NMDA receptor subunit NR1-immunoreactivity in the rat pons and brainstem and colocalization with Fos induced by nasal stimulation

In the present study, we examined the distribution of neurons in the parabrachial nucleus (PB), the Kölliker-Fuse nucleus (KF), the spinal trigeminal nucleus caudalis (Sp5C), the nucleus of the solitary tract (NTS) and the ventrolateral medulla (VLM), which are activated by evoking the nasotrigeminal reflex and which exhibit immunoreactivity for the N-methyl-D-aspartate (NMDA) receptor subunit NR1. By stimulating the nasal mucosa with saline, we induced the expression of the immediate early gene c-fos and combined the immunocytochemical detection of the Fos protein with the detection of the NR1 subunit. Cell counts revealed that nasal stimulation, compared to anesthesia controls, resulted in highly significant increases (p < or = 0.001) of Fos-immunoreactive (-ir) neurons in the midlevel KF, the external lateral PB, and the Sp5C. In the central lateral PB, the rostral ventrolateral medulla including the Bötzinger/pre-Bötzinger complex, and in the ventrolateral and commissural NTS the increases were only moderately significant (p < or = 0.05). With respect to the numbers of NR1-/Fos-ir double-labeled neurons, significant increases were only observed in a subset of these pontomedullary nuclei. Increases were highly significant in the Sp5C (p < or = 0.001) and the midlevel KF (p < or = 0.01) and moderately significant (p < or = 0.05) in the external lateral PB, Bötzinger/pre-Bötzinger complex, and ventrolateral NTS. The present study revealed that nasotrigeminally activated neurons in mandatory and potential relay sites of the nasotrigeminal reflex circuit express the NR1 subunit. This finding strongly suggests that NMDA-type glutamate receptors are involved in the mediation of the nasotrigeminally evoked cardiovascular and respiratory responses.

GABA, GABA transporters, GABA(A) receptor subunits, and GAD mRNAs in the rat parabrachial and Kölliker-Fuse nuclei

In the present study, we investigated the key molecules that determine gamma-aminobutyric acid (GABA)ergic signal transduction in the parabrachial/Kölliker-Fuse complex (PB/KF) by means of immunocytochemistry and in situ hybridization. Our data demonstrate a dense plexus of GABA-immunoreactive (-ir) varicosities throughout the nuclei of the PB and the KF. The number of neurons expressing GAD65 or GAD67 mRNA was fairly low in the PB, whereas caudally in the KF an accumulation of GAD-expressing neurons was observed. The GABA transporter-3 (GAT-3) was detected in all parts of the PB/KF, whereas immunolabeling for GAT1 was not observed. All nuclei of the PB and the KF exhibited immunoreactivity for the gamma2-, alpha2-, and alpha3-subunits of the GABA(A) receptor. Gamma2-ir was strong and similar in all PB/KF nuclei. In contrast, alpha2-labeling was particularly intense in the superior lateral PB, and alpha3-labeling was most prominent in the external lateral and external medial PB, compared with the remaining nuclei. With respect to the subcellular localization, we found gamma2-ir in cell bodies and higher order dendrites, whereas alpha2- and alpha3-ir was predominantly found in cell bodies. Immunolabeling for the beta2/3- and the alpha1-subunit was seen in cell bodies and presumed dendritic profiles. The staining intensity was strongest in the dorsal lateral PB. Most importantly, the external lateral PB and the waist area were totally devoid of beta2/3- and alpha1-ir. Our data suggest that neural processing in the PB/KF is under a strong GABAergic inhibition that is apparently mediated by different types of GABA(A) receptors in functionally different pathways through the PB/KF.

NMDA and GABAA receptors in the rat Kolliker-Fuse area control cardiorespiratory responses evoked by trigeminal ethmoidal nerve stimulation

1. Electrical stimulation (10 s) of the ethmoidal nerve (EN5) evokes the nasotrigeminal reflex responses, including apnoea, bradycardia and rise in arterial blood pressure. In the present study, we examined the involvement of N-methyl-D-aspartate (NMDA), AMPA/kainate, (gamma-aminobutyric acidA (GABAA) and glycine receptors in the Kolliker-Fuse (KF) nucleus in the mediation of the nasotrigeminal reflex responses. 2. Unilateral injections (n = 6) of 50-100 nl of the NMDA receptor antagonist AP5 into the KF area led to a significant blockade of the EN5-evoked respiratory depression and bradycardia. Injections placed into the midlevel of the KF area were most effective (80-90 % blockade). The rise in arterial blood pressure remained unaffected. 3. Unilateral injections (n = 6) of the AMPA/kainate receptor antagonist CNQX into the KF area failed to block EN5-evoked autonomic responses significantly. 4. Unilateral injections (n = 5) of the GABAA receptor antagonist bicuculline enhanced the EN5-evoked respiratory depression and bradycardia. The effect persisted for up to 30 s after stimulation. Bicuculline injections into the midlevel of the KF area were most effective. The increase in arterial blood pressure remained unaffected. 5. Unilateral injections (n = 5) of the glycine receptor antagonist strychnine into the KF area did not produce any significant effects on EN5-evoked autonomic responses. 6. Our results suggest that the KF area represents a mandatory relay for the nasotrigeminally induced apnoea and bradycardia which are predominantly mediated by NMDA receptors in the KF. Furthermore, it appears that KF neurons are under a potent GABAergic inhibitory control. The EN5-evoked rise in arterial blood pressure was not altered by any of the drugs and, therefore, appears not to be mediated via the KF.

