Competitive Inhibition of Rat Brain Hexokinase by 2-Deoxyglucose, Glucosamine, and Metrizamide

Abstract: The effects of metrizamide on the kinetics of rat brain hexokinase were compared in vitro with those of 2-deoxyglucose and glucosamine. Although metrizamide, 2-deoxyglucose, and glucosamine are known to be competitive inhibitors of approximately equal potency for glucose of yeast hexokinase ( K ₁ approximately 0.7 m m for all three), metrizamide is a much weaker competitive inhibitor ( K _i about 20 m m ) of rat brain hexokinase than either 2-deoxyglucose or glucosamine ( K _i about 0.3 m m for both). This indicates a greater active site specificity of rat brain hexokinase than of yeast hexokinase. Rat brain hexokinase activity is enhanced approximately threefold in the presence of 0.05, 0.2, and 0.8 mg/ml bovine serum albumin, while yeast hexokinase is only enhanced by 50% under these conditions. Despite the high K _i value for metrizamide, interference with glucose metabolism may occur whenever metrizamide is present in much higher concentrations than glucose. Myelography in humans is one such situation.     

Local Cerebral Glucose Utilization During Development and Aging of the Fischer-344 Rat

Abstract: Local cerebral glucose utilization was measured in brain regions of awake Fischer-344 rats. Measurements were taken in 15 regions of 1-month-old rats, and 19 regions of 3-, 12-, 24-, and 34-month-old rats. Between 1 and 3 months, glucose utilization tended to increase in all brain regions; statistically significant increases occurred in seven regions. Between the ages of 3 and 12 months, glucose utilization decreased significantly in 12 regions. The greatest reductions (25% or more) occurred in the striatum, inferior colliculus, and pons, but the hypothalamus and thalamus, nucleus accumbens, and septum showed no statistically significant change. Cerebral glucose utilization did not change between 12 and 24 months or between 24 and 34 months of age. The results demonstrate a rise in cerebral glucose utilization with development from 1 to 3 months, a decline between 3 and 12 months, and a constancy in the second and third years that does not reflect reported senescence-associated neurochemical and morphological cerebral changes.     

Brain Slice Glucose Utilization

The metabolism of 2-deoxyglucose has been studied in 540 micron and 1,000 micron hypothalamic brain slices. Slice 2-deoxyglucose (2DG) and 2-deoxyglucose-6-phosphate (2DG6P) levels were measured after tissue homogenization and perchloric acid extraction. By analyzing the uptake and washout kinetics with nonlinear least-squares methods, we have determined the rate constants for three-, four-, or five-parameter kinetic models and obtained a value for the in vitro lumped constant (LC). The kinetic analysis reveals a small, slowly decaying, 2DG component that is not predicted by any of the models. If this component is treated as a separate, parallel compartment, then the four- and five-parameter models are essentially equivalent. To compare our data to prior in vivo data, we combined 2DG and 2DG6P to produce Ci*, the total slice radioactivity, and analyzed the first 45 min of uptake. These data were fit best by a three-parameter model and the slowly decaying pool was not identified. Calculation of glucose utilization from total tissue radioactivity, measured by whole slice homogenization and by image analysis of autoradiograms, showed excellent correlation between the two methods. Image analysis of radioactivity in the suprachiasmatic nucleus, which is present in these slices, revealed a spontaneous diurnal variation in in vitro glucose utilization in close quantitative agreement with prior in vivo measurements. The kinetic analysis of the 1,000 micron slice was qualitatively similar to that of the 540 micron slice but revealed an increase in the LC and a large decrease in k1 as well as the expected large increase in the hexokinase rate constant, k3. Overall, in vitro glucose utilization increased by about 60%. These results are consistent with our prior studies of the 1,000 micron slice and support our interpretation that the 1,000 micron slice is an excellent in vitro model for brain ischemia without infarction.     

