Bilateral descending hypothalamic projections to the spinal trigeminal nucleus caudalis in rats

Abstract

Several lines of evidence suggest that the hypothalamus is involved in trigeminal pain processing. However, the organization of descending hypothalamic projections to the spinal trigeminal nucleus caudalis (Sp5C) remains poorly understood. Microinjections of the retrograde tracer, fluorogold (FG), into the Sp5C, in rats, reveal that five hypothalamic nuclei project to the Sp5C: the paraventricular nucleus, the lateral hypothalamic area, the perifornical hypothalamic area, the A11 nucleus and the retrochiasmatic area. Descending hypothalamic projections to the Sp5C are bilateral, except those from the paraventricular nucleus which exhibit a clear ipsilateral predominance. Moreover, the density of retrogradely FG-labeled neurons in the hypothalamus varies according to the dorso-ventral localization of the Sp5C injection site. There are much more labeled neurons after injections into the ventrolateral part of the Sp5C (where ophthalmic afferents project) than after injections into its dorsomedial or intermediate parts (where mandibular and maxillary afferents, respectively, project). These results demonstrate that the organization of descending hypothalamic projections to the spinal dorsal horn and Sp5C are different. Whereas the former are ipsilateral, the latter are bilateral. Moreover, hypothalamic projections to the Sp5C display somatotopy, suggesting that these projections are preferentially involved in the processing of meningeal and cutaneous inputs from the ophthalmic branch of the trigeminal nerve in rats. Therefore, our results suggest that the control of trigeminal and spinal dorsal horn processing of nociceptive information by hypothalamic neurons is different and raise the question of the role of bilateral, rather than unilateral, hypothalamic control.

Figure 1. Localization of Fluorogold (FG) injection sites within the spinal trigeminal nucleus caudalis (Sp5C).

A: Digitized photomicrographs of bulbo-cervical coronal sections showing examples of FG injection sites into the mandibular (V3), maxillary (V2) and ophthalmic (V1) areas of Sp5C; Scale Bar = 300 µm. B: Schematic representation illustrating the rostrocaudal distribution of all FG injection sites (n = 21) – confined to V1 (n = 4; red) V2 (n = 8; blue) or V3 (n = 3; green) areas or more spread (n = 6; grey), from -1.2 to -2.4 mm relative to the most caudal tip of the subnucleus interpolaris⁄Sp5C transition region and their ventro-dorsal extent within the Sp5C.

Figure 2. Rostrocaudal distribution of retrogradely FG-labeled cells in the hypothalamus following FG injections in Sp5C.

A–E: Bar histograms of the number of FG neurons (mean ± SEM; n = 21) at the ipsilateral (filled bars) and contralateral (empty bars) side on coronal sections plotted as a function of the posterior distance (mm posterior to bregma) within the five hypothalamic nuclei: RCA: retrochiasmatic area (A), PVN: paraventricular nucleus (B), LH: lateral hypothalamus area (C), PFX: perifornical area (D) and A11 (E). Abscissa in black are for the maximum rostrocaudal extent (mm posterior to bregma) of each hypothalamic nucleus: RCA (-1.6 to -1.8 mm), PVN (-0.9 to -2.1 mm), LH (-1.6 to 4.8 mm), PFX (-2.8 to -3.6 mm) and A11 (-3.3 to -4.8 mm). A-E right: Pie charts of the bilateral distribution of FG neurons in each hypothalamic nuclei.

Figure 3. Rostrocaudal distribution of retrogradely FG-labeled neurons in the hypothalamus after a large FG injection into the Sp5C.

A: Semi-schematic drawings of coronal sections at different levels, from -1.8 to -4.5 mm posterior to bregma. Retrogradely FG-labeled neurons in the hypothalamus and adjacent areas are indicated by red dots. B: A representative injection site of FG in Sp5C. PVN: paraventricular nucleus, RCA: retrochiasmatic area, LH: lateral hypothalamus area, PFX: perifornical area. Scale bar: 300 µm.

Figure 4. Examples of retrogradely FG-labeled neurons in the PVN, RCA, LH, PFX and A11 hypothalamic nuclei following a large FG injection into the Sp5C.

A–D: Representative photomicrographs of coronal sections showing retrogradely FG-labeled neurons in the RCA (A), PVN (B), LH and PFX (C) and A11 (D) following a large FG microinjection into the Sp5C (the same as Figure 3). These photomicrographs show clearly that FG-labeled neurons are present on both ipsilateral (on the right in A, B C, and D) and contralateral sides (on the left in A, B C, and D). 3V: third Ventricle, mt: mammillothalamic tract, fx: fornix, Fr: Fasciculus retroflexus . Scale bar: 50 µm.

Figure 5. Example of retrogradely FG-labeled neurons in the ipsilateral PVN following a large FG microinjection into the Sp5C.

A–D: Representative photomicrographs of coronal sections showing the retrogradely labeled neurons (left) and the corresponding cresyl violet stainings (right) through the parvi- and magnocellular PVN at -1.6 (A), -1.8 (B), -1.9 (C) and -2.1 mm (D) posterior to bregma following large FG microinjection into the Sp5C (the same as Figure 3). pv: periventricular part; am: anterior magnocellular part; ap: anterior parvicellular part; lp: lateral parvicellular part; pm: posterior magnocellular part, mp: medial parvicellular part; dp: dorsal parvicellular part of PVN. Scale bar: 50 µm.

Figure 6. Distribution of FG neurons in both the ipisilateral and contralateral hypothalamus as a function of the injection site in Sp5C.

Bar histogram of the number of FG neurons (mean number of neurons per section ± SEM; n = 15) in the five hypothalamic nuclei (PVN, LH, PFX, A11 and RCA) as a function of the localization of the Sp5C injection site: mandibular (V3, n=3), maxillary (V2, n=8) and ophthalmic (V1, n=4) areas.