Variation of pain and vasomotor responses evoked by intramuscular infusion of hypertonic saline in human subjects: Influence of gender and its potential neural mechanisms

Critical Points in the Methodology of Preparing Copper (II) Histidinate Injections and their Quality Assessment Applying Color Measurement

Copper (II) histidinate injection solution, applied in Menkes disease treatment, is characterized by low stability due to sensitivity to oxidation. The aim of this article was to determine the critical points of the injection preparation procedure, taking into account selection of appropriate packaging, determining the solution pH or application of an excess of L-histidine. In order to assess the stability of the Cu(His)2 complex, the spectrophotometric method (VIS: 400-800 nm), and the colorimetric method using a reflectance colorimeter were applied. The color changes observed using the CIELAB color system made it possible to determine: the differences in the observed color (ΔΕ) and the color chroma (C*) and hue (h°). It was found that the following parameters: λmax and ΔE enable fast and objective assessment of copper (II) histidinate injection solution quality. The advantage of the colorimetric method is the non-invasiveness of the analysis which is performed through the packaging material (transparent vial). The developed methodology of preparing Cu(His)2 injections in hospitals or community pharmacies guarantees their stability for at least 6 months, provided that the solution is stored at lower temperatures (2-8°C or 4°C).

Not just sensitization: sympathetic mechanisms contribute to expand experimental referred pain

Background

Widespread pain partially depends upon sensitization of central pain mechanisms. However, mechanisms controlling pain distribution are not completely known. The present study sought to assess skin temperature variations in the area of experimentally-induced pain and potential sex differences.

Methods

Pressure-pain thresholds (PPTs) were measured on the right infraspinatus muscle. At the end of Day 0, all participants performed an eccentric exercise of the shoulder external rotators to induce muscle soreness 24 hours after. On Day 1, participants indicated on a body chart the area of pain induced by 60 seconds of suprathreshold pressure stimulation (STPS; PPT + 20%) on the right infraspinatus muscle. Skin temperature variations in the area of referred pain were recorded with an infrared thermography camera, immediately before and after the STPS.

Results

Twenty healthy, pain-free individuals (10 females) participated. On Day 0, the pre-STPS temperature was higher than the post-STPS temperature on the arm (P = 0.001) and forearm (P = 0.003). On Day 1, the pre-STPS temperature was higher than the post-STPS temperature on the shoulder (P = 0.015), arm (P = 0.001), and forearm (P = 0.010). On Day 0, the temperature decrease after STPS in females was greater than in males on the forearm (P = 0.039). On Day 1, a greater temperature decrease was found amongst females compared with males at the shoulder (P = 0.018), arm (P = 0.046), and forearm (P = 0.005).

Conclusions

These findings indicate that sympathetic vasomotor responses contribute to expand pressure-induced referred pain, especially among females.

Spatiotemporal patterns of pain distribution and recall accuracy: a dose-response study

OBJECTIVES

Clinical decisions rely on a patient's ability to recall and report their pain experience. Monitoring pain in real-time (momentary pain) may reduce recall errors and optimize the clinical decision-making process. Tracking momentary pain can provide insights into detailed changes in pain intensity and distribution (area and location) over time. The primary aims of this study were (i) to measure the temporal changes of pain intensity, area, and location in a dose-response fashion and (ii) to assess recall accuracy of the peak pain intensity and distribution seven days later, using a digital pain mapping application. The secondary aims were to (i) evaluate the influence of repeated momentary pain drawings on pain recall accuracy and (ii) explore the associations among momentary and recall pain with psychological variables (pain catastrophizing and perceived stress).

METHODS

Healthy participants (N=57) received a low (0.5 ml) or a high (1.0 ml) dose of hypertonic saline (5.8%) injection into the right gluteus medius muscle and, subsequently, were randomized into a non-drawing or a drawing group. The non-drawing groups reported momentary pain intensity every 30-s. Whereas the drawing groups reported momentary pain intensity and distribution on a digital body chart every 30-s. The pain intensity, area (pixels), and distribution metrics (compound area, location, radiating extent) were compared at peak pain and over time to explore dose-response differences and spatiotemporal patterns. All participants recalled the peak pain intensity and the peak (most extensive) distribution seven days later. The peak pain intensity and area recall error was calculated. Pain distribution similarity was determined using a Jaccard index which compares pain drawings representing peak distribution at baseline and recall. The relationships were explored among peak intensity and area at baseline and recall, catastrophizing, and perceived stress.