Kinetics of Regional Blood–Brain Barrier Transport and Brain Phosphorylation of Glucose and 2-Deoxyglucose in the Barbiturate–Anesthetized Rat

Abstract: Recent studies indicate the lumped constant (LC), which defines the relative rates of brain utilization of glucose and 2-deoxyglucose (2-DG), doubles to values > 1.0 under conditions of hypoglycemia. Since changes in the LC should be predictable given the kinetic parameters of blood-brain barrier (BBB) transport and brain phosphorylation of glucose and 2-DG, the present studies were designed to measure the necessary kinetic parameters. The carotid injection technique was used to determine cerebral blood flow and the K_m , V_max, and K _D of glucose and 2-DG transport through the BBB in seven brain regions in rats anesthetized with 50 mg/kg i.p. pentobarbital. Regional glucose transport through the BBB was characterized by an average K_m = 6.3 m m , average V_max = 0.53 μmol min⁻¹g⁻¹, and average K _D= 0.022 ml min⁻¹g⁻¹. The nonsaturable route of transport of glucose represented on the average 40% of the total glucose influx into brain regions at an arterial glucose concentration of 10 m m . In addition, the rate constants of phosphorylation of glucose and 2-DG were measured for each region. Substitutions of the measured kinetic parameters for sugar transport and phosphorylation into equations defining the LC confirm the observation that the LC would be expected to vary under extreme conditions such as hypoglycemia and to exceed values of 1.0 under these conditions.     

L-Glutamate and Insulin Enhance Glycogen Synthesis in Cultured Astrocytes from the Rat Brain Through Different Intracellular Mechanisms

The effects of L-glutamate and insulin on glycogen synthesis in astrocytes were examined. L-Glutamate and insulin both stimulated glycogen synthesis in primary cultures of rat astrocytes in a dose-dependent manner, as measured by the incorporation of 14C from [14C]glucose into glycogen. D-Aspartate also increased the incorporation of 14C into glycogen. When insulin and L-glutamate were added together, the glycogen synthesis as well as glycogen content of the cells was additively increased. Wortmannin, an inhibitor of phosphatidylinositol 3-kinase, had little effect on glycogen synthesis induced by L-glutamate, whereas it suppressed the insulin-induced glycogen synthesis. These results suggest that the insulin- and L-glutamate-induced glycogen syntheses are mediated by different intracellular mechanisms. In fact, insulin stimulated the conversion of glycogen synthase b to glycogen synthase a, which was suppressed by wortmannin. L-Glutamate and D-aspartate, however, did not increase the level of glycogen synthase a activity. By contrast, L-glutamate increased 2-deoxy-D-[3H]glucose uptake by the astrocytes, whereas insulin did not affect the uptake. These results suggest that insulin stimulates glycogen synthesis in astrocytes by activating glycogen synthase, which is dependent on a wortmannin-sensitive signaling pathway. L-Glutamate, however, enhances the glucose uptake, which contributes to the increase in glycogen synthesis in the cells.     

In Vivo Microdialysis of 2-Deoxyglucose 6-Phosphate into Brain

Abstract : A unique method for simultaneously measuring interstitial (pH_e) as well as intracellular (pH_i) pH in the brains of lightly anesthetized rats is described. A 4-mm microdialysis probe was inserted acutely into the right frontal lobe in the center of the area sampled by a surface coil tuned for the collection of ³¹P-NMR spectra. 2-Deoxyglucose 6-phosphate (2-DG-6-P) was microdialyzed into the rat until a single NMR peak was detected in the phosphomonoester region of the ³¹P spectrum. pH_e and pH_i values were calculated from the chemical shift of 2-DG-6-P and inorganic phosphate, respectively, relative to the phosphocreatine peak. The average in vivo pH_e was 7.24 ± 0.01, whereas the average pH_i was 7.05 ± 0.01 (n = 7). The average pH_e value and the average CSF bicarbonate value (23.5 ± 0.1 mEq/L) were used to calculate an interstitial Pco₂ of 55 mm Hg. Rats were then subjected to a 15-min period of either hypercapnia, by addition of CO₂ (2.5, 5, or 10%) to the ventilator gases, or hypocapnia (Pco₂ < 30 mm Hg), by increasing the ventilation rate and volume. pH_e responded inversely to arterial Pco₂ and was well described (r² = 0.91) by the Henderson-Hassel-balch equation, assuming a pK_a for the bicarbonate buffer system of 6.1 and a solubility coefficient for CO₂ of 0.031. This confirms the view that the bicarbonate buffer system is dominant in the interstitial space. pH_i responded inversely and linearly to arterial Pco₂. The intracellular effect was muted as compared with pH_e (slope = -0.0025, r² = 0.60). pH_e and pH_i values were also monitored during the first 12 min of ischemia produced by cardiac arrest. pH_e decreases more rapidly than pH_i during the first 5 min of ischemia. After 12 min of ischemia, pH_e and pH_i values were not significantly different (6.44 ± 0.02 and 6.44 ± 0.03, respectively). The limitations, advantages, and future uses of the combined microdialysis/³¹P-NMR method for measurement of pH_e and pH_i are discussed.     

