Identification of biologically active δ-lactone eicosanoids as paraoxonase substrates

The mammalian paraoxonases (PONs 1, 2 and 3) are a family of esterases that are highly conserved within and between species. They exhibit antioxidant and anti-inflammatory activities. However, their physiological function(s) and native substrates are uncertain. Previous structure-activity relationship studies demonstrate that PONs have a high specificity for lipophilic lactones, suggesting that such compounds may be representative of native substrates. This report describes the ability of PONs to hydrolyze two bioactive δ-lactones derived from arachidonic acid, 5,6-dihydroxy-eicosatrienoic acid lactone (5,6-DHTL) and cyclo-epoxycyclopentenone (cyclo-EC). Both lactones were very efficiently hydrolyzed by purified PON3. PON1 efficiently hydrolyzed 5,6-DHTL, but with a specific activity about 15-fold lower than PON3. 5,6-DHTL was a poor substrate for PON2. Cyclo-EC was a poor substrate for PON1 and not hydrolyzed by PON2. Studies with the PON inhibitor EDTA and a serine esterase inhibitor indicated that the PONs are the main contributors to hydrolysis of the lactones in human and mouse liver homogenates. Studies with homogenates from PON3 knockout mouse livers indicated that >80% of the 5,6-DHTL and cyclo-EC lactonase activities were attributed to PON3. The findings provide further insight into the structural requirements for PONs substrates and support the hypothesis that PONs, particularly PON1 and PON3, evolved to hydrolyze and regulate a class of lactone lipid mediators derived from polyunsaturated fatty acids.

Methods for measuring serum activity levels of the 192 Q and R isoenzymes of paraoxonase 1 in QR heterozygous individuals

BACKGROUND

Paraoxonase 1 (PON1), an esterase that hydrolyzes toxic organophosphates and has antioxidative and antiatherogenic properties, contains a common polymorphism at position 192: glutamine (Q) or arginine (R). The Q and R isoenzymes exhibit different physical and protective properties. We describe 2 methods for quantifying their serum activity levels.

METHODS

We measured serum hydrolytic activity with paraoxon [paraoxonase (PXN) activity], phenylacetate [arylesterase (AE) activity], and diazoxon [diazoxonase (DZN) activity] with standard automated assays. We determined PON1 Q192R genotypes with PCR and Q192R phenotypes using the PXN/AE and PXN/DZN ratios. Interpolation equations were empirically derived to predict the percentage of total PON1 hydrolytic activity due to the Q isoenzyme (%Q) from the PXN/AE and PXN/DZN ratios; %R is 100 - %Q. We estimated Q and R isoenzyme activity levels in sera from 2095 veterans by multiplying AE activity, a measure of total PON1 hydrolytic activity, by %Q and %R.

RESULTS

In all 2095 samples, the PXN/AE and PXN/DZN ratios predicted Q192R phenotypes with nearly identical accuracy (κ = 0.997). In the 925 QR heterozygotes, the 2 interpolation methods predicted Q and R isoenzyme activity levels with excellent agreement (intraclass correlation 0.94). After excluding a few genotype/phenotype-discordant samples, the percentage of total PON1 activity due to the Q isoenzyme ranged from 22% to 70%.

CONCLUSIONS

These new interpolation methods allow accurate estimation of PON1 192 Q and R isoenzyme activity levels, increasing specificity and power for studying susceptibility to disease.

Angiotensin II Exerts Positive Feedback on the Intrarenal Renin-Angiotensin System by an Angiotensin Converting Enzyme-Dependent Mechanism1

BACKGROUND

Plasma angiotensin II (ANG II) is not increased significantly in renovascular hypertension (RVH), but tissue ANG II levels are elevated in both kidneys of renovascular rats. Because the contralateral, non-ischemic kidney is critical for maintenance of hypertension in RVH, this study sought to understand the mechanism by which intrarenal ANG II levels are augmented in the non-ischemic kidney. This study tested the hypothesis that an incremental increase in plasma ANG II induces the intrarenal renin-angiotensin system (RAS) in the non-ischemic kidney by an angiotensin converting enzyme (ACE) dependent mechanism.

METHODS

To simulate the incremental increase in plasma ANG II induced by the ischemic kidney in RVH, an ANG II infusion model was used. This model used a chronic infusion of ANG II (40 ng/min) or vehicle by osmotic minipump into uninephrectomized rats. Parallel groups were treated with the ACE inhibitor Enalaprilat (200 mg/kg/day). Intrarenal ACE activity was measured by radioenzymatic assay. ANG II levels were quantified by radioimmunoassay.

RESULTS

Hypertension was evident in ANG II-infused rats, compared to control rats (155 +/- 4 versus 112 +/- 1 mmHg; P < 0.001). Concurrent treatment with Enalaprilat reversed the hypertension induced by ANG II infusion (98 +/- 3 versus 155 +/- 4 mmHg; P < 0.001). ANG II up-regulated intrarenal ACE activity in the non-ischemic kidney (59.2 +/- 11.9 versus 25.2 +/- 6.8 units/mg protein; P < 0.01). Enalaprilat significantly decreased renal ACE activity in ANG II-treated rats, compared to ANG II alone (11.4 +/- 1.0 versus 59.2 +/- 11.9 units/mg protein; P < 0.001). Intrarenal ANG II was increased in ANG II-infused rats, compared to control animals (52.9 +/- 7.1 versus 23.0 +/- 3.2 fmol/mg tissue; P < 0.001), and Enalaprilat prevented ANG II-induced increases in intrarenal ANG II (29.9 +/- 2.6 versus 52.9 +/- 7.1 fmol/mg tissue; P < 0.05).

CONCLUSION

Incremental changes in plasma ANG II induce de novo production of ANG II in the non-ischemic kidney to augment intrarenal ANG II content. ACE inhibition blocks this positive feedback loop, suggesting that ANG II activates the intrarenal RAS by an ACE-dependent mechanism. The impact of ACE inhibition on blood pressure suggests that this feedback loop may be an important mechanism for maintenance of hypertension in RVH.

Angiotensin converting enzyme-independent angiotensin ii production by chymase is up-regulated in the ischemic kidney in renovascular hypertension

BACKGROUND

Tissue angiotensin II (ANG II) levels are elevated in both kidneys in renovascular hypertension (RVH). It has been demonstrated previously that intrarenal ANG II is augmented by an angiotensin converting enzyme (ACE) dependent mechanism in the non-ischemic kidney, but the role of ACE-independent production of ANG II in the kidney by the enzyme chymase is unknown. This study tested the hypothesis that intrarenal chymase activity is up-regulated in RVH.

