BOLD and blood volume-weighted fMRI of rat lumbar spinal cord during non-noxious and noxious electrical hindpaw stimulation

Spinal cord fMRI is a useful tool for studying spinal mechanisms of pain, hence for analgesic drug development. Its technical feasibility in both humans and rats has been demonstrated. This study investigates the reproducibility, robustness, and spatial accuracy of fMRI of lumbar spinal cord activation due to transcutaneous noxious and non-noxious electrical stimulation of the hindpaw in alpha-chloralose-anesthetized rats. Blood oxygenation level-dependent (BOLD) and blood volume-weighted fMRI data were acquired without and with intravenous injection of ultra small superparamagnetic iron oxide particles (USPIO), respectively, using a gradient echo (GE) echo planar imaging (EPI) technique at 4.7 T. Neuronal activation in the spinal cord induced by noxious stimulation to the hindpaw (2 ms wide, 5 mA amplitude, known to activate C-fibers) can be robustly detected by both fMRI techniques with excellent reproducibility and peaked at the stimulus frequency of 40 Hz. However, both fMRI techniques were not sensitive to neuronal activation in spinal cord induced by non-noxious stimulation (0.3 ms, 1.5 mA, known only to activate A-fibers). Spatially, the fMRI signal extended approximately 5 mm in the longitudinal direction, covering L(3)-L(5) segments. In the cross-sectional direction, the highest signal change of blood volume-weighted fMRI was in the middle of the ipsilateral dorsal horn, which roughly corresponds to laminae V and VI, while the highest signal change of BOLD fMRI was in the ipsilateral dorsal surface. This study demonstrates that spinal cord fMRI can be performed in anesthetized rats reliably and reproducibly offering it as a potential tool for analgesic drug discovery.

The role of MRI and PET/SPECT in Alzheimer's disease

Alzheimer's disease (AD) is difficult to diagnose in its early stages, and even if detected early, there is no preventative treatment. Imaging modalities such as MRI, PET, and SPECT have the potential to contribute to both the diagnosis of Alzheimer's disease, as well as assist in the search for more effective treatments. A number of AD-related biomarkers have been proposed and evaluated. The use of PET imaging to detect alterations in regional brain metabolism using [(18)F]FDG has enabled more sensitive and accurate early diagnosis of AD, especially in conjunction with traditional medical evaluation. Additionally, magnetic resonance imaging and spectroscopy provide a wide range of biomarkers that have been shown to correlate with the progression of AD. Some of these markers have been pursued in clinical trials. Progress has been made toward the evaluation of other more AD-specific biomarkers. However, many questions remain concerning the validity and sensitivity of these imaging biomarkers to aid in the assessment of potential new treatments, especially those related to increased levels of amyloid peptides in the brain.

Visualization and quantification of neurokinin-1 (NK1) receptors in the human brain

PURPOSE

This study was conducted to develop a new positron emission tomography (PET) method to visualize neurokinin-1 (NK(1)) receptor systems in the human brain in vivo in order to examine their neuroanatomical distribution and facilitate investigations of the role of substance P, NK(1) receptors, and NK(1) receptor antagonists in central nervous system (CNS) function and dysfunction.

METHODS

PET studies were conducted in 10 healthy male volunteers using a novel selective, high-affinity NK(1) receptor antagonist labeled with fluorine-18 to very high specific radioactivity (up to 2000 GBq/micromol) [F-18]SPA-RQ. Data were collected in 3D mode for greatest sensitivity. Different modeling methods were compared and regional receptor distributions determined for comparison with in vitro autoradiographic studies using postmortem human brain slices with [F-18]SPA-RQ.

RESULTS

The studies showed that the highest uptake of [F-18]SPA-RQ was observed in the caudate and putamen. Lower binding was found in globus pallidus and substantia nigra. [F-18]SPA-RQ uptake was also widespread throughout the neocortex and limbic cortex including amygdala and hippocampus. There was very low specific uptake of the tracer in the cerebellar cortex. The distribution pattern was confirmed using in vitro receptor autoradiography with [F-18]SPA-RQ on postmortem human brain slices. Kinetic modeling of the [F-18]SPA-RQ uptake data indicated a binding potential between 4 and 5 in the basal ganglia and between 1.5 and 2.5 in the cortical regions.

