Ascending projections of the locus coeruleus in the rat. II. Autoradiographic study.

The ascending projections of the locus coeruleus were studied using an autoradiographic method. The major projection of locus coeruleus neurons ascends in a dorsal pathway traversing the midbrain tegmentum in a position ventrolateral to the periaqueductal gray. At the caudal diencephalon the locus coeruleus axons descend to enter the medial forebrain bundle at a caudal tuberal hypothalamic level. They are jointed in the medial forebrain bundle by a much smaller locus coeruleus projection which takes a ventral course through the midbrain tegmentum and enters the medial forebrain bundle via the mammillary peduncle and ventral tegmental area. Terminal projections are evident in the midbrain to the periaqueductal gray, tegmentum and raphe nuclei. There are widespread projections to the dorsal thalamus. The heaviest of these are to the intralaminar nuclei, the anteroventral and anteromedial nuclei, the dorsal lateral geniculate and the paraventricular nucleus. In the hypothalamus the largest projections are to the lateral hypothalamic area, periventricular nucleus, supraoptic nucleus and paraventricular nucleus. As the locus coeruleus projection ascends in the medial forebrain bundle, fibers leave it to traverse the lateral hypothalamus and zona incerta and enter the internal capsule, the ventral amygdaloid bundle and ansa peduncularis. These appear to terminate in the amygdaloid complex and, via the external capsule, in the lateral and dorsal neocortex. At the level of the septum 4 projections are evident. One group of fibers enters the stria medullaris to terminate in the paraventricular nucleus and habenular nuclei. A second group joins the stria terminalis to terminate in the anygdaloid complex. The third group turns into the diagonal band and medial septum; some fibers terminate in the septal nuclei and others continue into the fornix to termimate in hippocampus. A large component continues around the corpus callosum into the cingulum to terminate in the cingulate and adjacent neocortex, the subiculum and hippocampus. The remaining fibers continue rostrally in the medial forebrain bundle to terminate in olfactory forebrain and frontal neocortex. Commissural projections arise at 4 locations. The first decussation occurs in the dorsal tegmentum just below the central gray rostral to the locus coeruleus. The crossing fibers enter the contralateral dorsal bundle. A second group of fibers leaves the ipsilateral dorsal pathway, crosses in the posterior commissure and enters the contralateral dorsal pathway at the level. The third commissural projection arises more rostrally and crosses in the dorsal supraoptic commissure to enter the contralateral medial forebrain bundle. The fourth commissural projection is through the anterior commissure. The termination of the contralateral projection appears similar to that of the ipsilateral projection.     

Topographic organization of the brainstem afferents to the mediodorsal thalamic nucleus

The afferent projections from the brainstem to the mediodorsal thalamic nucleus (MD) were studied in the cat, by means of retrograde transport of horseradish peroxidase. A topographical arrangement of these projections is described. The medial part of MD is the area of the nucleus which receives fewer afferents from the brainstem. After injections in this part, labeled neurons were observed mainly in the interpeduncular nucleus, the ventral tegmental area and the substantia nigra. After injections of HRP in the intermediate part of the MD, labeled cells were seen mainly in the interpeduncular nucleus, substantia nigra, dorsal and centralis superior raphe nuclei, dorsal tegmental nucleus, and coeruleus complex. Less conspicuous was the number of labeled cells in the central gray and the dorsolateral portion of the tegmentum of the mesencephalon and pons. After injections in the lateral part of MD, labeled neurons were observed mainly in the deep layers of the superior colliculus, central gray, the oral paramedian pontine reticular tegmentum, and the interpeduncular nucleus. Labeled cells were also observed in the substantia nigra, locus coeruleus, dorsal tegmental nucleus, cuneiform area, and the mesencephalic reticular formation. These findings show the MD as a thalamic link of three different groups of brainstem structures projecting to different cortical areas with different functional significance.     