The medical nucleus of the solitary tract mediates the trigeminally evoked pressor response

The present study examined whether the nucleus of the solitary tract (NTS) plays a role in mediating the trigeminally evoked pressor response which occurs after noxious perturbation of the nasal mucosa or electrical stimulation of the ethmoidal nerve (EN5). We recorded arterial blood pressure responses to EN5 stimulations before and after injections of the calcium channel blocker CoCl2 into the NTS. Unilateral and bilateral injections of CoCl2 into the medial NTS resulted in significant blockade (p < 0.05 and p < 0.01, respectively) of the EN5-evoked pressor responses. Recovery of the pressor responses was observed 15-25 min after the CoCl2 injections. CoCl2 injections into the caudal commissural NTS remained largely ineffective with respect to blocking the EN5-evoked pressor responses. These findings strongly suggest that the medial NTS is a crucial link for the trigeminally induced pressor response.

Effects of anesthetics on Fos protein expression in autonomic brain nuclei related to cardiovascular regulation

We tested the influence of urethane, chloral hydrate and a mixture of ketamine/xylazine on Fos protein expression in autonomic brain regions related to blood pressure. We conclude that ketamine/xylazine is a suitable anesthetic to be used in studies looking at c-fos expression in neural circuits serving cardiovascular regulation.

Fos expression in the rat parabrachial and Kölliker-Fuse nuclei after electrical stimulation of the trigeminal ethmoidal nerve and water stimulation of the nasal mucosa

The present study examined the location of neurons in the lateral parabrachial nucleus (PBL), Kölliker-Fuse nucleus (KF), nucleus of the solitary tract (NTS), spinal trigeminal nucleus (Sp5C) and upper cervical cord possibly involved in the transmission of autonomic responses (apnea, bradycardia and rise in arterial blood pressure) elicited by nasotrigeminal stimulation in the rat. To identify these neurons we employed immunocytochemical detection of the transcription factor Fos. To induce the expression of Fos protein, two kinds of stimuli and experimental controls were performed in chloralose/urethane-anesthetized animals: (i) electrical stimulation of the trigeminal ethmoidal nerve (EN5) and, as sham controls, dissection of the EN5 without electrical stimulation, (ii) stimulation of the nasal mucosa with water and, as control experiments, no stimulation. Both forms of stimulation lead to a consistent pattern of Fos-positive neurons in the PBL and KF. Differences could be observed rostrally in the PBL and KF, where significantly higher numbers of Fos-positive neurons were present after EN5 versus water stimulation. The EN5-stimulated group had a significantly higher number of Fos-immunoreactive neurons in the KF than the sham controls, especially in the midlevel region. In the PBL significant differences in the numbers of activated cells could be observed between EN5-stimulated versus sham controls. In the water-stimulated rats compared with the anesthesia controls, a significantly higher number of Fos-immunoreactive neurons was always observed at all rostrocaudal levels in the KF and in the midlevel PBL. Electrical EN5 stimulation induced Fos expression in the Sp5C at the level of the area postrema and caudally in the upper cervical cord. In contrast, after water stimulation Fos-positive neurons were exclusively found in the Sp5C. In addition, all forms of stimuli and controls induced strong expression of Fos in the medial and commissural NTS. Linear correlations were found between the numbers of Fos-immunoreactive neurons in the Sp5C versus the KF and the NTS versus the PBL. The activated neurons may belong to two functionally discrete pathways: the nasotrigeminal reflex circuit, which is activated by nasal sensory afferents running through the EN5 via the Sp5C to the KF, and a pathway activated most likely by baro- and chemoreceptor afferents running through the NTS to the PBL. Our results indicate that the PB/KF plays a pivotal role in the mediation and maintenance of the autonomic responses induced by the nasotrigeminal reflex.