METHODS

A two-kidney, one-clip (2K1C) rat model was used to induce RVH (n = 6 rats/group). Regulation of intrarenal chymase activity by plasma ANG II was investigated using an ANG II-infusion model. At sacrifice 14 days post-operatively, steady-state ANG II levels in plasma and kidney were quantified by radioimmunoassay. ANG II production was quantified in kidney homogenates by incubating at 37 degrees C for 60 min with enzyme substrate (200 microm ANG I) alone or substrate containing the chymase inhibitor chymostatin. ANG II was separated and quantitated by HPLC. Chymase activity was defined as the fraction of ANG II production inhibited by Chymostatin.

RESULTS

2K1C and ANG II-infused rats developed significant hypertension, compared to control rats (P = 0.0001 and P = 0.001, respectively). Chymase-dependent ANG II production was increased in the ischemic kidney, but not the non-ischemic kidney, of 2K1C rats compared to control animals (*P < 0.05). Intrarenal chymase activity was unchanged by ANG II infusion (P = NS).

CONCLUSIONS

Chymase activity is up-regulated in the ischemic kidney of 2K1C rats. Plasma ANG II does not appear to regulate intrarenal chymase activity, suggesting that ischemia per se up-regulates chymase activity in the kidney. ACE-independent ANG II production by chymase may provide a mechanism for augmenting intrarenal ANG II in the ischemic kidney in RVH.

Depletion of tissue angiotensin-converting enzyme differentially influences the intrarenal and urinary expression of angiotensin peptides

The relative contribution of circulating versus tissue renin-angiotensin systems to the tissue expression of angiotensin peptides in the kidney remains unresolved. To address this issue, intrarenal and urinary levels of the peptide products of the renin-angiotensin system were assessed in a tissue angiotensin-converting enzyme knockout (tisACE-/-) mouse model. Systolic blood pressure was significantly lower (64.6+/-3.6 versus 81.4+/-4.5 mm Hg; P<0.02) and urinary volume was increased (7.25+/-0.86 versus 2.86+/-0.48 mL/d; P<0.001) in tisACE-/- mice compared with wild-type mice. Intrarenal angiotensin II was 80% lower in tisACE-/- mice compared with wild-type mice (5.17+/-0.60 versus 25.5+/-2.4 fmol/mg protein; P<0.001). Intrarenal angiotensin I levels also declined by a comparable extent (73%) in the tisACE-/- mice (P<0.01). Intrarenal angiotensin-(1-7) concentrations were similar between the strains, but the ratio of intrarenal angiotensin-(1-7) to angiotensin II and angiotensin I in tisACE-/- mice increased 470% and 355%, respectively, compared with wild-type mice. Urinary excretion of angiotensin II and angiotensin-(1-7) were not different, but the excretion of angiotensin I increased 270% in tisACE-/- mice (P<0.01). These studies suggest 2 potential mechanisms for the reduction of intrarenal angiotensin II in tisACE-/- mice: (1) an attenuated capacity to form angiotensin II by renal angiotensin-converting enzyme and (2) significant depletion of its direct precursor angiotensin I in renal tissue. Sustained intrarenal levels of angiotensin-(1-7) may contribute to chronic hypotension and polyuria in tisACE-/- mice, particularly in the context of depleted angiotensin II in the kidney.

Dopamine receptors and learned helplessness in the rat: an autoradiographic study

(1) Disturbances of mesolimbic and mesocortical dopamine (DA) function have been implicated in the pathophysiology of several psychiatric disorders, including major depressive disorder. (2) Utilizing the learned helplessness (LH) animal model of clinical depression and quantitative autoradiography, the authors studied the densities of D1 and dopamine-2-like receptors (D2-like receptors) in medial prefrontal cortex, septum, nucleus accumbens and caudate nucleus in rats that received inescapable stress and were subsequently tested for LH behavior. (3) Dopamine-1 receptor (D1 receptor) densities were significantly higher in the core and shell of the nucleus accumbens and in the medial caudate nucleus of rats that did not become helpless after stress, compared to rats that developed LH. (4) Densities of D2-like receptors were significantly lower in the core of the nucleus accumbens in both the LH and the nonhelpless (NH) rats compared to controls. Densities of D2-like receptors were also lower in the medial and lateral caudate nuclei in LH rats compared to the other groups. (5) Increased D1 receptor densities in NH rats in the nucleus accumbens may be associated with an adaptive or protective role of this brain region in the prevention of escape deficits after exposure to inescapable stress. (6) Decreased D2-like receptor densities in the caudate nucleus in helpless rats may reflect a motor deficit associated with LH behavior, while decreases of D2-like receptor densities in the core of the nucleus accumbens may reflect a generalized effect of exposure to inescapable stress. (7) This study highlights the importance of the mesolimbic/nigrostriatal dopaminergic systems in mediating behavioral responses to inescapable stress.

Cloning, expression, and regulation of a glucocorticoid-induced receptor in rat brain: effect of repetitive amphetamine

Behavioral sensitization to psychostimulants involves neuroadaptation of stress-responsive systems. We have identified and sequenced a glucocorticoid-induced receptor (GIR) cDNA from rat prefrontal cortex. The full-length GIR cDNA encodes a 422 amino acid protein belonging to G-protein-coupled receptor superfamily. Although the ligand for GIR is still unknown, the dendrogram construction indicates that GIR may belong to peptide receptor subfamily (e.g., substance P receptor), with more distant relationship to subfamilies of glycoprotein hormone receptors (e.g., thyrotropin receptor) and biogenic amine receptors (e.g., dopamine receptor). GIR shares 31-34% amino acid identity to the tachykinin receptors (substance P receptor, neurokinin A receptor, and neurokinin B receptor). GIR mRNA is expressed preferentially in brain, and its neuronal expression is relegated to limbic brain regions, particularly in forebrain. GIR transcript levels are increased significantly and persistently in prefrontal cortex for 7 d after discontinuation of chronic amphetamine exposure. The induction of GIR expression by amphetamine is associated with augmented behavioral activation. These findings suggest that modulation of GIR expression may be involved in behavioral sensitization, and GIR may play a role at the interface between stress and neuroadaptation to psychostimulants.