CONCLUSIONS

[F-18]SPA-RQ is a novel tool for exploration of the functions of NK(1) receptors in man. [F-18]SPA-RQ can be used to define receptor pharmacodynamics and focus dose selection of novel NK(1) receptor antagonists in clinical trials thereby ensuring adequate proof of concept testing particularly in therapeutic applications related to CNS dysfunction.

Image-derived input function for [11C]flumazenil kinetic analysis in human brain

PURPOSE

We describe a method for analysis of [11C]flumazenil data using an input curve directly derived from the positron emission tomography (PET) images.

PROCEDURE

The shape of the tracer plasma curve was obtained from the product of the intact flumazenil fraction in plasma in six arterial samples and the internal carotid artery time-activity curve (TAC). The resulting curve was calibrated using the [11C]flumazenil concentration in three of the six samples. The curve peak was recovered by adding an exponential function to the scaled curve whose parameters were estimated from simultaneous fittings of several tissue TACs assuming that all regions share the same input.

RESULTS

Good agreement was found between the image-derived and the experimental plasma curves in six subjects. Distribution volumes were highly correlated with linear regression slope and intercept values between [0.94, 1.03] and [-0.10, 0.16], respectively.

CONCLUSION

The proposed method is suitable for benzodiazepine receptor quantification requiring only a few blood samples.

Further evidence that a shared efferent collicular pathway drives separate circuits for smooth eye movements and saccades

The aim of the present study was to find out whether smooth eye movements (SEMs) evoked by superior colliculus (SC) stimulation are, as suggested by Breznen et al. (1996), artefactual eye movements resulting from a non-physiological response of the saccadic generator. This question was reinvestigated in head-restrained cats. Long-lasting SC stimulation was found to evoke, in a comparable proportion, either a single saccade followed by an uninterrupted SEM or a staircase of two or three saccades interleaved with SEMs. These two different patterns of eye movements could be elicited at a near-threshold current and at low stimulation frequencies. In most cases, SEM direction clearly differed from that of the preceding saccade. This difference between SEM and saccade directions varied in a systematic way as a function of the initial saccade direction. As demonstrated by computer simulation, this observation can be explained if the neural circuit controlling SEMs reaches a saturation level earlier than the saccadic burst-generator. Our results in cats were reminiscent of those reported by Breznen et al. (1996) in the monkey only in some instances, when high frequency stimulation (400-600 Hz) was applied. Indeed, in the case of near-threshold stimulation-elicited staircase saccades, increasing the stimulation frequency led to a progressive disappearance of the smaller subsequent saccades that were substituted by uninterrupted SEM-like movements. Altogether, the present results confirm the view that SEMs are genuine eye movements. These results rule out the hypothesis that SEMs result from a saturation of the saccadic generator and strengthen the hypothesis that SEMs and saccades are distinct movements. We suggest that the same collicular efferent cells carry out the motor command to saccadic and SEM circuits and that the position error originating from the SC may be distributed amongst separate downstream motor systems.

Increase in GABAergic Cells and GABA Levels in the Spinal Cord in Unilateral Inflammation of the Hindlimb in the Rat

The effects of chronic peripheral inflammation on spinal cord gamma-aminobutyric acid (GABA) were examined in the rat. Following the injection of complete Freund's adjuvant in the left hindlimb footpad an increased number of immunoreactive cells occurred in ipsilateral laminae I - III of the dorsal horn from L3 to L5. GABA-immunoreactive cells were more numerous than contralaterally 1 week after the onset of the inflammation, reached maximal numbers after 3 - 4 weeks, and declined thereafter. Differences from control sides were statistically significant except at week 6. GABA levels in homogenates of the ipsilateral lumbar enlargement were increased significantly at 4 weeks. Since increases in GABA occurred in the spinal cord zone of projection of the nerves supplying the inflamed foot, the central response is surmised to result from the increased nociceptive input arriving from the periphery. However, the transmission from primary axons to GABA interneurons is not likely to be monosynaptic since profiles containing glutamate decarboxylase or GABA immunoreactivity are known to be predominantly presynaptic, and rarely postsynaptic, to primary afferent endings in electron micrographs in the rat. The findings support the function attributed to spinal GABA in modulating nociceptive input at segmental level.

