Effects of atipamezole on selected physiologic parameters in cynomolgus macaques (Macaca fascicularis)

BACKGROUND

Atipamezole, an α-2 adrenergic receptor antagonist, reverses the α-2 agonist anesthetic effects. There is a dearth of information on the physiological effects of these drugs in cynomolgus macaques (Macaca fascicularis). We assessed atipamezole's physiologic effects. We hypothesized atipamezole administration would alter anesthetic parameters.

METHODS

Five cynomolgus macaques were sedated with ketamine/dexmedetomidine intramuscularly, followed 45 min later with atipamezole (0.5 mg/kg). Anesthetic parameters (heart rate, blood pressure [systolic (SAP), diastolic (DAP), and mean (MAP) blood pressure], body temperature, respiratory rate, and %SpO2) were monitored prior to and every 10 min (through 60 min) post atipamezole injection.

RESULTS

While heart rate was significantly increased for 60 min; SAP, DAP, MAP, and temperature were significantly decreased at 10 min.

CONCLUSIONS

This study indicates subcutaneous atipamezole results in increased heart rate and transient blood pressure decrease. These findings are clinically important to ensure anesthetist awareness to properly support and treat patients as needed.

Geometry of rubrospinal, rubroolivary, and local circuit neurons in the macaque red nucleus

The primate red nucleus consists of three main neuron subpopulations, namely, rubrospinal neurons in the magnocellular nucleus, rubroolivary cells in the parvocellular nucleus, and local circuit neurons in both subnuclei: Each subpopulation has unique cerebellar and neocortical inputs. The structural framework for the interactions of these rubral subpopulations remains poorly defined and was the focus of this study in six macaques. Somata of rubrospinal neurons, dorsolateral-spinal (DL-spinal) neurons, as defined in the accompanying paper (Burman et al. [2000] J. Comp. Neurol., this issue), and rubroolivary neurons were labeled retrogradely first with Fast Blue injected either into the cervical spinal cord or the inferior olive. The soma/dendrite profiles of selected cells (53 rubrospinal, 19 DL-spinal, and 17 rubroolivary cells) were visualized by the intracellular injection of Lucifer Yellow/biocytin in fixed slices (400 microm thick) of midbrain. The descriptive statistics of the somata and the dendritic arborization of each rubral neuron type were established. Projection neuron subpopulations had similar but differentiable soma/dendrite profiles, with four to six slender, spine-bearing dendritic trees radiating out approximately 400 microm from the soma. Twelve presumed interneurons, all in the parvocellular nucleus, differed from projection neurons in that they had smaller somata and many slender, spine-bearing segments that constituted the multibranching dendrite profile that radiated out approximately 250 microm from the soma. A tentative model of the macaque rubral microcircuitry was developed, and its functional implications were explored. It incorporated 1) the known topography of the nucleus and its connections, 2) our data specifying the soma/dendrite morphology of the three main rubral neuron types, and 3) the ultrastructure reported by other laboratories of intrarubral synaptic connections.

Macaque red nucleus: origins of spinal and olivary projections and terminations of cortical inputs

The cerebellar, spinal, bulbar, and cortical connections of the mammalian red nucleus imply a motor role. However, what information the red nucleus receives, processes, and distributes is poorly understood, partly because the rubral microcircuitry, especially in primates, remains incompletely defined. Multiple retrogradely transported fluorescent tracers were injected into the spinal cord and inferior olive of the macaque to label rubrospinal and rubroolivary neuron populations, respectively. Anterograde dextran amines were used to label the terminals of corticorubral neurons. These data provided the topographic framework for examining the morphology of rubral neurons in the accompanying paper (Burman et al. [2000]). Soma profiles of rubrospinal and rubro-olivary neurons were respectively segregated in the magnocellular and parvocellular nuclei. A subpopulation of neurons (DL-spinal cells) with their somas immediately dorsolateral to the rostral magnocellular nucleus and its capsule, also projected to the spinal cord, as did clusters of neurons in the periaqueductal grey matter. Terminals of corticorubral axons originating from ipsilateral primary motor area 4 (the densest projection), the supplementary motor area, cingulate area 24, area 8, and posterior parietal area 5, were each mapped in the parvocellular red nucleus. Only area 4 projected to the magnocellular red nucleus, and this projection as small. DL-spinal neurons had no cortical input. The somatotopic organization of rubral connections was examined only in (a) the corticorubral input from motor area 4, and (b) the rubrospinal and DL-spinal projections. These connections and their somatotopic alignment, were mapped in a 3-dimensional reconstruction of the red nucleus.