Afferent and efferent connections of the ventrolateral tegmental area in the rat

The present study examined the organization of afferent and efferent connections of the rat ventrolateral tegmental area (VLTg) by employing the retrograde and anterograde axonal transport of Fluorogold and Phaseolus vulgaris-leucoagglutinin, respectively. Our interest was focused on whether the anatomical connections of the VLTg would provide evidence as to the involvement of this reticular area in audiomotor behavior. Our retrograde experiments revealed that minor inputs to the VLTg arise in various telencephalic structures, including the cerebral cortex. Stronger projections originate in the lateral preoptic area, the zona incerta, the nucleus of the posterior commissure and some other thalamic areas, the lateral substantia nigra, the deep layers of the superior colliculus, the dorsal and lateral central gray, the deep mesencephalic nucleus, the paralemniscal zone, the intercollicular nucleus, the external cortex of the inferior colliculus, the oral and caudal pontine reticular nucleus, the deep cerebellar nuclei, the gigantocellular and lateral paragigantocellular reticular nuclei, the prepositus hypoglossal nucleus, the spinal trigeminal nuclei, and the intermediate layers of the spinal cord. Most importantly, we disclosed strong auditory afferents arising in the dorsal and ventral cochlear nuclei and in the cochlear root nucleus. The efferent projections of the VLTg were found to be less widespread. Telencephalic structures do not receive any input from the VLTg. Moderate projections were seen to diencephalic reticular areas, the zona incerta, the nucleus of the posterior commissure, and to various other thalamic areas. The major VLTg projections terminate in the deep layers of the superior colliculus, the deep mesencephalic nucleus, the intercollicular nucleus and external cortex of the inferior colliculus, the oral and caudal pontine reticular nucleus, the gigantocellular and lateral paragigantocellular reticular nuclei, and in the medial column of the facial nucleus. From our data, we conclude that the VLTg might play a role in sensorimotor behavior.

Spinal afferents to functionally distinct periaqueductal gray columns in the rat: an anterograde and retrograde tracing study

The segmental and laminar organization of spinal projections to the functionally distinct ventrolateral (vlPAG) and lateral periaqueductal gray (lPAG) columns was examined by using retrograde and anterograde tracing techniques. It was found 1) that spinal input to both vlPAG and lPAG columns arose predominantly from neurons in the upper cervical (C1-4) and sacral spinal cord; 2) that there was a topographical separation of vl-PAG projecting and lPAG-projecting neurons within the upper cervical spinal cord; but 3) that below spinal segment C4, vlPAG-projecting and lPAG-projecting spinal neurons were similarly distributed, predominantly within contralateral lamina I, the nucleus of the dorsolateral fasciculus (the lateral spinal nucleus) and the lateral (reticular) part of lamina V. Consistent with the retrograde results, the greatest density of anterograde label, within both the vlPAG and lPAG, was found after tracer injections made either in the superficial or deep dorsal horn of the upper cervical spinal cord. Tracer injections made within the thoraco-lumbar spinal cord revealed that the vlPAG column received a convergent input from both the superficial and deep dorsal horn. However, thoraco-lumbar input to the lPAG was found to arise uniquely from the superficial dorsal horn; whereas the deep dorsal horn was found to innervate the "juxta-aqueductal" PAG region rather than projecting to the lPAG. These findings suggest that similar to spino-parabrachial projections, spinal projections to the lPAG (and juxta-aqueductal PAG) are topographically organised, with distinct subgroups of spinal neurons projecting to specific lPAG or juxta-aqueductal PAG subregions. In contrast, the vlPAG receives a convergent spinal input which arises from the superficial and deep dorsal horn of cervical, thoracic, lumbar, and sacral spinal segments.

The Kölliker-Fuse nucleus mediates the trigeminally induced apnoea in the rat

The present study examined whether the Kölliker-Fuse nucleus (KF) plays a role in mediating the trigeminally induced apnoea which occurs after noxious perturbation of the nasal mucosa. We stimulated the ethmoidal nerve (EN5) electrically and recorded respiratory responses before and after injections of the calcium channel blocker CoCl2 into the KF. Unilateral EN5 stimulations resulted in an apnoea or in a reduction of respiratory frequency and tidal volume. EN5 stimulations immediately after ipsilateral CoCl2 injections into the caudal KF caused only minor respiratory suppression, indicating a blockade of synaptic transmission. Recovery of the respiratory responses was observed 15-120 min after the CoCl2 injection. Our data strongly suggest that the caudal KF is an obligatory relay site for trigeminally induced apnoea.

Analysis of projections from the cochlear nucleus to the lateral paragigantocellular reticular nucleus in the rat

Golgi-staining, retrograde and anterograde tract-tracing, and a two-color immunoperoxidase technique have been employed, at the light- and electron-microscopic levels, to analyze the auditory projections from the cochlear nucleus (CN) to the lateral paragigantocellular reticular nucleus (LPGi) in the rat. We have found that the auditory input originates predominantly in the posteroventral and cochlear root nuclei. The auditory axons terminate in the cell-poor, ventral portion of the LPGi, which is strongly invaded by beaded dendritic profiles, originating from parent cell bodies located in the dorsal half of the LPGi. Ultrastructural analysis has revealed that the anterogradely labeled auditory axons form functional synapses preferentially with dendritic shafts. These axo-dendritic contacts are apparently excitatory in nature. By means of a sequential two-color immunoperoxidase staining method, we have further characterized potential postsynaptic neurons in the LPGi. Black-stained auditory fibers intermingle with brown-stained serotonergic or adrenergic neurons. Varicose auditory axons are often closely apposed to immunoreactive dendritic profiles of serotonergic and adrenergic neurons, indicating the presence of possible synaptic contacts of auditory terminal fibers with these transmitter-classified cells. The monosynaptic auditory in- put from the CN may modulate the activity of B3 sero-tonergic and C1 adrenergic cells in the LPGi and may thus induce adaptive changes in response to acoustic stimuli.