RESULTS

The pain intensity, area, distribution metrics, and the duration of pain were lower for the 0.5 mL than the 1.0 mL dose over time (p<0.05). However, the pain intensity and area were similar between doses at peak pain (p>0.05). The pain area and distribution between momentary and recall pain drawings were similar (p>0.05), as reflected in the Jaccard index. Additionally, peak pain intensity did not correlate with the peak pain area. Further, peak pain intensity, but not area, was correlated with catastrophizing (p<0.01).

CONCLUSIONS

This study showed differences in spatiotemporal patterns of pain intensity and distribution in a dose-response fashion to experimental acute low back pain. Unlike pain intensity, pain distribution and area may be less susceptible in an experimental setting. Higher intensities of momentary pain do not appear to influence the ability to recall the pain intensity or distribution in healthy participants.

IMPLICATIONS

The recall of pain distribution in experimental settings does not appear to be influenced by the intensity despite differences in the pain experience. Pain distribution may add additional value to mechanism-based studies as the distribution reports do not vary with pain catastrophizing. REC# N-20150052.

New insight into the importance of formulation variables on parenteral growth hormone preparations: potential effect on the injection-site pain

Reducing injection-site pain (ISP) in patients with chronic conditions such as growth hormone deficiency is a valuable strategy to improve patient compliance and therapeutic efficiency. Thus understanding different aspects of pain induction following subcutaneous injection of biotherapeutics and identifying the responsible factors are vital. Here we have discussed the effects of formulation's viscosity, concentration, osmolality, buffering agents, pH, and temperature as well as injection volume, dosing frequency, and different excipients on ISP following subcutaneous injection of commercially available recombinant human growth hormone products. Our literature review found limited available data on the effects of different components of parenteral rhGH products on ISP. This may be due to high cost associated with conducting various clinical trials to assess each excipient in the formulation or to determine the complex interactions of different components and its impact on ISP. Recently, conducting molecular dynamics simulation studies before formulation design has been recommended as an alternative and less-expensive approach. On the other hand, the observed inconsistencies in the available data is mainly due to different pain measurement approaches used in each study. Moreover, it is difficult to translate data obtained from animal studies to human subjects. Despite all these limitations, our investigation showed that components of parenteral rhGH products can significantly contribute to ISP. We suggest further investigation is required for development of long acting, buffer-free, preservative-free formulations. Besides, various excipients are currently being investigated for reducing ISP which can be used as alternatives for common buffers, surfactants or preservatives in designing future rhGH formulations.

Pain referral area is reduced by remote pain

BACKGROUND

Endogenous pain inhibitory mechanisms are known to reduce pain intensity, but whether they influence the size and distribution of pain referral is unclear. This study aimed to determine if referred pain is reduced by applying a remote, conditioning painful stimulus.

METHODS

Twenty-four healthy men participated in this randomized, crossover study with a control and conditioning session. Referred pain was induced from the infraspinatus muscle (dominant side) by a painful pressure for 60 s. When applying pressure, the intensity was adjusted to a local pain intensity of 7/10 on a numerical rating scale. In the conditioning session, tonic painful pressure was simultaneously applied to the non-dominant leg during induction of referred pain. The area of referred pain was drawn onto a digital body chart and size extracted for data analysis.

RESULTS

For the total group and in a subgroup with distinct patterns of referred pain (n = 15/24), the pain area perceived in the back and front+back was smaller during the conditioning compared with the control (p < 0.05). No significant difference was found between sessions in a subgroup only demonstrating local pain (n = 9/24).

CONCLUSIONS

Engaging the descending noxious inhibitory control reduced the size of pain areas predominately when distinct pain referral was present. Assuming a conditioning effect of descending inhibitory control acting on dorsal horn neurons, these findings may indicate that mechanisms underlying pain referral can be modulated by endogenous control. The findings may indicate that referred pain may be a useful proxy to evaluate sensitivity of central pain mechanisms as previously suggested.

SIGNIFICANCE

The current results indicate a link between endogenous inhibition and pain referral. Descending inhibitory control effects on pain referral support a spinal mechanism involved in pain referral. Future studies should investigate whether the spatial characteristics of referred pain (e.g. size, frequency of affected body regions and distribution away from the primary nociceptive stimulus) can useful to evaluate the efficiency of endogenous pain modulation.