Functional changes at periphery and cortex following dorsal root lesions in adult monkeys

Chronic peripheral nerve injuries produce neural changes at different levels of the somatosensory pathway, but these responses remain poorly defined. We selectively removed cutaneous input from the index finger and thumb in young adult macaque monkeys by lesioning dorsal rootlets to examine both immediate and long-term systemic responses to this deficit. Corresponding digit representations within somatosensory cortex (SI) were initially silenced, but two to seven months later again responded to cutaneous stimulation of the 'deafferented' digits. We remapped cutaneous receptive fields (RFs) within adjacent intact dorsal rootlets two to four months after lesioning. RF distributions had greatly expanded, so that rootlets previously innervating adjacent hand regions now responded to stimulation of the index finger and/or thumb. Thus our results demonstrate peripherally mediated central reorganization.

Parallel pathways mediating manual dexterity in the macaque

Transmission of information along appropriately structured parallel pathways ensures that a great deal of information can be transferred from the source to the target very quickly, and with great security-essential features of any motor control system. Studies over the last two decades have established that the corticospinal and corticocerebellar pathways mediating manual dexterity in the primate are structurally organized to sustain the parallel transmission of sensorimotor information in multiple pathways. Serial, hierarchical control systems now seem insufficient to regulate voluntary hand movements. To achieve the required coordination, and precision and speed of execution, they must be combined with parallel control systems, which themselves incorporate elaborate feedforward and feedback controls. To illustrate these issues, two aspects of the structural organization of parallel sensorimotor pathways mediating manual dexterity in the macaque are reviewed. First, we examine the structure of the multiple corticospinal neuron subpopulations projecting from different areas of the frontoparietal cortex and how they are modified following hemisection of the cervical spinal cord. The remarkable recovery of hand function following spinal hemisection, despite the absence of any structural 'bridging' of the interrupted spinal pathways, and the fact that this is accountable in a parallel but not in a purely serial transmission system, are then reviewed. The second aspect of parallel distributed transmission examined is its occurrence within a single population of relay neurons. Our recent structural analysis of the somatic/dendritic organization of rubrospinal neurons in macaque red nucleus is used. The very large dendritic fields of individual neurons, extending over one-third or more of the nucleus, provide a framework for extracting precise somatotopic information from an input population whose axon terminal arbors overlap extensively, and, which, without effective filtering, would provide poor spatial resolution.

Comparing thalamocortical and corticothalamic microstructure and spatial reciprocity in the macaque ventral posterolateral nucleus (VPLc) and medial pulvinar

The detailed morphology of thalamocortical (TC) and corticothalamic (CT) pathways connecting the ventral posterolateral nucleus (VPLc) with the primary somatosensory cortex (areas 3b and 1) and the thalamic pulvinar with the posterior parietal cortex (primarily area 7a), was compared. Each pathway processes information relevant to directed reaching tasks, but whereas VPLc receives its major input from the spinal cord and external environment, the primary afferent to the pulvinar is cortical. Using combined tracer and thick fixed slice procedures, the soma/dendritic morphology of TC neuron populations (with known destination) was shown to be quantitatively similar within VPLc and the pulvinar. This implies that differences in information processing in VPLc (a primary relay) and the pulvinar (an integrative thalamic nucleus) are not defined by a distinctive TC morphology, but rather by the connections of these neuron populations. Two morphologically distinct types of CT axon were observed within the medial pulvinar and VPLc. The more common "Type E" were fine, had boutons en passant and diffuse terminal bifurcations ending in masses of tiny boutons. "Type R" axons were thicker, smooth, and terminated in localised clusters of large terminal boutons. Each type had a unique pattern of termination reflecting a distinct action on target neuron populations. The spatial relationship between TC distribution territories and CT terminal fields was examined within the medial pulvinar and VPLc by using anterograde and retrograde tracers injected together within cortical areas 7a, and 3b/1, respectively. Spatial overlap was incomplete within both thalamic nuclei. Our findings show a more complex relationship between TC and CT neuron populations than previously demonstrated.

