Behavioral and biochemical effects of knife cuts that preferentially interrupt principal afferent and efferent connections of the striatum in the rat

Metoclopramide in the treatment of diabetic gastroparesis

Gastroparesis is a chronic disorder that affects a significant subset of the population. Diabetes mellitus is a risk factor for the development of gastroparesis. Currently, metoclopramide is the only US FDA-approved medication for the treatment of gastroparesis. However, the FDA recently placed a black-box warning on metoclopramide because of the risk of related side effects, including tardive dyskinesia, the incidence of which has been cited to be as high as 15% in the literature. This review will investigate the mechanisms by which metoclopramide improves the symptoms of gastroparesis and will focus on the evidence of clinical efficacy supporting metoclopramide use in gastroparesis. Finally, we seek to document the true complication risk from metoclopramide, especially tardive dyskinesia, by reviewing the available evidence in the literature. Potential strategies to mitigate the risk of complications from metoclopramide will also be discussed.

Reelin haploinsufficiency reduces the density of PV+ neurons in circumscribed regions of the striatum and selectively alters striatal-based behaviors

RATIONALE

Reelin, a large extracellular matrix glycoprotein, is down-regulated in the brain of schizophrenic patients and of heterozygous reeler mice (rl/+). The behavioral phenotype of rl/- mice, however, matches only partially the schizophrenia hallmarks.

OBJECTIVES

We recently reported (Marrone et al., Eur J Neurosci 24:20062-22070, 2006) that homozygous reeler mutants (rl/rl) exhibit reduced density of parvalbumin-positive (PV+) GABAergic interneurons in anatomically circumscribed regions of the neostriatum. Assuming that in rl/+ mice may also show regional reduction of striatal GABAergic interneurons, behavioral impairments should selectively emerge in tasks depending on specifically altered striatal circuits.

MATERIALS AND METHODS

We mapped the density of striatal PV+ interneurons in rl/+ and wild-type (+/+) mice and measured their performance in tasks depending on distinct striatal subregions.

RESULTS

Our findings show that, contrary to what would be expected on the basis of gene dosage criteria, the striatal regions in which rl/rl mice exhibited decreased density of PV+ interneurons were either unaltered (rostral striatum) or equally altered (dorsomedial and ventromedial intermediate striatum, caudal striatum) in rl/+ mice. The anatomical findings were paralleled by behavioral deficits in fear extinction and latent inhibition, respectively, requiring the dorsomedial and ventromedial striatal regions. Conversely, active avoidance performance, which requires the dorsolateral region, was unaffected.

CONCLUSIONS

Reelin haploinsufficiency alters the density of PV+ neurons in circumscribed regions of the striatum and selectively disrupts behaviors sensitive to dysfunction of these targeted regions. This aspect should be considered when designing experiments aimed at evaluating the impact of reelin haploinsufficiency in schizophrenia-associated cognitive disturbances in rl/+ mutants.

Altered cortico-striatal synaptic plasticity and related behavioural impairments in reeler mice

Reelin-deficient mice have been used to investigate the role of this extracellular protein in cortico-striatal plasticity and striatum-related behaviours. Here we show that a repetitive electrical stimulation of the cortico-striatal pathway elicited long-term potentiation (LTP) in homozygous reeler (rl/rl) mice, while causing long-term depression in their wild-type (+/+) littermates. The N-methyl-D-aspartic acid (NMDA) receptor antagonist D-(-)-2 amino-5-phosphonopentanoic acid prevented the induction of LTP in (rl/rl) mice, thus confirming that this form of synaptic plasticity was NMDA receptor-dependent. Interestingly, in the presence of tiagabine, a blocker of gamma-aminobutyric acid (GABA) re-uptake system, the probability that (rl/rl) mice showed LTP decreased significantly, thus suggesting an impaired GABAergic transmission in reeler mutants. Consistent with this view, a decreased density of parvalbumin-positive GABAergic striatal interneurons was found in (rl/rl) mice in comparison to (+/+) mice. Finally, compatible with their abnormal striatal function (rl/rl) mice exhibited procedural learning deficits. Our data, showing alterations in cortico-striatal plasticity largely depending on a depressed GABAergic tone, delineate a mechanism whereby the lack of reelin may affect cognitive functions.

