Opioids, respiratory function, and dyspnea

Put It in the Air? Nebulized Opioids in the Emergency Department

The emergency department (ED) is a frequent utilizer of alternative routes of medication administration (e.g., intranasal) for a variety of indications. Over the last several years, investigations into the use of medications via the nebulization route have greatly increased, with varying degrees of efficacy identified. This route has multiple theoretical advantages. Medications affecting bronchopulmonary function or secretions can be administered directly to the site of action, possibly utilizing a lower dose and hence minimizing side effects. It is also possible to have a faster onset of action compared with other routes, given the enhanced surface area for absorption. One group of medications that has been explored via this route of administration, and is frequently administered in EDs across the nation, is opioids, most notably fentanyl, hydromorphone, and morphine. However multiple questions exist regarding the implementation of these therapies via this route, including efficacy, dosing, and the functional aspects of medication administration that are more complex than that of more traditional routes. The intent of this review is to explore the supporting literature behind the use of nebulized opioids, most specifically fentanyl, hydromorphone, and morphine, in the ED for the treatment of acute pain presentations and provide the most up-to-date guidance for practitioners.

Evaluation of diagnostic and treatment approaches to acute dyspnea in a palliative care setting among medical doctors with different educational levels

Background

Dyspnea is common in patients with advanced cancer. Diagnostic procedures in patients with dyspnea are mandatory but often time-consuming and hamper rapid treatment of the underlying refractory symptoms. Opioids are the first-line drugs for the treatment of refractory dyspnea in palliative care patients with advanced lung cancer.

Methods

To evaluate the knowledge levels of medical doctors with different educational levels on the diagnosis of and treatment options for dyspnea in patients with advanced lung cancer in a palliative care setting, a case report and survey were distributed to physicians at the University Hospital Krems, describing acute dyspnea in a 64-year-old stage IV lung cancer patient. A total of 18 diagnostic and 22 therapeutic options were included in the survey. The physicians were asked to suggest and rank in order of preference their diagnosis and treatment options. Statistical analyses of the data were performed, including comparison of the responses of the senior doctors and the physicians in training.

Results

A total of 106 surveys were completed. The respondents were 82 senior physicians and 24 physicians in training (response rates of 86% and 80%, respectively). Regarding diagnostic investigations, inspection and reading the patient’s chart were the most important diagnostic tools chosen by the respondents. The choices of performing blood gas analysis (p = 0.01) and measurement of oxygen saturation (p = 0.048) revealed a significant difference between the groups, both investigations performed more frequently by the physicians in training. As for non-pharmacological treatment options, providing psychological support was one of the most relevant options selected. A significant difference was seen in choosing the option of improving a patient’s position in relation to level of training (65.9% senior physicians vs. 30.4% physicians in training, p = 0.04). Regarding pharmacological treatment options, oxygen application was the most chosen approach. The second most frequent drug chosen was a ß-2 agonist. Only 9.8% of the senior physicians and 8.7% of the physicians in training suggested oral opioids as a treatment option, whereas intravenous opioids were suggested by 43.9% of the senior physicians and 21.7% of the physicians in training (p = 0.089). For subcutaneous application of opioids, the percentage of usage was significantly higher for the physicians in training than for the senior physicians (78.3% vs. 48.8%, p = 0.017, respectively).

Conclusion

The gold standard treatment for treating refractory dyspnea in patients with advanced lung cancer is opioids. Nevertheless, this pharmacological treatment option was not ranked as the most important. Discussing hypothetical cases of patients with advanced lung cancer and refractory dyspnea with experienced doctors as well as doctors at the beginning of their training may help improve symptom control for these patients.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00520-022-06996-6.

