively.

Morphine effects in brainstem-transected cats: II. Behavior and sleep of the decerebrate cat

Previous studies have shown that opiates suppress both non-rapid eye movement (NREM) and rapid eye movement (REM) sleep. Furthermore, during the induced insomnia period, characteristic species-specific behaviors occur which are associated with high voltage slow waves in the EEG. This paper investigates the lower brainstem mechanisms involved in the generation of these effects, and describes the action of single morphine doses (0.5, 2.0 or 3.0 mg/kg, i.p.) on the behavior and REM sleep of chronic decerebrate cats. The effects of morphine in the decerebrate cat followed a 3-stage time course similar to that seen in intact cats: (1) autonomic manifestations (3-8 min postdrug); (2) a quiet state (10-60 min postdrug) with behavioral signs of NREM; and (3) a state of activated behavior (1-6 h postdrug), including motor activity and variations in muscle tone. The decerebrate cats also showed a dose-dependent suppression of REM sleep. The present results indicate that: (1) the lower brainstem provides the basic mechanisms for the behavioral deactivation-activation and the autonomic effects of the drug; (2) hypnogenic and synchronizing influences arising from the brainstem might induce the high voltage, slow burst EEG produced by opiates; (3) REM sleep suppression originates only partially in the lower brainstem; (4) the subsidiary action of the prosencephalon seems to be required for the full expression of the drug's effect on behavior and the EEG.

Morphine effects in brainstem-transected cats: I. EEG and 'sleep-wakefulness' in the isolated forebrain

In order to examine the prosencephalic mechanisms that might sustain the effects of opiates on EEG and sleep-wakefulness, the actions of morphine sulfate on the EEG and the pupil size were examined in the chronically isolated forebrain of brainstem-transected cats. Single morphine doses (0.5, 2.0 or 3.0 mg/kg, i.p.) administered to these animals produced a long-lasting EEG desynchronization in the isolated forebrain which was associated with pupil mydriasis. The specificity of these morphine effects was shown by the fact that naloxone blocks both the EEG and pupillary effects of the drug. After morphine, spontaneous synchronized EEG with delta waves normally seen in the isolated forebrain preparation was suppressed for 6-18 h, followed by a strong rebound. Both the suppression and rebound in synchronization with delta waves occurred in a dose-dependent manner. The duration of these effects closely paralleled previously reported morphine effects on non-rapid eye movement (NREM) sleep in intact cats. Therefore, in relation to the effects of morphine on EEG and sleep-wakefulness in intact animals, this study suggests that: (1) Morphine suppression of NREM sleep and the subsequent arousal state of the animal are mediated by prosencephalic structures; (2) the generation of the typical neocortical EEG slow burst activity produced by opiates depends on lower brainstem structures.

Studies on the effects of intrathalamically injected DADL and morphine on nociceptive thresholds and electroencephalographic activity: a thalamic delta receptor syndrome

Bilateral microinjections of DADL (D-Ala2-D-Leu5-enkephalin) and morphine were carried out in rats in a systematic fashion at histologically identified medial and lateral thalamic sites. DADL produced a dose-dependent (1.5-15.0 nmol), naloxone-reversible (1 mg/kg, i.p.) increase in the hot-plate (HP), tail-flick (TF) and catalepsy (CAT) response latencies with a predominance of activity occurring at lateral as opposed to medial thalamic sites. These effects were seen within 5 min of microinjection. At a significant number of sites, DADL precipitated convulsive seizure activity. Equimolar doses of morphine had a negligible effect on nociceptive indices and were not productive of seizures even at sites where DADL was found to be active. To further examine seizure activity, rats were prepared with bilateral frontal cortical electrodes and microinjected also at medial and lateral thalamic sites with equimolar doses of DADL and morphine (15 nmol). DADL was found to produce electrographically defined seizures unaccompanied by convulsive motor behavior (cataleptic seizures), as well as convulsive seizures. All animals in this group exhibiting analgesia and catalepsy had electrographic evidence of a seizure with markedly abnormal EEG tracings showing postictal spiking and changes in baseline frequency and amplitude. These seizures appeared to be naloxone-reversible. Morphine on the other hand was not productive of seizures, but did produce changes in electroencephalographic activity including spindle bursting, high-voltage slow-frequency activity as well as spiking. As noted, these changes were not associated with any effects on nociceptive measures.

