Melittin, the Major Pain-Producing Substance of Bee Venom

Abstract

Melittin is a basic 26-amino-acid polypeptide that constitutes 40-60% of dry honeybee (Apis mellifera) venom. Although much is known about its strong surface activity on lipid membranes, less is known about its pain-producing effects in the nervous system. In this review, we provide lines of accumulating evidence to support the hypothesis that melittin is the major pain-producing substance of bee venom. At the psychophysical and behavioral levels, subcutaneous injection of melittin causes tonic pain sensation and pain-related behaviors in both humans and animals. At the cellular level, melittin activates primary nociceptor cells through direct and indirect effects. On one hand, melittin can selectively open thermal nociceptor transient receptor potential vanilloid receptor channels via phospholipase A2-lipoxygenase/cyclooxygenase metabolites, leading to depolarization of primary nociceptor cells. On the other hand, algogens and inflammatory/pro-inflammatory mediators released from the tissue matrix by melittin's pore-forming effects can activate primary nociceptor cells through both ligand-gated receptor channels and the G-protein-coupled receptor-mediated opening of transient receptor potential canonical channels. Moreover, subcutaneous melittin up-regulates Nav1.8 and Nav1.9 subunits, resulting in the enhancement of tetrodotoxin-resistant Na(+) currents and the generation of long-term action potential firing. These nociceptive responses in the periphery finally activate and sensitize the spinal dorsal horn pain-signaling neurons, resulting in spontaneous nociceptive paw flinches and pain hypersensitivity to thermal and mechanical stimuli. Taken together, it is concluded that melittin is the major pain-producing substance of bee venom, by which peripheral persistent pain and hyperalgesia (or allodynia), primary nociceptive neuronal sensitization, and CNS synaptic plasticity (or metaplasticity) can be readily induced and the molecular and cellular mechanisms underlying naturally-occurring venomous biotoxins can be experimentally unraveled.

Keywords: Algogen; Melittin; Nociceptor; Pain; Spinal dorsal horn.

Fig. 1

Direct actions of melittin on primary nociceptor cells. Following subcutaneous injection of a low concentration of melittin, phospholipase A2 (PLA2) is activated and catalyzes the production of arachidonic acid (AA) from phospholipids. AA is further catalyzed by lipoxygenases (LOXs) to produce hydroperoxyeicosatetraenoic acids (HETEs) that serve as endogenous ligands to open TRPV1 receptor channels, leading to Ca²⁺ influx and membrane depolarization. Meanwhile, AA is also catalyzed by cyclooxygenases (COXs) to produce PGs that enhance the activity of TRPV1, leading to sensitization of the primary nociceptors.

Fig. 2

Indirect actions of melittin on primary nociceptor cells. A Following subcutaneous injection of a high concentration of melittin, the tissue matrix and mast cells are damaged by its pore-forming effect, resulting in the release of algogens such as H⁺, ATP, and 5-HT. These pain-inducing substances open the ligand-gated receptor channels such as the purinergic P₂X₃ receptor, acid-sensing ion channels, and 5-HT₃, leading to cation influx. B On the other hand, histamine, bradykinin, and ATP released by the pore-forming effect on the tissue matrix also activate GPCRs such as H1, P2Y, and B1/B2 that result in the phosphorylation of PLC. PLC cleaves phosphatidylinositol 4,5-bisphosphate into diacyl glycerol (DAG) and inositol 1,4,5-trisphosphate (IP3). DAG is an endogenous activator of TRPC. When TRPC is activated by DAG, a subpopulation of primary nociceptor cells that do not express TRPV1 are depolarized. Mitogen-activated protein kinases, including extracellular signal-regulated kinase 1/2 (ERK1/2), c-Jun N-terminal kinase (JNK) and p38 MAPK are also involved in the process of melittin-induced pain and hypersensitivity, possibly through activation of PLC as well.