Somatosensory projection to the mesencephalon: an anatomical study in the monkey

Signal processing in the C2 module of the flocculus and its role in head movement control

The major novel findings described and reviewed in the present study have all been demonstrated in the C2 module, which is formed by the rostral medial accessory olive, posterior interposed nucleus of the cerebellum, and zone C2. We show (1) that expression of dendritic lamellar bodies and dendrodendritic gap junctions in the rostral medial accessory olive are both down regulated by removal of the GABAergic input from the posterior interposed nucleus of the cerebellum to electrotonically coupled olivary dendrites; (2) that the high density of dendritic lamellar bodies in the rostral medial accessory olive can be correlated with a relatively high level of CS synchrony in the C2 zone of the flocculus; and (3) that the C2 zone of the flocculus is involved in head movements and probably gaze control. These results support the hypothesis that dendritic lamellar bodies are associated with dendrodentritic gap junctions, and they suggest that appropriate executions of compensatory head and eye movements require particular levels of complex spike synchrony in the flocculus.

Distinct cell groups in the lumbosacral cord of the cat project to different areas in the periaqueductal gray

The periaqueductal gray (PAG) is involved in aggressive and defensive behavior, micturition, and lordosis. Especially for the latter two functions, PAG afferents from the lumbosacral cord are of vital importance because, in addition to information regarding homeostasis and thermoregulation, they convey information from the pelvic viscera and sex organs. In the present retro- and antero-grade tracing study, the projection patterns of different lumbosacral cell groups in the PAG were determined. In the retrograde study, wheatgerm agglutinin-horseradish peroxidase (WGA-HRP) injections were made in the PAG and/or adjacent tegmentum, and in the anterograde study, WGA-HRP was injected in different lumbosacral segments. The results revealed that lumbosacral-PAG neurons could be divided into three groups. The first and largest group was present in lumbar 7-sacral 3 segments (L7-S3) and consisted of small, oval, and fusiform neurons. It extended from the dorsolateral part of lamina I in L7, along the lateral part of the dorsal horn in S1, and into lamina V of S2. In the lateral part of S2, some of its neurons formed clusters with intervals of +/- 230 microns. The location of the first group overlapped extensively with the termination area of pelvic and pudendal afferents. The main midbrain target of the first group was the medial part of the lateral PAG. The second group consisted of small to large multipolar neurons in laminae VIII and medial VII of caudal L6, L7, and rostral S1. This group projected strongly to a distinct region in the lateral part of the lateral PAG and the laterally adjacent tegmentum. About 10% of the labeled neurons did not fit in the two groups. They were evenly distributed throughout lumbar 4-coccygeal 3 segments (L4-Co3) and consisted of large multipolar lamina V neurons and small lamina I neurons that projected diffusely to the lateral and dorsal PAG. The large lamina V neurons also targeted the laterally adjacent tegmentum. The possible involvement of the lumbosacral-PAG projections in micturition, lordosis, and defensive and aggressive behavior is discussed.

Gracile, cuneate, and spinal trigeminal projections to inferior olive in rat and monkey

In the cat, somatosensory nuclei send substantial projections to the inferior olive, where they terminate in a somatotopic fashion. Although the organization of the cat inferior olive has been used to interpret data from other species, published data suggest this organization may not occur universally. The present study investigated whether the inferior olive in albino rats and cynomolgus monkeys receives the same brainstem somatosensory inputs, whether these inputs are organized somatotopically and, if so, how the organization compares with that in the cat. Projections from the gracile, cuneate and spinal trigeminal nuclei were labeled with wheat germ agglutinin conjugated to horseradish peroxidase or with biotinylated dextran. The results were compared with data from cats (Berkley and Hand [1978] J. Comp. Neurol. 180:253-264). In the rat and monkey, the gracile, cuneate and spinal trigeminal nuclei all project to the contralateral inferior olive, where each nucleus has a distinct preferred terminal field. As in the cat, projections to the medial accessory olive and caudal dorsal accessory olive did not terminate in a precisely organized fashion. Projections to the rostral dorsal accessory olive, however, formed a clear somatotopic map. These somatotopic maps differed from those in the cat in that input from the trigeminal nucleus was confined rostrally, so that the caudal end only received input from the gracile and cuneate nuclei. These data indicate that similar organizational principles characterize the somatosensory projections to the inferior olives of the three species. Nevertheless, distinct species differences occur with regard to the details of this organization.

Association of spinal lamina I projections with brainstem catecholamine neurons in the monkey

In addition to giving primary projections to the parabrachial and periaqueductal gray regions, ascending lamina I projections course through and terminate in brainstem regions known to contain catecholaminergic cells. For this reason, double-labeling experiments were designed for analysis with light and electron microscopy. The lamina I projections in the Cynomolgus monkey were anterogradely labeled with Phaseolus vulgaris leucoagglutinin (PHA-L) and catecholamine-containing neurons were labeled immunocytochemically for tyrosine hydroxylase (TH). Light level double-labeling experiments revealed that the terminations of the lamina I ascending projections through the medulla and pons strongly overlap with the localization of catecholamine cells in: the entire rostrocaudal extent of the ventrolateral medulla (A1 caudally, C1 rostrally); the solitary nucleus and the dorsomedial medullary reticular formation (A2 caudally, C2 rostrally); the ventrolateral pons (A5); the locus coeruleus (A6); and the subcoerulear region, the Kölliker-Fuse nucleus, and the medial and lateral parabrachial nuclei (A7). At the light microscopic level, close appositions between PHA-L-labeled lamina I terminal varicosities and TH-positive dendrites and somata were observed, particularly in the A1, A5 and the A7 cell groups on the contralateral side. At the electron microscopic level, examples of lamina I terminals were found synapsing on cells of the ventrolateral catecholamine cell groups in preliminary studies. The afferent input relayed by these lamina I projections could provide information about pain, temperature, and metabolic state as described previously. Lamina I input could impact interactions of the catecholamine system with higher brain centers modulating complex autonomic, endocrine, sensory, motor, limbic and cortical functions such as memory and learning. Nociceptive lamina I input to catecholamine cell regions with projections back to the spinal cord could form a feedback loop for control of spinal sensory, autonomic and motor activity.

The raccoon lateral cervical nucleus: mediolateral organization of GABA-positive and GABA-negative neurons and fibers

In the lateral cervical nucleus (LCN) of the cat, GABA-immunoreactive neurons and substance P-immunoreactive fibers are concentrated in the medial part of the nucleus, whereas in the monkey LCN no preferential locations have been identified. In raccoons, substance P-immunoreactive fibers display a distribution pattern similar to that in cats. However, the presence and distribution of GABA-immunoreactive neurons in the raccoon LCN has not been examined, and it is therefore not known whether raccoons are similar to cats or primates in this respect. Thus, in the present study, the raccoon LCN was examined for the presence and distribution of GABA-immunoreactive cells with respect to their numbers, locations, and sizes. The distribution of GABA-positive fibers and varicosities within the LCN was also investigated. The results of measurements of cross-sectional areas of LCN neurons indicate a trend toward decreasing cell size along the dorsolateral to medial axis of the raccoon LCN. Compared to neurons of the centrally located ventromedial division, neurons are statistically significantly larger in the dorsolateral division and smaller in the medial division of the nucleus. Cell counts in post-embedding-stained semithin sections through the nucleus revealed an average of 8,700 neurons per LCN. Approximately 4% of LCN neurons are GABA-immunoreactive. These neurons are small and most (80%) of them are located in the medial third of the LCN. In contrast, GABA-immunoreactive fibers and varicosities are present in about equal density throughout the raccoon LCN. Thus, the distributions of GABA-immunoreactive neurons and neuron sizes in the raccoon LCN conform closely to those in cats. Together with previous observations in cats and raccoons, the present findings support the notion that these small GABA-immunoreactive neurons may be local circuit inhibitory neurons and indicate the presence of a mediolateral segregation that may be of fundamental importance for the functional organization of the carnivore LCN.

