Effects of DSP-4 on monoamine and monoamine metabolite levels and on beta adrenoceptor binding kinetics in rat brain at different times after administration

Effects of DSP-4 on noradrenaline (NA), 3-methoxy-4-hydroxyphenyl glycol (MHPG), serotonin (5-HT) and 5-hydroxyindole acetic acid (5-HIAA) levels and on beta adrenoceptor binding kinetics (Bmax and K_D) in rat hippocampus, cortex and hypothalamus were studied between 24 hours and 14 days after systemic administration. Beta adrenoceptor numbers in hippocampus and cortex, but not in hypothalamus, were significantly increased after DSP-4. No significant changes in K_D values were observed in hypothalamus, but significant increases in this parameter were measured in hippocampus and cortex. NA and MHPG levels were significantly decreased in all three brain regions, but MHPG/NA ratios were increased in hippocampus, decreased in cortex and unchanged in hypothalamus. Very prominent increases in 5-HIAA levels were observed in all three brain regions, but only at one day after DSP-4. The greatest increases in 5-HIAA levels occurred in the hippocampus, but this effect of DPS-4 appeared to be slightly diminished by pre-treatment with fluoxetine. In cortex and hippocampus 5-HT levels were slightly, but significantly decreased after DSP-4.     

Aspects of early postnatal development of cortical neurons that proceed independently of normally present extrinsic influences

To examine the contribution of local versus extrinsic influences on postnatal development of cortical neurons, we compared the maturation of deep (infragranular) layer neurons in isolated slices of neocortex grown in organotypic culture to a similar population of neurons developing in vivo. All slice cultures were prepared from sensorimotor cortices of newborn mice (P0) and neurons in these cultures were examined at daily intervals during the first 9 days in vitro (DIV). The maturational state of neurons developing in vivo over this same time period was assessed in acute slices prepared from animals of equivalent postnatal age, P1–P9. Electrophysiological recordings were obtained from neurons in both cultured and acute slices, using Lucifer yellow filled whole-cell recording electrodes, enabling subsequent morphometric analysis of the labeled cells. We report significant changes in both cellular morphology and electrical membrane properties of these deep layer cortical neurons during the frist week in culture. Morphological maturation over this time period was characterized by a two- to three-fold increase in cell body size and total process length, and an increase in dendritic complexity. In this same population of cells a three-fold decrease in input resistance and changes in the action potential waveform, including a two-fold decrease in the AP duration, also occur. The degree of morphological and electrophysiological differentiation of individual neurons was highly correlated across developmental ages, suggesting that the maturational state of a cell is reflected in both cellular morphology and intrinsic membrane properties. A remarkably similar pattern of neuronal maturation was observed in neurons in layers V, VI/SP examined in acute slices prepared from animals between P1–P9. Because our culture system preserves many aspects of the local cortical environment while eliminating normal extrinsic influences (including thalamic, brainstem, and callosal connections), our findings argue that this early phase of neuronal differentiation, including the rate and extent of dendritic growth and development of AP waveform, results from instructive and/or permissive local influences, and appears to proceed independently of the many normally present extrinsic factors. © 1993 John Wiley & Sons, Inc.     

Iron Regulation in the Developing Rat Brain: Effect of In Utero Ethanol Exposure

Abstract: Fetal alcohol syndrome produces defects that parallel abnormalities associated with early iron deficiency. Hence, we examined the effects of prenatal exposure to ethanol on iron, transferrin, and ferritin concentrations. The subjects were the offspring of pregnant rats fed an ethanol-containing diet (Et), pair-fed an isocaloric control diet (Ct), or fed chow and water. The amounts of iron, transferrin, and ferritin were assessed in three CNS regions (cerebral cortex, subcortical forebrain, and brain-stem). In all three segments of the control rats, iron, transferrin, and ferritin levels decreased during the first 2 postnatal weeks, reached a minimum during week 3, and then rose to adult levels. This pattern was delayed by ethanol treatment, e.g., the minimal concentrations in iron, transferrin, and ferritin in the Et-treated rats were achieved later (3 days, 7 days, and 2 weeks, respectively) than they were in the Ct-treated rats. Ethanol-induced alterations in iron homeostasis persisted into adulthood; iron concentration was reduced, transferrin concentration was unaffected, and ferritin concentration was increased. The net result was that the timely delivery and bioavailability of iron were compromised by ethanol exposure. The defects in iron regulation are permanent and may underlie ethanol-induced abnormalities in iron-dependent growth processes such as myelination.     

Serotonergic Regulation of Acetylcholine Release in Rat Frontal Cortex

Abstract: The extent to which serotonin regulates the activity of cortically projecting cholinergic neurons was studied using in vivo microdialysis to monitor interstitial concentrations of acetylcholine in the frontal cortex of freely moving rats. Systemic administration of the serotonin release-inducing agent fenfluramine (3 or 10 mg/kg, i.p.) increased acetylcholine release by 110–130%. The fenfluramine-induced increase in acetylcholine release was significantly attenuated by pretreatment with the selective serotonin uptake inhibitor fluoxetine (10 mg/kg, i.p.). Pretreatment with the selective dopamine D₁ receptor antagonist SCH-23390 (0.3 mg/kg, s.c.) failed to prevent the fenfluramine-induced increase in acetylcholine release. In contrast, the serotonin 5-HT_2A receptor antagonist ketanserin (5 mg/kg, i.p.) blocked fenfluramine-induced increases in acetylcholine release. In contrast to previous studies that have concluded that serotonin has inhibitory actions on cortical acetylcholine release, the present results indicate that fenfluramine increases cortical acetylcholine release in vivo by its ability to enhance serotonin transmission and that serotonin produces these effects at least in part via actions at serotonin 5-HT_2A receptors.     

