Forebrain projections of the rostral nucleus raphe magnus shown by iontophoretic application of choleratoxin b in rats.

The nucleus raphe magnus belongs to the thermoafferent system. Following iontophoretic choleratoxin b injections in its rostral part, a substantial to large number of anterogradely labeled varicose fibres were observed in the medial and lateral preoptic areas, the bed nucleus, the substantia innominata, the ventral pallidum, the median preoptic nucleus, the paraventricular hypothalamic nucleus, the central amygdaloid nucleus and the lateral and dorsal hypothalamic areas. A small to moderate number were seen in the septal nuclei, the diagonal band, the magnocellular preoptic nucleus, the anterior hypothalamic area and the paraventricular and intralaminar thalamic nuclei. After choleratoxin b injections in the preoptic, dorsal and lateral hypothalamic areas, a substantial number of retrogradely labeled serotonin immunonegative neurones were specifically found in the rostral nucleus raphe magnus. Thus, non-serotonergic rostral nucleus raphe magnus cells might directly modulate hypothalamic thermointegrative neurones.

Thermoregulatory noradrenergic and serotonergic pathways to hypothalamic units.

1. In guinea-pigs hypothalamic single units were extracellularly tested for their response to thermal stimulation of the skin and to electrical stimulation of two different pontine areas, the nucleus raphé magnus and the dorsomedial reticular formation. Furthermore, thermoregulatory control actions were measured during the stimulations.2. Electrical stimulation of those reticular formation areas containing noradrenaline cells caused an increase of oxygen uptake, electrical muscle activity and body temperature, while stimulation of the nucleus raphé magnus, known to contain serotonin cells, brought about inhibition or had no effect.3. The recorded units could be subdivided into three groups. Cell type a. Neurones on the boundary of preoptic and anterior hypothalamic regions which increased their firing rate when the skin was cooled and decreased it when the nucleus raphé magnus was stimulated. Cell type b. Neurones in the anterior hypothalamus which did not respond to brain-stem stimulation. Cell type c. More posterior neurones which increased their firing rate when the skin was warmed or when the nucleus raphé magnus was stimulated and decreased their firing rate when the reticular formation was stimulated.4. Cell type a seems to represent interneurones which are connected to the ascending serotonergic thermoregulatory pathway. As for cell type c, it is inferred that it could represent interneurones which control the threshold for shivering and non-shivering thermogenesis.