projections from the parabrachial nucleus to the vestibular nuclei: potential substrates for...

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Brain Research 996 (2004) 126–137

Research report

Projections from the parabrachial nucleus to the vestibular nuclei:

potential substrates for autonomic and limbic influences on

vestibular responses

Carey D. Balaban*

Departments of Otolaryngology and Neurobiology, Eye and Ear Institute, University of Pittsburgh, 203 Lothrop Street, Pittsburgh, PA 15213, USA

Accepted 20 October 2003

Abstract

Previous anatomical studies in rabbits and rats have shown that the superior vestibular nucleus (SVN), medial vestibular nucleus (MVN)

and inferior vestibular nucleus (IVN) project to the parabrachial nucleus (PBN) and Kolliker–Fuse (KF) nucleus. Adult male albino rabbits

and Long–Evans rats received iontophoretic injections of biotinylated dextran amine, Phaseolus vulgaris leucoagglutinin, Fluoro-Gold or

tetramethylrhodamine dextran amine into either the vestibular nuclei or the PBN and KF nuclei. The results were similar in both rats and

rabbits. Injections of retrograde tracers into the vestibular nuclei produced retrogradely labeled neurons bilaterally in caudal third of the

medial, external medial, and external lateral PBN in both species, with more variable labeling in KF. Rats also had consistent bilateral

(predominantly contralateral) labeling in the ventrolateral PBN. The most prominent labeling was produced from injections that included the

SVN, with fewer labeled neurons observed from injections in the caudal MVN and the IVN. Anterograde transport of BDA from injections

into the PBN and KF nuclei of rabbits revealed prominent projections to the SVN, dorsal aspect of the rostral MVN, caudal MVN, pars beta

of the LVN and IVN. These connections appear to contain a component that is reciprocal to the vestibulo-parabrachial pathway and a non-

reciprocal component to regions connected with the vestibulocerebellum and vestibulo-motor reflex pathways. These connections support

the concept that a synthesis of autonomic, vestibular and limbic information is an integral property of pathways related to balance control in

both the brain stem and forebrain. It is suggested that these projections may contribute broadly to both performance tradeoffs in vestibular-

related pathways during variations in the behavioral context and affective state and the close association between anxiety and balance

function.

D 2003 Published by Elsevier B.V.

Theme: Motor systems and sensorimotor integration

Topic: Vestibular system

Keywords: Vestibular nucleus; Parabrachial nucleus; Anxiety; Balance

1. Introduction nucleus (LVN), and the caudal half of the medial vestibular

Recent anatomic and physiologic studies have demon-

strated direct connections between the vestibular nuclei and

brain stem regions that influence sympathetic and parasym-

pathetic outflow (review: [8]). These pathways originate

from a region within the vestibular nuclei, which includes

the dorsal aspect of the superior vestibular nucleus (SVN),

pars alpha (or caudoventral aspect) of the lateral vestibular

0006-8993/$ - see front matter D 2003 Published by Elsevier B.V.

doi:10.1016/j.brainres.2003.10.026

* Tel.: +1-412-647-2298; fax: +1-412-647-0108.

E-mail address: [email protected] (C.D. Balaban).

nucleus (MVN) and the inferior vestibular nucleus (IVN)

[3,8,40,41,46,47]. The caudal MVN and the IVN can

influence parasympathetic and sympathetic outflow, either

directly via projections to the brain stem or indirectly via

relays in the parabrachial nucleus (PBN). Projections from

the caudal MVN and IVN to the nucleus of the solitary tract

and the rostral ventrolateral medullary reticular formation

are likely to contribute to sympathetic components of

responses to body movements with respect to gravity, such

as blood pressure changes, heart rate changes and alter-

ations in muscle sympathetic nerve activation [10,32].