Manual dexterity: how does the cerebral cortex contribute?

1. Manual dexterity, of great evolutionary significance to the primates, ranges in complexity from the precise opposition of finger and thumb to Brendal playing Mozart. All dexterity depends on a sustained and rapid transfer of sensorimotor information between the cerebral cortex and the cervical spinal cord. 2. Multiple separate corticospinal neuron populations originate from cortical areas four, the supplementary motor area, anterior cingulate, postarcuate, parietal and insular cortex. Each corticospinal neuron population projects in parallel to all spinal segments, and has a distinctive pattern of terminations. 3. Each corticospinal neuron population has a unique thalamic input which can relay particular sensorimotor information from the sense organs, cerebellum and basal ganglia. The overall structural framework of these sensorimotor pathways, with many parallel corticospinal channels, with interconnections in the cerebral cortex and spinal cord to enable crosstalk between the channels, is that needed for parallel distributed processing, which would enable the very rapid transfer of information between the cerebral cortex and spinal cord needed for any sophisticated use of the hand. 4. Hemisection of the cervical spinal cord in the macaque results in an immediate hemiplegia, with subsequent remarkable although incomplete recovery of hand and finger movements. The only direct corticospinal input to the hemicord caudal to the hemisection, even after 3 years, is the approximately 10% of fibres which cross the midline caudal to the lesion: the fibres 'spared' by the hemisection. A matching 'sparing' of somatosensory input from the paresed limb also occurs. No regeneration of the interrupted pathways has been visualized using modern tracer techniques. 5. Cervical hemisection permanently reduces the number of parallel channels which transmit information between cortex and spinal cord, but does not reduce their cortical origins nor the neuron populations targeted in the spinal cord. We infer that the content of the information that can be transmitted between the cortex and spinal cord is not greatly changed, but the rate of transmission of this information is sharply reduced, and is the 'bottleneck' that limits the complete recovery of dexterity following hemisection. The remarkable recovery that does occur presumably reflects more economic transmission of information by the few spared channels. We guess that this involves substantial synaptic reorganization not visualized by the procedures we have used.

Topographic reorganization in the striate cortex of the adult cat and monkey is cortically mediated

In primary sensory and motor cortex of adult animals, alteration of input from the periphery leads to changes in cortical topography. These changes can be attributed to processes that are intrinsic to the cortex, or can be inherited from alterations occurring at stages of sensory processing that are antecedent to the primary sensory cortical areas. In the visual system, focal binocular retinal lesions initially silence an area of cortex that represents the region of retina destroyed, but over a period of months this area recovers visually driven activity. The retinotopic map in the recovered area is altered, shifting its representation to the portion of retina immediately surrounding the lesion. This effectively shrinks the representation of the lesioned area of retina, and expands the representation of the lesion surround. To determine the loci along the visual pathway at which the reorganization takes place, we compared the course of topographic alterations in the primary visual cortex and dorsal lateral geniculate nucleus (LGN) of cats and monkeys. At a time when the cortical reorganization is complete, the silent area of LGN persists, indicating that changes in cortical topography are due to alterations that are intrinsic to the cortex. To explore the participation of thalamocortical afferents in the reorganization, we injected a series of retrogradely transported fluorescent tracers into reorganized and surrounding cortex of each animal. Our results show that the thalamocortical arbors do not extend beyond their normal lateral territory and that this physical dimension is insufficient to account for the reorganization. We suggest that the long-range intrinsic horizontal connections are a likely source of visual input into the reorganized cortical area.

Axonal sprouting accompanies functional reorganization in adult cat striate cortex

Removal of sensory input from a focal region of adult neocortex can lead to a large reorganization of cortical topography within the deprived area during subsequent months. Although this form of functional recovery is now well documented across several sensory systems, the underlying cellular mechanisms remain elusive. Weeks after binocular retinal lesions silence a corresponding portion of striate cortex in the adult cat, this cortex again becomes responsive, this time to retinal loci immediately outside the scotoma. Earlier findings showed a lack of reorganization in the lateral geniculate nucleus and an inadequate spread of geniculocortical afferents to account for the cortical reorganization, suggesting the involvement of intrinsic cortical connections. We investigated the possibility that intracortical axonal sprouting mediates long-term reorganization of cortical functional architecture. The anterograde label biocytin was used to compare the density of lateral projections into reorganized and non-deprived cortex. We report here that structural changes in the form of axonal sprouting of long-range laterally projecting neurons accompany topographic remodelling of the visual cortex.

