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PubMed1985

Distribution of primary afferent fibres in the cochlear nuclei. A silver and horseradish peroxidase (HRP) study.

Horseradish peroxidase, when injected intracochlearly, is transported transganglionically to the brain stem cochlear nuclei, thus providing an excellent method for tracing the central projection of the spiral ganglion neurons. Silver impregnation using the Cajal-de Castro method, which stains axons even when inside the bone, was used as a reference technique. The combination of both procedures led to the following conclusions. Primary cochlear afferents are found only in the ventral zone of the dorsal cochlear nucleus. In this area they cover the deep and fusiform cell layers. The molecular layer shows no HRP label. The higher concentration of primary cochlear afferents in the ventral cochlear nucleus appears in its central zone; wide areas in this nucleus are not labelled at all. A thin bundle of primary cochlear afferents runs parallel to, and beneath, the granular region.

Afferent Pathways↗
PubMed1987

Inhibitory connections between AVCN and DCN: evidence from lidocaine injection in AVCN.

Principal cells of the dorsal cochlear nucleus (DCN) receive a variety of inhibitory influences, including some that are associated with the anteroventral cochlear nucleus (AVCN). Electrical stimulation in the caudal part of the AVCN produces long-lasting inhibition of DCN units [(1973) Exp. Brain Res. 17, 428-442], including unit type IV, which is associated with principal cells. This inhibition may be mediated either by orthodromic activation of AVCN axons that terminate in DCN or by antidromic stimulation of DCN inhibitory interneurons whose axons send collaterals to the AVCN. These two possibilities cannot be distinguished with electrical stimulation alone. In this study, microinjections of lidocaine were made in the AVCN. These injections should block AVCN to DCN axons without directly affecting the activity, within the DCN, of DCN interneurons. Two effects of lidocaine injections were observed. In some cases, all activity of DCN cells was abolished by lidocaine. These cases involved either spread of lidocaine to the recording site or into the auditory nerve. In other cases, the inhibitory responses of DCN type IV units were weakened by lidocaine without changing the units' thresholds or spontaneous activity. The latter result demonstrates that there are inhibitory pathways originating in AVCN or passing through AVCN and terminating in the DCN.

Acoustic Stimulation↗
PubMed1995

Fitting the Nucleus-22 cochlear implant for Spanish speakers.

This work aimed to study the speech comprehension of 13 native Spanish-speaking patients implanted with the Nucleus-22 system. Experiments were performed under two conditions. First, tests of isolated vowel discrimination, bisyllable word recognition in a consonant-vowel-consonant context, and fluency speech comprehension were conducted with the speech processors (SPs) fitted with the frequency boundary distribution as specified by the diagnostic and programming system. This distribution was called default frequency boundary. For the second condition, the SP of each patient was refitted using a procedure based on changes in the frequency-to-electrode mapping, called modified frequency boundaries (MFB), taking into account the formant pattern of the Spanish Rioplatense vowel system. These changes intended to improve the vowel perception and therefore the fluency of speech, as the result of a more adequate placeformant distribution. Patients were retested, and as a consequence of the MFB, improvements in the results of the tests were observed for most of the patients. Counterbalance mode tests were conducted to avoid the influence of training effects in the results. This study suggested that speech comprehension could be improved by adjusting this SP according to the patients' specific language.

Adult↗
PubMed1991

Afferents to the medial nucleus of the trapezoid body and their collateral projections.

Cells and axons that supply direct afferent input to the medial nucleus of the trapezoid body are described. Afferents were intracellularly labeled in brainstem tissue slices of two rodent and two bat species. The main afferents are calyciferous axons from globular bushy cells of the ventral cochlear nucleus. Calyciferous axons were highly consistent across species, projecting directly from the cochlear nucleus, across the midline in the trapezoid body, to the contralateral medial nucleus of the trapezoid body. Within the target nucleus, a typical axon turned sharply away from horizontal to form a large ending, the calyx of Held, around the soma of a single principal cell. Three groups of calyciferous axons were classified based on the path taken from bend to calyx. In subjects younger than four weeks, single axons often formed two calyces, each on a different cell. These calyx pairs were often found on adjacent or vertically aligned cells. In older animals, calyx pairs were more closely aligned, but fewer double calyx axons were seen. A secondary focus of this study was the system of thin collateral branches that characterizes calyciferous axons in all species. The projection patterns of these collaterals suggest that calyciferous axons may provide ascending input to periolivary cell groups with descending projections. In addition to calyciferous afferents, labeled cells that provide input to the medial nucleus of the trapezoid body from adjacent periolivary cell groups are described. Also described is a type of afferent that descends from the level of the lateral lemniscus to the medial nucleus of the trapezoid body.

