Stress increases noradrenaline release in the rat frontal cortex: prevention by diazepam

Chronic social isolation during adolescence augments catecholamine response to acute ethanol in the basolateral amygdala

Adolescent social isolation (SI) results in numerous behavioral alterations associated with increased risk of alcoholism. Notably, many of these changes involve the basolateral amygdala (BLA), including increased alcohol seeking. The BLA sends a strong glutamatergic projection to the nucleus accumbens and activation of this pathway potentiates reward-seeking behavior. Dopamine (DA) and norepinephrine (NE) exert powerful excitatory and inhibitory effects on BLA activity and chronic stress can disrupt the excitation-inhibition balance maintained by these catecholamines. Notably, the impact of SI on BLA DA and NE neurotransmission is unknown. Thus the aim of this study was to characterize SI-mediated catecholamine alterations in the BLA. Male Long Evans rats were housed in groups of four (GH) or in SI for 6 weeks during adolescence. DA and NE transporter levels were then measured using Western blot hybridization and baseline and ethanol-stimulated DA and NE levels were quantified using microdialysis. DA transporter levels were increased and baseline DA levels were decreased in SI compared to GH rats. SI also increased DA responses to an acute ethanol (2 g kg(-1)) challenge. While no group differences were noted in NE transporter or baseline NE levels, acute ethanol (2 g kg(-1)) only significantly increased NE levels in SI animals. Collectively, these SI-dependent changes in BLA catecholamine signaling may lead to an increase in BLA excitability and a strengthening of the glutamatergic projection between the BLA and NAc. Such changes may promote the elevated ethanol drinking behavior observed in rats subjected to chronic adolescent stress.

Molecular mechanisms of stress-induced prefrontal cortical impairment: implications for mental illness

The symptoms of mental illness often involve weakened regulation of thought, emotion, and behavior by the prefrontal cortex. Exposure to stress exacerbates symptoms of mental illness and causes marked prefrontal cortical dysfunction. Studies in animals have revealed the intracellular signaling pathways activated by stress exposure that induce profound prefrontal cortical impairment: Excessive dopamine stimulation of D1 receptors impairs prefrontal function via cAMP intracellular signaling, leading to disconnection of prefrontal networks, while excessive norepinephrine stimulation of alpha1 receptors impairs prefrontal function via phosphatidylinositol-protein kinase C intracellular signaling. Genetic studies indicate that the genes disrupted in serious mental illness (bipolar disorder and schizophrenia) often encode for the intracellular proteins that serve as brakes on the intracellular stress pathways. For example, disrupted in schizophrenia 1 (DISC1) normally regulates cAMP levels, while regulator of G protein signaling 4 (RGS4) and diacylglycerol kinase (DGKH)-the molecule most associated with bipolar disorder- normally serve to inhibit phosphatidylinositol-protein kinase C intracellular signaling. Patients with mutations resulting in loss of adequate function of these genes likely have weaker endogenous regulation of these stress pathways. This may account for the vulnerability to stress and the severe loss of PFC regulation of behavior, thought, and affect in these illnesses. This review highlights the signaling pathways onto which genetic vulnerability and stress converge to impair PFC function and induce debilitating symptoms such as thought disorder, disinhibition, and impaired working memory.

Blockade of IP3-mediated SK channel signaling in the rat medial prefrontal cortex improves spatial working memory

Planning and directing thought and behavior require the working memory (WM) functions of prefrontal cortex. WM is compromised by stress, which activates phosphatidylinositol (PI)-mediated IP3-PKC intracellular signaling. PKC overactivation impairs WM operations and in vitro studies indicate that IP3 receptor (IP3R)-evoked calcium release results in SK channel-dependent hyperpolarization of prefrontal neurons. However, the effects of IP3R signaling on prefrontal function have not been investigated. The present findings demonstrate that blockade of IP3R or SK channels in the prefrontal cortex enhances WM performance in rats, suggesting that both arms of the PI cascade influence prefrontal cognitive function.

Effects of chlordiazepoxide on footshock- and corticotropin-releasing factor-induced increases in cortical and hypothalamic norepinephrine secretion in rats