Thalamic projections to areas 3a, 3b, and 4 in the sensorimotor cortex of the mature and infant macaque monkey

Area 3a in the macaque monkey, located in the fundus of the central sulcus, separates motor and somatosensory cortical areas 4 and 3b. The known connections of areas 4 and 3b differ substantially, as does the information which they receive, process, and transfer to other parts of the central nervous system. In this analysis the thalamic projections to each of these three cortical fields were examined and compared by using retrogradely transported fluorescent dyes (Fast Blue, Diamidino Yellow, Rhodamine and Green latex microspheres) as neuron labels. Coincident labeling of projections to 2-3 cortical sites in each monkey allowed the direct comparison of the soma distributions within the thalamic space of the different neuron populations projecting to areas 3a, 3b, and 4, as well as to boundary zones between these cortical fields. The soma distribution of thalamic neurons projecting to a small circumscribed zone (diameter = 0.5-1.0 mm) strictly within cortical area 3a (in region of hand representation) filled out a "territory" traversing the dorsal half of the cytoarchitectonically defined thalamic nucleus, VPLc (abbreviations as in Olszewski [1952] The Thalamus of the Macaca mulatta. Basel: Karger). This elongate, rather cylindrical, territory extended caudally into the anterior pulvinar nucleus, but not forward into VPLo. The rostrocaudal extent of the thalamic territory defining the soma distribution of neurons projecting to small zones of cortical area 3b was similar, but typically extended into the ventral part of VPLc, filling out a medially concavo-convex laminar space. Two such territories projecting to adjacent zones of areas 3a and 3b, respectively, overlapped and shared thalamic space, but not thalamic neurons. Contrasting with the 3a and 3b thalamic territories, the soma distribution of thalamic neurons projecting to a circumscribed zone in the nearby motor cortex (area 4) did not penetrate into VPLc, but instead filled out a mediolaterally flattened territory extending from rostral VLo, VLm, VPLo to caudal and dorsal VLc, LP, and Pul.o. These territories skirted around VPLc. All three cortical areas 4, 3a, and 3b) also received input from distinctive clusters of cells in the intralaminar Cn.Md. It is inferred that, in combination, the thalamic territories enveloping those neuron somas projecting to, say, the sensorimotor hand representation in areas 3a, 3b, and 4 (and also areas 1 and 2), which would be coactive during the execution of a manual task, constituted a lamellar space extending from VLo rostrally to Pul.o caudally.(ABSTRACT TRUNCATED AT 400 WORDS)

Ipsilateral cortical projections to areas 3a, 3b, and 4 in the macaque monkey

In the macaque monkey area 3a of the cerebral cortex separates area 4, a primary motor cortical field, from somatosensory area 3b, which has a subcortical input mainly from cutaneous mechanoreceptive neurons. That each of these cortical areas has a unique thalamic input was illustrated in the preceding paper. In the present experiments the cortical afferent projections to these 3 areas of the sensorimotor cortex monkey were visualized and compared, using 4 differentiable fluorescent dyes as axonal retrogradely transported labels. The cortical projection patterns to areas 3a, 3b, and 4 were similar in that they each consisted of (a) a "halo" of input from the immediately surrounding cortex, and (b) discrete projections from one or more remote cortical areas. However, the pattern of remote inputs from precentral, mesial, and posterior parietal cortex was different for each of the 3 cortical target areas. The cortical input configuration was least complex for area 3b, its remote input projecting mainly from insular cortex. The pattern of discrete cortical inputs to the motor area 4, however, was more complex, with projections from the cingulate motor area (24c/d), the supplementary motor area, postarcuate cortex, insular cortex, and postcentral areas 2/5. Area 3a, in addition to the proximal projections from the immediately surrounding cortex, also received input from the supplementary motor area, cingulate motor cortex, insular cortex, and areas 2/5. Thus, this pattern of cortical input to area 3a resembled more closely that of the adjacent motor rather than that of the somatosensory area 3b. Contrasting with this, however, the thalamic input to area 3a was largely from somatosensory VPLc (abbreviations from Olszewski [1952] The Thalamus of the Macaca mulatta. Basel: Karger) and not from VPLo (with input from cerebellum, and projecting to precentral motor areas).