Projections from the same vestibular nuclear regions to

Table 1

Rabbits: summary of locations of injection sites and retrograde labeling loci

Case Site in VN Retrograde labeling in PBN and KF

95014 S, Lb, Lg mpb(bi), em(bi), el(bi)

95016 S, Lg, La mpb(bi), em(bi), el(bi), kf(bi)

98003 M, Lb mpb(bi), em(bi)

98006 S, Lb, M mpb(bi), em(bi), el(bi), kf (cont)

98007 Lb mpb(bi), em(bi), el(i), lat(i)

99013 S mpb(bi), em(bi)

99015 S mpb(bi), em(bi), el(i)

99019 S mpb(bi), em(bi), el(bi), kf(i)

20002 S mpb(bi), em(bi), el(i)

20004 S, Lb mpb(i), em(i), el(i)

Abbreviations: La, pars alpha of LVN; Lb, pars beta of LVN; Lg, pars

gamma of LVN; M, medial vestibular nucleus; S, superior vestibular

nucleus; mpb, medial parabrachial nucleus; em, external medial PBN; el,

external lateral PBN; lat, lateral PBN; KF, Kolliker–Fuse nucleus. The

laterality of retrograde labeling is indicated parenthetically for each nuclear

region: I, ipsilateral; cont, contralateral; bi, bilateral.

C.D. Balaban / Brain Research 996 (2004) 126–137 127

preganglionic parasympathetic neurons in the dorsal motor

vagal nucleus and nucleus ambiguous have been suggested

to contribute to alterations in gastrointestinal function and

Fig. 1. Charting of four BDA injection sites in the vestibular nuclei that produced r

halo region, are indicated by case numbers and are charted on a standard series o

reference brain (sectioned at 10 Am). The sections are arranged in series from a m

SVN (g, upper left). The nomenclature for the vestibular nuclei includes the SVN (

(or Deiters nucleus, Lg), and group y (y). The inferior cerebellar peduncle (ICP)

‘vasovagal’ features of vestibular disorders. A recently

described projection from the SVN, MVN and LVN (pars

alpha) to preganglionic parasympathetic neurons innervat-

ing the eye [6] may contribute to accommodative vergence

and papillary constriction during linear vestibulo-ocular

reflexes and to intraocular blood flow control during

postural shifts.

An ascending pathway also originates from the dorsal

aspect of the SVN, pars alpha of the LVN, and the caudal

half of the MVN and the IVN. This ascending projection

terminates densely in a caudal, vestibulo-recipient region of

the PBN [3,11,40], in a region that includes the medial,

external medial and external lateral parabrachial nuclei and

the Kolliker–Fuse (KF) nucleus. Neurons in this region

respond to whole body angular velocity and position (rela-

tive to gravity) in alert monkeys, indicating that these

neurons receive both semicircular canal- and otolith organ-

derived signals [11]. The presence of vestibular responses is

significant because the PBN forms a bi-directional link

between brain stem autonomic and telencephalic structures:

it has reciprocal connections with the amygdala, hypothal-

etrograde labeling in the PBN. The sites, defined as both the dense core and

f camera lucida drawings of transverse sections from a paraffin embedded

iddle level of the vestibular nuclei (a, lower right) to the rostral pole of the

S), MVN (M), LVN pars alpha (La), LVN pars beta (Lb), LVN pars gamma

is also indicated in this figure.

C.D. Balaban / Brain Research 996 (2004) 126–137128

amus and prefrontal cortex and descending projections to

autonomic output and spinal pathways. In particular, inter-

connections between the PBN, central amygdaloid and

infralimbic and prefrontal cortex are believed to be impor-

tant for the development and expression of conditioned

aversion and fear responses, and panic disorder [17,24,

34]. Hence, it has been proposed that these structures may

also be a substrate for the close clinical linkage between

Fig. 2. Camera lucida drawings of transverse sections through the ipsilateral ca

SVN. Sections are arranged from rostral (A) to caudal (B). Anterogradely label

parabrachial (m) and external medial parabrachial (em) nuclei and intercalated

also shown.

balance disorders and panic with agoraphobia [4,9,10]. This

study demonstrates the existence of a descending projection

from the PBN to the vestibular nuclei in the two species

with most extensively studied PBN connections, rats and

rabbits. This parabrachio-vestibular pathway may provide

integrated sensory and limbic contextual information to

vestibular nucleus neurons that influence autonomic and

affective responses.

udal third of the parabrachial region after an injection of BDA into the

ed axons and retrogradely labeled cell bodies are illustrated in the medial

among fibers of the SCP. The borders of the ventrolateral PBN (vl) are


2. Materials and methods

2.1. Surgical procedures

The experimental protocols were reviewed and approved

by the University of Pittsburgh Institutional Animal Care

and Use Committee.