The morphology and divergent axonal organization of midbrain raphe projection neurons in the rat

The morphology of dorsal raphe neurons was examined using intracellular injections of horseradish peroxidase (HRP) and the Golgi technique. Light microscopic examination of HRP-labeled projection neurons revealed a neuron type with radiating, poorly branched and sparsely spined dendrites and terminal dendritic thickets. The stem axon of these neurons left the nucleus ventrally but gave off a beaded collateral while still within the parent cell's dendritic domain. Somatodendritic morphology from Golgi-Kopsch stained material coincided with intracellular HRP findings and the dorsal raphe may consist of varieties of one basic morphological type of neuron. Intracellular recordings made during the HRP injection experiments confirmed that stimulation of the ventral medial tegmentum elicited an antidromic action potential and an inhibitory postsynaptic potential in dorsal raphe projection neurons. The order of axonal projections arising from the midbrain raphe nuclei was examined using a double retrograde axonal tracing technique. After paired HRP and [3H] wheat germ agglutinin injections within certain projection targets of the dorsal and median raphe neurons (caudate-putamen, amygdala, hippocampus, substantia nigra and locus coeruleus), each target structure was found to have its own unique representation within a topographically distinct portion of one or more of the raphe subgroups. Neurons projecting to the caudate-putamen and substantia nigra occupied rather rostral portions. Neurons projecting to the hippocampus and locus coeruleus resided more caudally. Neurons projecting to the amygdala were situated intermediately. Overall, rostrocaudal topography in the intranuclear distributions of raphe projection neurons resulted in the formation of complex overlap zones where collateralized neurons always resided.     

Dorsal raphe, substantia nigra and locus coeruleus: Interconnections with each other and the neostriatum

Using a retrograde axonal transport method, direct projections to the neostriatum were demonstrated from the dorsal raphe nucleus, a large area of the ventral midbrain tegmentum (including the ventral tegmental area of Tsai, the substantia nigra pars compacta, reticulata and suboculomotoria), and the tegmentum ventral to the caudal red nucleus. A direct projection was also found from the mediodorsal part of the substantia nigra to the rostral part of the dorsal raphe nucleus. Projections from the entopeduncular nucleus (pallidum) and the lateral hypothalamic area to the lateral habenular nucleus, and from the latter to the dorsal raphe nucleus were also found. This habenular projection arises primarily from large neurons in the medial part of the lateral habenula and also from another group of small cells immediately adjacent to the medial habenular nucleus. A non-reciprocal connection of the dorsal raphe nucleus to the locus coeruleus was also found. On the basis of these results and the data available in the literature on the possible neurotransmitters used by these various structures, it is suggested that the dorsal raphe nucleus may play an important role in brain stem modulation of neostriatal function.     

Serotonergic synaptic input to cholinergic neurons in the rat mesopontine tegmentum

Serotonergic synaptic inputs to cholinergic neurons in the laterodorsal and pedunculopontine tegmental nuclei were examined with pre-embedding dual-label immunoelectron microscopy. Numerous serotonin-immunoreactive axon terminals visualized with a silver-enhanced immunogold method were present in both of these tegmental nuclei. Serotonergic terminals occasionally made synaptic contacts with the soma and proximal dendrites of cholinergic tegmental neurons labelled with a choline acetyltransferase-immunoreactive peroxidase-anti-peroxidase diaminobenzidine reaction product. In the rostralmost region of the laterodorsal tegmental nucleus, a few serotonergic neurons of the dorsal raphe nucleus were interspersed among cholinergic neurons. Some dendrites of these serotonergic neurons appeared to contain synaptic vesicles. Both myelinated and unmyelinated serotonergic axons were present in the mesopontine tegmentum. The presence of serotonergic synapses onto tegmental cholinergic neurons is consistent with previous behavioral and electrophysiological findings suggesting an inhibitory role of serotonin in the induction of rapid eye movement sleep and its phenomenology through an action on cholinergic neurons in the mesopontine tegmentum.     

Efferent connections of the substantia nigra and ventral tegmental area in the rat