Afferent Pathways↗
PubMed1989

Auditory brainstem prosthesis: biocompatibility of stimulation.

As part of a program to develop a central auditory system prosthesis, we have examined the reaction of tissue to different levels of stimulation from electrodes chronically implanted in the cochlear nucleus of the guinea pig. Tolerance and histologic reaction to 20 hours of periodic electrical stimulation of the cochlear nucleus were analyzed. Intraoperative and postoperative electrically evoked middle latency responses were monitored during stimulus trials. The threshold necessary to generate the middle latency responses was frequently below 50 microA. In the animals that received 50 and 100 microA of biphasic charge-balanced stimulation (corresponding to approximately 200 and 400 microC/cm2 phase), adverse tissue reaction was minimal, and glial proliferation along the electrode tract never exceeded 25 microns in width. Stimulation intensities of 150 and 200 microA (approximately 600 and 800 microC/cm2 phase) produced significant tissue response at the site of the electrode terminus, with necrosis, cell loss, and reactive cells present. Therefore neuronal damage was observed to occur at an intensity far greater than that required for eliciting an electrophysiologic response.

Animals↗
PubMed1997

Localization of AMPA-selective glutamate receptors in the auditory brainstem of the barn owl.

AMPA receptor subunit-specific antibodies were used to determine if the distribution of excitatory amino acid receptors in the owl's auditory brainstem and midbrain nuclei reflected specializations for temporal processing. Each auditory nucleus displays characteristic levels of immunostaining for the AMPA receptor subunits GluR1-4, with high levels of the subtypes which exhibit rapid desensitization (GluR4 and 2/3). In the auditory brainstem, levels of GluR2/3 and GluR4 were very high in the cochlear nucleus magnocellularis and the nucleus laminaris. The different cell types of the cochlear nucleus angularis and the superior olive were characterized by heterogeneous GluR2/3 and 4 immunostaining. GluR1 levels were very low or undetectable. In the lemniscal nuclei, most neurons contained low levels of GluR1, and dense GluR2/3 and GluR4 immunoreactivity, with high levels of GluR4 in the dendrites. Levels of GluR4 were higher in the anterior portion of the ventral nucleus of the lateral lemniscus. The divisions of the inferior colliculus could be distinguished on the basis of GluR1-4 immunoreactivity, with high levels of GluR4 and moderate levels of GluR1 in the external nucleus. No major differences were observed between the pathways for encoding time and sound level cues.

Animals↗
PubMed1985

Surface topography of the cochlear nuclei in humans: two- and three-dimensional analysis.

We used three-dimensional reconstruction to study the cochlear nuclear complex (CN) in postmortem adult brains. Resulting data show that the largest part of the CN surface, particularly the dorsal cochlear nucleus (DCN), is fully within the lateral recess of the fourth ventricle. The surface of another subdivision, the ventral cochlear nucleus (VCN), is also almost entirely within the recess, except for a narrow zone adjacent to the caudoventral border of the nucleus. The caudal portion of the exposed zone of the VCN is in the vicinity of the rootlets of the glossopharyngeal (IX) nerve, and the ventral portion is close to the terminal part of the vestibulocochlear (VIII) nerve. The border between the intraventricular part of the CN and the extraventricular portion and also the terminal part of the VIII nerve approximately coincides with the line of attachment of the inferior medullary velum of the fourth ventricle (tenia of the choroid plexus). In the narrow strip of this ventral most part of the tenia we did not observe big blood vessels or neurons. Accordingly this could be a reasonably safe surgical route to the intraventricular surface of the CN.

Adult↗
PubMed1998

Distribution of a metabotropic glutamate receptor, mGluR2, in the central nervous system of the rat and mouse: an immunohistochemical study with a monoclonal antibody.