Opioid receptor alterations in a genetic model of generalized epilepsy

Autoradiography was used to examine opioid receptor binding in the Mongolian gerbil, a genetic model of the epilepsies. Coronal brain sections of seizure-resistant (SR) and seizure-sensitive (SS) (both pre- and post-seizure conditions) gerbils were labeled with [3H]dihydromorphine. SS (pre-seizure) gerbils demonstrated overall greater brain opioid binding when compared to SR animals. The periaqueductal gray, substantia nigra and medial geniculate body were specific areas in SS (pre-seizure) gerbils which demonstrated highly significant increases in opioid binding compared to SR animals (% increase vs SR were 98%, 91.3% and 42.9%, respectively). Scatchard analysis demonstrated that the increase in opioid binding was due to an increase in the total number of receptors without a significant change in receptor affinity (i.e. periaqueductal gray area: total number of binding sites was 12.7 (SR) and 18.0 fmol/mg tissue (SS pre-seizure), while Kd values were 4.0 (SR) and 4.0 mM (SS pre-seizure). Opioid binding was also increased in the SS (post-seizure) animals when compared to SR animals, especially in the substantia nigra. However, when compared to SS (pre-seizure) gerbils, there was a general but not significant, decrease in opioid binding in SS post-seizure gerbils. The increased opioid binding in the SS (pre-seizure) gerbil compared to SR gerbils could reflect an up-regulation due to a deficit in endogenous ligand (e.g. a deficit in synthesis or decreased release) which could underlie the seizure diathesis in the gerbil.

Inhibition of penicillin-induced EEG discharges by low doses of morphine or naloxone in the rabbit. Evidence for a possible non-opioid receptor-mediated mechanism at the sensorimotor cortex

In rabbits, pretreatment by intravenous (IV) and intracortical (IC) routes with low doses of morphine (250 micrograms/kg IV or 60 pmoles/rabbit IC) and naloxone (1-50 micrograms/kg IV or 0.3 pmoles/rabbit IC) antagonizes the EEG and behavioural seizures due to the IC injection of penicillin (150 Units) at the level of the sensorimotor cortex. Pretreatment with naloxone (20 micrograms/kg IV) did not alter the anticonvulsant effect of morphine (250 micrograms/kg IV). The similar anticonvulsant effect of the two drugs together with the absence of any antagonism by naloxone on the effect of morphine seem to suggest that both drugs act through a non-opioid receptor-mediated mechanism. Further, in light of the low effective doses of the drugs and of the absence of any additive effect after their combined administration, one might speculate that morphine and naloxone do not act through different pharmacological receptors. However, the presence of distinct EEG patterns with either morphine or naloxone, injected IC and IV, in animals fully protected against penicillin-induced seizures, does not seem to be in favour of the latter possibility.

The effects of methadone on cortical and subcortical EEG in the rat

(1) Administration of methadone in rats elicited high voltage, slow activity in the cerebral cortex and low voltage irregular waves in the hippocampus. Intravenous administration of methadone (0.2-0.5 mg/kg) markedly increased the threshold for cortical desynchronization by stimulation of the MRF. This increase by methadone was dose dependent. (2) Cortical desynchronization following mechanical stimulation of the tail or foot was blocked by methadone at a dose level of 0.5 mg/kg. At this dose level, the cortical desynchronizing threshold for MRF stimulation increased more than 8-fold whereas the threshold for MT and DH stimulations showed only small but measurable increases. A minimum dose of 0.5 mg/kg was needed to raise the threshold of these structures significantly (3) The increased threshold following injection of methadone was completely blocked by prior injection of the antagonist naloxone, indicating this response to be specific. Naloxone alone had no effect on electrical activity at any site. (4) The incidence of dissociation of the cortical response from the limbic system response after stimulation of the dorsal hypothalamus was approximately 4 times greater in methadone-treated than in untreated animals.