Small injections of tritiated leucine and proline confined to the ventral tegmental area (AVT) were found to label fibers ascending: (a) to the entire ventromedial half of the striatum, but most massively to the ventral striatal zone that includes the nucleus accumbens; (b) to the thalamus: lateral habenular nucleus, nuclei reuniens and centralis medius, and the most medial zone of the mediodorsal nucleus; (c) to the posterior hypothalamic nucleus and possibly the lateral hypothalamic and preoptic region; (d) to the nuclei amygdalae centralis, lateralis and medialis; (e) to the bed nucleus of the stria terminalis, the nucleus of the diagonal band, and the medial half of the lateral septal nucleus; (f) to the anteromedial (frontocingulate) cortex; and (g) to the entorhinal area. Further AVT efferents descend to the medial half of the midbrain tegmentum including an anterior region of the median raphe nucleus, to the ventral half of the central grey substance including the dorsal raphe nucleus, to the parabrachial nuclei, and to the locus coeruleus. Similar injections centered in the pars compacta of the substantia nigra (SNC) label fibers that are distributed in the striatum in an orderly medial-to-lateral arrangement, and almost entirely avoid the nucleus accumbens and olfactory tubercle. With the exception of the lateral quarter of the substantia nigra, which apparently does not project to the extreme rostral pole of the striatum, each small SNC locus, regardless of its anteroposterior localization, distributes nigrostriatal fibers throughout the length of the striatum. Descending SNC efferents are distributed to the same general regions that receive descending AVT projections, except that no SNC fibers appear to enter the locus coeruleus. Isotope injections confined to the pars reticulata (SNR) label sparse nigrostriatal fibers, and numerous nigrothalamic fibers ascending mainly to the nucleus ventromedialis and in lesser number to the parafascicular nucleus and the paralamellar zone of the nucleus mediodorsalis. Descending SNR fibers leave the nigra as a voluminous fiber bundle that bifurcates into a large nigrotectal and a smaller nigrotegmental component, the latter terminating largely in the pedunculopontine nucleus of the pontomesencephalic tegmentum.     

Brainstem projections to midline and intralaminar thalamic nuclei of the rat

The projections from the brainstem to the midline and intralaminar thalamic nuclei were examined in the rat. Stereotaxic injections of the retrograde tracer cholera toxin beta -subunit (CTb) were made in each of the intralaminar nuclei of the dorsal thalamus: the lateral parafascicular, medial parafascicular, central lateral, paracentral, oval paracentral, and central medial nuclei; in the midline thalamic nuclei-the paraventricular, intermediodorsal, mediodorsal, paratenial, rhomboid, reuniens, and submedius nuclei; and, in the anteroventral, parvicellular part of the ventral posterior, and caudal ventral medial nuclei. The retrograde cell body labeling pattern within the brainstem nuclei was then analyzed. Nearly every thalamic site received a projection from the deep mesencephalic reticular, pedunculopontine tegmental, dorsal raphe, median raphe, laterodorsal tegmental, and locus coeruleus nuclei. Most intralaminar thalamic sites were also innervated by unique combinations of medullary and pontine reticular formation nuclei such as the subnucleus reticularis dorsalis, gigantocellular, dorsal paragigantocellular, lateral, parvicellular, caudal pontine, ventral pontine, and oral pontine reticular nuclei; the dorsomedial tegmental, subpeduncular tegmental, and ventral tegmental areas; and, the central tegmental field. In addition, most intralaminar injections resulted in retrograde cell body labeling in the substantia nigra, nucleus Darkschewitsch, interstitial nucleus of Cajal, and cuneiform nucleus. Details concerning the pathways from the spinal trigeminal, nucleus tractus solitarius, raphe magnus, raphe pallidus, and the rostral and caudal linear raphe nuclei to subsets of midline and intralaminar thalamic sites are discussed in the text. The discussion focuses on brainstem-thalamic pathways that are likely involved in arousal, somatosensory, and visceral functions.     

Locus coeruleus neuron density and parkinsonism in older adults without Parkinson's disease

Previous work has showed that nigral neuron density is related to the severity of parkinsonism proximate to death in older persons without a clinical diagnosis of Parkinson's disease (PD). We tested the hypothesis that neuron density in other brain stem aminergic nuclei is also related to the severity of parkinsonism. We studied brain autopsies from 125 deceased older adults without PD enrolled in the Memory and Aging Project, a clinicopathologic investigation. Parkinsonism was assessed with a modified version of the Unified Parkinson's Disease Rating Scale (UPDRS). We measured neuron density in the substantia nigra, ventral tegmental area, locus coeruleus, and dorsal raphe, along with postmortem indices of Lewy body disease, Alzheimer's disease and cerebrovascular pathologies. Mean age at death was 88.0 years, and global parkinsonism was 14.8 (SD, 9.50). In a series of regression models that controlled for demographics and neuron density in the substantia nigra, neuron density in the locus coeruleus (estimate, ?0.261; SE, 0.117; P = .028) but not in the ventral tegmental area or dorsal raphe was associated with severity of global parkinsonism proximate to death. These findings were unchanged in models that controlled for postmortem interval, whole‐brain weight, and other common neuropathologies including Alzheimer's disease and Lewy body pathology and cerebrovascular vascular pathologies. In older adults without a clinical diagnosis of PD, neuron density in locus coeruleus nuclei is associated with the severity of parkinsonism and may contribute to late‐life motor impairments. © 2012 Movement Disorder Society     

