Changes in the dorsal root potential with diazepam and with the analgesics aspirin, nitrous oxide, morphine and meperidine

Ketoprofen: i.c.v. injection and electrophysiological aspects of antinociceptive effect

The direct action of ketoprofen on the CNS was investigated by electrophysiological techniques in rats made arthritic. Between the 10th and 16th days after the administration of the complete Freund's adjuvant the rats were anesthetized and the drug was injected directly into the cerebral ventricles. The changes in spontaneous and evoked firing of thalamic neurons were investigated. Ketroprofen very rapidly inhibited the nociceptive activity induced by ankle mobilization. The inhibitory effects were maximal at the doses of 20 and 40 micrograms within 5 min and spontaneous activity was also reduced. The time courses of the action of the increasing doses were also measured. In view of the site of injection, the doses and the very rapid start of the effects, the inhibition of spontaneous and evoked neuronal firing supports the view of a direct effect of this drug on the electrical behaviour of the neurons.

[Use of morphinomimetics in regional anesthesia]

Pain relief is one of medicine's most important challenges and the first aim of anaesthesia. The most common technique of postoperative analgesia remains intramuscular or subcutaneous opiates. There has been a better understanding of the mechanisms of action of opiates over the last decade, and new techniques and methods of administration have been developed especially their regional application. In 1979, two reports acted as catalysts to promote further studies. Wang et al. reported on the efficacy of intrathecal morphine to relieve unbearable malignant pain in 8 patients whereas Behar et al reported on the efficacy of morphine by epidural route. More recently, several studies pointed out the usefulness of the peripheral perineural route for opiates. However, this remains controversial, as some discrepancies persist in the results. The classification of opiate receptors and their relationship to opiate analgesia, as well as the mechanisms of action of regionally administered opiates are analyzed. The dual pre- and postsynaptic actions of spinal opiates are then considered. The parts played by the different neurotransmitters and pathways are set out. The evidence for opiate receptors at peripheral nerve sites and the different hypotheses suggested to explain the effect of opiates given by the perineural route are discussed. The pharmacokinetics and pharmacodynamics of opiates given by the subarachnoid and epidural routes are considered, in particular with respect to the comparative pharmacology of the commonly used opiates. The adverse effects of spinal opiates are reviewed, with their potential risks, and their clinical and therapeutic implications. Opiates and local anaesthetics given by the spinal route are compared. The clinical applications of intrathecal and epidural opiates are discussed, especially in the fields of postoperative analgesia, treatment of chest trauma, and cancer pain. Lastly, the few controlled studies concerning the use of opiates in peripheral nerve blocks, especially brachial plexus blocks, and the prospects of this new technique of giving opiates regionally are discussed.

Mechanisms of halothane action on synaptic transmission in motoneurons of the newborn rat spinal cord in vitro

Action of halothane on synaptic transmission was studied on the isolated newborn rat spinal cord. Clinical doses of halothane (less than or equal to 3%) suppressed mono- and polysynaptic reflexes, dorsal root reflexes, excitatory and inhibitory postsynaptic potentials as well as the spontaneous synaptic potentials caused by impulse bombardment. However, the spontaneous miniature inhibitory postsynaptic potentials observed after blocking impulse activities by tetrodotoxin were not all suppressed by halothane. During halothane administration, the membrane potential of motoneurons was hyperpolarized by several millivolts, associated with an increase in input conductance. However, the threshold potential level for spike generation was virtually unaffected. Depression of synaptic transmission in spinal motoneurons by halothane is suggested to be due to two factors: a reduction in the amount of transmitter release secondary to interference with Ca2+ entry into nerve terminals, either by partial blockade of impulse invasion or voltage-dependent Ca2+ channels; and an increase in the depolarizing current necessary for excitation of motoneurons owing to hyperpolarization and decreased input resistance.

