Increase of Extracellular Glutamate and Expression of Fos-Like Immunoreactivity in the Ventral Tegmental Area in Response to Electrical Stimulation of the Prefrontal Cortex

Abstract: Electrical stimulation of the medial prefrontal cortex caused glutamate release in the ventral tegmental area (VTA) of freely moving animals. Cathodal stimulation was given through monopolar electrodes in 0.1-ms pulses at an intensity of 300 µA and frequencies of 4–120 Hz. Glutamate was measured in 10-min perfusate samples by HPLC coupled with fluorescence detection following precolumn derivatization with o -phthaldialdehyde/β-mercaptoethanol. The stimulation-induced glutamate release was frequency dependent and was blocked by the infusion of the sodium channel blocker tetrodotoxin (10 µ M ) through the dialysis probe. The stimulation also induced bilateral Fos-like immunoreactivity in ventral tegmental neurons, with a significantly greater number of Fos-positive cells on the stimulated side. These findings add to a growing body of evidence suggesting that the medial prefrontal cortex regulates dopamine release in the nucleus accumbens via its projection to dopamine cell bodies in the VTA.     

Differential Regulation of GABAA Receptor Gene Expression by Ethanol in the Rat Hippocampus Versus Cerebral Cortex

Abstract: Previous research has shown that chronic ethanol consumption dramatically alters GABA_A receptor α₁ and α₄ subunit gene expression in the cerebral cortex and GABA_A receptor α₁ and α₆ subunit gene expression in the cerebellum. However, it is not yet known if chronic ethanol consumption produces similar alterations in GABA_A receptor gene expression in other brain regions. One brain region of interest is the hippocampus because it has recently been shown that a subset of GABA_A receptors in the hippocampus is responsive to pharmacologically relevant concentrations of ethanol. Therefore, we directly compared the effects of chronic ethanol consumption on GABA_A receptor subunit gene expression in the hippocampus and cerebral cortex. Furthermore, we investigated whether the duration of ethanol consumption (14 or 40 days) would influence regulation of GABA_A receptor gene expression in these two brain regions. Chronic ethanol consumption produced a significant increase in the level of GABA_A receptor α₄ subunit peptide in the hippocampus following 40 days but not 14 days. The relative expression of hippocampal GABA_A receptor α₁, α₂, α₃, α_2/3, or γ₂ was not altered by either period of chronic ethanol exposure. In marked contrast, chronic ethanol consumption for 40 days significantly increased the relative expression of cerebral cortical GABA_A receptor α₄ subunits and significantly decreased the relative expression of cerebral cortical GABA_A receptor α₁ subunits. This finding is consistent with previous results following 14 days of chronic ethanol consumption. Hence, chronic ethanol consumption alters GABA_A receptor gene expression in the hippocampus but in a different manner from that in either the cerebral cortex or the cerebellum. Furthermore, these alterations are dependent on the duration of ethanol exposure.     

Serotonin Metabolism and Release in Frontal Cortex of Rats on a Vitamin E-Deficient Diet

Abstract: Rats were fed a control or vitamin E (all- rac -α-tocopheryl acetate)-deficient diet for 3 or 12 weeks. Serotonin (5-HT), 5-hydroxyindoleacetic acid (5-HIAA), tryptophan, and α-tocopherol concentrations were determined in the frontal cortex using HPLC. α-Tocopherol concentrations fell significantly to 27% of control values at 12 weeks. Tissue 5-HT, 5-HIAA, and tryptophan concentrations were not significantly altered by the vitamin E-deficient diet at either time point. In vivo microdialysis revealed normal basal and K⁺-stimulated concentrations of 5-HT and 5-HIAA, but extracellular concentrations of tryptophan were significantly decreased after 3 weeks on the vitamin E-deficient diet, which resulted in an increase in the tissue/extracellular ratio and suggested a change in compartmentation. However, after 12 weeks on the deficient diet these values had returned to normal. Results in general indicate that a prolonged and substantial depletion of brain vitamin E can occur without major disturbance of serotonergic function.     

