Low concentrations of amitriptyline inhibit nicotinic receptors in unmyelinated axons of human peripheral nerve

Amitriptyline Decreases GABAergic Transmission in Basal Forebrain Neurons Using an Optogenetic Model of Aging

The antidepressant drug amitriptyline is used in the treatment of clinical depression and a variety of neurological conditions such as anxiety, neuropathic pain disorders and migraine. Antidepressants are associated with both therapeutic and untoward effects, and their use in the elderly has tripled since the mid-1990s. Because of this widespread use, we are interested in testing the acute effects of amitriptyline on synaptic transmission at therapeutic concentrations well below those that block voltage-gated calcium channels. We found that 3 μM amitriptyline reduced the frequency of spontaneous GABAergic inhibitory postsynaptic currents (IPSCs) and reduced quantal content in mice at ages of 7-10 mo. and 23-25 mo., suggesting a presynaptic mechanism of action that does not diminish with age. We employed a reduced synaptic preparation of the basal forebrain (BF) and a new optogenetic aging model utilizing a bacterial artificial chromosome (BAC) transgenic mouse line with stable expression of the channelrhodopsin-2 (ChR2) variant H134R specific for GABAergic neurons [VGAT-ChR2(H134R)-EYFP]. This model enables optogenetic light stimulation of specific GABAergic synaptic terminals across aging. Age-related impairment of circadian behavior was used to confirm predictable age-related changes associated with this model. Our results suggest that low concentrations of amitriptyline act presynaptically to reduce neurotransmitter release and that this action is maintained during aging.

Examination and characterisation of the effect of amitriptyline therapy for chronic neuropathic pain on neuropeptide and proteomic constituents of human cerebrospinal fluid

INTRODUCTION

Amitriptyline is prescribed to reduce the intensity of chronic neuropathic pain. There is a paucity of validated in vivo evidence in humans regarding amitriptyline's mechanism of action. We examined the effect of amitriptyline therapy on cerebrospinal fluid (CSF) neuropeptides and proteome in patients with chronic neuropathic pain to identify potential mechanisms of action of amitriptyline.

METHODS

Patients with lumbar radicular neuropathic pain were selected for inclusion with clinical and radiological signs and a >50% reduction in pain in response to a selective nerve root block. Baseline (pre-treatment) and 8-week (post-treatment) pain scores with demographics were recorded. CSF samples were taken at baseline (pre-treatment) and 8 weeks after amitriptyline treatment (post-treatment). Proteome analysis was performed using mass spectrometry and secreted cytokines, chemokines and neurotrophins were measured by enzyme-linked immunosorbent assay (ELISA).

RESULTS

A total of 9/16 patients experienced a >30% reduction in pain after treatment with amitriptyline and GO analysis demonstrated that the greatest modulatory effect was on immune system processes. KEGG analysis also identified a reduction in PI3K-Akt and MAPK signalling pathways in responders but not in non-responders. There was also a significant decrease in the chemokine eotaxin-1 (p ​= ​0.02) and a significant increase in the neurotrophin VEGF-A (p ​= ​0.04) in responders.

CONCLUSION

The CSF secretome and proteome was modulated in responders to amitriptyline verifying many pre-clinical and in vitro models. The predominant features were immunomodulation with a reduction in pro-inflammatory pathways of neuronal-glia communications and evidence of a neurotrophic effect.

Low doses of amitriptyline, pregabalin, and gabapentin are preferred for management of neuropathic pain in India: is there a need for revisiting dosing recommendations?

Background

Current therapy for the treatment of neuropathic pain is often unsatisfactory. Considerable variation in treatment pattern still exists in spite of availability of sufficient literature from various guidelines. Recent Indian market data suggested that the utilization (sale) of drugs such as amitriptyline, pregabalin, and gabapentin was more for low-dose unit packs than that of the high-dose unit packs, raising the belief that these drugs are prescribed at a lower dose than is actually recommended in the guidelines. To test this hypothesis, a survey was conducted across speciality throughout the country to observe the prescription pattern of these drugs amongst the health care providers in India.

Methods

Three hundred fifty survey forms were distributed of which 281 forms were included for analysis.

Results

It was observed that the commonly used initiation and maintenance dose for amitriptyline, pregabalin, and gabapentin was 5–10 mg/day, 50–75 mg/day, and 100–300 mg/day, respectively. The reason to select the lower dosages was to have a balancing effect to achieve good efficacy with minimum side effects. Care-givers reported no side effects/not many side effects as a reason in 22.2%, 16.88%, and 23.86% patients with amitriptyline, pregabalin, and gabapentin, respectively. Sedation and giddiness were commonly reported with all three drugs.

Conclusions

Commonly prescribed drugs for management of neuropathic pain, such as amitriptyline, pregabalin, and gabapentin are preferred at lower doses in Indian clinical settings. Acceptable efficacy and low tolerance to the standard dosage is believed to be the reason behind the prescribed dose.

