Pain and analgesia: the outlook for more rational treatment

Recent advances have been made in research on the physiology of pain, especially that focusing on the primary afferent nociceptor and endogenous analgesia systems. These advances have shed new light on the mechanisms of action of some long-used methods of managing pain, have led to the development of several methods, and have suggested new lines of investigation that may lead to more rational treatment of pain. The earlier that we can intervene in the pain-transmission pathways, the more likely we are to produce adequate and lasting control of pain. The peripheral nervous system is an important target for this therapeutic approach. The elucidation of the central neural pathways, which contain endogenous opioids and mediate the analgesia elicited by opiates, has raised the possibility of more rational use of this class of analgesics. The rationale for the use of current therapies needs to be reexamined in light of these new findings, and both current and new methods need to be tested in controlled trials.

Multiple opioid receptors: evidence for mu-delta binding site interactions in endotoxic shock

Using antagonists with selectivity for the delta (ICI 154,129) and mu (beta-funaltrexamine) binding sites, evidence was obtained to indicate that delta receptors within the brain mediate the endogenous opioid component of endotoxic hypotension. The therapeutic actions of intravenous ICI 154,129 were dose related, with effective doses between 15-60 mg/kg. Evidence for a functional interaction between mu and delta binding sites was obtained: prior occupancy of the mu binding site by beta-funaltrexamine prevented the usual therapeutic response to the delta antagonist ICI 154,129 in endotoxemic rats. These data indicate that mu and delta binding sites may be a part of the same macromolecular complex which interact through allosteric coupling.

Effects of N-methylnaloxone and N-methylnaltrexone on nociception and precipitated abstinence in mice

Subcutaneous administrations of naloxone and naltrexone have already been shown to enhance nociceptive reactions in mice. The present study was undertaken to examine the effects of N-methyl-naloxone and N-methylnaltrexone on nociception using the hot plate test (dose range: 0.3 to 30 mg kg-1s.c.). The latter compounds were selected to differentiate the central and peripheral components of hyperalgesia. Unlike naloxone, N-methyl-naloxone did not produce hyperalgesia. Similarly low doses of N-methylnaltrexone did not enhance the jumping response. However, a high dose of N-methylnaltrexone (30 mg kg-1 s.c.) significantly reduced the jumping latencies 2 h after its administration. This phenomenon indicated that it might be converted to an active metabolite. Further, N-methylnaloxone and N-methylnaltrexone were very weak in precipitating the signs of abstinence in mice rendered acutely dependent on morphine. Two factors, poorer penetration into the CNS and steric hindrance, might render the N-methylated antagonists weak. Hence, both these factors should be considered when interpreting the effects after quaternary derivatives of opioid antagonists.

Coma vigil masquerading as psychiatric illness

A patient is described who had the hyperpathic variety of akinetic mutism (coma vigil), which was mistaken for a dissociative state. The case discussion emphasizes the importance of differentiating neuropsychiatric syndromes from those which are purely psychiatric in nature in order that appropriate treatment may be given.

Beneficial action of a new opiate antagonist (Win 44,441-3) in hemorrhagic shock

The new opiate antagonist Win 44,441-3 (-)-isomer was infused intravenously in cats at a rate of 2 mg . kg-1 . h-1 to determine its effect in hemorrhagic shock. Hemorrhaged cats treated with Win 44,441-3 maintained post reinfusion mean arterial blood pressure (MABP) at a higher value compared to cats receiving only the vehicle. Final MABP was 70 +/- 11 mm Hg for cats receiving vehicle compared to 103 +/- 7 mm Hg for cats receiving Win 44,441-3. These values represent 60 +/- 9% and 85 +/- 6% of initial MABP for the vehicle- and Win 44,441-3-treated cats respectively. Win 44,441-2 (+)-isomer, the inactive stereoisomer of Win 44,441-3, was also infused at 2 mg . kg-1 . h-1 in cats subjected to hemorrhagic shock. The final pressure in this group was 72 +/- 8 mm Hg which is 61 +/- 8% of the initial pressure for this group. Win 44,441-3 and Win 44,441-2 were both ineffective in moderating increases in circulating lysosomal hydrolase activity in shocked cats. Neither isomer stabilized lysosomal membranes or retarded proteolysis in vitro. Plasma myocardial depressant factor was significantly reduced by the opiate antagonist, Win 44,441-3 during shock. Our results show that the systemic infusion of an opiate antagonist improves the hemodynamic state of cats subjected to hemorrhagic shock while the (+)-isomer which lacks opiate antagonist activity produces no such improvement.

