The mast cell degranulator compound 48/80 directly activates neurons.

BACKGROUND: Compound 48/80 is widely used in animal and tissue models as a "selective" mast cell activator. With this study we demonstrate that compound 48/80 also directly activates enteric neurons and visceral afferents. METHODOLOGY/PRINCIPAL FINDINGS: We used in vivo recordings from extrinsic intestinal afferents together with Ca(++) imaging from primary cultures of DRG and nodose neurons. Enteric neuronal activation was examined by Ca(++) and voltage sensitive dye imaging in isolated gut preparations and primary cultures of enteric neurons. Intraluminal application of compound 48/80 evoked marked afferent firing which desensitized on subsequent administration. In egg albumen-sensitized animals, intraluminal antigen evoked a similar pattern of afferent activation which also desensitized on subsequent exposure to antigen. In cross-desensitization experiments prior administration of compound 48/80 failed to influence the mast cell mediated response. Application of 1 and 10 µg/ml compound 48/80 evoked spike discharge and Ca(++) transients in enteric neurons. The same nerve activating effect was observed in primary cultures of DRG and nodose ganglion cells. Enteric neuron cultures were devoid of mast cells confirmed by negative staining for c-kit or toluidine blue. In addition, in cultured enteric neurons the excitatory action of compound 48/80 was preserved in the presence of histamine H(1) and H(2) antagonists. The mast cell stabilizer cromolyn attenuated compound 48/80 and nicotine evoked Ca(++) transients in mast cell-free enteric neuron cultures. CONCLUSIONS/SIGNIFICANCE: The results showed direct excitatory action of compound 48/80 on enteric neurons and visceral afferents. Therefore, functional changes measured in tissue or animal models may involve a mast cell independent effect of compound 48/80 and cromolyn.

Critical evaluation of the streptozotocin model of painful diabetic neuropathy in the rat.

Streptozotocin (STZ)-induced diabetes in the rat has been increasingly used as a model of painful diabetic neuropathy to assess the efficacies of potential analgesic agents. We have established this model, and here we question whether the changes in nocifensive reflex activity, used as a measure of hyperalgesia, are genuinely indicative of peripheral neuropathy or may rather be attributed to the extreme poor health of the animals. For comparison we have examined animals with peripheral neuropathy induced by partial ligation of the sciatic nerve. Diabetic animals were chronically ill, with reduced growth rate, polyuria, diarrhoea, and had enlarged and distended bladders. Indicative of their poor health, diabetic animals showed markedly reduced motor activity. In contrast, following partial sciatic nerve ligation rats showed none of these adverse effects and their motor activity was not different to naive animals. Diabetic animals displayed marked mechanical hyperalgesia, and some thermal hypoalgesia. Morphine and L-baclofen partially reversed established STZ-induced mechanical hyperalgesia, whilst the NK-1 receptor-antagonist RP-67580, the NMDA-antagonists MK801 and ketamine, and the nitric oxide synthase inhibitor L-NAME were without significant effect. Morphine and L-baclofen produced greater reversal of mechanical hyperalgesia following partial nerve ligation, although RP67580 and MK801 showed little or no activity. These data confirm previous findings that STZ-induced diabetes in rats produces long-lasting mechanical, but not thermal hyperalgesia. In our experience this mechanical hyperalgesia is largely resistant to a range of pharmacological tools. However, we feel that the profound ill-health of the animals, together with the poor activity of a range of potential analgesic drugs, must raise questions on the predictive value of these animals as a model for the human condition of chronic diabetic pain seen in patients receiving long-term insulin treatment, as well as ethical concerns on the use of the animals themselves.