Distribution of alpha 2-adrenergic binding sites in the parabrachial complex of the rat

The present study describes the distribution of alpha 2-adrenoceptors in the parabrachial and Kölliker-Fuse nucleus of the rat by employing the tritium-labeled alpha 2-receptor antagonist rauwolscine ([3H]-RAUW) as a ligand. The [3H]-RAUW binding was densitometrically quantified in five nuclei of the parabrachial (PB) complex in serial coronal sections. We found that cytoarchitectonically and anatomically distinct nuclei of the PB complex exhibit different numbers of [3H]-RAUW-binding sites. The largest number of binding sites was observed over the external lateral PB and caudally over the waist area of the PB. Lower numbers of binding sites were found in the remaining lateral PB nuclei, followed by the medial PB and the Kölliker-Fuse nucleus. In addition we disclosed that the internal lateral PB contains a very low number of binding sites while the external medial PB is marked by dense [3H]-RAUW binding. Also, the affinities of the binding sites differed between the PB areas. High affinities were observed in the external lateral PB, the remaining lateral PB nuclei and in the waist area of the PB, while the medial PB and the Kölliker-Fuse nucleus exhibited only low affinities for the ligand. Furthermore, saturation curves demonstrated non-linear profiles, indicating the presence of more than one population of binding sites in the PB nuclei for the radioligand. Our data demonstrate that the PB exhibits a distinct distribution of alpha 2-adrenergic binding sites. These correlate well with the cytoarchitectonically defined nuclei of the PB complex and with the pattern of ascending axons from the medial nucleus of the solitary tract and the area postrema terminating in the PB. Since a large number of these projection neurons utilize adrenaline or noradrenaline as their transmitters, we conclude that solitary-parabrachial neurotransmission to the forebrain is, at least in part, mediated via alpha 2-adrenoceptors.

Morphology and dendritic domains of neurons in the lateral parabrachial nucleus of the rat

The present study provides a description of the dendritic morphology and the dendritic domains of neurons in the lateral parabrachial nucleus (PB) of the rat. The cells were intracellularly stained in vitro with Lucifer yellow. A subpopulation of these cells was characterized beforehand as neurons projecting to the amygdaloid complex by retrograde transport with rhodamine beads. With respect to their dendritic arborization, different types of "spatially" organized PB neurons were discriminated. One major cell type in the external lateral PB (PBel) is characterized by long, elongated dendritic trees that are preferentially oriented parallel to the superior cerebellar peduncle. The majority of their dendrites appears to respect subnuclear boundaries, yet their distal dendrites often exceed the limits of the PBel to encroach upon adjacent subnuclei located dorsally and ventrolaterally to the PBel. Another prominent cell type in the PBel has fairly small and locally restricted dendritic trees that are also elongated, running with their main axis from ventrolateral to dorsomedial. The dendrites of the majority of these neurons apparently stay within the confines of the PBel. A distinct group of neurons is found in the ventral portion of the PBel. The majority of their dendrites is mediolaterally oriented and not confined to the PBel subnucleus. In addition, we found a smaller number of neurons scattered within the lateral PB whose dendrites do not show a preferential orientation but travel across subnuclear boundaries into several different PB subnuclei. Our data show that the dendrites of a large proportion of neurons in the lateral PB either stay within the confines of a particular subnucleus or slightly extend across subnuclear limits. In any case, they appear to match with terminal territories of afferent axons and, thus, maintain the functional specificity of inputs by their relay through the PB. In contrast, PB neurons that extend their dendrites across subnuclear boundaries or known terminal territories are likely to receive inputs of different qualities from a variety of sources and therefore transmit a more general, integrated signal to the forebrain.

Topographic organization of spinal and trigeminal somatosensory pathways to the rat parabrachial and Kölliker-Fuse nuclei

We examined the organization of somatosensory projections to the parabrachial (PB) and Kölliker-Fuse (KF) nuclei by employing the retrograde and anterograde axonal transport of Fluorogold and Phaseolus vulgaris-leucoagglutinin (PHA-L), respectively. Small PHA-L injections were made into different parts of the spinal trigeminal complex, including the paratrigeminal nucleus, and into different segments and laminae of the spinal dorsal horn. The subnuclear distribution of axonal labeling in the PB and KF was mapped with a camera lucida. Our results show that the somatosensory input to the PB and KF is highly organized. Neurons in the spinal trigeminal nuclei project predominantly to the KF and to the ventral portion of the external lateral PB. Neurons in the paratrigeminal nucleus project to the ventral lateral PB, the external medial PB, and to caudal aspects of the medial PB. These findings were supported by retrograde tracing experiments with Fluorogold. Spinal cord neurons located in the superficial dorsal horn (laminae I-II) of upper cervical segments project specifically to the ventral portion of the external lateral PB and, although more sparsely, to various other lateral PB nuclei. In contrast, neurons in the superficial dorsal horn of thoracic and lumbar spinal segments project mainly to the dorsal lateral and the central lateral PB. Finally, neurons in the lateral reticulated area and the lateral spinal nucleus of all spinal segments project almost exclusively to the internal lateral PB, whereas neurons in the respective nuclei of upper cervical segments also project to the KF. From our data we conclude that the somatosensory projections to the PB and KF are topographically organized. It is assumed that these pathways, which run from trigeminal and spinal neurons through the PB and KF to various forebrain, medullary, and spinal nuclei, form functionally different neural circuits that are involved in somatoautonomic processing.

