5-HT(1D) receptor immunoreactivity in the sphenopalatine ganglion: implications for the efficacy of triptans in the treatment of autonomic signs associated with cluster headache

Abstract

Objective: To determine if 5-HT(1D) receptors are located in the sphenopalatine ganglion.

Background: While the 5-HT(1D) receptor has been described in sensory and sympathetic ganglia in the head, it was not known whether they were also located in parasympathetic ganglia.

Methods: We used retrograde labeling combined with immunohistochemistry to examine 5-HT(1D) receptor immunoreactivity in rat sphenopalatine ganglion neurons that project to the lacrimal gland, nasal mucosa, cerebral vasculature, and trigeminal ganglion.

Results: We found 5-HT(1D) receptor immunoreactivity in nerve terminals around postganglionic cell bodies within the sphenopalatine ganglion. All 5-HT(1D) -immunoreactive terminals were also immunoreactive for calcitonin gene-related peptide but not vesicular acetylcholine transporter, suggesting that they were sensory and not preganglionic parasympathetic fibers. Our retrograde labeling studies showed that approximately 30% of sphenopalatine ganglion neurons innervating the lacrimal gland, 23% innervating the nasal mucosa, and 39% innervating the trigeminal ganglion were in apparent contact with 5-HT(1D) receptor containing nerve terminals.

Conclusion: These data suggest that 5-HT(1D) receptors within primary afferent neurons that innervate the sphenopalatine ganglion are in a position to modulate the excitability of postganglionic parasympathetic neurons that innervate the lacrimal gland and nasal mucosa, as well as the trigeminal ganglion. This has implications for triptan (5-HT(1D) receptor agonist) actions on parasympathetic symptoms in cluster headache.

Figure 1

(A) Confocal Z-projection of a section of rat SPG showing 5-HT_1D receptor immunoreactivity confined to fibers traversing the SPG (arrowheads) and to terminals that were in close apposition to neuronal cell bodies (numbered 0–3 according to the innervation score they were given). (B–D) Confocal Z-projection of a section of rat SPG showing double labeling for 5-HT_1D receptor (B) and CGRP (C) immunoreactivity. All 5-HT_1D receptor immunoreactive fibers and terminals were CGRP immunoreactive (D; yellow). (E–G) Confocal Z-projection of a section of rat SPG showing double labeling for 5-HT_1D receptor (E) and VAChT (F) immunoreactivity. No 5-HT_1D receptor immunoreactive fibers or terminals were VAChT immunoreactive (G). Confocal Z-projection of a section of rat SPG showing 5-HT_1D receptor immunoreactivity in fibers and terminals (red) in close apposition to retrogradely labeled cell bodies (blue) of postganglionic neurons that innervate the lacrimal gland (H) and the trigeminal ganglion (I). In H and I, the numbers 0–3 indicate the innervations score each of the retrograde labeled cells were given.

Figure 2

Summary histogram of the percentage of retrogradely labeled cells from each target that were in close apposition to 5-HT_1D receptor immunoreactive nerve terminals.

Figure 3

Schematic of connections between cranial sensory and parasympathetic pathways putatively involved in pain and autonomic symptoms associated with cluster headache and migraine. Trigeminal primary afferent neurons with somata in the trigeminal ganglion (TG) relay information from intracranial vasculature to the brainstem trigeminal complex (TC), which passes this information to higher pain regulatory centers in the brain. Reflex connections with brainstem parasympathetic outflow nuclei, such as the superior salivatory nucleus (SSN), can influence postganglionic parasympathetic neurons in the SPG that project to the lacrimal gland and nasal mucosa and are likely responsible for some of the autonomic symptoms of cluster headache and migraine. Postganglionic parasympathetic neurons that innervate the lacrimal gland and nasal mucosa can also receive direct inputs from collateral fibers of 5-HT_1D receptor immunoreactive trigeminal primary afferent neurons (dotted line). These trigeminal fibers have previously been shown to be from the maxillary (V₂) division (see Suzuki et al. 1989). This peripheral connection (in the SPG) may also be responsible, at least in part, for some of the autonomic signs associated with cluster headache and migraine. Furthermore, triptans could act on 5-HT_1D receptor immunoreactive terminals at this site to reduce the autonomic signs associated with these conditions. A reciprocal connection between the two ganglia may provide a mechanism for feedback to modulate the activity of trigeminal primary afferent neurons in cluster headache and migraine.