Afferent connections of septal nuclei of the domestic chick (Gallus domesticus): a retrograde pathway tracing study

The afferents to the septum of the domestic chicken were studied using retrograde tracers, rhodamine conjugated latex bead or Fast Blue, placed in different septal subregions. The results were verified by anterograde tracer injections deposited to selected areas. The main telencephalic afferents to the septum arise ipsilaterally from the hippocampal formation, dorsolateral corticoid area, piriform cortex, amygdaloid pallium, and the ventral pallidum. Contralateral afferents originate from the lateral septum and the amygdaloid pallium. A massive bilateral projection arises from the lateral hypothalamus. Other hypothalamic afferents arise from the periventricular, paraventricular and anterior medial nuclei, and the premammillary and mammillary areas. The dorsal thalamic nuclei (dorsal medial anterior and posterior) and the reticular dorsal nuclei also contribute septal afferents. Brainstem afferents arise bilaterally from the ventral tegmental area, substantia nigra, central gray, A8, locus coeruleus, ventral subcoeruleus nucleus, and raphe nuclei. The main terminal fields for septal afferents lie in the lateral septal nucleus and the belt of medial septal nucleus. The core of the latter is invaded mainly by fibers from the brainstem, presumably belonging to the ascending activating system. The septal afferents of the chicken are largely similar to those of other avian and nonavian species. The most prominent differences with previous pigeon data were found in the subregional selectivity of the hippocampal formation, dorsolateral corticoid area, mammillary nuclei, some dorsal thalamic nuclei, substantia nigra, and subcoeruleus nuclei in their projections to defined septal nuclei.     

Distribution of aromatic L-amino acid decarboxylase mRNA in mouse brain by in situ hybridization histology

Aromatic L-amino acid decarboxylase (AAAD) is the second enzyme in the sequence leading to the synthesis of catecholamines or serotonin. Antisense riboprobes for aromatic L-amino acid decarboxylase mRNA were used to map the gene in mouse brain by in situ hybridization. The substantia nigra, the ventral tegmental nucleus, the dorsal raphe nucleus, the locus coeruleus, and the olfactory bulb contained the highest signal for AAAD mRNA. After treatment with the dopaminergic neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), the signal disappeared in the substantia nigra, decreased somewhat in the ventral tegmental area, and remained unchanged in the dorsal raphe nucleus. Hypothalamic and cerebellar Purkinje neurons known to contain histidine decarboxylase or glutamic acid decarboxylase, respectively, were unlabeled by the probes. However, neurons in the deep layers of the frontal cortex, many thalamic nuclei, and the pyramidal neurons of the hippocampus were lightly to moderately labeled for mouse AAAD mRNA. The presence of AAAD message in these neurons suggests that the enzyme has functions other than that for the synthesis of the classical biogenic amine neurotransmitters. © 1993 Wiley-Liss,Inc.     

Some anatomical observations on the projections from the hypothalamus to brainstem and spinal cord: an HRP and autoradiographic tracing study in the cat