Optimal Duration of Experimental Period in Measurement of Local Cerebral Glucose Utilization with the Deoxyglucose Method

The time course and magnitude of the effects of product loss on the measurement of local cerebral glucose utilization (LCGU) by the 2-[14C]deoxyglucose (DG) method were studied by determination of LCGU in 38 rats with 25-120 min experimental periods after a [14C]DG pulse and in 45 rats with experimental periods of 2.5-120 min during which arterial plasma [14C]DG concentrations (C*P) were maintained constant. LCGU was calculated by the operational equation, which assumes no product loss, with the original set of rate constants and with a new set redetermined in the rats used in the present study; in each case the rate constants were those specific to the structure. Data on local tissue 14C concentrations and C*P were also plotted according to the multiple time/graphic evaluation technique ("Patlak Plot"). The results show that with both pulse and constant arterial inputs of [14C]DG the influence of the rate constants is critical early after onset of tracer administration but diminishes with time and becomes relatively minor by 30 min. After a [14C]DG pulse calculated LCGU remains constant between 25 and 45 min, indicating a negligible effect of product loss during that period; at 60 min it begins to fall and declines progressively with increasing time, indicating that product loss has become significant. When C*P is maintained constant, calculated LCGU does not change significantly over the full 120 min. The "Patlak Plots" reinforced the conclusions drawn from the time courses of calculated LCGU; evidence for loss of product was undetectable for at least 45 min after a pulse of [14C]DG and for at least 60 min after onset of a constant arterial input of [14C]DG.     

Changes in Brain Glycogen During Slow-Wave Sleep in the Rat

Abstract: During slow-wave sleep, rat brain glycogen increases within a few minutes to about 70% above waking levels. Upon awakening, the increment is lost within 2–5 min. After repeated episodes of sleep, brain glycogen levels are comparable to those observed after only a single episode of sleep. Liver glycogen is unaffected by slow-wave sleep.     

Measurement of Cerebral Glucose Utilization Using Washout After Carotid Injection in the Rat

Abstract: The carotid injection technique, used previously to quantitate the kinetics of blood-brain barrier transport of metabolic substrates, may be modified to analyze the rate of cerebral glucose utilization. A 0.2-ml solution of [¹⁴C]glucose (G_F) and [³H]methylglucose (M), an internal reference, is rapidly injected into the carotid artery, followed by microwave fixation of brain at various times up to 4 min after injection. The brain radioactivity is separated into a fraction containing neutral hexoses (G_F and M) and a fraction containing metabolites of glucose. The G_F/M ratio is related to the rate constant (k₃) of brain glucose utilization by the simple, linear equation: In(G_F/M) = In(G_F°/M°) –k₃t, where G_F°/M°= the brain uptake index of glucose, relative to methylglucose, at 5-15 s after injection, and t= the time after carotid injection, e.g., 1–4 min. It is assumed that (a) the rate of influx due to recirculation of label is minimal during the 4-min circulation period; and (b) the rate constants of glucose efflux (k₂) and methylglucose efflux (k₂*) are identical. Independent estimates of k₂ and k₂* showed these parameters to be identical: k₂= 0.14 + 0.08 min-I; k₂*= 0.14 ± 0.02 min-I. A logarithmic plot of G_F/M ratios versus time was linear (r = 0.99), and was described by the slope k₂= 0.21 ± 0.02 min^?1. Assuming glucose is uniformly distributed in brain, then the glycolytic rate = k₃× brain glucose = (0.21 min^?1) (2.6 μmol g^?1) = 0.55 μmol min^?1 g^?1 for the cortex of the barbiturate-anesthetized rat. These studies provide the basis for a simple method of measurement of regional brain glycolysis that does not require either the use of correction factors, e.g., the lumped constant, or the use of differentially labeled glucose.     

2-Deoxyglucose Incorporation in the Cerebellum of Weaver and Nervous Mutant Mice

Abstract: The 2-deoxyglucose autoradiographic method has been used to study activity in cerebellum of the weaver and nervous mutant mice. Patterns of 2-deoxyglucose incorporation into the cerebral hemispheres from weaver and nervous strains did not differ significantly from those of the controls. In the normal cerebellum, 2-deoxyglucose incorporation was maximal in the granular layer, where mossy fibers form synapses with the dendrites of granule cells. In the cerebellum of nervous mice, which lacks Purkinje cells, the incorporation of the 2-deoxyglucose was maximal in the granular layer, but the incorporation into the molecular layer appeared less than in the control. The incorporation into the cerebellum from weaver, which lacks granule cells, was much higher than that of the control, the maximal incorporation being found in the Purkinje cell layer and in cell masses located in the white matter. These data suggest that the heterologous synapses that mossy fibers or climbing fibers form with the cells in the Purkinje cell layer and the cells in the white matter in the weaver cerebellum are functional.     

