Activation of transient receptor potential vanilloid 1 (TRPV1), a calcium permeable channel expressed in primary sensory neurons, induces the release of glutamate from their central and peripheral afferents during normal acute and pathological pain. However, little information is available regarding the glutamate release mechanism associated with TRPV1 activation in primary sensory neurons. To address this issue, we investigated the expression of vesicular glutamate transporter (VGLUT) in TRPV1-immunopositive (+) neurons in the rat trigeminal ganglion (TG) under normal and complete Freund’s adjuvant (CFA)-induced inflammatory pain conditions using behavioral testing as well as double immunofluorescence staining with antisera against TRPV1 and VGLUT1 or VGLUT2. TRPV1 was primarily expressed in small and medium-sized TG neurons. TRPV1+ neurons constituted approximately 27% of all TG neurons. Among all TRPV1+ neurons, the proportion of TRPV1+ neurons coexpressing VGLUT1 (VGLUT1+/ TRPV1+ neurons) and VGLUT2 (VGLUT2+/TRPV1+ neurons) was 0.4% ± 0.2% and 22.4% ± 2.8%, respectively. The proportion of TRPV1+ and VGLUT2+ neurons was higher in the CFA group than in the control group (TRPV1+ neurons: 31.5% ± 2.5% vs. 26.5% ± 1.2%, VGLUT2+ neurons: 31.8% ± 1.1% vs. 24.6% ± 1.5%, p < 0.05), whereas the proportion of VGLUT1+, VGLUT1+/TRPV1+, and VGLUT2+/TRPV1+ neurons did not differ significantly between the CFA and control groups. These findings together suggest that VGLUT2, a major isoform of VGLUTs, is involved in TRPV1 activation-associated glutamate release during normal acute and inflammatory pain.

Previous studies suggested that myelinated axons innervating rat molar pulps undergo morphological changes in their peripheral course. However, little information is available on the morphological feature of the parent axons at the site of origin. We therefore investigated the size of the myelinated parent axons and their morphological features at the proximal sensory root of the trigeminal ganglion by horseradish peroxidase (HRP) injection into rat upper molar pulps and subsequent light and electron microscopy. A total of 248 HRP-labeled myelinated axons investigated were highly variable in the size. Fiber area, fiber diameter, axon area (axoplasm area), axon diameter (axoplasm diameter), and myelin thickness were 11.32 ± 8.36 μm2 (0.80~53.17 μm2), 3.99 ± 1.53 μm (1.08~9.26 μm), 8.70 ± 6.30 μm2 (0.70~41.83 μm2), 3.13 ± 1.13 μm (0.94~7.20 μm) and 0.43 ± 0.23 μm (0.07~1.06 μm), respectively. The g-ratio (axon diameter / fiber diameter) of the labeled axons was 0.79 ± 0.05 (0.61~0.91). Axon diameter was highly correlated with myelin thickness (correlation coefficients, r=0.83) but little correlated with g-ratio (r=−0.33) of individual myelinated parent axons. These results indicate that myelin thickness of the myelinated parent axons innervating rat molar pulps increase with increasing axon diameter, thus maintaining a constant g-ratio.

Various voltage-gated K+ currents were recently described in dorsal root ganglion (DRG) neurons. However, the characterization and diversity of voltage-gated K+ currents have not been well studied in trigeminal root ganglion (TRG) neurons, which are similar to the DRG neurons in terms of physiological roles and anatomy. This study was aimed to investigate the characteristics and diversity of voltage-gated K+ currents in acutely isolated TRG neurons of rat using whole cell patch clamp techniques. The first type (type I) had a rapid, transient outward current (IA) with the largest current size having a slow inactivation rate and a sustained delayed rectifier outward current (IK) that was small in size having a fast inactivation rate. The IA currents of this type were mostly blocked by TEA and 4-AP, K channel blockers whereas the IK current was inhibited by TEA but not by 4-AP. The second type had a large IA current with a slow inactivation rate and a medium size-sustained delayed IK current with a slow inactivation rate. In this second type (type II), the sensitivities of the IA or IK current by TEA and 4-AP were similar to those of the type I. The third type (type III) had a medium sized IA current with a fast inactivation rate and a large sustained IK current with the slow inactivation rate. In type III current, TEA decreased both IA and IK but 4-AP only blocked IA current. The fourth type (type IV) had a smallest IA with a fast inactivation rate and a large IK current with a slow inactivation rate. TEA or 4-AP similarly decreased the IA but the IK was only blocked by 4-AP. These findings suggest that at least four different voltage-gated K+ currents in biophysical and pharmacological properties exist in the TRG neurons of rats.

LAR-RPTP (leukocyte common antigen-related receptor protein tyrosine phosphatase) is an important regulator in the nervous system, but little is known about its expression pattern in rat trigeminal ganglion (TG) neurons. To examine whether LAR-RPTP is expressed in the TG in the current study, we sacrificed rats at 0, 7, 10 and 56 day postpartum (dpp) and a second group of rats at 3 and 5 days after an experimental tooth extraction as a TG injury model. RT-PCR was then used to determine the level of LAR-RPTP expression in the TG and immunohistology was employed to detect the subcellular localization of the protein. The mRNA expression of LAR-RPTP during the developmental stages in the TG was found to gradually increase. After experimental tooth extraction however, these transcript levels had significantly decreased at three days. LAR-RPTP protein signals in the TG were found to be cytoplasmic in the normal animals but interestingly, at five days after an experimental tooth extraction, these signals were rare. These results indicate that LAR-RPTP may be regulated during both the developmental as well as regenerative processes that take place in the TG. This further suggests that LAR-RPTP is not only involved in primary axonogenesis but possibly also in the molecular control of axons during TG repair.

