Viscerotopic organization of neurons subserving hypotensive reactions within the midbrain periaqueductal grey: a correlative functional and anatomical study

Microinjection of the excitatory amino acid D,L-homocysteic acid (40 nmol, in 200 nl) made into the ventrolateral part of the caudal half (A2.5-P1.5) of the midbrain periaqueductal gray (PAG) of the decerebrate cat evoked a hypotensive reaction associated with a slowing of the heart and a decrease in either external iliac or renal vascular resistance. The decrease in iliac vascular resistance was elicited from the pretentorial portion (A2.5-A0.6) of the PAG hypotensive area, whereas the decrease in renal vascular resistance was elicited from the subtentorial portion (A0.6-P1.5). Anatomical experiments using the method of retrograde transport of rhodamine-labelled microspheres or wheat germ agglutinin-horseradish peroxidase demonstrated topographically organized projections from the ventrolateral PAG to the subretrofacial (SRF) pressor nucleus in the rostral ventrolateral medulla. The pretentorial part of the ventrolateral PAG projected mainly to the caudal part of the SRF nucleus, which preferentially controls iliac vascular resistance. The subtentorial part of the ventrolateral PAG projected mainly to the rostral part of the SRF nucleus, which preferentially controls renal vascular resistance. Taken together, these findings suggest: (i) that neurons within the ventrolateral PAG are viscerotopically organized; and (ii) that their hypotensive function may be mediated by an inhibition of SRF pressor neurons. The results are discussed in relation to the recently described PAG hypertensive area which also is viscerotopically organized and projects to the SRF nucleus.     

Rostrocaudal differences in morphology and neurotransmitter content of cells in the subretrofacial vasomotor nucleus.

The rostral ventrolateral medulla (RVLM) contains sympathoexcitatory neurons that exert a powerful control over the sympathetic outflow to the cardiovascular system. In the cat there is a concentration of such neurons (but not neurons subserving other functions) within a narrow longitudinal column in the RVLM termed the subretrofacial (SRF) nucleus. Furthermore, it has been suggested that there are subgroups of cells, located at different rostrocaudal levels of the SRF nucleus, that preferentially or exclusively control different vascular beds (e.g. in the kidney and hindlimb). The aim of this study was to map quantitatively the rostrocaudal distribution within the nucleus of different cell types, defined according to morphological and/or chemical criteria, and to correlate this with the regional vasomotor effects (in hindlimb and kidney) evoked by stimulation of SRF cells at the corresponding rostrocaudal levels. SRF cells were highly heterogeneous with respect to both their morphology and chemical properties. They varied greatly in size (equivalent diameter ranging from 10-40 microns) as well as in shape and orientation. An immunohistochemical examination using the avidin-biotin procedure revealed that many SRF cells (estimated 57% of all SRF cells) were immunoreactive for tyrosine hydroxylase (TH, a marker of catecholamine cells). In addition, there were SRF cells immunoreactive for neuropeptide Y (NPY, 11% of total), enkephalin (ENK, 16% of total), and serotonin (5HT, 10% of total), but not for substance P, galanin or somatostatin. Different cell types, defined according to their morphology and/or chemical properties, were unevenly distributed throughout the nucleus. In the most caudal part of the SRF nucleus, virtually all cells were TH-positive, and the large majority (estimated 80%) were NPY-positive, suggesting that many cells at this level contained both TH and NPY. In contrast, in the most rostral part of the SRF nucleus, only 30% of cells were TH-positive, and no NPY-positive cells were observed. Both 5HT- and ENK-positive cells were found throughout the rostrocaudal extent of the nucleus, but predominantly within its rostral part. Furthermore, TH-positive cells in the rostral SRF nucleus were on average significantly larger (mean equivalent diameter 18-43% greater) than TH/NPY-positive cells in the caudal part of the nucleus, but smaller than 5HT- or ENK-positive cells at the same level. Overall, rostral cells (regardless of their chemical type) were larger than caudal cells within the SRF nucleus (mean equivalent diameter 13-28% greater).(ABSTRACT TRUNCATED AT 400 WORDS)     

The efferent projections of the periaqueductal gray in the rat: A Phaseolus vulgaris-leucoagglutinin study. II. Descending projections