Traveling Waves and the Processing of Weakly Tuned Inputs in a Cortical Network Module

Recent studies have shown that local cortical feedback can havean important effect on the response of neurons in primary visualcortex to the orientation of visual stimuli. In this work, westudy the role of the cortical feedback in shaping thespatiotemporal patterns of activity in cortex. Two questionsare addressed: one, what are the limitations on the ability ofcortical neurons to lock their activity to rotatingoriented stimuli within a single receptive field? Two, can thelocal architecture of visual cortex lead to the generation ofspontaneous traveling pulses of activity? We study theseissues analytically by a population-dynamic model of ahypercolumn in visual cortex. The order parameter thatdescribes the macroscopic behavior of the network is thetime-dependent population vector of the network. We firststudy the network dynamics under the influence of a weakly tunedinput that slowly rotates within the receptive field. We showthat if the cortical interactions have strong spatialmodulation, the network generates a sharply tuned activityprofile that propagates across the hypercolumn in a path thatis completely locked to the stimulus rotation. The resultantrotating population vector maintains a constant angular lagrelative to the stimulus, the magnitude of which grows with thestimulus rotation frequency. Beyond a critical frequency thepopulation vector does not lock to the stimulus but executes aquasi-periodic motion with an average frequency that is smallerthan that of the stimulus. In the second part we consider thestable intrinsic state of the cortex under the influence of isotropic stimulation. We show that if the local inhibitoryfeedback is sufficiently strong, the network does not settleinto a stationary state but develops spontaneous travelingpulses of activity. Unlike recent models of wave propagation incortical networks, the connectivity pattern in our model isspatially symmetric, hence the direction of propagation ofthese waves is arbitrary. The interaction of these waves withan external-oriented stimulus is studied. It is shown that thesystem can lock to a weakly tuned rotating stimulus if thestimulus frequency is close to the frequency of the intrinsic wave.     

Cortical control of reaching movements

Recent studies provide further support for the hypothesis that spatial representations of limb position, target locations, and potential motor actions are expressed in the neuronal activity in parietal cortex. In contrast, precentral cortical activity more strongly expresses processes involved in the selection and execution of motor actions. As a general conceptual framework, these processes may be interpreted in terms of such formalisms as sensorimotor transformation and ‘internal models’.     

Cyclic AMP-Mediated Enhancement of High-Affinity Choline Transport and Acetylcholine Synthesis in Brain

Abstract: Intracerebroventricular administration of N⁶, 2′-O-dibutyryladenosine 3′,5′-cyclic monophosphate (db-cyclic AMP) to mice increased high-affinity choline transport (HAChT) into synaptosomal preparations from the hippocampus, striatum, and frontal cortex in a time-dose-, and brain region-dependent manner. Similar observations were made when the cyclic AMP analogue 8-bromo-cyclic AMP, the adenylyl cyclase activator forskolin, and the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine were administered. Inhibition of phosphatase 1 and 2A, with okadaic acid, increased basal choline transport and enhanced the response to db-cyclic AMP. The early increase of HAChT activity induced by db-cyclic AMP was blocked by H-7 and H-89, protein kinase A inhibitors, but not by cycloheximide, a protein synthesis inhibitor. Kinetic analysis of the early changes of HAChT revealed an increase in the apparent V_max without a change of the K_m for choline. Hemicholinium-3 (HC-3) binding was not altered when studied 1 h after db-cyclic AMP administration. In contrast, HC-3 binding and HAChT activity were both elevated when estimated 3 h after the treatment, and pretreatment with cycloheximide partially prevented the db-cyclic AMP-induced HAChT rise. As evidence that enhanced HAChT is associated with a direct action of cyclic AMP-dependent pathways on the cholinergic nerve terminals, addition of 8-bromocyclic AMP to isolated hippocampal synaptosomes induced an increase of HAChT that was prevented by H-89. Choline acetyltransferase activity was not affected at any time during the studies. The synthesis of acetylcholine, however, was enhanced 1 h after db-cyclic AMP addition. Our studies show that cyclic AMP-mimetic compounds appear to modulate the choline carrier by a dual mode: an early increase of the maximal velocity without a change of the number of HC-3 binding sites and a late rise of transport that is accompanied by an increase of HC-3 binding. We postulate that HAChT and consequently acetylcholine synthesis in vivo is modulated, in part, by protein kinase A.     

Alterations in Glutamate Transporter Protein Levels in Kindling-Induced Epilepsy

Abstract: There is increasing evidence that levels of glutamate are elevated in certain brain regions immediately prior to and during induction and propagation of seizures. Modulation of high-affinity glutamate uptake is a potential mechanism responsible for the elevated levels observed with seizures. To date, three distinct Na⁺-dependent glutamate transporters have been cloned from rat and rabbit: GLT-1, GLAST, and EAAC-1. We performed a series of experiments to determine whether levels of these transporters are altered in amygdala-kindled rats. Levels of GLT-1, GLAST, and EAAC-1 were examined in three brain regions (hippocampus, piriform cortex/amygdala, and limbic forebrain) by quantitative immunoblotting using subtype-specific antibodies. GLAST protein was down-regulated in the piriform cortex/amygdala region of kindled rats as early as 24 h after one stage 3 seizure and persisting through multiple stage 5 seizures. In contrast, kindling induced an increase in EAAC-1 levels in piriform cortex/amygdala and hippocampus once the animals had reached the stage 5 level. No changes in GLT-1 were observed in any region examined. Changes in transporter levels could contribute to the changes in glutamate levels seen with kindling.