Olanzapine treatment for post-traumatic stress disorder: an open-label study

Post-traumatic stress disorder (PTSD) is a common and increasingly diagnosed mental illness. Recent pharmacotherapeutic research on treatments for this condition has focused on antidepressant drugs with serotonergic actions. However, the presence of intrusive, psychotic-like symptoms in a substantial portion of PTSD patients raises the possibility that antipsychotics with serotonergic properties might also prove useful in treating PTSD. We conducted an open-label 8-week study of olanzapine treatment in veterans with combat-induced PTSD. Primary outcome measures in this study were the Clinician Administered PTSD Scale (CAPS) and the Clinical Global Impressions Improvement scale. Secondary outcome measures included the Hamilton Rating Scales for Depression (HRSD) and Anxiety (HRSA). Forty-eight patients enrolled in the study, and 30 completed the 8-week trial. Results of intent-to-treat and completer analyses demonstrated that all outcome measures improved significantly during treatment. Secondary analyses indicate that improvement in the intrusive symptom cluster of the CAPS was independent of improvement on the HRSD and HRSA. In conclusion, the study indicates that olanzapine treatment is useful in alleviating the symptoms of combat-induced PTSD.

The influence of gender and the estrous cycle on learned helplessness in the rat

Although the etiology of clinical depression is unknown, women are more likely to suffer from major depressive disorder than men. In addition, in some women, there is a clear association between depression and specific phases of the menstrual cycle. Surprisingly little research has examined gender differences and the influences of the estrous cycle in this and other animal behavioral models of clinical depression. Learned helplessness is a valid animal model of stress-induced behavioral depression in which prior exposure to inescapable stress produces deficits in escape testing. Learned helplessness was studied in rats using an inescapable tail shock stress followed by a shuttle box test to determine escape latencies. Animals with mean escape latencies of >or=20 s after shuttle-box testing are defined as learned helpless. Males and normal cycling female rats in the estrus and diestrus II phases were studied. Female rats in the diestrus II phase had significantly higher escape latencies and exhibited a more helpless behavior than female rats in the estrus phase. Male rat escape latencies were intermediate between the two female phases. These results suggest a role for gonadal hormones in the development of stress-induced behavioral depression or 'learned helplessness.'

Plasma GABA levels correlate with aggressiveness in relatives of patients with unipolar depressive disorder

Plasma gamma-aminobutyric acid (GABA) levels are decreased in some patients with depression, mania and alcoholism. Medications which increase plasma GABA improve symptoms of mood disorders and can decrease aggression. We examined the relationship between plasma GABA and aggressiveness on the Buss-Durkee Hostility Inventory in 77 psychiatrically healthy adults. In subjects selected for having a first-degree relative with primary unipolar depressive disorder (FH+, n=33), plasma GABA was negatively correlated with aggressiveness (beta=-0.338, P=0.036), as was age (beta=-0.483, P=0.005). A relationship between plasma GABA levels and aggressiveness was not observed in subjects with no such family history (FH-, n=44). Moreover, FH+ subjects had significantly lower plasma GABA concentrations than FH- subjects. These data suggest that low GABA levels may correlate with some aspects of aggressiveness and may be genetically regulated.

Activation of skeletal muscle afferents evokes release of glutamate in the subretrofacial nucleus (SRF) of cats

The subretrofacial nucleus (SRF) is a region of the rostral ventrolateral medulla known to play a crucial role in sympathoexcitation. SRF neurons send direct projections to the intermediolateral cell columns of the spinal cord where they form synaptic contact with preganglionic sympathetic motor neurons. Activation of this neural pathway increases sympathetic outflow to the heart and blood vessels affecting cardiac function and vasomotor tone. Previous studies utilizing electrophysiological recording techniques and c-Fos expression have established that the activity of SRF neurons is increased during skeletal muscle contraction. However, the excitatory neurotransmitter mediating this increased activity remains in question. In the present study, static contraction of the triceps surae was induced by electrical stimulation of L7 and S1 ventral roots in anesthetized cats (n=12). Endogenous release of glutamate (Glu) from the SRF was recovered by microdialysis and measured by HPLC. Static muscle contraction for 4 min increased mean arterial pressure (MAP) 38+/-4 mmHg from a control level of 102+/-12 mmHg (P< 0.05). During muscle contraction the extracellular concentration of Glu recovered from the SRF increased from 623+/-117 to 1078+/-187 nM (P<0.05). To determine the effect of muscle contraction on Glu release in the absence of synaptic input from other reflexogenic areas, contraction was repeated following acute sinoaortic denervation and vagotomy. Following this denervation, muscle contraction increased MAP 41+/- 4 mmHg (P < 0.05) and Glu concentration from 635+/-246 to 1106+/-389 nM (P < 0.05). Muscle paralysis prevented the increases in MAP and Glu concentration during ventral root stimulation. These results suggest that: (i) Glu is released in the SRF during activation of contraction-sensitive skeletal muscle afferent fibers in the cat; and (ii) synaptic input from other reflexogenic areas appears to be ineffective in modulating the release of Glu in the SRF during static muscle contraction.

Effects of learned helplessness on brain GABA receptors

GABA is involved in both clinical depression and in animal models of depression; however, the roles of GABA(A) and GABA(B) receptors in specific brain regions are not clear. Changes in densities of both GABA(A) and GABA(B) receptors have been reported with the learned helplessness animal model of depression and with chronic antidepressant drug treatment. However, some of these findings are discrepant. Thus, we used quantitative autoradiography to study the GABA(A) and GABA(B) receptors in learned helplessness and we used an experimental paradigm that allows non-specific effects of stress to be differentiated from learned helplessness. Densities of GABA binding were measured in prefrontal cortex, septum, hippocampus, hypothalamus and amygdala. In the septum, learned helpless rats had increased densities of GABA(A) receptors and rats that did not become helpless after inescapable stress had decreased GABA(B) receptor densities. No significant group differences of GABA(A) or GABA(B) receptor densities were observed in any other brain region studied. These results suggest a unique role for the septum in modulating GABA in the learned helplessness animal model of depression.

Effect of basal ganglia injury on central dopamine activity in Gulf War syndrome: correlation of proton magnetic resonance spectroscopy and plasma homovanillic acid levels

BACKGROUND

Many complaints of Gulf War veterans are compatible with a neurologic illness involving the basal ganglia.

METHODS

In 12 veterans with Haley Gulf War syndrome 2 and in 15 healthy control veterans of similar age, sex, and educational level, we assessed functioning neuronal mass in both basal ganglia by measuring the ratio of N-acetyl-aspartate to creatine with proton magnetic resonance spectroscopy. Central dopamine activity was assessed by measuring the ratio of plasma homovanillic acid (HVA) and 3-methoxy-4-hydroxyphenlyglycol (MHPG).

