Retrograde HRP identification of neurons in the rhombencephalon and spinal cord of the rat that project to the dorsal mesencephalon

The tracts, cytoarchitecture, and neurochemistry of the spinal cord

The human spinal cord can be described using a range of nomenclatures with each providing insight into its structure and function. Here we have comprehensively reviewed the key literature detailing the general structure, configuration of tracts, the cytoarchitecture of Rexed's laminae, and the neurochemistry at the spinal segmental level. The purpose of this review is to detail current anatomical understanding of how the spinal cord is structured and to aid researchers in identifying gaps in the literature that need to be studied to improve our knowledge of the spinal cord which in turn will improve the potential of therapeutic intervention for disorders of the spinal cord.

Possible involvement of histamine, dopamine, and noradrenalin in the periaqueductal gray in electroacupuncture pain relief

Acupuncture and electroacupuncture are used in pain relief; however, the mechanism underlying the analgesic effect of acupuncture is unclear. Several lines of evidence propose that the periaqueductal gray (PAG), which is one of the regions that contributes to the endogenous pain inhibitory system, is involved in the analgesic effect of acupuncture, and the region receives several neural projections such as histamine and noradrenalin and contains the dopamine cell bodies. The current study examined the effects of electroacupuncture at Zusanli (ST36) and Shangjuxu (ST37) acupoints, which are used for clinical pain control, on the release of neurotransmitters in the PAG in rats. Histamine and dopamine release was increased after pain stimulus, while the changes were completely abolished by electroacupuncture. Pain stimulus had no effect on noradrenalin release, but electroacupuncture increased its release. These findings indicate that acupuncture at Zusanli and Shangjuxu exerts an antinociceptive effect via the activation of neurons in the PAG and that the histaminergic, dopaminergic, and noradrenalinergic systems in the PAG are related to electroacupuncture-induced pain relief.

Site and behavioral specificity of periaqueductal gray lesions on postpartum sexual, maternal, and aggressive behaviors in rats

Bilateral electrolytic lesions of the lateral and ventrolateral caudal periaqueductal gray (cPAGl,vl) of lactating rats are known to severely reduce suckling-induced kyphosis (upright crouched nursing), which is necessary for maximal litter weight gains, and impair sexual behavior during the postpartum estrous, while heightening nursing in other postures and attacks on unfamiliar adult male intruders. In the present report, the site specificity of the cPAG with respect to the control of these behaviors was determined by comparing lesions of the cPAGl,vl with similarly sized lesions within the rostral PAG (rPAG) and surrounding mesencephalon. The previously seen effects of prepartum cPAGl,vl lesions on kyphotic nursing, sexual proceptivity and receptivity, maternal aggression, and daily litter weight gains were replicated. Additionally, the post-lesion facilitation of aggression was found to be behaviorally specific, first by being directed toward an adult, but not to a nonthreatening juvenile male rat, and second, by requiring the recent presence of the pups, being eliminated or decreased 24 h after removal of the litter. Damage to the rPAG did not affect nursing or sexual behaviors, and had only a minimal effect on maternal aggression. Lesions of the rPAG, however, greatly impaired the dams' ability to rapidly release pups held in the mouth, but not to pick them up or carry them directly to the nest during retrieval. Separate regions of the PAG, therefore, are differentially involved in the control of specific components of behaviors in lactating rats.

Somatosensory contributions to c-fos activation within the caudal periaqueductal gray of lactating rats: effects of perioral, rooting, and suckling stimuli from pups

In lactating rats, the immediate-early gene c-fos was previously shown to be highly activated in several brain sites by physical interaction with either suckling or nonsuckling pups but not by distal stimuli from pups, a non-pup stimulus, or no stimulation. Further, even greater levels of Fos-immunoreactivity (ir) occurred following suckling versus nonsuckling contact with pups in only 1 of over 25 sites--the caudal periaqueductal gray (cPAG) at an intercollicular level, lesions of which severely reduced the typical suckling-induced kyphotic nursing posture. Herein we further evaluated the effects and site-specificity of various somatosensory cues received from pups during 60 min on Fos-ir in the PAG of day 7 postpartum rats after a 48-h dam-litter separation. Dams interacting with suckling versus nonsuckling pups showed relatively high numbers of Fos-ir cells in the intercollicular cPAG site identified earlier, but not in three other rostrocaudal planes of the PAG. Elimination of rooting on the dam's ventrum by use of fully anesthetized pups did not further diminish Fos-ir in maternally behaving, nonsuckled dams. Perioral anesthesia of dams prior to reunion with the litter prevented retrieval and licking of pups but not pup-initiated nursing behavior, the duration of which was positively correlated with Fos-ir levels within the intercollicular cPAG. Thus, various somatosensory stimuli from pups activate c-fos in a discrete region of the cPAG but only interactions that include suckling and its behavioral consequences elicit maximal expression, consistent with a role for this midbrain site in the sensorimotor control of kyphotic nursing in rats.