Muscle pain from an intramuscular injection of hypertonic saline increases variability in knee extensor torque reproduction

The intensity of exercise-induced pain (EIP) reflects the metabolic environment in the exercising muscle, so during endurance exercise, this may inform the intelligent regulation of work rate. Conversely, the acute debilitating effects of EIP on motor unit recruitment could impair the estimation of force produced by the muscle and impair judgement of current exercise intensity. This study investigated whether muscle pain that feels like EIP, administered via intramuscular injection of hypertonic saline, interferes with the ability to accurately reproduce torque in a muscle group relevant to locomotive exercise. On separate days, 14 participants completed an isometric torque reproduction task of the knee extensors. Participants were required to produce torque at 15% and 20% maximal voluntary isometric torque (MVIT), without visual feedback before (baseline), during (pain/no pain), and after (recovery) an injection of 0.9% isotonic saline (Control) or 5.8% hypertonic saline (Experimental) into the vastus lateralis of the right leg. An elevated reported intensity of pain, and a significantly increased variance in mean contraction torque at both 15% (P = 0.049) and 20% (P = 0.002) MVIT was observed in the Experimental compared to the Control condition. Both 15 and 20% target torques were performed at a similar pain intensity in the Experimental condition (15% MVIT: 4.2 ± 1.9; 20% MVIT: 4.5 ± 2.2; P > 0.05). These findings demonstrate that the increased muscle pain from the injection of hypertonic saline impeded accurate reproduction of knee extensor torque. These findings have implications for the detrimental impact of EIP on exercise regulation and endurance performance.NEW & NOTEWORTHY We provide novel data demonstrating that the presence of muscle pain interferes with estimations of torque produced by the knee extensors, which could impair judgment of work rate during endurance exercise. The novelty of our study is in the application of the hypertonic saline experimental model into a quadriceps muscle during short, submaximal isometric contractions at an intensity that provides a more translatable assessment of the impact of exercise-induced pain on work-rate regulation during whole body exercise.

Effects of Intramuscular Heating-needle Stimulation in Controlling Adjuvant-induced Muscle Nociception in Rats: Differential Roles of Thalamic Purinergic P2X3 Receptors

Here we investigated effects of intramuscular (i.m.) heating-needle stimulation on persistent muscle nociception evoked by i.m. injection of different doses (50-200 µl) of complete Freund's adjuvant (CFA) in rats. Paw withdrawal reflexes evoked by noxious mechanical and heat stimulation as well as hind limb swelling were determined prior to and two weeks after the CFA injection. The unilateral injection of CFA induced a dose-related and long-lasting (5-14 d), bilateral secondary mechanical hyperalgesia and heat hypoalgesia associated with long-term limb swelling. A period of 30-45 min 43 °C heating-needle stimulation significantly enhanced the i.m. CFA-induced bilateral heat hypoalgesia and alleviated hind limb swelling. In contrast, 30-45 min 46 °C heating-needle stimulation markedly enhanced both mechanical hyperalgesia and heat hypoalgesia, but failed to influence the CFA-induced hind limb swelling. Microinjection of P2X3 receptor antagonist A-317491 (0.5-4.5 nmol/0.5 µl) into the thalamic ventromedial (VM) nucleus dose-dependently inhibited the 43 °C and 46 °C heating-needle stimulation-induced heat hypoalgesia, whereas the 46 °C heating-needle stimulation-induced mechanical hyperalgesia was significantly prevented by microinjection of A-317491 into the thalamic mediodorsal (MD) nucleus. In contrast, the hind limb swelling was not affected by the microinjection of A-317491 into the thalamic VM or MD nucleus. The present study indicates that in the CFA-induced persistent muscle nociception condition, 43 °C heating-needle stimulation selectively increases descending inhibition, which effect is modulated by the thalamic VM nucleus. In addition to the antinociceptive role of P2X3 receptors in the thalamic VM nucleus, P2X3 receptors within the thalamic MD nucleus participate in the descending facilitation evoked by i.m. 46 °C heating-needle stimulation.

Sex differences and mechanisms of muscle pain

Clinical conditions resulting in musculoskeletal pain show important sex differences in both prevalence and degree of functional disability. The underlying mechanisms for these distinctions in pain manifestation are not fully known. However, recent preclinical studies have shown at the primary afferent level that males and females present fundamental differences in their peripheral response properties and injury-related gene expression patterns that may underlie observed afferent sensitization. At the spinal cord level, studies in various models of pain suggest important roles for the immune system, glutamate signaling and hormones in modulating sex differences. While preclinical studies have been able to characterize some of the basic underlying molecular mechanisms of sex differences in muscle pain, human studies have relied mainly on functional brain imaging studies to explain differences. Further complicating our understanding of how sex influences muscle pain is the notion that the type of injury sustained, or clinical condition may differentially activate distinct mechanisms of muscle pain development in males versus females. More research is necessary to better understand how the sexes differ in their perception of muscle pain. This review highlights recent advances in both human and animal studies of sex differences in muscle pain.