The present study investigated the role of ERK in the onset of mechanical and cold allodynia in a rat model of compression of the trigeminal ganglion by examining changes in the air-puff thresholds and number of scratches following the intracisternal injection of PD98059, a MEK inhibitor. Male Sprague Dawley rats weighing between 250 and 260 g were used. Under anesthesia, the rats were mounted onto a stereotaxic frame and received 4% agar (10μℓ) solution to compress the trigeminal ganglion. In the control group, the animals were given a sham operation without the application of agar. Changes in behavior were examined at 3 days before and at 3, 7, 10, 14, 17, 21, 24, 30, and 40 days after surgery. Compression of the trigeminal ganglion significantly decreased the air-puff thresholds. Mechanical allodynia was established within 3 days and persisted over postoperative day 24. To evaluate cold allodynia, nociceptive scratching behavior was monitored after acetone application on the vibrissa pad of the rats. Compression of the trigeminal ganglion was found to produce significant cold allodynia, which persisted for more than 40 days after surgery. On postoperative day 14, the intracisternal administration of 1 μg or 10 μg of PD98059 in the rat model significantly decreased the air-puff thresholds on both the ipsilateral and contralateral side. The intracisternal administration of 10 μg of PD98059 also significantly alleviated the cold allodynia, compared with the vehicle-treated group. These results suggest that central ERK plays an important role in the development of mechanical and cold allodynia in rats with compression of the trigeminal ganglion and that a targeted blockade of this pathway is a potential future treatment strategy for trigeminal neuralgia-like nociception.

We recently described a novel animal model of trigeminal neuropathic pain following compression of the trigeminal ganglion (Ahn et al., 2009). In our present study, we adapted this model using male Sprague-Dawley rats weighing between 250-260 g and then analyzed the behavioral responses of these animals following modified chronic compression of the trigeminal ganglion. Under anesthesia, the rats were mounted onto a stereotaxic frame and a 4% agar solution (10μL) was injected in each case on the dorsal surface of the trigeminal ganglion to achieve compression without causing injury. In the control group, the rats received a sham operation without agar injection. Air-puff, acetone, and heat tests were performed at 3 days before and at 3, 7, 10, 14, 17, 21, 24, 30, 40, 55, and 70 days after surgery. Compression of the trigeminal ganglion produced nociceptive behavior in the trigeminal territory. Mechanical allodynia was established within 3 days and recovered to preoperative levels at approximately 60 days following compression. Mechanical hyperalgesia was also observed at 7 days after compression and persisted until the postoperative day 40. Cold hypersensitivity was established within 3 days after compression and lasted beyond postoperative day 55. In contrast, compression of the trigeminal ganglion did not produce any significant thermal hypersensitivity when compared with the sham operated group. These findings suggest that compression of the trigeminal ganglion without any injury produces prolonged nociceptive behavior and that our rat model is a useful system for further analysis of trigeminal neuralgia.

The present study investigated inflammatory hypersensitivity following compression of the trigeminal ganglion in rats. Experiments were carried out on male Sprague-Dawley rats weighing 250-260 g. Under anesthesia, rats were mounted on a stereotaxic frame and injected with 8μL of 4% agar solution through a stainless steel injector to compress the trigeminal ganglion. In the control group, rats underwent a sham operation without agar injection. Injection sites were examined with a light micrograph after compression of the trigeminal ganglion. Air-puff thresholds (mechanical allodynia) were evaluated 3 days before surgery and 3, 7, 10, 14, 17, 21, 24, 30, and 40 days after surgery. Air-puff thresholds significantly decreased after compression of the trigeminal ganglion. Mechanical allodynia was established within 3 days and remained strong over 24 days, returning to preoperative levels approximately 40 days following compression. After subcutaneous injection of 5% formalin (50μL) in the compression of the trigeminal ganglion-treated rats, nociceptive scratching behavior was recorded for 9 successive 5-min internals. Injection of formalin into the vibrissa pad significantly increased the number of scratches and duration of noxious behavioral responses in sham-treated rats. Noxious behavioral responses induced by subcutaneous formalin administration were significantly potentiated in rats with trigeminal ganglion compression. These findings suggest that compression of the trigeminal ganglion enhanced formalin-induced infla-mmatory pain in the orofacial area.

R-type(Cav2.3) calcium channel contributes to pain sensation in peripheral sensory neurons. Six isoforms of Cav2.3 that result from combinations of presence or deletion of three inserts(insert I and insert in the II-III loop, and insert III in N-terminal regions) have been demonstrated to be present in different mammalian tissues. However, the molecular basis of Cav2.3 in trigeminal ganglion(TG) neurons is not known. In the present study, we determined which isoforms of Cav2.3 are expressed in rat TG neurons using the RT-PCR analysis. Whole tissue RT-PCR analyses revealed that only two isoforms, Cav2.3a and Cav2.3e, were present in TG neurons. From single-cell RT-PCR, we found that Cav2.3e rather than Cav2.3a was the major isoform expressed in TG neurons, and Cav2.3e was preferentially detected in small-sized neurons that express nociceptive marker, transient receptor potential vanilloid 1(TRPV1). Our results suggest that Cav2.3e in trigeminal neurons may be a potential target for the pain treatment.