Sharon A. Welner

Peripheral benzodiazepine binding sites in Alzheimer's disease frontal and temporal cortex

Much evidence has accumulated to suggest that the peripheral type benzodiazepine (PBZ) binding site has a predominantly glial localization. Elevated PBZ binding densities have been reported in various models of brain damage, apparently reflecting glial proliferation in response to neurodegeneration. In the present study, PBZ receptor densities were examined in frontal and temporal cortex of Alzheimers disease (AD) patients using the ligand [3H]PK 11195. There was a highly significant (p less than 0.01) increase in PBZ binding sites in the temporal cortex from AD patients. In the frontal cortex, a moderate increase was observed that approached statistical significance (p = 0.07). Decreased choline acetyltransferase activity was observed in both regions. These findings offer support for the potential use of the PBZ binding site as a marker for gliosis associated with neuronal cell death.

Further evidence for differential affinity states of the serotonin1A receptor in rat hippocampus

The binding profile of [3H]8-hydroxy-2-(di-N-propylamino)-tetralin ([3H]8-OH-DPAT) to serotonin1A (5-HT1A) sites in rat hippocampal, frontocortical and striatal membranes has been compared. In these regions, [3H]8-OH-DPAT labels both a high and a low-affinity binding site; the affinity values for each of the two sites are comparable in the different brain regions, but have different maximal capacity. By modifying the experimental conditions in a series of hippocampal membrane preparations, reciprocal changes in the proportion of the two sites were observed suggesting that they represent, at least in this region, different conformations or affinity states of a single receptor protein. In contrast to the lower affinity state, it appears that the high-affinity state is stabilized by coupling with a G-protein. Evidence supporting this statement is provided by addition of the guanine nucleotide Gpp(NH)p, breakage of labile disulfide bonds using N-ethylmaleimide and increasing membrane rigidity with ascorbate-induced lipid peroxidation, conditions which all reduced the density of receptors in the high-affinity state. Moreover, the high-affinity state appears to be stabilized at the expense of the lower affinity state in the presence of Mn2+. On the other hand, a complete shift to the low-affinity binding state was observed after a 24 h in vivo treatment with inhibitors of monoamine oxidase A (phenelzine or clorgyline) but not of monoamine oxidase B (deprenyl). This disappearance of the high-affinity state with a concomitant increase in the binding capacity of the low-affinity state was reproduced by inhibiting monoamine oxidase A in vitro, as well as by reducing preincubation washout periods. Also, competitors of the [3H]8-OH-DPAT binding site, such as serotonin and unlabelled 8-OH-DPAT, display two affinity states while others like (+/-)-propranolol, tryptamine and spiperone recognize a single affinity component. These results suggest that the 5-HT1A binding site may exhibit at least two different affinity states depending upon its microenvironment and the intrinsic activity of the ligand used.

Chromaffin cell grafts to rat cerebral cortex reverse lesion-induced memory deficits

Adrenal chromaffin cells were isolated from donor adult rats and transplanted to the cerebral cortex of bilaterally nucleus basalis magnocellularis-lesioned rats. Chromaffin cell grafts to lesioned animals completely reversed the spatial memory deficit seen in lesioned alone animals on a T-maze alternation task. Although chromaffin cell grafts have been used previously to reverse motor abnormalities arising from defective nigro-striatal aminergic transmission, the present report is the first evidence that chromaffin cell transplants can reverse deficits in memory function. Grafts also enhanced cortical acetylcholinesterase staining.

Glutamate Stimulation of [3H]Dopamine Release from Dissociated Cell Cultures of Rat Ventral Mesencephalon

Abstract In dissociated cell cultures of fetal rat ventral mesencephalon preloaded with [3H]dopamine, glutamate (10‐5‐10‐3M) stimulated the release of [3H]dopamine. Glutamate stimulation of [3H]dopamine release was Ca2+ dependent and was blocked by the glutamate antagonist, cis‐2,3‐piperidine dicarboxylic acid. Glutamate stimulation of [3H]dopamine release was not due to glutamate neurotoxicity because (1) glutamate did not cause release of a cytosolic marker, lactate dehydrogenase, and (2) preincubation of cultures with glutamate did not impair subsequent ability of the cells to take up or release [3H]dopamine. Thus, these dissociated cell cultures appear to provide a good model system to characterize glutamate stimulation of dopamine release. Release of [3H]dopamine from these cultures was stimulated by verat‐ridine, an activator of voltage‐sensitive Na+ channels, and this stimulation was blocked by tetrodotoxin. However, glutamate stimulated [3H]dopamine release was not blocked by tetrodotoxin or Zn2+. Substitution of NaCl in the extracellular medium by sucrose, LiCl, or Na2SO4 had no effect on glutamate stimulation of [3H]dopamine release; however, release was inhibited when NaCl was replaced by choline chloride or N‐methyl‐d‐glucamine HCl. Glutamate‐stimulated [3H]‐dopamine release was well maintained (60‐82% of control) in the presence of Co2+, which blocks Ca2+ action potentials, and was unaffected by the local anesthetic, lidocaine. These results are discussed in terms of the receptor and ionic mechanisms involved in the stimulation of dopamine release by excitatory amino acids.