The hypothalamus is closely involved in a wide variety of behavioral, autonomic, visceral, and endocrine functions. To find out which descending pathways are involved in these functions, we investigated them by horseradish peroxidase (HRP) and autoradiographic tracing techniques. HRP injections at various levels of the spinal cord resulted in a nearly uniform distribution of HRP-labeled neurons in most areas of the hypothalamus except for the anterior part. After HRP injections in the raphe magnus (NRM) and adjoining tegmentum the distribution of labeled neurons was again uniform, but many were found in the anterior hypothalamus as well. Injections of 3H-leucine in the hypothalamus demonstrated that: The anterior hypothalamic area sent many fibers through the medial forebrain bundle (MFB) to terminate in the ventral tegmental area of Tsai (VTA), the rostral raphe nuclei, the nucleus Edinger-Westphal, the dorsal part of the substantia nigra, the periaqueductal gray (PAG), and the interpeduncular nuclei. Further caudally a lateral fiber stream (mainly derived from the lateral parts of the anterior hypothalamic area) distributed fibers to the parabrachial nuclei, nucleus subcoeruleus, locus coeruleus, the micturition-coordinating region, the caudal brainstem lateral tegmentum, and the solitary and dorsal vagal nucleus. Furthermore, a medial fiber stream (mainly derived from the medial parts of the anterior hypothalamic area) distributed fibers to the superior central and dorsal raphe nucleus and to the NRM, nucleus raphe pallidus (NRP), and adjoining tegmentum. The medial and posterior hypothalamic area including the paraventricular hypothalamic nucleus (PVN) sent fibers to approximately the same mesencephalic structures as the anterior hypothalamic area. Further caudally two different fiber bundles were observed. A medial stream distributed labeled fibers to the NRM, rostral NRP, the upper thoracic intermediolateral cell group, and spinal lamina X. A second and well-defined fiber stream, probably derived from the PVN, distributed many fibers to specific parts of the lateral tegmental field, to the solitary and dorsal vagal nuclei, and, in the spinal cord, to lamina I and X, to the thoracolumbar and sacral intermediolateral cell column, and to the nucleus of Onuf. The lateral hypothalamic area sent many labeled fibers to the lateral part of the brainstem and many terminated in the caudal brainstem lateral tegmentum, including the parabrachial nuclei, locus coeruleus, nucleus subcoeruleus, and the solitary and dorsal vagal nuclei.(ABSTRACT TRUNCATED AT 400 WORDS)     

Pedunculopontine tegmental nucleus of the rat: cytoarchitecture, cytochemistry, and some extrapyramidal connections of the mesopontine tegmentum

The pedunculopontine tegmental nucleus (PPTn) was originally defined on cytoarchitectonic grounds in humans. We have employed cytoarchitectonic, cytochemical, and connectional criteria to define a homologous cell group in the rat. A detailed cytoarchitectonic delineation of the mesopontine tegmentum, including the PPTn, was performed employing tissue stained for Nissl substance. Choline acetyltransferase (ChAT) immunostained tissue was then analyzed in order to investigate the relationship of cholinergic perikarya, dendritic arborizations, and axonal trajectories within this cytoarchitectonic scheme. To confirm some of our cytoarchitectonic delineations, the relationships between neuronal elements staining for ChAT and tyrosine hydroxylase were investigated on tissue stained immunohistochemically for the simultaneous demonstration of these two enzymes. The PPTn consists of large, multipolar neurons, all of which stain immunohistochemically for ChAT. It is present within cross-sections that also include the A-6 through A-9 catecholamine cell groups and is traversed by catecholaminergic axons within the dorsal tegmental bundle and central tegmental tract. The dendrites of PPTn neurons respect several nuclear boundaries and are oriented perpendicularly to several well-defined fiber tracts. Cholinergic axons ascend from the mesopontine tegmentum through the dorsal tegmental bundle and a more lateral dorsal ascending pathway. A portion of the latter terminates within the lateral geniculate nucleus. It has been widely believed that the PPTn is reciprocally connected with several extrapyramidal structures, including the globus pallidus and substantia nigra pars reticulata. Therefore, the relationships of pallidotegmental and nigrotegmental pathways to the PPTn were investigated employing the anterograde autoradiographic methodology. The reciprocity of tegmental connections with the substantia nigra and entopeduncular nucleus was investigated employing combined WGA-HRP injections and ChAT immunohistochemistry. The pallido- and nigrotegmental terminal fields did not coincide with the PPTn, but, rather, were located just medial and dorsomedial to it (the midbrain extrapyramidal area). The midbrain extrapyramidal area, but not the PPTn, was reciprocally connected with the substantia nigra and entopeduncular nucleus. We discuss these results in light of other cytoarchitectonic, cytochemical, connectional, and physiologic studies of the functional anatomy of the mesopontine tegmentum.     