Noradrenergic and corticotropin-releasing factor (CRF) neuronal systems within the brain have been implicated in stress and anxiety. Synaptic release of cerebral norepinephrine (NE) is increased during stress, and following intracerebral CRF administration. Benzodiazepines are commonly used anxiolytic drugs but information on their effects on the stress- and CRF-related release of NE is limited. We have used in vivo microdialysis to test the effects of the benzodiazepine, chlordiazepoxide (CDP) on the noradrenergic responses to footshock and intracerebroventricular CRF in the medial hypothalamus and the medial prefrontal cortex (PFM) of freely moving rats. Footshock (60 x 0.1-0.2 mA shocks in 20 min) significantly increased microdialysate concentrations of NE in the first sample collected after initiating the footshock. In the hypothalamus, microdialysate NE was augmented 64% above baseline. A second footshock session (100 min after the first footshock) increased microdialysate NE to 313% of the baseline. Thus the noradrenergic responses to footshock were enhanced by preceding footshocks. CRF (100 ng) administered into the locus coeruleus (LC) almost tripled microdialysate concentrations of NE in the PFM. CDP (5mg/kg, i.p.) had no statistically significant effects on the basal dialysate concentrations of NE, but it significantly attenuated both footshock- and CRF-induced increases in dialysate NE. CDP may exert a direct inhibitory effect on the noradrenergic neurons, alter the input to LC noradrenergic neurons, or alter the ability of CRF to activate the LC noradrenergic system.

Involvement of the medial prefrontal cortex in central cardiovascular modulation in the rat

The medial prefrontal cortex (MPFC) and specifically its ventral portion (vMPFC) have been reported to modulate autonomic responses. On the cardiovascular system, this modulation is characterized by an influence on arterial blood pressure, regional blood flow as well as cardiac sympathetic and parasympathetic responses. The vMPFC also modulates baroreflex activity. Several neurotransmitters are present in the vMPFC. Among them L-glutamate, acetylcholine and noradrenaline are involved with cardiovascular modulation. In the present review, we describe evidences on the mechanisms involved in the vMPFC-related cardiovascular modulation.

Cortical/hippocampal monoamines, HPA-axis changes and aversive behavior following stress and restress in an animal model of post-traumatic stress disorder

Post-traumatic stress disorder (PTSD) is characterized by monoaminergic and hypothalamic-pituitary-adrenal (HPA)-axis abnormalities. Understanding monoamine-HPA-axis responses following stress and restress may provide a greater understanding of the neurobiology of PTSD and of its treatment. Hippocampal and frontal cortex serotonin, noradrenaline and dopamine, plasma corticosterone and aversive behavior were studied in rats on day 1 and day 7 post acute stress (AS = sequential restraint stress, swim stress and halothane exposure), and on day 1 and day 7 post restress (RS = swim stress). After AS, there was an early increase in both avoidant behavior and corticosterone (1 h after stress), with subsequent normalisation (day 7), suggesting an adequate adaptive response to the stressor. However, restress (RS) evoked a significant early HPA-axis hyporesponsiveness (1 h after RS) and a later significant increase in avoidant behavior on day 7 post RS. Hippocampal serotonin, noradrenaline and dopamine concentrations were unchanged 1 h post AS, but were significantly raised on day 7 post AS. Restress, however, reduced serotonin and noradrenaline levels 1 h after and on day 7 post RS, respectively, while dopamine was unchanged. In the frontal cortex only dopamine levels were altered, being significantly elevated 1 h after AS, and reduced on day 7 post RS. AS and RS thus differently effect the HPA-axis, evoking regional-specific brain monoamine changes that underlie maladaptive behavior and other post stress-related sequelae.

Blockade of tachykinin NK1 receptors attenuates stress-induced rise of extracellular noradrenaline and dopamine in the rat and gerbil medial prefrontal cortex

Substance P receptor antagonists cause antidepressant- and anxiolytic-like effects in rodents that are thought to involve brain monoamines. In the present study, we examined the effects of the NK1 receptor antagonist GR-205,171 on basal and stress-induced rise of extracellular noradrenaline (NA) and dopamine (DA) in the medial prefrontal cortex (mPFC) of conscious rats and gerbils with the in vivo microdialysis technique. GR-205,171 given intraperitoneally to rats (10 and 30 mg/kg) and gerbils (0.3 and 1 mg/kg) did not affect extracellular NA in either species and increased extracellular DA in rats. Forty minutes of immobilization increased extracellular NA and DA by, respectively, 179% and 188% of baseline values in rats and 222% and 316% of baseline values in gerbils. At 10 mg/kg, GR-205,171 attenuated the stress-induced increase of extracellular NA in the rat. At 30 mg/kg, GR-205,171 suppressed the effect of stress on extracellular DA but had no effect on NA. A lower dose (1 mg/kg) attenuated the stress-induced rise of extracellular NA and DA in the mPFC of gerbils. The results show that blockade of NK1 receptors marginally increased basal extracellular DA in rats but had no effect in gerbils, whereas the stress-induced rise of extracellular NA and DA was markedly attenuated in both species. It is suggested that catecholamines may contribute to the functional effects of GR-205,171.