Autoradiographic distribution of multiple classes of opioid receptor binding sites in human forebrain

Receptor binding parameters and autoradiographic distribution of various opioid receptor sites have been investigated in normal human brain, post-mortem. [3H]DAGO, a highly selective mu ligand, binds to a single class of high affinity (Kd = 1.1 nM), low capacity (Bmax = 160 fmol/mg protein) sites in membrane preparations of frontal cortex. These sites show a ligand selectivity profile that resembles that of the mu opioid receptor. On the other hand, [3H]bremazocine, in presence of saturating concentrations of mu and delta blockers, appears to selectively bind to a single population of kappa opioid sites (Kd = 0.13 nM; Bmax = 93.0 fmol/mg protein) in human frontal cortex. Whole hemisphere in vitro receptor autoradiography reveals that [3H]DAGO-mu, [3H]DSLET-delta and [3H]bremazocine (plus blockers)-kappa binding sites are discretely and differentially distributed in human forebrain. In the cortex, mu sites are concentrated in laminae I and IV, delta sites in laminae I and II while kappa sites are found in deeper layers (laminae V and VI). In subcortical nuclei, high densities of mu and delta sites are seen in the caudate and putamen while high amounts of kappa sites are present in the claustrum and amygdala. The nucleus basalis of Meynert is enriched in all three classes of sites while the globus pallidus only contains moderate densities of kappa sites. Thus, the possible alterations of these various classes of opioid receptors in neurological and psychiatric diseases certainly deserve further investigation.

Long-term effects of cysteamine on cognitive and locomotor behavior in rats: relationship to hippocampal glial pathology and somatostatin levels.

Peroxidase-positive astrocytic inclusions, derived from effete, iron-laden mitochondria, accumulate in the rat hippocampus, striatum and other subcortical brain regions as a function of advancing age. The sulfhydryl agent, cysteamine (CSH), accelerates the appearance of this senescent glial phenotype both in primary astrocyte cultures and in the aging subcortical brain in situ. Earlier experiments have shown that short-term administration of CSH results in reversible depletion of brain somatostatin (SS) levels, cognitive deficits and decreases in locomotor activity. In the present study, we tested spatial learning/memory and motor functioning in rats at 4-5 weeks following cessation of chronic (6 week) CSH treatment to determine whether behavioral deficits may be associated with gliopathic changes within the dorsal hippocampus distinct from the behavioral abnormalities accruing to the immediate effects of the drug. CSH-treated rats displayed significantly impaired performance in the Morris water maze 4-5 weeks following termination of prolonged CSH treatment. In contrast, locomotor activity was not affected in this experimental paradigm. CSH-treated animals exhibited significantly higher numbers of peroxidase-positive astrocyte granules as well as total numbers of GFAP-positive astrocytes in the CA1 sector of the dorsal hippocampus relative to saline-treated controls. In the hilus of the dentate gyrus, numbers of both peroxidase-positive glial inclusions and astrocytes were unaffected by CSH exposure. At 5 weeks following cessation of CSH treatment, SS levels in the hippocampus and hypothalamus (but not cerebral cortex) were elevated relative to those of saline-treated controls. Our results indicate that chronic CSH exposure induces senescence-like changes in CA1 astrocytes which are associated with deficits in cognitive, but not locomotor, behavior and elevated levels of hippocampal and hypothalamic SS. Pathological glial-neuronal interactions within the hippocampus and other subcortical brain regions may play an important role in the cognitive decline observed during normal senescence and in aging-related neurodegenerative disorders.