A quantitative analysis of the correlations between eye movements and neural activity in the pretectum

The study of the saccadic system has focused mainly on neurons active before the beginning of saccades, in order to determine their contribution in movement planning and execution. However, most oculomotor structures contain also neurons whose activity starts only after the onset of saccades, the maximum of their activity sometimes occurring near saccade end. Their characteristics are still largely unknown. We investigated pretectal neurons with saccade-related activity in the alert cat during eye movements towards a moving target. They emitted a high-frequency burst of action potentials after the onset of saccades, irrespective of their direction, and will be referred to as "pretectal saccade-related neurons". The delay between saccade onset and cell activity varied from 17 to 66 ms on average. We found that burst parameters were correlated with the parameters of saccades; the peak eye velocity was correlated with the peak of the spike density function, the saccade amplitude with the number of spikes in the burst, and burst duration increased with saccade duration. The activity of six pretectal saccade-related neurons was studied during smooth pursuit at different velocities. A correlation was found between smooth pursuit velocity and mean firing rate. A minority of these neurons (2/6) were also visually responsive. Their visual activity was proportional to the difference between eye and target velocity during smooth pursuit (retinal slip). These results indicate that the activity of pretectal saccade-related neurons is correlated with the characteristics of eye movements. This finding is in agreement with the known anatomical projections from premotor regions of the saccadic system to the pretectum.

Central afferent pathways conveying nociceptive input to the hypothalamic paraventricular nucleus as revealed by a combination of retrograde labeling and c-fos activation

Previous data have shown that noxious thermal stimulation of the hind leg in the anesthetized rat causes c-fos activation in the paraventricular nucleus of the hypothalamus (PVN); in other brain nuclei, including the cathecholaminergic cell groups of the caudal medulla; and in the adenohypophysis. Stimulation was followed by adrenocorticotropic hormone plasma release but did not provoke cardiovascular changes. In the current study, the afferent central pathways conveying the nociceptive input to the PVN were studied throughout the brain by using double labeling for the Fos-protein and the retrograde tracer cholera toxin subunit B (CTb) injected into the PVN. Although double labeling occurred in several hypothalamic nuclei, the periaqueductal gray, the lateral parabrachial area, and the catecholaminergic medullary groups, high rates of double labeling occurred only in the cells of the A1 region of the ventrolateral medulla ( approximately 83% of CTb-labeled cells expressing c-fos). Further triple labeling with tyrosine hydroxylase (TH) revealed that > 80% of the double-labeled cells were TH-immunoreactive. The spinal cord had the usual strong c-fos expression but showed no retrograde labeling from the PVN. Noxious stimulation caused corticosterone plasma release. To ascertain a possible link of spinofugal neurons with the A1 cells, biotinylated dextran amine was injected into the spinal dorsal horn. Numerous anterogradely labeled fibers with bouton-like structures were observed, with the latter apposed to double- and triple-labeled cells in the A1 region. It is suggested that a dysynaptic route relayed in the A1 region conveys the nociceptive somatic input from the spinal cord to the PVN. Noxious stimulation may act as a systemic stressor, activating the hypothalamic-pituitary-adrenal axis.

Intravesical resiniferatoxin desensitizes rat bladder sensory fibres without causing intense noxious excitation. A c-fos study

In this study the desensitizing power of increasing concentrations of resiniferatoxin applied topically to the bladder mucosa, and the irritating properties of the most effective desensitizing dose, were determined with the aid of the spinal expression of the proto-oncogene c-fos. Desensitization was assessed by the decrease in the number of Fos-immunoreactive spinal neurons induced by the intravesical instillation of 1% acetic acid, when the latter was preceded by resiniferatoxin in concentrations between 1 and 1000 nM. Irritation, as shown by the noxious excitation of vesical sensory innervation, was measured by the c-fos response evoked by a single application of resiniferatoxin. As to the desensitizing power, resiniferatoxin produced a dose-dependent effect with a maximum at 100 nM, which decreased Fos-immunoreactive cell numbers to less than 10% of controls. No further decrease of c-fos activation occurred at 1000 nM. As to the irritating power, the saturation dose of resiniferatoxin (100 nM) produced a very weak c-fos activation in lumbosacral spinal cord segments. These data show that in an effective desensitizing concentration, resiniferatoxin is virtually devoid of nociceptive effects, in agreement with current clinical observations.