Sodium Channel Nav1.8 Underlies TTX-Resistant Axonal Action Potential Conduction in Somatosensory C-Fibers of Distal Cutaneous Nerves

Voltage-gated sodium (Na_V) channels are responsible for the initiation and conduction of action potentials within primary afferents. The nine Na_V channel isoforms recognized in mammals are often functionally divided into tetrodotoxin (TTX)-sensitive (TTX-s) channels (Na_V1.1-Na_V1.4, Na_V1.6-Na_V1.7) that are blocked by nanomolar concentrations and TTX-resistant (TTX-r) channels (Na_V1.8 and Na_V1.9) inhibited by millimolar concentrations, with Na_V1.5 having an intermediate toxin sensitivity. For small-diameter primary afferent neurons, it is unclear to what extent different Na_V channel isoforms are distributed along the peripheral and central branches of their bifurcated axons. To determine the relative contribution of TTX-s and TTX-r channels to action potential conduction in different axonal compartments, we investigated the effects of TTX on C-fiber-mediated compound action potentials (C-CAPs) of proximal and distal peripheral nerve segments and dorsal roots from mice and pigtail monkeys (Macaca nemestrina). In the dorsal roots and proximal peripheral nerves of mice and nonhuman primates, TTX reduced the C-CAP amplitude to 16% of the baseline. In contrast, >30% of the C-CAP was resistant to TTX in distal peripheral branches of monkeys and WT and Na_V1.9^-/- mice. In nerves from Na_V1.8^-/- mice, TTX-r C-CAPs could not be detected. These data indicate that Na_V1.8 is the primary isoform underlying TTX-r conduction in distal axons of somatosensory C-fibers. Furthermore, there is a differential spatial distribution of Na_V1.8 within C-fiber axons, being functionally more prominent in the most distal axons and terminal regions. The enrichment of Na_V1.8 in distal axons may provide a useful target in the treatment of pain of peripheral origin.SIGNIFICANCE STATEMENT It is unclear whether individual sodium channel isoforms exert differential roles in action potential conduction along the axonal membrane of nociceptive, unmyelinated peripheral nerve fibers, but clarifying the role of sodium channel subtypes in different axonal segments may be useful for the development of novel analgesic strategies. Here, we provide evidence from mice and nonhuman primates that a substantial portion of the C-fiber compound action potential in distal peripheral nerves, but not proximal nerves or dorsal roots, is resistant to tetrodotoxin and that, in mice, this effect is mediated solely by voltage-gated sodium channel 1.8 (Na_V1.8). The functional prominence of Na_V1.8 within the axonal compartment immediately proximal to its termination may affect strategies targeting pain of peripheral origin.

Population Pharmacokinetics of Amitriptyline After Intrathecal, Epidural, and Intravenous Administration in Sheep

BACKGROUND

Amitriptyline (AMI) is a lipophilic, tricyclic antidepressant with analgesic properties that could potentially be used for epidural (EPI) analgesia. However, no pharmacokinetic data are available for AMI in spinal spaces. The objective of this study was to evaluate the spinal disposition and intrathecal (IT) bioavailability of AMI after IT and EPI administration.

METHODS

Six Lacaune ewes received 3 consecutive administrations of AMI. They initially received 10 mg of AMI administered intravenously, then 5 mg of AMI administered intrathecally, and 50 mg of AMI injected into the EPI space. Consecutive administrations were separated by intervals of 2 hours. A simultaneous microdialysis technique was used to determine the EPI and IT concentrations of AMI. Population analysis with S-ADAPT software was used to evaluate the pharmacokinetic parameters.

RESULTS

Following intravenous administration, the clearance and central compartment (Vc) in plasma were 1.32 L/min and 147 L, respectively. Concentration-time profiles for the IT and EPI compartments were highly variable after transmeningeal diffusion. The IT Vc after IT administration and the EPI Vc after EPI administration were 2.4 and 48.9 mL, respectively. Less AMI transferred from the EPI to the IT space than from the IT to the EPI compartment, with bioavailabilities of 1.3% and 55%, respectively.

CONCLUSIONS

Simultaneous population analysis for AMI demonstrated differences in EPI and IT pharmacokinetics following the EPI and IT administration of this drug. The IT bioavailability of AMI after EPI administration is relatively low.

Autonomic testing of women with interstitial cystitis/bladder pain syndrome

Purpose

Interstitial cystitis/bladder pain syndrome (IC/BPS) is characterized by urinary urgency, frequency, nocturia, pain worse as the bladder fills and improved after emptying. These features might suggest abnormal autonomic bladder control mechanisms. We compared the structural integrity of the autonomic nervous system (ANS) in IC/BPS and control subjects.

Methods

IRB-approved study at University Hospitals Case Medical Center, Cleveland, OH to evaluate the structural integrity of the ANS in adult females. Testing included cardiovascular response to deep breathing, Valsalva maneuver, 30 min head up tilt, and sudomotor test.