Cognitive dysfunction and dementia in Parkinson’s disease

Parkinson's disease (PD) is a slowly progressive neurodegenerative disorder mainly characterized by degeneration of dopaminergic neurons in the substantia nigra and the ventral tegmental area, in combination with a varying loss of central noradrenergic (locus coeruleus), cholinergic (nucleus basalis of Meynert) and serotonergic (dorsal raphe nuclei) integrity, leading to a multitude of motor and non-motor behavioral disturbances. Apart from the clinical motor hallmarks, in the early stages of disease, subtle cognitive dysfunction might be seen comprising mainly executive dysfunction, with secondary visuospatial and mnemonic disturbances. In about 20-40% of patients, these problems may eventually proceed to dementia, which constitutes an important risk factor for caregiver distress, decreased quality of life and nursing home placement. Dementia in PD is typically characterized by a progressive dysexecutive syndrome with attentional deficits and fluctuating cognition, often accompanied by psychotic symptoms. It is thought to be the result of a combination of both subcortical and cortical changes. PD-related dopaminergic deficiency in the nucleus caudatus and mesocortical areas (due to degeneration of projections from the substantia nigra and ventral tegmental area) and cholinergic deficiency in the cortex (due to degeneration of ascending projections from the nucleus basalis of Meynert), combined with additional Alzheimer-pathology and cortical Lewy bodies, may greatly contribute to dementia.Current treatment of dementia in PD is based on compensation of the profound cholinergic deficiency. Recent studies with the cholinesterase inhibitors galantamine, donepezil and rivastigmine show promising results in improving cognition and ameliorating psychotic symptoms, which must further be confirmed in randomized controlled trials.     

Brainstem innervation of prefrontal and anterior cingulate cortex in the rhesus monkey revealed by retrograde transport of HRP

The cells of origin of projections from the brainstem to the dorsolateral and orbital prefrontal granular cortex and to the anterior cingulate cortex of the rhesus monkey were analyzed by means of retrograde axonal transport of the enzyme horseradish peroxidase (HRP). Following injections in various portions of the dorsolateral prefrontal and in the cingulate cortex, HRP-positive neurons were found in three main locations: (1) the ventral midbrain including the anterior ventral tegmental area, the medial one-third of the substantia nigra pars compacta, and the retrorubral nucleus; (2) the central superior nucleus and the dorsal raphe nucleus, primarily in its caudal subdivision; and (3) the locus coeruleus and adjacent medial parabrachial nucleus. Labeled neurons in the raphe nuclei and locus coeruleus were distributed bilaterally. A basically similar pattern of labeled somata was found in the brainstem with HRP injections in the orbital prefrontal cortex. Scattered HRP-positive cells were found throughout the ipsilateral ventral tegmental area and in ventromedial portions of the retrorubral nucleus, and a large number of HRP-positive cells were distributed bilaterally in the dorsal raphe and central superior nuclei as well as the dorsolateral pontine tegmentum. However, in contrast to the results obtained with injections on the dorsolateral and medial aspects of the hemisphere, labeled neurons were not found in any portion of the substantia nigra. The neurons labeled retrogradely after injection of HRP in these various regions of the frontal lobe in rhesus monkey correspond both in location and morphology to the monoamine-containing neurons of the brainstem and are thus very likely the source of dopamine, norepinephrine, and serotonin found in the frontal cortex of the same species.     

Origins of the glycinergic inputs to the rat locus coeruleus and dorsal raphe nuclei: a study combining retrograde tracing with glycine immunohistochemistry

The amino acid glycine is a major inhibitory neurotransmitter in the brainstem and is likely involved in the tonic inhibition of the monoaminergic neurons during all sleep-waking stages. In order to determine the neurons at the origin of the glycinergic innervation of the two principal monoaminergic nuclei, the locus coeruleus and the dorsal raphe of the rat, we applied a double-labelling technique, combining retrograde transport of cholera-toxin B subunit with glycine immunohistochemistry. Using this technique, we found that the locus coeruleus and dorsal raphe nuclei receive a common glycinergic innervation from the ventral and ventrolateral periaqueductal grey, including the adjacent deep mesencephalic reticular nucleus. Small additional glycinergic inputs to these nuclei originated from the lateral paragigantocellular nucleus and the rostral ventromedial medullary reticular formation. The potential role of these glycinergic inputs in the control of the excitability of the monoaminergic neurons of the locus coeruleus and dorsal raphe nuclei is discussed.     