Functional characteristics of the midbrain periaqueductal gray

The major functions of the midbrain periaqueductal gray (PAG), including pain and analgesia, fear and anxiety, vocalization, lordosis and cardiovascular control are considered in this review article. The PAG is an important site in ascending pain transmission. It receives afferents from nociceptive neurons in the spinal cord and sends projections to thalamic nuclei that process nociception. The PAG is also a major component of a descending pain inhibitory system. Activation of this system inhibits nociceptive neurons in the dorsal horn of the sinal cord. The dorsal PAG is a major site for processing of fear and anxiety. It interacts with the amygdala and its lesion alters fear and anxiety produced by stimulation of amygdala. Stimulation of PAG produces vocalization and its lesion produces mutism. The firing of many cells within the PAG correlates with vocalization. The PAG is a major site for lordosis and this role of PAG is mediated by a pathway connecting the medial preoptic with the PAG. The cardiovascular controlling network within the PAG are organized in columns. The dorsal column is involved in pressor and the ventrolateral column mediates depressor responses. The major intrinsic circuit within the PAG is a tonically-active GABAergic network and inhibition of this network is an important mechanism for activation of outputs of the PAG. The various functions of the PAG are interrelated and there is a significant interaction between different functional components of the PAG. Using the current information about the anatomy, physiology, and pharmacology of the PAG, a model is proposed to account for the interactions between these different functional components.

Anticonvulsant role of nigrotectal projection in the maximal electroshock model of epilepsy--I. Mapping of dorsal midbrain with bicuculline

Previous work has indicated that the anticonvulsant effect of nigral inactivation on the maximal electroshock model of generalized seizures is mediated by the projection from substantia nigra to superior colliculus. In accordance with this idea, and with the GABAergic nature of the nigrotectal pathway, microinjections of the GABAA antagonist bicuculline methiodide into the superior colliculus have been reported to block tonic hindlimb extension induced by maximal electroshock. To characterize the relevant circuitry more precisely, the present study sought to determine which region of the superior colliculus was important for the anticonvulsant effect of bicuculline by systematic mapping in the rat. Bilateral injections of bicuculline methiodide (50 pmol in 400 nl/side) were most effective in the caudal deep layers of the superior colliculus and adjoining midbrain reticular formation. These results suggest that the well-known projection from substantia nigra pars reticulata to the superior colliculus may not be involved in the anticonvulsant effect of nigral inactivation in the electroshock model, because this pathway terminates primarily in the intermediate layers of the superior colliculus throughout its rostrocaudal extent. Instead, some other pathway from ventral midbrain to a dorsal midbrain anticonvulsant zone appears to be part of the brain's anticonvulsant circuitry. The following paper [Redgrave et al. (1991) Neuroscience 46, 391-406] describes an anatomical study to characterize this pathway.

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.

Patterns of 2-deoxyglucose uptake reflect the neural processing of lordosis-inducing somatosensory stimuli in hamsters