Effects of morphine on sensory-evoked responses recorded from central gray, reticular formation, thalamus, hypothalamus, limbic system, basal ganglia, dorsal raphe, locus ceruleus, and pineal body

Field potential recordings of acoustic and photic-evoked responses were obtained from 15 brain sites of freely behaving unanesthetized rats previously implanted stereotaxically with permanent electrodes. Several dosages of morphine (1, 5, 10, 30, and 50 mg/kg) were examined. The activities recorded from all the structures in this study, except the cochlear nucleus (CoN), were affected by morphine. Different sensitivities to morphine threshold were observed between structures, and several structures exhibited dose-related patterns (ventromedial hypothalamus (VMH), caudate nuucleus (CN), central gray (CG), hippocampus (Hipp), and lateral septum (Spt)). Several brain sites, after the initial dose of morphine, did not recruit more responses to subsequent doses of the drug, ie, exhibited all-or-none responses (pineal body (PB), medial thalamus (MTh), anterior hypothalamus (AH), mesencephalic reticular formation (MRF), and the dorsal raphe (DR)). In some structures, morphine induced increases in the response amplitudes, while in other sites decreases in response amplitudes were elicited. Biphasic responses, ie, increases in response amplitude after low doses of the drug and decreases in response amplitude after higher dosages, were also observed (VMH, CN, DR, CG, and MRF). The acoustic-evoked responses were affected by morphine more than the photic responses. The present observations indicated that 1) morphine exerts effects in many parts of the central nervous system (CNS); 2) some structures are more sensitive to morphine than others; 3) only a few structures exhibit dose-related patterns and, thus, may represent sites of direct morphine action; 4) some structures exhibit all-or-none responses; and 5) morphine depressed activity in some structures and increased activity in others, ie, morphine elicited different effects in different structures.

Dual action effects of morphine on the electrical activity of the dorsal tegmentum

The midbrain tegmentum has been identified as an important locus for development of negative reinforcement with electrical stimulation of the brain. It also plays a central role in the motivational-affective component of pain, and is a site of the analgesic action of morphine. The present study reports the effects of morphine on the electrical activity of areas of the dorsal tegmentum of rats that were also tested for the aversive effects of brain stimulation. The results of spectral analysis of the EEG indicated that IP injections of 16 mg/kg of morphine significantly depressed intensity of EEG, while 8 mg/kg of morphine tended to increase intensity. The results were interpreted in terms of the dual action hypothesis of morphine action and Winters' model of drug effects on electrical activity of the brain. It was concluded that morphine may produce complementary inhibitory and excitatory effects on the negative and positive reinforcement systems of the brain respectively.

Epileptic properties of leucine- and methionine-enkephalin: comparison with morphine and reversibility by naloxone

Morphologically similar epileptic seizures were recorded from the cortex of rats after injections into the lateral ventricle of 100 microgram of leucine-enkephalin, methionine-enkephalin, and morphine. Seizures were either greatly attenuated or blocked completely by prior systemic administration of naloxone (10 mg/kg). These findings suggest that such seizures result from an interaction of these compounds with opiate receptors in the brain. The epileptogenic potency of the enkephalins was illustrated by the observation that seizures and other pathological manifestations could still be elicited by doses as low as 10 microgram. Leucine-enkephalin was seen to have greater epiliptic potency than methionine-enkephalin. At doses of 1 microgram both enkephalins typically evoked cortical spindles resembling those seen in drowsy animals. Enkephalin-induced analgesia was seen in only one animal at the 100 microgram dose. Results obtained with repeated injections of morphine suggest that the epileptogenic effect of opiates may be subject to either tolerance or potentiation, depending on the prior occurrence of seizures. A synthesis of the present findings with several other lines of evidence suggests both that endogenous enkephalins play some role in normal mechanisms of reward, and that, when regulatory processes are disturbed, they may contribute as well to the elaboration of certain epileptic phenomena.

Differential morphine effects on evoked impulse activity in the caudate and central grey

This study had two primary objectives: (1) to develop a new physiological method for investigating opiate action, based on the effect of morphine on evoked impulse activity in small neuronal populations, and (2) to test the hypothesis that morphine's effects would vary with response site (caudate and central grey), and with stimulation site (sciatic nerve, substantia nigra). Bipolar recordings were made from curarized, artificially respired rats. Morphine increased stimulus thresholds for sciatic evoked responses in both brain areas, with especially marked effects in the central grey. Sciatic stimulation produced phasic (transient) responses in both brain areas to mild stimulation and tonic (sustained) responses to more intense stimulation; the tonic responses were depressed more effectively than phasic responses. Substantia nigra stimulation produced only tonic responses in both areas, and morphine did not alter these stimulation thresholds. Morphine effects were blocked and reversed by naloxone. Thus, clear differential depressive effects of morphine were demonstrated in terms of function (phasic vs. tonic responses), in terms of stimulus site, and in terms of responding site.