Blockade of FG 7142-Induced Increased Dopamine Utilization by the Glycine/NMDA Receptor Antagonist (+)-HA 966

Abstract: Systemic administration of the anxiogenic benzodiazepine inverse agonist FG 7142 has been shown to increase selectively dopamine utilization in the medial prefrontal cortex and the shell, but not core, subregion of the nucleus accumbens. In the present study, we examined the functional interaction between benzodiazepine and N -methyl- d -aspartate receptor influences on dopamine utilization in these areas. Male Sprague-Dawley rats were pretreated with the glycine receptor antagonist (+)-HA 966 (15 mg/kg, i.p.) or saline 15 min before FG 7142 (20 mg/kg, i.p.) or vehicle administration. Subjects were killed 30 min later and assayed for tissue concentrations of dopamine and its major metabolite 3,4-dihydroxyphenylacetic acid in the core and shell subdivisions of the nucleus accumbens and the medial prefrontal cortex. (+)-HA 966 administration blocked FG 7142-induced increased dopamine utilization in both the medial prefrontal cortex and the shell subdivision of the nucleus accumbens. Results are discussed in terms of N -methyl- d -aspartate receptor influences on the response of mesoaccumbal dopamine neurons to stress.     

Modulation of [3H]Muscimol Binding in Rat Cerebellar and Cerebral Cortical Membranes by Picrotoxin, Pentobarbitone, and Etomidate

Modulation of [3H]muscimol binding by picrotoxin, pentobarbitone, and etomidate was investigated in rat cerebellar and cerebral cortical membranes. In cerebellum, at 37 degrees C in the presence of chloride ions (150 mM), picrotoxin and picrotoxinin decreased specific [3H]muscimol binding to 43 +/- 3% of control, with an EC50 of 1.2 +/- 0.1 microM. [3H]Muscimol saturation experiments in the presence and absence of picrotoxin indicated that the picrotoxin effect was primarily due to a loss of high-affinity muscimol sites with KD approximately equal to 10 nM. Pentobarbitone enhanced specific [3H]muscimol binding to 259 +/- 3% of control, with EC50 = 292 +/- 37 microM, and etomidate increased binding to 298 +/- 18%, with EC50 = 7.1 +/- 1.0 microM. The influence of temperature and chloride ion concentration on these effects was investigated by comparing experiments at 37 and 0 degrees C in the presence or absence of chloride at constant ionic strength. The results indicate that studies at 0 degrees C underestimate the coupling between GABA receptors and barbiturate sites and that they greatly overestimate the importance of chloride ions in this phenomenon. In cerebral cortical membranes (37 degrees C, 150 mM Cl-), the effect of picrotoxin was similar to that observed in cerebellum, whereas the effects of pentobarbitone and etomidate were greater, but occurred at higher concentrations.     

Effects of Thyroxine on Methionine Adenosyltransferase Activity in Rat Cerebral Cortex and Cerebellum During Postnatal Development

Abstract: Methionine adenosyltransferase (MAT) activity was evaluated in cerebral cortex and cerebellum in controls and in rats treated with thyroxine. In controls the enzyme showed a different pattern in cerebral cortex and cerebellum during neonatal and late suckling periods. Hyperthyroid rats showed a significant increase of the enzyme in cerebral cortex only at the 2nd day of the neonatal period; in cerebellum the developmental pattern of MAT in neonatal period was anticipated temporally by 2–4 days. During the late suckling period thyroxine treatment produced in cerebellum a significant decrease in MAT activity at the 15th day after birth. From these data, we propose that hyperthyroidism may cause precocious induction of MAT both in cerebral cortex and in cerebellum and that the increased availability of S -adenosyll-methionine during the neonatal period could be related to its utilization also in polyamine biosynthesis.     

Effect of Lesions of the Olfactory Bulb on the Levels of Amino Acids and Related Enzymes in the Olfactory Cortex of the Guinea Pig

Abstract: Olfactory bulb removal and consequential degeneration of the lateral olfactory tract led to a decreasein the levels of glutaminase and malate dehydrogenase inthe ipsilateral olfactory cortex. These changes in enzyme activity may account for the well established decrease inthe levels of aspartate and glutamate in the olfactory cortex following ipsilateral bulbectomy. The level of glutamine synthetase, a glial marker enzyme, was slightly-increased while the activities of glutamate decarboxylase, glutamate dehydrogenase, and glutamate oxaloacetic transaminase were unchanged.     