2.1.1. Rabbits

In addition to a series of animals produced for this study,

this report includes data from animals utilized in previous

studies of the organization of vestibular nuclear output

pathways [3,8]. New Zealand white rabbits (2.7–4.4 kg

body weight) were premedicated with atropine methylni-

trate (0.06–0.2 mg, s.c.) and anesthetized with sodium

pentobarbital (40 mg/kg, 4 ml/kg total volume, i.v.). The

rabbits were given 13% mannitol (2.3–3.0 ml/kg, i.v.) to

increase the exposure of the floor of the fourth ventricle by

osmotically shrinking the cerebellum and brain stem. Sur-

Fig. 3. Photomicrographs of retrogradely labeled cells in the PBN. (A) Large mu

injection into the ipsilateral SVN. (B) Medium-sized multipolar neuron in the rabbi

SVN. (C) Medium-sized neuron in the rabbit medial PBN, labeled retrogradely afte

rabbit medial PBN labeled retrogradely from a BDA injection into the ipsilateral S

retrogradely from the ipsilateral vestibular nuclei. (F) A cluster of retrogradely la

injection of Fluoro-Gold. Calibration bar: 100 Am in A–D, 200 Am in E–F.

gical procedures were performed under aseptic conditions.

The head was fixed with zygoma clamps in a stereotaxic

apparatus (Narishige Instruments, Tokyo, Japan) with the

head tilted 45j nose-down. Lidocaine (1–2%, s.c.) was

injected along the incision line. The cervicoauricularis

muscle aponeurosis was divided and the underlying

muscles were retracted to expose the occipital bone, atlas

and atlanto-occipital membrane. The atlanto-occipital mem-

brane was removed and the foramen magnum was enlarged

with rongeurs medulla to expose the medulla and posterior

aspect of the cerebellum.

Using the obex and the floor of the fourth ventricle as

landmarks, rabbits were given an iontophoretic injection of

Phaseolus vulgaris leucoagglutinin (PHAL, Vector Labo-

ratories, 2.5% solution in sodium phosphate buffered saline

(PBS), pH 8.0) and/or biotinylated dextran amine (BDA,

10,000 MW, Molecular Probes, 7–10% solution in PBS,

pH 7.0) into the vestibular nuclei and/or nucleus prepositus

hypoglossi (10–15 Am tip diameter, 4 AA positive current,

ltipolar neuron in the rabbit medial PBN, labeled retrogradely after a BDA

t medial PBN, labeled retrogradely after a BDA injection into the ipsilateral

r a BDA injection into the contralateral SVN. (D) Medium-sized neuron in

VN. (E) Fluoro-Gold labeled neurons in the rat external medial PBN labeled

beled neurons in the rat external PBN contralateral to a vestibular nuclear


10 min). This communication is restricted to data from

sites confined to the vestibular nuclei and nucleus prepos-

itus hypoglossi, with no evidence of spread to the cere-

bellum, nucleus tractus solitarius or the dorsal medullary

reticular formation.

After completion of the injections, the craniotomy was

packed with Gelfoam or Surgicel and the soft tissues were

sutured in layers. Postsurgical analgesia was provided with a

single injection of ketaprofen (2 mg/kg, s.c.). Penicillin

Fig. 4. Chartings of retrogradely labeled neurons in the rabbit parabrachial nuclea

injection sites are shown in Fig. 1. The labeling is charted on a series of transvers

caudal third of the parabrachial nuclear region. Note bilateral labeling in the med

parabrachial (el) nuclei. The locations of the lateral (l) and ventrolateral (vl) parabra

SCP are also shown.

(80,000–100,000 U/day, i.m.) was administered during the

survival period if a break in sterility was suspected during

the surgery.