Lidocaine prevents noxious excitation of bladder afferents induced by intravesical capsaicin without interfering with the ensuing sensory desensitization: an experimental study in the rat

PURPOSE

The effects of the local anesthetic lidocaine on the noxious excitation and subsequent desensitization of bladder sensory fibers, produced by intravesical capsaicin, were evaluated through c-fos activation in the spinal cord.

MATERIALS AND METHODS

Noxious excitation was demonstrated by counting Fos-immunoreactive (IR) cells occurring in the rat spinal cord 2 hours after intravesical administration of 1 mM. capsaicin, preceded or not by 2% lidocaine. Desensitization was studied by comparing the number of Fos-IR cells induced by 1% acetic acid in rats treated 24 hours before with 1 mM. intravesical capsaicin preceded or not by 2% lidocaine.

RESULTS

Lidocaine instilled previously markedly reduced the number of Fos-IR spinal cells responding to capsaicin-induced bladder afferent excitation. Numbers of Fos-IR cells induced by acetic acid instillation in bladders desensitized by capsaicin administrated 24 hours before were not changed by lidocaine application prior to capsaicin.

CONCLUSIONS

These findings suggest that local anesthetic pretreatment of the bladder with lidocaine reduces the capsaicin-induced noxious excitation of the sensory fibers without decreasing their subsequent desensitization.

Sites of renal pain processing in the rat spinal cord. A c-fos study using a percutaneous method to perform ureteral obstruction

The sites of renal pain processing in the rat spinal cord were studied by mapping the spinal cord neurons expressing c-fos after acute ureteral distension due to obstruction. A new experimental model is presented. A nylon knot was loosely placed around the ureter and the ends of the thread exteriorized through the retroperitoneal wall. Eight days later, when all c-fos expression due to nociceptive input from the abdominal wound and the manipulation of the intestines had disappeared, the nylon ends were pulled to produce ureteral occlusion. C-fos activation occurred at spinal segments T10-L4 with a peak at L1-L2. The activated neurons were concentrated in laminae I, lateral IV-V, medial VII and X. While in lamina I nearly all Fos-immunoreactive cells were ipsilateral, in the deeper laminae taken together 60% cells were ipsilateral and 40% contralateral to the distended ureter. It is suggested that renal nociceptive input giving rise to conscious pain perception is transmitted through ipsilateral lamina I, whereas input triggering autonomic reflexes may be mainly processed, ipsi- and contralaterally, in the deep laminae.

The pontine A5 noradrenergic cells which project to the spinal cord dorsal horn are reciprocally connected with the caudal ventrolateral medulla in the rat

A disynaptic pathway linking the caudal ventrolateral medulla (VLM) to the spinal cord via the A5 noradrenergic cell group of the pons has recently been described in the rat. In the present work, the projections of the A5 to the VLM and to the spinal dorsal horn were studied with double-tracing techniques combined with immunostaining of the noradrenaline-synthesizing enzyme dopamine-beta-hydroxylase. Cholera toxin subunit B (CTb) injected into the VLM and fluoro-gold injected into the spinal dorsal horn produced double retrograde labelling of A5 neurons immunoreactive for dopamine-beta-hydroxylase, which received appositions of fibre varicosities labelled anterogradely with CTb injected into the VLM. After injecting CTb into the A5, retrogradely labelled neurons occurred in the VLM. These neurons were contacted by anterogradely labelled fibres from the A5 group. These observations indicate that the VLM cells acting upon the A5 spinally projecting neurons, which are likely to exert an alpha2-adrenoreceptor-mediated inhibition on the spinal cord, are targeted by collaterals of the A5 spinal cord-bound axons. The A5-VLM pathway may be the anatomical substrate of a negative feedback circuit whereby the modulatory action of the VLM on the spinal cord is self-inhibited through activation of the A5.

