Site- and test-dependent antinociceptive efficacy of amitriptyline in rats

Cold nociception as a measure of hyperalgesia during spontaneous heroin withdrawal in mice

Opioids are powerful analgesic drugs that are used clinically to treat pain. However, chronic opioid use causes compensatory neuroadaptations that result in greater pain sensitivity during withdrawal, known as opioid withdrawal-induced hyperalgesia (OWIH). Cold nociception tests are commonly used in humans, but preclinical studies often use mechanical and heat stimuli to measure OWIH. Thus, further characterization of cold nociception stimuli is needed in preclinical models. We assessed three cold nociception tests-thermal gradient ring (5-30 °C, 5-50 °C, 15-40 °C, and 25-50 °C), dynamic cold plate (4 °C to -1 °C at -1 °C/min, -1 °C to 4 °C at +1 °C/min), and stable cold plate (10 °C, 6 °C, and 2 °C)-to measure hyperalgesia in a mouse protocol of heroin dependence. On the thermal gradient ring, mice in the heroin withdrawal group preferred warmer temperatures, and the results depended on the ring's temperature range. On the dynamic cold plate, heroin withdrawal increased the number of nociceptive responses, with a temperature ramp from 4 °C to -1 °C yielding the largest response. On the stable cold plate, heroin withdrawal increased the number of nociceptive responses, and a plate temperature of 2 °C yielded the most significant increase in responses. Among the three tests, the stable cold plate elicited the most robust change in behavior between heroin-dependent and nondependent mice and had the highest throughput. To pharmacologically characterize the stable cold plate test, we used μ-opioid and non-opioid receptor-targeting drugs that have been previously shown to reverse OWIH in mechanical and heat nociception assays. The full μ-opioid receptor agonist methadone and μ-opioid receptor partial agonist buprenorphine decreased OWIH, whereas the preferential μ-opioid receptor antagonist naltrexone increased OWIH. Two N-methyl-d-aspartate receptor antagonists (ketamine, MK-801), a corticotropin-releasing factor 1 receptor antagonist (R121919), a β2-adrenergic receptor antagonist (butoxamine), an α2-adrenergic receptor agonist (lofexidine), and a 5-hydroxytryptamine-3 receptor antagonist (ondansetron) had no effect on OWIH. These data demonstrate that the stable cold plate at 2 °C yields a robust, reliable, and concise measure of OWIH that is sensitive to opioid agonists.

Treatment of Chronic Refractory Cough in Adults: Focus on Neuromodulators and Other Therapeutic Modalities

Objective: To evaluate the literature and educate health care professionals about the different management options for chronic refractory cough (CRC). Data Sources: Searches were performed through MEDLINE (1966 to July 2020) using OVID and EBSCOhost and EMBASE (1947 to July 2020) using OVID. Search terms included chronic cough, neurogenic cough, neuromodulators, chronic cough management, and chronic cough treatment. References of all relevant articles were further used to obtain additional articles. Study Selection and Data Extraction: This review includes articles in the English language and human trial literature. Twenty-three trials explored the use of oral neuromodulators in the management of CRC. Data Synthesis: CRC is a poorly understood disease that may have a complex neuropathic etiology. Oral neuromodulators, such as amitriptyline, baclofen, gabapentin, and pregabalin, continue to be the most commonly used agents for the management of CRC. Alternative pharmacological therapies such as botulinum toxin, anesthetic agents (benzonatate, lidocaine), and narcotic agents have efficacy data, and further research into effective doses and routes of administration is warranted. Alternative nonpharmacologic therapies more commonly used included speech pathology treatment. Conclusion: Several medication and alternative treatment interventions may be effective in managing CRC. Current studies used wide dosing and titration strategies, making it challenging to standardize therapy. No consistent method of assessing cough reflex was used between studies, as well as a lack of consistent randomization and small sample sizes. Additional research is needed to standardize treatment durations, optimum doses, and place in therapy of the available interventions in the management of CRC.

Differential Effects of Neuropathic Analgesics on Wind-up-like Pain and Somatosensory Function in Healthy Volunteers

OBJECTIVES

To investigate the effects of gabapentin, carbamazepine, and amitriptyline on temporal summation, simple nociceptive pain, and innocuous touch sensation in healthy volunteers.