Subnuclear localization of FOS-like immunoreactivity in the rat parabrachial nucleus after nociceptive stimulation

The effect of noxious stimulation on the expression of FOS-like immunoreactivity (FOS-LI) in neurons of the parabrachial nucleus (PB) was studied in awake, freely moving rats. In one series of experiments, the rats were subjected to noxious mechanical stimulation (pinch) of either the nape of the neck or the base of the tail for 20 seconds every 5 minutes for 90 minutes, and then they were killed by transcardial perfusion after 45-210 minutes. Control animals received innocuous mechanical stimulation (brush) of the tail. Noxious stimuli resulted in FOS-LI in neurons in the dorsal part of the lateral PB, with heavy labeling in the superior lateral (PBsl) and the dorsal lateral (PBdl) subnuclei. FOS-LI was also elicited in the central lateral subnucleus (PBcl) and, although much more sparsely, in the external lateral subnucleus and the Kölliker-Fuse nucleus. Tail and neck stimulation resulted in similar labeling patterns, but more neurons, particularly in PBsl, expressed FOS-LI after pinch of the tail than of the neck. In another series of experiments, rats received injection of 5% formalin into one hindpaw. After 75-90 minutes, FOS-LI was seen in the same parts of PB as after noxious mechanical stimulation. The heaviest labeling was seen on the side contralateral to the injection side, with statistically significant (P < 0.05) side differences present in PBsl and PBdl. In a third series of experiments, rats were hemisected at low cervical-upper thoracic segments, allowed 2 weeks to recover, and then given formalin injections in both hindpaws. Significantly more neurons were FOS-labeled in PBdl, PBsl, and PBcl on the side contralateral to the hemisection than on the ipsilateral side. These observations are discussed in relation to the organization of the spinal afferent input and the efferent connections of PB. It is concluded that the FOS-LI expression in PBdl and PBsl and probably also in PBcl, to a large extent, is evoked by the ascending spinal nociceptive input to PB. Because these subnuclei project to several hypothalamic regions, it is suggested that neurons in PB that express FOS after noxious mechanical and chemical stimulation primarily are involved in autonomic and homeostatic responses to behavioral situations that involve tissue-damaging stimuli.

Common patterns of increased and decreased fos expression in midbrain and pons evoked by noxious deep somatic and noxious visceral manipulations in the rat

Immunohistochemical detection of the protein product (Fos) of the c-fos immediate early gene was used to study neuronal activation in the rostral pons and midbrain of halothane-anesthetised rats following noxious deep somatic or noxious visceral stimulation. In animals exposed only to halothane anesthesia, Fos-like immunoreactive (IR) neurons were located in the midbrain periaqueductal gray matter, tectum, and parabrachial nucleus. Following noxious stimulation of hindlimb muscle, knee joint, vagal cardiopulmonary, or peritoneal nociceptors, there was, compared to halothane-only animals, a significant increase in the numbers of Fos-like (IR) cells in the caudal ventrolateral periaqueductal gray and the intermediate gray lamina of the superior colliculus. Given the general agreement that increased Fos expression is a consequence of increased neuronal activity, the finding that a range of noxious deep somatic and noxious visceral stimuli evoked increased neuronal activity in a discrete, caudal ventrolateral periaqueductal gray region is consistent with previous suggestions that this region is an integrator of deep noxious evoked reactions. The noxious deep somatic and noxious visceral manipulations also evoked, compared to halothane-only animals, reductions in the numbers of Fos-like IR cells in the stratum opticum of the superior colliculus and the unlaminated portion of the external subnucleus of the inferior colliculus. To our knowledge this is the first report of reductions in Fos-expression in the tectum evoked by noxious stimulation. In separate experiments, the effects of noxious deep somatic and noxious visceral manipulations on arterial pressure and heart rate were measured. The noxious visceral manipulations evoked substantial and sustained falls in arterial pressure (15-45 mmHg), and heart rate (75-100 bpm), whereas the depressor and bradycardiac effects of the noxious deep somatic manipulations were weaker, not as sustained, or entirely absent. As similar distributions and numbers of both increased and decreased Fos-like IR cells were observed after each of the deep noxious manipulations, it follows that the deep noxious evoked increases and decreases in Fos expression were not secondary to the evoked depressor or bradycardiac effects.

Projections from visual areas of the cerebral cortex to pretectal nuclear complex, terminal accessory optic nuclei, and superior colliculus in macaque monkey

The purpose of this study was to analyze the projections from visually related areas of the cerebral cortex of rhesus monkey to subcortical nuclei involved in eye-movement control; i.e., the pretectal nuclear complex, the terminal nuclei of the accessory optic system (AOS), and the superior colliculus (SC). The anterograde tracer 3H-leucine was pressure injected bilaterally into the cortex of six monkeys (for a total of 12 cases) involving the primary visual cortex (area 17); the medial prestriate cortex (medial 18/19); dorsomedial area 19; the caudal portion of the cortex of the superior temporal sulcus, upper bank (cytoarchitectural area OAa) and lower bank (area PGa); the lower bank of the caudal lateral intraparietal sulcus (area POa); and the inferior parietal lobule (area 7). The results revealed that the pretectal nucleus of the optic tract received inputs from medial prestriate cortex, dorsomedial part of area 19, OAa, and PGa. The posterior pretectal nucleus received sparse projections from area 7 and the cortex lining the intraparietal sulcus (dorsomedial part of area 19 and POa). The pretectal olivary nucleus was targeted by neurons in cortex of dorsomedial area 19, and the anterior pretectal nucleus was targeted by neurons in both dorsomedial 19 and area 7. The nuclei of the AOS (dorsal terminal; lateral terminal; and interstitial nuclei of the superior fasciculus, posterior and medial fibers) received projections exclusively from areas OAa and PGa. Furthermore, in one case with PGa injection, the medial terminal nucleus, dorsal portion, was also labeled. The visual cortical areas studied projected differentially upon the SC laminae. The primary visual area 17 projected only to the superficial laminae, i.e., stratum zonale (SZ), stratum griseum superficiale (SGS), and stratum opticum (SO). On the other hand, the medial portion of the prestriate cortex and caudal OAa and PGa targeted the superficial and intermediate laminae, i.e., SZ, SGS, SO, and stratum griseum intermediale (SGI), whereas caudal area POa projected primarily to the intermediate layer SGI. Rostral area 7 (mainly 7b) neurons terminated in the stratum album intermediale (SAI); no SC terminals were found in a case in which caudal area 7 (mainly 7a) was injected.

Distribution of brainstem projections from spinal lamina I neurons in the cat and the monkey

The distribution of terminal projections in the brainstem from lamina I neurons in the spinal dorsal horn was investigated with the anterograde tracer Phaseolus vulgaris-leucoagglutinin in the cat and the cynomolgus monkey. Iontophoretic injections made with physiological guidance were restricted to lamina I or to laminae I-III in the cervical (C6-8) or lumbar (L6-7) enlargement. The distribution of terminal labeling was essentially identical in the cat and the monkey, although consistently of greater intensity in the monkey. Terminations were observed in the solitary nucleus, the dorsomedial medullary reticular formation, the entire rostrocaudal extent of the ventrolateral medulla, the locus coeruleus, the subcoerulear region and the Kölliker-Fuse nucleus, the lateral and medial portions of the parabrachial nucleus, the cuneiform nucleus, the ventrolateral and lateral portions of the periaqueductal gray, and the intercollicular nucleus. Lamina I terminations were generally bilateral in the medulla but more dense contralaterally in the pons and mesencephalon. The density and laterality of labeling in the medulla varied between cases independently from that in the pons and mesencephalon, suggesting that the lamina I projections to these regions may originate from different subsets of neurons. A clear topographic organization was observed only in the lateral column of the periaqueductal gray, where lumbar lamina I terminations were found caudal to cervical terminations. These observations indicate that spinal lamina I neurons project to a variety of brainstem sites involved in autonomic (cardiovascular, respiratory) and homeostatic processing and the control of behavioral state. These projections provide an afferent substrate for spino-bulbo-spinal somatoautonomic reflex arcs activated by nociceptive, thermoreceptive activity and for a spino-bulbo-hypothalamic relay of such activity by cells in the caudal ventrolateral medulla. These observations support the general concept that lamina I projections distribute modality-selective sensory information relevant to the physiological status and maintenance of the tissues and organs of the entire organism.

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.