2.1.2. Rats

Adult male Long–Evans rats were anesthetized with

sodium pentobarbital (25 mg/kg, i.p.) combined with

either Innovar Vet (0.02 mg/kg, i.m.) or ketamine (75

mg/kg, i.m.). Two surgical approaches were used: (1) a

r complex after injections of BDA in the vestibular nuclei. The respective

e sections from caudal (lower section) to rostral (upper section) through the

ial parabrachial (m), external medial parabrachial (em) and external lateral

chial and KF nuclei, the mesencephalic trigeminal nucleus (5m), LC and the

C.D. Balaban / Brain Researc

stereotaxically guided injection through a burr hole in the

dorsal surface of the cranium or (2) a direct approach

through an occipital craniotomy. Micropipettes (20–40

Am O.D.) filled with either 10% Fluoro-Gold or 10%

tetramethylrhodamine dextran amine in PBS were intro-

duced to make iontophoretic injections bilaterally (4–5

AA DC, tip positive, 7–10 min). The craniotomy was

then packed lightly with Gelfoam and the skin was

sutured.

Fig. 5. Camera lucida drawing of anterograde transport of BDA from the caudal

vestibular nuclei. The sections are arrayed from caudal (lower section) to rostral (u

(S), MVN (M), LVN pars alpha (La), LVN pars beta (Lb), and IVN (I). Nucleu

indicated in this figure.

2.2. Histological, immunohistochemical and histochemical

procedures

2.2.1. Rabbits

After survival times ranging from 4 to 10 days, the

rabbits were euthanized with a pentobarbital overdose and

perfused transcardially with PBS followed by the parafor-

maldehyde–lysine–sodium metaperiodate (PLP) fixative of

McLean and Nakane [37]. The brains were post-fixed for

h 996 (2004) 126–137 131

aspect of the external medial and external lateral parabrachial nuclei to the

pper section). The nomenclature for the vestibular nuclei includes the SVN

s prepositus hypoglossi (PH) and nucleus tractus solitarius (NTS) are also


18–24 h at 4 jC in a solution of 4% paraformaldehyde–

30% sucrose in 50 mM phosphate buffer and cryoprotected

in a 30% sucrose–50 mM phosphate buffer solution for 2–3

days. Frozen sections (40 Am, transverse plane) were cut on

a sliding microtome and sets of every fourth to sixth section

were placed in 50 mM phosphate buffer (pH 7.2–7.4). For

longer term storage, sections were maintained at � 20 jC in

a solution of 30% sucrose–30% ethylene glycol solution in

50 mM phosphate buffer.

Axonally transported PHA-L was visualized immunohis-

tochemically by standard published methods [3]. For visu-

alizing BDA transport, free-floating 40 Am frozen sections

were rinsed successively in distilled water (3� 10 min),

0.9% H2O2 and distilled water to suppress endogenous

peroxidase activity, followed by a preincubation for 2 h at

room temperature in 0.5% Triton X-100 in PBS. After a

rinse in PBS, the sections were incubated for 1 h in

VectastainR ABC peroxidase (avidin–biotin conjugate

horseradish peroxidase) reagent (Vector Laboratories),

rinsed in buffer and reacted for visualizing sites of perox-

idase activity with either a nickel-enhanced DAB or a

standard DAB (2 mg DAB, 8.3 Al H2O2 (ACS reagent

grade, 30.8%, Sigma Chemical Co.) in 10 ml 500 mM

sodium acetate buffer, pH 6.0) chromagen. Sections were

mounted on subbed slides, dehydrated through a graded

alcohol series, cleared in xylene and coverslipped with

either permount or non-fluorescent DPX (Fluka).

2.2.2. Rats

After survival times ranging from 2 to 3 days, rats were

euthanized by sodium pentobarbital overdose) and perfused

transcardially with PBS followed by a paraformaldehyde–

lysine–periodate (PLP) fixative solution [37]. The brains

Fig. 6. Photomicrographs of rabbit parabrachiovestibular axons labeled anterogra

terminal varicosities and en passage varicosities in the SVN. Panel C shows an ex

shows a larger caliber fiber parabrachiovestibular fiber and terminals in the rostra

were removed from the cranium and cryoprotected for at

least 2 days at 4 jC in a 50 mM phosphate-buffered, 4%

paraformaldehyde solution containing 30% sucrose. The

brains were then sectioned in the transverse plane at a

thickness of 40 Am on a sliding microtome equipped with

a dry ice freezing stage. Sections were mounted on gelatin-

chromalum subbed slides, dried, cleared in xylene and

coverslipped with DPX.