RESULTS

The logarithm of the age-standardized HVA/MHPG ratio was inversely associated with functioning neuronal mass in the left basal ganglia (R(2) = 0.56; F(1,27) = 33.82; P<.001) but not with that in the right (R(2) = 0. 04; F(1,26) = 1.09; P =.30). Controlling for age, renal clearances of creatinine and weak organic anions, handedness, and smoking did not substantially alter the associations.

CONCLUSIONS

The reduction in functioning neuronal mass in the left basal ganglia of these veterans with Gulf War syndrome seems to have altered central dopamine production in a lateralized pattern. This finding supports the theory that Gulf War syndrome is a neurologic illness, in part related to injury to dopaminergic neurons in the basal ganglia.

Increased septal 5-HIAA efflux in rats that do not develop learned helplessness after inescapable stress

Learned helplessness is a behavioral deficit that can be induced by exposure to inescapable stress. Previous studies have implicated the lateral septum in mediating this phenomenon, and in this brain region, serotonin plays an important role in the development, maintenance, prevention, and reversal of learned helplessness behavior. Using the technique of in vivo microdialysis, we measured the efflux of serotonin (5-HT), dopamine (DA), and their respective metabolites, 5-hydroxyindoleacetic acid (5-HIAA) and 3, 4-dihydroxyphenylacetic acid (DOPAC), from the lateral septum of rats that either developed or did not develop learned helplessness. During the microdialysis session all rats were subjected to restraint stress. Control groups included naïve, home cage rats as well as tested control rats that were subjected to the identical handling, restraint, and shuttlebox testing as the rats that received inescapable shock. Overall, levels of 5-HIAA were significantly higher in non-helpless rats. There were no significant effects of restraint or differences in levels of 5-HT, DA, or DOPAC. We propose that this increase in 5-HIAA is indicative of an overall increase in serotonin metabolism in the lateral septum of rats that do not become helpless after inescapable stress. This increased serotonin metabolism in the lateral septum may protect the animal from adverse behavioral consequences of inescapable stress. J. Neurosci. Res. 61:101-106, 2000. Published 2000 Wiley-Liss, Inc.

Nefazodone treatment for chronic posttraumatic stress disorder: an open trial

Currently, there is no standard treatment for posttraumatic stress disorder (PTSD) because of a deficit of systematic treatment trials. The symptom overlap with other mood and anxiety disorders that respond to antidepressants and the results of a limited number of antidepressant trials indicate promise for psychopharmacologic treatment. Several open trials and one controlled trial with selective serotonin reuptake inhibitors have reported improvement in the symptomatology of PTSD. In this study, a relatively new serotonergic antidepressant, nefazodone, was tested as a treatment for PTSD. Veterans with chronic PTSD (N = 36) were enrolled in an 8-week open-label trial of nefazodone. The primary outcome measure was the Clinician-Administered PTSD Scale (CAPS). Thirty-one patients completed at least 4 weeks of treatment, which was considered to be an adequate trial, and 26 patients completed the 8-week study. During treatment, there was a significant decrease in the total CAPS score and in each of three CAPS subscale scores, with most of the improvement occurring during the first 4 weeks. Comparable improvements were also seen on the Hamilton Rating Scales for Anxiety and for Depression. Nefazodone treatment was well tolerated by this patient population, with only four patients discontinuing because of adverse effects. In summary, nefazodone treatment improved the symptoms of PTSD, including the core symptoms. Placebo-controlled studies should be undertaken to further elucidate the efficacy of nefazodone in the treatment of PTSD.

Acute stress does not alter 5-HT1A receptor density

Previous research suggests that 5-HT1A receptors are altered with exposure to chronic stress. No previous studies have examined the effect of acute stress on 5-HT1A. Using receptor autoradiography it was observed that there were no differences in [3H]-8-OH-DPAT binding between control rats and rats that received 20 minutes of restraint stress 2 hours prior to sacrifice. This study suggests that the changes in 5-HT1A receptor density associated with chronic stress develop over the course of repeated stress.

Evidence for the segregation of a major gene for human plasma GABA levels

Gamma-aminobutryic acid (GABA) is a major neurotransmitter in the central nervous system, and plasma levels of GABA may reflect brain GABA activity. In 35-40% of patients with mood disorders, plasma GABA levels are low compared to psychiatrically normal controls. Low plasma GABA in this subgroup of patients has characteristics of a biological trait marker for mood disorders. Low plasma GABA is also found in a subset of patients with alcohol dependence, but not in schizophrenia, anxiety, or eating disorders, suggesting some diagnostic specificity. Previous data from a small study of monozygotic twins are consistent with the hypothesis that plasma GABA levels are under genetic control. To better understand these mechanisms, we conducted a segregation analysis of plasma GABA levels in a sample of 157 individuals from 50 nuclear families. Analysis using the Class D regressive model indicated that the familial transmission of plasma GABA levels is compatible with the segregation of a recessive major gene. Our results suggest that plasma GABA levels are under single gene control. Future research should address the precise mechanisms which may account for the abnormality in GABA levels seen in a subset of patients with mood disorders.

D-fenfluramine challenge in posttraumatic stress disorder

BACKGROUND

Much progress has been made in understanding the role of catecholamines in the pathophysiology of posttraumatic stress disorder (PTSD). Recent research has broadened the scope of neuroregulation of PTSD to include serotonin.

METHODS

We used the serotonin-releasing agent and reuptake inhibitor, d-fenfluramine, to assess the integrity of the serotonin-mediated prolactin release in 8 men with combat-induced PTSD and 8 healthy men.

RESULTS

The veterans with PTSD had a significantly lower prolactin response to d-fenfluramine as compared to healthy control subjects. The prolactin response to d-fenfluramine was inversely correlated with the patient's level of PTSD symptomatology and measures of aggression.

CONCLUSIONS

This study demonstrates a central serotonin dysfunction, as reflected in a lower prolactin response to d-fenfluramine, in patients with PTSD.

Decrease in stress-induced c-Fos-like immunoreactivity in the lateral septal nucleus of learned helpless rats

Inescapable stress can induce learned helplessness in many species of animals. Learned helplessness is a phenomenon which has some behavioral and neurotransmitter analogies with human clinical depression. Stress can also induce the expression of immediate early genes, including c-fos in many areas of the central nervous system. We examined stress-induced c-Fos-like immunoreactivity (FLI) using the learned helplessness paradigm. Naive rats showed significantly higher FLI than the tested groups in all the amygdaloid regions and in the hypothalamic paraventricular nucleus. However, in the lateral septal nucleus, helpless animals showed significantly reduced FLI in response to stress, compared to the other groups. These, and other previous data, highlight the importance of the septal area in mediating behavioral responses to inescapable stress.