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

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

Organization of the efferent projections from the spinal cervical enlargement to the parabrachial area and periaqueductal gray: a PHA-L study in the rat

The organization of efferent projections from the spinal cervical enlargement to the parabrachial (PB) area and the periaqueductal gray (PAG) was studied in the rat by using microinjections of Phaseolus vulgaris-leucoagglutinin (PHA-L) into different laminae around the C7 level. The results demonstrated two areas of cervical enlargement which project in different ways to the PB area and PAG. First, the superficial laminae (I, II) showed a very dense projection, with a clear contralateral dominance at the coronal level where the inferior colliculus merges with the pons, to a restricted "superficial" portion of the PB area, namely the lateral crescent area, the dorsal lateral, the superior lateral (PBsl), and the outer portion of the external lateral PB subnuclei. Less dense projections were observed in the Kölliker-Fuse nucleus (KF) and in the ventrolateral/lateral quadrant of the caudal and mid PAG. By contrast, the labeling was weak or absent in the other PB subnuclei and the outer adjacent regions; in particular, no, or very little, labeling was found in the cuneiform nucleus. The PB area appeared to be the supraspinal target that received the densest projection from laminae I and II. Projections were less dense in the PAG and the thalamus and markedly less in other sites such as the ventrolateral medulla, the subnucleus reticularis dorsalis, and the nucleus of the solitary tract. Second, the reticular portion of lamina V, the medial portion of laminae IV-VI up to X and lamina VIII, showed bilateral projections with a weak ipsilateral dominance and a high to medium density on a very restricted portion of the PB area, namely the internal lateral PB subnucleus. A lesser projection was also observed in the adjacent portion of the PBsl, the KF, and the lateral quadrant of the PAG. These results suggest that signals carried by neurons from lamina I-II converge on a restricted superficial portion of the PB area and the ventral part of the lateral quadrant of the PAG. These results are discussed in the context of the role of the spino-PB and spino-PAG pathways in nociception.

The efferent projections of the periaqueductal gray in the rat: a Phaseolus vulgaris-leucoagglutinin study. I. Ascending projections

This study has examined the ascending projections of the periaqueductal gray in the rat. Injections of Phaseolus vulgaris-leucoagglutinin were placed in the dorsolateral or ventrolateral subregions, at rostral or caudal sites. From either region, fibers ascended via two bundles. The periventricular bundle ascended in the periaqueductal and periventricular gray matter. At the posterior commissure level, this bundle divided into a dorsal component that terminated in the intralaminar and midline thalamic nuclei, and a ventral component that supplied the hypothalamus. The ventral bundle formed in the deep mesencephalic reticular formation and supplied the ventral tegmental area, substantia nigra pars compacta, and the retrorubral field. The remaining fibers were incorporated into the medial forebrain bundle. These supplied the lateral hypothalamus and forebrain structures, including the preoptic area, the nuclei of the diagonal band, and the lateral division of the bed nucleus of the stria terminalis. The dorsolateral subregion preferentially innervated the centrolateral and paraventricular thalamic nuclei and the anterior hypothalamic area. The ventrolateral subregion preferentially innervated the parafascicular and central medial thalamic nuclei, the lateral hypothalamic area, and the lateral division of the bed nucleus of the stria terminalis. Although the dorsolateral and ventrolateral subregions gave rise to differential projections, the projections from both the rostral and caudal parts of either subregion were similar. This suggests that the dorsolateral and ventrolateral subregions are organized into longitudinal columns that extend throughout the length of the periaqueductal gray. These columns may correspond to those demonstrated in recent physiological studies.

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.

Columnar organization in the midbrain periaqueductal gray: modules for emotional expression?

Independent discoveries in several laboratories suggest that the midbrain periaqueductal gray (PAG), the cell-dense region surrounding the midbrain aqueduct, contains a previously unsuspected degree of anatomical and functional organization. This organization takes the form of longitudinal columns of afferent inputs, output neurons and intrinsic interneurons. Recent evidence suggests: that the important functions that are classically associated with the PAG--defensive reactions, analgesia and autonomic regulation--are integrated by overlapping longitudinal columns of neurons; and that different classes of threatening or nociceptive stimuli trigger distinct co-ordinated patterns of skeletal, autonomic and antinociceptive adjustments by selectively targeting specific PAG columnar circuits. These findings call for a fundamental revision in our concept of the organization of the PAG, and a recognition of the special roles played by different longitudinal PAG columns in co-ordinating distinct strategies for coping with different types of stress, threat and pain.

The role of the periaqueductal grey in vocal behaviour

This is a review of our current knowledge about the role of the periaqueductal grey (PAG) in vocal control. It shows that electrical stimulation of the PAG can evoke species-specific calls with short latency and low habituation in many mammals. The vocalization-eliciting region contains neurones the activity of which is correlated with the activity of specific laryngeal muscles. Lesioning studies show that destruction of the PAG and laterally bordering tegmentum can cause mutism without akinesia. Neuroanatomical studies reveal that the PAG lacks direct connections with the majority of phonatory motoneurone pools but is connected with the periambigual reticular formation, an area which does have direct connections with all phonatory motor nuclei. The PAG receives a glutamatergic input from several sensory areas, such as the superior and inferior colliculi, solitary tract nucleus and spinal trigeminal nucleus. Glutamatergic input, in addition, reaches it from numerous limbic structures the stimulation of which also produces vocalization, such as the anterior cingulate cortex, septum, amygdala, hypothalamus and midline thalamus. Pharmacological blocking of this glutamatergic input causes mutism. The glutamatceptive vocalization-controlling neurones are under a tonic inhibitory control from GABAergic neurones. Removal of this inhibitory input lowers the threshold for the elicitation of vocalization by external stimuli. A modulatory control on vocalization threshold is also exerted by glycinergic, opioidergic, cholinergic, histaminergic and, possibly, noradrenergic and dopaminergic afferents. It is proposed that the PAG serves as a link between sensory and motivation-controlling structures on the one hand and the periambigual reticular formation coordinating the activity of the different phonatory muscles on the other.

Neurotransmitters in subcortical somatosensory pathways

Investigations during recent years indicate that many different neuroactive substances are involved in the transmission and modulation of somesthetic information in the central nervous system. This review surveys recent developments within the field of somatosensory neurotransmission, emphasizing immunocytochemical findings. Increasing evidence indicates a widespread role for glutamate as a fast-acting excitatory neurotransmitter at different levels in somatosensory pathways. Several studies have substantiated a role for glutamate as a neurotransmitter in primary afferent neurons and in corticofugal projections, and also indicate a neurotransmitter role for glutamate in ascending somatosensory pathways. Other substances likely to be involved in somatosensory neurotransmission include the neuropeptides. Many different peptides have been detected in primary afferent neurons with unmyelinated or thinly myelinated axons, and are thus likely to be directly involved in primary afferent neurotransmission. Some neurons giving rise to ascending somatosensory pathways, primarily those with cell bodies in the dorsal horn, are also immunoreactive for peptides. Recent investigations have shown that the expression of neuropeptides, both in primary afferent and ascending tract neurons, may change as a result of various kinds of peripheral manipulation. The occurrence of neurotransmitters in intrinsic neurons and neurons providing modulating inputs to somatosensory relay nuclei (the dorsal horn, the lateral cervical nucleus, the dorsal column nuclei and the ventrobasal thalamus) is also reviewed. Neurotransmitters and modulators in such neurons include acetylcholine, monoamines, GABA, glycine, glutamate, and various neuropeptides.