Opioids in COVID-19: Two Sides of a Coin

Introduction: The treatment of most severe COVID-19 patients included the large-scale use of sedatives and analgesics-possibly in higher doses than usual-which was reported in the literature. The use of drugs that decrease mortality is necessary and opioids are important agents in procedures such as orotracheal intubation. However, these drugs seem to have been overestimated in the COVID-19 pandemic. We performed a review of the PubMed-Medline database to evaluate the use of opioids during this period. The following descriptors were used to enhance the search for papers: "Opioids", "COVID-19," "COVID-19 pandemic," "SARS-CoV-2," "Opioid use disorder," "Opioid dependence" and the names of the drugs used. We also evaluated the distribution of COVID-19 patients in Brazil and the applicability of opioids in our country during the COVID-19 pandemic. Results: Several positive points were found in the use of opioids in the COVID-19 pandemic, for instance, they can be used for analgesia in orotracheal intubation, for chronic pain management, and as coadjutant in the management of acute intensification of pain. However, high doses of opioids might exacerbate the respiratory depression found in COVID-19 patients, their chronic use can trigger opioid tolerance and the higher doses used during the pandemic might result in greater adverse effects. Unfortunately, the pandemic also affected individuals with opioid use disorder, not only those individuals are at higher risk of mortality, hospitalization and need for ventilatory support, but measures taken to decrease the SARS-CoV-2 spread such as social isolation, might negatively affect the treatment for opioid use disorder. In Brazil, only morphine, remifentanil and fentanyl are available in the basic health care system for the treatment of COVID-19 patients. Out of the 5,273,598 opioid units used in this period all over the country, morphine, fentanyl, and remifentanil, accounted for, respectively, 559,270 (10.6%), 4,624,328 (87.6%), and 90,000 (1.8%) units. Many Brazilian regions with high number of confirmed cases of COVID-19 had few units of opioids available, as the Southeast region, with a 0.23 units of opioids per confirmed COVID-19 case, and the South region, with 0.05 units. In the COVID-19 pandemic scenario, positive points related to opioids were mainly the occurrence of analgesia, to facilitate intubation and their use as coadjutants in the management of acute intensification of pain, whereas the negative points were indiscriminate use, the presence of human immunosuppressor response and increased adverse effects due to higher doses of the drug. Conclusion: The importance of rational and individualized use of analgesic hypnotics and sedative anesthetics should be considered at all times, especially in situations of high demand such as the COVID-19 pandemic.

A double-edged sword of using opioids and COVID-19: a toxicological view

Today, COVID-19 is spreading around the world. Information about its mechanism, prognostic factors, and management is minimal. COVID-19, as a human disease, has several identifying phases. Physicians of patients with COVID-19 may be interested in knowing whether opioid use disorder may affect their patients' course or prognosis. This information may be crucial when considering the opioid epidemic in the US and other parts of the world. Opioid use at high doses and over several months duration can mitigate the immune system's function, which may complicate the course of COVID-19 disease. Potential suppression of parts of the immune response may be important in prevention, clinical support, and therapeutic use of medications in various phases of the COVID-19. Specifically, opioid use disorders via an inhalation route may enhance the "late hyper-inflammatory phase" or result in end-organ damage. It is well established that opioids decrease ventilation as their effect on the medullary respiratory centers increases the risk of pneumonia. This increased risk has been associated with immune-suppressive opioids. The ultimate role of opioids in COVID-19 is not clear. This paper endorses the need for clinical studies to decipher the role and impact of chronic opioid use on viral diseases such as COVID-19.

Yoga in Burn: Role of pranayama breathing exercise on pulmonary function, respiratory muscle activity and exercise tolerance in full-thickness circumferential burns of the chest

BACKGROUND

Circumferential burn of chest (CBC) is a significant type of burn and considers as a major cause of restrictive lung disease (RLD). Patient who has CBC with RLD leads to respiratory symptoms such as breathing difficulty, airway obstruction, reduced exercise capacity and altered pulmonary functions. However, studies examining the role of pranayama breathing exercise on pulmonary function, respiratory muscle activity and exercise tolerance in full thickness circumferential burn of chest are lacking.

OBJECTIVE

To find the short term effects of pranayama breathing exercise on pulmonary function, respiratory muscle activity and exercise tolerance in full thickness circumferential burns of chest.

METHODS

Through simple random sampling method thirty subjects (N = 30) with RLD following CBC were allocated to pranayama breathing exercise group (PBE-G; n = 15) and conventional breathing exercise group (CBE-G; n = 15). They received pranayama breathing exercise and conventional breathing exercise for 4 weeks respectively. All the subjects received chest mobility exercise as common treatment. Primary (Numeric Pain Rating Scale - NPRS, forced expiratory volume (FEV1), forced vital capacity (FVC) and maximum voluntary ventilation (MVV) and secondary (Electromyogram of sternocleidomastoid, scalene, external intercostal and diaphragm muscle, 6 min walk test & Global Rating of Change - GRC) outcome measures were measured at baseline, after four weeks and after three months follow up.

