Study of the µ opioid receptor in cutaneous ulcers of leishmaniasis and sporotrichosis according to the complaints of local pain

Patients with cutaneous leishmaniasis or sporotrichosis with ulcerated lesions may present similar epidemiological and clinical characteristics. Local pain is often referred to in the sporotrichosis lesions, but not in cutaneous leishmaniasis. The µ Opioid Receptor (MOR) is indirectly associated to the production of cytokines, and is related to the epidermal proliferation.

Sporotrichosis in the state of Rio de Janeiro is mainly caused by Sporothrix brasiliensis [6]. In sporotrichosis, dermis presents at an early stage infl ammation with infi ltration of neutrophils, plasma cells, and lymphocytes that may or may not be intense. Gradually, the usual ulcerated cutaneous lesion typically exhibits a granulomatous dermatitis surrounding a suppurative abscess, with the presence of a central zone composed of neutrophils and a few eosinophils, and an outer zone of lymphocytes and plasma cells. At a later stage, granulomas mainly consist of epithelioid cells. Small abscesses may be seen within granulomas. The histopathological fi ndings are generally nonspecifi c and variable in different stages of the disease. The histopathological pattern is usually a combination of pyogenic and granulomatous reaction and may display epidermal hyperplasia, papillomatous acanthosis, hyperkeratosis, and intraepidermal microabscesses [7,8].
Experimental immunologic studies in the cutaneous lesions of leishmaniasis and sporotrichosis also show similar in situ profi les with high levels of activated type 2 macrophages and production of IL-4 and IL-10 [9].
Infl ammatory mediators are released and tissue acidifi cation activates nociceptive primary afferent neurons that stimulate the sensation of pain causing hyperalgesia [10].
Immunocytes are recruited and release interleukins performing their functions in the process of healing the cutaneous lesion in an orchestrated way. The cytokine cascade results in the activation of COX-2 dependent prostanoid and in the release of catecholamine from sympathetic fi bers [11]. Cytokines such as IL-1, IL-6, TNF- and IL-8 are related to the pain threshold.
On the other hand, opioid peptides render nociceptors less sensitive to excitation and thus inhibit the action of multiple excitatory mediators. Opioid peptides do not bind exclusively to one unique opioid receptor, but instead exhibit affi nity for various opioid receptors including μ-, ∂-and -opioid receptors [10,12]. Endogenous opioids as endorphins and encephalin act primarily on μ and ∂ opioid receptors. They are synthesized in vivo in order to modulate pain mechanisms and infl ammatory pathways, and mediate analgesia in response to painful stimuli by binding to opioid receptors on sensitive cutaneous nerves. Opioids produced by cells of the immune system and keratinocytes are capable of exerting additional effects, such as immunomodulation in cutaneous infl ammation. -endorphin is present in macrophages, monocytes, granulocytes, and lymphocytes, in secretory granules arranged at the cell periphery, ready for exocytose. During the early stages of infl ammation, as the leukocytes migrate to the site of infection, they (along with the resident cells) secrete various chemokines such as IL-1, IL-6, IL-8, which lead to hyperalgesia. In the late infl ammation stage, macrophages and lymphocytes secrete IL-4, IL-10 and IL-13 inhibiting the hyperalgesic pathways leading to hypoalgesia [13]. Pain perception depends upon the activation of specialized peripheral neurons called Primary Afferent Nociceptors (PANs) from primary afferent fi bers (A-, A-, and C-fi bers) [14].
The peripheral anti-nociceptive action of μ-opioid receptors (MOR) agonists is greatly increased in infl amed tissues. This is in part due to stimulation of the MOR synthesis in the Dorsal Root Ganglion (DRG) induced by cytokines, especially Neural Growth Factor (NGF), and its transport to peripheral terminals. In summary, MOR analgesia depends upon a set of widely distributed neural targets that include NAPs, ascending pain projection neurons and a top-down pain modulatory circuit [14].
Some experimental studies on nociception in leishmaniasis have been performed in order to better understand the profi le of cytokines related to pain, without a satisfactory conclusion [15,16]. The purpose of this study was to determine the profi le of MOR staining in the well established cutaneous ulcerated lesions of leishmaniasis and sporotrichosis in patients from Rio de Janeiro, Brazil, and to associate the MOR staining profi le with the presence or absence of pain in the cutaneous lesions of both diseases.
Reaction was detected by phosphatase alkaline kit (GBI Labs, Bothel, Washington, USA) and revealed with permanent red [17]. The sections were counterstained with hematoxylin.

Compact granuloma (mature epithelioid cells in clusters)
were present in 50% of the leishmaniasis cutaneous lesions, in There was no association between the clinical complaint of pain in the lesions and the intensity of the MOR staining.  Table   2).

Discussion
The keratinocytes are surrounded by infl ammatory infi ltrate being stimulated and releasing cytokines and growing factors, including NGF and opioids. Probably the high intensity These data suggest that epidermal cytokine expression may be    [14].
A weakness of this study is to be a retrospective study.
Further studies with larger number of patients and with gradation of local pain sensation would allow a better understanding on the issue of local pain in granulomatous infectious diseases of the skin.
In conclusion, the general concept that leishmaniasis presents mainly painless lesions or sporotrichosis presents painful lesions could not be directly demonstrated in this study. There was no association between MOR staining and the presence or absence of local pain in the cutaneous lesions of both diseases. Certainly, there must be pain modulation by local opioids, but the main modulation seems to be originated from the peripheral and central nervous system.

Financial Support
Coordenação de Aperfeiçoamento de Pessoal de Ensino Superior (CAPES) through code 001.