2-Methyl-6-(phenylethynyl)-pyridine (MPEP), a potent, selective and systemically active mGlu5 receptor antagonist

In the present paper we describe 2-methyl-6-(phenylethynyl)-pyridine (MPEP) as a potent, selective and systemically active antagonist for the metabotropic glutamate receptor subtype 5 (mGlu5). At the human mGlu5a receptor expressed in recombinant cells, MPEP completely inhibited quisqualate-stimulated phosphoinositide (PI) hydrolysis with an IC50 value of 36 nM while having no agonist or antagonist activities at cells expressing the human mGlu1b receptor at concentrations up to 30 microM. When tested at group II and III receptors, MPEP did not show agonist or antagonist activity at 100 microM on human mGlu2, -3, -4a, -7b, and -8a receptors nor at 10 microM on the human mGlu6 receptor. Electrophysiological recordings in Xenopus laevis oocytes demonstrated no significant effect at 100 microM on human NMDA (NMDA1A/2A), rat AMPA (Glu3-(flop)) and human kainate (Glu6-(IYQ)) receptor subtypes nor at 10 microM on the human NMDA1A/2B receptor. In rat neonatal brain slices, MPEP inhibited DHPG-stimulated PI hydrolysis with a potency and selectivity similar to that observed on human mGlu receptors. Furthermore, in extracellular recordings in the CA1 area of the hippocampus in anesthetized rats, the microiontophoretic application of DHPG induced neuronal firing that was blocked when MPEP was administered by iontophoretic or intravenous routes. Excitations induced by microiontophoretic application of AMPA were not affected.

SIB-1757 and SIB-1893: selective, noncompetitive antagonists of metabotropic glutamate receptor type 5

Cell lines expressing the human metabotropic glutamate receptor subtype 5a (hmGluR5a) and hmGluR1b were used as targets in an automated high-throughput screening (HTS) system that measures changes in intracellular Ca2+ ([Ca2+]i) using fluorescence detection. This functional screen was used to identify the mGluR5-selective antagonist, SIB-1757 [6-methyl-2-(phenylazo)-3-pyridinol], which inhibited the glutamate-induced [Ca2+]i responses at hmGluR5 with an IC50 of 0.37 microM compared with an IC50 of >100 microM at hmGluR1. Schild analysis demonstrated a noncompetitive mechanism of inhibition. Pharmacophore mapping was used to identify an additional compound, SIB-1893 [(E)-2-methyl-6-(2-phenylethenyl)pyridine], which was also shown to block glutamate-induced increases in [Ca2+]i at hmGluR5 with an IC50 of 0.29 microM compared with an IC50 of >100 microM at hmGluR1. SIB-1757 and SIB-1893 showed little or no activity when tested for agonist and antagonist activity at the other recombinant human mGluR subtypes, alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid, kainate, and N-methyl-D-aspartate receptors. In rat neonatal brain slices, SIB-1757 and SIB-1893 inhibited (S)-3,5-dihydroxyphenylglycine (DHPG)-evoked inositol phosphate accumulation in hippocampus and striatum by 60% to 80%, with a potency similar to that observed on recombinant mGluR5. However, in the cerebellum, a brain region with low mGluR5 expression, SIB-1757 failed to inhibit DHPG-evoked inositol phosphate accumulation. In cultured rat cortical neurons, SIB-1757 and SIB-1893 largely inhibited DHPG-evoked [Ca2+]i signals, revealing a population of neurons that were less sensitive to SIB-1757 and SIB-1893. This is the first description of highly selective, noncompetitive mGluR5 antagonists. These compounds will be useful tools in evaluating the role of mGluR5 in normal physiology and in animal models of disease.

(S)-4-carboxy-3-hydroxyphenylglycine activates phosphatidyl inositol linked metabotropic glutamate receptors in different brain regions of the neonatal rat