METHODS

A placebo controlled four-way crossover double-blind randomized protocol was followed. Seventeen healthy subjects, male and female, aged 18 to 24, took part. Punctate pain, temporal summation pain to repeat punctate stimulation, and vibration detection threshold were assessed in triplicate. Study drugs were given as bedtime and early morning doses with assessments carried out midmorning.

RESULTS

Gabapentin and carbamazepine significantly reduced the intensity of temporal summation pain (P < 0.001 and P < 0.01 respectively), whereas amitriptyline significantly increased temporal summation pain (P < 0.001). None of the drugs affected pain produced by a single punctate stimulus (P > 0.05). Carbamazepine increased vibration detection thresholds (P < 0.05), but neither gabapentin nor amitriptyline had any detectable effect on vibration.

DISCUSSION

We have shown that gabapentin, carbamazepine, and amitriptyline, three pharmacologically different drugs, have distinct and quantifiable effects on somatosensory pathways in healthy volunteers. These findings provide a link between pharmacology of the study drugs and clinical effectiveness. The effects of gabapentin and carbamazepine on temporal summation pain show that these drugs can block centrally amplified wind-up pain in the absence of a neuropathic disorder.

Antinociceptive effects of tricyclic antidepressants and their noradrenergic metabolites

This study evaluates the antinociceptive effect of several tricyclic antidepressants in four nociceptive tests which employ either thermal (hot plate and tail flick tests) or chemical (formalin and acetic acid tests) stimuli. Forced swimming test was also performed as a model of depression and an activity test was also performed. Mixed antidepressants in current clinical use: amitriptyline, imipramine and clorimipramine and their respective main secondary metabolites which preferentially inhibit noradrenaline reuptake: nortriptyline, desipramine and desmethylclorimipramine, were tested (2.5-20 mg/kg, i.p.) in mice. The results show a stronger antinociceptive effect in chemical tests induced by all the drugs, compared with thermal tests. The doses needed to produce antinociception were lower than those inducing an antidepressive effect, both effects being mutually independent. The overall results show that preferentially noradrenergic tricyclics induced an antinociceptive effect comparable with that of mixed tricyclics, indicating that noradrenaline reuptake plays an important role in tricyclic-induced antinociception.

Antinociception by tricyclic antidepressants in the rat formalin test: differential effects on different behaviours following systemic and spinal administration

The present study (1) examined analgesic effects of systemically and spinally administered antidepressants (ADs) on phase 2 flinching and biting/licking behaviours in the rat formalin test, a model considered to be of greater relevance to clinical pain than acute threshold tests, and (2) determined whether motor or anti-inflammatory effects contributed to such actions. Systemic administration of amitriptyline (3-20 mg/kg) produced a dose-related enhancement of flinching behaviours, while at the same time suppressing biting/licking behaviours. Imipramine (except for 20 mg/kg), nortriptyline, desipramine and fluoxetine had no significant effect on flinching behaviours, while producing a dose-related suppression of biting/licking behaviours. When administered spinally, either by acute lumbar puncture or via chronically implanted intrathecal cannulas, amitriptyline similarly augmented flinching behaviours. When given by lumbar puncture, amitriptyline suppressed biting/licking behaviours, but when intrathecal cannulas were used, this behaviour was not expressed in the formalin group. Other ADs also suppressed biting/licking behaviours without affecting flinching when given by lumbar puncture. Effects on paw volume were determined at the end of behavioural testing. Systemic administration of all ADs produced a dose-related reduction in paw volume. Spinal administration of nortriptyline by lumbar puncture also reduced paw volume, but for other agents, the reduction was not significant. Motor effects were noted qualitatively throughout these experiments, and considered in relation to nociceptive behaviours. These results indicate (a) a marked dissociation between the effects of systemic ADs on flinching and biting/licking behaviours in the formalin test, (b) spinal efficacy of ADs that essentially reproduces effects seen with systemic administration when given by lumbar puncture, (c) a lack of causality between anti-inflammatory effects of ADs and their analgesic properties in the formalin test, and (d) a contribution of motor effects to analgesic actions at higher doses affecting biting/licking but not flinching behaviours.