RESULTS

Baseline demographic and clinical variables show homogenous distribution between the groups (p > 0.05). Four weeks following different breathing exercises, PBE-G group shows more significant changes in pain intensity, pulmonary function, respiratory muscle activity, exercise tolerance and global rating of change than CBE-G group (p ≤ 0.05) at four weeks and three months follow up.

CONCLUSION

Both groups showed improvement over time. However, differences between the groups were noticed small. Still physiotherapy management, which included pranayama breathing exercises with chest mobilization program, had an effective strategy in the treatment of restrictive lung disease following circumferential burn of chest.

Development of palliative care attitude and knowledge (PCAK) questionnaire for physicians in Kuwait

Background

Over the past five decades, palliative care has changed from helping patients at the end of life into a highly dedicated service focused on delivering supportive care to patients with life-limiting illnesses throughout the disease trajectory. To date there is no common agreement on universally applicable measurement tool to know the areas of weakness in physicians’ understanding of palliative care and identifying misconceptions about palliative care. This paper describes the development of a reliable and valid questionnaire to provide a measure of the attitude and knowledge of physicians toward palliative care (PCAK).

Methods

Item pool was generated paying particular attention to content and face validity. The initial version of the questionnaire was piloted and assessed based on psychometric criteria. Items which did not reach acceptable validity were excluded, and the final 37 item version was administered to two groups differing in their palliative care attitude and knowledge on two occasions to assess the construct validity and test-retest reliability. Two hundred thirty two physicians working in primary care clinics and general hospitals completed the questionnaire at the piloting stage. The final version (PCAK) was administered to 35 oncologists and 76 physicians. SPSS v20 was used for statistical analysis.

Results

Of the Pilot study, 20 items were excluded because didn’t meet the criteria for item difficulty and discrimination. Item-to-total-score correlations (r) was ranging from 0.347 up to 0.806. Internal consistency (Cronbach’s alpha) was high ranging from 0.636 to 0.824. While testing the final PCAK, oncologist scored consistently higher than the other physicians on all sections of the questionnaire (P < 0.001) suggesting good construct validity. Test to retest reliability for each section was very high, ranging from 0.879 to 0.97 and the overall reliability was 0.95. The internal consistency reliability of each section was very good ranging from 0.681 ± 0.893.

Conclusion

The findings demonstrate that PCAK meets psychometric criteria for reliability and construct validity. It provides a useful scale to assess the attitude and knowledge of physicians about palliative care helping in planning of educational programs for physicians.

Electronic supplementary material

The online version of this article (10.1186/s12904-019-0430-9) contains supplementary material, which is available to authorized users.

Nebulized opioids for the palliation of dyspnea in terminally ill patients

PURPOSE

The use of nebulized opioids for the palliation of dyspnea in terminally ill patients is reviewed.

SUMMARY

More than 50% of patients with advanced diseases experience dyspnea during their final stages of life. Systemically administered opioids are recommended for the management of dyspnea in these patients, but adverse effects may limit their use. Nebulization offers an alternative route for administering opioids, providing relief of dyspnea while minimizing adverse events. An extensive literature search was conducted to identify publications evaluating nebulized opioids for the palliation of dyspnea in patients at end-of-life. Ten studies that evaluated nebulized morphine, fentanyl, hydromorphone, and morphine-6-glucuronide were reviewed; 1 of these studies evaluated 4 different opioids. Of these 10 studies, 2 had double-blind, placebo-controlled, randomized crossover designs; 1 was retrospective, and the remaining 7 were prospective studies. A total of 181 patients, all adults, were evaluated. Subjective improvement in dyspnea from baseline was observed in 9 of the 10 studies. Nebulized morphine 20 mg every 4 hours was the most common opioid studied. Other doses of nebulized opioids included fentanyl 25 and 100 μg and hydromorphone 5 mg. Nine studies reported subjective improvement of dyspnea from baseline after administering nebulized opioids. Six studies evaluated objective outcomes and showed decreased respiratory rate (morphine, fentanyl, and hydromorphone) and heart rate (hydromorphone) and increased oxygen saturation (fentanyl). Mild-to-moderate adverse effects such as claustrophobia due to nebulizer mask, drowsiness, cough, and bitter taste were described.