GABAergic drugs can enhance or attenuate chlordiazepoxide-induced sleep time in a heterogeneous strain of mice

Evidence supports the notion that differences between the Long-Sleep and Short-Sleep selectively-bred lines of mice are attributable to differences in brain excitability and that these differences are mediated by activity of the GABAergic system. The general applicability of this hypothesis to other populations of mice was tested by using an outbred strain of mice. Specifically, a heterogeneous strain of mice was administered several doses of the hypnotic chlordiazepoxide. Additionally, the indirect GABA agonist AOAA, and the GABA antagonists bicuculline, picrotoxin and pentylenetetrazol were administered to independent groups in conjunction with chlordiazepoxide. The results clearly demonstrate that chlordiazepoxide dose-dependently increased hypnosis, while AOAA enhanced, and the antagonists attenuated sleep time. These findings can be used to support the contention that GABA mediates the bidirectional response of Long-Sleep and Short-Sleep mice to CNS hypnotic-depressants; and, further, show that GABA mediation of sleep time in mice is a general phenomenon.

Depressant effects of suprofen, a new non-steroidal anti-inflammatory drug on thalamic evoked neuronal firing in arthritic rats

The effects of suprofen, a new non-steroidal anti-inflammatory drug, (NSAID), the activity of which is mainly antinociceptive, were compared with those of aspirin (as a reference drug) in a study of spontaneous and evoked firing of thalamic neurons (nucleus lateralis and ventrobasalis) in rats rendered arthritic by injection of Freund's adjuvant into the paw. Suprofen (3.7 mg/kg, i.v.) induced a marked decrease in the firing evoked in arthritic rats by ankle mobilization. This effect, after a rapid onset, lasted on the average for 60 min. A similar effect was obtained with aspirin, but with 54 mg/kg (i.v.) (14 times more than suprofen). With increasing doses of suprofen, it was possible to obtain an increased long-lasting inhibition of the evoked activity, with a significant dose-effect linear regression. The possibility that there are both CNS and peripheral effects of suprofen is discussed in relation to the possible role of aspirin (the reference standard for NSAIDs) in enhancing presynaptic inhibition, thus reducing the effectiveness of incoming sensory stimuli.

The stereospecific effect of naloxone on rat dorsal horn neurones; inhibition in superficial laminae and excitation in deeper laminae

The effect of systemic naloxone on the activity evoked by C-fibre stimulation in dorsal horn neurones of the rat spinal cord has been investigated. Recordings were made in unanaesthetized, decerebrate spinalized rats. Fifteen units were recorded from laminae 4 and 5 of the dorsal horn, 11 of these units were excited by naloxone (0.2--1.0 mg/kg). The onset of this excitation was after 20 sec to 5 min and recovery to control levels occurred within 15--40 min. Of 17 units recorded in substantia gelatinosa of the dorsal horn, 13 were inhibited by the naloxone. The latency of onset of this inhibition was short (2--10 sec) and the effect persisted for 5--10 min. The effects were largely restricted to C-fibre evoked activity although sometimes A delta responses were similarly altered. Neurones stimulated by A beta-fibre threshold, or whose sole afferent input were A beta-fibres, were unaffected by the naloxone. The stereoisomer of naloxone, (+)naloxone which is inactive in opiate receptor binding tests, failed to produce the same changes found with (-)naloxone in 17 units. These results show a differential effect of naloxone on neurones in the dorsal horn which respond to C-fibre input. Units in the substantia gelatinosa are inhibited while units in deeper laminae are excited by naloxone. These effects are likely to be mediated by the blockade of endogenous opioids in the spinal cord.