Serotonin and learned helplessness: a regional study of 5-HT1A, 5-HT2A receptors and the serotonin transport site in rat brain

Serotonin (5-HT) plays a central role in the neurochemistry of the learned helplessness animal model of depression. Using quantitative autoradiography, we measured the density of 5-HT1A and 5-HT2A receptors and of 5-HT transport sites in medial prefrontal cortex, dorsal hippocampus, septum, hypothalamus, and amygdala in learned helpless rats, and in rats that were nonhelpless after inescapable stress, as well as in shuttlebox-tested and nonhandled controls. We found no changes in 5-HT1A receptor density among the groups in any region studied. In dorsal hippocampus, 5-HT2A receptor density was decreased in nonhelpless rats, while in amygdala 5-HT2A receptor density was decreased in both groups of stressed rats, whether helpless or nonhelpless. In the hypothalamus 5-HT2A receptor density, was decreased in helpless rats as compared to controls. In medial prefrontal cortex, the serotonin transport sites showed decreased density in helpless rats as compared to controls but not to nonhelpless rats. These findings further highlight the complexity of regional 5-HT effects in the learned helplessness animal model.

GABA and brain abnormalities in schizophrenia

Some recent autopsy studies indicate that gamma-aminobutyric acid (GABA) function is decreased in brain areas that involve some of the well-described structural changes observed in schizophrenia. The current study examined the relationship between CSF and plasma GABA levels and brain structural measures in schizophrenia. Sixty-two drug-free, physically healthy male patients with schizophrenia (DSM-IIIR) were evaluated for plasma and CSF GABA, as well as brain structural measures on CT scans. Plasma levels of GABA were associated with prefrontal sulcal widening and VBRs, but not global sulcal widening in the schizophrenic patients. CSF GABA measures were not associated with brain structural measures, but were associated with age and age of onset. The significant relationship between plasma GABA, but not CSF GABA, and specific brain morphology measures in schizophrenic patients suggests that if GABA transmission is impaired in schizophrenia, it is a local, but not global, phenomenon.

Plasma GABA in children and adolescents with mood, behavior, and comorbid mood and behavior disorders: a preliminary study

Plasma GABA concentrations (pGABA) were measured in 115 inpatients (aged 7-17) with child psychiatric disorders. Group mean pGABAs were compared for 38 patients with mood disorders only (MOOD), 29 with behavior disorders only (BEH), 48 with comorbid mood and behavior disorders (MOOD + BEH), and 14 normal controls (CON, aged 14-17). The BEH group was characterized by (a) high mean pGABAs (157 vs. 133 pmol/ml), (b) lower mean pGABAs in BEH subjects who had been receiving pharmacotherapy with SSRIs or other medications (p < 0.026), and (c) decreased pGABA with increasing age (p = 0.019). These features were not found in controls or in groups of patients with mood disorders (MOOD or MOOD + BEH). Elevated mean pGABA in the BEH group appeared specifically in patients with comorbid CD and ADHD, not in patients with ADHD or CD alone (p = 0.004). No patient in BEH (or CON) had pGABA below 100 pmol/ml, but low pGABAs were found in 15% of MOOD patients (who had no behavior disorder) and in 16% of MOOD + BEH patients. Pharmacotherapy did not change pGABAs in the MOOD or the MOOD + BEH groups. No pGABA differences were found among the anxiety disorders, either alone or with mood or behavior comorbidity. The finding that plasma GABA levels are elevated in nonmedicated behavior disorders that present in the absence of mood disorders, and appear to lower following medication treatments, merits increased attention to the pharmacological study of nonaffective behavior disorders.

Benzodiazepine prevention of swim stress-induced sensitization of cortical biogenic amines: an in vivo microdialysis study

In vivo microdialysis was used to determine the effect of diazepam, flumazenil and FG-7142 upon the biogenic amine response to acute and repeated swim stress in the medial prefrontal cortex of the rat. Acute swim stress increased norepinephrine levels, although dopamine and serotonin levels remained stable. Upon re-exposure to swim stress twenty-four hours later, sustained increases (200-300% of baseline) in all three biogenic amines were detected. This enhanced response to re-stress was not seen in rats pretreated with either a benzodiazepine: agonist (diazepam, 2 mg/kg), an antagonist (flumazenil, 10 mg/kg), or an inverse agonist (FG-7142, 10 mg/kg) given prior to the first swim stress. Therefore, the sensitization of biogenic amine response to re-stress may be prevented by compounds which differ in their activity at the benzodiazepine receptor.

Plasma gamma-aminobutyric acid (GABA) predicts outcome in patients with alcohol dependence

1. Previous studies have suggested that low plasma GABA levels (< or = 100 pmol/ml) may characterize a subset of patients with alcohol dependence. 2. In order to assess the clinical relevance of this biologic finding, the authors followed 49 alcohol dependent patients for up to 18 months following inpatient treatment. Treatment outcome was assessed by continuous abstinence and continued contact with research personnel. 3. Alcohol dependent patients with low plasma GABA had significantly better outcome than patients with plasma GABA in the normal control range (101-150 pmol/ml). 4. These findings suggest that plasma GABA measures may prove to be clinically useful in identifying alcohol dependent patients at risk for relapse.

Growth hormone response to the GABAB agonist baclofen in major depressive disorder

Growth hormone (GH) response to the gamma-aminobutyric acid B receptor agonist, baclofen, was measured in 16 male patients with major depressive disorder and in 16 age-matched healthy male controls. No significant differences were found in the GH response to baclofen between the depressed patients and controls. On repeat testing, the GH response to baclofen showed significant retest reliability in both groups. There was no significant correlation between serum baclofen levels and the GH response to baclofen. Age significantly correlated with GH response, with older subjects having lower GH response to baclofen. These data do not suggest that a blunted GH response to baclofen. represents a specific neuroendocrine feature of major depression.