Compartmentation of glutamate and glutamine in the lateral cervical nucleus: further evidence for glutamate as a spinocervical tract neurotransmitter

Previous observations indicate that spinocervical tract terminals contain relatively high levels of glutamate. To examine whether these high glutamate levels are likely to represent a neurotransmitter pool or an elevated metabolic pool, the distributions of glutamate- and glutamine-like immunoreactivities were examined in adjacent immunogold-labeled sections of the lateral cervical nucleus. Spinocervical tract terminals were identified by anterograde transport of horseradish peroxidase and wheat germ agglutinin-horseradish peroxidase conjugate from the spinal cord. Spinocervical tract terminals were found to contain significantly higher levels of glutamate-like immunoreactivity than other examined tissue compartments (large neuronal cell bodies, terminals with pleomorphic vesicles, astrocytes, and average tissue level). In contrast, the highest levels of glutamine-like immunoreactivity were detected in astrocytes. The different analyzed tissue elements formed three groups with respect to glutamate:glutamine ratios: one high ratio group including spinocervical tract terminals, a second group with intermediate ratios consisting of neuronal cell bodies and terminals containing pleomorphic synaptic vesicles, and a third low ratio group including astrocytes. Our findings indicate the presence of a compartmentation of glutamate and glutamine in the lateral cervical nucleus, similar to that postulated in biochemical studies of the central nervous system. The results also show that spinocervical tract terminals have high glutamate: glutamine ratios, similar to those previously observed in putative glutamatergic terminals in the cerebellar cortex. Thus, spinocervical tract terminals display biochemical characteristics that would be expected of glutamatergic terminals and the present findings therefore provide further evidence for glutamate as a spinocervical tract neurotransmitter.

Spinal cord and trigeminal projections to the pontine parabrachial region in the rat as demonstrated with Phaseolus vulgaris leucoagglutinin

In order to determine the regions within the parabrachial nucleus that receive synaptic input from nociceptive regions of the spinal cord and medulla in the rat, we analyzed the "Golgi-like" labeling produced by anterograde transport of Phaseolus vulgaris leucoagglutinin (PHA-L) from discrete iontophoretic injections confined to either the superficial dorsal horn of the lumbar spinal cord or to the superficial dorsal horn of the trigeminal nucleus at the level of the obex. Labeled fibers from both the spinal cord and the medulla ascended through the ventral lateral pons and coursed with the ventral spinocerebellar tract toward the parabrachial nuclei. Spinal cord injections led to labeling of fine caliber fibers and en passant and terminal enlargements in the rostral part of the contralateral lateral parabrachial nucleus (PBL), mostly in the central lateral and dorsal lateral subnuclei. Medullary injections revealed fiber and enlargement labeling primarily in the ipsilateral caudal PBL, mostly in the central lateral, external lateral, and medial subnuclei. Injections in both regions resulted in labeled terminations in the Kölliker-Fuse nucleus. These results indicate that the nociceptive regions of the spinal cord and medulla terminate in regions of the parabrachial nucleus that have been associated with autonomic functions because of their interconnections with the hypothalamus, brainstem cardiovascular and respiratory control centers, and the amygdala.

The anterior pretectal nucleus: a proposed role in sensory processing

Four nuclei of the pretectal complex, the olivary pretectal nucleus, the medial pretectal nucleus, the nucleus of the optic tract and the posterior pretectal nucleus, all have a demonstrated role in visual function. In contrast, the anterior pretectal nucleus (APtN) has no inputs from retina and has few outputs to visual accessory nuclei. The APtN has connections with areas associated with sensory functions and it has been suggested that this nucleus may have a role to play in somatosensory processing. An increasing number of behavioural and electrophysiological studies support this view. Brief low-intensity electrical or chemical stimulation of the APtN causes antinociception in the tail flick test in both unanaesthetised and anaesthetised animals. This inhibition of the tail flick response is attenuated by naloxone, alpha-adrenoceptor antagonists and muscarinic cholinergic receptor antagonists. Electrical stimulation of the APtN is similarly effective in the paw pressure and formalin tests. APtN stimulation also causes a brief inhibition of the tooth pulp-evoked jaw opening reflex. studies with [C14]2-deoxyglucose indicate that peripheral noxious stimuli will cause an increase in metabolic activity within the APtN. Animals with electrodes placed in the APtN will self-administer electrical stimulation and this can reduce the aversive and autonomic effects of stimulating the ventromedial hypothalamus. Part of the antinociceptive effects of stimulating the APtN are due to a descending inhibition of spinal dorsal horn projection neurones. Multireceptive neurones deep in the dorsal horn are inhibited by APtN stimulation. In contrast, superficial projection neurones that respond to intense cutaneous stimuli are excited by APtN stimulation. The APtN receives an excitatory input from low-threshold afferents via the dorsal column pathway and a high-threshold excitatory drive from superficial cells projecting through the dorsolateral funiculus. The excitatory input from the dorsal columns may well participate in the long-term inhibition of spinal projection neurones evoked by dorsal column stimulation. These ascending excitatory pathways may also be important to the long-term activation of descending inhibition from the APtN.

The organization of the efferent projections from the pontine parabrachial area to the amygdaloid complex: a Phaseolus vulgaris leucoagglutinin (PHA-L) study in the rat

The organization of the efferent projections from the pontine parabrachial (pPB) area to the amygdala has been studied in the rat by using microinjections of Phaseolus vulgaris leucoagglutinin (PHA-L), a sensitive and selective anterograde axonal marker, into restricted subregions of the pPB area. The results confirmed that the pPB area primarily projected onto the ipsilateral nucleus centralis of the amygdala (Ce), and to a lesser extent onto the ipsilateral posterior basolateral (BLP), anterior basomedial (BMA), and amygdaloid cortical (ACo) nuclei of the amygdala. Substantial projections were also found in the substantia innominata dorsal/ventral portion of the globus pallidus (SId/GPv), substriatal (SStr), and fondus striatal (FStr) regions which continue the amygdala rostrally. The results demonstrated that the projections of the pPB area onto the Ce were topically organized: 1) The region of the pPB area mainly including the medial subnucleus (pPBm), the waist area (pPBwa), and a thin rostral lamina of the ventral lateral subnucleus (pPBvl) projects primarily to the medial portion of the Ce (CeM). Dense projections were also found in the BLP, BMA, and ACo nuclei of the amygdala, and in the SId/GPv, SStr, and FStr rostral areas. 2) The region of the pPB mainly including the rostral portion of the central lateral subnucleus (pPBcl) and the outer-rostral portion of the external lateral subnucleus (pPBel) projects primarily to the lateral portion of the Ce (CeL). 3) The region of the pPB mainly including the dorsolateral subnucleus (pPBdl), the remaining pPBel, and the external medial (pPBem) subnuclei projects primarily to the lateral capsular portion of the Ce (CeLC) and bilaterally to its rostral portion. Dense projections were also found in the regions which extend the CeLC rostrally and in the SId/GPv, SStr, and FStr rostral areas. The possible role of each of the three parabrachio-amygdaloid pathways described is discussed. It was suggested that the pPB-CeM pathway is mainly implicated in gustatory processes; the pPB-CeL pathway is mainly implicated in visceral and chemosensitive processes; and the pPB-CeLC pathway is mainly implicated in respiratory, cardiovascular, and nociceptive processes.

Serotoninergic innervation of the dorsal column nuclei and its relation to cytoarchitectonic subdivisions: an immunohistochemical study in cats and monkeys (Aotus trivirgatus)

The serotoninergic innervation of the dorsal column nuclei (DCN) was investigated in cats and owl monkeys (Aotus trivirgatus) with immunohistochemical methods. A dense network of serotonin-immunoreactive fibers was present in the reticular regions of DCN in cats, and in the pars triangularis of the cuneate nucleus and the peripheral and caudal regions of the gracile nucleus in owl monkeys. The cat's cluster regions and the monkey's rotund regions were more sparsely innervated. Electron microscopic examination showed that the labeled fibers were thin and unmyelinated. Vesicle-containing, terminal-like structures were small. They were in contact with dendrites, other terminals and cell bodies, but synapses were rare. The results demonstrate that the serotoninergic projection to the DCN in both cats and owl monkeys is heterogeneously distributed in a pattern that is faithfully related to the cytoarchitectonic subdivisions of the DCN. The densely innervated reticular regions in the DCN of cats and the corresponding regions in monkeys are predominantly involved in the processing of sensory information to the cerebellum, either directly, or indirectly through projections to the inferior olive, pontine gray, tectum, pretectum, red nucleus, or zona incerta. Thus, the present findings suggest that the serotoninergic innervation of the DCN is primarily related to the DCN's involvement in motor functions.