The distribution of a metabotropic glutamate receptor mGluR2 in the central nervous system was immunohistochemically examined in the rat and mouse with a monoclonal antibody raised against an N-terminal sequence of rat mGluR2 (amino acid residues 87-134). Neuronal cell bodies with mGluR2-like immunoreactivity (mGluR2-LI) were clearly shown in the horizontal cells of Cajal in the cerebral cortex, neurons in the triangular septal nucleus and medial mammillary nucleus, Golgi cells and the unipolar brush cells in the cerebellar cortex, and Golgi-like and unipolar brush-like cells in the cochlear nucleus. Neuropil was intensely immunostained in the accessory olfactory bulb, bed nucleus of the accessory olfactory tract, neocortex, cingulate cortex, retrosplenial cortex, subicular and entorhinal cortices, stratum lacunosum-moleculare of CA1 and CA3, molecular layer of the dentate gyrus, periamygdaloid cortex, basolateral amygdaloid nucleus, bed nucleus of the anterior commissure, caudate-putamen, accumbens nucleus, thalamic reticular nucleus, anteroventral and paraventricular thalamic nuclei, granular layer of the cerebellar cortex, anterior and ventral tegmental nuclei, granular layer of the cochlear nucleus, and parvicellular part of the lateral reticular nucleus. Many axons in the white matter and fiber bundles were also immunostained. No glial cells with mGluR2-LI were found. No particular species differences were found in the distribution pattern of mGluR2-LI between the rat and mouse. The results indicate that mGluR2 is expressed not only in somato-dendritic domain, but also in axonal domain of excitatory and inhibitory neurons.

Animals↗
PubMed1999

A comparison of the growth of open-set speech perception between the nucleus 22 and nucleus 24 cochlear implant systems.

OBJECTIVE: To compare the short-term development of open-set speech perception between implementations of the spectral peak (SPEAK) strategy in adults using the Nucleus 22 (N22) Spectra processor and the Nucleus 24 (N24) system with the SPrint processor. STUDY DESIGN: Two sets of patients with a follow-up period of 3 months were compared. SETTING: This study was conducted at the New York University (NYU) Medical Center. PATIENTS: Thirty-six profoundly hearing-impaired adults were included in this study. INTERVENTION: Sixteen adults received the N22 cochlear implant with the Spectra speech processor and twenty adults received the N24 cochlear implant with the SPrint speech processor. MAIN OUTCOME MEASURES: Open-set speech perception for the patients with N22 implants was evaluated preoperatively and postoperatively using the following measures: NU-6 monosyllabic words, Iowa sentences, and CID sentences. CNC words, CUNY sentences, and HINT sentences were used to assess the patients with N24 implants. RESULTS: Results revealed that the N22 and N24 cochlear implant recipients had significant open-set speech recognition 3 and 6 months after implantation. The apparent favorable absolute scores for the patients with N24 implants were equalized when a multivariate analysis was performed to account for their younger age at time of implantation and shorter length of deafness. CONCLUSIONS: The growth of open-set word and sentence recognition in similar between the N22 and N24 cochlear implants. Longitudinal studies are needed to determine if higher ceiling scores are present in the N24. Further exploration of monopolar versus bipolar stimulation is warranted.

Adult↗
PubMed1989

Diverse afferents converge on the nucleus paragigantocellularis in the rat ventrolateral medulla: retrograde and anterograde tracing studies.

The nucleus paragigantocellularis in the ventrolateral medulla has been implicated in cardiovascular, pain, and analgesic functions; and it has also been found to be a major afferent to the pontine nucleus locus coeruleus. In the present study, afferents to the nucleus paragigantocellularis were identified in the rat by means of the retrograde tracers wheat germ agglutinin-conjugated horseradish peroxidase or Fluoro-Gold. Projections to the nucleus paragigantocellularis arise from a wide variety of nuclei with autonomic, visceral, and sensory-related functions. Major afferents with consistent and robust retrograde labeling include most laminae of the spinal cord, the caudal lateral medulla, the contralateral paragigantocellularis, the nucleus of the solitary tract, the A1 area, the lateral parabrachialis, the Kölliker-Fuse nucleus, the periaqueductal gray, and a preoculomotor nucleus in the ventral central gray, the supraoculomotor nucleus. Other notable afferents, seen only after large caudal injections into the nucleus paragigantocellularis, include the lateral hypothalamus, the paraventricular nucleus of the hypothalamus, and the medial prefrontal cortex. Minor afferents include the gigantocellular nucleus, the area postrema, the caudal raphe groups, the inferior colliculus, the A5 area, and the locus coeruleus. The projection from the supraoculomotor nucleus, not previously reported as an afferent to the ventrolateral medulla, was confirmed with anterograde tracing by means of Phaseolus vulgaris-leucoagglutinin. Iontophoretic deposits of Phaseolus vulgaris-leucoagglutinin into the nucleus of the solitary tract (commissuralis level) or into the periaqueductal gray also yielded terminal fiber labeling in the nucleus paragigantocellularis. Fibers from the supraoculomotor nucleus and the nucleus of the solitary tract were densest in the lateral aspect of the nucleus paragigantocellularis (corresponding to the rostroventrolateral reticular nucleus), while fibers from the periaqueductal gray were more medially located. Previous studies have defined inputs to the rostral ventrolateral medulla from the cochlear nucleus as well as from the colliculi. In the present study, deposits of wheat germ agglutinin-conjugated horseradish peroxidase or Phaseolus vulgaris-leucoagglutinin into the cochlear nucleus or the superior colliculus yielded only sparse anterograde labeling in the nucleus paragigantocellularis, but heavily labeled adjacent areas. The inferior collicular injections yielded strong but restricted anterograde labeling in the rostromedial paragigantocellularis, medial to the facial nucleus. These results indicate that the paragigantocellularis area receives inputs from diverse brain structures. Neurons in the nucleus paragigantocellularis afferent to the locus coeruleus, being distributed throughout this region, may provide a channel where several types of information are integrated and transmitted to the extensive locus coeruleus noradrenergic efferent network...