Consistent with NIH requirements, the procedures in

these rat and rabbit studies have been reviewed and ap-

proved by the University of Pittsburgh Institutional Animal

Care and Utilization Committee.

3. Results

Ten rabbits had BDA injections confined to the rostral

half of the vestibular nuclei, with no evidence of spread into

the cerebellar white matter, superior cerebellar peduncle

(SCP), reticular formation, locus coeruleus (LC) or the

caudal pole of the parabrachial nuclear complex. The

injection sites involved the SVN, MVN, and both pars beta

and pars gamma of the LVN. The sites and findings are

summarized in Table 1; four representative sites are shown

in Fig. 1. Since all ten cases displayed the same basic pattern

of anterograde and retrograde labeling in the PBN complex,

four representative cases have been illustrated to show the

range of variability in retrograde labeling between animals

(Figs. 1, 2 and 4).

Fig. 2 shows a camera lucida drawing of the ipsilateral

PBN after an injection of BDA in the SVN. Anterogradely

labeled varicose axons and terminal arborizations were

located caudally within the medial parabrachial, external

dely with BDA. Panels A and B show larger caliber fibers that contributed

ample of a smaller caliber parabrachiovestibular fiber in the SVN. Panel D

l MVN (rMVN). Calibration bar: 50 Am.

Table 2

Rats: summary of locations of injection sites and retrograde labeling loci

Case Site in VN Retrograde labeling in PBN and KF

329A S, La, Lg, I,

y (grazed Lb, M)

m (bi), em (I), vl (cont)

329C La, I m (bi), em (bi), el (bi), vl (bi)

607B Mc, I m (i), em (i), el (i), vl (cont), kf(i)

711B Mc, I m (i), vl (i)

712B M, Lb (grazed La) m (bi), em (bi), el (bi), vl (bi)

712C M, La, Lb, Lg, I m (i), em (i), el (i), vl (i), kf(i)

712D S, M, La, Lg m (i), em (i), el (i), vl (cont), kf(i)

396113 Mc, La, I m (bi), em (bi), el (bi), vl (bi), kf(bi)

Abbreviations: I, inferior vestibular nucleus; La, pars alpha of LVN; Lb,

pars beta of LVN; Lg, pars gamma of LVN; M, medial vestibular nucleus;

S, superior vestibular nucleus; y, group y; m, medial PBN; em, external

medial PBN; el, external lateral PBN; vl, ventrolateral PBN; KF, Kolliker–

Fuse nucleus. The laterality of retrograde labeling is indicated parentheti-

cally for each nuclear region: I, ipsilateral; cont, contralateral; bi, bilateral.


medial parabrachial, external lateral parabrachial and KF

nuclei. Retrogradely labeled neurons were also interspersed

within this terminal region, ranging from heavily labeled

multipolar neurons (Fig. 3a and b) to more lightly labeled

cuboid (Fig. 3c) and fusiform (Fig. 3d) cells. The labeled

somata confined almost exclusively within the medial para-

brachial, external medial parabrachial and external lateral

parabrachial nuclei (Fig. 4). These labeled neurons were

present bilaterally, with more labeled cells ipsilateral than

contralateral to the iontophoretic injection site. Significantly,

the retrogradely labeled neurons were confined to the region

of the parabrachial nuclear complex that contained ante-

rogradely labeled fibers in each animal.

3.1. Anterograde and retrograde transport from PBN

Two rabbits had iontophoretic injections of BDA con-

fined within the parabrachial nuclear complex. These ani-

Fig. 7. Charting of injection sites in the rat vestibular nuclei that produced retrog

sections are arranged from caudal (lower drawing) to rostral (upper drawing). Ab

mals displayed both anterogradely labeled axons and

retrogradely labeled somata in the vestibular nuclei. The

distribution of retrogradely labeled neurons reproduced the

pattern reported previously [3].

