Effect of etorphine, morphine and diprenorphine on neurones of the cerebral cortex and spinal cord of the rat

Etorphine inhibition of pancreatic exocrine secretion in rats: comparison with methadone

The effects of etorphine, a potent opiate agonist without preferential affinity for mu, delta or kappa receptors, on exocrine pancreatic secretion were studied in rats fitted with chronic or acute pancreatic fistulas and compared to those of methadone, a well-documented mu agonist. In conscious rats etorphine (3 micrograms/kg s.c.) inhibited basal pancreatic secretion by about 50% for volume and bicarbonate output and by 70% for protein output. Pancreatic secretion returned to its basal level within 2 h. Methadone (5 mg/kg s.c.) was about equipotent but the inhibition lasted longer. The effects of both etorphine and methadone were completely antagonized by naloxone (1 mg/kg s.c.) and to a lesser extent by diprenorphine (10 microgram/kg s.c.). Yohimbine did not suppress the inhibitory effect of etorphine on protein output but showed some antagonism against the effects of etorphine on water and bicarbonate output. In anaesthetized rats etorphine (3 micrograms/kg) inhibited the pancreatic secretion stimulated by 2-deoxy glucose, a centrally acting vagal stimulatory agent, by 50-60% for volume and bicarbonate output and totally for protein output. The same dose of etorphine did not inhibit the pancreatic secretion evoked by vagal electrical stimulation, a peripheral stimulus. Methadone (5 mg/kg) inhibited the pancreatic secretion stimulated by 2-deoxy glucose to the same extent, but for a longer time than etorphine, and at the same dose did not suppress the pancreatic pancreatic response to vagal electrical stimulation. The inhibitory effects of etorphine and methadone in anaesthetized rats were completely suppressed by naloxone (1 mg/kg s.c.) and only reduced by diprenorphine (10 micrograms/kg s.c.).(ABSTRACT TRUNCATED AT 250 WORDS)

Behavioral steering in dual and social states of conation by the amygdala, hypothalamus, ventral striatum, and thalamus

Conative information relating to goal states is proposed as regulating columnar information structures in the neocortex. These structures may handle a spatiotemporal hierarchy of sensorimotor information and conative information. In basic behavioral states, a subcortical system selects a dominant modality of conation (corresponding to a specific goal state) from cortical and subcortical sources, and enhances or suppresses activity in neocortical information structures, yielding the selected modality. These adjustments steer behavior relative to the goal state, allowing assembly of new behavior patterns. For opposing or cooperative modalities of conation, a dual-state system is proposed that modulates the basic-state system. For more extended combinations of conative modalities, a further function is needed that interrelates the basic-state system and the cortex to provide hierarchical conation. Basic-state system control may be exercised by the hypothalamus, influencing the cortex via the thalamus. Other conative control functions may be implemented by the amygdala and the ventral striatum. Opiatergic circuits may be involved in modifying conative components of information.

Opiate receptor gradients in monkey cerebral cortex: correspondence with sensory processing hierarchies

In order to obtain information on the possible functions of endogenous opiates in the primate cerebral cortex, we assessed the distribution of mu-like opiate receptors (which selectively bind 3H-labeled naloxone) and delta-like opiate receptors (which selectively bind 3H-labeled D-Ala2, D-Leu5-enkephalin) throughout the cerebral cortex of the rhesus monkey. Stereospecific [3H]naloxone binding sites increased in a gradient along hierarchically organized cortical systems that sequentially process modality-specific sensory information of a progressively more complex nature. Specific [3H]enkephalin binding sites, in contrast, were relatively evenly distributed throughout the cerebral cortex. These results, in combination with electrophysiological studies of monkeys and humans, suggest that mu-like opiate receptors may play a role in the affective filtering of sensory stimuli at the cortical level, that is, in emotion-induced selective attention.

The effects of GABA-ergic drugs on enkephalin-induced motor seizure phenomena in the rat

1. The relationship between the effects of GABA-ergic drugs and D-ala2-met-enkephalinamide (DALA)-induced myoclonic contractions of inframandibular muscles has been studied in the rat. 2. GABA-ergic drugs altered enkephalin-induced myoclonic contractions in the following manner: (a) The GABA-mimetic drugs, muscimol, gabaculine and baclofen, decreased DALA-induced myoclonic contractions. (b) The GABA antagonist, bicuculline and the anticonvulsant substance, sodium valproate (dipropylacetic acid, DPA) potentiated DALA-induced myoclonic contractions. The potentiating effect of DPA is probably due to its opiate-like activity, since naloxone abolished this effect. 3. The modulatory effect of the GABA-mimetic drug on enkephalin-induced myoclonic contractions may give grounds for further study to test the possible use of other GABA-mimetic drugs and possibly opiate antagonists for the treatment of myoclonic syndromes.