Afferent Pathways↗
PubMed2003

Changes in [14C]-2-deoxyglucose uptake in the auditory pathway of hamsters previously exposed to intense sound.

The current study evaluated changes in [14C]-2-deoxyglucose (2-DG) uptake along the auditory pathways of hamsters that were exposed unilaterally to intense sound. The measurement of the acoustically evoked auditory brainstem responses indicated that intense sound exposure caused asymmetrical hearing loss. The 2-DG results revealed some changes in metabolic activity in exposed animals, as compared to unexposed animals. Significant decreases in 2-DG uptake were found in the ipsilateral anteroventral and posteroventral cochlear nucleus, with respect to the exposed left ears. Exposed animals also showed significant increases in the ipsilateral nucleus of the lateral lemniscus, central nucleus of inferior colliculus and medial geniculate body. No significant changes in uptake were observed in the ipsilateral dorsal cochlear nucleus, superior olivary complex, auditory cortex and any contralateral structures. The mechanisms for the observed changes in 2-DG uptake are discussed.

Acoustic Stimulation↗
PubMed1987

[Anatomy of the central auditory pathway. Demonstration with horseradish peroxidase in the guinea pig].

The neurones of the afferent and efferent auditory pathways were labelled by axonally transported horseradish peroxidase. The first neurone of the ascending system is the acoustic nerve with the spiral ganglion. These fibres terminate mainly in the ventral cochlear nucleus (VCN). The neurones of the latter chiefly run to the contralateral superior olivary complex (SOC), whereas the neurones of the dorsal cochlear nucleus (DCN) terminate mainly in the central nucleus of the contralateral inferior colliculus (IC). The nerve cells of the superior olivary complex and nuclei of the lateral lemniscus (DLL, VLL) project to the IC. The IC neurones run mainly uncrossed to the medial geniculate body (MGB), i.e. the neurones of the central nucleus of the inferior colliculus terminate in the ventral nucleus of the medial geniculate body. This latter nucleus projects to the primary region A I of the auditory cortex, whereas the neurones in the pericentral medial geniculate body terminate in the associated auditory regions A II, EP and SF. The auditory areas of both hemispheres is well interconnected by commissural fibres. In addition to this ascending pathway there is a descending system. The parent cells correspond to pyramidal neurones in layer V in the auditory cortex. These neurones terminate in the medial geniculate body and in the pericentral nuclei of the inferior colliculus. From this, efferent neurones project to the DCN and SOC. The neurones of the olivocochlear bundles originate in the SOC; these bundles terminate predominantly on the surface of the outer hair cells in the organ of Corti.

Animals↗
PubMed1987

Repeated TTS exposures in monkeys: alterations in hearing, cochlear structure, and single-unit thresholds.