The pattern of anterograde transport to the vestibular

nuclei is summarized in a series of camera lucida drawings

from an injection centered in the external medial and

external lateral subnuclei of the PBN in Fig. 5. Anterog-

radely labeled axons were traced caudally from the injection

site to the dorsal border and the dorsolateral margin of the

rostral pole of the SVN. The fibers along the dorsal border

continued caudally, forming both en passage varicosities

and terminal varicosities in the medial aspect of the SVN

and in the rostral MVN. Coarse axons formed both terminal

varicosities and varicosities en passage in the neuropil and

near somata in superior (Fig. 6a and b) and rostral medial

(Fig. 6d) vestibular nuclei. Finer caliber fibers formed a

more extensive plexus in both regions (Fig. 6c). Some fibers

continued caudally from the dorsolateral margin of the SVN

to form en passage and terminal varicosities in the lateral

vestibular (pars alpha and beta), caudal medial vestibular

and inferior vestibular nuclei. The distal branches of these

caudally projecting fibers were almost exclusively of fine

caliber.

A few anterogradely labeled axons were also observed in

the contralateral vestibular nuclei. These fibers originated

from large caliber axons that entered the ipsilateral SCP and

traveled caudally, dorsally and medially to enter the cere-

bellar white matter. These fibers followed a trans-cerebellar

course similar to crossed parabrachio-PBN projections in

the subfastigial bundle in rats [38]. The axons traveled

medially, decussated in the white matter rostral to the

fastigial nucleus and entered the medial aspect of the

contralateral SCP. The fibers then turned caudally and

contributed a sparse projection to the contralateral vestibular

rade labeling in the caudal aspect of the parabrachial nuclear region. The

breviations are identical to Fig. 1.


nuclei, following the same pattern as the ipsilateral projec-

tions (Fig. 5).

3.1.1. Rats

This analysis is based upon the results from eight rats

with Fluoro-Gold injections confined to the vestibular nuclei

(Table 2). Retrogradely labeled neurons were observed in

the medial (m), external medial (em), ventral lateral (vl), and

external lateral (el) parabrachial subnuclei and the KF

nucleus. Examples of labeled neurons in the external medial

and external lateral parabrachial nuclei are shown in Fig. 3e

and f. There were no discernable differences in the distri-

bution of retrogradely labeled neurons associated with

Fig. 8. Retrogradely-labeled parabrachiovestibular somata are charted on a series o

spaced sections through the caudal half of the PBN. The lower section in each ser

� 0.50 and the upper section is at approximately level ear bar � 0.20. The abbre

differences in the locations of the injection sites. All cases

displayed retrogradely labeled neurons ipsilaterally in the

medial parabrachial nuclei; half of the cases also showed

contralateral labeling. Four representative injection sites are

shown in Fig. 7 and the distributions of retrogradely labeled

neurons in the PBN are charted in Fig. 8. The retrogradely

labeled neurons were all distributed in the caudal third of the

parabrachial nuclear complex, corresponding to the region

that receives vestibular nucleus input [40]. Seven of the

eight cases had retrogradely labeled neurons in the ipsilat-

eral external medial and external lateral parabrachial nuclei,

with three cases also showing bilateral labeling. The later-

ality of retrograde labeling in the ventral lateral PBN

f transverse sections through the rat PBN. Each case is charted on a series of

ies is at approximately ear bar � 0.80, the middle section is at level ear bar

viations are identical to Fig. 4.


showed more inter-animal variability: it was bilateral in

three cases, strictly ipsilateral in two cases and strictly

contralateral in three cases. A few retrogradely labeled cells

were present in the caudal aspect of KF in only four rats

(three strictly ipsilateral to the injection and one bilaterally).

4. Discussion

This study has demonstrated the existence of descending

projections from the caudal aspect of the PBN (and, to a

lesser extent, the KF nucleus) to the vestibular nuclei.