Nucleus cuneiformis and pain modulation: anatomy and behavioral pharmacology

The anatomical substrate and behavioral pharmacology of stimulation-produced analgesia resulting from electrical stimulation of the pontomesencephalic nucleus cuneiformis (NCF) was determined in the present study. Maximum increase in nociceptive tail-flick latencies following NCF stimulation occurred during the first 5 min post stimulation and decreased afterwards. The increased reflex latency could be attenuated by prior treatment with the narcotic antagonist, naloxone or the cholinergic antagonist, scopolamine. The anatomical projections of NCF were identified in autoradiographic and histochemical studies. Ipsilateral fibers coursed caudal from the NCF injection site through the ventral pontine reticular formation to innervate nucleus raphe magnus and the ipsilateral nucleus magnocellularis. At rostral medullary levels fibers coursed dorsolateral to innervate the ipsilateral nucleus reticularis parvocellularis. Descending contralateral fibers crossed through the decussation of the superior cerebellar peduncle, then coursed ventrolaterally projecting to the contralateral nucleus magnocellularis. Two primary groups of ascending fibers were observed. The dorsally located group ascended through the central tegmental tract projecting to the dorsal raphe, ipsilateral periaqueductal gray, nucleus parafascicularis and centromedianus, the intermediolateral and lateral thalamic nuclei. The ventral group coursed ventrolateral from the injection site projecting to the substantia nigra, zona compacta, ventral tegmental area of Tsai, zona incerta, Fields of Forel, lateral hypothalamic nucleus and nucleus reuniens. These anatomic and behavioral data suggest that NCF plays an important role in sensory/motor integration relevant to pain transmission.     

Patterns of afferent projections to the dentate gyrus studied in organotypic co-cultures

Cholinergic axons originating from the septum form a characteristic layer of preterminal axons and apparent termination in the molecular layer of the hippocampal dentate gyrus. The present study explored the specificity of this characteristic axonal pattern, through the use of organotypic slice co-cultures. Slices of hippocampus were co-cultured with a slice from one of a variety of other potential sources of afferents, and the afferent axons were labeled histochemically or immunocytochemically to determine which afferents distribute within the dentate molecular layer in a pattern similar to that formed by septal cholinergic projections. Acetylcholinesterase (AChE) histochemistry demonstrated that cholinergic axons from septum, substantia innominata, and striatum all consistently targeted the inner molecular layer of the dentate gyrus. AChE-labeled cholinergic axons from dorsal lateral pontine tegmentum and from spinal cord sometimes formed this pattern, while axons from the habenula failed to extend into the dentate gyrus. Immunocytochemically identified monoaminergic axons from the substantia nigra, locus coeruleus, and raphe extended into co-cultured hippocampus; each of these afferent systems displayed a prominent axonal plexus within the hilus of the dentate, but only the raphe axons projected prominently to the molecular layer. These data demonstrate that the molecular layer of the dentate gyrus provides an attractive target zone for some cholinergic and monoaminergic afferents, but not all. Commonalities between neuronal populations that preferentially project to the molecular layer in vitro may offer clues regarding the axon guidance mechanisms that normally direct cholinergic axons to target sites in the dentate gyrus molecular layer.     

Cytoarchitecture and efferent projections of the nucleus incertus of the rat

The nucleus incertus is located caudal to the dorsal raphe and medial to the dorsal tegmentum. It is composed of a pars compacta and a pars dissipata and contains acetylcholinesterase, glutamic acid decarboxylase, and cholecystokinin-positive somata. In the present study, anterograde tracer injections in the nucleus incertus resulted in terminal-like labeling in the perirhinal cortex and the dorsal endopyriform nucleus, the hippocampus, the medial septum diagonal band complex, lateral and triangular septum medial amygdala, the intralaminar thalamic nuclei, and the lateral habenula. The hypothalamus contained dense plexuses of fibers in the medial forebrain bundle that spread in nearly all nuclei. Labeling in the suprachiasmatic nucleus filled specifically the ventral half. In the midbrain, labeled fibers were observed in the interpeduncular nuclei, ventral tegmental area, periaqueductal gray, superior colliculus, pericentral inferior colliculus, pretectal area, the raphe nuclei, and the nucleus reticularis pontis oralis. Retrograde tracer injections were made in areas reached by anterogradely labeled fibers including the medial prefrontal cortex, hippocampus, amygdala, habenula, nucleus reuniens, superior colliculus, periaqueductal gray, and interpeduncular nuclei. All these injections gave rise to retrograde labeling in the nucleus incertus but not in the dorsal tegmental nucleus. These data led us to conclude that there is a system of ascending projections arising from the nucleus incertus to the median raphe, mammillary complex, hypothalamus, lateral habenula, nucleus reuniens, amygdala, entorhinal cortex, medial septum, and hippocampus. Many of the targets of the nucleus incertus were involved in arousal mechanisms including the synchronization and desynchronization of the theta rhythm.     