In the present investigation, effects of several agonists and antagonists of metabotropic glutamate receptors (mGluRs) which are coupled to phosphatidyl inositol (PI) hydrolysis were evaluated in slices of neonatal rat hippocampus, striatum, cortex and cerebellum. The rank order of potency of agonists in the PI hydrolysis assay was identical in all brain regions: quisqualic acid (Quis) > (RS)-3,5-dihydroxyphenylglycine (3,5-DHPG) > 1S, 3R-aminocyclopentane dicarboxylic acid (1S,3R-ACPD) >> L-glutamate (Glu). All agonists were equiefficacious in the four brain regions tested. The responses to 3,5-DHPG, a highly selective Class I mGluR agonist, were attenuated by (S)-4-carboxyphenylglycine ((S)-4CPG), (+)-alpha-methyl-4-carboxyphenylglycine ((+)-MCPG) and 1-aminoindan-1,5-dicarboxylic acid (UPF-523) with a rank order of potency of (+)-MCPG > or = (S)-4CPG > or = UPF-523 in the different brain regions. These results suggest little selectivity among these putative mGluR antagonists in the different brain regions studied. Interestingly, (S)-4-carboxy-3-hydroxyphenylglycine ((S)-4C3HPG), a compound reported to act as antagonist at Class I mGluRs, produced concentration-dependent increases in PI hydrolysis in all four brain regions suggesting that (S)-4C3HPG acts as an agonist. In striatum, hippocampus and cortex, (S)-4C3HPG was equiefficacious to Quis, 3,5-DHPG, 1S,3R-ACPD and Glu. However, in the cerebellum, (S)-4C3HPG displayed weak agonist activity (37% of that of a maximally effective concentration of Quis). The effects of (S)-4C3HPG in the PI hydrolysis assay appeared to be mediated by the activation of an mGluR subtype since it was significantly blocked by (S)-4CPG, an mGluR antagonist. In addition, the agonistic effects of (S)-4C3HPG appear to be unrelated to inhibition of [3H]-Glu uptake into rat hippocampal or cerebellar synaptosomes. These results demonstrate a unique pharmacological profile of (S)-4C3HPG which can be interpreted as (S)-4C3HPG being a highly selective mGluR5 agonist or alternatively, that the effects of (S)-4C3HPG may be mediated through a novel Class I mGluR subtype(s), yet to be identified.

A three-dimensional multimodality brain map of the nemestrina monkey

A three-dimensional multimodality computerized map of the nemestrina monkey brain was created with serial sectioning and digital imaging techniques. An adult female Macaca nemestrina (pigtail macaque) weighing 7.2 kg was used in constructing this atlas. CT, PET, and MRI were performed on the monkey before the specimen's head was frozen and cryoplaned. Closely spaced (50 microns) images of the specimen blockface were then digitally acquired and modified to produce whole head and brain-only 3D image sets. The resulting data sets were organized into a digital volume and repositioned into a stereotaxic coordinate system defined by Horsley and Clark in 1908 [7]. Orthogonal images were obtained by digitally resampling the volume in order to produce a full set of coronal, sagittal, and horizontal images. Stereotaxic reference grids were applied to each image indicating the A/P, M/L, or Ho position within the digital volume. Specific anatomic structures were outlined from the cryosection data set and 3D surface models reconstructed. Structural labels indicating nuclei, tracts, and other neuroanatomical features were incorporated into coronally sliced cryosection images spaced at 500 microns. The CT, PET, and MRI data sets were reconstructed into a digital volume and coregistered to the cryosection volume. All images constructed from this 3D map are available for public access via the internet using an anonymous file transfer protocol (FTP) and the World Wide Web (http:@www.loni.ucla.edu). The foremost advantage of this digital map is an integrated multimodality three-dimensional representation of the Macaca nemestrina brain, which is not possible with traditional atlases.

Pharmacological characterization of SIB-1765F: a novel cholinergic ion channel agonist

Nicotine, the prototypical agonist for neuronal nicotinic acetylcholine receptors (NAChR), nonselectively activates NAChR limiting its use in elucidating the function of NAChR subtypes. SIB-1765F is a subtype selective NAChR agonist that displaces [3H]-nicotine binding with an IC50 of 4.6 nM and [3H]-cytisine binding with an IC50 of 12.2 nM which is 2000- to 6000-fold lower than its displacement of [3H]-QNB or [125I]-alpha-bungarotoxin. SIB-1765F did not inhibit human or rat cholinesterases or the uptake of [3H]-DA in synaptosomal preparations. SIB-1765F mimicked (-)-nicotine in stimulating [3H]-DA release from rat striatal and olfactory tubercle slices, with EC50 values of 99.6 and 39.6 microM, respectively. Such stimulation was sensitive to mecamylamine and DH beta E. SIB-1765F also released endogenous DA in the striatum and the nucleus accumbens as measured by in vivo microdialysis. SIB-1765F was less efficacious than (-)-nicotine at stimulating [3H]-NE release from rat hippocampal slices; in contrast, SIB-1765F increased [3H]-NE release from rat thalamic and cortical slices with efficacies approaching those of (-)-nicotine. Similar to (-)-nicotine and (+/-)-epibatidine, subcutaneous administration of SIB-1765F increased the turnover rate of dopamine ex vivo both in the striatum and olfactory tubercles in a mecamylamine-sensitive manner. Because the release of striatal DA and hippocampal NE appears to be regulated by distinct NAChR, differential effects of SIB-1765F on striatal DA and hippocampal NE release supports the NAChR subtype selectivity of SIB-1765F compared to (-)-nicotine. This is further demonstrated by observations showing that SIB-1765F has a higher affinity for h alpha 4 beta 2 NAChR relative to h alpha 4 beta 4 NAChRs in displacing [3H]-epibatidine binding and increasing cytosolic CA+2 concentration in cell lines stably expressing h alpha 4 beta 2 or h alpha 4 beta 4.