Retrograde tracing data indicated that these projections

originate bilaterally from the medial, external medial and

external lateral parabrachial subnuclei in both rats and

rabbits, with a less prominent contribution from the caudal

aspect of the KF nucleus. In rats, there was also a contribu-

tion from the caudal aspect of the ventrolateral parabrachial

subnucleus. The significance of this apparent species differ-

ence is unclear. All of these regions of the parabrachial

nuclear complex receive afferents from the vestibular nuclei

in the respective species [3]; corresponding PBN regions of

alert primates respond to whole body rotation, displaying

sensitivities to rotational velocity and static position that are

consistent with dynamic response properties of vestibular

nucleus neurons [5,11]. Further, these PBN regions all

contain neurons that project to the central amygdaloid

nucleus in the respective species; the origin of projections

to the central amygdaloid nucleus includes the ventrolateral

parabrachial subnucleus in rats [23,30] but not rabbits [31].

Hence, despite the species differences in projections from the

ventrolateral subnucleus, parabrachiovestibular projections

appear to originate from groups of neurons with similar

connectivity in both rats and rabbits.

The projections from the caudal parabrachial and KF

nuclei to the SVN, rostral MVN, IVN and caudal MVN are

consistent with reciprocal connections between a component

of the parabrachiovestibular pathway and vestibular nucleus

regions that project to the PBN. This type of PBN-brainstem

reciprocal connectivity was reported previously by Herbert

et al. [29]. They found reciprocal connections between the

‘respiratory part’ of nucleus of the solitary tract (dorsal

respiratory group) and the KF nucleus and between the

rostral ventrolateral reticular nucleus, periambiguus region

and parvicellular reticular area and the parabrachial and KF

nuclei. These brainstem connections share the property of

involvement in sensorimotor integration, either for automat-

ic movements (e.g. respiration) or autonomic control. The

ascending vestibulo-autonomic path to mediate the auto-

matic panic-like aspects associated with falling (or a per-

ception of falling) and the clinical linkage between balance

disorders and panic disorder with agoraphobia [4,9]. Hence,

a reciprocal pattern of organization of vestibuloparabrachial

and parabrachiovestibular connections is consistent with

parabrachial connections with other brainstem sensorimotor

integration pathways for relatively automatic responses.

The projections to the SVN, rostral MVN, IVN and

caudal MVN are consistent with reciprocity between a

component of the parabrachiovestibular pathway and vesti-

buloparabrachial connections [3,40]. These connections are

likely to influence information processing in the ascending

vestibulo-autonomic pathway. However, it is likely that

these connections function as more than a reciprocal pro-

cessing loop in the ascending vestibulo-autonomic path.

Parabrachiovestibular projections to the IVN and caudal

MVN also have the potential to influence (1) descending

vestibulo-autonomic projections to the solitary nucleus,

nucleus ambiguus/parambiguus, rostral ventrolateral medul-

la and lateral medullar tegmentum [8,40] and (2) vestibulo-

spinal motor projections to abdominal musculature [14,48].

These connections of the vestibular-related regions of the

PBN are consistent with the view that they are involved in

coordinating somatic, autonomic and affective responses to

linear and angular acceleration challenges to factors as

diverse as blood distribution, respiratory movements and

control of body segments (review: [10]).

The primary sites of origin of parabrachiovestibular

connections (the caudal aspect of the medial, external

medial, external lateral and ventral lateral parabrachial

subnuclei) have three common features: they receive affer-

ents from the vestibular nuclei [3,40], paratrigeminal nucle-

us [22,42] and the infralimbic and insular cortex [39].

Because the paratrigeminal nucleus receives chemoceptive,

mechanoreceptive and nociceptive afferents from the oral

cavity, nasal cavity and pharynx [42], it seems reasonable to

suggest that a global characteristic of parabrachiovestibular

projection regions may be integration of information about

head motion, oropharyngeal (i.e. head-referenced) visceral

and somatic sensation, and descending signals from ‘limbic’

cortex. Nested within the termination region of these com-

mon input sources, though, are smaller regions that were

classified by Herbert et al. [29] as sites showing predomi-

nant gustatory or respiratory patterns of connectivity.