Pain Processing: Paradoxes and Predictions

During the last 25 years, there have been substantial advances in our understanding of the physiology and pathophysiology of pain. The development of animal models that more closely mimic clinical pain in humans has helped elucidate the putative mechanisms by which chronic pain develops and is maintained. However, our increased understanding of the neurobiology of pain has not translated into breakthrough treatments for pain management. As such, chronic pain is still primarily managed by drugs whose primary indication does not include pain (eg, antidepressants, anticonvulsants, antiarrhythmics, local anesthetics). These adjuvant analgesics have come into favor despite the fact that the mechanisms through which these drugs provide pain relief remain either largely unknown or are not selective for a single target. Moreover, the efficacy of adjuvant analgesics in animal models of pain is often validated only after case studies or clinical trials have been reported. This retrospective validation of "novel" analgesics in animal models of pain raises a question of the predictive validity of these models. This article reviews the use of several adjuvant and standard analgesics currently used to treat difficult-to-manage pain. What can these drugs teach us about the development of novel pain medicines? Within this context, the use of animal models of pain to predict analgesic efficacy in clinical pain conditions is considered.

The relationship between pupil diameter and pain by the administration of morphine and antidepressant drugs in mice

Because the pain sensation is subjective, it is difficult to evaluate the responses to analgesic drugs. Some analgesics that affect the central nervous system are known to change the pupil diameter. The pupil diameter is a more objective criterion that shows the drug effect. We studied the relation between the pupil diameter and analgesia responses to morphine and antidepressants by using the selective micro-receptor agonist morphine (2 and 4 mg/kg), the noradrenaline reuptake inhibitor desipramine (7.5 and 10 mg/kg), the mixed serotonergic and noradrenergic uptake inhibitor and cholinergic receptor antagonist amitriptyline (2.5 and 5 mg/kg), and the selective serotonin reuptake inhibitor sertraline (2.5 and 5 mg/kg) in mice. Both monocular microscopy to assess pupil measurement and the hot-plate test to assess nociceptive thresholds were used in the same animals. We found that morphine played an important role in both mydriasis and analgesia, whereas amitriptyline and desipramine had a greater effect on pupil response than on nociception. Sertraline produced antinociception without causing a change in pupil diameter. As a result, although the pupil response is an important criterion in evaluating the analgesic effect of morphine, it is not possible to put forward the same criterion for the antidepressant drugs. Because different neurotransmitters are involved in pupil and pain mechanisms of antidepressant drugs, it is difficult to evaluate the analgesic response with the pupil diameter.

Differential effects of amitriptyline on perception of somatic and visceral stimulation in healthy humans

Tricyclic antidepressants treat chronic pain both in patients with somatic illness and with functional bowel disorders. We compared the effects of amitriptyline on perception of cutaneous and gastrointestinal stimulation to assess differential analgesic effects of tricyclics on somatic and visceral pain. Cutaneous electrical stimulation and rectal and esophageal distension were performed before and after 21 days of double-blind 50 mg amitriptyline vs. placebo in healthy volunteers. Amitriptyline increased currents that elicited cutaneous threshold, moderate discomfort, and moderate pain compared with basal (P < 0.05), whereas placebo had no effect. Amitriptyline had no effect on perception of rectal and esophageal distension and did not alter luminal compliance; thus the lack of effect on perception is not due to altered visceral elastic wall properties. In conclusion, amitriptyline reduces perception of cutaneous stimulation but does not alter visceral perception or compliance. This investigation demonstrates differential effects of tricyclics on somatic and visceral afferent function in healthy humans and provides insight into mechanisms of action in chronic pain both from somatic disease and from functional bowel disorders.

Effects of clomipramine and desipramine on a C-fiber reflex in rats

A C-fiber nociceptive reflex evoked by electrical stimulation within the territory of the sural nerve, was recorded from the ipsilateral biceps femoris muscle in urethane anesthetized rats. Intravenously administered clomipramine and desipramine produced a dose-dependent depression of the C-fiber reflex. High doses of intrathecal desipramine also inhibited the C-fiber reflex, while similar intrathecal doses of clomipramine produced only a modest inhibition of the response. Intracerebroventricular administration of clomipramine decreased dose-dependently the C-fiber reflex whereas intracerebroventricular desipramine increased this reflex. These findings suggest that tricyclic antidepressants with noradrenergic selectivity, as desipramine, inhibit the spinal processing of C inputs by acting directly at the spinal cord level, while those with serotonergic spectra, as clomipramine, depress the C-fiber-evoked spinal reflex by acting at a supraspinal modulatory site.