Semi-quantitative [14C]2-deoxyglucose (2DG) autoradiography was used to map the neural responses of female hamsters to lordosis-inducing flank stimuli. Specifically, manual stimulation of one flank was used to maintain estrous females in lordosis for 20 min after an IV injection of 200 muCi/kg of 2DG. Hemispheric differences in 2DG uptake then were sought in brain nuclei implicated in the programming of lordosis, or in the mediation of somatosensory or hormonal influences on this response. The responses to lateralized flank stimulation included reliable contralateral elevations in 2DG uptake in the ventral posterior lateral nucleus of the thalamus (VPL), the dorsal mesencephalic central gray (dCG), and the tectum. Elevated activity on the part of the VPL may not be crucial for lordosis. However, the effects of flank stimulation on 2DG uptake by the dCG and tectum confirm and extend much previous evidence implicating the dorsal midbrain in the mediation of tactile and hormonal effects on sexual responses. For example, these results suggest that somatosensory influences on hamster lordosis are mediated by both the dCG and tectum. In addition, they suggest that these influences are strongly lateralized until at least this stage of sensory processing, leaving for some subsequent element of neural circuitry the task of translating these lateralized inputs into the bilaterally symmetric outputs ultimately required to program the normal, bilaterally symmetric, lordosis response.

Spinocerebellar neurones in the guinea pig--a morphological study

The morphology and distribution of spinocerebellar neurones were examined in the guinea pig. Horseradish peroxidase was injected into the cerebellum, and after a survival time of 72 h retrogradely labelled cells were examined in the spinal cord. The distribution of spinocerebellar cells was similar to that previously demonstrated in the rat. Three major groups of neurones were distinguished: the central cervical nucleus (C1-C2), Clarke's column (T2-L3) and spinal border cells (L3-L6). Neurones in the central cervical nucleus were multipolar, had mean equivalent diameters of about 24 microns, and their axons ascended on the contralateral side of the spinal cord. Neurones in Clarke's column were spindle-shaped, approximately 25 X 35 microns, and their axons ascended on the ipsilateral side of the spinal cord. Spinal border cells were multipolar, with mean equivalent diameters of about 35 microns; their axons were predominantly crossed.

Spinomesencephalic tract: projections from the lumbosacral spinal cord of the rat, cat, and monkey

Anterograde transport of wheat germ agglutinin conjugated to horseradish peroxidase was used to determine the terminal domain of the projection from the lumbosacral spinal cord to the midbrain in the rat, cat, and monkey. Results have shown that several midbrain regions receiving afferent input from this level of the spinal cord are common to the three species examined. Structures innervated by this projection were located throughout the full rostrocaudal extent of the midbrain. The strongest projections were to the intercollicular region and caudal midbrain contralateral to injection sites in the spinal cord. Terminal labeling in the rostral midbrain, except that observed in the nucleus of Darkschewitsch, was substantially less than that observed at more caudal midbrain levels. Structures receiving the strongest input from the spinal cord included the central gray, nucleus cuneiformis, the deep and intermediate layers of the superior colliculus, and the intercollicular nucleus. Other structures receiving afferent input from the lumbosacral spinal cord included the anterior and posterior pretectal nuclei, red nucleus, Edinger-Westphal nucleus, interstitial nucleus of Cajal, and the mesencephalic reticular formation. It is concluded that the spinal projection to the midbrain is a multicomponent projection consisting of several pathways terminating in discrete midbrain regions. Considering the diverse functions associated with midbrain regions receiving spinal input and the response and receptive field properties of cells belonging to this pathway, the results of the present study are discussed in relation to the potential role of the spinomesencephalic tract in somatic, visceral, and motor function.

Afferent projections to the periaqueductal gray in the rabbit

The afferents to the periaqueductal gray in the rabbit have been described following hydraulic pressure injection of horseradish peroxidase at various sites throughout this structure. Every third section was reacted with tetramethylbenzidine, for the localization of afferent neurons. At the site of the deposit alternate sections were reacted with tetramethylbenzidine, Hanker-Yates reagent, or diaminobenzidine, for comparative assessment of the injection site. A large number of retrogradely labelled cells, assessed by bright- and dark-field microscopy, were observed in a wide range of areas throughout the brain. Major labelled areas within the telencephalon were cortical areas 5, 20, 21, 32 and 40. Within the diencephalon, the hypothalamus contained quantitatively by far the largest number of labelled cells. Of these nuclei, the dorsal pre-mammillary nucleus contained the largest number of labelled cells. Considerable labelling was also found within medial and lateral preoptic nuclei, anterior hypothalamic area, and ventromedial hypothalamic nucleus. Another diencephalic region containing a significant number of retrogradely labelled neurons was the zona incerta. At midbrain, pontine and medullary levels, additional labelled regions were: the substantia nigra, cuneiform nucleus, parabigeminal nucleus, raphe magnus, and reticular areas. Heavy labelling was seen within the periaqueductal gray itself, rostral and caudal to deposits placed within each subdivision. In addition, a large number of other areas labelled throughout the brain (Tables 2A-D). Not only were some differences noted in the pattern of labelled cells with deposits placed rostrally or caudally within periaqueductal gray, but certain topographical differences with respect to the degree of labelling within nuclei were also seen with injection sites ventral, lateral or dorsal to the aqueduct. In addition, a further difference was noted, in that over one third of the areas labelled with deposits in just one or other of the "divisions" within periaqueductal gray. The results therefore suggest that the periaqueductal gray might be divisible to some extent on the basis of connectivity with intrinsic subdivisions of the complex. It is hoped that, with time, it might prove possible to resolve any such differential input in functional terms. The wide variety of afferent input to the periaqueductal gray, and its strategic location, would seem to place it in a unique position for integrating and modifying a diversity of motor, autonomic, hormonal, sensory and limbic influences.(ABSTRACT TRUNCATED AT 400 WORDS)