The descending projections of the periaqueductal gray (PAG) have been studied in the rat using the anterograde tracer Phaseolus vulgaris-leucoagglutinin. The tracer was injected into the dorsolateral or ventrolateral subdivisions of the PAG at rostral or caudal sites. It was found that the patterns of the descending projections of the rostral and caudal parts of the dorsolateral PAG were the same and that the patterns of the descending projections of the rostral and caudal parts of the ventrolateral PAG were the same. However, the patterns of projections of the dorsolateral and ventrolateral PAG subregions were substantially different. These results suggest that the dorsolateral and ventrolateral parts of the PAG are organized into longitudinal columns that extend throughout the length of the PAG. The axons of PAG neurons descended through the pons and medulla via two routes. A small fiber bundle was present in the periaqueductal gray and in the periventricular area. This bundle distributed fibers and terminals locally within the periaqueductal gray and in the locus coeruleus and Barrington's nucleus. A larger bundle had a diffuse arrangement in the pontine reticular formation, however, and it had a more restricted distribution in the medulla, where it occupied a position dorsolateral to the pyramid. This bundle supplied structures in the pontine and medullary tegmentum. The dorsolateral column preferentially supplied the locus coeruleus, subcoeruleus, the gigantocellular nucleus pars alpha, the rostral part of the paragigantocellular nucleus, and the region of the A5 noradrenergic cell group. The ventrolateral column preferentially supplied the nucleus raphe magnus, the caudal part of the lateral paragigantocellular nucleus, and the rostroventrolateral reticular nucleus. © 1995 Willy-Liss, Inc.     

Anatomical evidence that hypertension associated with the defence reaction in the cat is mediated by a direct projection from a restricted portion of the midbrain periaqueductal grey to the subretrofacial nucleus of the medulla

Wheat germ agglutinin-horseradish peroxidase (WGA-HRP) injections were made at sites within a restricted portion of the midbrain periaqueductal grey region (PAG) of the cat at which microinjection of the excitant amino acid, d,l-homocysteic acid, elicits the strongest form of a defence reaction, including a hypertensive response. Among the revealed projections, significant anterograde labelling was found in a discrete region of the rostral ventrolateral medulla, the subretrofacial nucleus (SRF). In the cat, the SRF contains pressor neurones which project to the spinal preganglionic sympathetic outflow. The labelling was most marked ipsilaterally, although substantial contralateral labelling was also observed. To verify that the projection to the SRF originated from the restricted ‘defence region’ of the PAG, WGA-HRP or rhodamine-labelled microspheres were injected into physiologically-identified sites in the SRF. In all experiments, labelled neurones were found in the same restricted region of the PAG at which DLH injection evokes hypertension and behavioural signs of the defence reaction. The results are consistent with the hypothesis that a discrete cell group within the PAG mediates both somatic and autonomic components of the defence reaction and that the characteristic hypertensive response is mediated by a direct pathway from these PAG cells to pressor neurones in the SRF.     

Nucleus retroambiguus projections to the periaqueductal gray in the cat

The nucleus retroambiguus (NRA) of the caudal medulla is a relay nucleus by which neurons of the mesencephalic periaqueductal gray (PAG) reach motoneurons of pharynx, larynx, soft palate, intercostal and abdominal muscles, and several muscles of the hindlimbs. These PAG-NRA-motoneuronal projections are thought to play a role in survival behaviors, such as vocalization and mating behavior. In the present combined antero- and retrograde tracing study in the cat, we sought to determine whether the NRA, apart from the neurons projecting to motoneurons, also contains cells projecting back to the PAG. After injections of WGA-HRP in the caudal and intermediate PAG, labeled neurons were observed in the NRA, with a slight contralateral preponderance. In contrast, after injections in the rostral PAG or adjacent deep tectal layers, no or very few labeled neurons were present in the NRA. After injection of [(3)H]leucine in the NRA, anterograde labeling was present in the most caudal ventrolateral and dorsolateral PAG, and slightly more rostrally in the lateral PAG, mainly contralaterally. When the [(3)H]leucine injection site extended medially into the medullary lateral tegmental field, labeling was found in most parts of the PAG as well as in the adjoining deep tectal layers. No labeled fibers were found in the dorsolateral PAG, and only a few were found in the rostral PAG. Because the termination pattern of the NRA fibers in the PAG overlaps with that of the sacral cord projections to the PAG, it is suggested that the NRA-PAG projections play a role in the control of motor functions related to mating behavior.     

Subregions of the periaqueductal gray topographically innervate the rostral ventral medulla in the rat.