The effects of methionine- and leucine-enkephalin on spinal neurones of the cat

Extra- and intracellular recordings were obtained from physiologically identified, spinal neurones in the 6th and 7th lumbar segments of the pentobarbitone-anaesthetized cat. Microiontophoretically applied methionine- and leucine-enkephalin reversibly inhibited the spontaneous, synaptically induced, and L-glutamate-induced activity in the majority of dorsal horn neurones studied in laminae 4, 5 and 6 of Rexed. Most of these depressant effects were antagonized by the prior microiontophoretic application of the opiate antagonist naloxone. Intracellular studies performed on dorsal horn neurones and motoneurones revealed that microiontophoretically applied methionine- and leucine-enkephalin caused no change in the resting membrane potential or the membrane resistance. Neither spike initiation nor spike overshoot were detectably altered by either enkephalin. The membrane depolarization and associated decrease in membrane resistance following microiontophoretic L-glutamate application were effectively blocked by the prior application of enkephalin. Naloxone, which by itself had no detectable effect on the membrane resistance, antagonized this effect. We propose that [enkephalinergic] cells in lamina II and III may modulate cells subserving somatosensory perception, including pain.

The effect of morphine tolerance on the incorporation of 3H-leucine into proteins of rat synaptic membranes

The rate of incorporation of 3H-leucine into proteins of the synaptic junctional and nonjunctional membranes and synaptic vesicles of rat brain has been examined in control and morphine-tolerant rats. There are no discernible differences between control and tolerant animals in amount of protein as measured by densitometric tracings of dye-stained proteins separated by acrylamide gel electrophoresis from the three membrane fractions of whole brain areas. However, there are differences in the turnover of membrane protein: three vesicle protein bands and one junctional-membrane protein band are significantly more highly labeled, and one junctional-membrane protein is significantly less highly labeled by 3H-leucine in samples from tolerant rats. Of these, the two junctional-membrane proteins can be tentatively identified as components of the post synaptic densities, and one of the vesicle proteins as tubulin.

Specific-opiate-induced depression of transmitter release from dorsal root ganglion cells in culture

The opiate etorphine depresses monosynaptic excitatory postsynaptic potentials (EPSP's) elicited in spinal cord cells by activation of dorsal root ganglion cells in murine neuronal cell culture. The depression is reversed by naloxone. Statistical analysis of the synaptic responses reveals that the opiate reduces EPSP quantal content at this synapse without altering quantal size. Therefore, the opiate action is presynaptic and affects transmitter release rather than postsynaptic responsiveness.

Naloxone as a GABA antagonist: evidence from iontophoretic, receptor binding and convulsant studies

From the following three lines of evidence, it is proposed that at least part of the convulsant activity of naloxone is a result of GABA receptor blockade. Firstly, iontophoretic naloxone reversibly antagonized GABA-evoked depression of firing rate in 21 of 27 neurons tested in the rat olfactory tubercle-nucleus accumbens region, without blocking inhibition evoked in the same cells by glycine (15 cells) or morphine (6 cells). Secondly, i.p. naloxone in high doses caused convulsions in mice, and potentiated the convulsant activity of bicuculline, but not that of strychnine. Diazepam, which protected mice against convulsions elicited by bicuculline, but not by strychnine, also protected mice against naloxone. Thirdly, naloxone, morphine, levorphanol and its non-analgesic enantiomer dextrorphan displaced 3H-GABA from GABA receptor sites in homogenates of human cerebellum, all with comparable low potencies (IC50 = 250--400 micron). There was no correlation with affinities at the stereospecific receptor sites that mediate opiate-induced analgesia, since the potent opiates etorphine and diprenorphine were relatively inactive (IC50 greater than 3 mM). In addition naloxone displaced 3H-GABA from receptor sites in rate forebrain and cerebellum, with similar low potency.

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.

