Probing the diversity of serotonin neurons

Abstract

The serotonin (5-HT) system is generally considered as a single modulatory system, with broad and diffuse projections. However, accumulating evidence points to the existence of distinct cell groups in the raphe. Here, we review prior evidence for raphe cell heterogeneity, considering different properties of 5-HT neurons, from metabolism to anatomy, and neurochemistry to physiology. We then summarize more recent data in mice and zebrafish that support a genetic diversity of 5-HT neurons, based on differential transcription factor requirements for the acquisition of the 5-HT identity. In both species, PET1 plays a major role in the acquisition and maintenance of 5-HT identity in the hindbrain, although some 5-HT neurons do not require PET1 for their differentiation, indicating the existence of several transcriptional routes to become serotoninergic. In mice, both PET1-dependent and -independent 5-HT neurons are located in the raphe, but have distinct anatomical features, such as the morphology of axon terminals and projection patterns. In zebrafish, all raphe neurons express pet1, but Pet1-independent 5-HT cell groups are present in the forebrain. Overall, these observations support the view that there are a number of distinct 5-HT subsystems, including within the raphe nuclei, with unique genetic programming and functions.

Figure 1.

Transcripts for (a) tph2 and (b) pet1 revealed by in situ hybridization performed on brains dissected from 6-day-old zebrafish. tph2 is present in the rostral (r) and caudal (c) raphe populations, as well as in pretectal (p) neurons and the pineal gland (not shown). In contrast, pet1 is only detectable in the hindbrain populations. Ventral view, anterior to the left. Adapted from [31].

Figure 2.

Schematic drawing illustrating the location of serotoninergic neurons in the adult zebrafish brain (lateral view). 5-HT-immunoreactive cells are also present in the retina (not shown). The main projections originating from the raphe populations are indicated. AP, area postrema; Cer, cerebellum; Hyp, hypothalamus; MO, medulla oblongata; NLV, nucleus lateralis valvulae; OB, olfactory bulb; PG, preglomerular complex, Po, preoptic region; PT, posterior tuberculum; SC, spinal cord; Tel, telencephalon; TeO, tectum opticum; Th, thalamus; V, ventral telencephalic area. Adapted from [19].

Figure 3.

Transcripts for Tph2, Vglut3 and R-crf2 (receptor for CRF) on consecutive coronal sections through the dorsal raphe of control (a–c) and Pet1 knockout (e–g) mice. An overlay using false colour code is shown in d and h. In control raphe (d), the three transcripts delimit nested domains within the dorsal raphe, outlining in particular the Tph2-positive neurons in the lateral wings, which do not contain Vglut3 or R-crf2. In Pet1 KO raphe, the number of Tph2-positive neurons is reduced by 70%, while Vglut3-containing neurons are reduced by 30%. However, double combining Vglut3 in situ hybridization with TPH immunocytochemistry [90] indicated that most of these remaining Vglut3 neurons are not serotoninergic. Micrographs taken by Vera Kiyasova.

Figure 4.

Schematic illustrating the residual serotonin innervation in the Pet1 KO. (a) The rostral and caudal raphe cell groups send projections to the forebrain and to the brainstem and spinal cord, respectively. In Pet1 KO mice, all 5-HT neurons and projections schematized in dark grey fail to differentiate; they do not acquire a 5-HT phenotype and do not send projections to the hippocampus and cerebral cortex. In contrast, the neurons and projections schematized in light grey differentiate and send topographically appropriate projections. (b) Drawing summarizing the main morphological characteristics of the residual 5-HT innervation in the Pet1 KO. The residual 5-HT terminals target highly selective regions in the forebrain: the basolateral nucleus of the amygdala (BL), paraventricular nucleus of hypothalamus (PV) and mildline thalamic nuclei (IL). In these areas, the residual 5-HT labelled axons display large varicosities (upper insert) and differentiated synaptic junctions (lower insert).