Learned helplessness increases 5-hydroxytryptamine1B receptor mRNA levels in the rat dorsal raphe nucleus

Learned helplessness is a behavioral condition induced by exposure to inescapable stress that models aspects of stress-related disorders including depression and posttraumatic stress disorder, and has been associated with diminished serotonin release in the rat frontal cortex. Our hypothesis was that presynaptic 5-hydroxytryptamine1B (5-HT1B) receptors, which inhibit the synthesis and release of serotonin in nerve terminals, may be increased in learned helplessness. Postsynaptic 5-HT1B mRNA hybridization levels in the hippocampus or frontal cortex were unchanged following induction of learned helplessness; however, presynaptic 5-HT1B mRNA hybridization signal in the dorsal raphe nucleus of helpless rats was 25% higher than control values. There was no change in dorsal raphe serotonin transporter mRNA level. The detection of increased 5-HT1B mRNA levels in the dorsal raphe nucleus suggests an increased capacity to synthesize presynaptic 5-HT1B receptors and could account for diminished serotonin neurotransmission in learned helplessness.

Whole-blood serotonin in children and adolescents with mood and behavior disorders

Whole-blood serotonin (5-hydroxytryptamine, 5-HT) levels were measured in 118 children and adolescents with DSM-III-R mood disorders (n = 30) or behavior disorders (n = 27), a mixed group who met criteria for both mood and behavior disorders (n = 47), and a small sample of normal control subjects (n = 14). The patients were selected from consecutive admissions to an inpatient state hospital setting and the control subjects were recruited from a local high school. Levels of whole-blood 5-HT were significantly higher in the behavior disorder group (193 +/- 120) than in the mood disorder (122 +/- 83) or mixed mood and behavior (137 +/- 95) patient groups, but did not differ from control levels (170 +/- 48). A subsample of patients irrespective of diagnostic classification who had been on a selective serotonin reuptake inhibitor (SSRI) before admission had significantly lower whole-blood 5-HT concentrations (97.8 +/- 78.4) than those in patients who had been receiving some other type of psychotropic medication at admission (159.8 +/- 109.2) and from those in unmedicated patients (161.9 +/- 101.4). The 5-HT concentrations for patients receiving non-SSRI psychotropic medications did not differ from those of unmedicated patients. The frequency analysis of 5-HT concentration by psychiatric disorder group suggests that patients with mood disorders have the lowest values (below 100 ng/ml) and patients with behavior disorders have the highest values (above 300 ng/ml). Levels in the limited sample of normal subjects were all between 100 and 300 ng/ml. These findings were not accounted for by age, sex, gender, race, or season and lend support to accumulating research on simple neurobiological indicators in blood that help to distinguish these child/adolescent psychiatric disorders from each other and from individuals without these disorders.

Effect of benzodiazepines on plasma levels of homovanillic acid in anxious patients and control subjects

The effects of four logarithmically increasing doses of intravenous diazepam or placebo on plasma homovanillic acid (HVA) were determined in benzodiazepine-naive patients with panic disorder (PD) or generalized anxiety disorder (GAD), and in healthy controls. Plasma HVA was measured at baseline and 3 min after the first and fourth doses of diazepam/placebo. Mean baseline plasma HVA levels were significantly lower in PD patients compared with GAD patients and controls. Although plasma HVA levels decreased significantly with time in all groups, there was no diazepam effect. This study suggests that low dopaminergic activity may occur in a subset of anxious patients (PD), and that diazepam does not significantly affect dopaminergic activity as measured by plasma HVA in humans.

Plasma levels of gamma-aminobutyric acid and panic disorder

Low levels of gamma-aminobutyric acid (GABA) in plasma have been associated with the presence of mood disorders in patients with major depressive disorder. We examined plasma GABA in patients with panic disorder, a disorder that is often comorbid with major depression, and in a group of control subjects. Patients with panic disorder had plasma GABA levels that did not differ significantly from levels in controls subjects. These data support the specificity of low plasma GABA as a marker for mood disorders.

Excitatory amino acid concentrations in the spinal dorsal horn of cats during muscle contraction

In anesthetized cats, static hindlimb muscle contraction reflexly increases mean arterial pressure (MAP) and heart rate (HR). Pharmacological and immunohistochemical evidence suggests that excitatory amino acids are involved in the spinal transmission of this reflex. Using microdialysis and high-performance liquid chromatography technology, we tested the hypothesis that static contraction of the triceps surae muscle increases the extracellular concentration of glutamate (Glu) and aspartate (Asp) at the L7 level of the dorsal horn of the spinal cord. With the exception of the L7 dorsal root, the L5-S2 dorsal and ventral roots were cut ipsilateral to the contracting muscle. After the insertion of microdialysis probes and a 3-h recovery period, a 2-min static contraction was electrically evoked. MAP and HR increased by 53 +/- 8 mmHg and 20 +/- 4 beats/min. The concentration of Glu increased from 324 +/- 59 to 857 +/- 80 nM, whereas Asp increased from 199 +/- 57 to 499 +/- 113 nM. These results were repeatable, in that Glu and Asp rose by similar amounts in two subsequent contractions. In both of these latter contractions, MAP and HR were also significantly increased. By contrast, in a subset of cats whose L7 dorsal roots were cut after the first contraction, neither MAP, HR, Glu, nor Asp was significantly increased over baseline levels. These data demonstrate that static contraction of the hindlimb increases the extracellular concentration of Glu and Asp in the dorsal horn. In summary, the results from this study are in agreement with previous findings suggesting that excitatory amino acids are involved in the spinal transmission of sensory information from the hindlimb muscle.

Low plasma gamma-aminobutyric acid levels during the late luteal phase of women with premenstrual dysphoric disorder

OBJECTIVE

Plasma gamma-aminobutyric acid (GABA) levels have been reported to be low in some patients with major depressive disorder. Premenstrual dysphoric disorder is often associated with major depressive disorder. Therefore, the authors sought to determine whether women with premenstrual dysphoric disorder with or without prior major depressive disorder also had low plasma GABA levels.

METHOD

Plasma GABA levels were measured in 27 women with premenstrual dysphoric disorder and 21 comparison women during the the mid-follicular and late luteal phases of the menstrual cycle.

RESULTS

In comparison women, plasma GABA levels increased from the mid-follicular to the late luteal phase. Women with premenstrual dysphoric disorder and a past history of major depressive disorder had low plasma GABA levels during both phases. In women with premenstrual dysphoric disorder but no past major depressive disorder, plasma GABA levels decreased from the nonsymptomatic, mid-follicular phase to the symptomatic, late luteal phase.

CONCLUSIONS

Decreased GABA function may represent a common biological link between subtypes of depressive and premenstrual dysphoric disorders. A trait in major depressive disorder and a state-dependent decrease in premenstrual dysphoric disorder might imply a possible continuum between the two disorders.