Thalamic projections to sensorimotor cortex in the macaque monkey: use of multiple retrograde fluorescent tracers

We used several fluorescent dyes (Fast Blue, Diamidino Yellow, Rhodamine Latex Microspheres, Evans Blue, and Fluoro-Gold) in each of eight macaques, to examine the patterns of thalamic input to the sensorimotor cortex of macaques 12 months or older. Inputs to different zones of motor, premotor, and postarcuate cortex, supplementary motor area, and areas 3b/1 and 2/5 in the postcentral cortex, were examined. Coincident labeling of thalamocortical neuron populations with different dyes (1) increased the precision with which their soma distributions could be related within thalamic space, and (2) enabled the detection by double labeling, of individual thalamic neurons that were common to the thalamic soma distributions projecting to separate, dye-injected cortical zones. Double-labeled thalamic neurons projecting to sensorimotor cortex were rarely seen in mature macaques, even when the injection sites were only 1-1.5 mm apart, implying that their terminal arborizations were quite restricted horizontally. By contrast, separate neuron populations in each thalamic nucleus with input to sensorimotor cortex projected to more than one cytoarchitecturally distinct cortical area. In ventral posterior lateral (oral) (VPLo), for example, separate populations of cells sent axons to precentral medial, and lateral area 4, medial premotor, and postarcuate cortex, as well as to supplementary motor area. Extensive convergence of thalamic input even to the smallest zones of dye uptake in the cortex (approximately 0.5 mm3) characterized the sensorimotor cortex. The complex forms of these projection territories were explored using 3-dimensional reconstructions from coronal maps. These projection territories, while highly ordered, were not contained by the cytoarchitectonic boundaries of individual thalamic nuclei. Their organization suggests that the integration of the diverse information from spinal cord, cerebellum, and basal ganglia that is needed in the execution of complex sensorimotor tasks begins in the thalamus.

Thalamic projections to sensorimotor cortex in the newborn macaque

In the present experiments thalamocortical projections to different functional areas of the newborn (or prematurely delivered) macaque's sensorimotor cortex were labeled using retrogradely transported fluorescent dyes. Several dyes were used in each animal to (1) enable the direct comparison of the soma distributions of different thalamocortical projections within thalamic space, and (2) identify by double labeling neurons shared between these distributions. The projection patterns in the newborn macaque were compared with those of the mature animal reported by Darian-Smith et al. (J. Comp. Neurol. 1990;298:000-000). The main observations were (1) all thalamocortical projections to the sensorimotor cortex of the mature macaque are well established by embryonic days 146-150, as was shown by labeling these pathways in infants delivered by cesarean section, (2) a significant number of thalamocortical neurons in the newborn were double-labeled following dye injections into different pre- or postcentral areas, and where the margins of the dye uptake zones were separated by 3-8 mm, and (3) extensive projections from the anterior pulvinar nucleus to the motor and premotor cortex, and to the supplementary motor cortex were labeled in the newborn macaque. Both the exuberant terminal arborizations, and the precentral pulvinar projections were diminished by the 6th postnatal month, and absent in the mature macaque. The role of epigenetic determinants of these postnatal events is briefly considered.

An evolutionary link for developing mammalian lungs

Lungs of the human infant and those of other mammals are filled with fluid immediately prior to birth. Studies of the ionic composition of this fluid indicate that active ionic transport processes occur in the epithelial cells of the potential airspaces. The purpose of this study was to see if these active ion pumps were present in developing species other than mammals thus providing a possible evolutionary link to mammals. A series of samples of lung liquid, amniotic fluid, and plasma were taken from embryonic marine turtles gathered from clutches incubating in the beach at Mon Repos, Queensland, Australia during the summer of 1986-87. The concentrations of sodium, potassium and chloride ions and protein measured in these liquids indicated that active pumping processes similar to that seen in the mammalian lung were present in the developing lungs of these marine reptiles and further, circumstantial evidence was gathered to suggest that this liquid was partially reabsorbed prior to hatching. The results support the notion that processes responsible for the normal development of the human lung and lungs of other mammals are also present in the hollow lungs of marine turtles. Thus there is an evolutionary counterpart controlling lung development in more ancient species. It may be possible to generalize this observation to the development of hollow lungs of other species.