Cholangiographic demonstration of relief of narcotic-induced spasm of the sphincter of Oddi

Narcotics are known to cause spasm of the sphincter of Oddi. This spasm may be difficult to distinguish from obstruction of the distal common bile duct on operative cholangiograms in cases where narcotics are used perioperatively. A case is presented in which narcotic-induced spasm of the sphincter of Oddi, clearly demonstrated in an operative cholangiogram, is reversed by a narcotic antagonist, thereby avoiding an unnecessary common duct exploration.

Antipruritic effect of an opiate antagonist, naloxone hydrochloride

Central elicitation of itch by morphine may result from binding to opiate receptors, mimicking the physiological binding of endorphins and enkephalins to these receptors. Pretreatment of normal subjects with naloxone hydrochloride resulted in diminution or abolition of histamine-provoked itch. These results suggest an important role for central opioid peptides as mediators of the itch sensation.

Circulatory and vestibular implications of central angiotensin mechanisms in physiological adaptation to weightlessness

The discovery of an iso-renin angiotensin (AII) system within the central nervous system (CNS) led investigators to theorize a physiological and/or pathological role for the central actions of AII. Activation of central AII receptors in specific brain regions elicits antidiuretic, dipsogenic and pressor responses subsequently producing net body fluid retention. Removal of hydrostatic forces during weightlessness causes a massive translocation of body fluids to the thoraco-abdominal region; physiological adaptation to such a change is manifested as adipsia and net body fluid loss. It is suggested that these events may result from a decrease in the CNS-mediated effects of AII due to suppression of circulating AII levels. Depressed AII activity in the area postrema (AP) may also be responsible for the atypical nausea characteristic of space sickness.

Psychopathology and endorphins

Suggestive evidence exists linking endorphins to the schizophrenic syndrome; narcotic antagonists appear to slightly attenuate some symptoms, attentional performance is improved and CSF opiate-binding substances are reported to be elevated in a sub-group of patients. Many fewer affectively ill patients have been studied and little evidence has accumulated suggesting a relationship between symptoms of affective illness and endorphins although CSF endorphins appear elevated in some manic-depressive patients and "pain patients" with depression have higher CSF endorphins than pain patients without depression. Catatonic symptoms as well as other psychomotor functions remain promising areas for study. Opioid effects on manic symptoms have been reported by only a few research groups and would benefit from study with longer-acting antagonists administered daily.

Antagonists of morphine-induced respiratory depression. A study in postoperative patients

This double blind study showed the effect of four drugs, levallorphan, naloxone, doxapram and amiphenazole, an opiate-induced analgesia and respiratory depression. Satisfactory analgesia was induced by administration of morphine intravenously in a dose of up to 0.33 mg/kg; such a dose, however, produced significant depression of respiration. Administration of levallorphan, naloxone, and amiphenazole produced reversal of respiratory depression and analgesia. Doxapram reversed the respiratory depression but did not alter analgesia.

Pharmacological and clinical importance of narcotic antagonists and mixed antagonists--use in cardiology

1 The treatment of pain of cardiac origin requires a knowledge of the haemodynamic action of the analgesic agents used. 2 The haemodynamic effects of morphine, diamorphine, pavaveretum, pethidine and pentazocine are reviewed. 3 Clinical experience with the new antagonist analgesic buprenorphine is reported. 4 These studies indicate that buprenorphine may be the agent of choice for the relief of severe pain in patients with unstable circulation.

Analgesic effectiveness of the narcotic agonist-antagonists

1 Two fundamentally different types of narcotic-antogonists have been found to be very effective analgesics with relatively low dependence-producing potentials.

2 These two drug classes can be distinguished as being either morphine-like or nalorphine-like on the basis of their subjective and objective effects after single doses and on chronic administration, and by the character of their abstinence syndromes on abrupt withdrawal or on precipitation by other antagonists.

3 To explain differences in side effects associated with their analgesic actions, the existence of three types of receptors has been postulated: a μ receptor which is believed to be associated with euphoria and other typical morphine-like effects and a kappa (χ) and a sigma (σ) receptor which are believed to be associated with the sedative and psychotomimetic effects, respectively, of the nalorphine-like drugs.

4 The antagonist-analgesics of the morphine-type have the characteristics of being agonists of low intrinsic activity but with high affinity for the μ receptor. Representative analgesics of this type are profadol, propiram and buprenorphine.