Effects of morphine of human spinal cord and peripheral nervous activities

The depressant effects of morphine on the evoked electrospinogram (EESG), evoked electromyogram (EEMG) and nerve action potential (NAP) were studied in surgical patients. The EESG was recorded with an epidural electrode in the posterior epidural space in the lumbar enlargement. A clinical dose of morphine (1 mg/kg) depressed the amplitude of all components (P1, N1, P2) of the EESG, elicited by both weak and strong stimuli to the tibial nerve. The depressant effects of morphine were most pronounced on the P2 component of EESG, which is believed to correspond to primary afferent depolarization. The H-relfex was augmented by the drug, while the M-wave and the NAP were unaffected. The changes induced by morphine in both the EESG and the H-reflex were reversed completely, partially or even potentiated by naloxone (0.1 mg/kg). These depressant effects of morphine on the EESG were minimized under nitrous oxide (75%) anaesthesia, indicating the existence of a degree of interaction between these two drugs. Thus, morphine in a clinical anaesthetic dose, affects both spinal cord function and afferent volleys along the roots in normal man.

Opiate analgesics and endorphins inhibit rat dorsal root potential in vitro

The effects of endorphins and opiate analgesics on the dorsal root potential (DRP) were studied in vitro using the isolated spinal cord of newborn rat. Bath-applied beta-endorphin and [D-Ala2]-Met-enkephalinamide (D-Ala) greatly depressed the DRP and the depressant effects were abolished by prior perfusion with naloxone. The potency of Met-enkephalin, Leu-enkephalin and alpha-endorphin was much weaker than that of D-Ala. Morphine and levorphanol depressed the DRP and these effects were also antagonized by naloxone. The isolated rat spinal cord appears to be a convenient in vitro preparation for analysing the effects of opiates on synaptic transmission in the central nervous system.

Effects of enkephalin analogue and naloxone on cat spinal cord dorsal root potentials

A systemically active enkephalin analogue, FK33824, given intravenously depressed dorsal root potentials in cat spinal cord. The negative DR V and positive DR VI, measured by computer, were both decreased; this effect was reversed by small doses of intravenous naloxone. Naloxone, given alone, with no previous analogue produced no changes in dorsal root potentials suggesting the absence of a basal enkephalin tone. A second injection of FK33824 was much less effective that the first dose. The results were discussed in relation to presynaptic mechanisms for analgesia: we proposed that FK33824 causes presynaptic inhibition by modulation rather than by depolarization of primary afferent fibers.

Presynaptic excitability changes induced by morphine in single cutaneous afferent C- and A-fibers

Intraspinal microstimulation was used to test the excitability of single cutaneous (sural) A- and C-fibers at their central terminals in the cat. Systemic morphine (1.0-2.6 mg/kg) increased the antidromic threshold to 110% (mean value) of control in C-fibers. In some of the C-fibers this effect was reversed by naloxone. A-fibers were not consistently affected by mophine.

Morphine and met-enkephalin effects on sural Adelta afferent terminal excitability

Morphine (30--80 nA) and met-enkephalin (30--80 nA) decreased the excitability of single sural Adelta afferent terminals and potentiated the enhancement of the terminal excitability produced by superficial peroneal nerve stimulation, in mid-collicular decerebrate and spinalized cats. Naloxone (20--40 nA) antagonized the actions of both substances on the unconditioned and the conditioned terminal excitabilities. These results indicate that morphine and met-enkephalin hyperpolarize Adelta sural afferent terminals and facilitate the terminal depolarization during presynaptic inhibition. This enhancement of presynaptic inhibition may be, at least partly, responsible for the analgesic action of these agents.

The effect of diazepam on presynaptic inhibition in patients with complete and incomplete spinal cord lesions

The effect of diazepam on presynaptic inhibition in man has been examined in 5 patients with complete spinal transections and 7 patients with incomplete lesions. The inhibition of the H reflex by vibration applied to the tendo Achilles was used to assess presynaptic inhibition of the Ia monosynaptic pathway. Diazepam increased this inhibition in the patients with incomplete lesions, but had no significant effect on the inhibition in the patients with complete spinal transections. Evidently diazepam can enhance presynaptic inhibition in man. The effect, however, cannot be demonstrated in patients with longstanding complete spinal lesions possibly because of some alteration in the segmental presynaptic inhibitory mechanism in this group.