Preserved fronto-striatal plasticity and enhanced procedural learning in a transgenic mouse model of Alzheimer's disease overexpressing mutant hAPPswe

Mutations in the amyloid precursor protein (APP) gene inducing abnormal processing and deposition of beta-amyloid protein in the brain have been implicated in the pathogenesis of Alzheimer's disease (AD). Although Tg2576 mice with the Swedish mutation (hAPPswe) exhibit age-related Abeta-plaque formation in brain regions like the hippocampus, the amygdala, and the cortex, these mice show a rather specific deficit in hippocampal-dependent learning and memory tasks. In view of recent findings showing that neural systems subserving different forms of learning are not simply independent but that depressing or enhancing one system affects learning in another system, we decided to investigate fronto-striatal synaptic plasticity and related procedural learning in these mutants. Fronto-striatal long-term depression (LTD) induced by tetanic stimulation of the cortico-striatal input was similar in Tg2576 and wild-type control mice. Behavioral data, however, pointed to an enhancement of procedural learning in the mutants that showed robust motor-based learning in the cross maze and higher active avoidance scores. Thus, in this mouse model of AD, an intact striatal function associated with an impaired hippocampal function seems to provide neural conditions favorable to procedural learning. Our results suggest that focusing on preserved or enhanced forms of learning in AD patients might be of interest to describe the functional reorganization of the brain when one memory system is selectively compromised by neurological disease.

Altered long-term corticostriatal synaptic plasticity in transgenic mice overexpressing human CU/ZN superoxide dismutase (GLY(93)-->ALA) mutation

Apart from the extensive loss of motor neurons, degeneration of midbrain dopaminergic cells has been described in both familial and sporadic forms of amyotrophic lateral sclerosis (ALS). Mice overexpressing the mutant human Cu/Zn superoxide dismutase (SOD1) show an ALS-like phenotype in that they show a progressive death of motor neurons accompanied by degeneration of dopaminergic cells. To describe the functional alterations specifically associated with this dopaminergic dysfunction, we have investigated the corticostriatal synaptic plasticity in mice overexpressing the human SOD1 (SOD1+) and the mutated (Gly(93)-->Ala) form (G93A+) of the same enzyme. We show that repetitive stimulation of the corticostriatal pathway generates long-term depression (LTD) in SOD1+ mice and in control (G93A-/SOD1-) animals, whereas in G93A+ mice the same stimulation generates an N-methyl-D-aspartic acid receptor-dependent long-term potentiation. No significant alterations were found in the intrinsic membrane properties of striatal medium spiny neurons and basal corticostriatal synaptic transmission of G93A+ mice. Bath perfusion of dopamine or the D(2) dopamine receptor agonist quinpirole restored LTD in G93A+ mice. Consistent with these in vitro results, habituation of locomotor activity and striatal-dependent active avoidance learning were impaired in G93A+ mice. Thus, degeneration of dopaminergic neurons in the substantia nigra of G93A+ mice causes substantial modifications in striatal synaptic plasticity and related behaviors, and may be a cellular substrate of the extrapyramidal motor and cognitive disorders observed in familial and sporadic ALS.

Extended amygdala and basal forebrain

The basal forebrain is a confluence of systems that are crucial to understanding some of the most important functions of the brain, including reward and punishment, learning and cognition, and feeding and reproduction. Basic to understanding this broad spectrum of behavior is untangling the interwoven functional systems in basal forebrain. This has been grounded by the appreciation that the major nearby structures, that is, amygdala and basal ganglia, provide a context for interpreting basal forebrain areas that are best viewed as extensions of either of these larger regions. The components of basal forebrain, the ventral striatopallidal system and the medial and central divisions of extended amygdala, are subcortical relays for information garnered from brain stem, thalamus, and cortical areas. With respect to the classically defined amygdala of the temporal lobe, the lateral-basolateral complex, and the superficial amygdaloid nuclei may tentatively be viewed as specialized cortical regions. Their output targets both the striatopallidal complex and the extended amygdala, with some of the most massive basal forebrain efferents originating in the basolateral amygdaloid complex. The subcortical projections of the basolateral nucleus, at least in the rat, appear to be dichotomous, with anterior (or magnocellular) portions of the nucleus preferentially targeting striatum and ventral striatum (including the core of the nucleus accumbens), while the posterior (small-celled) portions of the basolateral nucleus target the extended amygdala as well as the shell of the nucleus accumbens. This divergence represents a particular opportunity for behavioral neuroscientists analyzing basal forebrain functions. Studies exploiting the dual subcortical projection of basolateral amygdala indicate distinct functional roles for striatum versus extended amygdala. These reinforce the identification of extended amygdala as a functional-anatomical entity distinct from the striatopallidal system.