Role of capsaicin- and heat-sensitive afferents in stimulation of acupoint-induced pain and analgesia in humans

We investigated role of capsaicin-sensitive afferents within and without the areas of Zusanli (ST36)/Shangjuxu (ST37) acupoints along the stomach (ST) meridian in the perception and modulation of pain assessed by visual analog scale of pain and its distribution rated by subjects, pressure pain threshold (PPT), and heat pain threshold (HPT) in humans. Compared with the treatment of non-acupoint area, capsaicin (100µg/50µl) administered into either ST36 or ST37 acupoint caused the strongest pain intensity and the most extensive pain distribution, followed by rapid onset, bilateral, long-lasting secondary mechanical hyperalgesia and slower onset secondary heat hypoalgesia (1day after the capsaicin treatment). Between treatments of different acupoints, capsaicin administrated into the ST36 acupoint exhibited the stronger pain intensity and more widespread pain distribution compared with the treatment of ST37 acupoint. A period of 30- to 45-min, but not 15-min, 43°C heating-needle stimulation applied to the ST36 acupoint significantly enhanced the HPT, and had no effect on PPT. Upon trapezius muscle pain elicited by the i.m. injection of 5.8% saline, pre-emptive treatment of the contralateral ST36 acupoint with 43°C heating-needle stimulation alleviated the ongoing muscle pain, reduced painful area, and reversed the decrease in HPT. It is suggested that (1) pain elicited from the acupoint and non-acupoint areas differs significantly, which are supposed to be dependent on the different distributions and contributions of capsaicin-sensitive afferents. (2) Non-painful heat stimulation is a valid approach in prevention of ongoing muscle pain with associated post-effects of peripheral and central sensitization.

Role of thalamic nuclei in the modulation of Fos expression within the cerebral cortex during hypertonic saline-induced muscle nociception

It has been proposed that thalamic mediodorsal (MD) and ventromedial (VM) nuclei form thalamic 'nociceptive discriminators' in discrimination of nociceptive afferents, and specifically govern endogenous descending facilitation and inhibition. The present study conducted in rats was to explore the role of thalamic MD and VM nuclei in modulation of cerebral neuronal activities by means of detection of spatiotemporal variations of Fos expression within the cerebral cortex. Following a unilateral intramuscular injection of 5.8% saline into the gastrocnemius muscle, Fos expression within the bilateral, different areas of the cerebral cortex except S2 was significantly increased (P<0.05). Particularly, the increases in Fos expression within the cingulate cortex and the insular cortex occurred at 0.5h, 4h and reached the peak level at 4h, 16h, respectively. Electrolytic lesion of the contralateral thalamic MD and VM nuclei significantly blocked the 5.8% saline intramuscularly induced increases in Fos expression within the bilateral cingulate and insular cortices, respectively. Additionally, the 5.8% saline-induced Fos expression in the cingulate cortex and the insular cortex were dose-dependently attenuated by microinjection of μ-opioid antagonist β-funaltrexamine hydrochloride into the thalamic MD and VM nuclei. It is suggested that (1) the neural circuits of 'thalamic MD nucleus - cingulate cortex' and 'thalamic VM nucleus - insular cortex' form two distinct pathways in the endogenous control of nociception, (2) mirror or contralateral pain is hypothesized to be related to cross-talk of neuronal activities within the bilateral cerebral cortices modulated by μ-opioid receptors within the thalamic MD and VM nuclei.

Influences of gender on postoperative morphine consumption

BACKGROUND

Gender related differences on morphine consumption during postoperative period following abdominal surgeries.

MATERIALS AND METHODS

Four hundred and fifty ASA I & II patients of either sex (male =231, female = 219), between the age group of 18-65 y undergoing elective intra abdominal surgeries under general anaesthesia were included for the study. Patients with preexisting pain either acute or chronic preoperatively were excluded from the study. Anaesthesia and analgesia protocol during surgery was standardized. Postoperatively, a loading dose was given to achieve the visual analogue scale (VAS) of ≤30 and subjects were connected to patient control analgesia (PCA) pump containing 0.4 mg/ml of morphine configured to deliver a bolus dose (1 mg) with a 5 min lock-out period. The total analgesic requirements along with VAS (visual analog scale) score were analysed between males and females in the first 24 h postoperatively. All demographic data and between group comparisons were analysed with student t-test. Within group comparisons were done by using one-way-ANOVA test and Tukey's Honestly significant Difference test.

