Group Ⅰ Metabotropic Glutamate Receptors Are Involved In The Activation Of Peripheral Nociceptors

It is generally accepted that central nociception is based on the initiation of centripetal signals from peripheral nociceptors activated by peripheral noxious stimulation. Glutamate is the major excitatory transmitter in mammalian nervous system and plays a significant role in the nociceptive processes including generation of noxious signals from peripheral nociceptors and transmission of nociceptive inputs in the spinal cord. Glutamate receptors are divided into two types of receptors named ionotropic receptors (iGluRs, ligand-gated channels) and metabotropic receptors (mGluRs, G protein-coupled receptors). Increasing evidence indicated that peripheral iGluRs play an important role in the process of activation of nociceptors. Recently, Bhave et al. showed that Group I mGluRs, including mGluR1 and mGluR5, are also located in terminals of primary afferent fibers by morphological examinations. Thus, the present study is to investigate if these mGluRs are functionally involved in the process of activation of nociceptors as iGluRs do by using formalin tests combined with a behavioraland Fos expression examinations.There were two parts of experiments. In the first part, both behavioral responses and Fos expression in dorsal horn induced by peripherally injected formalin were observed. Animals were divided into three groups (n=5): one of them was control with injection of vehicle (HEPES) into the plantar aspect of the left hindpaw; and two of them were experimental groups with injection of selective mGluR, and mGluR5 antagonists, CPCCOEt or MPEP, respectively. Formalin (5%, 50ul) was injected 15 min after the pretreatment of antagonists or vehicle at the same site of drug injected paw in all three groups. The behavioral observation of nociceptive responses were carried out immediately after formalin administration by examining the total time used for licking and biting towards the injected hindpaw by rats, and then, one hour later after formalin injection, rats were killed for immunocytochemical procedures for Fos expression in the dorsal horn. In the second part of the experiment, formalin was injected in both hindpaws and one of them was administered with CPCCOEt or MPEP, respectively. In this case, only the Fos expression in both sides of the spinal cord were examined by immunocytochemical procedures.Behavioral observation in control group showed that, injection of formalin into one plantar aspect of hindpaw resulted in biphasic behavioral responses of licking and biting towards the injected paw. The first phasestarted immediately after injection formalin and lasted around 6-9 min,named acute phase. The second phase occurred following a short interval of 9-15 min and lasted about 20-40 min, named tonic phase. Peripheral administration of CPCCOEt and MPEP produced significant suppression in the tonic phase of nociceptive responses, while both antagonists showed no effects On the acute phase responses. In the immunocytochemical examination of Fos expression, subcutaneous injection of formalin induced significant increase of Fos-like immunoreactive (FLI) neurons which could be found mainly in superficial laminae of the dorsal horn ipsilateral to the injection of formalin, while combined pretreatment of CPCCOEt or MPEP significantly suppressed the Fos expression in the drug-injected groups or injected side in both parts of experiments. These results indicated that:1. Peripheral injection of formalin into plantar aspect of hindpaw produced both nociceptive behavioral responses and Fos expression in the spinal dorsal horn.2. Formalin-induced nociceptive behavioral responses included two phases, and subcutaneous pretreatment of CPCCOEt or MPEP significantly suppressed the tonic responses without effects on the acute nociceptive responses.3. Peripheral injection of formalin induced increasing Fos expression in superficial laminae of the dorsal horn, while the combined subcut...