The findings of a number of studies investigating the effects of excessive sound on hearing have indicated that the correspondence between behavioral, physiological, and histological measures of noise-induced hearing loss may be markedly dependent upon the sensitivity of the particular measure. Recent studies demonstrating significant changes in the responses of single auditory neurons following brief exposures to pure tones suggest that single-unit activity may be a sensitive indicator of physiological insult to the organ of Corti's sensory cells. In addition, the long-lasting nature of the changes in neural responsiveness suggests that each temporary threshold shift (TTS) episode may produce an increment of damage to the ear that eventually contributes to a measurable permanent threshold shift (PTS). A logical extension of this implication is the proposal that repeated episodes of TTS would first affect single-unit thresholds, and that such damage would eventually manifest itself as PTS. A test of this notion was performed by repeatedly exposing monkeys to short-lasting TTS sounds for many months. Behavioral thresholds were monitored using a reaction-time task before and after each inducement of TTS. Two subjects participated in exposure sessions for 18 months, while the remaining monkey was exposed to identical stimuli for 6 months. At the end of behavioral testing, the monkeys were prepared for chronic recording from single cells of the cochlear nucleus. Following the recording period, cochleas were prepared for examination as plastic-embedded whole mounts. Flat preparations of the cochlear duct were made and the position and extent of damage to the organ of Corti and myelinated nerve fibers were determined. No elevations in behavioral threshold were noted for the monkey receiving 6 months of sound-exposure experience, while for both subjects exposed for 18 months, a significant high-frequency hearing loss became apparent during the final months of exposure. For damaged ears, the thresholds of ipsilateral cochlear nucleus units were elevated for characteristic frequencies (CFs) corresponding to the frequency regions where behavioral thresholds were shifted. Thresholds for units with high-frequency CFs in the animal exposed for 6 months also demonstrated a loss in sensitivity. Histological examination of the cochleas of monkeys with permanent hearing losses revealed corresponding damage to the high-frequency region of the organ of Corti. The monkey exposed for 6 months, which demonstrated only elevated unit thresholds, also had high-frequency lesions.(ABSTRACT TRUNCATED AT 400 WORDS)

Acoustic Stimulation↗
PubMed1994

Serotonin and serotonin receptors in the central auditory system.

Immunohistochemical and ligand-binding techniques were used to visualize the neurotransmitter serotonin and one of its receptors, the 5-HT1A subtype, in auditory nuclei of the brainstem. Serotonergic fibers and terminal endings were found in all auditory nuclei extending from the cochlear nucleus to the inferior colliculus, including the superior olivary complex and the nuclei of the lateral lemniscus. The density of the innervation varied between and within each nucleus. All serotonergic cell bodies were located outside the auditory nuclei. The 5-HT1A receptor subtype was found in the cochlear nucleus as well as in the inferior colliculus. With no serotonergic cell bodies present in the auditory nuclei, the present neuroanatomic and neurochemical findings support behavioral and neurophysiologic findings that the serotonergic system may modulate central auditory processing.

Animals↗
PubMed1992

Structure and function of the cochlea in the African mole rat (Cryptomys hottentotus): evidence for a low frequency acoustic fovea.

The cochlea of the mole rat Cryptomys hottentotus was investigated with physiological and anatomical methods. In order to reveal the place-frequency map of the cochlea, iontophoretic HRP-applications were made in the cochlear nucleus at physiologically characterized locations. Subsequent HRP-transport in auditory nerve fibres and labeling patterns of spiral ganglion cells within the cochlea were evaluated. A cochlear place-frequency map was constructed from 17 HRP-applications in the cochlear nucleus at positions where neurons had characteristic frequencies between 0.1 and 12.6 kHz. As in other mammals, high frequencies were found to be represented at the cochlear base, low frequencies at the cochlear apex. The place-frequency map had three distinct parts which were characterized by their different slopes. A clear over-representation of the frequencies between 0.6 and 1 kHz was revealed, in this frequency range the slope of the place-frequency map amounted to 5.3 mm/octave. As calculated from the regression analysis, below 0.6 kHz the slope of the cochlear place-frequency map amounted to 0.24 mm/octave, above 1 kHz to 0.9 mm/octave. As in other mammals width of the basilar membrane (BM) increased from the cochlear base towards the cochlear apex. Also in concordance with the findings in other mammals, BM-thickness decreased from the cochlear base to the apex. However, it was remarkable to find that there was no or little change in BM-width and thickness between 40 and 85% BM-length. It was also revealed that scala tympani was only 1/10th the size found in the rat or other mammals of similar body size.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1991

Neurogenesis in the brain auditory pathway of a marsupial, the northern native cat (Dasyurus hallucatus).