Previous anatomical and physiological studies have revealed a substantial projection from the periaqueductal gray (PAG) to the nucleus paragigantocellularis (PGi). In addition, physiological studies have indicated that the PAG is composed of functionally distinct subregions. However, projections from PAG subregions to PGi have not been comprehensively examined. In the present study, we sought to examine possible topographic specificity for projections from subregions of the PAG to PGi. Pressure or iontophoretic injections of wheat germ agglutinin-conjugated horseradish peroxidase, or of Fluoro-Gold, placed into the PGi of the rat retrogradely labeled a substantial number of neurons in the PAG from the level of the Edinger-Westphal nucleus to the caudal midbrain. Retrogradely labeled neurons were preferentially aggregated in distinct subregions of the PAG. Rostrally, at the level of the oculomotor nucleus, labeled neurons were i) compactly aggregated in the ventromedial portion of the PAG corresponding closely to the supraoculomotor nucleus of the central gray, ii) in the lateral and ventrolateral PAG, and iii) in medial dorsal PAG. More caudally, retrogradely labeled neurons became less numerous in the dorsomedial PAG but were more widely scattered throughout the lateral and ventrolateral parts of the PAG. Only few retrogradely labeled neurons were found in the ventromedial part of the PAG at caudal levels. Injections of retrograde tracers restricted to subregions of the PGi suggested topography for afferents from the PAG. Injections into the lateral portion of the PGi yielded the greatest number of labeled neurons within the rostral ventromedial PAG. Medially placed injections yielded numerous retrogradely labeled neurons in the lateral and ventrolateral PAG. Injections placed in the rostral pole of the PGi (medial to the facial nucleus) produced the greatest number of retrogradely labeled neurons in the dorsal PAG. To examine the pathways taken by fibers projecting from PAG neurons to the medulla, and to further specify the topography for the terminations of these afferents in the PGi, the anterograde tracer Phaseolus vulgaris-leucoagglutinin was iontophoretically deposited into subregions of the PAG that contained retrogradely labeled neurons in the above experiments. These results revealed distinct fiber pathways to the rostral medulla that arise from the dorsal, lateral/ventrolateral, and ventromedial parts of the PAG. These injections also showed that there are differential but overlapping innervation patterns within the PGi. Consistent with the retrograde tracing results, injections into the rostral ventromedial PAG near the supraoculomotor nucleus yielded anterograde labeling immediately ventral to the nucleus ambiguus in the ventrolateral medulla, within the retrofacial portion of the PGi.(ABSTRACT TRUNCATED AT 400 WORDS)     

Neurons in the lateral sacral cord of the cat project to periaqueductal grey, but not to thalamus

Previous work of our laboratory has shown that neurons in the lateral sacral cord in cat project heavily to the periaqueductal grey (PAG), in all likelihood conveying information from bladder and genital organs. In humans this information usually does not reach consciousness, which raises the question of whether the lateral sacral cell group projects to the thalamus. After wheatgerm agglutinin-horseradish peroxidase (WGA-HRP) injections into the sacral cord, anterogradely labelled fibers were found in the thalamus, specifically in the ventral anterior and ventral lateral nuclei, the medial and intralaminar nuclei, the lateral ventrobasal complex/ventroposterior lateral nucleus, and the nucleus centre median, lateral to the fasciculus retroflexus. Much denser projections were found to the central parts of the PAG, mainly to its dorsolateral and ventrolateral parts at caudal levels and lateral parts at intermediate levels. In a subsequent retrograde tracing study, injections were made in those parts of the thalamus that received sacral fibers, as found in the anterograde study. Labelled neurons were observed in the sacral cord, but not in the lateral sacral cell group. In contrast, a small control injection in the caudal PAG resulted in many labelled neurons in the lateral sacral cord. These results suggest that afferent information regarding micturition and sexual behaviour is relayed to the PAG, rather than to the thalamus.     

Fos-like immunoreactivity in the periaqueductal gray of rats exposed to a natural predator

In the present study we examined, in rats exposed to a predator (cat), the distribution of neurons expressing Fos along the continuum formed by the central gray surrounding the caudal pole of the third ventricle and the periaqueductal gray (PAG). After the predatory encounter, a distinct cluster of Fos-immunoreactive cells was observed in the precommissural nucleus. In the rostral two-thirds of the PAG, Fos expression was mostly seen in the dorsomedial and dorsolateral regions. In contrast, at caudal levels of the PAG, most of the Fos-labelled neurons were distributed in the lateral and ventrolateral PAG. These results are discussed and compared with the pattern of the PAG activation after fear conditioned to a context or elicited by aversive foot shock.     

Reciprocal connections between the medial preoptic area and the midbrain periaqueductal gray in rat: a WGA-HRP and PHA-L study.

