Rat Brain Cerebral Cortex Research Articles

Structural basis of information capacity changes of sensory-motor cerebral cortex of rat brain during post-resuscitation period

In a rat model of asphyxial cardiac arrest the volume of the sensory-motor cortex and the number of neurons and synapses were determined 90 min, 6 h, and 1, 3, 7, 14, and 30 days post-resuscitation. The number of synapses was determined from serial fragmental sections, using selective contrast in ethanol solution of phosphorotungsten acid (PTA), and by cytoarchitectonic analysis on medium-thick sections. The sensory-motor cortex (SMC) volume did not change significantly during the subsequent 30 days after resuscitation. The number of neurons decreased from (2.462 ± 0.082) × 10 6 pre-insult to (1.441 ± 0.098) × 10 6 30 days after resuscitation. Damage was most severe in the small neuronal cell complexes of layers III–IV, which serve as an afferent cortical ‘entrance’. Damage was least in the large neuronal cell complexes of layer V, which serves as an efferent cortical ‘outlet’. The number of SMC synapses decreased from (5.920 ± 0.51) × 10 9 pre-insult to (3.441 ± 0.305) × 10 9 30 days after resuscitation. Damage was most severe in the synaptic pool of the cortical SMC ‘entrance’. An increase in the number of high-efficiency hypertrophic synaptic contacts was observed during the post-resuscitation period, which may significantly change interneuronal relationships.

Modulation in acetylcholinesterase of rat brain by pyrethroids in vivo and an in vitro kinetic study.

The modulation of acetylcholinesterase (AChE) of rat brain by two pyrethroids--permethrin (PM) and cypermethrin (CPM)--was studied both in vivo and in vitro. PM inhibited AChE activity in all regions of the rat brain (cerebral cortex, cerebellum, corpora striata, brain-stem, hippocampus, and hypothalamus) at 4, 8, and 12 h after gastric intubation, whereas CPM elevated the enzyme activity in vivo. Substrate-dependent enzyme kinetic studies have shown that PM and CPM behave as mixed-type inhibitors, as evidenced by alterations in both Michaelis-Menten constant (Km) and maximal velocity (Vmax) values. This indicates that both PM and CPM and substrate acetylcholine interact at hydrophobic subsites and may be able to bind simultaneously to the enzyme.

Expression of growth inhibitory factor (GIF) in normal and injured rat brains

Immunohistochemical study on growth inhibitory factor (GIF) in rat brain has revealed that a glial cell layer on the surface of cerebral cortex and the cells surrounding Purkinje cells has been reported. In addition, neurons in gray matter were weakly immunostained for GIF. In situ hybridization using digoxigenin-labeled single-strand RNA probes also demonstrated that most of the neurons and small round cells, which were presumably astrocytes, expressed GIF mRNA in the cerebral cortex of rat brain. These findings indicate that GIF is produced in neurons as well as in astrocytes. The most prominent findings in this study are, a very strong reaction of GIF and GIF mRNA in the reactive astrocytes around the site of injury induced by stab wound or kainic acid injection. These results raised the possibility that GIF may act as an acute-phase protein in reactive astrocytes and have a role in tissue repair.

Simultaneous increases of extracellular monoamines in microdialysates from hypothalamus of conscious rats by duloxetine, a dual serotonin and norepinephrine uptake inhibitor.

Duloxetine (LY248686, [+]-N-methyl-3-(1-napthalenyloxy)-2-thiophene-propanamine) is a potent dual inhibitor of serotonin (5-hydroxytryptamine, 5-HT) and norepinephrine (NE) uptake in hypothalamus and cerebral cortex of rat brain (Wong et al. 1993; Fuller et al. 1994). Consistent with the dual mechanisms of inhibiting 5-HT and NE uptake, duloxetine at 15 mg/kg IP produced large increases in extracellular levels of 5-HT (250%) and NE (1,100%) 30 minutes after systemic administration. Levels of 3-methoxy-4-hydroxy-phenylethyleneglycol (MHPG) and 5-hydroxyindoleacetic acid (5-HIAA), metabolites of NE and 5-HT, respectively, were reduced, whereas those of dopamine (DA) and its metabolite 3, 4-dihydroxyphenylacetic acid (DOPAC) were not significantly altered. Duloxetine at 7 mg/kg produced less pronounced increases while no consistent effects were observed at 4 mg/kg. In this dose range, duloxetine inhibited 5-HT uptake in platelets ex vivo without inhibiting striatal dopamine (DA) uptake. In the present study we also found that the primary amine (a racemate) of duloxetine is about one-fourth as active as duloxetine to inhibit 5-HT and NE uptake. The potential primary amine metabolite of duloxetine might contribute, in part, to the inhibition of 5-HT and NE uptake in vivo. Thus the ability to produce robust increases of extracellular 5-HT and NE levels suggests that duloxetine may potentially be a highly effective antidepressant agent.