A 3D digital map of rat brain

A three dimensional (3D) computerized map of rat brain anatomy created with digital imaging techniques is described. Six male Sprague-Dawley rats, weighing 270-320 g, were used in the generation of this atlas. Their heads were frozen, and closely spaced cryosectional images were digitally captured. Each serial data set was organized into a digital volume, reoriented into a flat skull position, and brought into register with each other. A volume representative of the group following registration was chosen based on its anatomic correspondence with the other specimens as measured by image correlation coefficients and landmark matching. Mean positions of lambda, bregma, and the interaural plane of the group within the common coordinate system were used to transform the representative volume into a 3D map of rat neuroanatomy. images reconstructed from this 3D map are available to the public via Internet with an anonymous file transfer protocol (FTP) and World Wide Web. A complete description of the digital map is provided in a comprehensive set of sagittal planes (up to 0.031 mm spacing) containing stereotaxic reference grids. Sets of coronal and horizontal planes, resampled at the same increment, also are included. Specific anatomic features are identified in a second collection of images. Stylized anatomic boundaries and structural labels were incorporated into selected orthogonal planes. Electronic sharing and interactive use are benefits afforded by a digital format, but the foremost advantage of this 3D map is its whole brain integrated representation of rat in situ neuroanatomy.

3D spatial relationships of histochemical activity in the rat superior colliculus

Recent evidence points to the existence of lattices of acetylcholinesterase (AChE) and cytochrome oxidase (CYO) in deep layers of the superior colliculus. We sought to further describe their localization within the rat superior colliculus using new three dimensional (3D) digital imaging techniques. Texture maps of AChE staining within the stratum griseum intermediate (SGI) suggest a sheet of high enzyme activity punctuated by areas of pallor. This lattice of dense staining covered the caudal end of the colliculus and extended rostrally along the medial bank. Discrete regions of low staining were most apparent within the IVc subdivision of SGI. A similar examination of CYO activity failed to reveal any compartmentalization.

Imaging optical reflectance in rodent barrel and forelimb sensory cortex

Novel neuroimaging techniques are extending the scope for studying dynamic brain function. We have developed a system which enables the repeatable imaging of rapid function in rodent primary somatosensory cortex (S-I), based on activity-related changes in its optical reflectance (intrinsic signals). The S-I cortices of anesthetized male Sprague-Dawley rats were exposed. Images were acquired with a slow-scan, cooled, charge-coupled device camera (CCD) through filters at 550, 610, and 850 nm before, during, and after contralateral stimulation (vibrissal deflection or forepaw stimulation). Images were divided by prestimulus controls and then averaged across 9-27 trials to produce maps of stimulus-related reflectance change. Optical activity had magnitude 10(-3) of baseline reflectance and consistently comprised two distinct spatiotemporal components over cortex, depending on paradigm. The diffuse signal at 610 nm begins 0.5-1 s after stimulus onset and has a duration of 4-5 s. The second signal is macrovenous and is delayed by 1 s. Similar response patterns were observed at 550 and 850 nm. Evoked potentials, recorded at sites inside and outside the zone of optical activity, confirmed the functional nature of these signals. Using a CCD we have imaged functional reflectance changes over rodent S-I which commence, peak, and extinguish over a time scale of seconds. This optical activity is consistent with the etiologies of microvascular recruitment and chromophore redox change.