Comparison of intracranial infusions of colchicine and ibotenic acid as models of neurodegeneration in the basal forebrain

Colchicine and ibotenic acid were compared for their ability to produce neurodegeneration and cognitive deficit after bilateral infusions into the nucleus basalis magnocellularis of male Long-Evans rats. Four weeks post-lesion, there was no difference in locomotor activity following infusion of either neurotoxicant or vehicle. In a passive avoidance task, both treated groups had significantly shorter step-through latencies compared with vehicle. Five weeks post-lesion, rats were killed for neurochemistry or histochemistry. Choline acetyltransferase (ChAT) activity in both the frontal and parietal cortex was significantly decreased (25-35%) in the colchicine- and ibotenic acid-infused rats when compared to control. There was no effect of either neurotoxicant on ChAT activity in the hippocampus or striatum. Both neurotoxicants produced damage in the general area of the ventromedial pallidum, although ibotenic acid infusion consistently produced a larger area of damage as assessed in Nissl-stained sections. Analysis of the number of ChAT-immunoreactive cells in the nucleus basalis magnocellularis (NBM) showed an average 60% cell loss following colchicine infusion and a 75% cell loss after ibotenic acid infusion. Area of glutamic acid decarboxylase (GAD) staining was significantly decreased in several regions surrounding the NBM for ibotenic acid (51% average decrease), and showed non-significant decreases (28%) following colchicine infusion. Colchicine infusion decreased dopamine and 3,4-dihydroxyphenylacetic acid (DOPAC) in the striatum; ibotenic acid had no effect on brain catechol of indoleamine levels. The results indicate that although similar cholinergic hypofunction and behavioral deficits were achieved, several non-cholinergic differences between the neurotoxicants were detected.

The interaction of L-deprenyl and scopolamine on spatial learning/memory in rats

L-Deprenyl, a specific MAO-B inhibitor, has been reported to improve learning/memory in some cognitive tests in aged rats. The present study investigated whether L-deprenyl could alleviate the spatial learning deficit induced by muscarinic blockade and aging in OFA rats. Scopolamine (0.25 mg/kg) impaired the acquisition of a water maze task in adult rats and increased their swimming speeds. L-Deprenyl (0.25 mg/kg, 14 days) had no effect on water maze performance in saline treated adult rats, but markedly alleviated the learning deficit induced by scopolamine and increased the time and distance of swimming in the training quadrant when the platform was removed (spatial probe trial). L-Deprenyl partly reduced the effect of scopolamine on speed of swimming. Nevertheless, administration of l-deprenyl (0.25 mg/kg, 14 days) had no effect on spatial learning/memory in aged rats. We suggest that the l-deprenyl-scopolamine interaction in the water maze test may be considered as a premise for further investigations of l-deprenyl as cognition enhancer.

Brain systems and long-term memory

This paper focuses mainly on those findings derived from lesion studies on the rat which help to identify ensembles of neural structures concerned with the expression of previously learned responses. At the outset, the use of the lesion method in the search for those neurological circuits underlying memory is defended. This is followed by an evaluation of neocortical and subcortical systems in long-term memory. Subsequently, a modest list of tentative functional neural "complexes" involved in the maintenance of certain classes of learned responses is given, based largely upon the author's own research. It is concluded that the key to the understanding of the neurological substrates of long-term memory lies in the identification of those subcortical sites which interact with neocortical sites in the performance of complex learned tasks. The most likely subcortical sites involved in this interaction appear to inhabit the regions of the basal ganglia, limbic midbrain area, and ventral portions of the brainstem reticular formation.