Neurogenesis in the auditory pathway of the marsupial Dasyurus hallucatus was studied. Intraperitoneal injections of tritiated thymidine (20-40 microCi) were made into pouch-young varying from 1 to 56 days pouch-life. Animals were killed as adults and brain sections were prepared for autoradiography and counterstained with a Nissl stain. Neurons in the ventral cochlear nucleus were generated prior to 3 days pouch-life, in the superior olive at 5-7 days, and in the dorsal cochlear nucleus over a prolonged period. Inferior collicular neurogenesis lagged behind that in the medial geniculate, the latter taking place between days 3 and 9 and the former between days 7 and 22. Neurogenesis began in the auditory cortex on day 9 and was completed by about day 42. Thus neurogenesis was complete in the medullary auditory nuclei before that in the midbrain commenced, and in the medial geniculate before that in the auditory cortex commenced. The time course of neurogenesis in the auditory pathway of the native cat was very similar to that in another marsupial, the brushtail possum. For both, neurogenesis occurred earlier than in eutherian mammals of a similar size but was more protracted.

Aging↗
PubMed1988

Connections of the superior olivary complex in the rufous horseshoe bat Rhinolophus rouxi.

In the rufous horseshoe bat (Rhinolophus rouxi), the superior olivary complex contains four main divisions. In comparison with other species, the most lateral division is clearly homologous to the lateral superior olive (LSO); the most medial division is homologous to the medial nucleus of the trapezoid body (MNTB). Lying between these landmarks, in approximately the position of the medial superior olive (MSO) of other mammals, are two additional divisions that are cytoarchitecturally distinct from one another yet do not greatly resemble the MSO of nonecholocating mammals such as the cat. We refer to these nuclei as the dorsal medial superior olive (DMSO) and the ventral medial superior olive (VMSO). We examined the afferent and efferent connections of all of these cell groups with retrograde and anterograde transport of WGA-HRP from the superior olivary complex. In the same animals we recorded the binaural response properties of single units in the superior olivary complex. Virtually all units recorded in LSO were excitatory to the ipsilateral ear and inhibitory to the contralateral ear (EI); all of the units sampled in the MNTB and most of those sampled in the VMSO responded only to the contralateral ear (OE). In DMSO the binaural properties of units were varied: the number of units that were inhibitory to the ipsilateral ear and excitatory to the contralateral ear (IE) was about equal to the number of units excitatory to both ears (EE); a few units had OE responses; no units had EI responses. Connectional correlates for these binaural response properties are seen in the patterns of retrograde transport from WGA-HRP injections in the divisions of the superior olive. The LSO receives projections from the ipsilateral cochlear nucleus and MNTB; MNTB receives projections from the contralateral cochlear nucleus. The DMSO and VMSO both receive bilateral projections from the cochlear nuclei. The results of retrograde and anterograde transport suggest that VMSO, in addition, receives projections from the ipsilateral MNTB. The LSO, DMSO, and VMSO all project to the ventral two-thirds of the central nucleus of the inferior colliculus, and their targets are approximately coextensive. However, the LSO projects bilaterally to the inferior colliculus, whereas the medial cell groups project mainly ipsilaterally.

Acoustic Stimulation↗
PubMed2001

Hypocretin receptor protein and mRNA expression in the dorsolateral pons of rats.

The hypocretins (also known as orexins) are hypothalamic peptides that have been implicated in feeding and sleep regulation. Previous reports have described the distribution of the mRNAs encoding two hypocretin receptors (HCRT-R), but the pattern of protein expression has not been investigated. Here we examine the distribution of the mRNA and protein for the HCRT receptor 1 (HCRT-R1) and HCRT receptor 2 (HCRT-R2) in the pontine brainstem and demonstrate that they are present in many pontine nuclei including those associated with REM sleep. Immunohistochemistry indicates that one or both of the receptor subtypes are expressed in the dorsal raphe, the lateral dorsal tegmental (LDT), the pedunculo pontine (PPT), the locus coeruleus (LC), the locus subcoeruleus, pontis oralis, Barrington's, the trigeminal complex (mesencephalic trigeminal and motor nucleus of the trigeminal nerve), the dorsal tegmental nucleus of Gudden (DTG), the ventral cochlear nucleus (VCA), trapezoid nucleus (TZ), pontine raphe nucleus and the pontine reticular formation. These regions have been shown to be involved in mastication, bladder control, gastrointestinal function and in arousal. Given these projection sites and the functions associated with these sites, we suggest that HCRT may play a role in maintaining alertness and vigilance while the animal is engaged in consummatory behavior.

Animals↗