Chemical sensory deafferentation abolishes hypothalamic pituitary activation induced by noxious stimulation or electroacupuncture but only decreases that caused by immobilization stress. A c-fos study

We have shown in previous c-fos studies that noxious stimulation or electroacupuncture in deeply anaesthetized rats activate the hypothalamic pituitary corticotrope axis in a specific way. C-fos expression was more pronounced in the arcuate than the paraventricular hypothalamic nuclei, and none occurred in the pituitary intermediate lobe. The absence of the usual autonomic responses to psychological stress, such as tachycardia or blood pressure elevation, suggested a specific action of the somatosensory input on the hypothalamic pituitary axis. To prove this hypothesis, c-fos expression was examined in the paraventricular, arcuate and other hypothalamic nuclei, the pituitary gland, and the A1 and A2 medullary catecholaminergic cell groups of animals deprived of nociceptive primary afferent input by neonatal capsaicin. After noxious stimulation or electroacupuncture, no c-fos enhancement occurred in any of those sites in capsaicin-treated animals, and there was no increased plasma release of adrenocorticotropic hormone. In contrast, the hypothalamic pituitary c-fos activation provoked by immobilization stress though markedly decreased, was not abolished by capsaicin, whereas plasma release of adrenocorticotropic hormone remained undiminished. These findings suggest that noxious stimulation or electroacupuncture act on the hypothalamic pituitary corticotrope axis through an exclusively physical effect depending on the noxious signal elicited in the somatosensory pathway. They also demonstrate the occurrence of a minor somatosensory physical component after forced immobilization, acting on the hypothalamic pituitary axis probably together with the prevalent component of emotional arousal elicited by this form of stress.

Desensitization of bladder sensory fibers by intravesical capsaicin has long lasting clinical and urodynamic effects in patients with hyperactive or hypersensitive bladder dysfunction

PURPOSE

Capsaicin was used to treat symptomatic patients with hyperactive or hypersensitive bladders.

MATERIALS AND METHODS

Capsaicin solution (1 mM.) was instilled into the bladder of 16 patients with spinal hyperreflexia, bladder instability or hypersensibility.

RESULTS

Frequency decreased in 14 patients and continence was achieved in 10 of 14 incontinent cases. Mean first desire to void and maximal cystometric capacity increased significantly. These effects lasted for 6 to 12 months and were renewed following repeat treatment.

CONCLUSIONS

Bladder afferent desensitization with capsaicin is promising in patients with motor or sensory bladder dysfunction, although initial pungency might limit its use.

The ventrolateral medulla of the rat is connected with the spinal cord dorsal horn by an indirect descending pathway relayed in the A5 noradrenergic cell group

The pathway conveying the descending inhibitory noradrenergic input elicited from the caudal ventrolateral medulla (VLM) onto the spinal cord dorsal horn was studied in the rat. Retrograde labeling with cholera toxin subunit B (CTb) injected into the dorsal horn was combined with immunostaining for dopamine-beta-hydroxylase (DBH) in the VLM and other brainstem nuclei containing noradrenergic cells. CTb-labeled neurons occurred in the lateral part of the VLM (VLMlat), located ventrolaterally to the DBH-immunoreactive cells of the A1 noradrenergic cell group. Neuronal profiles stained for CTb and DBH (double labeled) occurred in the A5 (31%), A6 (57%), and A7 (12%) noradrenergic cell groups. To ascertain whether noradrenergic cells targeting the spinal cord in those groups received projections from the VLMlat, this area was injected with the anterograde tracer biotinylated dextran amine (BDA). Labeled terminal fibers with boutons en passant were apposed to numerous double-stained neurons in the A5 cell group. Similar appositions occurred in small amounts in the ventral subcoerulear component of the A6. Correlated light and electron microscopic analyses of the labeled appositions revealed that the BDA-labeled axonal boutons contained spherical vesicles and were presynaptic at asymmetrical contacts to somata and dendritic profiles of the double-stained A5 neurons. These data indicate the occurrence of an indirect dysynaptic pathway connecting the VLM to the spinal cord, with a relay in the A5 cells. This pathway may convey the antinociceptive effects mediated by alpha 2-adrenoreceptors, which have been previously observed in the spinal cord following VLM stimulation.