Physiological and morphological characteristics of spinal neurons projecting to the parabrachial region of the cat

Neurons in the lumbosacral, superficial spinal dorsal horn in the cat were recorded extra- and intracellularly, using dorsal root stimulation as a search stimulus. Isolated neurons were tested for antidromic activation from the contra- and ipsilateral parabrachial region. Seventy-one nociceptive-specific neurons, 11 innocuous cooling neurons, and 8 multireceptive neurons were antidromically activated from the lateral parabrachial region. The receptive fields and response properties were typical of other lamina I and lamina II neurons, in that the receptive fields were usually discrete and relatively small, and the responses ranged from sluggish and decrementing to brisk and augmenting with afterdischarge. The conduction velocity to the parabrachial region averaged 3.7 m/sec for the nociceptive-specific neurons, 3.9 m/sec for the innocuous cooling neurons, and 13.5 m/sec for the multireceptive neurons. Intracellularly labeled neurons were mostly medium to large Waldeyer-like neurons in lamina I. Some had axon collaterals that distributed varicosities in laminae I, II, and V. These data indicate that a slowly conducting nociceptive-specific and thermoreceptive pathway exists between the superficial dorsal horn and the parabrachial region at the pontine-midbrain junction.

Tectal and related target areas of spinal and dorsal column nuclear projections in hedgehog tenrecs

The terminal distributions of spinal and dorsal column nuclear projections to tectum, pretectum, and central gray of hedgehog tenrecs (Echinops telfairi and Setifer setosus) were investigated using anterograde axonal flow and various tracer substances. In the inferior colliculus, the densest and most extensive mesencephalic projections were found within the pericentral regions. One target area, referred to as the external portion of the inferior colliculus, was represented as a semicircle of grain patches lateral and caudal to the central nucleus. This region received somesthetic afferents from the dorsal column nuclei and from spinal segments at various levels. In contrast, after high cervical injections, the pericentral portion dorsomedial to the rostral half of the central nucleus was labeled almost exclusively. This area of labeling was distinct from the labeling in the central gray and might be best compared with the intercollicular zone in other species. The superior colliculus received projections predominantly from the high cervical cord; minor projections also arose from lumbar spinal segments and the dorsal column nuclei. The terminal field covered roughly the caudal half of the colliculus and involved the stratum griseum intermediale in a patch-like fashion. Some labeling was also found in the stratum griseum profundum and in the stratum griseum superficiale. Other than in the colliculi, weak pretectal projections were observed following dorsal column nuclear injections, while the nucleus of Darkschewitsch was labeled best following lumbosacral injections. All mesencephalic target areas were labeled consistently on the contralateral side, while their ipsilateral side was involved to a varying degree: The relatively most prominent ipsilateral labeling was seen in the central gray, being roughly similar on both sides; scarcely any labeling was noted in the ipsilateral superior colliculus. Tectal injections of retrograde tracer, in addition, revealed a considerable number of labeled neurons in a relatively cell-poor region immediately ventral to the high cervical dorsal horn. This region might correspond to the lateral cervical nucleus, an aggregation of neurons that so far has only been demonstrated in higher mammals.

Trigeminal and dorsal column nuclei projections to the anterior pretectal nucleus in the rat

The projections of the trigeminal (V) sensory nuclei (VSN) and the dorsal column nuclei (DCN) to the anterior pretectal nucleus (APT) of the rat were investigated by the use of anterograde and retrograde transport of wheat-germ agglutinin-conjugated horseradish peroxidase (WGA-HRP). Injections of WGA-HRP into the APT retrogradely labeled neurons in the contralateral VSN and DCN. The labeled neurons in the VSN were most concentrated in the rostral V subnucleus interpolaris (Vi), but were also found in caudal V subnucleus oralis (Vo). No labeled neurons were seen in V subnucleus caudalis. In the DCN, retrogradely labeled neurons were observed in rostral portions of both the cuneate (Cu) and gracile (Gr) nuclei. Injections of WGA-HRP into the rostral Vi or caudal Vo resulted in dense anterograde terminal labeling in the ventral two-thirds of the APT; the labeling was maximal in the ventromedial part of the caudal half of the APT and did not extend into its most rostral portion. Labeling resulting from injections of tracer into Cu or Gr was located primarily in the ventral half of the APT, was maximal in the mid-levels of the nucleus and extended into its rostral portions. These results indicate the existence of prominent somatosensory projections to the APT and are consistent with recent findings suggesting a role for the APT in sensorimotor integration.

Ultrastructure of the periaqueductal grey matter of the rat: an electron microscopical and horseradish peroxidase study

The neurons of the mesencephalic periaqueductal grey substance (PAG) in the rat are small and medium sized. The cells are frequently located in small clusters, without interdigitating glial elements and may be connected by direct membrane appositions or by gap junctions. The inner zone of the PAG is cell poor. In many cases, the cytoplasm of the cells is filled with extensive rough endoplasmic reticulum, free ribosomes, Golgi apparatus, and large lysosome-like granules. The nuclei show large indentations. The cells have a high nucleus-cytoplasm ratio. The neuropil is very extensive and particularly rich in large numbers of small unmyelinated axons, dendrites, axonal varicosities, and synaptic connections. Myelinated fibres are relatively scarce. The orientation of the fibres was studied in transverse and horizontal sections, in combination with HRP track tracing experiments. It appeared that throughout the PAG most of the fibres were orientated longitudinally. Quantitation showed that most fibres were present in the inner zones of the PAG. Moreover, the diameter of the fibres adjacent to the aqueduct was smaller than that of the fibres in the peripheral parts of the PAG. The thin unmyelinated fibres made extensive synaptic connections within the PAG. Many synaptic varicosities were found in the neuropil of the PAG. There were four types of synaptic varicosities, characterized by different populations of clear and dense-core secretory granules and by the different morphology of the synaptic specializations. In general, the different types of varicosity were homogeneously distributed in the different parts of the PAG. Electron dense secretory granules, when present, were located at some distance from the synaptic junction. Serial sections revealed varicosities which contained only dense-core secretory granules, without synaptic specializations. The dendrites of PAG neurons generally lacked synaptic spines. Many dendrites, particularly those of neurons located in the peripheral parts of the PAG, were directed toward the aqueduct. The present study shows that the PAG is a very complex brain area. The crisscrossing of axons and dendrites with synaptic connections at considerable distances from the cell bodies render it very difficult to unravel the relationships between the possible sources and destinations of ongoing information. This structure complicates the search for relationships between the functional organization and the cytoarchitectural borders in the PAG area.

Factors influencing neural activity in parabrachial regions during cat vocalizations

The parabrachial nucleus in mammals is intimately connected with other vocalization controlling brainstem structures. It, along with ventromedially adjacent structures, also has been identified as the pneumotaxic center, and as such shows strong respiratory related activity in the anesthetized cat. The current study examines the neuronal activity in cat parabrachial regions during production of instrumentally conditioned vocalizations. Most of the units in our sample show considerable activity during periods between vocalizations. For many units, firing rate fluctuates during the respiratory cycle, although apparently not as strongly as reported in the decerebrate cat. Also, there is often strong phasic activity during periods where animals are licking to ingest their food rewards. During the peri-vocalization period, various neural activity patterns can be recorded. Most common is an activity increase during the vocalization itself. Moreover, in some units, this activity increase has an auditory component. A smaller number of units show other activity patterns, including a suppression of activity during vocalization and activity increases preceding the vocalization. Overall, our results suggest that the parabrachial region's involvement in vocal control is quite complex, involving convergence of respiratory, acoustic, vocalization-related, and perhaps somatosensory influences.

Bulbar reticular neurons relaying somatosensory information to the mesencephalic parabrachial area of the cat

Somatosensory neurons projecting to the mesencephalic parabrachial area (MPBA), which is located ventral to the inferior colliculus and dorsal to the brachium conjunctivum, were recorded from the bulbar reticular formation of adult cats anesthetized with alpha-chloralose. The majority (41 of 50 neurons) were nociceptive-specific neurons responding only to noxious mechanical and/or thermal stimuli to the skin, cornea and/or oral mucosa. The size of their receptive fields was smaller than that of the intrinsic MPBA-neurons, but larger than that of the trigeminal sensory nucleus neurons. Twenty-three neurons received input from the tooth pulp nerve and 10 of 32 neurons tested responded to electrical stimulation of the vagal nerve. These results indicate that these bulbar reticular neurons receive noxious inputs and transmit them to the MPBA, which also receives input from spinal or trigeminal sensory nucleus neurons projecting directly to the MPBA.