RESULTS

During the first 24 h, males consumed significantly higher amount of morphine (34.35 ± 6.68 mg) when compared to females (26.78 ± 7.14 mg), p < 0.001.

CONCLUSION

We conclude that men require more morphine in the postoperative period than women.

Effects of simulated weightlessness on intramuscular hypertonic saline induced muscle nociception and spinal Fos expression in rats

We assessed the effects of simulated weightlessness, hindlimb unloading (HU) by 7 days of tail suspension, on noxious mechanically and heat evoked spinal withdrawal reflexes and spinal Fos expression during muscle nociception elicited by intramuscular (i.m.) injection of hypertonic (HT; 5.8%) saline into gastrocnemius muscle in rats. In HU rats, i.m. HT saline-induced secondary mechanical hyperalgesia was enhanced, and secondary heat hypoalgesia was significantly delayed. After 7 days of HU, basal Fos expression in spinal L4-6 segments was bilaterally enhanced only in superficial (I-II) but not middle and deep laminae (III-VI) of the spinal dorsal horn, which finding was not influenced by tail denervation. Unilateral i.m. HT saline injection increased spinal Fos expression bilaterally in both the control rats and 7 days of HU rats. The HT saline-induced bilateral increase of spinal Fos occurred within 0.5h and reached its peak within 1h, after which it gradually returned to the control levels within 8h. Spatial patterns of spinal Fos expression differed between the control group and 7 days of HU group. In superficial laminae, the HT saline-induced increases in Fos expression were higher and in the middle and deep laminae V-VI lower in the 7 days of HU than control rats. It is suggested that supraspinal mechanisms presumably underlie the effects of HU on spinally-organized nociception. Simulated weightlessness may enhance descending facilitation and weaken descending inhibition of nociception.

Effects of interocclusal distance on bite force and masseter EMG in healthy participants

The aim of this study was to investigate effects of interocclusal distance (IOD) on bite force and masseter electromyographic (EMG) activity during different isometric contraction tasks. Thirty-one healthy participants (14 women and 17 men, 21·2 ± 1·8 years) were recruited. Maximal Voluntary Occlusal Bite Force (MVOBF) between the first molars and masseter EMG activity during all the isometric-biting tasks were measured. The participants were asked to bite at submaximal levels of 20%, 40%, 60% and 80% MVOBF with the use of visual feedback. The thickness of the force transducer was set at 8, 12, 16 and 20 mm (= IOD), and sides were tested in random sequence. MVOBF was significantly higher at 8 mm compared with all other IODs (P < 0·001). Only in women, IOD always had significant influence on the corresponding root-mean-square (RMS) value of EMG (P < 0·011). When biting was performed on the ipsilateral side to the dominant hand, the working side consistently showed higher masseter EMG activity compared with the balancing side (P < 0·020). On the contralateral side, there was no difference between the masseter EMG at any IODs. The results replicated the finding that higher occlusal forces can be generated between the first molars at shorter IODs. The new finding in this study was that an effect of hand dominance could be found on masseter muscle activity during isometric biting. This may suggest that there can be a general dominant side effect on human jaw muscles possibly reflecting differences in motor unit recruitment strategies.

Involvement of spinal orexin A in the electroacupuncture analgesia in a rat model of post-laparotomy pain

BACKGROUND

Orexin A (OXA, hypocretin/hcrt 1) is a newly discovered potential analgesic substance. However, whether OXA is involved in acupuncture analgesia remains unknown. The present study was designed to investigate the involvement of spinal OXA in electroacupuncture (EA) analgesia.

METHODS

A modified rat model of post-laparotomy pain was adopted and evaluated. Von Frey filaments were used to measure mechanical allodynia of the hind paw and abdomen. EA at 2/15 Hz or 2/100 Hz was performed once on the bilateral ST36 and SP6 for 30 min perioperatively. SB-334867, a selective orexin 1 receptor (OX1R) antagonist with a higher affinity for OXA than OXB, was intrathecally injected to observe its effect on EA analgesia.

RESULTS

OXA at 0.3 nmol and EA at 2/15 Hz produced respective analgesic effects on the model (P<0.05). Pre-surgical intrathecal administered of SB-334867 30 nmol antagonized OXA analgesia and attenuated the analgesic effect of EA (P<0.05). However, SB-334867 did not block fentanyl-induced analgesia (P>0.05). In addition, naloxone, a selective opioid receptor antagonist, failed to antagonize OXA-induced analgesia (P>0.05).

CONCLUSIONS

The results of the present study indicate the involvement of OXA in EA analgesia via OX1R in an opioid-independent way.