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A study conducted in an animal model suggests there is cross-talk between the two pruritogens, serotonin and histamine, in their pathway for itch induction within the spinal cord. Based on their findings, the investigators propose combination treatment targeting serotonin receptor antagonism and histamine-3-receptor stimulation may provide relief of intractable pruritus associated with dermatologic disease.
National report - Treatment that combines serotonin (5-HT) receptor antagonism with stimulation of the histamine-3-receptor (H3R) may offer superior relief of 5-HT-induced itch compared with 5-HT antagonism alone, according to Hui Wang, M.D., and colleagues from Guangdong Medical College, China.
Their conclusion is based on the results of an animal study providing evidence that the H3R may act as a cross-talk point in the afferent pathway of 5-HT-induced itch.
"5-HT and histamine are both potent pruritogens. However, in animal studies where each is injected separately into the skin, treatment with an agent that blocks the activity of the individual chemical mediator or its binding to its respective receptor does not always completely abolish itch," Dr. Wang says.
The study involved 29 Sprague-Dawley rats that were divided into four groups to receive intradermal injection in the rostral part of the back with: 1) 5-HT (eight animals); 2) 5-HT plus thioperamide, an H3R antagonist (eight animals); 3) 5-HT plus R-alpha-methylhistamine, an H3R agonist (eight animals); or 4) saline (five animals).
The animals were monitored with videotaping for a period of 60 minutes after injection to allow counting of the number of scratching bouts to the injected site. Thereafter, the animals were euthanized and the spinal cord from C3 to C5 along with the corresponding dorsal root ganglia were harvested and processed for immunohistochemical evaluation of the H3R. Cells staining positive for H3R were measured with optical density analysis using commercially available image-processing software.
The researchers found that, compared with animals injected with 5-HT alone, concomitant injection with the H3R agonist reduced scratching behavior and expression of H3R in the primary afferent neurons, whereas co-injection with the H3R antagonist resulted in increased scratching and H3R expression.
Animals injected with 5-HT alone experienced an average of about 110 scratching bouts compared with just two scratching bouts among the saline-injected controls. The mean number of scratching bouts recorded in animals injected with 5-HT plus the H3R agonist was 147; concomitant injection of 5-HT plus the H3R agonist was associated with a mean of 61 scratching bouts. The mean number of scratching bouts in both of those latter groups was significantly different compared with the animals injected with 5-HT alone.
H3R expression in the superficial posterior horn was not significantly different in the control animals compared with those injected with 5-HT only. It was significantly greater in the group of animals injected with the H3R antagonist plus 5-HT, and significantly lower in those injected with serotonin plus the H3R agonist compared with both the controls and the animals injected with 5-HT only.
Analysis of H3R expression in the anterior horn showed a significant increase in the animals injected with serotonin alone compared with the controls, and an even greater increase in animals injected with serotonin plus the H3R antagonist. Among the four treatment groups, H3R expression was lowest in the animals injected with 5-HT plus the H3R agonist, and the difference in expression compared with all other groups was statistically significant.
"The increased expression of H3R in the motor neurons of the anterior horns following serotonin injection suggests these two pruritogens may share the same efferent pathway," Dr. Wang notes.
The dorsal root ganglia were also examined microscopically to categorize the cells staining positive for H3R into three groups by size (small, meaning >200 to <700 microns2; middle, meaning 700 to 1,250 microns2; and large, meaning >1,250 microns2). Then, the proportion of small plus middle-sized cells relative to the total was calculated for each treatment group.
That analysis showed the ratio of small and middle positive cells was greatest in animals treated with 5-HT and the H3R antagonist (84 percent), followed in descending order by those treated with 5-HT only (79.88 percent), 5-HT plus the H3R agonist (78.45 percent) and the controls (75.59 percent).