Organization of the pontine nuclei

The pontine nuclei provide the cerebellar hemispheres with the majority of their mossy fiber afferents, and receive their main input from the cerebral cortex. Even though the vast majority of pontine neurons send their axons to the cerebellar cortex, and are contacted monosynaptically by (glutamatergic) corticopontine fibers, the information-processing taking place is not well understood. In addition to typical projection neurons, the pontine nuclei contain putative GABA-ergic interneurons and complex synaptic arrangements. The corticopontine projection is characterized by a precise but highly divergent terminal pattern. Large and functionally diverse parts of the cerebral cortex contribute; in the monkey the most notable exception is the almost total lack of projections from large parts of the prefrontal and temporal cortices. Within corticopontine projections from visual and somatosensory areas there is a de-emphasis of central vision and distal parts of the extremities as compared with other connections of these sensory areas. Subcorticopontine projections provide only a few percent of the total input to the pontine nuclei. Certain cell groups, such as the reticular formation, project in a diffuse manner whereas other nuclei, such as the mammillary nucleus, project to restricted pontine regions only, partially converging with functionally related corticopontine connections. The pontocerebellar projection is characterized by a highly convergent pattern, even though there is also marked divergence. Neurons projecting to a single cerebellar folium appear to be confined to a lamella-shaped volume in the pontine nuclei. The organization of the pontine nuclei suggests that they ensure that information from various, functionally diverse, parts of the cerebral cortex and subcortical nuclei are brought together and integrated in the cerebellar cortex.

Alternative spinal, somatosensory pathways investigated with the tactile orienting reaction in the cat

Previous results indicated the possibility of abolishing the orienting reaction to light tactile stimulation of specific areas below lesions encompassing three sectors of the transverse spinal plane, if all sectors were transected simultaneously. Hence presumably interrupting three different ascending pathways. Two sectors corresponded to the sites of the well known, somatosensory, dorsal column and spino-cervical pathways. Single stage lesion technique now has been used to pinpoint the site of the third pathway. Immediate orienting reactions to both sides were seen before surgery. The orienting reactions remained postoperatively to stimuli applied on the hind limb contralateral to the dorsal column and the spino-cervical lesions. When the hind limb ipsilateral to the dorsal column and the spino-cervical lesions was stimulated five cats showed an absence of orienting reactions. The cats' lesions included the dorsal column and the spino-cervical on one side and the border area between the lateral and ventral funiculi on the other side of the cord. The remaining cats showed either partial or no deficiency of the orienting reactions. These cats' spinal lesions spared the area between the ventral and lateral funiculi. The findings show the possibility of abolishing the tactile orienting reactions from one hind limb with single stage lesions, which include the dorsal column and the spino-cervical pathway on one side, and a pathway located in the border area between the contralateral lateral, and ventral funiculi. This site corresponds to the morphological position of ascending spino-mesencephalic and/or spino-thalamic fibres. Consequently, all of these pathways might provide alternative routes for information about the place of tactile stimuli, which may evoke orienting reactions.

Reciprocal connections between the periaqueductal gray matter and other somatosensory regions of the cat midbrain: a possible mechanism of pain inhibition

Lectin-conjugated horseradish peroxidase was injected or implanted in crystalline form into various parts of the periaqueductal gray matter (PAG) in the cat. After varying survival periods, the animals were fixed and the mesencephalon was sectioned and incubated for HRP histochemistry. Outside PAG, labelled cells and terminal labelling were observed in the cuneiform, parabrachial and intercollicular nuclei, in the deep and intermediate gray layers of the superior colliculus, in the anterior and posterior pretectal nuclei and in the nucleus of Darkschewitsch. This study has shown that the region of PAG that is known to receive heavy ascending somatosensory input from the spinal cord and to be part of descending pain-inhibiting systems, also has reciprocal connections with other somatosensory areas of the midbrain. The results are discussed in relation to nociception and nociceptive inhibiting mechanisms.

Somatotopy of spinal nociceptive processing

Relatively little is known about the spatial organization of spinal nociceptive processing. This study has employed the expression of c-fos-like protein as a marker for neuronal activity and has analyzed the patterns of immunoreactivity seen within the rodent spinal cord following noxious mechanical stimulation of various portions of one hindlimb. The results indicate that noxious mechanical stimulation induces distinct, somatotopic patterns of immunolabeling in laminae I-IV. Individual digits of a foot are represented medially in the dorsal horn over a short rostrocaudal distance, with the most lateral digit represented approximately one segment caudal to the most medial digit. Representation of the hip region is more lateral, is centered at L2, and extends rostrocaudally over many segments. The patterns of neuronal excitation seen in laminae V-IX following noxious peripheral stimulation were similar to those noted in laminae I-IV but were less tightly organized. C-fos-like immunoreactivity was noted both medially and laterally in the deeper laminae following stimulation of any portion of the hindlimb, but stimulation of different areas produced different columns of labeled cells extending from the superficial dorsal horn into lamina VII. In the rostrocaudal direction, immunolabeling in lamina V-IX was maximal at the same segmental level as in laminae I-IV, but the more ventral laminae exhibited increases in c-fos-like immunoreactivity over longer rostrocaudal distances. Experiments in spinally transected animals indicated that long, descending pathways contributed little or nothing to the pattern of immunolabeling. The results of this study imply that spinal nociceptive processing is spatially organized not only in laminae I-IV, but also in more ventral regions of the spinal cord.

Thalamically projecting cells of the lateral cervical nucleus in monkey

The number, location, and morphology of thalamically projecting lateral cervical nucleus (LCN) cells were determined in monkey using retrograde transport of wheatgerm agglutinin-conjugated horseradish peroxidase. These data were compared to the total population of LCN neurons as determined by Nissl stain. In 4 Macaca fascicularis and one Saimiri sciureus the average size of the thalamic projection from LCN was found to be 506 +/- 94 cells contralateral to the injections. Thalamically projecting LCN neurons were located between the lower medulla and the third cervical segment; approximately 90% of these cells were in the first two cervical segments. Morphologic analysis of thalamically projecting LCN cells showed that they were smaller in size, and more oblong in shape in caudal regions of the nucleus. In 3 macaques, the average total number of LCN cells was determined to be 1617 +/- 908 on one side, in Nissl material. In these Nissl-stained preparations LCN neurons were found as far caudal as the fourth cervical segment; 68% were located in the first two cervical segments. Hence, thalamically projecting LCN neurons in the monkey are located in the rostral portion of the nucleus and comprise about one-third of the total population. Comparison of these data with reports in the literature imply that, unlike the cat, the major projection from LCN in monkeys is to the mesencephalon rather than to the thalamus.

Acetylcholinesterase and NADH-diaphorase chemoarchitectonic subdivisions in the rabbit medial geniculate body

The distribution of acetylcholinesterase and NADH-diaphorase activities was studied histochemically in the rabbit medial geniculate body, yielding new data useful for the definition of the common structural pattern of this thalamic complex in mammals. Four chemoarchitectonic subdivisions could be detected in transversal, horizontal and sagittal sections that corresponded to the previously described ventral, dorsal and internal nuclei, and to a fourth subdivision, defined as the mediorostral nucleus of the medial geniculate complex in the rabbit. The topography and cellular typology of the mediorostral nucleus suggest its homology with the so-called magnocellular nucleus of other mammals, an identity that was previously assigned to the internal nucleus. The relative position of the rabbit internal and dorsal nuclei and comparative connectional data are combined to suggest their correspondence with the anterodorsal and posterodorsal subnuclei, respectively, of the cat and the monkey. Global functional interpretations of these nuclei as sites of visuoacoustic and somatoacoustic polymodal integration support the notion of a shell region of the medial geniculate, surrounding the principal cochleotopic ventral nucleus and interconnected to the cortical acoustic belt around the primary auditory area. Acetylcholinesterase and NADH-diaphorase chemoarchitectony may be useful for the detection of similar partitions in species where cytoarchitectonic differentiation of the medial geniculate is less clear.