Substance P and other putative transmitters modulate the activity of reticular pontine neurons: an electrophysiological and immunohistochemical study

In this study we investigated the effects of possible modulatory transmitters on acoustically responsive neurons of the caudal pontine reticular nucleus (PnC). From previous work in our laboratory it has been suggested that the acoustically responsive giant neurons of this nucleus are the sensorimotor interface mediating the acoustic startle response. Furthermore they are the site of some of the modulatory influence impinging on this response. Besides a possibly glutamatergic excitation from the amygdala a cholinergic input from the midbrain has been described which may use substance P as cotransmitter. Therefore we used electrophysiological and histochemical methods to study this possible modulatory influence in the caudal pontine reticular nucleus. In the first part of this study we recorded extracellularly from single units in the PnC in vivo and studied the effects of iontophoretically applied transmitters. Substance P elicited a long lasting excitation. This excitatory effect of SP was potentiated by acetyl-beta-methylcholine (AMCh, an acetylcholine agonist), whereas single application of AMCh showed no uniform response. Glutamate elicited a potent brief excitation, while application of GABA showed a potent brief inhibition of PnC neurons. In the second part of this study we employed immunoperoxidase staining for substance P, which revealed a fairly dense network of substance P-immunoreactive (SP-ir) fibers in the lateral and ventral aspects of the PnC. Combining retrograde tracing and immunocytochemistry for substance P, we demonstrated that the SP-ir axons in the PnC originate mainly in the laterodorsal tegmental nucleus. We therefore conclude that activation of the laterodorsal tegmental nucleus may facilitate the acoustic startle response by a long lasting excitation of neurons in the caudal pontine reticular nucleus.

Cholinergic neurons in the pedunculopontine tegmental nucleus are involved in the mediation of prepulse inhibition of the acoustic startle response in the rat

The amplitude of the acoustic startle response (ASR) is markedly reduced when the startle eliciting pulse is preceded by a weak, non-startling stimulus at an appropriate lead time, usually about 100 ms. This phenomenon is termed prepulse inhibition (PPI) and has received considerable attention in recent years as a model of sensorimotor gating. We report here on experiments which were undertaken in order to investigate some of the neural mechanisms of PPI. We focused on the characterization of the cholinergic innervation of the pontine reticular nucleus, caudal part (PnC), an obligatory relay station in the primary startle pathway. The combination of retrograde tracing with choline acetyltransferase-immunocytochemistry revealed a cholinergic projection from the pedunculopontine tegmental nucleus (PPTg) and laterodorsal tegmental nucleus (LDTg) to the PnC. Extracellular recording from single PnC units, combined with microiontophoretic application of the acetylcholine (ACh) agonists acetyl-beta-methylcholine (AMCH) and carbachol revealed that ACh inhibits the majority of acoustically responsive PnC neurons. Neurotoxic lesions of the cholinergic neurons of the PPTg significantly reduced PPI without affecting the ASR amplitude in the absence of prepulses. No effect on long-term habituation of the ASR was observed. The present data indicate that the pathway mediating PPI impinges upon the primary acoustic startle circuit through an inhibitory cholinergic projection from the PPTg to the PnC.

Topography of descending projections from the inferior colliculus to auditory brainstem nuclei in the rat

We examined the organization of descending projections from the inferior colliculus (IC) to auditory brainstem nuclei and to pontine and reticular nuclei in the rat by employing the anterograde axonal tracer Phaseolus vulgaris-leucoagglutinin (PHA-L). Small PHA-L injections into cytologically defined subnuclei of the IC revealed that each subnucleus has a unique pattern of efferent projections. The central nucleus of the IC projects in a topographic order to the dorsal nucleus of the lateral lemniscus (DLL), the rostral periolivary nucleus (RPO), the ventral nucleus of the trapezoid body (VNTB), and the dorsal cochlear nucleus (DCN). It is assumed that this topography represents a cochleotopic arrangement. The external cortex of the IC projects to the nucleus sagulum (Sag), the RPO, the VNTB, and the DCN. Minor projections were found to pontine and reticular nuclei. Efferent fibers from the dorsal cortex of the IC terminate mainly in the Sag, while other nuclei of the auditory and extra-auditory brainstem receive only minor projections. The intercollicular zone sends a moderate number of fibers to the DLL and very few, if any, to the remaining auditory brainstem nuclei. In contrast, fairly strong projections from the intercollicular zone to the reticular formation were found. The present data demonstrate that the four subnuclei of the IC have a differential pattern of descending projections to nuclei in the pontine and medullary brainstem. These parallel colliculofugal pathways, assumed to belong to functionally separate circuits, may modulate auditory processing at different levels of the auditory neuraxis.