CONCLUSION

Nebulized opioids may provide subjective relief of dyspnea in terminally ill patients with mild-to-moderate adverse effects.

The effects of breathing exercise types on respiratory muscle activity and body function in patients with mild chronic obstructive pulmonary disease

[Purpose] Fragmentary studies on characteristics of respiratory muscles are being done to increase respiratory capacity by classifying exercises into voluntary respiratory exercise which relieves symptoms and prevents COPD and exercise using breathing exercise equipment. But this study found changes on respiratory pattern through changes on the activity pattern of agonist and synergist respiratory muscles and studied what effect they can have on body function improvement. [Subjects and Methods] Fifteen subjects in experimental group I that respiratory exercise of diaphragm and 15 subjects in experimental group II that feedback respiratory exercise were randomly selected among COPD patients to find the effective intervention method for COPD patients. And intervention program was conducted for 5 weeks, three times a week, once a day and 30 minutes a session. They were measured with BODE index using respiratory muscle activity, pulmonary function, the six-minute walking test, dyspnea criteria and BMI Then the results obtained were compared and analyzed. [Results] There was a significant difference in sternocleidomastoid muscle and scalene muscle and in 6-minute walk and BODE index for body function. Thus the group performing feedback respiratory had more effective results for mild COPD patients. [Conclusion] Therefore, the improvement was significant regarding the activity of respiratory muscles synergists when breathing before doing breathing exercise. Although, it is valuable to reduce too much mobilization of respiratory muscles synergists through the proper intervention it is necessary to study body function regarding improvement of respiratory function for patients with COPD.

Hospitalist attitudes toward the assessment and management of dyspnea in patients with acute cardiopulmonary diseases

BACKGROUND

Dyspnea is a common symptom in patients hospitalized with acute cardiopulmonary diseases. Routine assessment of dyspnea severity is recommended by clinical guidelines based on the evidence that patients are not treated consistently for dyspnea relief.

OBJECTIVE

To evaluate attitudes and beliefs of hospitalists regarding the assessment and management of dyspnea.

DESIGN

Cross-sectional survey.

SETTINGS

Nine hospitals in the United States.

MEASUREMENTS

Survey questions assessed the following domains regarding dyspnea: importance in clinical care, potential benefits and challenges of implementing a standardized assessment, current approaches to assessment, and how awareness of severity affects management. A 5-point Likert scale was used to assess the respondent's level of agreement; strongly agree and agree were combined into a single category.

RESULTS

Of the 255 hospitalists invited to participate, 69.8% completed the survey; 77.0% agreed that dyspnea relief is an important goal when treating patients with cardiopulmonary conditions. Approximately 90% of respondents stated that awareness of dyspnea severity influences their decision to intensify treatment, to pursue additional diagnostic testing, and the timing of discharge. Of the respondents, 61.0% agreed that standardized assessment of dyspnea should be part of the vital signs, and 64.6% agreed that awareness of dyspnea severity influences their decision to prescribe opioids. Hospitalists who appreciated the importance of dyspnea in clinical practice were more likely to support the implementation of a standardized scale.

CONCLUSIONS

Most hospitalists believe that routine assessment of dyspnea severity would enhance their clinical decision making and patient care. Measurement and documentation of dyspnea severity may represent an opportunity to improve dyspnea management.

An overview of end-of-life issues in the intensive care unit

The population of the earth is aging, and as medical techniques, pharmaceuticals, and devices push the boundaries of human physiological capabilities, more humans will go on to live longer. However, this prolonged existence may involve incapacities, particularly at the end-of-life, and especially in the intensive care unit. This arena involves not only patients and families, but also care givers. It involves topics from economics to existentialism, and surgery to spiritualism. It requires education, communication, acceptance of diversity, and an ultimate acquiescence to the inevitable. Here, we present a comprehensive overview of issues in the care of patients at the end-of-life stage that may cause physicians and other healthcare providers, medical, ethical, social, and philosophical concerns in the intensive care unit.

End-of-life issues in the acute and critically ill patient

The challenges of end-of-life care require emergency physicians to utilize a multifaceted and dynamic skill set. Such skills include medical therapies to relieve pain and other symptoms near the end-of-life. Physicians must also demonstrate aptitude in comfort care, communication, cultural competency, and ethical principles. It is imperative that emergency physicians demonstrate a fundamental understanding of end-of-life issues in order to employ the versatile, multidisciplinary approach required to provide the highest quality end-of-life care for patients and their families.