Results

Differences in ANS integrity for IC/BPS subjects and controls were determined by modified Composite Autonomic Severity Score (CASS) that includes sudomotor, adrenergic and cardiovascular indices. Baseline heart rate (HR) and HRs from each of three 10 min upright segments of a tilt test were compared and trend analyses performed using t tests. Healthy and IC/BPS subjects were demographically similar. The two groups did not differ in modified-CASS scores but elevated average peak heart rate was evident during baseline (supine; p = 0.057) for IC/BPS subjects prior to a tilt test. Difference at baseline was maintained at each interval during the tilt, with nearly identical slopes across intervals. The preliminary nature of this report denotes a small sample size and important differences may not be detected.

Conclusions

The findings show no structural ANS abnormalities in IC/BPS subjects. Higher baseline HR supports the concept of functional rather than structural change in the ANS, such as abnormality of sympathetic/parasympathetic balance that will require further evaluation.

Neuronal nicotinic receptors as analgesic targets: it's a winding road

Along with their well known role in nicotine addiction and autonomic physiology, neuronal nicotinic receptors (nAChRs) also have profound analgesic effects in animal models and humans. This is not a new idea, even in the early 1500s, soon after tobacco was introduced to the new world, its proponents listed pain relief among the beneficial properties of smoking. In recent years, analgesics that target specific nAChR subtypes have shown highly efficacious antinociceptive properties in acute and chronic pain models. To date, the side effects of these drugs have precluded their advancement to the clinic. This review summarizes the recent efforts to identify novel analgesics that target nAChRs, and outlines some of the key neural substrates that contribute to these physiological effects. There remain many unanswered mechanistic questions in this field, and there are still compelling reasons to explore neuronal nAChRs as targets for the relief of pain.

Segmental spinal root avulsion in the adult rat: a model to study avulsion injury pain

Road traffic accidents are the most common cause of avulsion injury, in which spinal roots are torn from the spinal cord. Patients suffer from a loss of sensorimotor function, intractable spontaneous pain, and border-zone hypersensitivity. The neuropathic pains are particularly difficult to treat because the lack of a well-established animal model of avulsion injury prevents identifying the underlying mechanisms and hinders the development of efficacious drugs. This article describes a hindlimb model of avulsion injury in adult rats where the L5 dorsal and ventral spinal root are unilaterally avulsed (spinal root avulsion [SRA]), leaving the adjacent L4 spinal root intact. SRA produced a significant ipsilateral hypersensitivity to mechanical and thermal stimulation by 5 days compared with sham-operated or naïve rats. This hypersensitivity is maintained for up to 60 days. No autotomy was observed and locomotor deficits were minimal. The hypersensitivity to peripheral stimuli could be temporarily ameliorated by administration of amitriptyline and carbamazepine, drugs that are currently prescribed to avulsion patients. Histological assessment of the L4 ganglion cells revealed no significant alterations in calcitonin gene-related peptide (CGRP), IB4, transient receptor potential cation channel subfamily V member 1 (TrpV1), or N52 staining across groups. Immunohistochemistry of the spinal cord revealed a localized glial response, phagocyte infiltration, and neuronal loss within the ipsilateral avulsed segment. A comparable response from glia and phagocytes was also found in the intact L4 spinal cord, supporting the role for central mechanisms within the L4-5 spinal cord in contributing to the generation of the pain-related behavior. The SRA model provides a platform to investigate possible new pharmacological treatments for avulsion injuries.

GABA increases electrical excitability in a subset of human unmyelinated peripheral axons

Background

A proportion of small diameter primary sensory neurones innervating human skin are chemosensitive. They respond in a receptor dependent manner to chemical mediators of inflammation as well as naturally occurring algogens, thermogens and pruritogens. The neurotransmitter GABA is interesting in this respect because in animal models of neuropathic pain GABA pre-synaptically regulates nociceptive input to the spinal cord. However, the effect of GABA on human peripheral unmyelinated axons has not been established.

Methodology/Principal Findings

Electrical stimulation was used to assess the effect of GABA on the electrical excitability of unmyelinated axons in isolated fascicles of human sural nerve. GABA (0.1–100 µM) increased electrical excitability in a subset (ca. 40%) of C-fibres in human sural nerve fascicles suggesting that axonal GABA sensitivity is selectively restricted to a sub-population of human unmyelinated axons. The effects of GABA were mediated by GABA_A receptors, being mimicked by bath application of the GABA_A agonist muscimol (0.1–30 µM) while the GABA_B agonist baclofen (10–30 µM) was without effect. Increases in excitability produced by GABA (10–30 µM) were blocked by the GABA_A antagonists gabazine (10–20 µM), bicuculline (10–20 µM) and picrotoxin (10–20 µM).

Conclusions/Significance

Functional GABA_A receptors are present on a subset of unmyelinated primary afferents in humans and their activation depolarizes these axons, an effect likely due to an elevated intra-axonal chloride concentration. GABA_A receptor modulation may therefore regulate segmental and peripheral components of nociception.