Functional properties of spinomesencephalic tract (SMT) cells in the upper cervical spinal cord of the cat

Response and receptive field properties were evaluated for 62 spinomesencephalic tract cells in the upper cervical spinal cord (C1-C3) of cats anesthetized with sodium pentobarbital and alpha-chloralose. Recordings were made from cells in laminae I-VIII and X contralateral to antidromic stimulating electrodes positioned in the rostral, caudal and intercollicular region of the midbrain. The mean antidromic threshold for all cells was 185 +/- 132 microA, and conduction velocities ranged from 2.3 to 38.6 m/sec. Twelve cells were backfired from both midbrain and diencephalic stimulation sites. Receptive fields ranged from simple, i.e., ipsilateral forelimb or face, to complex, i.e., excitatory and/or inhibitory responses from large portions of the body. Peripheral receptive fields included muscles, joints, cornea, dura, forelimbs, hind limbs, tail, and/or testicles. Five functional classes of cells were observed: (a) wide dynamic range (14 cells); (b) high threshold (2 cells): (c) low threshold (4 cells); (d) deep/tap (11 cells); and (e) non-responsive (31 cells). Eight cells were evaluated for responses to different doses (5-150 micrograms) of intravenous (i.v.) serotonin. Two of the 8 cells exhibited excitatory effects, whereas 2 cells classified as deep/tap and 4 cells classified as non-responsive were not affected. The results of this study have shown the upper cervical component of the spinomesencephalic tract is made up of a heterogenous population of cells involved in the integration of varied inputs from large portions of the body. It is proposed that the large population of SMT cells in the upper cervical spinal cord may be involved in pain mechanisms, especially those related to the affective consequences of acute and chronic pain.

Spatial relationships of axons arising from the substantia nigra, spinal trigeminal nucleus, and pedunculopontine tegmental nucleus within the intermediate gray of the cat superior colliculus

We have utilized two different anterograde transport methods (Phaseolus vulgaris leucoagglutinin [PHA-L] immunocytochemistry and autoradiography) in the same experiment to compare the sublaminar location and arrangement of tectopetal axons arising from the substantia nigra pars reticulata, the spinal trigeminal nucleus, and the pedunculopontine tegmental nucleus. Our findings reveal that the nigrotectal projection terminates in a patchy fashion within three horizontally oriented sublaminae of the stratum griseum superficiale (SGI), the dorsal, middle and ventral. The middle tier of nigrotectal axons exhibits an exquisite, puzzle-like, complementary spatial relationship with trigeminotectal axons. In contrast, axons arising from the pedunculopontine tegmental nucleus overlap with patches of nigrotectal axons within the middle tier. Thus the middle tier of the SGI consists of domains of overlapping nigral and pedunculopontine tegmental inputs which interdigitate with domains rich in somatosensory inputs.

Spinal distribution and collateral projections of rat spinomesencephalic tract cells

The distribution of cells belonging to the rat spinomesencephalic tract was studied by means of the retrograde transport of fluorescent dyes. Bilateral midbrain injections of cytoplasmic and nuclear tracers were made in order to evaluate the location of ipsilateral, contralateral, or bilaterally projecting cells. Spinal neurons with ascending projections to midbrain and descending propriospinal projections were identified by midbrain and spinal injections of different cytoplasmic labels. The locations of spinomesencephalic tract cells included seven regions of the spinal gray matter: marginal zone, lateral neck of the dorsal horn, nucleus proprius, the region around the central canal, the lateral cervical and spinal nuclei and the ventral horn. Cells projecting to the ipsilateral or contralateral midbrain had similar distributions and were frequently found in clusters with overlapping dendritic fields. Approximately 75% of spinomesencephalic cells projected to the contralateral midbrain. The largest contribution to the spinomesencephalic tract cell population was found in cervical cord segments 1-4. Cells with bilateral projections accounted for nearly 2% of all labeled cells, whereas 5% had both ascending and descending projections. Spinomesencephalic cells were found to have varying dendritic fields and morphology, e.g. fusiform, pyramidal, round/oval, and multipolar. The results of the present study lend further support to the view that the spinomesencephalic tract is a multi-component pathway with varied origins and projection targets.

Ultrastructure of substance P immunoreactive elements in the periaqueductal gray matter of the rat

Substance P (SP) is a non-opioid peptide that generates a potent analgesia when injected into the periaqueductal gray matter (PAG). The aim of this study was to investigate the fine neuronal structures and synaptic circuits involved in SP action in rats by means of electron microscopy, using immunocytochemical (ICC) pre-embedding methods. A conventional ultrastructural study, carried out to interpret the ICC data correctly, shows small sized nerve cell bodies with a high nucleus-cytoplasmic ratio; absence of an extensive granular endoplasmic reticulum; and few axo-somatic contacts having symmetrical and asymmetrical junctions in equal proportions. The large neuropil is characterized by numerous thin unmyelinated axons and axo-dendritic synapses mainly showing pleomorphic vesicles and asymmetrical junctions. The ICC analysis showed moderately labeled nerve cell bodies with the same structural, synaptic, and dimensional features as the negative cells. In the neuropil SP immunoreactivity is shown by dendrites, synapses, and thin elements which are unidentifiable structurally. No SP terminals synapsing on SP nerve cell bodies were found and only occasional SP light labeled terminals synapsing on negative perikarya were seen. The SP boutons generally have pleomorphic vesicles and asymmetrical junctions. On the basis of these data a possible excitatory activity of PAG SP synapses could be hypothesized. This activity would take place on postsynaptic neurons generally at a dendritic level. Our ultrastructural findings give support to an excitatory role carried out by SP neurons of the PAG, as suggested by the role of PAG circuitry on spinal nociception.

Ascending afferents to the lateral cervical nucleus are enriched in glutamate-like immunoreactivity: a combined anterograde transport-immunogold study in the cat

To investigate whether glutamate (Glu) may be a transmitter in terminals of ascending afferents to the lateral cervical nucleus (LCN), these terminals were identified by anterograde transport of wheatgerm agglutinin-horseradish peroxidase from the spinal cord, and their content of Glu-like immunoreactivity (Glu-LI) was assessed at the ultrastructural level by the immunogold technique. The gold particle density over the peroxidase-positive terminals of the spinocervical tract (SCT) was significantly higher (by a factor of 2.44) than over a reference terminal population containing flattened or pleomorphic vesicles. Further, LCN neurons were densely labeled by the Glu antiserum, although the gold particle density over neuronal cell bodies was not as high as in the SCT terminals. Previous investigations have shown enrichment of Glu-LI in putative glutamatergic terminals in other parts of the CNS. Hence, the present observations indicate that Glu may be a transmitter in the synapses between SCT terminals and LCN neurons. The cell body labeling in the LCN is more difficult to interpret because of possible interference of metabolic pools of glutamate.

Collaterals of primate spinothalamic tract neurons to the periaqueductal gray

Collateral projections are an important feature of the organization of ascending projections from the spinal cord to the brain. Primate spinothalamic tract (STT) neurons with collaterals to the periaqueductal gray (PAG) were studied by means of a fluorescent double-labeling method. Granular Blue and rhodamine-labeled latex microspheres were placed in the ventral posterior lateral (VPL) nucleus of the thalamus and the periaqueductal gray, respectively. Single and double labeled neurons were studied in the upper cervical cord, cervical enlargement, thoracic cord, lumbar enlargement, and sacral segments. The laminar distribution of double labeled neurons was similar to that of spinomesencephalic tract (SMT) neurons. Most double labeled (STT-SMT) neurons were located in contralateral laminae I, V, VII, and X. Relatively more lamina I STT-SMT neurons were found in the cervical enlargement and more lamina V STT-SMT neurons in the lumbar enlargement. The density of STT-SMT neurons in the upper cervical segments and cervical enlargement was almost equal. The density of STT-SMT neurons in the lumbar enlargement was 40% of that in the cervical enlargement. The thoracic and sacral segments had the lowest density of STT-SMT neurons, about 10% of that in the cervical enlargement. STT-SMT neurons constituted 14.7% of SMT neurons and 6% of STT neurons in the cervical enlargement and 15.3% of SMT neurons and 2.9% of STT neurons in the lumbar enlargement. The branch points of eight STT-SMT axons were studied electrophysiologically. The average percentage of conduction time spent in the parent axon was more than 85% for an antidromic action potential from the VPL nucleus and 91% from the PAG. Branch points of STT-SMT axons were calculated to be 9-13 mm caudal to the PAG, in the pons or rostral medulla.