Activation of anterior lobe corticotrophs by electroacupuncture or noxious stimulation in the anaesthetized rat, as shown by colocalization of Fos protein with ACTH and beta-endorphin and increased hormone release

A marked expression of the c-fos proto-oncogene has been recently reported in cells of the anterior lobe of the pituitary gland in rats subject to electroacupuncture or noxious thermal stimulation under pentobarbital anaesthesia. The present study was undertaken to identify the activated pituitary cells. Following both kinds of stimulation, most Fos-immunoreactive anterior lobe cells showed colocalization with adrenocorticotropic hormone or beta-endorphin immunoreactivity. No c-fos expression occurred in pituitary cells immunoreactive for growth hormone, prolactin, luteinizing hormone, or thyrotropin-stimulating hormone. A marked rise of adrenocorticotropic hormone and beta-endorphin concentrations occurred in plasma. In the hypothalamus, c-fos expression was increased in the mediobasal nuclei-namely, the arcuate nucleus-and in the paraventricular nucleus, but more in the former. It is suggested that somatosensory noxious input, or the partly noxious input evoked by electroacupuncture, activate the hypothalamo-pituitary-adrenocortical axis as in common forms of stress, but with a specific activation of the mediobasal hypothalamic nuclei and no stimulation of intermediate lobe cells. Opiate release from the pituitary gland may contribute to acupuncture analgesia or the intrinsic antinociceptive reactions triggered by noxious stimulation.

The medullary dorsal reticular nucleus facilitates acute nociception in the rat

The influence on pain processing caused by destruction or stimulation of the dorsal reticular nucleus (DRt) was studied using the tail-flick and the increasing temperature hot-plate tests. Lesions of the DRt were obtained by injecting quinolinic acid (180 nmol/microliters) unilaterally or bilaterally, and nociceptive responses were evaluated by both tests. Following unilateral lesions, the tail-flick latencies and the hot-plate response temperatures were increased, values differing statistically from controls in the latter test. Bilateral lesions resulted in statistically significant increases of both tail-flick latency and hot-plate response temperature. Stimulation of the DRt was performed by injecting glutamate (100 nmol/microliters) unilaterally, which was followed 1 min later by a significant decrease in the tail-flick latency compared to saline injected controls. These results suggest that the DRt is involved in the facilitation of nociception after acute thermal noxious stimulation. This effect may be mediated through a spino-DRt-spinal loop causing a rebound of excitation in lamina I cells receiving noxious input from their own receptive field.

C-fos expression in the hypothalamo-pituitary system induced by electroacupuncture or noxious stimulation

In the anaesthetized rat, low frequency electrical stimulation of the Zusanli acupoint (S36) or noxious thermal stimulation caused by immersing the footpad in water at 52 degrees C caused marked expression of c-fos in the anterior lobe of the pituitary gland, as well as in the arcuate and some nearby hypothalamic nuclei. A similar anterior lobe response was caused by immobilization stress in awake rats but in this case Fos-immunoreactive cells extended into the intermediate lobe and were very abundant in the paraventricular nucleus. It is suggested that the anterior pituitary cells that respond to stress are also activated by acupuncture or painful stimulation. However, the mechanisms of pituitary cell activation seem distinct from those occurring in stress, since different hypothalamic nuclei are involved.

Dental noxious input reaches the subnucleus caudalis of the trigeminal complex in the rat, as shown by c-fos expression upon thermal or mechanical stimulation

The first upper molar tooth was subject to noxious thermal or mechanical stimulation in anaesthetised rats. In some animals, the red-heated metal end of a dental filling instrument was placed upright, briefly, on the occlusal surface. In other rats, a fine explorer was introduced three times into the pulp through a drilling cavity. Brainstem and cervical cord of treated and control animals were immunoreacted for the Fos protein. Numerous Fos cells occurred in the dorsomedial part of the ipsilateral trigeminal subnucleus caudalis including the transition zone with the subnucleus interpolaris, and in lamina I at spinal segments C1-C2, but were lacking in more rostral trigeminal subdivisions. Findings support the exclusive processing of nociceptive dental input in the medullary and upper cervical dorsal horn.