Mapping functional activity in rodent cortex using optical intrinsic signals

We have investigated the dynamic response of rodent posteromedial barrel subfield (PMBSF) cortex to mechanical whisker deflection, using optical intrinsic signal imaging. While electrophysiologic response in barrel cortex has been well studied, dynamic metabolic changes affecting activity-related perfusion or oxidative enzymes are not well understood. Male Sprague-Dawley rats were anesthetized. Contralateral single and multiple vibrissae were deflected while images of somatosensory cortex were acquired with a charge-coupled-device camera. Intrinsic signals were observed over PMBSF as stimulus-related reflectance decreases (10(-3) of baseline) comprising two distinct spatiotemporal components. At 610 nm the first, diffuse, component begins 0.5-1 sec after stimulus onset, peaks at 2.5-3 sec, and returns to baseline by 4-5 sec. The second component is macrovascular, beginning at 1-1.5 sec, peaking at 3 sec, and dissipating by 5-6 sec. Similar patterns were observed at 550 nm and 850 nm. Signal size and location varied with the stimulus. Evoked potentials were found to have maximal amplitude in the region of maximal optical signals, diminishing toward the optical periphery. We have demonstrated PMBSF response to vibrissal deflection using optical reflectance methods. These intrinsic signals overlie regions of maximal electrophysiologic response, but commence, peak, and extinguish over a time scale of seconds from stimulus onset. They most likely indicate activity-related microvascular recruitment and chromophore redox changes.

Superpositioning of 3-dimensional neuroanatomic data sets

Superpositioning of multiple 3-dimensional digital volumes is essential to extending neuroimaging capabilities from single-subject analysis to group evaluation. In this study, 6 rat heads were cryoplaned and digital images of the specimen blockface were organized into data volumes. The midline plane of each brain guided coronal and horizontal rotations and medial-lateral positioning. Four different methods of determining sagittal rotation, dorsoventral position, and anteroposterior position were tested. The first technique uses lambda and bregma as fiducial points; the second method employs bony sutures in the base of the skull; and the third strategy fits both sets of alignments points. The fourth method differs from the previous landmark-based approaches by employing cross-correlation of image densities. A composite of midsagittal cutplanes through the volumes aligned by the first method revealed significant misalignment. Use of ventral fiducials resulted in improved correlation statistics for orthogonal cutplanes through the volumes and significantly reduced positional variability of anteroposterior landmarks. Combining dorsal and ventral fiducials produced a slight improvement in midsagittal image correlation but had mixed effects on landmark variability. Following the fourth method of registration, correlation statistics and the alignment of reference points were similar to those obtained with the 4-point approach.

Animated visualization of a high resolution color three dimensional digital computer model of the whole human head

The interactive visualization of animated images through a computerized three dimensional (3D) full color model of an unstained cadaveric human head is presented. Serial full color images were taken of the blockface of a cryomicrotomed frozen human head every 200 microns. From this series of images a three dimensional digital model with a resultant pixel resolution of 200 microns3 was created on a UNIX workstation. Using this database, resampled images were computed along orthogonal axes and written sequentially to a write-once-read-many times (WORM) videodisc unit. Playback of this customized videodisc dataset provides animations of the digitally reconstructed slices and 3D reconstructed surface models. An interactive interface to the animated sequences is provided through a PC based tutorial package. This tutorial program is able to access videodisc frames to display animations and labeled still images in a software window to illustrate various neuroanatomic topics. The technique of animation as applied to this high resolution 3D model provides insight into complex spatial relationships and has great potential in research and as a teaching tool in the neurosciences.

Expression of mRNA for glial fibrillary acidic protein after experimental cerebral injury

This study was undertaken to determine whether a mRNA for glial fibrillary acidic protein (GFAP) was present in increased amounts as a response to injury and, if so, how was its temporal expression related to the demonstration of GFAP by immunocytochemical techniques. A cerebral freeze-injury was produced in mice and at intervals thereafter the animals were anesthetized, perfused with formalin and histological sections of the brain through the injured area were prepared. A riboprobe for GFAP mRNA labeled with S35 and an immunocytochemical probe for GFAP were utilized to localize mRNA and GFAP immunoreactivity, respectively. For mRNA studies, the histological slide exposed to either sense or antisense probe was overlaid with x-ray film or dipped in photographic emulsion. The developed film was quantitated by digital image analysis. Emulsions were examined by dark-field microscopy. The results indicate that mRNA for GFAP is increased in the cortex in the environs of the injury by 6 hours, becomes maximal at 4-5 days, and is present in increased amounts up to 14 days. The message is enhanced in the adjacent cortex, the subpial region, the adjacent corpus callosum and in the ipsilateral and contralateral callosal radiations. This pattern of enhancement follows the distribution of post-injury edema. Glial fibrillary acidic protein is demonstrable at 24-48 hours after injury. Thus, there is a rapid response of the astrocyte to injury with increased mRNA expression that is followed by expression of GFAP immunoreactivity.