Activation of Adenylyl Cyclase by Preincubation of Rat Cerebral-Cortical Membranes Under Phosphorylating Conditions: Role of ATP, GTP, and Divalent Cations

Abstract: The effects of preincubation under phosphorylating conditions on adenylyl cyclase activity were studied in preparations containing synaptic membranes from rat cerebral cortex. Preincubation of the membranes with 2 mM ATP and 10 mM MgCl₂ resulted in a 50% increase of adenylyl cyclase activity which withstood sedimentation and washing. This activation was maximal after 5 min of preincubation, was reversed after longer preincubations, and paralleled the time course of endogenous phosphorylation-dephosphorylation of proteins observed under these conditions. The activation showed a critical requirement for Mg²⁺ ions and was dependent on ATP concentration. Similar activation was observed after preincubation of cerebral-cortical membranes with adenosine-5′-0-(3-thiophosphate) (ATPγS), but this activation was not reversed by prolonged preincubation times. The activation by ATPγS was potentiated severalfold by including synaptoplasm in the preincubation. Further experiments indicated that the activity of nucleoside diphosphokinase, which converts ATPγS to guanosine-5′-0-(3-thiophosphate) (GTPγS), could account for this potentiation. Preincubation of washed membranes for 5 min with 10 μ.M GTP and 10 mM MgCl₂ also produced a 50% activation of adenylyl cyclase which withstood sedimentation and washing and was reversed by longer preincubations. Endogenous phosphorylation of specific protein components in the membranes during the preincubation was examined by including radioactively labeled nucleoside thiophosphates in the preincubation medium. Incorporation of ³⁵S from [³⁵S]ATPγS into a protein component with apparent M_r of 54,000 daltons (54K) correlated significantly with the activation of adenylyl cyclase by ATPγS. Thiophosphorylation of the 54K protein was potentiated by addition of GDP to reactions carried out with [³⁵S]ATPγS. Endogenous activity utilizing [γ-³²P]GTP as a phosphate donor also preferentially phosphorylated the 54K protein band. These results support previous suggestions that protein phosphorylation plays a role in the regulation of adenylyl cyclase activity. Among the numerous membrane-bound phosphoproteins in rat brain, we have identified a specific protein component with an apparent M_r of 54,000 daltons as the most likely candidate for involvement in this mode of regulation. This 54K protein, which is a principal substrate for a GTP-preferring protein kinase activity in brain membranes, can now be at the focus of investigations attempting to demonstrate a direct role for protein phosphorylation in adenylyl cyclase regulation.     

Morphological and functional correlates of VIP neurons in cerebral cortex

Vasoactive Intestinal Polypeptide (VIP) promotes the hydrolysis of ³H-glycogen newly synthesized from ³H-glucose by mouse cortical slices. This effect occurs rapidly, approximately 50% of the maximal effect being reached within one minute. The maximal effect is achieved after 5 minutes and maintained for at least 25 minutes. Furthermore the glycogenolytic effect of VIP is reversible, and pharmacologically specific. Thus several neuropeptides present in cerebral cortex such as cholecystokinin-8, somatostatin-28, somatostatin-14, met-enkephalin, leu-enkephalin, do not affect ³H-glycogen levels. VIP fragments 6–28, 16–28 and 21–28 are similarly inactive. Furthermore, among the peptides which share structural homologies with VIP, such as glucagon, secretin, PHI-27 and Gastric Inhibitory Peptide, only secretin and PHI-27 promote ³H-glycogen hydrolysis, with EC₅₀ of 500 and 300 nM respectively, compared to an EC₅₀ of 25 nM for VIP. Immunohistochemical observations indicate that each VIP-containing bipolar cell is identified with a unique radial cortical volume, which is generally between 15–60 μm in diameter and overlaps with the contiguous domains of neighbouring VIP-containing bipolar cells. Thus this set of biochemical and morphological observations support the notion that VIP neurons have the capacity to regulate the availability of energy substrates in cerebral cortex locally, within circumscribed, contiguous, radial domains.