The midbrain periaqueductal gray (PAG) participates in diverse functions such as analgesia, autonomic regulation, sexual behavior, and defense/escape responses. Anatomical studies of the circuits involved in such functions have largely focused on the connections of PAG with the medulla. Projections to PAG from forebrain structures are extensive, but their organization has received little attention. Previous anatomic studies indicate that the medial preoptic area (MPO), involved in a variety of physiological and behavioral functions, is a major source of afferent input to the periaqueductal gray. Here, we have examined the topography of reciprocal connections between these two structures in the rat by using wheat germ agglutinin conjugated horseradish peroxidase (WGA-HRP) and Phaseolus vulgaris leucoagglutinin (PHA-L). Multiple WGA-HRP injections at several rostrocaudal levels of PAG retrogradely labeled large numbers of neurons in the medial preoptic area; labeled cells were primarily located in the medial preoptic nucleus, the median preoptic nucleus, and the region lateral to the medial preoptic nucleus. The distribution of labeled cells shifted medially to laterally along the rostral to caudal axis of the medial preoptic area. Rostrally, there was selective retrograde labeling in the central and lateral divisions of medial preoptic nucleus, whereas caudally, labeled cells were primarily located only in the lateral subdivision of medial preoptic nucleus. Tracer injections in PAG also produced strong anterograde labeling in MPO. WGA-HRP and PHA-L injections in the medial preoptic area resulted in dense anterograde labeling along the entire rostrocaudal axis of PAG. The terminal labeling in PAG from the medial preoptic area was not uniformly distributed throughout PAG, however. Instead, this projection formed one or two rostrocaudally oriented longitudinal columns that terminated in different subregions of PAG along the entire rostrocaudal axis of this structure. Rostrally, inputs from the medial preoptic area project heavily to dorsomedial PAG, and at mid-PAG levels, the projection becomes distinctly bipartite with two discrete longitudinal terminal columns in dorsomedial and lateral PAG; caudally, the heaviest labeling is in ventrolateral PAG. The projection also exhibited a central to peripheral (radial) gradient; labelled fibers and terminals were heaviest near the aqueduct and much lower in the peripheral parts of PAG. WGA-HRP injections in MPO also produced retrograde labeling of neurons at all rostrocaudal levels of PAG; more neurons were labeled in the rostral than the caudal half of PAG. The majority of labeled cells were located in dorsomedial and ventral/ventrolateral parts of PAG; only a few neurons in the dorsal raphe region appear to project to MPO.(ABSTRACT TRUNCATED AT 400 WORDS)     

Periaqueductal gray neurons monosynaptically innervate extranuclear noradrenergic dendrites in the rat pericoerulear region

Previous reports using light microscopy have provided anatomical evidence that neurons in the ventrolateral periaqueductal gray (PAG) innervate the medial pericoerulear dendrites of noradrenergic neurons in the nucleus locus coeruleus (LC). The present study used anterograde tracing and electron microscopic analysis to provide more definitive evidence that neurons in the ventrolateral PAG form synapses with the somata or dendrites of noradrenergic LC neurons. Deposits of either biotinylated dextran amine or Phaseolus vulgaris leucoagglutinin into the rat ventrolateral PAG labeled a moderate to high number of axons in the region of the medial pericoerulear region and Barrington's nucleus, but a relatively low number were labeled in the nuclear core of the LC. Ultrastructural analysis of anterogradely labeled terminals at the levels of the rostral (n = 233) and caudal (n = 272) subdivisions of the LC indicated that approximately 20% of these form synapses with tyrosine hydroxylase-immunoreactive dendrites; most of these were located in the medial pericoerulear region. In rostral sections, about 12% of these were symmetric synapses, 9% were asymmetric synapses, and 79% were membrane appositions without clear synaptic specializations. In caudal sections, about 30% were symmetric synapses, 11% were asymmetric synapses, and 59% were appositions. In both rostral and caudal sections, 60% of the anterogradely labeled terminals formed synapses with noncatecholamine dendrites, and 20% formed axoaxonic synapses. These results provide direct evidence for monosynaptic projections from neurons in the ventrolateral PAG to the extranuclear dendrites of noradrenergic LC neurons. This monosynaptic pathway may mediate in part the analgesia, reduced responsiveness to external stimuli, and decreased excitability of somatic motoneurons produced by stimulation of neurons in the ventrolateral PAG.     