Localization of Glycogen Synthase in Brain

Antisera against glycogen synthase from canine brain were prepared and used for investigation of the localization of the enzyme in the brain. Antisera cross-reacted only with the 88-kilodalton protein that is the subunit of brain glycogen synthase. Immunoreactivity of glycogen synthase was universally distributed in all regions of the brain, although hippocampus, cerebral cortex, caudatoputamen, and cerebellar cortex had relatively high immunoreactivity. Light microscopic examination revealed that the immunoreactivity was found in all cell types, such as neurons in several regions, astrocytes, ependymal cells surrounding the ventricle, oligodendrocytes, and epithelial cells of the choroid plexus in the ventricle. Immunoreactive intensity was more prominent in neurons than glial cells. Immunostaining may be a useful tool for investigation of the state of glycogen metabolism under normal and pathological conditions.     

Comparison of Freeze-Blowing and Funnel-Freezing of Rat Brain for the Measurement of Cerebral Glucose Concentration In Vivo

The efficacy of funnel-freezing of rat brain to inactivate metabolic processes and preserve in vivo tissue glucose concentration was validated by comparing the results obtained by funnel-freezing with those obtained with freeze-blowing of brain. The arterial plasma glucose level was clamped at 9 mM in halothane-anesthetized rats to produce identical glucose levels in brain tissue prior to freeze fixation. In funnel-frozen and freeze-blown brains, tissue glucose concentrations were 2.47 +/- 0.05 and 2.47 +/- 0.06 mumol/g (means +/- SEM), respectively. Lactate levels in funnel-frozen brains were slightly but significantly higher than those in freeze-blown brains, i.e., 1.56 +/- 0.05 mumol/g versus 1.30 +/- 0.05 mumol/g (means +/- SEM; p less than 0.05). Regional analysis in funnel-frozen brains revealed that glucose concentrations in superficial and basal brain areas remained approximately equal at 2.30 +/- 0.1 mumol/g and 2.31 +/- 0.09 mumol/g (means +/- SEM), respectively. Our findings indicate that in the anesthetized rat, funnel-freezing of brain is suitable for the measurement of regional in vivo glucose concentrations.     

The Effects of Apomorphine upon Local Cerebral Glucose Utilization in Conscious Rats and in Rats Anesthetized with Chloral Hydrate

The effects of the dopaminergic agonist apomorphine (1 mg . kg-1 i.v.) upon local cerebral glucose utilization in 43 anatomically discrete regions of the CNS were examined in conscious, lightly restrained rats and in rats anesthetized with chloral hydrate by means of the quantitative autoradiographic [14C]2-deoxyglucose technique. In animals anesthetized with chloral hydrate, glucose utilization was reduced throughout all regions of the CNS from the levels observed in conscious animals, although the magnitude of the reductions in glucose use displayed considerable regional heterogeneity. With chloral hydrate anesthesia, the proportionately most marked reductions in glucose use (by 40-60% from conscious levels) were noted in primary auditory nuclei, thalmaic relay nuclei, and neocortex, and the least pronounced reductions in glucose use (by 15-25% from conscious levels) were observed in limbic areas, some motor relay nuclei, and white matter. In conscious, lightly restrained rats, the administration of apomorphine (1 mg . kg-1) effected significant increased in glucose utilization in 15 regions of the CNS (e.g., subthalamic nucleus, ventral thalamic nucleus, rostral neocortex, substantia nigra, pars reticulata), and significant reductions in glucose utilization in two regions of the CNS (lateral habenular nucleus and anterior cingulate cortex). In rats anesthetized with chloral hydrate, the effects of apomorphine upon local glucose utilization were less widespread and less marked than in conscious animals. In only two of the regions (the globus pallidus and septal nucleus), which displayed increased glucose use following apomorphine in conscious rats, were significant increases in local glucose utilization observed with this agent in chloral hydrate-anesthetized rats. In the pars compacta of the substantia nigra, in which apomorphine increased glucose utilization in conscious animals, significant reductions in glucose utilization were observed following apomorphine in rats anesthetized with chloral hydrate. The profound effects of chloral hydrate anesthesia upon local cerebral glucose use, and the modification by this anesthetic regime of the local metabolic responses to apomorphine, emphasize the difficulties which exists in the extrapolation of data from anesthetized animals to the conditions which prevail in the conscious animal.     