Effect of diazepam on plasma gamma-aminobutyric acid in sons of alcoholic fathers

A subgroup of abstinent alcoholics, display low levels of plasma gamma-aminobutyric acid (GABA). Two previous studies of plasma GABA in sons of alcoholic fathers (SOAs) have yielded conflicting results. The aim of the current study was to measure plasma GABA both at baseline and after challenge with diazepam, a GABAA receptor agonist, in a group of SOAs already shown to display decreased eye movement, memory, and sedative effects of diazepam. Twenty-seven SOAs and 23 male control subjects received four logarithmically increasing doses of diazepam or placebo in randomized order on 2 days at least 1 week apart. Plasma GABA was measured at baseline and after the last dose. There were no significant differences between SOAs and controls in baseline plasma GABA levels. In the whole sample, there were significant correlations between baseline plasma GABA and both high novelty-seeking and low-harm avoidance scores on the Tridimensional Personality Questionnaire. Both SOAs and controls displayed decreases in plasma GABA over time on both testing days, but there was no effect of diazepam on plasma GABA and no significant difference between groups in plasma GABA response to diazepam. These results suggest that neither low plasma GABA at baseline nor altered plasma GABA response to diazepam is associated with increased genetic risk for alcoholism.

Growth hormone response to baclofen: a comparison of 10-mg and 20-mg doses in healthy men

Growth hormone (GH) response and baclofen levels were measured in seven healthy adult men following a 10-mg and a 20-mg dose of oral baclofen (gamma-aminobutyric acidB agonist) to determine the preferred dose in baclofen challenge studies. Multivariate analysis of variance (ANOVA) with repeated measures revealed no differences between the doses. However, when a univariate ANOVA with repeated measures was performed for each dose, the 10-mg dose showed no significant GH response over time, whereas the 20-mg dose showed a significant GH response over time. The average delta GH (change in GH from baseline) was 7.84 ng/ml (SD = 10.17) for the 10-mg dose and 3.34 ng/ml (SD = 3.64) for the 20-mg dose. The variability in the delta GH response to the 10-mg dose was significantly greater than the response to the 20-mg dose of baclofen. This variance in GH response was not explained by the differences in serum baclofen levels. Thus, a 20-mg baclofen dose appears to be preferable to a 10 mg-dose in baclofen challenge studies.

Plasma GABA predicts acute response to divalproex in mania

Bipolar I, manic phase inpatients were treated with divalproex sodium, lithium, or placebo in a previously reported parallel group multicenter, double-blind, randomized, controlled acute phase treatment trial. Plasma concentrations of gamma aminobutyric acid (GABA) were measured before and after treatment. Higher pretreatment plasma GABA levels were significantly (p = .04) related to a better clinical response to divalproex (n = 19). Pretreatment plasma GABA levels did not correlate with response to either lithium (n = 13) or placebo (n = 31). Following treatment with divalproex sodium, plasma GABA levels decreased significantly (p < .05), compared to placebo. Pretreatment plasma GABA levels were not related to overall severity of manic symptoms. Plasma GABA may predict response to pharmacologic agents acting on the GABA system.

Serotonin dysfunction disorders: a behavioral neurochemistry perspective

The spectrum of efficacy of the serotonin selective reuptake inhibitor (SSRI) antidepressant drugs continues to expand. In fact, no psychiatric syndrome seems to worsen with these agents, and few studies fail to demonstrate clinical improvement in some patients, regardless of any nosologic nicety, such as precise DSM diagnosis. This suggests that the biological rubric of psychopathology is dimensional rather than categorical. New research using in vivo microdialysis shows differences in neurochemistry among SSRIs, wherein fluoxetine blocks reuptake of dopamine and norepinephrine, as well as serotonin, in medial prefrontal cortex, and fluvoxamine has a relatively more selective neurochemical profile. In the animal model of learned helplessness, which is a biobehavioral model for stress-induced anxiety causing depression, the SSRIs including fluvoxamine prevent helplessness. From these and other data, a neurotransmitter balance theory of biopsychopathology is formulated. In this hypothetical construct, dopamine, norepinephrine, and GABA modulate thought, anxiety, and mood, respectively. Serotonin is a stabilizing agent, which assists in returning the mind to its homeostatic setpoint.

Levels of gamma-aminobutyric acid in cerebrospinal fluid and plasma during alcohol withdrawal

Aminobutyric acid (GABA) is implicated in the biochemical pathophysiology of alcohol intoxication, dependence and withdrawal. We therefore measured GABA in both cerebrospinal fluid (CSF) and plasma from 14 male alcohol-dependent patients during acute alcohol withdrawal (day 1) and again after 21 days of inpatient treatment (day 21). Plasma GABA levels on admission correlated with indices of liver function. When corrected for differences in liver function, plasma levels of GABA levels on day 1 were significantly higher than on day 21. CSF GABA concentrations were also significantly higher during withdrawal compared with concentrations after 3 weeks of abstinence. The change in plasma GABA levels correlated significantly with the change in CSF GABA levels, although there was no correlation between plasma and CSF levels at either time. These findings demonstrate that changes in CSF GABA may be reflected in plasma GABA, and they highlight the potential importance of the GABA system in alcohol dependence and withdrawal.

Stability of plasma GABA at four-year follow-up in patients with primary unipolar depression

The biology of mood disorders involves gamma-aminobutyric acid (GABA), a neurotransmitter whose levels in plasma likely reflect brain GABA activity. Previous research has shown that a subset of patients with primary unipolar major depression have low plasma GABA levels, which parallels findings from studies of cerebrospinal fluid. We have completed a 4-year follow-up on 46 male patients with primary unipolar depression. Plasma levels of GABA were stable over this time. For the group, mean plasma GABA levels on follow-up did not change significantly from entry levels. Plasma GABA levels measured on follow-up were significantly (p < .001) correlated with entry levels. Patients with low plasma GABA levels (< 100 pmol/ml) on entry into the study were likely to remain low on follow-up, and patients with plasma GABA levels in the control range (> or = 100 pmol/ml) at entry similarly remained in this category (chi 2 = 7.23, p = .007). This was true whether or not the patient had recovered from depression on follow-up. Levels of plasma GABA did not significantly correlate with severity of depression at either entry (p = .40) or follow-up (p = .52), nor was there a significant correlation between change in plasma GABA and change in the 17-item Hamilton Depression Rating Scale score from entry to follow-up (p = .89). These data are consistent with the notion that plasma GABA is independent of clinical state in patients with primary unipolar depression. Low plasma GABA may be a trait marker of illness in a subset of patients with mood disorder.