Heterogeneous distribution of neurotensin-like immunoreactive neurons and fibers in the midbrain periaqueductal gray of the rat

The midbrain periaqueductal gray (PAG) has been shown to be a site where various manipulations induce pain suppression. Recent physiological evidence (Behbehani and Pert, 1984; Behbehani et al., 1987) suggests that neurotensin has pronounced physiological actions in PAG and effects pain suppression. We have performed immunohistochemical studies in order to determine the magnitude and distribution of neurotensin-like immunoreactive (NT-IR) cell bodies and fibers in PAG. NT-IR cell bodies were common throughout PAG, although there were more in the caudal than the rostral half. NT-IR neurons were much more numerous in the ventral than the dorsal half of PAG, and some appeared to be located within the dorsal raphe nucleus. The pattern of NT-IR fibers was analyzed with the aid of image enhancement/analysis and densitometry. The fibers were found to be heterogeneously distributed, being most heavily concentrated in the region adjacent to the cerebral aqueduct in the caudal two-thirds of PAG. The distribution of NT fibers closely matches sites where exogenously applied NT elicits long-lasting excitation of PAG neurons (Behbehani et al., 1987). Based on the known physiological and behavioral actions of NT in PAG, the present anatomical results suggest that NT acts on elements located predominantly in the medial and ventrolateral parts of PAG. Neurons activated by NT may project directly to the nucleus raphe magnus and adjacent ventral medulla (Behbehani and Pert, 1984) to activate neurons that project to the spinal cord and modulate nociceptive circuits.     

Autoradiographic localization of substance P ligand binding sites and distribution of immunoreactive neurons in the periaqueductal gray of the rat

The autoradiographic localization of substance P (SP) binding sites and the distribution of SP immunoreactive (SP-ir) neurons in the periaqueductal gray (PAG) of the rat were studied. The autoradiograms revealed an uneven distribution of specific SP binding sites in the PAG. Throughout the rostrocaudal extent, the densest ligand binding sites were observed in the medial PAG adjacent to the aqueduct, and extended into the dorsal medullary region and to the dorsal raphe nucleus midline region. The distribution of binding sites were denser in the dorsal PAG than the ventral half. In the cuneiform nucleus, a lesser and a denser binding site were observed in the medial and lateral halves respectively. Optical density readings of autoradiograms also supported the differences between these areas. The distribution of SP-ir neurons was also found uneven. In the rostral PAG, SP-ir neurons were found in the entire dorsoventral region. In the caudal PAG, SP-ir neurons were found as 3 clusters: in the dorsomedial, dorsolateral and ventrolateral regions. The present study revealed more SP-ir neurons in the PAG than previously reported.     

Distribution of medullary respiratory neurons in the rat

In Nembutal-anesthetized and spontaneously breathing rats, a total of 226 respiratory neurons were recorded in the medulla extending from the caudal end of the facial nucleus to 1 mm caudal to the obex. They were classified into inspiratory (I) and expiratory (E) neurons by their temporal relationships to diaphragm EMGs. One hundred and seventeen I and 108 E neurons were identified. I and E neurons were further classified into augmenting, decrementing, and other types based on their firing patterns. Almost all the respiratory neurons recorded were located around the nucleus ambiguus and the nucleus retroambigualis, corresponding to the ventral respiratory group (VRG) of the cat. On the other hand, only a few respiratory neurons were identified around the ventrolateral nucleus of the solitary tract, corresponding to the dorsal respiratory group of the cat. In the VRG, 3 subgroups were distinguished rostrocaudally. One group of E neurons was located ventrally to the rostral part of the nucleus ambiguus, presumably corresponding to the B?tzinger complex defined in the cat. Another group of E neurons extended caudally beyond the obex, from the caudal portion of the nucleus ambiguus through the nucleus retroambigualis. Between these two groups of E neurons, an assembly of predominantly I neurons existed in the vicinity of the nucleus ambiguus. These characteristics of distributions were basically similar to those of the VRG of the cat.     