Evidence for projections from medullary nuclei onto serotonergic and dopaminergic neurons in the midbrain dorsal raphe nucleus of the rat

The anterograde tracer Phaseolus vulgaris-leucoagglutinin was injected into the medial nucleus of the solitary tract and into the rostral dorsomedial medulla. A sequential two-color immunoperoxidase staining was accomplished in order to demonstrate the co-distribution of presumed terminal axons with chemically distinct neurons in the dorsal raphe nucleus of the midbrain central gray, i.e., B7 serotonergic and A10dc dopaminergic neurons. Black-stained efferent fibers from the medial nucleus of the solitary tract and the rostral dorsomedial medulla intermingled with brown-stained serotonergic (5-hydroxytryptamine-immunoreactive) or dopaminergic (tyrosine hydroxylase-immunoreactive) neurons. Light microscopy revealed that the black-stained efferent axons exhibited numerous en passant and terminal varicosities that were often found in close apposition to brown-stained serotonergic and dopaminergic somata, and to proximal and distal dendrites and dendritic processes. The close association of immunoreactive elements suggests the presence of axo-somatic and axo-dendritic synaptic contacts of medullary fibers with serotonergic and dopaminergic neurons in the dorsal raphe nucleus. These projections could be involved in the modulation of dorsal raphe neurons, depending on the autonomic status of an animal.

Organization of medullary adrenergic and noradrenergic projections to the periaqueductal gray matter in the rat

The periaqueductal or midbrain central gray matter (CG) in the rat contains a dense network of adrenergic and noradrenergic fibers. We examined the origin of this innervation by using retrograde and anterograde axonal tracers combined with immunohistochemistry for the catecholamine biosynthetic enzymes tyrosine hydroxylase (TH), dopamine beta-hydroxylase (DBH), and phenylethanolamine N-methyltransferase (PNMT). Following injections of the fluorescent tracers Fast Blue or Fluorogold into the CG, double-labeled neurons in the medulla were identified mainly in the noradrenergic A1 group in the caudal ventrolateral medulla (VLM) and A2 group in the medial part of the nucleus of the solitary tract (NTS); and in the adrenergic C1 group in the rostral ventrolateral medulla and C3 group in the rostral dorsomedial medulla. Injections of Phaseolus vulgaris-leucoagglutinin (PHA-L) into these cell groups resulted in a distinct pattern of axonal labeling in various subdivisions of the CG. Anterogradely labeled fibers originating in the medial NTS were predominantly found in the lateral portion of the dorsal raphe nucleus and in the adjacent part of the lateroventral CG (CGlv). Following PHA-L injections into the C3 region the anterogradely labeled fibers were diffusely distributed in the CGlv and the dorsal raphe nucleus at caudal levels, but rostrally tended to be located laterally in the CGlv. In contrast, ascending fibers from the caudal and rostral VLM terminated in the rostral dorsal part of the CGlv and in the dorsal nucleus of the CG, whereas ventral parts of the CG, including the dorsal raphe nucleus, contained few afferent fibers. Double-label studies with antisera against DBH and PNMT confirmed that noradrenergic neurons in the A1 and A2 groups and adrenergic neurons in the C1 and C3 groups contributed to these innervation patterns in the CGlv. Noradrenergic and adrenergic projections from the medulla to the CG may play an important role in a variety of autonomic, sensory and behavioral processes.

Auditory projections from the cochlear nucleus to pontine and mesencephalic reticular nuclei in the rat

We investigated projections from the cochlear nucleus in the rat using the anterograde tracer Phaseolus vulgaris-leucoagglutinin. We focused on nuclei in the brainstem which are not considered to be part of the classical auditory pathway. In addition to labeling in auditory nuclei, we found presumed terminal fibers in 4 pontine and mesencephalic areas: (1) the pontine nucleus (PN), which receives bilateral projections from the antero- and posteroventral cochlear nuclei; (2) the ventrolateral tegmental nucleus (VLTg), which receives a contralateral projection from the rostral portion of the anteroventral cochlear nucleus; (3) the caudal pontine reticular nucleus (PnC), which receives bilateral input originating predominantly in the dorsal cochlear nucleus; and (4) the lateral paragigantocellular nucleus (LPGi), which receives projections from all subdivisions of the cochlear nuclei. In the VLTg and PnC, anterogradely labeled varicose axons were often found in close apposition to the primary dendrites and somata of large reticular neurons. Injections of the retrograde fluorescent tracer Fluoro-Gold into the VLTg demonstrated that the neurons of origin are mainly located contralaterally in the rostral anteroventral cochlear nucleus and in the cochlear root nucleus. The relevance of these auditory projections for short-latency audio-motor behaviors and acoustically elicited autonomic responses is discussed.