Alteration of brain nicotinic receptors induced by immobilization stress and nicotine in rats

Immobilization stress (2 h· day −1, for 2 weeks) caused downregulation of the nicotinic receptors ([ 3H]cytisine binding sites) in the rat brain (cerebral cortex and midbrain). However, chronic nicotine treatment abrogated stress induced downregulation of the [ 3H]cytisine binding sites, suggesting that chronic nicotine treatment may block stress activated nicotinic receptor-mediated neurotransmission.

Characterization and distribution of somatostatin SS-1 and SRIF-1 binding sites in rat brain: identitity with SSTR-2 receptors

Somatostatin (SRIF) SS-1 binding sites were initially defined in radioligand binding studies performed in rat brain cerebral cortex membranes using [ 125I]204-090 (a radiolabelled Tyr 3 analogue of SMS 201-995, octreotide). SRIF-1 recognition sites were defined in binding studies performed with [ 125I]MK 678 (seglitide). Both SS-1 and SRIF-1 sites were characterized by their high affinity for SRIF-14, SRIF-28 and for cyclic peptides such as octreotide and seglitide, in marked contrast to SS-2 and SRIF-2 sites which have very low affinity for these synthetic SRIF analogues. In the present study, SS-1 and SRIF-1 radioligand binding studies were performed in rat cortex membranes and compared to results obtained in cloned Chinese hamster ovary cells expressing human SSTR-2 receptors using [ 125I]204-090 and/or [ 125I]MK-678. The rank orders of affinity of a variety of SRIF analogues and synthetic peptides for SS-1/SRIF-1 binding sites and recombinant SSTR-2 receptors were very similar and correlated highly significantly ( r = 0.94–0.99); by contrast, correlation between SS-1 and SSTR-5 ( r = 0.44) or SSTR-3 binding ( r = 0.07) was not significant. Autoradiographic studies were performed in rat brain using both radioligands [ 125I]204-090 and [ 125I]MK-678 and compared with the distribution of SSTR-2 receptor mRNA determined using in situ hybridization. A clear overlap was observed between the distribution of SSTR-2 mRNA and binding sites labelled with both radioligands. SSTR-2 receptor-mediated inhibition of forskolin-stimulated adenylate cyclase in Chines hamster ovary cells by a variety of SRIF analogues and short synthetic peptides displayed a rank order of potency highly similar to their rank order of affinity at SS-1/SRIF-1 binding sites. It is concluded that SS-1 and SRIF-1 binding sites respectively labelled with [ 125I]204-090 and [ 125I]MK 678, both display the pharmacological profile of SSTR-2 receptors, that the distribution of [ 125I]204-090 and [ 125I]MK-678 binding sites in rat brain is superimposable and largely comparable to that of SSTR-2 mRNA expression. It is also shown that neither [ 125I]204-090 nor [ 125I]MK-678 label SSTR-3 or SSTR-5 receptors in rat brain. Finally, it is demonstrated that SSTR-2 receptors can very efficiently couple to adenylate cyclase activity in an inhibitory manner.

Pharmacological identity between somatostatin SS-2 binding sites and SSTR-1 receptors