Glutamate Increases Glycogen Content and Reduces Glucose Utilization in Primary Astrocyte Culture

The glycogen content of primary cultured astrocytes was approximately doubled by incubation with 1 mM L-glutamate or L-aspartate. Other amino acids and excitatory neurotransmitters were without effect. The increase in glycogen level was not blocked by the glutamate receptor antagonist kynurenic acid but was completely blocked by the glutamate uptake inhibitor threo-3-hydroxy-D,L-aspartate and by removal of Na+ from the medium. Incubation with radiolabeled glucose and glutamate revealed that the increased glycogen content was derived almost entirely from glucose. Glutamate at 1 mM was also found to cause a 53 +/- 12% decrease in glucose utilization and a 112 +/- 69% increase in glucose-6-phosphate levels. These results suggest that the glycogen content of astrocytes is linked to the rate of glucose utilization and that glucose utilization can, in turn, be affected by the availability of alternative metabolic substrates. These relationships suggest a mechanism by which brain glycogen accumulation occurs during decreased neuronal activity.     

Incorporation of Glycerol and Ethanolamine into Glycerophospholipid in Rat Brain Areas During Bicuculline-Induced Convulsive Seizures

The effect of bicuculline-induced convulsive seizures on lipid metabolism has been studied in four brain areas (cerebellum, cerebral cortex, hippocampus, and brainstem) using [2-3H]glycerol and [1,2-14C]ethanolamine as radioactive lipid precursors administered simultaneously with bicuculline. Twelve minutes after the administration, the uptake of radioactivity depended both on brain area and treatment, being generally higher in convulsing rats. The uptake of glycerol was influenced to a larger extent than that of ethanolamine and increased during convulsions, but its incorporation into lipids did not. In contrast, the amount of ethanolamine incorporated into lipids increased during bicuculline-induced seizures. The difference in behavior of glycerol and of ethanolamine is also indicated by the decrease of the 3H/14C ratio of phosphatidyl-ethanolamine in various brain areas during convulsions. It is, therefore, evident that the metabolism of the two precursors is affected differently by seizures.     

Regulation of Glycogen Content in Primary Astrocyte Culture: Effects of Glucose Analogues, Phenobarbital, and Methionine Sulfoximine

Compounds known to affect glycogen metabolism in vivo or in cell-free preparations were used to investigate the regulation of glycogen content in intact astrocytes cultured from newborn rat cortex. Compounds were added with fresh medium to culture dishes, and astrocyte glucose and glycogen content determined 24 h later. Increasing the medium glucose concentration from 7.5 mM to 30 mM increased cell glycogen content 80%. Addition of 2-deoxyglucose or 3-O-methyl glucose (2.5-10 mM) also increased cell glycogen content, 50-100%, suggesting a regulatory rather than mass action effect of glucose on astrocyte glycogen content. The phosphorylase b inhibitors 2,2',4,4',5,5'-hexabromobiphenyl and riboflavin had no effect on astrocyte glycogen content, consistent with negligible phosphorylase b activity in normal astrocytes. Phenobarbital and L-methionine-DL-sulfoximine (MSO) are both known to induce astrocyte glycogen accumulation in vivo. The addition of phenobarbital (2 mM) had no effect on the glycogen content of cultured astrocytes, suggesting an indirect mechanism for the in vivo effect. MSO at 1 mM, however, induced a 300% increase in glycogen content. The time course of glucose and glycogen content after MSO administration suggests this increase to be the result of slowed glycogenolysis rather than accelerated glycogen synthesis.     

Phosphorylation and Inactivation of Brain Glycogen Synthase by a Multifunctional Calmodulin-Dependent Protein Kinase

Glycogen synthase was partially purified from canine brain to about 70% purity. The purified enzyme showed differences from the properties of the skeletal muscle enzyme with respect to molecular weights of the holoenzyme and subunit and phosphopeptide mapping. The multifunctional calmodulin-dependent protein kinase from the brain phosphorylated brain glycogen synthase with concomitant inactivation of the enzyme. Although about 1.3 mol of phosphate/mol subunit was maximally incorporated into glycogen synthase, 0.4 mol of phosphate/mol subunit was sufficient for the maximal inactivation of the enzyme. The results indicate that brain glycogen synthase is regulated in a calmodulin-dependent manner similarly to the skeletal muscle enzyme, but that the brain enzyme is different from the skeletal muscle enzyme.     