GABA and hypothalamic feeding systems. II. A comparison of GABA, glycine and actylcholine agonists and their antagonists

Microinjections of various compounds into the paraventricular hypothalamic nucleus (PVH) were made and the effects on feeding observed. During the light phase of the lighting cycle, injections of 0.3 microliter of muscimol (100 ng) and flurazepam diHCl (20 micrograms) increased feeding. Similar injections of glycine (500 ng) did not influence feeding during the light phase. During the dark phase, 0.3 microliter injections of bicuculline methiodide (30 ng) and picrotoxin (160 ng) suppressed feeding. Similar i-jections of carbachol increased drinking during the dark phase. Injections of strychnine during this phase were without effect. Tilt box activity levels were not altered by injection of picrotoxin (160 ng) into the PVH.

Effect of brain lesions on rat shuttle behavior in four different tests

Rats were trained to perform shuttle responses to a buzzer in four different situations: pseudoconditioning or D test (buzzers and footshocks presented at random), classical conditioning or DP test (buzzers and footshocks paired on every trial), avoidance without stimulus pairing or DC test (buzzer-shock intervals varied at random, shocks contingent upon non-emission of a shuttle response to the preceding buzzer), and standard two-way avoidance or DPC test (buzzers paired to shocks, but the latter omitted every time there was shuttling to the buzzer). The letters in each test disignate the factors involved in the emission of responses in each one, which were shown in previous papers to be: a non-associative factor or "drive" (D), the Pavlovian or stimulus-stimulus relation ("pairing", P), and the shuttle-no shock or main avoidance contingency (C). The effects of various brain lesions on these behaviors were studied. Ventral caudate and amygdala lesions depress both the Pavlovian (P) and the avoidance (C) component. Dorsal caudate lesions have an opposite influence on these two factors. Septal (n.medialis + lateralis, and n.accumbens) and tuberculum olfactorium lesions enhance the non-associative component (D); accumbens lesions, in addition, impair operation of the C factor. The effect of the diverse lesions on jumping responses to the buzzer or on performance of intertrial crossings does not correlate with the effect on shuttle responses.

Conditioning compensates the neglect due to unilateral 6-OHDA lesions of substantia nigra in rats

The degree of the contralateral sensory neglect in rats with unilateral 6-OHDA lesions of substantia nigra was assessed by a conditioning procedure, employing lateralized CS. In the first experiment visual neglect (revealed by failure of the visually elicited placing reaction contralateral to the lesion) was shown to be accompanied by slower acquisition of a brightness discrimination task. The impairment was due to ipsilateral turning tendency rather then to visual deficit, however, since monocular relearning yielded equal savings with the ipsilateral and contralateral eyes. The second experiment showed that rats anesthetized with urethane reacted to noxious skin stimulation contralateral to the lesion with shorter-lasting EEG arousal than to ipsilateral stimulation of the same intensity. The electrophysiological asymmetry could be compensated by classical conditioning, i.e. by pairing habituated tactile stimuli with noxious tail shock. The conditioned arousal reaction could be elicited with the same efficiency from the neglected and intact body surface. It is concluded that neglect is due neither to a sensory nor to a motor failure, but that 6-OHDA lesions of substantia nigra in one hemisphere reduce the arousing efficiency of unconditioned stimuli and interfere with sensorimotor integrating mechanisms on the side contralateral to the lesion. Compensation of the neglect by conditioning indicates that the role of the nigrostriatal system can be partly substituted by other circuits.

GABA stimulation and blockade in the hypothalamus and midbrain: effects on feeding and locomotor activity

Microinjections of the gamma-aminobutyric acid (GABA) antagonist, bicuculline methiodide (BM) (100 ng), into the anterolateral hypothalamus (LH) increased ingestion of sweet milk. A subsequent injection of BM 48 hrs. later produced a type of kindling effect consisting of feeding related automatisms, such as gnawing and biting. The behavioral effects of injections of 100 ng of GABA into the LH were variable. GABA injections into the ventromedial hypothalamus (VMH) reliably increased food intake. GABA injections into the origin of the nigrostriatal dopamine (DA) neurons in the substantia nigra (SN) suppressed it. Similar injections into the origin of the mesolimbic DA cells in the ventral tegmental area (VTA) had no effect on feeding behavior. Following BM injections into the SN, a moderate increase in tilt box activity was observed. A second injection of the GABA blocker 6 days later exaggerated this effect. Short latency extreme hyperactivation was accompanied by unidirectional barrel rolling which persisted until blocked by local injections of GABA.