Antidepressant-anxiolytic interactions: Involvement of the benzodiazepine-GABA and serotonin systems

1. Recent studies have demonstrated that antidepressant drugs are actually more effective than BZs in the treatment of anxiety states. The role of two major neurochemical substrates that may be implicated in the anxiolytic activity of antidepressants, the benzodiazepine (BZ)-GABA receptor chloride ionophore complex and central serotonergic pathways, are focused on in this review. 2. A wide range of antidepressants elicit a reduction in BZ receptors and display anxiolytic effects within a conflict paradigm. 3. The anxiolytic activity of antidepressants, however, does not appear to be mediated via the BZ receptor, but possibly via another component of the complex such as the chloride channel-associated with the GABAA receptor. 4. Additionally, as possible candidates for the mechanism of anxiolytic activity of these compounds, results of pharmacological, behavioral and clinical studies point to the importance of serotonin (5-HT)1A receptors and 5-HT transporter sites as targets for the action of antidepressants, triazolobenzodiazepines and anxioselective piperazine derivatives.

The effect of quisqualic acid-induced lesions of the nucleus basalis magnocellularis on latent inhibition.

Latent inhibition (LI) is a reduction in the rate of acquisition of a Pavlovian conditioned response that results from prior nonreinforced preexposure to a conditioned stimulus (CS). LI has been suggested to reflect the operation of mechanisms involved in stimulus selection for subsequent cognitive processing. The present experiment was conducted to assess the effect of bilateral lesions of the nucleus basalis magnocellularis (NBM) on LI employing a conditioned emotional response paradigm. Bilateral lesions of the NBM were produced by administration of 0.12 M quisqualic acid and resulted in decreased cortical acetylcholinesterase staining, as well as a 40% reduction in cortical choline acetyltransferase activity. Following lever press training, preexposed animals received 40 presentations of a 60-s tone CS. Nonpreexposed animals received no tone presentations. Acquisition of conditioned suppression was then assessed over the course of 4 tone-shock (0.6 mA, 0.5 s) pairings. Control, preexposed animals displayed a retarded rate of acquisition in comparison to nonpreexposed controls, thereby demonstrating that the parameters used in the present experiment produced LI. In contrast, lesioned animals preexposed to the CS acquired conditioned suppression as readily as nonpreexposed lesioned animals. However, the acquisition of conditioned suppression in both lesioned groups was found to be similar to that displayed in the preexposed control group. This pattern of results was interpreted as being attributable to a lesion-induced impairment in the ability to maintain stimulus processing, rather than a deficit in the ability to filter a stimulus.

Amelioration of sensory attention and sensorimotor deficits by chromaffin cell grafts to the cerebral cortex of nucleus basalis magnocellularis lesioned rats

Rats that have received lesions to the nucleus basalis magnocellularis display with a variety of behavioral deficits; among these are decreases in performance of maze tests as well as deficiencies on measures of general health, sensory attention and sensorimotor abilities. We have previously shown that grafts of chromaffin cells placed in the cerebral cortex of nucleus basalis magnocellularis lesioned rats can ameliorate the lesion-induced deficits in performance of a task involving spatial memory. In the present study, we find that lesion-induced deficits in the sensory attention measure of exploration of the environment (head scanning) as well as the sensorimotor behavior involving a rat righting itself when placed nose down on an inclined grid are evident at 8 weeks post-lesion in lesioned-alone rats; these deficits are significantly ameliorated by chromaffin cell grafts in the cerebral cortex placed two weeks following the lesion procedure. These findings may have relevance to the use of chromaffin cells for grafting in neurodegenerative disorders in which sensorimotor or attention deficit components are involved.

Excitatory and inhibitory amino acids in the cerebral cortex of nucleus basalis magnocellularis lesioned rats.

It is well known that pathways arising from the nucleus basalis magnocellularis in the basal forebrain which terminate in the cerebral cortex are involved in cognitive function. The cholinergic system is generally thought to play a large part in these processes from lesion, pharmacological and transplantation studies. With increasing evidence suggesting the involvement of amino acid transmitters in learning and memory processes, it is of interest to also evaluate possible changes in the levels of amino acid transmitters in the cortex of nucleus basalis magnocellularis-lesioned rats. In the present study, 9 cortical amino acids were measured in rats with bilateral lesions of the nucleus basalis magnocellularis. We measured significant reductions in aspartate, alanine and gamma-aminobutyric acid; these were 80%, 75%, and 81%, respectively, of control brain values. These results suggest that changes in the amino acid content of the cerebral cortex following lesion of the nucleus basalis magnocellularis-lesioned rat should perhaps also be considered when evaluating behavioral effects in this model.