Somatostatin (SRIF) SS-2 binding sites were originally defined in rat brain cerebral cortex membranes using [ 125I]Tyr 11-SRIF-14 in the presence of 120 mM NaCl. These sites were characterized by their high affinity for SRIF-14 and SRIF-28, but very low affinity for cyclic peptides such as octreotide (SMS 201-995) and seglitide (MK 678). The characteristics of SS-2 sites are reminiscent of 125I]CGP 23996-labelled sites in rat brain which have been termed SRIF-2 sites. In the present study, the pharmacological profile of SS-2 sites was determined in radioligand binding studies performed in rat cortex membranes using [ 125I]SRIF-14 in the presence of 120 mM NaCl and compared to that of human SSTR-1 receptors expressed in human embryonic kidney (HEK 293) cells, using [ 125I]SRIF-14. The rank orders of affinity of a variety of SRIF analogues and synthetic peptides for SS-2 binding sites and recombinant human SSTR-1 receptors were very similar and correlated highly significantly ( r = 0.99). However, SS-2 binding correlated also with binding to recombinant SSTR-4 receptors ( r = 0.91). Autoradiographic studies were performed using the radioligand [ 125I]CGP 23996 which has been claimed to label selectively SRIF-2 binding sites and compared with the distribution of SSTR-1 receptor mRNA determined using in situ hydridization in rat brain. Although some overlap was observed between the distribution of SSTR-1 mRNA and [ 125I]CGP 23996 binding sites, the latter were clearly more widespread, suggesting this ligand to label SSTR-1 and other sites. In addition, inhibition of forskolin-stimulated adenylate cyclase was investigated in HEK 293 cells transfected with human SSTR-1 receptors; a variety of SRIF analogues and short synthetic peptides behaved as agonists at adenylate cyclase and displayed a rank order of potency highly similar to that observed for these compounds at SS-2 binding sites. Seglitide acted as an antagonist at SSTR-1 receptor mediated inhibition of adenylate cyclase activity with a pK B of 4.42. It is concluded that the pharmacological profile of SS-2 binding sites resembles most closely that of SSTR-1 receptors (although similarities with SSTR-4 receptors were observed), that [ 125I]CGP 23996 labels presumably several SRIF receptors in rat brain, and that SSTR-1 receptors are negatively and efficiently coupled to adenylate cyclase activity.

Short and Long Form γ2 Subunits of the GABAA/Benzodiazepine Receptors

Three novel antisera to the gamma 2 subunit of the gamma-aminobutyric acidA (GABAA) receptor/benzodiazepine receptor (GABAAR/BZDR) complex have been made. Anti-gamma 2S and anti-gamma 2L are specific antibodies to synthetic peptides that recognize the gamma 2S (short) and gamma 2L (long) forms, respectively, of the gamma 2 subunit. An antibody (anti-gamma 2IL2) to staphylococcal protein A fusion protein of the large intracellular loop (gamma 2IL) located between the putative transmembrane segments M3 and M4 of gamma 2S recognizes both gamma 2S and gamma 2L subunits. The antibodies immunoprecipitated both the solubilized and affinity-purified GABAAR/BZDR from rat and bovine brain. Immunoblots with membranes from rat brain cerebral cortex as well as with affinity-purified receptor from bovine cortex show that anti-gamma 2S and anti-gamma 2L recognize peptides of 45,000 and 47,000 M(r), respectively. Immunoprecipitation experiments indicate that gamma 2S is more prevalent in hippocampus, whereas gamma 2L is more abundant in cerebellum. Intermediate values for each form are found in the cerebral cortex. The results suggest that in the rat brain there is a considerable amount of colocalization of gamma 2S and gamma 2L in the same receptor complex. In the cerebral cortex, 15% of the BZDRs contain both gamma 2S and gamma 2L subunits and 41-48% of the gamma 2L subunit coexists with gamma 2S in the same receptor complex. In cerebellum, in 27% of the clonazepam-sensitive and 39% of the clonazepam-insensitive BZDRs the gamma 2S and gamma 2L coexist in the same receptor complex. The latter are presumably localized in granule cells and also contain alpha 6. In addition, almost all (93%) the clonazepam-insensitive BZDRs that contain gamma 2L also contain a gamma 2S subunit in the same receptor complex. The most likely interpretation of the results is that there is an important population of granule cell receptors that contain alpha 6, gamma 2S, and gamma 2L coexisting in the same receptor complex. Nevertheless, 31% of the cerebellar receptors that contain alpha 6 subunit(s) have neither gamma 2S nor gamma 2L subunits. There are also species differences with respect to the relative abundance of gamma 2S and gamma 2L. These results might be relevant for understanding the molecular mechanisms underlying some of the GABAAR/BZDR-mediated effects of ethanol intoxication involving cerebellar granule cells.

Preproenkephalin gene expression in the rat cerebral cortex during chronic tracheal stenosis.