Imaging peripheral benzodiazepine receptors in brain tumors in rats: in vitro binding characteristics

Peripheral benzodiazepine binding constants for transplanted RG-2 gliomas and HD and LK Walker 256 tumors (metastatic breast carcinoma) were determined in Wistar rats using autoradiography. In addition, Kd and Bmax parameters for peripheral benzodiazepine receptors on RG-2 tumors were directly visualized using digital image analysis of autoradiograms. High specific binding of [3H]PK11195, a selective peripheral benzodiazepine ligand, had excellent topographical correlation to areas of histologically verified tumor. Scatchard analysis suggested a single class of peripheral binding sites with similar binding affinities in RG-2 and LK Walker 256 tumors and normal cortex. Bmax was 20-fold greater in glial tumors and 11.6- and 10.6-fold greater in LK and HK Walker 256 tumors, respectively, compared to normal cortex. The location of metastatic tumors, either intracerebrally or subcutaneously, did not effect their Kd or Bmax values. Kd and Bmax values for RG-2 tumors were similar whether determined densitometrically or by direct visualization with image analysis. Binding parameters within normal brain were difficult to visualize by image analysis due to the low level of specific binding. The ability to label specifically intracerebral tumor cells and to characterize the binding parameters shown in this study suggest that peripheral benzodiazepine receptor ligands could be utilized by PET to analyze directly a variety of tumors in humans.

Three-dimensional comparison of peripheral benzodiazepine binding and histological findings in rat brain tumor

Experiments were undertaken to determine the in vivo utility of the mixed benzodiazepine ligand [3H]flunitrazepam and the selective peripheral benzodiazepine ligand [3H]PK 11195 [1-(2-chlorophenyl)-N-methyl-N-(1-methylpropyl)-3-isoquinoline carboxamide] to outline the borders of rat C6 glial tumors in three dimensions. Intravenous injection of [3H]flunitrazepam resulted in a tumor/cortex ratio of radioactive densities between 2.7 and 1.5 within the first 60 minutes after injection. [3H]PK 11195 demonstrated a higher tumor/cortex ratio (5.3) than [3H]flunitrazepam. For three-dimensional studies, images were generated from thionin-stained histological sections and autoradiograms. The mixed type benzodiazepine ligand [3H]flunitrazepam was superior in showing some of the normal anatomical structures surrounding the tumor, whereas [3H]PK 11195, a specific peripheral ligand, demonstrated higher tumor/brain contrast and superior topographical correlation between histological and autoradiographic images. Implications of peripheral benzodiazepine receptor ligands for positron emission tomography are discussed.

Effects of chronic cortical seizures on GABA and benzodiazepine receptors within seizure pathways

The effects of chronic cortical seizures on the autoradiographic distributions of two markers of the gamma-aminobutyric acid (GABA)/benzodiazepine receptor--chloride ionophore complex within local and long circuits connected to the focus were examined. Rats were subjected to electrically triggered seizures of the forelimb--sensorimotor overlap zone either daily or once every other day. At the time of sacrifice the rats had received a mean of 39 +/- 3 stimulations and their seizure responses had grown in intensity and duration. [3H]Muscimol binding and [3H]flunitrazepam binding, at near saturating ligand concentrations were unchanged in the focus, mirror focus, dorsolateral caudate, globus pallidus, ventrolateral and ventrobasal thalamic nuclei, and the substantia nigra pars reticularis. These results indicate that the progressive increases in strength and duration of recurrent focal cortical seizures are not accompanied by changes in the density of either GABA receptors or benzodiazepine receptors within the focus or projection pathways.

Regional distribution of flunitrazepam binding constants: visualizing Kd and Bmax by digital image analysis

We demonstrate direct visualization of benzodiazepine binding parameters collected in a 3H-flunitrazepam saturation study. In this way the relationship between anatomy and specific binding, Kd and Bmax, can be appreciated. These experiments have resulted in image representations of specific flunitrazepam binding, Kd and Bmax. The images retain the anatomic localization inherent in the original autoradiogram and hence make these parameters amenable to visual survey. Binding constants were consistent with data presented elsewhere in the literature. The image of Bmax showed regional heterogeneity. Kd, on the other hand, was relatively homogeneous. These results demonstrate a means by which autoradiographic binding experiments can be extended to saturation studies without significant loss of data.