The parabrachiovestibular projections from ‘gustatory’

PBN regions may constitute a head-centered representation

for information processing within vestibulo-autonomic path-

ways. The overlap between the vestibulo-recipient and

gustatory PBN regions encompasses the caudal aspects of

the medial parabrachial and external medial parabrachial

subnuclei. Anatomical data indicate that this region receives

convergent vestibular information and oropharyngeal che-

moceptive, mechanoreceptive and nociceptive inputs from

both the rostral pole of the nucleus of the solitary tract [29]

and the paratrigeminal nucleus [42]. Infralimbic and insular

cortex also contribute projections to these parabrachial

subnuclei [39]. Since the gustatory region of the PBN

mediates the development of conditioned taste aversions

[25–27,44], it is possible that this vestibulo-oropharyngeal

reciprocal circuit may contribute to both the development of

conditioned taste aversions with environments evoking

motion sickness and, subsequently, the detection of ade-

quate stimuli to trigger the conditioned response.


By contrast, the participation of respiratory regions of the

parabrachial complex in reciprocal connections with the

vestibular nuclei may constitute a torso and airway-related

representation within vestibulo-autonomic pathways. These

regions of the parabrachial complex include the external

lateral parabrachial, ventral lateral parabrachial and caudal

KF nuclei. They receive projections from the ventrolateral

nucleus of the solitary tract (or ‘‘dorsal respiratory group’’)

[29], the medial nucleus of the solitary tract (a visceral

sensory relay region) [29] and the amygdala [29,39]. The

ventral lateral PBN also receives projections from the

infralimbic and insular cortex [39]. A convergence of

respiratory-related, visceral sensory and vestibular informa-

tion in this reciprocal circuit would be a logical contributor

to phenomena such as the entrainment of the respiratory

cycle with otolith organ stimulation during off-vertical axis

rotation [32] or the entrainment of respiratory movements

with locomotor activity [15]. In a more general sense,

though, muscles involved in ventilation are active during

both postural adjustments and activities as diverse as defe-

cation, deglutition, vocalization, and emesis [10]. Therefore,

their patterns of activation reflect dynamic trade-offs be-

tween voluntary and automatic task demands; for example,

the trading off a less urgent need to take a breath for a more

urgent desire to speak or to generate a postural response to a

slip on an icy street. The multimodal information processing

in reciprocal vestibuloparabrachial connections is a candi-

date mechanism for matching the appropriate responses to a

complex behavior context.

The parabrachiovestibular projection also extends to a

region of the vestibular nuclei beyond the location of

vestibuloparabrachial cells and their dendritic fields. This

is particularly evident for projections to pars alpha and pars

beta of the LVN, which are not involved in vestibulopar-

abrachial pathways. Further, much of the projection field in

the inferior and caudal medial vestibular nuclei lies outside

the region of dendrites and somata of cells that project to

either the PBN or the solitary nucleus [3,8]. These terminal

regions, though, are likely to be associated with flocculo-

nodular lobe terminal regions (e.g. [2,28]), related vesti-

bulo-ocular reflex pathways (e.g. [1,2,7]), the origins of

vestibular projections the anteromedian nucleus [6] and

sites of origin of secondary vestibulo-flocculonodular lobe

pathways [12]. The mechanisms are currently unknown for

well-documented phenomena such as modulation of vesti-

bulo-ocular reflex performance as a function of ‘arousal’

[19–21], ‘mental set’ [18] and a subject’s frame of refer-

ence [13], context-dependence of vestibulo-ocular reflex

adaptation [16] and [33,43,45] context-related alterations of

velocity storage characteristics of vestibulo-ocular and

optokinetic responses [35,36]. As in the case the reciprocal

component of vestibulo-parabrachial connections, it is

suggested that these projections may contribute broadly

to performance tradeoffs in vestibular-related pathways

during variations in the behavioral context and affective

state.

Acknowledgements

The author wishes to thank Maria Freilino, Gloria

Limetti and Jean Betsch for expert surgical and histological

assistance. These studies were supported by R01 DC00739

and P01 DC03417. A Core Grant for Vision Research

(EY08098) provided technical support for maintenance of

critical laboratory equipment.

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