Inhibition by 2-Deoxyglucose and 1,5-Gluconolactone of Glycogen Mobilization in Astroglia-Rich Primary Cultures

Abstract: The presence of glycogen in astroglia-rich primary cultures derived from the brains of newborn rats depends on the availability of glucose in the culture medium. On glucose deprivation, glycogen vanishes from the astroglial cultures. This decrease of glycogen content is completely prevented if 2-deoxyglucose in a concentration of > 1 m M or 1,5-gluconolactone (20 m M ) is present in the culture medium. 2-Deoxyglucose itself or 3- O -methylglucose, a glucose derivative that is not phosphorylated by hexokinase, does not reduce the activity of glycogen phosphorylase purified from bovine brain or in the homogenate of astroglia-rich rat primary cultures. In contrast, deoxyglucose-6-phosphate strongly inhibits the glycogen phosphorylase activities of the preparations. Half-maximal effects were obtained at deoxyglucose-6-phosphate concentrations of 0.75 (phosphorylase a, astroglial culture), 5 (phosphorylase b, astroglial culture), 2 (phosphorylase a, bovine brain), or 9 m M (phosphorylase b, bovine brain). Thus, the block of glycogen degradation in these cells appears to be due to inhibition of glycogen phosphorylase by deoxyglucose-6-phosphate rather than deoxyglucose itself. These results suggest that glucose-6-phosphate, rather than glucose, acts as a physiological negative feedback regulator of the brain isoenzyme of phosphorylase and thus of glycogen degradation in astrocytes.     

Relationship Between Regional Brain Angiotensinogen a Local Blood Flow and Volume in the Adrenalectomize Rat: Application of Approach to Quantification of Brain Corticosterone Receptors

Abstract: Prior studies from this laboratory have established that angiotensinogen, the prohormone of angiotensin, is unevenly distributed in the rat brain and that adrenalectomy selectively perturbs levels of the prohormone in regions associated with cardiovascular neural pathways. However, plasma angiotensinogen levies are 10²-10³ times higher in plasma than in brain, so that the observation of a unique distribution of brain angiotensinogen may reflect variable plasma contamination. Studies were therefore undertaken to establish whether adrenalectomy selectively alters regional blood flow, blood volume, or plasma contamination of brain tissue, thereby artifactitiously altering apparent angiotensinogen levels. Radioactive 2-deoxyglucose, iodoantipyrine, and inulin were employed in these analyses. We conclude that variations in blood flow do not explain the selective effects of adrenalectomy, but that a variable extent of residual plasma contamination (remaining after transcardiac perfusion) is partially reflected in our earlier data. However, after correcting for plasma contamination, we still find significant changes in selected areas of the rat brain following adrenalectomy. Finally, our results demonstrate the necessity for direct quantitation of plasma contamination of brain tissue segments. This is shown to have relevance in other situations, such as corticosterone binding globulin contamination of brain corticosterone receptor binding.     

Local Cerebral Glucose Utilisation Following Indoleamine- and Piperazine-Containing 5-Hydroxytryptamine Agonists

Substances with varying structural components have been shown to have 5-hydroxytryptamine (5-HT)-like properties in the CNS. In this study, putative 5-HT agonists with indoleamine moeities--lysergic acid diethylamide (LSD) and 5-methoxy-N,N-dimethyltryptamine (5-MeODMT)--and with piperazine moieties--quipazine (Quip) and 6-chloro-2-(1-piperazinyl)pyrazine (6-CPP) were administered to rats. Local cerebral glucose utilisation was measured using the [14C]2-deoxyglucose autoradiographic technique. It was found that in most cerebral structures, these substances produced dose-dependent reductions in glucose utilisation. However, Quip and 6-CPP increased glucose utilisation in specific areas of the diencephalon (e.g., nucleus reuniens) and produced a biphasic effect in some but not all extrapyramidal structures (e.g., ventromedial caudate nucleus). No such increases in local cerebral glucose utilisation were measured following LSD or 5-MeODMT administration. These results indicate that although similarities exist between the effects of indoleamine- and piperazine-containing 5-HT agonists on local cerebral glucose utilisation there are also significant differences in the overall patterns of response produced.