The mechanism of inhibition of neuronal activity by opiates in the spinal cord of cat

Extra- and intracellular recordings from motoneurones, interneurones and dorsal horn neurones (laminae 4 and 5) were obtained from the lumbar segments (L6-L7) of spinalised (Th 9/10) or pentobarbital-anaesthetised and anaemically decorticated cats. In the majority of spinal neurones microelectrophoretically applied morphine and levorphanol reversibly depressed spontaneous as well as stimulus-evoked and L-glutamate- or acetylcholine-induced activity. There is evidence that opiates block L-glutamate-induced depolarisations by impairing the Na+-influx triggered at the postsynaptic membrane. These depressant effects of opiates could be antagonised by naloxone, and, except in a few cases, were not associated with hyperpolarisation of the cell. Dextrorphan, the D+ enantiomer of levorphanol, displayed no such depressant actions, indicating that stereospecific receptors mediate the depressant effects of opiates. Phoretically applied atropine, procaine and Ca2+ ions have anti-glutamate and anti-acetylcholine actions similar to opiates, but these actions were not antagonised by naloxone. The hyperpolarising effect of glycine was not influenced at dose levels of opiates sufficient to suppress depolarisation induced by L-glutamate or acetylcholine. Microelectrophoretically administered morphine and levorphanol slowed the rate of rise of mono- and polysynaptic EPSPs by a naloxone-antagonisable mechanism at dose levels where almost no alteration in spike shape was detectable. Increased doses of morphine and levorphanol reduced the amplitude of IPSPs and completely blocked or reduced the amplitude of both direct- and antidromically-evoked spikes. These effects of increased doses of opiates were not antagonised by naloxone. Intravenous injection of 2 mg/kg of morphine or 20 mug/kg of Fentanyl mimicked the suppression of spontaneous and evoked neuronal activity observed after phoretic administration. This depressant action of systemically applied opiates could be transiently antagonised by phoretic administration of naloxone. The results are discussed with respect to a stereospecific action of opiates at a postsynaptic receptive site in the spinal cord.

Actions of opiates upon single unit activity in the cortex of naive and tolerant rats

The effect of microelectrophoretically and systemically applied opiates on neuronal discharge activity in the sensorimotor cortex of naive and morphine tolerant/dependent rats has been studied. In naive rats depression of spontaneous discharge activity was the predominant effect of low doses of phoretically applied morphine. Higher doses and repeated application frequently converted this effect into excitation. Only the depressant effect was antagonised by naloxone. Naloxone itself had no effect on spontaneous discharge activity when applied at dose-levels sufficient to antagonise the depressant effect of morphine. Levorphanol mimicked the action of morphine whereas dextrorphan was inactive. Morphine depressed the excitatory action of L-glutamate and of acetylcholine by a naloxone-antagonisable mechanism. Systemic application of Fentanyl mimicked the inhibitory effect of phoretically applied morphine upon transcallosally evoked discharge activity. The late response was markedly depressed whereas the primary response was little affected. Phoretically applied naloxone antagonised the effects of systemically applied Fentanyl. In chronically morphinised rats the depressant effect of microelectrophoretically administered morphine was almost lacking and a naloxone-resistant excitation became the predominant effect. In these animals the excitant effect of naloxone was also increased and the anti-glutamate effect and the anti-acetylcholine effect of morphine was abolished. The present data speak in favour of a postsynaptically located stereospecific receptor which mediates the inhibitory effects of opiates and which may be involved in the development of acute and chronic tolerance to these drugs.

An investigation of the foetal rat spinal cord. II. An ultrastructural study on the development of synapses with the aid of observations on some electrophysiological properties

Electrophysiological and ultrastructural studies were carried out on foetal rat spinal cord. The electrophysiological observations allowed certain identification of the site of second order sensory neurones, regions of the most functionally mature ventral horn cells and the adequacy of reflex conduction at 18 days. In the ultrastructural studies we made use of these identifications. No definitive synapses were found at 13-14.5 days in dorsal and ventral horn neuropil though some possible precursors were seen. Immature axodendritic synapses are found first in both dorsal and ventral marginal zones at 14.5 days and in both dorsal and ventral neuropil regions at 15-16 days. At 17 days there is an abrupt increase in frequency and maturity of synaptic profiles in all regions; synapses containing pleomorphic populations of vesicles are first seen in the ventral horn neuropil at this age as rare axo-somatic synapses. At 18 days the synapses population increases and multiple contacts involving axons or dendrites commonly occur. Furthermore, axo-somatic synapses are seen for the first time in the dorsal horn. From 20 days onwards mature synapses were commonplace and all earlier stages can be found. In addition axo-dendritic synapses with pleomorphic populations of vesicles were first seen in the dorsal horn. Axo-somatic synapses in the dorsal horn remained immature in appearance at this time. These findings are discussed particularly in relationship to previous studies by others on the development of motility in the rat. It appears that in the rat lumbar cord, onset of formation of different synapse types in specific locations precedes the onset of possible related functions by 1-2 days.