Differential origin of brainstem serotoninergic projections to the midbrain periaqueductal gray and superior colliculus of the rat

Previous studies have shown that both the midbrain periaqueductal gray (PAG) and the superior colliculus receive a significant serotoninergic (5-HT) innervation. In the present study the origins of these 5-HT projections to the rodent PAG and superior colliculus were analyzed by using a combined immunohistochemical-retrograde transport technique. Thirteen brainstem regions were found to contain double-labelled 5-HT-like immunoreactive neurons following HRP injections into the PAG while only four brainstem nuclei contained double-labelled neurons following superior collicular injections. After HRP deposits into the ventral PAG, the largest percentage of double-labelled neurons was identified in nucleus raphe magnus, pars alpha of the nucleus gigantocellularis, and the paragigantocellular nucleus. The dorsal PAG, on the other hand, received the largest percentage of its 5-HT projections from nuclei raphe dorsalis, raphe obscurus, raphe pontis, and raphe medianis. The 5-HT input to the superior colliculus was found to arise exclusively from nuclei raphe dorsalis, raphe medianis, and raphe pontis and from the contralateral periaqueductal gray. Raphe nuclei were found to contribute serotoninergic projections to both the PAG and the superior colliculus while reticular nuclei contributed 5-HT projections only to the PAG. Injections of the fluorescent retrograde tracers true blue and nuclear yellow were then made into the PAG and superior colliculus to ascertain if neurons located in raphe nuclei that projected to both structures provided axon collaterals to both areas. Generally, less than 10% of raphe neurons projecting to the superior colliculus were identified as providing axon collaterals to the PAG. The present results demonstrate major quantitative and qualitative differences in the origin of 5-HT projections to the ventral PAG and superior colliculus. The origin of 5-HT input to the dorsal PAG, on the other hand, showed many similarities to the origin of 5-HT innervation of the superior colliculus. These data also indicate that approximately 35% of raphe neurons provide nonserotoninergic projections to the PAG and superior colliculus.

An autoradiographic study of projections ascending from the midbrain central gray, and from the region lateral to it, in the rat

Ascending projections from the midbrain central gray (CG) and from the region lateral to it were traced in the rat using tritiated amino acid autoradiography. Leucine or a cocktail of amino acids (leucine, proline, lysine, histidine, and tyrosine) were used as tracers. In addition to projections within the midbrain, ascending fibers follow three trajectories. The ventral projection passes through the ventral tegmental region of Tsai and the medial forebrain bundle to reach the hypothalamus, preoptic area, caudoputamen, substantia innominata, stria terminalis, and amygdala. There are labeled fibers in the diagonal bands of Broca and medial septum, and terminal labeling in the lateral septum, nucleus accumbens, olfactory tubercle, and frontal cortex. The dorsal periventricular projection terminates in the midline and intralaminar thalamic nuclei. The ventral periventricular projection follows the ventral component of the third ventricle into the hypothalamus, passing primarily through the dorsal hypothalamic area and labeling the rostral hypothalamus and preoptic area. Projections from the region lateral to the CG are similar, but exhibit stronger proximal, and weaker distal, projections. Rostral levels of the CG send heavier projections to the fields of Forel and the zona incerta, but fewer fibers through the supraoptic decussation, than do caudal levels. Ascending projections from the CG are both strong and widespread. Strong projections to the limbic system and the intralaminar thalamic nuclei provide an anatomical substrate for CG involvement in nociception and affective responses.