Functional metabolic mapping during forelimb movement in rat. II. Stimulation of forelimb muscles

Repetitive electrical stimulation of wrist extensor muscles in rat was combined with quantitative 14C-deoxyglucose autoradiography to study sensory systems functionally activated during forelimb movement. Metabolism increased ipsilaterally in the wrist extensors, the dorsal horn of the cervical spinal cord, the cuneate nucleus and cerebellar hemisphere. The metabolic activation in cerebellum occurred in cortex surrounding the primary fissure anteriorly (lobules simplex and V), and the prepyramidal fissure posteriorly (lobules paramedian and copula pyramis). Metabolism was increased in both granule cell and molecular layers and was uniform throughout the zone of activation. Hindlimb stimulation primarily activated the medial aspect of copula pyramis, demonstrating the somatotopic specificity of changes. Forelimb stimulation also activated contralateral sites in the dorsal accessory nucleus of the inferior olive, ventrobasal thalamus, and SI and SII in cortex. Studies of the relationship between the magnitude of the response and the frequency of the stimulation revealed a positive correlation in muscle, dorsal horn and cuneate nucleus. Other activated sites only showed a significant change at the highest rates of stimulation. Comparison of the pattern of metabolic activation during forelimb movements induced centrally (Collins et al., 1986) with the pattern induced peripherally revealed overlap primarily in the paramedian zone of anterior and posterior cerebellum, and the granular cortex of SI and SII. These studies suggest that forelimb movement initiated centrally would have considerable influence on feedback sensation from the moving limb in these sites.

Functional metabolic mapping during forelimb movement in rat. I. Stimulation of motor cortex

The quantitative 14C-deoxyglucose (DG) autoradiographic technique has been used to study changes in cerebral metabolism during forelimb movements induced by graded stimulation of motor cortex. Experiments were directed at studying basic physiologic and anatomic aspects of the metabolic changes. Single shocks caused movement without metabolic change, whereas low-frequency trains caused seizures. Repetitive high-frequency train stimuli of short duration (500 Hz for 20 msec) caused jerk movements coupled with DG uptake in pathways. With stimulation of the forelimb motor zone at frequencies of 15-30/min there was prominent activation of cortical columns and strips in ipsilateral SI, SII, and MII, and contralateral MI and SI. Higher frequencies (120/min) were required to cause significant changes in DG in subcortical circuits. The most prominent changes occurred within a longitudinal corridor in dorsal thalamus and a ventral corridor in second-order sites in basal ganglia. Metabolic activation also occurred in contralateral cerebellum, the cuneate nucleus, and dorsal horn of the cervical spinal cord. Changes in these latter two sites were largely eliminated by removing feedback sensory activity. Stimulation of the forelimb sensory zone activated different sites in caudatoputamen and thalamus but similar zones in midbrain and cerebellum. The magnitude of the metabolic response in distant sites depended on the frequency of cortical stimulation. Different frequency-response relationships in different sites seemed to reflect the nature of the cortical input as well as differential effects of anesthesia. The pattern of the metabolic response was studied by comparing sites of activation with sites of the anatomic projections from motor and sensory cortical zones. 3H- and 14C-labeled amino acids were used to map the site and relative strength of pathways. Results revealed good correlation between the site of anatomic projection and the site of DG uptake but no consistent relationship between the relative strength of a projection and the magnitude of metabolic change within its field. Changes in glucose utilization with metabolic mapping experiments depend on the nature, strength, and frequency of stimulation; the site and nature of anatomic projection; the effects of anesthesia; and the strength of sensory feedback associated with the induced behavior.

Failure of muscarinic blockade to antagonize analepsis induced by thyrotropin-releasing hormone and MK-771 in the rat

In the rat, the antimuscarinics atropine and scopolamine failed to block the reduction in pentobarbital-induced sleep time produced by either thyrotropin-releasing hormone (TRH) or MK-771 (a TRH analog). Previous reports have indicated that the marked analeptic effect produced by TRH is antagonized by such agents. It is not clear at this time whether the difference between our findings and these previous studies indicates a reduced sensitivity of cholinergic receptors in the rat to muscarinic blockade, a different neurochemical mechanism of action of TRH in the rat, or other unknown factors.