Distribution and origin of noradrenergic and serotonergic fibers in the cochlear nucleus and inferior colliculus of the rat

We examined the monoaminergic innervation of the rat cochlear nucleus (CN) and the inferior colliculus (IC) by using retrograde transport of the fluorescent dye Fluoro-Gold combined with immunohistochemistry. We used antisera against the catecholamine synthesizing enzymes tyrosine hydroxylase (TH), dopamine-beta-hydroxylase (DBH) and phenylethanolamine-N-methyltransferase (PNMT), and one against the transmitter serotonin (5-HT). Each substance revealed a distinct pattern of immunoreactive staining in the CN and the IC. In the CN, DBH-immunoreactive (-ir) fibers were present in all subnuclei. The molecular layer of the dorsal CN and the granular layer of the ventral CN, however, were largely devoid of DBH-ir fibers. In contrast, 5-HT-ir fibers were abundant in the molecular layer and the granular cell layer of the CN. In the dorsal CN and the postero- and anteroventral CN, however, this innervation was less dense and evenly distributed across subnuclei. In the IC, the DBH-ir fibers were slightly more numerous in layer 2 of the dorsal cortex than in other subnuclei, while the layer 1 of both the dorsal and the external cortex contained only a few fibers. In contrast, the 5-HT-ir fibers formed a dense network in both the dorsal and external cortices of the IC, while they were less abundant in the remaining subnuclei. PNMT-ir fibers were not found in any of the auditory brainstem nuclei. Following Fluoro-Gold injections into the CN or IC, retrogradely labeled DBH-ir neurons were found in the A6 noradrenergic cell group (locus coeruleus). The CN received additional projections from the A5 noradrenergic cell group, as well as sparse projections from the A4 and A7 cell groups. The serotonergic innervation of the CN and IC originated largely in the B7 serotonergic cell group (dorsal raphe nucleus). Serotonergic neurons in other groups of the raphe nuclei were only occasionally labeled. Our data indicate that both noradrenaline and serotonin may play a role in central auditory processing. Their differential distribution in the IC and CN subnuclei suggests that these transmitter systems might influence different functional circuits.

Efferent projections of the infralimbic cortex of the rat

On the basis of stimulation studies, it has been proposed that the infralimbic cortex (ILC), Brodmann area 25, may serve as an autonomic motor cortex. To explore this hypothesis, we have combined anterograde tracing with Phaseolus vulgaris leucoagglutinin (PHA-L) and retrograde tracing with wheat germ aggutinin conjugated to horseradish peroxidase (WGA-HRP) to determine the efferent projections from the ILC. Axons exit the ILC in one of three efferent pathways. The dorsal pathway ascends through layers III and V to innervate the prelimbic and anterior cingulate cortices. The lateral pathway courses through the nucleus accumbens to innervate the insular cortex, the perirhinal cortex, and parts of the piriform cortex. In addition, some fibers from the lateral pathway enter the corticospinal tract. The ventral pathway is by far the largest and innervates the thalamus (including the paraventricular nucleus of the thalamus, the border zone between the paraventricular and medial dorsal nuclei, and the paratenial, reuniens, ventromedial, parafasicular, and subparafasicular nuclei), the hypothalamus (including the lateral hypothalamic and medial preoptic areas, and the suprachiasmatic, dorsomedial, and supramammillary nuclei), the amygdala (including the central, medial, and basomedial nuclei, and the periamygdaloid cortex) and the bed nucleus of the stria terminalis. The ventral efferent pathway also provides descending projections to autonomic cell groups of the brainstem and spinal cord including the periaqueductal gray matter, the parabrachial nucleus, the nucleus of the solitary tract, the dorsal motor vagal nucleus, the nucleus ambiguus, and the ventrolateral medulla, as well as lamina I and the intermediolateral column of the spinal cord. The ILC has extensive projections to central autonomic nuclei that may subserve a role in modulating visceral responses to emotional stimuli, such as stress.

Topography of projections from the auditory cortex to the inferior colliculus in the rat

We examined the organization of descending projections from auditory and adjacent cortical areas to the inferior colliculus (IC) in the rat by using the retrograde and anterograde transport of wheat germ agglutinin-horseradish peroxidase. Small tracer injections were placed into cytologically defined subnuclei of the IC. On the basis of the resulting pattern of retrogradely labeled neurons in the cortex, different cortical areas and fields were defined. Two secondary areas located ventrocaudally (Te2) and ventrally (Te3) to the primary auditory area (Te1) were delineated. The primary auditory area was subdivided into a posterior (Te1.p), a medial (Te1.m), and an anterior (Te1.a) auditory field. In addition, we outlined an area located rostrally to the auditory areas comprising a part of the secondary somatosensory cortex, as well as a dorsal belt surrounding dorsally the auditory areas. The following basic patterns of corticocollicular projections are revealed: 1) layers 2 and 3 of the dorsal cortex of the IC (DC2, DC3) are differentially innervated by the primary auditory fields (Te1.p and Te1.a project bilaterally to DC2, while Te1.m projects bilaterally and in topographical order to DC3); cells in Te1.m, arranged in caudal to rostral sequence, project to corresponding loci in DC3 arranged from dorsolateral to ventromedial; 2) the fibrocellular capsule of the IC, comprising layer 1 of the dorsal and external cortex of the IC, receives input from the secondary auditory area Te2; 3) layers 2 and 3 of the external cortex of the IC are only weakly innervated by the primary and secondary auditory cortex; 4) the intercollicular zone receives its major input from the secondary auditory area Te3, the secondary somatosensory cortex, and the dorsal belt; and 5) finally, the central nucleus of the IC receives no input from the temporal cortex at all. Our results demonstrate that the corticocollicular projections are highly organized. These pathways may modulate auditory processing in different functional circuits of the inferior colliculus.

