Griseum centrale, a homologue of the periaqueductal gray in the lamprey

Abstract

Fear, a response to threatening stimuli and important for survival, is a behavior found throughout the animal kingdom. One critical structure involved in the expression of fear-related behavior is the periaqueductal gray (PAG) in mammals, and in the zebrafish, the griseum centrale. Here, we show in the lamprey, belonging to the oldest now living group of vertebrates, that a bilateral periventricular nucleus in the ventral mesencephalon has a similar location to that of the PAG and griseum centrale. It targets the pretectum and the substantia nigra pars compacta (SNc), expresses the dopamine D1 and D2 receptors and receives input from the pallium (cortex in mammals), hypothalamus, the raphe area and SNc. These are all hallmarks of the mammalian PAG. In addition, like in the zebrafish, there is an input from the interpeduncular nucleus. Our results thus suggest that a structure homologous to the PAG/griseum centrale was present very early in vertebrate evolution.

Keywords: Evolution; Fear-related behavior; Interpeduncular nucleus; PAG; SNc.

Graphical abstract

Fig. 1

Identifying the PAG homologue. (A) Injection of Neurobiotin into the pretectum. (B–C) Bilaterally located retrogradely labeled cell body clusters in the periventricular area in the ventromedial mesencephalon. The squared area in B is shown in high magnification in C. (D) Injection of Neurobiotin into the SNc, confirmed by the presence of tyrosine hydroxylase (TH) immunoreactive cells (red). (E) High magnification photomicrograph of retrogradely labeled cell bodies (green) located periventricularly in the ventromedial mesencephalon with TH immunopositive fibers (red) in close proximity to the cells. (F and G) In situ hybridization of the dopamine D1 and D2 receptor transcripts showing expression of the dopamine D1 (F) and the D2 (G) receptor, respectively, in griseum centrale,. (H) Retrogradely labeled cells (green) in the griseum centrale following SNc injection (D) combined with 5-HT immunohistochemistry. (I) 5-HT immunoreactive fibers (red) in close proximity to the retrogradely labeled cells (green; high magnification photomicrograph of the squared area in H). All sections were counterstained with a fluorescent Nissl stain. 5-HT, serotonin; M1, Müller cell 1; M3, Müller cell 3; nIII, oculomotor nerve; IPN, interpeduncular nucleus; TH, tyrosine hydroxylase. Scale bars = A, B, D, F, G, H, 250 μm; C, E, I, 50 μm. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 2

Identification of afferents to the griseum centrale. (A–C) Dual injections of Neurobiotin into the SNc (A) and biotin-dextran amine into the IPN (B). (B–C) Anterogradely labeled nerve fibers from IPN injection (red) in close proximity to retrogradely labeled cell bodies from SNc injection (green). (D–F) Injection of Neurobiotin into IPN (D) resulted in retrogradely labeled cells in the medial habenula (E-F; magenta). (G–H) Injection of Neurobiotin into griseum centrale (G) resulted in retrogradely labeled cells in the hypothalamus (H, arrow). (I) Pallial fibers and presumed terminals (red) in close apposition to cells in GC (green) retrogradely labeled from pretectum. Note that pallial fibers also innervated the contralateral GC. (J) Higher magnification of the photomicrograph in (I). (K) Neurobiotin injection into the lateral pallium (LPal). (L) Dextran injection into pretectum (PT). (M) GABA immunoreactive cells in the griseum centrale. All sections were counterstained with a fluorescent Nissl stain. IPN, interpeduncular nucleus; lHb, lateral habenula; M3, Müller cell 3; M5, the M5 nucleus of Schober; mHb, medial habenula nIII, oculomotor nerve. Scale bars = A, B, D, E, G, H, K, L, 250 μm; F, I 100 μm; C, I, J, M 50 μm. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 3

Location of the lamprey griseum centrale. (A) Dorsal view of the lamprey brain indicating the level of the injection site (B), as well as the most rostral (C) and the most caudal section (M). (B) The bilateral injection sites in the pretectum. (C–M) Schematic drawing illustrating the rostrocaudal extent of griseum centrale (GC; green) and its relation to the serotonergic raphe nucleus (magenta). Aq, aqueduct; ARRN, anterior rhombencephalic reticular nucleus; dnIII, dorsal nucleus of the oculomotor nerve; Hyp, hypothalamus; I1, I1 Müller cell; IPN, interpeduncular nucleus; nIII, the oculomotor nerve; OT, optic tectum; pc, posterior commissure; PT, pretectum. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 4

The connectivity pattern of the griseum centrale and PAG. Diagram comparing the afferent and efferent connectivity of the griseum centrale in lamprey and zebrafish, and the PAG in mammals. GC, griseum centrale; IPN, interpeduncular nucleus; PAG, periaqueductal gray; SNc, substantia nigra pars compacta.