Recommendations for end-of-life care in the intensive care unit: a consensus statement by the American College [corrected] of Critical Care Medicine

BACKGROUND

These recommendations have been developed to improve the care of intensive care unit (ICU) patients during the dying process. The recommendations build on those published in 2003 and highlight recent developments in the field from a U.S. perspective. They do not use an evidence grading system because most of the recommendations are based on ethical and legal principles that are not derived from empirically based evidence.

PRINCIPAL FINDINGS

Family-centered care, which emphasizes the importance of the social structure within which patients are embedded, has emerged as a comprehensive ideal for managing end-of-life care in the ICU. ICU clinicians should be competent in all aspects of this care, including the practical and ethical aspects of withdrawing different modalities of life-sustaining treatment and the use of sedatives, analgesics, and nonpharmacologic approaches to easing the suffering of the dying process. Several key ethical concepts play a foundational role in guiding end-of-life care, including the distinctions between withholding and withdrawing treatments, between actions of killing and allowing to die, and between consequences that are intended vs. those that are merely foreseen (the doctrine of double effect). Improved communication with the family has been shown to improve patient care and family outcomes. Other knowledge unique to end-of-life care includes principles for notifying families of a patient's death and compassionate approaches to discussing options for organ donation. End-of-life care continues even after the death of the patient, and ICUs should consider developing comprehensive bereavement programs to support both families and the needs of the clinical staff. Finally, a comprehensive agenda for improving end-of-life care in the ICU has been developed to guide research, quality improvement efforts, and educational curricula.

CONCLUSIONS

End-of-life care is emerging as a comprehensive area of expertise in the ICU and demands the same high level of knowledge and competence as all other areas of ICU practice.

Complications in patients with severe emphysema

Thoracic surgeons frequently evaluate patients with severe emphysema and concomitant pathology requiring pulmonary resection. There are no absolute guidelines defining the suitability of a given patient for resection. In this review, we outline our approach to evaluating and treating patients with severe emphysema in need of resection. We describe the lessons learned from lung volume reduction surgery and apply that knowledge to the care of the patient with severe emphysema. Careful preoperative evaluation of the patient's lung anatomy, distribution of emphysematous changes in the lung, and overall health is essential to identifying the appropriate candidate for resection and avoiding postoperative complications.

[Palliation for respiratory syndrome]

Respiratory symptoms such as dyspnea, coughing, bronchorrhea, hemoptysis and death rattle are a common problem in palliative medicine. Their prevalence increases markedly in terminal phase oncological patients. In such cases, new therapeutic methods with lower toxicity and well documented efficiency in the control of respiratory syndrome should be considered.

Death, dying, and last wishes

Issues regarding patient care near the end of life can be challenging and rewarding for emergency physicians. Knowledge of the patient's wishes is essential, and may be accomplished by advance directives or communication with patients and surrogates. Resuscitative efforts are appropriate for many patients, but inappropriate for others. The goals of medicine remain the following: providing optimal health care, provision of the best possible symptom control, communication, empathy, and caring. As death approaches, provision of the best possible medical care, in accordance with the patient's wishes, can be rewarding for patients, families, and health care providers.

Patient reporting and doctor recognition of dyspnoea in a comprehensive cancer centre

The aim of this study was to examine different aspects of dyspnoea in an Australian acute cancer care population, specifically prevalence, recognition, reporting, symptom control methods and prognostic significance. Patients and treating hospital medical officer were concurrently asked to evaluate the experience of dyspnoea. The prevalence of dyspnoea was 33%, with discrepancies observed between patient and doctor reporting of the presence of dyspnoea (P = 0.021), as well as its intensity and distress. Symptomatic methods for the relief of cancer-related dyspnoea are underused, particularly opioids. The medical underestimation of dyspnoea is consistent with previous studies and potentially detracts from effective management of this symptom.

