Levodopa raises objective pain threshold in Parkinson's disease: a RIII reflex study

Levodopa analgesia in experimental neuropathic pain

Levodopa has been shown to produce analgesia in various clinical and experimental settings, but its use for chronic pain treatment has not been established. We have undertaken a study of the antiallodynic actions of levodopa in a rat model of painful mononeuropathy. When administered systemically, levodopa produced a decrease in tactile and cold allodynia lasting at least 3h. Direct intrathecal (i.t.) levodopa injection at lumbar levels produced a similar, though shorter, antiallodynic effect. This effect was blocked by the D2-type receptor antagonist sulpiride, which supports the involvement of the spinal dopaminergic system in the analgesic action of levodopa on neuropathic pain. These results provide experimental support on the antiallodynic effect of levodopa in neuropathic pain and suggest that at least part of the analgesic action takes place in the spinal cord and involves dopaminergic D2-type receptors.

Pain in Parkinson's disease

Parkinson's disease is characterized primarily as a neurodegenerative disorder that leads to disabling motor and cognitive impairment. PD is less widely appreciated as a disease causing a substantial variety of pain syndromes, although the prevalence of pain in PD is approximately 40%. In a minority of patients, pain is so severe and intractable that it overshadows the motor symptoms of the disorder. In recent years, descriptive surveys of non-motor symptoms in PD have led to a classification of painful sensations into one or more of several categories: musculoskeletal pain, radicular or neuropathic pain, dystonia-related pain, akathitic discomfort, and primary, central parkinsonian pain. A framework for diagnosing and treating painful PD is described in this review, together with recent insignts into the neurophysiological mechanisms and substrates of pain in PD.

Study of the neural basis of striatal modulation of the jaw-opening reflex

Previous experimental data from this laboratory demonstrated the participation of the striatum and dopaminergic pathways in central nociceptive processing. The objective of this study was to examine the possible pathways and neural structures associated with the analgesic action of the striatum. The experiments were carried out in rats anesthetized with urethane. The jaw-opening reflex (JOR) was evoked by electrical stimulation of the tooth pulp of lower incisors and recorded in the anterior belly of the digastric muscles. Intrastriatal microinjection of apomorphine, a nonspecific dopamine agonist, reduced or abolished the JOR amplitude. Electrolytic or kainic acid lesions, unilateral to the apomorphine-injected striatum, of the globus pallidus, substantia nigra pars reticulata, subthalamic nucleus and bilateral lesion the rostroventromedial medulla (RVM), blocked the inhibition of the JOR by striatal stimulation. These findings suggest that the main output nuclei of the striatum and the RVM may be critical elements in the neural pathways mediating the inhibition of the reflex response, evoked in jaw muscles by noxious stimulation of dental pulp.

Pain intensity on and off levodopa in patients with Parkinson's disease

Pain is frequently reported by patients with Parkinson's disease (PD). In this study, intensity of pain as measured by a visual analogue scale (VAS) was assessed on and off levodopa in 15 patients with PD. All patients had motor fluctuations and suffered from pain of various types. Description of pain was assessed with the McGill pain questionnaire. Ratings for pain intensity on the VAS were increased during off period for all patients but one (P = 0.001). There was a correlation (P = 0.04) between changes in motor performance (Unified Parkinson's Disease Rating Scale part III) and pain intensity (VAS). Compared with a historical sample of subjects with different pain syndromes without PD, terms related to fear and punishment were used more frequently by patients with PD in this study. In two patients, pain was exclusively limited to the off period. The majority of subjects suffered from secondary pain possibly related to lumbar osteoarticular degeneration. Secondary pain was relieved but not completely abolished by levodopa. The results of this study suggest that aggravation of secondary pain should be considered as a part of the spectrum of nonmotor off symptoms. Analgesics should not be given as first line drugs when pain occurs or increases in off conditions, and pain can be significantly alleviated or abolished by adjustments of dopaminergic medication.

A review of central neuropathic pain states

PURPOSE OF REVIEW

Central neuropathic pain is an important and disabling but often neglected problem following central nervous system lesions. The present review highlights recent advances in the understanding of the underlying mechanisms and in the diagnosis and treatment of central pain.

RECENT FINDINGS

Within the past year, the field of central pain has moved toward a more integrative understanding of central pain. The involvement of nonneuronal cells and interaction between multiple areas of the central nervous system has been recognized as important in the underlying mechanisms. The interest for conditions other than spinal cord injury, multiple sclerosis, and stroke has increased, and continued discussions on clear clinical diagnostic criteria are needed. The treatment of central pain is still a great challenge, but recent evidence points to tricyclic antidepressants, gabapentin and pregabalin, and selective serotonin-noradrenaline reuptake inhibitors as first-line drugs in central pain. An increased understanding of the psychosocial aspects of central pain also has implications for the treatment.