Long ascending projections to the midbrain from cells of lamina I and nucleus of the dorsolateral funiculus of the rat spinal cord

Small volumes (5-40 nl) of an aqueous solution of wheat-germ-agglutinin-conjugated horseradish peroxidase (WGA-HRP) were injected unilaterally into midbrain structures of 18 adult, albino rats. In 17 of these preparations cells of many types were found to be retrogradely labeled in cervical and lumbar spinal cord segments. The data reported here concern the number and location of labeled cells from injection sites in the midbrain that affected two distinct cell populations: neurons within the marginal layer (lamina I cells) and neurons of the nucleus of the dorsolateral funiculus (NDLF cells). In ten of the preparations, only nine of which are reported in detail here, a total of 1,831 labeled lamina I cells were identified. In the lumbar enlargement they reached a density of more than 60 cells/mm. Of these, 85% projected to medial portions of the caudal, contralateral midbrain. Injection sites that were centered in the caudal periaqueductal gray (PAG) and/or in the immediately adjacent region of nucleus cuneiformis labeled the largest numbers of lamina I cells. Cells of the NDLF were retrogradely labeled in all preparations in which lamina I cells were labeled but they were also observed in five cases in which lamina I cells were not labeled. A total of 1,914 NDLF cells were labeled from all injection sites. These cells were found to have essentially a bilateral distribution with 57% of the cells located in the contralateral DLF. Although there is substantial overlap between the terminal fields of lamina I and NDLF cells within the midbrain, NDLF cells had a more diffuse target area encompassing the reticular core of the midbrain and PAG, bilaterally, while the target area for lamina I cells was comparatively discrete, being largely restricted to the more medially situated midbrain structures, contralaterally. Whether the terminations of lamina I cells in and near the PAG are from collaterals of spinothalamic neurons originating in lamina I, or a subclass of lamina I neurons that project exclusively to the midbrain, is not known. It is significant, however, that lamina I cells, known to be activated by noxious stimuli to the skin, should project to a region of the brain stem from which analgesia can be produced by electrical stimulation or by local application of opiates.

Temporal pattern of HRP spread from an iontophoretic deposit site and description of a new HRP-gel implant method

In the course of examining afferents to ventromedial hypothalamic (VMH) neurons using horseradish peroxidase (HRP), we needed to know how close to an iontophoretic deposit site neurons could be proved to be retrogradely labeled. In evaluating cells near but clearly outside HRP deposit sites visualized after a 24-hour survival period, for example, neurons which had been filled with HRP by somal or dendritic uptake could not be treated as retrogradely labeled and thus would not add to studies of intrahypothalamic connections. Rats were given standardized iontophoretic applications of HRP into VMH (continuous positive current 0.25 mu amp for 1 minute) and sacrificed after 5 or 15 minutes, 1, 4, 8, 12, or 24 hours in order to examine the pattern of HRP spread. The chromogen was tetramethylbenzidine. The volume of the application site visualized at 24 hours was less than 10% of maximum site size, which occurred at 1 hour. Since the cells located within the maximal spread boundary are candidates for nonretrograde labeling, HRP data on local connections obtained even from small iontophoretic deposits must be evaluated in the light of the demonstrated expansion and subsequent contraction of the application site. These results may also hold implications for the precision with which distant connections can be examined using the HRP retrograde method, as sites that appear discrete when visualized after 24-hour survival may have overlapped at shorter times post-iontophoresis. Incorporation of retrograde tracers into polyacrylamide gels provides an effective alternative to pressure injection or iontophoresis of aqueous tracer solutions. We describe a method for filling micropipettes with HRP-polyacrylamide gel. The pipettes are then implanted into brain sites to provide a confined pool of HRP. With postimplantation survival of 24 hour or longer, this method produces sites comparable in size to iontophoretic sites examined at 24 hours and results in improved retrograde labeling. Some results obtained with this method concerning the afferent connections of the dorsomedial hypothalamus are described.