Immunohistochemical identification of noradrenaline- and adrenaline- synthesizing neurons in the cat ventrolateral medulla

The distribution and morphology of cell bodies containing the catecholamine biosynthetic enzymes dopamine-beta-hydroxylase (DBH) and phenylethanolamine N-methyltransferase (PNMT) in the ventrolateral medulla (VLM) of the cat were studied immunohistochemically after intracisternal administration of colchicine. Perikarya immunoreactive to DBH were found throughout the VLM extending from approximately the spinomedullary junction to the level of the superior olivary nucleus. In the caudal VLM DBH neurons were found primarily in the region immediately dorsal to the lateral reticular nucleus (LRN), although a few scattered DBH neurons were also found near the ventral surface of the medulla in and around the parvicellular division of the LRN. These DBH neurons in the caudal VLM were generally fusiform, fusiform-bipolar, or multipolar, with a mean somal area of 422 +/- 32 microns2, and with two to four branching processes. In the rostral VLM DBH neurons formed three distinct groups: one group was found in the nucleus paragigantocellularis lateralis in the region just ventromedial to the retrofacial nucleus (RFN) near the ventrolateral surface of the medulla; the second group was found in the region dorsomedial to the rostral aspects of the nucleus ambiguous and the RFN, and the third group was found in the region along the lateral aspect of the superior olivary nucleus. Perikarya immunoreactive to the adrenaline-synthesizing enzyme PNMT were localized to a more restricted region of the VLM that extended from approximately the rostral aspect of the caudal third of the inferior olivary complex (level of the obex) to the caudal pole of the facial nucleus. These PNMT neurons were fusiform or multipolar with a mean somal area of 273 +/- 21 microns2, and with two to five branching processes. The location, size, shape, and numbers of PNMT-immunoreactive neurons corresponded closely to the rostral groups of DBH neurons, with the exception of the group found along the lateral aspect of the superior olivary nucleus. These data indicate that noradrenaline-synthesizing neurons are primarily found in the caudal VLM and in the region near the superior olivary nucleus, whereas catecholamine neurons in the rostral VLM between these two noradrenergic cell groups synthesize adrenaline. As the VLM has previously been shown to have direct connections with spinal cord, brainstem, and hypothalamic areas implicated in cardiovascular and neuroendocrine regulation, this suggests that DBH- and PNMT-synthesizing neurons are components of neuronal circuits involved in these homeostatic mechanisms.     

Central pupillary light reflex circuits in the cat: II. Morphology,ultrastructure, and inputs of preganglionic motoneurons

Preganglionic motoneurons supplying the ciliary ganglion control lens accommodation and pupil diameter. In cats, these motoneurons make up the preganglionic Edinger‐Westphal population, which lies rostral, dorsal, and ventral to the oculomotor nucleus. A recent cat study suggested that caudal motoneurons control the lens and rostral motoneurons control the pupil. This led us to examine the morphology, ultrastructure, and pretectal inputs of these populations. Preganglionic motoneurons retrogradely labeled by introducing tracer into the cat ciliary ganglion generally fell into two morphologic categories. Fusiform neurons were located rostrally, in the anteromedian nucleus and between the oculomotor nuclei. Multipolar neurons were found caudally, dorsal and ventral to the oculomotor nucleus. The dendrites of preganglionic motoneurons within the anteromedian nucleus crossed the midline, providing a possible basis for consensual responses. Ultrastructurally, several different classes of synaptic profiles contact preganglionic motoneurons, suggesting that their activity may be modified by a variety of inputs. Furthermore, there were differences in the synaptic populations contacting the rostral vs. caudal populations, supporting the contention that these populations display functional differences. Anterogradely labeled pretectal terminals were observed in close association with labeled preganglionic motoneurons, particularly in the rostral population. Ultrastructural analysis revealed that these terminals, packed with clear, spherical vesicles, made asymmetric synaptic contacts onto motoneurons in the rostral population, indicating that these cells serve the pupillary light reflex. Thus, the preganglionic motoneurons found in the cat display morphologic, ultrastructural, and connectional differences suggesting that this rostral preganglionic population is specialized for pupil control, whereas more caudal elements control the lens. J. Comp. Neurol. 522:3978–4002, 2014. © 2014 Wiley Periodicals, Inc.     

Increased c-Fos expression in select lateral parabrachial subnuclei following chemical versus electrical stimulation of the dorsal periaqueductal gray in rats

