Afferent connections of the parabrachial nucleus in C57BL/6J mice

Abstract

Although the mouse is an experimental model with an increasing importance in various fields of neuroscience, the characteristics of its central gustatory pathways have not yet been well documented. Recent electrophysiological studies using the rat and hamster have revealed that taste processing in the brainstem gustatory relays is under the strong influence of inputs from forebrain gustatory structures. In the present study, we investigated the organization of afferent projections to the mouse parabrachial nucleus (PbN), which is located at a key site between the brainstem and gustatory, viscerosensory and autonomic centers in the forebrain. We made injections of the retrograde tracer fluorogold centered around the "waist" area of the PbN, whose neurons are known to be highly responsive to taste stimuli. Retrogradely labeled neurons were found in the infralimbic, dysgranular and agranular insular cortex as well as the claustrum; the bed nucleus of the stria terminalis and the substantia innominata; the central nucleus of the amygdala; the lateral and medial preoptic areas, the paraventricular, the dorsomedial, the ventromedial, the arcuate, and the lateral hypothalamic areas; the periaqueductal gray, the substantia nigra pars compacta, and the ventral tegmental area; the supratrigeminal nucleus, rostral and caudal nucleus of the solitary tract; the parvicellular intermediate and gigantocellular reticular nucleus; the caudal and interpolar divisions of the spinal trigeminal nucleus, dorsomedial spinal trigeminal nucleus, and the area postrema. Numbers of labeled neurons in the main components of the gustatory system including the insular cortex, bed nucleus of the stria terminalis, central nucleus of the amygdala, lateral hypothalamus, and rostral nucleus of the solitary tract were quantified. These results are basically consistent with those of the previous rat and hamster studies, but some species differences were found. Functional implications of these afferent inputs are discussed with an emphasis on their role in taste.

Figure 1

(A) Fluorescent images of a representative injection site centered around the taste-responsive (waist) region of the PbN from caudal (left) to rostral (right) level (mouse 3). Scale bar = 500 μm. (B) Schematic diagram indicating PbN subnuclei at a caudal-intermediate level (cf. Fig 3M). (C) The location of the injection site of FG in each mouse. ^Ppressure injection; ^Iiontophoretic injection.

Figure 2

Representative fluorescent images of labeled neurons in the major structures in the gustatory neural axis after an injection of FG into the PbN. All labeling is shown on the side ipsilateral to the injection site, except the PbN (E). A. Insular cortex. B. Bed nucleus of the stria terminalis. C. Central nucleus of the amygdala. D. Lateral hypothalamus. E. Parabrachial nucleus. F. Nucleus of the solitary tract. Scale bar = 500 μm.

Figure 3

Diagrams illustrating the distribution of labeled neurons in the brain (rostral to caudal, A–R) after FG injections into the PbN (in mouse 1). Each dot represents one neuron in a single 40-μm-thick section. Maps are adapted and modified from Paxinos and Franklin (2001). Scale bar = 1 mm

Figure 3

Diagrams illustrating the distribution of labeled neurons in the brain (rostral to caudal, A–R) after FG injections into the PbN (in mouse 1). Each dot represents one neuron in a single 40-μm-thick section. Maps are adapted and modified from Paxinos and Franklin (2001). Scale bar = 1 mm

Figure 3

Diagrams illustrating the distribution of labeled neurons in the brain (rostral to caudal, A–R) after FG injections into the PbN (in mouse 1). Each dot represents one neuron in a single 40-μm-thick section. Maps are adapted and modified from Paxinos and Franklin (2001). Scale bar = 1 mm

Figure 4

The mean number of retrogradely labeled neurons in the insular cortex (IC), rostral nucleus of the solitary tract (rNST), bed nucleus of the stria terminalis (BST), lateral hypothalamus (LH) and central nucleus of the amygdala (CeA). *p < 0.01.

Figure 5

(A and B) Fluorescent images of a representative elongate (A) and stellate (B) cells located in the rostral central subdivision of rNST. Arrowheads indicate cell body and arrows indicate dendrites. Scale bars = 20 μm. (C) Fluorescent image of rNST divided into the four subdivisions. Scale bar = 100 μm. (D) The mean number of retrogradely labeled neurons in the medial (M), rostral lateral (RL), ventral (V) and rostral central (RC) subdivisions of rNST. *p < 0.01.