5 The antagonist-analgesics of the nalorphine-type are drugs which are believed to have varying degrees of affinity and intrinsic activity at all three receptors, but characteristically seem to act merely as competitive antagonists with no intrinsic activity at the μ receptor. Representative analgesics of this type are pentazocine, nalbuphine and butorphanol.

6 There are considerable differences among the individual drugs of each type in terms of their analgesic and narcotic-antagonistic potencies. However, clear differences in analgesic efficacy among any of the antagonist-analgesics remain to be proved. All give evidence of being capable of relieving pain in nondependent patients in situations in which doses of morphine (or its surrogates) usually used would be effective.

7 The major advantages of the partial agonists of the morphine-type over the nalorphine-type drugs are that they have not been found to produce psychotomimetic reactions, and they seem to have fewer potentially deleterious effects in cardiac patients.

Use of narcotic antagonists in anaesthesia

1 The introduction of morphine antagonists into anaesthesiology has aroused great scientific interest, increased our knowledge of analgesia, and opened new frontiers for therapeutics. 2 The advantages and disadvantages of these compounds are described and assessed. 3 The best choices for treatment of pain are the most potent and long-acting drugs of this group with a wide safety margin. As an antidote to narcotic agonists, naloxone seems to be the most suitable drug in the majority of cases, although in certain conditions drugs having less rapid onset of action and of longer duration may be more desirable. 4 In all cases of treatment after anaesthesia when antagonists to narcotic analgesics are used, drug administration must depend on careful control, which necessitates specific and prolonged patient care by specifically trained and competent staff.

Human pharmacology of narcotic antagonists

1 Human studies at the Addiction Research Center enable narcotic antagonists to be classified into three subgroups: (1) nalorphine-like agents; (2) pure antagonists; and (3) morphine-like agents. 2 Six narcotic antagonists (pentazocine, nalbuphine, cyclazocine, butorphanol, propiram and buprenorphine) developed in recent years seem to have a lesser abuse potential than codeine or propoxyphene. 3 When adjusted for relative availability of the agents, epidemiological data shows that pentazocine is abused less than codeine or propoxyphene in the US. 4 Recent studies with buprenorphine indicate that this agent would find application both as an analgesic of low abuse potential and as a new type of drug for the treatment of addiction.

Endorphins, dopamine, and schizophrenia

The theory that alterations of dopaminergic synaptic transmission may play a role in the pathogenesis of schizophrenia is widely accepted. A more recent theory links the endorphin system to the etiology of schizophrenia. We propose that these two theories may be combined into a single model. Recent neurochemical and pharmacological findings have indicated close functional relationships between the endorphin and dopamine systems. Endorphins modulate dopaminergic synaptic transmission by exerting both presynaptic and postsynaptic effects. On the molecular level, this modulation may involve the activity of nucleotide cyclases and protein phosphorylation systems. Thus, the dopaminergic neuronal hyperactivity, currently believed to be related to schizophrenia, may be caused by a primary alteration in the endorphin system. Several hypotheses about the nature of that alteration have been advanced and tested in therapeutic experiments with schizophrenic patients. These experiments have not yet yielded definitive results.

Animal-human correlates of narcotic dependence: a brief review

Knowledge of opiate action in laboratory animals can be of great value in developing a rational clinical approach to the treatment of narcotic dependence. Physiological alterations experimentally produced by narcotics have been confirmed in human subjects, and classical laboratory effects of conditioning and operant behavior also have clinical counterparts. Realization of animal-human correlates of narcotic dependence allows a better understanding of the strengths and weaknesses of existing therapeutic modalities.

Endorphins in psychiatry: an overview and a hypothesis

This article presents an overview of the biochemistry, pharmacology, and physiology of endogenous opioid peptides (endorphins). Clinical psychopharmacology of exogenous opiate agonists and antagonists is reviewed. The evidence presented in the review is compatible with a hypothesis that the level of functional endorphins may be related to psychological events, with a normal level needed for psychological homeostasis. One corollary of this hypothesis is that the level of opioids in the brains of the mentally ill may be disturbed. Therapeutic implications of this hypothesis are considered.

Clinical aspects of the use of narcotic antagonists: the state of the art

The clinical application of narcotic antagonists would theoretically be of value to several major groups of users. However, while it is clear that supportive services are needed, these services have not been defined by controlled studies.