The parabrachial nucleus (PBN) is located in the rostral dorsolateral pons and has been identified as a critical relay for cardiovascular responses (sympathoexcitation and baroreflex attenuation) evoked by the dorsal periaqueductal gray (PAG). We examined the pattern of c-Fos protein immunoreactivity throughout the rostral-caudal extent of the PBN in four groups of anesthetized male Sprague-Dawley rats to identify the specific PBN regions activated by dorsal PAG stimulation. Both electrical stimulation and chemical (0.3 mM bicuculline methobromide) activation of the dorsal PAG elicited a selective increase in Fos-like immunoreactivity (FLI) in the superior lateral and central lateral subnuclei of the rostral lateral PBN (LPBN) relative to surgery and blood pressure control groups. In the middle LPBN chemical stimulation of the dorsal PAG selectively increased FLI in the central lateral subnucleus while electrical stimulation increased FLI in the Kolliker-Fuse area only. Finally, in the caudal LPBN only electrical stimulation of the dorsal PAG induced significant changes in FLI above control. Significant changes in FLI in the medial PBN were not observed under any experimental conditions. These results confirm neuroanatomical data demonstrating that neurons in superior lateral and central lateral subnuclei of the rostral and middle LPBN are the primary targets of the dorsal PAG. Our results also demonstrate that this descending projection to the central lateral and superior lateral subnuclei of the LPBN is in part excitatory. Finally, our results raise the possibility that neurons in the central lateral subnucleus of the middle and rostral LPBN are integrally involved in descending modulation of sympathetic drive associated with dorsal PAG activation.     

Afferents to the abducens nucleus in the monkey and cat

The abducens nucleus is a central coordinating element in the generation of conjugate horizontal eye movements. As such, it should receive and combine information relevant to visual fixation, saccadic eye movements, and smooth eye movements evoked by vestibular and visual stimuli. To reveal possible sources of these signals, we retrogradely labeled the afferents to the abducens nucleus by electrophoretically injecting horseradish peroxidase into an abducens nucleus in four monkeys and two cats. The histologic material was processed by the tetramethyl benzidine (TMB) method of Mesulam. In both species the largest source of afferents to the abducens nucleus was bilateral projections from the ventrolateral vestibular nucleus and the rostral pole of the medial vestibular nucleus. Scattered neurons were also labeled in the middle and caudal levels of the medial vestibular nucleus. Large numbers of neurons were labeled in the ventral margin of the nucleus prepositus hypoglossi in the cat and in the common margin of the nucleus prepositus and the medial vestibular nucleus in the monkey, a region we call the marginal zone. Substantial numbers of retrogradely labeled neurons were found in the dorsomedial pontine reticular formation both caudal and rostral to the abducens nuclei. In the monkey, large numbers of labeled neurons were present in the contralateral medial rectus subdivision of the oculomotor complex, while smaller numbers occurred in the ipsilateral medial rectus subdivision and elsewhere in the oculomotor complex. In the cat, large numbers of retrogradely labeled cells were present in a small periaqueductal gray nucleus immediately dorsal to the caudal pole of the oculomotor complex, and a few labeled neurons were also dispersed through the caudal part of the oculomotor complex. Occasional labeled neurons were present in the contralateral superior colliculus in both species. The size and distribution of the labeled neurons within the intermediate gray differed dramatically in the two species. In the cat, the retrogradely labeled neurons were very large and occurred predominantly in the central region of the colliculus, while in the monkey, they were small to intermediate in size and were distributed more uniformly within the middle gray. Among the afferent populations present in the monkey, but not in the cat, was a group of scattered neurons in the ipsilateral rostral interstitial nucleus of the medial longitudinal fasciculus and a denser, bilateral population in the interstitial nucleus of Cajal.(ABSTRACT TRUNCATED AT 400 WORDS)     

Descending projections of the posterior nucleus of the hypothalamus: Phaseolus vulgaris leucoagglutinin analysis in the rat

No previous report in any species has systematically examined the descending projections of the posterior nucleus of the hypothalamus (PH). The present report describes the descending projections of the PH in the rat by using the anterograde anatomical tracer, Phaseolus vulgaris leucoagglutinin. PH fibers mainly descend to the brainstem through two routes: dorsally, within the central tegmental tract; and ventromedially, within the mammillo-tegmental tract and its caudal extension, ventral reticulo-tegmental tracts. PH fibers were found to distribute densely to several nuclei of the brainstem. They are (from rostral to caudal) 1) lateral/ventrolateral regions of the diencephalo-mesopontine periaqueductal gray (PAG); 2) the peripeduncular nucleus; 3) discrete nuclei of pontomesencephalic central gray (dorsal raphe nucleus, laterodorsal tegmental nucleus, and Barrington's nucleus); 4) the longitudinal extent of the central core of the mesencephalic through medullary reticular formation (RF); 5) the ventromedial medulla (nucleus gigantocellularis pars alpha, nucleus raphe magnus, and nucleus raphe pallidus); 6) the ventrolateral medulla (nucleus reticularis parvocellularis and the rostral ventrolateral medullary region); and 7) the inferior olivary nucleus. PH fibers originating from the caudal PH distribute much more heavily than those from the rostral PH to the lower brainstem. The PH has been linked to the control of several important functions, including respiration, cardiovascular activity, locomotion, antinociception, and arousal/wakefulness. It is likely that descending PH projections, particularly those to the PAG, the pontomesencephalic RF, Barrington's nucleus, and parts of the ventromedial and ventrolateral medulla, serve a role in a PH modulation of complex behaviors involving an integration of respiratory, visceromotor, and somatomotor activity. © 1996 Wiley-Liss, Inc.     

