Evidence for the participation of peripheral 5-HT2A, 5-HT2B, and 5-HT2C receptors in formalin-induced secondary mechanical allodynia and hyperalgesia.

The role of 5-HT2A/2B/2C receptors in formalin-induced secondary allodynia and hyperalgesia in rats was assessed. Formalin produced acute nociceptive behaviors (flinching and licking/lifting) followed by long-term secondary mechanical allodynia and hyperalgesia. Pre-treatment for five consecutive days with compound 48/80 (1, 3, 10, 10, and 10 µg/paw) prevented formalin-induced secondary allodynia and hyperalgesia. Ipsilateral, but not contralateral, peripheral pre-treatment (nmol/paw) with the 5-HT2 receptor agonist DOI (3-30), 5-HT (10-100) or fluoxetine (0.3-3) significantly increased 0.5% formalin-induced secondary allodynia and hyperalgesia in both paws. The pronociceptive effect of DOI (10 nmol/paw), 5-HT (100 nmol/paw) and fluoxetine (1 nmol/paw) was blocked by selective 5-HT2A (ketanserin), 5-HT2B (RS-127445), and 5-HT2C (RS-102221) receptor antagonists. Furthermore, ipsilateral pre-treatment (nmol/paw) with ketanserin (1, 10, and 100), RS-127445 (0.01, 0.1 and 1) or RS-102221 (1, 10 and 100) prevented while post-treatment reversed 1% formalin-induced secondary allodynia and hyperalgesia in both paws. In marked contrast, contralateral injection of the greatest tested dose of 5-HT2A/2B/2C receptor antagonists did not modify long-lasting secondary allodynia and hyperalgesia. These results suggest that 5-HT released from mast cells after formalin injection sensitizes primary afferent neurons via 5-HT2A/2B/2C receptors leading to the development and maintenance of secondary allodynia and hyperalgesia.

Role of peripheral and spinal 5-HT6 receptors according to the rat formalin test.

The present study assessed the possible pronociceptive role of peripheral and spinal 5-HT(6) receptors in the formalin test. For this, local peripheral administration of selective 5-HT(6) receptor antagonists N-[3,5-dichloro-2-(methoxy)phenyl]-4-(methoxy)-3-(1-piperazinyl)-benzenesulphonamide (SB-399885) (0.01-1 nmol/paw) and 4-iodo-N-[4-methoxy-3-(4-methyl-1-piperazinyl)phenyl]benzene-sulfonamide hydrochloride (SB-258585) (0.001-0.1 nmol/paw) significantly reduced formalin-induced flinching. Local peripheral serotonin (5-HT) (10-100 nmol/paw) or 5-chloro-2-methyl-3-(1,2,3,6-tetrahydro-4-pyridinyl)-1H-indole hydrochloride (EMD-386088) (0.01-0.1 nmol/paw; a selective 5-HT(6) receptor agonist) augmented 0.5% formalin-induced nociceptive behavior. The local pronociceptive effect of 5-HT (100 nmol/paw) or EMD-386088 (0.1 nmol/paw) was significantly reduced by SB-399885 or SB-258585 (0.1 nmol/paw). In contrast to peripheral administration, intrathecal injection of 5-HT(6) receptor antagonists SB-399885 and SB-258585 (0.1-10 nmol/rat) did not modify 1% formalin-induced nociceptive behavior. Spinal 5-HT (50-200 nmol/rat) significantly reduced formalin-induced flinching behavior during phases 1 and 2. Contrariwise, intrathecal EMD-386088 (0.1-10 nmol/rat) dose-dependently increased flinching during phase 2. The spinal pronociceptive effect of EMD-386088 (1 nmol/rat) was reduced by SB-399885 (1 nmol/rat) and SB-258585 (0.1 nmol/rat). Our results suggest that 5-HT(6) receptors play a pronociceptive role in peripheral as well as spinal sites in the rat formalin test. Thus, 5-HT(6) receptors could be a target to develop analgesic drugs.

Decreased nitric oxide markers and morphological changes in the brain of arsenic-exposed rats.

Epidemiological studies demonstrate an association between chronic consumption of arsenic contaminated water and cognitive deficits, especially when the exposure takes place during childhood. This study documents structural changes and nitrergic deficits in the striatum of adult female Wistar rats exposed to arsenic in drinking water (3 ppm, approximately 0.4 mg/kg per day) from gestation, throughout lactation and development until the age of 4 months. Kainic acid injected animals (10mg/kg, i.p.) were also analyzed as positive controls of neural cell damage. Morphological characteristics of cells, fiber tracts and axons were analyzed by means of light microscopy as well as immunoreactivity to neuronal nitric oxide synthase (nNOS). As nitrergic markers, nitrite/nitrate concentrations, nNOS levels and expression of nNOS-mRNA were quantified in striatal tissue. Reactive oxygen species (ROS) and lipid peroxidation (LPx) were determined as oxidative stress markers. Arsenic exposure resulted in moderate to severe alterations of thickness, organization, surrounding space and shape of fiber tracts and axons, while cell bodies remained healthy. These anomalies were not accompanied by ROS and/or LPx increases. By contrast, except the expression of nNOS-mRNA, all nitrergic markers including striatal nNOS immunoreactivity presented a significant decrease. These results indicate that arsenic targets the central nitrergic system and disturbs brain structural organization at low exposure levels.