5-HT2A receptors are concentrated in regions of the human infant medulla involved in respiratory and autonomic control

Abstract

The serotonergic (5-HT) system in the human medulla oblongata is well-recognized to play an important role in the regulation of respiratory and autonomic function. In this study, using both immunocytochemistry (n=5) and tissue section autoradiography with the radioligand (125)I-1-(2,5-dimethoxy-4-iodo-phenyl)2-aminopropane (n=7), we examine the normative development and distribution of the 5-HT(2A) receptor in the human medulla during the last part of gestation and first postnatal year when dramatic changes are known to occur in respiratory and autonomic control, in part mediated by the 5-HT(2A) receptor. High 5-HT(2A) receptor binding was observed in the dorsal motor nucleus of the vagus (preganglionic parasympathetic output) and hypoglossal nucleus (airway patency); intermediate binding was present in the nucleus of the solitary tract (visceral sensory input), gigantocellularis, intermediate reticular zone, and paragigantocellularis lateralis. Negligible binding was present in the raphé obscurus and arcuate nucleus. The pattern of 5-HT(2A) immunoreactivity paralleled that of binding density. By 15 gestational weeks, the relative distribution of the 5-HT(2A) receptor was similar to that in infancy. In all nuclei sampled, 5-HT(2A) receptor binding increased with age, with significant increases in the hypoglossal nucleus (p=0.027), principal inferior olive (p=0.044), and medial accessory olive (0.038). Thus, 5-HT(2A) receptors are concentrated in regions involved in autonomic and respiratory control in the human infant medulla, and their developmental profile changes over the first year of life in the hypoglossal nucleus critical to airway patency and the inferior olivary complex essential to cerebellar function.

Figure 1

Autoradiographic images of ¹²⁵I DOI binding to 5-HT_2A receptors in transverse sections of the caudal (A) and rostral (B) human infant medulla. High 5-HT_2A receptor binding density is present in the DMX and HG, moderate binding density in the NTS and PGCL, and relatively low binding density in the ROB and Arc.

Figure 2

Bar graph displaying 5-HT_2A receptor binding density in medullary nuclei presented from the highest to lowest. 5-HT_2A receptor binding density is highest in medullary nuclei without 5-HT neurons (e.g., DMX, HG).

Figure 3

5-HT_2A receptor immunostaining in the human infant medulla at 10 postnatal months. Panels (A) and (B) show diagrams of horizontal sections of rostral and mid-medulla, respectively, showing the location of the component nuclei of the medullary 5-HT system at each level. 5-HT_2A receptor immunostaining is punctate and localized to neuronal cell bodies and processes, indicative of post-synaptic localization of the receptors. Figure shows intense immunostaining of neurons in the DMX (C), motor neurons in the HG (D), and spherical neurons in the PIO (E). Moderately stained neurons of heterogenous morphology were observed in the extra-raphé regions including the GC (F). Spherical 5-HT_2A immunoreactive neurons were observed in the lateral extent of the PGCL (G) consistent in location and morphology with preBötC neurons in rodents. Lightly stained spherical and fusiform 5-HT_2A receptor immunoreactive neurons were observed in the midline raphé (H). Midline of the medulla is labeled by double headed arrow; D, dorsal; V, ventral. Immunostaining to “classical” arcuate neurons (arrows) and astrocytes (arrow-heads) in the arcuate nucleus (I). VMS, ventral medullary surface. All images at ×40. Scale bar= 100 μm.

Figure 4

Double-label immunofluorescent images showing localization of 5-HT_2A receptors and 5-HT neurons in the PGCL (×40). A. 5-HT_2A immunofluorescent staining; B. TPOH immunofluorescent staining; C. Merged images. 5-HT_2A receptors co-localized to the soma and dendrites of a subset of 5-HT immunoreactive neurons (arrows) in the medulla, but not all 5-HT neurons expressed 5-HT_2A receptors (arrowheads), and not all neurons expressing 5-HT_2A receptor immunoreactivity were 5-HT (*). ×40. Scale bar=100μm.

Figure 5

Linear regression plots of the relationship between 5-HT_2A receptor binding density and postconceptional age (weeks) in HG (A), PIO (B) and MAO (C). In each nuclei, receptor binding density increases significantly with age. *p<0.05 linear regression analysis.

Figure 6

Developmental expression of 5-HT_2A receptor immunocytochemistry in the human medulla from 15 gestational weeks to term (41 gestational weeks). Immunostaining in the HG (A) and extra-raphé (B) (i.e., GC/PGCL) at 15 weeks gestation. Neurons in the extra-raphé were tightly clustered together with no obvious distinction between nuclei. Neurons are spherical and undifferentiated. At 22 weeks gestation 5-HT_2A receptor immunoreactivity was observed in distinct nuclei including the DMX (C), PIO (D) and arcuate nucleus (E) with evidence of neuronal differentiation. Motor neurons in the HG expressing 5-HT_2A receptor immunoreactivity at term (41 weeks gestation) (F). All images at ×40 (scale bar=100μm) except D at ×20 (scale bar=200μm). VMS, ventral medullary surface.