Fos expression induced by changes in arterial pressure is localized in distinct,longitudinally organized columns of neurons in the rat midbrain periaqueductal gray

The distribution of neurons expressing Fos within the periaqueductal gray (FAG) following pharmacologically induced high or low blood pressure was examined to determine (1) if PAG neurons are responsive to changes in arterial pressure (AP) and (2) the relationship of these cells to the functionally defined hypertensive and hypotensive columns in PAG. Changes in AP differentially induced robust Fos expression in neurons confined to discrete, longitudinally organized columns within PAG. Increased AP produced extensive Fos-like immunoreactivity within the lateral PAG, beginning at the level of the oculomotor nucleus. At the level of the dorsal raphe, Fos expression induced by increased AP shifted dorsally, into the dorsolateral division of PAG; this pattern of Fos labeling was maintained throughout the caudal one-third of PAG. Double-labeling for Fos and nicotinamide adenine dinucleotide phosphate diaphorase confirmed that Fos-positive cells induced by increased AP were located in the dorsolateral division of PAG at these caudal levels. Fos positive cells were codistributed, but not colocalized, with nicotinamide adenine dinucleotide phosphate diaphorase-positive cells. Decreased AP evoked a completely different pattern of Fos expression. Fos-positive cells were predominantly located within the ventrolateral PAG region, extending from the level of the trochlear nucleus through the level of the caudal dorsal raphe. Double-labeling studies for Fos and serotonin indicated that only 1–2 double-labeled cells per section were present. Saline infusion resulted in very few Foslike immunoreactive cells, indicating that volume receptor activation does not account for Fos expression in PAG evoked by changes in AP. These results indicate that (1) substantial numbers of PAG neurons are excited by pharmacologically induced changes in AP and (2) excitatory barosensitive PAG neurons are anatomically segregated based on their responsiveness to a specific directional change in AP. © 1995 Wiley-Liss, Inc.     

Output systems of the dorsal column nuclei in the cat

Numerous authors have demonstrated that the dorsal column nuclear complex (DCN) is functionally heterogeneous and has multiple terminal targets throughout the neuroaxis. In order to increase understanding of the functional significance of DCN's divergent connections, the present study used single and double light microscopic retrograde tracing strategies in the cat to characterize the location and morphology of DCN neurons that project to different portions of the diencephalon, rostral mesencephalon and spinal cord. These neuronal populations were then compared with those (previously reported from this and other laboratories) that project to the caudal mesencephalon, pons, inferior olive and cerebellum. When the results are considered together, a tentative picture of DCN emerges in which a population of clustered neurons that project exclusively to VPL form a core that is surrounded by and infiltrated with neurons projecting to other parts of the nervous system. Although the neuronal populations projecting to each of the different targets were individually separable anatomically by their location and/or morphological characteristics, previously reported physiological and other anatomical evidence permitted a preliminary grouping of these populations into 3 main systems. The first, a sensory tactile and kinesthetic 'cortical' system, consisted of 3 components: a double core of round, clustered medium-sized neurons (one each in the gracile and cuneate nuclei) and a variform rostral group projecting to the ventroposterolateral nucleus (VPL), a ventral group of unclustered large round neurons in the middle cuneate nucleus and a dense group of neurons in nucleus Z projecting to VPL's border with the ventrolateral nucleus (VPL/VL), and a group of mainly small-sized neurons located between the clusters of neurons or in the thin dorsal rim around the caudal and middle portions of the double cores and a populous, variform rostral group projecting indirectly (and possibly directly) to the posterior group through the intercollicular region of the tectum. The second, a sensorimotor 'cerebellar' system, consisted of multiple, subtly separable populations of neurons with different morphological characteristics all of which were located in different parts of the complex region that surrounds the cores on all sides. These neurons projected to restricted portions of interconnected targets within the zona incerta, tectum, pretectum, red nucleus, pontine grey, pontine raphe, inferior olive, and cerebellum.(ABSTRACT TRUNCATED AT 400 WORDS)