Organization of cortical, basal forebrain, and hypothalamic afferents to the parabrachial nucleus in the rat

In a previous study (Herbert et al., J. Comp. Neurol. [1990];293:540-580), we demonstrated that the ascending afferent projections from the medulla to the parabrachial nucleus (PB) mark out functionally specific terminal domains within the PB. In this study, we examine the organization of the forebrain afferents to the PB. The PB was found to receive afferents from the infralimbic, the lateral prefrontal, and the insular cortical areas; the dorsomedial, the ventromedial, the median preoptic, and the paraventricular hypothalamic nuclei; the dorsal, the retrochiasmatic, and the lateral hypothalamic areas; the central nucleus of the amygdala; the substantia innominata; and the bed nucleus of the stria terminalis. In general, forebrain areas tend to innervate the same PB subnuclei from which they receive their input. Three major patterns of afferent termination were noted in the PB; these corresponded to the three primary sources of forebrain input to the PB: the cerebral cortex, the hypothalamus, and the basal forebrain. Hypothalamic afferents innervate predominantly rostral portions of the PB, particularly the central lateral and dorsal lateral subnuclei. The basal forebrain projection to the PB ends densely in the external lateral and waist subnuclei. Cortical afferents terminate most heavily in the caudal half of the PB, particularly in the ventral lateral and medial subnuclei. In addition, considerable topography organization was found within the individual projections. For example, tuberal lateral hypothalamic neurons project heavily to the central lateral subnucleus and lightly to the waist area; in contrast, caudal lateral hypothalamic neurons send a moderately heavy projection to both the central lateral and waist subnuclei. Our results show that the forebrain afferents of the PB are topographically organized. These topographical differences may provide a substrate for the diversity of visceral functions associated with the PB.

Connections of the parabrachial nucleus with the nucleus of the solitary tract and the medullary reticular formation in the rat

We examined the subnuclear organization of projections to the parabrachial nucleus (PB) from the nucleus of the solitary tract (NTS), area postrema, and medullary reticular formation in the rat by using the anterograde and retrograde transport of wheat germ agglutinin-horseradish peroxidase conjugate and anterograde tracing with Phaseolus vulgaris-leucoagglutinin. Different functional regions of the NTS/area postrema complex and medullary reticular formation were found to innervate largely nonoverlapping zones in the PB. The general visceral part of the NTS, including the medial, parvicellular, intermediate, and commissural NTS subnuclei and the core of the area postrema, projects to restricted terminal zones in the inner portion of the external lateral PB, the central and dorsal lateral PB subnuclei, and the "waist" area. The dorsomedial NTS subnucleus and the rim of the area postrema specifically innervate the outer portion of the external lateral PB subnucleus. In addition, the medial NTS innervates the caudal lateral part of the external medial PB subnucleus. The respiratory part of the NTS, comprising the ventrolateral, intermediate, and caudal commissural subnuclei, is reciprocally connected with the Kölliker-Fuse nucleus, and with the far lateral parts of the dorsal and central lateral PB subnuclei. There is also a patchy projection to the caudal lateral part of the external medial PB subnucleus from the ventrolateral NTS. The rostral, gustatory part of the NTS projects mainly to the caudal medial parts of the PB complex, including the "waist" area, as well as more rostrally to parts of the medial, external medial, ventral, and central lateral PB subnuclei. The connections of different portions of the medullary reticular formation with the PB complex reflect the same patterns of organization, but are reciprocal. The periambiguus region is reciprocally connected with the same PB subnuclei as the ventrolateral NTS; the rostral ventrolateral reticular nucleus with the same PB subnuclei as both the ventrolateral (respiratory) and medial (general visceral) NTS; and the parvicellular reticular area, adjacent to the rostral NTS, with parts of the central and ventral lateral and the medial PB subnuclei that also receive rostral (gustatory) NTS input. In addition, the rostral ventrolateral reticular nucleus and the parvicellular reticular formation have more extensive connections with parts of the rostral PB and the subjacent reticular formation that receive little if any NTS input. The PB contains a series of topographically complex terminal domains reflecting the functional organization of its afferent sources in the NTS and medullary reticular formation.