To evaluate whether the endogenous opioid system is activated in the higher brain centre while a chronic resistance to airflow, we examined changes of mRNAs for preproenkephalin (PPE)-A, which is a precursor of enkephalin, and for 70 kD heat shock protein (HSP70) in the cerebral cortex of rat brain during chronic tracheal stenosis. Northern blot revealed that PPE-A mRNA was induced at 3 days of airway stenosis. In situ hybridization revealed that PPE-A mRNA was gradually induced in frontal cortex. The significant induction of PPE-A mRNA was observed at 3 and 7 days. However, no significant induction of HSP70 gene was observed. These results suggest that the endogenous opioid system may be at work as an important compensatory mechanism to reduce the respiratory sensation during chronic respiratory stress.

BW619C89, a glutamate release inhibitor, protects against focal cerebral ischemic damage.

The excitatory amino acid neurotransmitter glutamate is involved in excitotoxic brain injury and neurodegeneration after cerebral ischemia. Therefore, compounds that block the release of glutamate may be useful as cerebroprotective agents. The purpose of this study was to evaluate the cerebroprotective properties of a glutamate release inhibitor, BW619C89. In the studies reported here, the effect of BW619C89 [4-amino-2-(4-methyl-1-piperazinyl)-5-(2,3,5-trichlorophenyl)pyrimidine] on neurotransmitter release (endogenous amino acids, gamma-aminobutyric acid, and acetylcholine) from slices of rat brain cerebral cortex in vitro has been determined. The neuroprotective efficacy of BW619C89 has been evaluated using the middle cerebral artery occlusion model of focal cerebral ischemia in the Fischer 344 rat. In the in vitro studies, BW619C89 inhibited veratrine- (but not potassium-) evoked release of both endogenous glutamate and aspartate from rat cerebral cortex slices with IC50 values of approximately 5 microM. BW619C89 was approximately 10-fold less potent to inhibit veratrine-evoked 3H-gamma-aminobutyric acid release (IC50 = 51 microM), fourfold less potent to inhibit 3H-acetylcholine release (IC50 = 21 microM), and at 10 microM had only weak activity at excitatory amino acid (N-methyl-D-aspartate, kainate, and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) binding sites. When administered intravenously to Fischer 344 rats 5 minutes after permanent middle cerebral artery occlusion, BW619C89 produced marked reductions of both total (cortex and basal ganglia) and cortical infarct volumes. Cortical infarct size was reduced by 20% at a dose of BW619C89 of 5 mg/kg (n = 6, not significant); 43% at 10 mg/kg (n = 8, P < .01); 59% at 20 mg/kg (n = 8, P < .001); 61% at 30 mg/kg (n = 8, P < .001), and 53% at 40 mg/kg (n = 8, P < .001). BW619C89 at doses of 20 and 30 mg/kg also significantly reduced noncortical (basal ganglia) infarct volumes, demonstrating that a proportion of this tissue also appears to be salvageable. Behavioral effects observed were dose related, generally minor, and at doses of 20 mg/kg IV and above consisted of body tremor and mild ataxia lasting approximately 2 hours. These results suggest that glutamate release inhibitors such as BW619C89 may provide an alternative to excitatory amino acid receptor antagonists in the treatment of focal cerebral ischemia and stroke.

The development of neuropeptide Y-immunoreactive neurons in a model of pure cortical culture

The development of neuropeptide Y-immunoreactive neurons in the rat brain cerebral cortex was studied in a model of a pure cortical culture. In this model, development of neurons devoid of any afferents from other brain structures could be observed. Since mutual interactions between neuropeptide Y and catecholamines have been postulated, such a pure cortical culture offers a possibility of studying the development of neuropeptide Y neurons devoid of any brainstem monoaminergic afferents. A tissue dissected from 16-day-old rat fetuses and cultivated in a dissociated culture for 14 days was examined immunohistochemically for the presence of neuropeptide Y-immunoreactive neurons. Three main types of neuropeptide Y-immunoreactive neurons were found: unipolar, bipolar and multipolar. Cell processes and terminal varicose fibres were also observed. The results obtained indicate that neuropeptide Y-immunoreactive neurons and fibres may develop in a pure culture of the rat cerebral cortex without the influence of any other structures.

Incorporation of 15N from L-leucine into isoleucine by rat brain cerebral cortex slices.

The fate of leucine nitrogen in the central nervous system was investigated by incubating rat cerebral cortex slices in the presence of 0.5 mM each of L-[15N]-leucine, L-isoleucine, and L-valine. Analysis of the slices and incubation media for free amino acids by gas chromatography-mass spectrometry revealed that the 15N from leucine is incorporated into isoleucine only. No 15N was detected in valine or any other amino acid. These results suggest that leucine, valine, and their corresponding aminotransferases may be compartmentalized in brain cerebral cortex.