Beta-adrenergic receptor changes in learned helplessness may depend on stress and test parameters

Behavioral deficits following inescapable stress (learned helplessness) may serve as an animal model of depression. Previous studies using foot-shock stress to induce learned helplessness and a bar-press test for the stress-induced behavioral deficit have found increased beta-adrenergic receptor density in the hippocampus of learned helpless rats. We replicated these experiments using a tail-shock stress and the shuttle-box test. In our experiments, rats that developed learned helplessness after inescapable stress did not demonstrate any significant differences in beta-adrenergic receptor density or affinity in the frontal cortex, cerebellum, or hippocampus compared to the nonhelpless rats, nor to the tested control rats. These results suggest that beta-adrenergic receptor changes in learned helplessness may depend on the specific stress and test procedures used.

In vivo biogenic amine efflux in medial prefrontal cortex with imipramine, fluoxetine, and fluvoxamine

In vivo brain microdialysis was used to determine the effects of the standard tricyclic antidepressant imipramine and the two selective serotonin reuptake inhibitors (SSRIs) antidepressants, fluoxetine and fluvoxamine, on extracellular levels of norepinephrine (NE), dopamine (DA), and serotonin (5-HT) in rat medial prefrontal cortex. When given intraperitoneally (IP), imipramine increased NE in the microdialysis perfusate, and elevated DA and 5-HT to a lesser extent. Similar dose-dependent increases in DA and 5-HT were detected after IP fluoxetine, although NE was less affected. In contrast, IP fluvoxamine produced no change in basal NE nor DA, although a large increase in 5-HT occurred at an intermediate dose. When administered directly into cortex, all three antidepressants increased 5-HT by the same amount in a dose-dependent fashion. Intracortical imipramine and fluoxetine increased NE, and fluoxetine and fluvoxamine both increased DA, with fluoxetine doing so at a lower concentration. These data suggest that the SSRIs are not entirely selective for serotonin in vivo.

Previous stress increases in vivo biogenic amine response to swim stress

In vivo microdialysis was used to determine biogenic amines in medial prefrontal cortex of rats exposed to eight minutes of swim stress on two consecutive days. On the first day of stress, norepinephrine (NE) efflux increased by 183% over baseline after stress, while dopamine (DA) and serotonin (5-HT) remained stable throughout. On the second day of stress, a robust increase was observed in all 3 neurotransmitters measured, with (NE), (DA), and (5-HT) increasing by 310%, 441% and 496% respectively, and remaining elevated for an hour or more after stress. This suggests that the first exposure to swim stress, while not causing dramatic changes in biogenic amine release, may sensitize biogenic amines in medial prefrontal cortex to subsequent swim stress. Our results also serve as preliminary data concerning the neurochemical changes which might underlie the forced swimming model of "behavioral despair".

Desipramine does not alter plasma GABA in patients with major depression

Low plasma GABA is a biological marker for depression in a subset of patients tested. Plasma GABA has been shown to reflect brain GABA activity. This marker has many characteristics of a trait marker for depression, including stability with time, and lack of influence by coincident factors such as gender, season, time, activity or diet. We here report that plasma GABA remained stable after 4 weeks of treatment with desipramine in patients with major depression. Since the levels of plasma GABA did not change with time, nor with clinical improvement, plasma GABA is not a state marker of depression.

Low plasma GABA is a trait-like marker for bipolar illness

Plasma gamma-aminobutyric acid (pGABA) is an index of brain GABA activity and a peripheral marker of mood disorder. Previous research has indicated that pGABA is abnormally low in approximately 40% of patients symptomatic with primary unipolar depression. We have now measured pGABA in a series of patients with bipolar disorder. Blood samples for GABA determinations were collected soon after admission to hospital or clinic while patients were symptomatic. In both manic and depressed phase bipolar patients, mean levels of pGABA were significantly lower than in healthy control subjects. The distribution of pGABA in bipolar patients, whether manic or depressed, was similar to that in symptomatic unipolar depression, with 30% to 40% having pGABA levels lower than the control range. These data indicate that low pGABA is not specific to the depressed state, as it is also found in the manic phase of bipolar disorder. Low pGABA may represent a shared biologic correlate between bipolar and unipolar illness.

Intravenous sodium lactate decreases plasma GABA levels in man

Gamma aminobutyric acid (GABA), an inhibitory neurotransmitter, may play an important role in anxiety. We studied changes in plasma GABA levels in nine healthy subjects before and after infusions of sodium lactate and dextrose. Plasma GABA significantly decreased during infusions of sodium lactate (109.3 +/- 4.4 versus 91.6 +/- 5.1 pmol/ml; P = 0.0001) but not during infusions of dextrose.

Low plasma gamma-aminobutyric acid levels in male patients with depression

Plasma levels of gamma-aminobutyric acid (GABA) were significantly lower in males with primary unipolar major depressive disorder than in healthy controls. Although the difference in means between control and symptomatic depressed patient groups was small, the distribution of plasma GABA in the depressed patients was markedly different from controls. Forty percent of depressed patients had plasma GABA levels below those of controls. Plasma GABA levels correlated positively with duration of illness, and negatively with age at onset of the mood disorder and the total Endogenomorphic Symptom Score on the Hamilton Rating Scale. Plasma GABA levels may be a biochemical marker of vulnerability to depression, as opposed to a consequence of the illness. A low GABA condition in depression fits and complements the prevailing biogenic amine hypotheses of depression.

Effect of acute and chronic benzodiazepines on plasma GABA in anxious patients and controls

The acute effects of diazepam on plasma GABA were determined in 18 patients with panic disorder, 13 patients with generalized anxiety disorder and 20 healthy controls. All subjects were benzodiazepine-naive. Four logarithmically increasing doses of diazepam/placebo were administered intravenously at 15-min intervals on 2 separate days. Plasma GABA was measured at baseline and 3 min after the highest dose of diazepam/placebo. There was an overall decrease in plasma GABA that was significantly greater following diazepam compared with placebo, but no group differences in response. In a separate group of 18 panic disorder patients receiving chronic benzodiazepine treatment with alprazolam, the same diazepam infusion procedure (no placebo day) produced decreases in plasma GABA similar to those seen in the untreated panic disorder patients. The clinical and physiologic implications of these findings are discussed.