SUMMARY

Increased insight into the mechanisms of central pain will hopefully lead to increased efforts to study mechanism-based treatment of central pain.

Pain in Parkinson's disease: Prevalence and characteristics

Parkinson's disease is a chronic, progressive, incurable neurodegenerative disease. As the disease progresses, motor disturbances and non-motor symptoms represent considerable illness burdens. Symptom relief is the goal for the treatment. Pain is frequently observed in patients with Parkinson's disease, but its prevalence, characteristics and associations with Parkinson's disease are poorly documented. These were investigated in 176 home-living PD patients. They underwent a neurological examination and a structured interview for registration of pain characteristics in addition to responding to standardised questionnaires. Pain was reported by 146 (83%) patients. Compared to the general population, the Parkinson's disease patients experienced significantly more pain as measured by SF-36 Bodily Pain Scale. The average pain during the last 24h measured by the Brief Pain Inventory was 2.85. Fifty-three percent of the patients reported one, 24% reported two and 5% reported three pain types. Musculoskeletal pain was reported by 70%, dystonic pain by 40%, radicular-neuropathic pain by 20% and central neuropathic pain by 10%. Thirty-four percent were on analgesic medication. Pain was not associated with age, disease duration or severity of the disease; female gender was the only significant predictor of pain. Pain is frequent and disabling, independent of demographic and clinical variables except for female gender, and is significantly more common in Parkinson's patients compared to the general population. A minority of the Parkinson's disease patients with pain received analgesic medication. The findings call for improved attention to assessment and treatment of pain in the follow-up of Parkinson's disease patients.

Comparison of chronic analgesic drugs prevalence in Parkinson's disease, other chronic diseases and the general population

Patients with Parkinson's disease (PD) frequently experienced pain. Nevertheless, there are no epidemiological data about frequency of pain in PD. We compare pain prevalence using analgesic prescription in PD patients, in the general population and in two samples of painful patients: diabetics and osteoarthritis patients in France. Data were obtained from the French System of Health Insurance for the year 2005. Medications (antiparkinsonian, antidiabetics drugs and osteoarthritis drugs) were used for identification of PD, diabetic and osteoarthritis patients. We estimated the prevalence of analgesic drugs prescription (at least one analgesic drug) and the prevalence of chronic analgesic drugs prescription (more than 90 DDD of analgesic drug). The study included 11,466 PD patients. PD patients significantly received more prescription of analgesics than the general population (82% versus 77%,) and fewer than patients with osteoarthritis (82% versus 90%). No significant difference was found between PD and diabetic patients. The chronic prescription of analgesic drugs was more prevalent in PD patients (33%) than in the general population (20%) and in diabetic patients (26%) and similar to that in osteoarthritis patients. PD patients were more exposed than the general population and diabetics to opiates, acetaminophen, and adjuvant analgesics chronic use.

A common neurobiology for pain and pleasure

Pain and pleasure are powerful motivators of behaviour and have historically been considered opposites. Emerging evidence from the pain and reward research fields points to extensive similarities in the anatomical substrates of painful and pleasant sensations. Recent molecular-imaging and animal studies have demonstrated the important role of the opioid and dopamine systems in modulating both pain and pleasure. Understanding the mutually inhibitory effects that pain and reward processing have on each other, and the neural mechanisms that underpin such modulation, is important for alleviating unnecessary suffering and improving well-being.

Nociceptive behavioral responses to chemical, thermal and mechanical stimulation after unilateral, intrastriatal administration of 6-hydroxydopamine

The basal ganglia are involved not only with motor processes such as posture, pre-movement planning and movement initiation, but also with the processing and modulation of nociceptive somatosensory information. In the current studies, unilateral, intrastriatal 6-hydroxydopamine (6-OHDA) was used to investigate how dopamine depletion alters nociceptive behavioral responses to chemical, thermal and mechanical stimulation in rats. Compared to control rats injected with intrastriatal saline, rats depleted of dopamine displayed increased nociceptive responses to chemical stimulation of the face and hyperalgesic responses to thermal stimulation of the hind paw without alterations in rearing behavior or body weight gain. Minor changes were observed in the response to mechanical stimulation of the hind paws and face. These data provide further evidence that the dopaminergic nigrostriatal pathway plays a role in the modulation of nociceptive information.

Management of non-motor symptoms in advanced Parkinson disease

Progress in pharmacology has markedly improved the treatment of early Parkinson's disease. The management of advanced Parkinson's symptoms, however, remains a challenge. These symptoms are divided into motor and non-motor symptoms. Non-motor symptoms may appear early or late in the disease and sometimes even before the onset of the first motor symptoms confirming the diagnosis. The spectrum of non-motor symptoms encompasses autonomic dysfunctions, sleep disorders, mood disorders, impulse control disorders, cognitive dysfunction, dementia, paranoia and hallucinations. They are often less appreciated than motor symptoms but are important sources of disability for many PD patients. This review describes these non-motor symptoms and their managements.