Developmental changes in brain kynurenic acid concentrations

The cerebral distribution and regulation of excitatory amino acid levels may play a crucial role in neuronal development. In the present study we examined concentrations of the endogenous excitatory amino acid antagonist kynurenic acid and related substances during development in fetal and neonatal rat brain and fetal non-human primate cerebral cortex. Kynurenic acid concentrations in rat fetal whole brain were significantly increased 4–5 fold prenatally, then declined rapidly at 1 day after birth, and reached adult concentrations at 7 days after birth. l-Kynurenine concentrations were also markedly increased prior to birth and then declined to adult concentrations at 1 day after birth. l-Tryptophan was increased 3 fold before birth, and decreased to adult concentrations 1 day after birth. In contrast concentrations of dopamine, norepinephrine, 3,4-dihydroxyphenylacetic acid and homovanillic acid increased 1 day prior to birth and continued to increase following birth. Fetal baboon cerebral cortex showed significant increases in kynurenic acid concentrations both pre-term and near-term as compared with adult concentrations. These results show that marked changes in kynurenic acid concentrations occur prior to and following birth. It is possible that high levels of kynurenic acid prior to birth inhibit neurite branching and development of excitatory synapses, which then develop rapidly in parallel with the decrease in kynurenic acid levels.

Calretinin, a neuronal calcium binding protein, inhibits phosphorylation of a 39 kDa synaptic membrane protein from rat brain cerebral cortex

The neuronal calcium binding protein calretinin was studied for possible effects on brain protein phosphorylation. Calretinin (100 nM) inhibited the appearance of a calcium stimulated 39 kDa phosphoprotein within a synaptic membrane fraction following sucrose density centrifugation. Calmodulin or a specific protein kinase C inhibitor had no effect on either the phosphorylation of the 39 kDa protein or the inhibition produced by calretinin. At the same concentration, calretinin produced a slight increase in the phosphorylation of several other synaptic membrane proteins which appeared additive with the stimulation produced by either calmodulin or phosphatidylserine in the presence of calcium.

Synthesis and biochemical evaluation of tritium-labeled 1-methyl-N-(8-methyl-8-azabicyclo[3.2.1]oct-3-yl)-1H-indazole-3-carboxamide, a useful radioligand for 5HT3 receptors

The advent of potent, highly selective 5HT3 receptor antagonists has stimulated considerable interest in 5HT3 receptor mediated physiology and pharmacology. To permit detailed biochemical studies regarding interaction of the indazole class of serotonin (5HT) antagonists with 5HT3 receptors in multiple tissues, we synthesized 1-methyl-N-(8-methyl-8-azabicyclo[3.2.1]oct-3-yl)-1H-indazole- 3-carboxamide (LY278584, compound 9) in high specific activity, tritium-labeled form. This radioligand was selected as a synthetic target because of its potency as a 5HT3-receptor antagonist, its selectivity for this receptor viz a viz other 5HT-receptor subtypes, and the ability to readily incorporate three tritia via the indazole N-CH3 substituent. Alkylation of N-(8-methyl-8-azabicyclo[3.2.1]oct-3-yl)-1H-indazole-3-carboxamide (8) with sodium hydride and tritium-labeled iodomethane, followed by HPLC purification, resulted in [3H]-9 with a radiochemical purity of 99% and a specific activity of 80.5 Ci/mmol. This radioligand bound with high affinity to a single class of saturable recognition sites in membranes isolated from cerebral cortex of rat brain. The Kd was 0.69 nM and the Bmax was 16.9 fmol/mg of protein. The specific binding was excellent, and accounted for 83-93% of total binding at concentrations of 2 nM or less. The potencies of known 5HT3-receptor antagonists as inhibitors of [3H]-9 binding correlated well with their pharmacological receptor affinities as antagonists of 5HT-induced decreases in heart rate and contraction of guinea pig ileum, suggesting the central recognition site for this radioligand may be extremely similar to or identical with peripheral 5HT3 receptors.