Brainstem afferents to the magnocellular basal forebrain studied by axonal transport, immunohistochemistry, and electrophysiology in the rat

Brainstem afferents to the magnocellular basal forebrain were studied by using tract tracing, immunohistochemistry and extracellular recordings in the rat. WGA-HRP injections into the horizontal limb of the diagonal band (HDB) and the magnocellular preoptic area (MgPA) retrogradely labelled many neurons in the pedunculopontine and laterodorsal tegmental nuclei, dorsal raphe nucleus, and ventral tegmental area. Areas with moderate numbers of retrogradely labelled neurons included the median raphe nucleus, and area lateral to the medial longitudinal fasciculus in the pons, the locus ceruleus, and the medial parabrachial nucleus. A few labelled neurons were seen in the substantia nigra pars compacta, mesencephalic and pontine reticular formation, a midline area in the pontine central gray, lateral parabrachial nucleus, raphe magnus, prepositus hypoglossal nucleus, nucleus of the solitary tract, and ventrolateral medulla. A similar but not identical distribution of labelled neurons was seen following WGA-HRP injections into the nucleus basalis magnocellularis. The possible neurotransmitter content of some of these afferents to the HDB/MgPA was examined by combining retrograde Fluoro-Gold labelling and immunofluorescence. In the mesopontine tegmentum, many retrogradely labelled neurons were immunoreactive for choline acetyltransferase. In the dorsal raphe nucleus, some retrogradely labelled neurons were positive for serotonin and some for tyrosine hydroxylase (TH); however, the majority of retrogradely labelled neurons in this region were not immunoreactive for either marker. The ventral tegmental area, substantia nigra pars compacta, and locus ceruleus contained retrogradely labelled neurons which were also immunoreactive for TH. Of the retrogradely labelled neurons occasionally observed in the nucleus of the solitary tract, prepositus hypoglossal nucleus, and ventrolateral medulla, some were immunoreactive for either TH or phenylethanolamine-N-methyltransferase. To characterize functionally some of these brainstem afferents, extracellular recordings were made from antidromically identified cortically projecting neurons, mostly located in the HDB and MgPA. In agreement with most previous studies, about half (48%) of these neurons were spontaneously active. Electrical stimulation in the vicinity of the pedunculopontine tegmental and dorsal raphe nuclei elicited either excitatory or inhibitory responses in 21% (13/62) of the cortically projecting neurons.(ABSTRACT TRUNCATED AT 400 WORDS)     

Afferent connections to the amygdaloid complex of the rat with some observations in the cat. III. Afferents from the lower brain stem

The projections from the medulla oblongata, pons, and mesencephalon to each nucleus of the amygdaloid complex of the rat were investigated by the use of retrograde transport of horseradish peroxidase (HRP). The enzyme was injected stereotactically by microiontophoresis using four different approaches. The findings indicate that the majority of the ascending fibers terminate in the central and medial amygdalar nuclei. Injections in the central nucleus label neurons at the dorsal aspect of substantia nigra, pars compacta, and in the adjacent ventral tegmental area and peripeduncular nucleus. At more caudal levels, reactive neurons are found in the periaqueductal gray substance, various raphe nuclei, the locus coeruleus, the parabrachial nucleus, the nucleus of the solitary tract, and the mesencephalic and bulbar reticular formation. Injections in the medial nucleus lead to labeling of neurons in the peripeduncular nucleus, the dorsal raphe and superior central nuclei, the parabrachial nucleus, and in the dorsomedial extreme of the dorsal nucleus of the lateral lemniscus. The parabrachial nucleus is the most important lower brain stem source of amygdalopetal fibers. This nucleus projects to the ipsilateral as well as the contralateral amygdala in a topographical manner. Most of the lower brain stem structures found to project to the amygdala in the rat are identified as sources of amygdalopetal fibers in the cat as well.