Pulpal and cutaneous inputs to somatosensory neurons in the parabrachial area of the cat

The majority of somatosensory neurons recorded from the mesencephalic parabrachial area and pontine parabrachial nucleus of the cat responded exclusively to noxious mechanical stimuli to the skin. Their receptive fields were very large. Two-thirds of the neurons tested responded to electrical stimulation of the tooth pulp. These results suggest that neurons in this area have extensive convergence of spinal and trigeminal inputs, and contribute to the affective or autonomic aspects of pain.

A comparison of the distribution of the cerebellar and cortical connections of the nucleus of Darkschewitsch (ND) in the cat: a study using anterograde and retrograde HRP tracing techniques

Bidirectional transport of lectin conjugated horseradish peroxidase was employed to investigate the relative distribution of the cerebellar and cortical connections of the nucleus of Darkschewitsch in the cat. Injection of horseradish peroxidase into the deep cerebellar nuclei produced terminal labeling which extended throughout the length of the contralateral nucleus of Darkschewitsch and into the perifascicular region. Injection of horseradish peroxidase into the pericruciate cortex produced both ipsilateral terminal labeling which extended throughout the length of the nucleus of Darkschewitsch and into the perifascicular region, and ipsilateral retrograde neuronal labeling. Labeled neurons displayed a variety of shapes and sizes, were more numerous in sections cut at rostral levels of the nucleus of Darkschewitsch, and were located both within and outside fields of terminal labeling. Comparison of the distribution of labeling following cerebellar and cortical injections indicates that convergence and overlap of input from these two sources occur in the nucleus of Darkschewitsch. These findings provide the morphological basis for integration of cerebellar and cortical information in this nucleus which may, in turn, influence output from neurons which project to the cortex or to the inferior olivary nucleus.

Afferents and efferents of the rat cuneiformis nucleus: an anatomical study with reference to pain transmission

Small iontophoretic application of wheat-germ-agglutinin conjugated to horseradish peroxidase, was used to simultaneously observe, in the rat, the afferent and the efferent connections of the cuneiformis nucleus (Cnf), in the same animal. The results demonstrate that the main Cnf afferent connections originate from: (1) 6 regions of the forebrain: the nucleus centralis of the amygdala, the zona incerta, the dorsomedial, ventromedial and the lateral area of the hypothalamus and the periventricular gray matter and (2) 4 regions of the midbrain: the substantia nigra pars lateralis, the peripeduncular area, the periaqueductal gray matter and the other side Cnf. In contrast to the wealth of origins of afferent fibers, most projections of the Cnf are concentrated to the B3 area and the bordering reticular nuclei. It is proposed that the Cnf, which mostly receives afferent connections from the forebrain and the midbrain, and which in turn densely projects on the B3 area, is a relay for the modulation of pain processes.

GABA-immunoreactive neurons and terminals in the lateral cervical nucleus of the cynomolgus monkey

An antiserum against the inhibitory transmitter substance gamma-aminobutyric acid (GABA) was used to investigate the distribution of GABAergic nerve terminals and cell bodies in the lateral cervical nucleus (LCN) of the cynomolgus monkey. Light microscopic immunohistochemistry demonstrated GABA-immunoreactive puncta, suggestive of nerve terminals, scattered throughout the LCN. The terminal-like profiles are often present along the somata of unlabeled neurons, but most are located in the neuropil. GABA-immunoreactive neurons are present in the LCN, but constitute a very small number of the LCN neurons. Electron microscopy showed that the GABA-positive neurons are small with a relatively large nucleus. They are contacted by few somatic boutons. Numerous GABA-immunoreactive terminals containing densely packed round to oval synaptic vesicles were also found. Most GABA-positive terminals make synaptic contact with dendrites, but synapses with cell bodies are also present. Synaptic contacts between labeled and unlabeled terminals were not observed. Some GABA-positive terminals make contact with GABA-positive neurons. The present findings suggest that GABA is a major inhibitory transmitter substance in the LCN of the monkey. However, in comparison with other somatosensory relay nuclei, there are few GABA-immunoreactive neurons in the LCN. This may imply that the GABA-positive neurons branch extensively in the LCN or that an extrinsic source of GABAergic input exists.

The cerebral and cerebellar connections of pretecto-thalamic and pretecto-olivary neurons in the anterior pretectal nucleus of the cat

The neuronal connections of the anterior pretectal nucleus (PTA) were investigated in the cat. For the light microscopy, the retrograde double-labeling technique by means of Fluoro-Gold (FG) and horseradish peroxidase conjugated to wheat germ agglutinin (WGA-HRP) was used. Following injections of one tracer into the central lateral nucleus of the thalamus (CL) and the other into the dorsal accessory olivary nucleus (DAO), distributions of labeled neurons in the PTA were observed. Most of the labeled neurons were single-labeled either with FG or with WGA-HRP. The result indicated that pretecto-thalamic projection neurons were distributed throughout the whole extent of the PTA, whereas pretecto-olivary projection neurons were located in a restricted area of the ventral part of the PTA. Only a very small number of double-labeled neurons were found in the PTA. These two efferent projections thus seemed to be derived from different populations of PTA neurons. For the electron microscopy, a combination of retrograde transport of horseradish peroxidase (HRP) and anterograde degeneration technique was used. After HRP injections into the CL combined with lesions either in the motor cortex (MCx) or in the anterior interpositus nucleus of the cerebellum (Cbl), some degenerating axon terminals originating from the cerebrum or cerebellum were found to synapse with retrogradely labeled pretecto-thalamic projection neurons. We have already observed direct cerebral and cerebellar projections to the pretecto-olivary projection neurons (J. Comp. Neurol., 259 (1987) 348-363). We conclude that the two different populations of PTA neurons comprise two different kinds of neuronal circuitries, i.e. MCx-PTA-CL-MCx and Cbl-PTA-DAO-Cbl, and that these two circuitries might interrelate with each other in the PTA at the cellular level.

Spinal input to the parabrachial nucleus in the cat

The projections from the spinal cord to the parabrachial nucleus in the cat were investigated using both the degeneration method and the anterograde transport of wheat germ agglutinin-horseradish peroxidase conjugate. Both methods produced similar results. Spinal input to the parabrachial nucleus was bilateral, with a slight contralateral predominance. The termination area was localized predominantly in the dorsal part of the lateral parabrachial nucleus, with additional limited terminations in the Kölliker-Fuse subnucleus. Projections from different rostrocaudal levels of the spinal cord overlapped completely, suggesting that spinal input to the parabrachial nucleus is not topographically organized. Taking these results together with those of others indicating that spinal input to the parabrachial nucleus arises primarily from nociceptive-specific neurons in lamina I of the dorsal horn, it is concluded that the spinal projections to the parabrachial nucleus are likely to be involved in various generalized aspects of nociception.

Morphological types of spinomesencephalic neurons in the marginal zone (lamina I) of the rat spinal cord, as shown after retrograde labelling with cholera toxin subunit B

Retrogradely labelled lamina I neurons were studied after intramesencephalic injections of subunit B of cholera toxin. The tracer was visualized with a mixture of two monoclonal antibodies followed by the peroxidase-antiperoxidase technique that produced Golgi-like staining of the labelled cells. A morphological and morphometric analysis in the three anatomical viewing planes disclosed two structural neuronal types which were recognized as the fusiform and pyramidal cells of our Golgi-based classification of rat marginal cells. Fusiform cells had a bipolar longitudinally elongated dendritic arbor, were located in the lateral third of lamina I, and appeared to project mainly to the contralateral parabrachial nuclei. It is asserted that these cells may convey the spinal input which elicits visceral responses generated in that area. Pyramidal cells were longitudinally oriented pyramids with the triangular base straddling the dorsal horn/white matter border and a dendritic arbor extending lateromedially and mainly rostrocaudally throughout superficial lamina I and the dorsal funiculus. These cells occurred along the entire mediolateral extent of lamina I and seemed to have a prevalent projection to the contralateral caudal ventrolateral periaqueductal gray. They may represent the ascending branch of the spinal-midbrain loop centered in that zone which controls nociceptive transmission postsynaptically in the dorsal horn.