EXPRESSION OF SEROTONIN RECEPTOR SUBTYPE 3A (5HT3A) ON RAT TRIGEMINAL SENSORY NEURONS

Area of Science:

• Neuroscience
• Pain Research
• Neuropharmacology

Background:

• Serotonin (5-hydroxytryptamine, 5HT) is a key neurotransmitter and mediator of inflammation, initiating pain signaling through various receptors on nociceptors.
• Transient receptor potential vanilloid 1 (TRPV1) channels are expressed on a subset of sensory neurons involved in pain, and 5HT can directly activate and sensitize these channels.
• Craniofacial pain disorders disproportionately affect women, highlighting the need to investigate sex-specific pain mechanisms in trigeminal sensory neurons.

Purpose of the Study:

• To investigate whether 5HT acting via the 5HT3A receptor elicits sexually dimorphic orofacial pain behaviors in rats.
• To determine if 5HT3A receptor mRNA is expressed in trigeminal sensory neurons, including those co-expressing TRPV1, and if this influences pain signaling.
• To explore the role of 5HT3A in craniofacial pain mechanisms, particularly in the context of sex differences.

Main Methods:

• Assessment of orofacial pain behaviors in male and female rats following 5HT administration and 5HT3A receptor antagonism.
• In situ hybridization to detect and localize 5HT3A and TRPV1 mRNA expression in trigeminal sensory neurons from male and female rats.
• Evaluation of 5HT3A antagonism's effect on pain signaling in cultured trigeminal sensory neurons.

Main Results:

• 5HT evoked pain behaviors in both male and female rats, but 5HT3A receptor antagonism did not significantly reduce these behaviors.
• In situ hybridization revealed distinct, non-overlapping populations of trigeminal sensory neurons expressing either 5HT3A mRNA or TRPV1 mRNA in both sexes.
• Antagonism of the 5HT3A receptor did not alter pain signaling in cultured trigeminal sensory neurons.

Conclusions:

• The 5HT3A receptor on trigeminal sensory neurons innervating orofacial tissues does not appear to play a significant role in sexually dimorphic craniofacial pain.
• The distinct expression patterns of 5HT3A and TRPV1 mRNA suggest separate roles for these receptors in trigeminal sensory neurons.
• While 5HT3A may contribute to sex differences in specific pain conditions like myofascial pain, its role in broader craniofacial pain disorders seems limited.