Ultrastructural Immunocytochemical Evidence for Presynaptic Localization of Beta‐Adrenergic Receptors in the Striatum and Cerebral Cortex of Rat Braina

Ultrastructural Immunocytochemical Evidence for Presynaptic Localization of Beta‐Adrenergic Receptors in the Striatum and Cerebral Cortex of Rat Brain^a

Antioxidant enzymatic systems in neuronal and glial cell-enriched fractions of rat brain during aging

The activities of Cu,Zn superoxide dismutase, glutathione peroxidase, catalase and glutathione reductase in neuronal and glial cell-enriched fractions obtained from the cerebral cortex of rat brain during aging (15, 30, 90, 350, 750 days of age) were assayed. Our results showed that glutathione peroxidase, catalase and glutathione reductase activities varied little during the examined periods. Only the Cu,Zn superoxide dismutase activity decreased notably from 15th to 750th day of age in both neuronal and glial cells, moreover the activities of all enzymes studied were always detected at lower levels in neuronal cells with respect to glial cells. In agreement with diminished SOD activity, the lipid peroxidation showed an elevated increase with aging; this fact is more evident in neuronal than in glial cells. In conclusion our data show that Cu,Zn superoxide dismutase is the most affected antioxidant enzymatic system of brain aging and it could be responsible for the increased lipid peroxidation in both cell types examined.

The influence of substance P and its fragments on endogenous phosphorylation of synaptosomal membrane protein (Synapsin) from cerebral cortex of rat brain

1. The effects of substance P and its fragments and analogue of a C-terminal fragment on cyclic AMP-dependent phosphorylation of synapsin I in synaptosomal membranes (SM) from cerebral cortex were investigated. 2. SP (1–11) and sp (1–4) at 10 −3 M caused a marked stimulation of synapsin I phosphorylation. 3. A C-terminal fragment of SP (SP 6–11 had no effect on phosphorylation of synapsin 1. 4. Analogue of C-terminal fragment [(Tyr 8SP 6–11] at 10 −3M distinctly inhibits phosphorylation of synapsin I. 5. These data suggest that sp 1–11 and its C- and N-terminal fragments have a modulator function against the phosphorylation of some rat brain proteins.

Phospholipid hydrolysis and loss of membrane integrity following treatment of rat brain synaptosomes with β-bungarotoxin, notexin, and Naja naja atra and Naja nigricollis phospholipase A 2

The effects of the phospholipase A 2 (PLA 2) toxins, β-bungarotoxin and notexin, and the PLA 2 enzymes from Naja naja atra and Naja nigricollis snake venoms on the plasma membrane integrity of synaptosomes were examined. Synaptosomes were isolated from rat brain cerebral cortex, corpus striatum and hippocampus. Osmotic activity, lactate dehydrogenase leakage, and leakage of 2-deoxy- d-(1- 3H)-glucose-6-phosphate were monitored (37°C, 10–120 min) following incubation with 0.5, 5 and 50 nM concentrations of toxins and enzymes. Damage to the synaptosomal plasma membrane was time and concentration but not tissue dependent. The potencies of the treatments were as follows: N. n. atra PLA 2 ≥ N. nigricollis PLA 2 > notexin > β- bungarotoxin. Chelation of Ca 2+ with 5 mM EDTA completely inhibited plasma membrane disruption caused by β-bungarotoxin and N. n. atra PLA 2. One mg/ml of bovine serum albumin also blocked the disruptive action of N. n. atra PLA 2, while 8 mg/ml was required to antagonize β-bungarotoxin. A correlation between phospholipid hydrolysis and loss of membrane integrity was also observed. The generation of phospholipid hydrolytic products may be critical in the permeabilization of synaptic plasma membranes by these toxins and enzymes, however, they do not explain the presynaptic specificity and potency of β-bungarotoxin and notexin.

γ‐Aminobutyric AcidB Receptor Activation Modifies Agonist Binding to β‐Adrenergic Receptors in Rat Brain Cerebral Cortex

The interaction of isoproterenol with beta-adrenergic receptor (beta AR) binding sites was measured in membranes prepared from rat brain cerebral cortical slices previously incubated in the presence or absence of gamma-aminobutyric acid (GABA) receptor agonists. Both GABA and baclofen, but not isoguvacine, altered beta AR agonist binding by increasing the affinity of both the low- and high-affinity binding sites and by increasing the proportion of low-affinity receptors. The response to baclofen was stereoselective, and the effect of GABA was not inhibited by bicuculline. The results suggest that GABAB, but not GABAA, receptor activation modifies the coupling between beta AR and stimulatory guanine nucleotide-binding protein, which may in part explain the ability of baclofen to augment isoproterenol-stimulated cyclic AMP accumulation in brain slices.