Comprehensive Symptom Management in Patients with Advanced-Stage Non–Small-Cell Lung Cancer

Although we have made steady improvements in the survival rates of patients with advanced-stage lung cancer, the majority of patients still experience distress and suffering. Although the symptom burden is greatest in patients in the end stages of life, many patients living with lung cancer suffer from troubling symptoms and side effects of therapy. Even long-term survivors with early-stage non-small-cell lung cancer (NSCLC) often experience respiratory symptoms, such as dyspnea and cough. Because of the high prevalence of NSCLC and the frequency with which it presents in an incurable stage, symptom management is a large component of the care of these patients. Dyspnea, cough, fatigue, anorexia/cachexia, and pain are the most common symptoms in patients with advanced-stage NSCLC. Cancer-directed therapy can improve some of these symptoms but often incompletely and temporarily. Therefore, comprehensive care of patients with advanced-stage NSCLC must include therapies targeted at these difficult and distressing symptoms.

Terminal Care: The Last Weeks of Life

BACKGROUND

The care of patients in their last weeks of life is a fundamental palliative care skill, but few evidence-based reviews have focused on this critical period.

METHOD

A systematic review of published literature and expert opinion related to care in the last weeks of life.

RESULTS

The evidence base informing terminal care is largely descriptive, retrospective, or extrapolated. While home deaths and hospice use are increasing, medical care near death is becoming more aggressive and hospice lengths of stay remain short. Though the prediction of impending death remains imprecise, studies have identified several common terminal signs and symptoms. Decreased communication near death complicates the determination of patient wishes, and advanced directives prior to the terminal stage are recommended. Anorexia and cachexia are common in dying patients but there is no evidence that this process is painful or responsive to intervention. While there is general consensus that artificial nutrition is not beneficial in dying patients, the use of artificial hydration is controversial, especially in the setting of delirium. Breathlessness has been shown to benefit from oral and parenteral opioids but not anxiolytics. Accumulation of respiratory tract secretions (death rattle) is common and usually responds to antimuscarinics. Physical pain typically decreases toward death but its assessment in dying patients is difficult. Terminal delirium may occur in up to one-third of patients, may have a reversible cause, and may respond to antipsychotics or benzodiazepines. Palliative sedation is controversial but widely used, especially internationally. Caregiver stress and bereavement may benefit from improved communication and hospice involvement.

CONCLUSION

While the terminal care literature is characterized by varying quality, numerous knowledge gaps, and frequent inconsistencies, it supports several common clinical interventions. More research is needed to resolve controversies, define effective therapies, and improve the outcomes of dying patients.

Control of common, non-pain cancer symptoms

Despite major advances in cancer biology and therapeutics, cancer and its treatment continue to cause devastating suffering. Patients with advanced cancer most often experience multiple physical and psychological symptom concurrently. We review here some of the common non-pain cancer symptoms, focusing on the assessment and treatment of fatigue, anorexia and cachexia, dyspnea, and symptoms common near the end of life.

Changes in acetylcholinesterase activity and muscarinic receptor bindings in mu-opioid receptor knockout mice

Anatomical evidence indicates that cholinergic and opioidergic systems are co-localized and acting on the same neurons. However, the regulatory mechanisms between cholinergic and opioidergic system have not been well characterized. In the present study, we investigated whether there are compensatory changes of acetylcholinesterase activity and cholinergic receptors in mice lacking mu-opioid receptor gene. The acetylcholinesterase activity was determined by histochemistry assay. The cholinergic receptor binding was carried out by quantitative autoradiography using [3H]-quinuclidinyl benzilate (nonselective muscarinic receptors), N-[3H]-methylscopolamine (nonselective muscarinic receptors), [3H]-pirenzepine (M1 subtype muscarinic receptors) and [3H]-AF-DX384 (M2 subtype muscarinic receptors) in brain slices of wild-type and mu-opioid receptor knockout mice. The acetylcholinesterase activity of mu-opioid receptor knockout mice was higher than that of the wild-type in the striatal caudate putamen and nucleus accumbens, but not in the cortex and hippocampus areas. In addition, the bindings in N-[3H]-methylscopolamine and [3H]-AF-DX384 of mu-opioid receptor knockout mice were significantly lower when compared with that of the wild-type controls in the striatal caudate putamen and nucleus accumbens. However, there were no significant differences in bindings of [3H]-quinuclidinyl benzilate and [3H]-pirenzepine between mu-opioid receptor knockout and wild-type mice in the cortex, striatum and hippocampus. These data indicate that there are up-regulation of acetylcholinesterase activity and compensatory down-regulation of M2 muscarinic receptors in the striatal caudate putamen and nucleus accumbens of mu-opioid receptor knockout mice.