Differential projections to the intralaminar and gustatory thalamus from the parabrachial area: A PHA-L study in the rat

The organization of projections from the parabrachial (PB) area to the vent ral posterior parvicellular (VPpc) "gustatory" and intralaminar nuclei of t he thalamus was studied in the rat by using microinjections of Phaseolus vu lgaris leucoagglutinin (PHA-L), into subregions of the PB area. The present study is a follow-up of three former studies (Bernard et al. [1993] J. Com p. Neurol. 329:201-229; Alden et al. [1994] J. Comp. Neurol. 341:289-314; B ester et al. [1997a] J. Comp. Neurol. 383:245-281) that examined PB project ions onto the amygdala, the bed nucleus of the stria terminalis, and the hy pothalamus. Our data showed that (1) the region centered in the internal la teral PB subnucleus projects densely with a bilateral and symmetric pattern to the caudal portion of the paracentral and, to a lesser extent, to the a djacent portion of the central and parafascicular medial thalamic nuclei; ( 2) the mesencephalic PB region centered in the ventral lateral subnucleus a nd scattered neurons in the subjacent brachium conjunctivum project primari ly, although diffusely, to the central medial thalamic nucleus. The third r egion includes two subgroups: (3a) the medial subgroup, including the media l, the waist area, and the ventral lateral subnuclei of the pontine PB area , projects bilaterally but with a weak ipsilateral predominance to the VPpc , terminals bearing large varicosities. Additionally, a diffuse projection with small varicosities spreads in the area between the two VPpc nuclei and the central medial nucleus. (3b) The lateral subgroup, centered in the ext ernal medial subnucleus, projects with a contralateral predominance in the periphery of the VPpc nuclei, most terminals being located around the dorso medial tip. It is suggested that the PB projections to the intralaminar nucleus could b e involved in cortical limbic arousal processing in relation with nocicepti ve, (somatic, visceral, and intraoral) and gustatory aversive stimuli. The projection with large varicosities inside the VPpc could process gustatory discrimination. (C) 1999 Wiley-Liss, Inc.