In vivo neurochemical monitoring and the study of behaviour

In vivo neurochemical monitoring techniques measure changes in the extracellular compartment of selected brain regions. These changes reflect the release of chemical messengers and intermediates of brain energy metabolism resulting from the activity of neuronal assemblies. The two principal techniques used in neurochemical monitoring are microdialysis and voltammetry. The presence of glutamate in the extracellular compartment and its pharmacological characteristics suggest that it is released from astrocytes and acts as neuromodulator rather than a neurotransmitter. The changes in extracellular noradrenaline and dopamine reflect their role in the control of behaviour. Changes in glucose and oxygen, the latter a measure of local cerebral blood flow, reflect synaptic processing in the underlying neuronal networks rather than a measure of efferent output from the brain region. In vivo neurochemical monitoring provides information about the intermediate processing that intervenes between the application of the stimulus and the resulting behaviour but does not reflect the final efferent output that leads to behaviour.

Influence of naturally occurring variations in maternal care on prepulse inhibition of acoustic startle and the medial prefrontal cortical dopamine response to stress in adult rats

In rats, naturally occurring variations in maternal care contribute to the development of individual differences in the behavioral and neuroendocrine responses to stress during adulthood. The dopamine (DA) projection to the medial prefrontal cortex (mPFC) plays an important role in mediating stress responsivity and is thought to be involved also in regulating sensorimotor gating. In the present study, we compared prepulse inhibition (PPI) of acoustic startle as well as the left and right mPFC DA stress responses in the adult offspring of high- and low-licking/grooming (LG) dams. Our data indicate that the offspring of low-LG animals are impaired on measures of PPI compared with high-LG animals. We also observed in low-LG animals a significant blunting of the mPFC DA stress responses that was lateralized to the right hemisphere, whereas in high-LG animals, the left and right mPFC DA stress responses were equally attenuated. Although mPFC levels of DA transporter did not differ between the two groups of animals, mPFC levels of catechol-O-methyl transferase immunoreactivity of low-LG animals were significantly lower than those of high-LG animals. These data provide evidence that variations in maternal care can lead to lasting changes in mPFC DA responsivity to stress and suggest the possibility that such changes in mesocorticolimbic DA function can also lead to deficits in sensorimotor gating.

Blockade of the mGlu5 receptor decreases basal and stress-induced cortical norepinephrine in rodents

RATIONALE

Glutamate, the major excitatory neurotransmitter in the brain mediates its effects by both ionotropic and metabotropic receptor subtypes. Recently, the search for selective ligands for glutamate receptor subtypes has led to the discovery of 2-methyl-6-(phenylethynyl)pyridine (MPEP), an antagonist specific for metabotropic glutamate receptor 5 (mGlu5). This receptor is highly expressed in limbic forebrain regions and is thought to modulate anxiety-related processes. The noradrenergic nucleus locus coeruleus (LC) is an important mediator of stress responses and dysfunction of this system is implicated in affective disorders such as anxiety and depression.

OBJECTIVES

We sought to assess the effects of mGlu5 receptor antagonists, MPEP and 3-[(2-methyl-1,3-thiazol-4-yl)ethynyl]pyridine (MTEP) on cortical norepinephrine (NE) levels.

METHODS

In vivo microdialysis and high-pressure liquid chromatography with electrochemical detection (HPLC-ED) were used to assess the effects of mGlu5 antagonism on extracellular NE in the frontal cortex, a major terminal field of the LC.

RESULTS

Blockade of the mGlu5 receptor elicited significant reductions in extracellular NE in the frontal cortex. The benzodiazepine diazepam also reduced cortical NE. Furthermore, MPEP administration attenuated stress-induced increases in extracellular NE.

CONCLUSIONS

Taken together, these data show that MPEP and MTEP, through their blockade of the mGlu5, reduce extracellular norepinephrine, the impact of which may contribute to their anxiolytic actions.

Antagonism of the stress-induced increase in cortical norepinephrine output by the selective norepinephrine reuptake inhibitor reboxetine

We have previously shown that long-term treatment of rats with antidepressant drugs that affect the activity of noradrenergic and serotonergic neurons by different mechanisms, inhibits the increase in cortical norepinephrine output induced by stress. With the use of microdialysis, we have now evaluated the effects of reboxetine, an antidepressant drug that selectively inhibits norepinephrine reuptake, on the increase in cortical norepinephrine output elicited in rats by exposure to foot-shock stress or by the acute administration of N-methyl-beta-carboline-3-carboxamide (FG 7142) (20 mg/kg, i.p.). Foot-shock stress and FG 7142 each induced a marked increase in the cortical extracellular concentration of norepinephrine (+200 and +90%, respectively) in control rats. Long-term treatment with reboxetine (10 mg/kg, i.p., once a day for 21 days) reduced the effect of foot-shock stress and completely antagonized the effect of FG 7142 on cortical norepinephrine output. Our results suggest that changes in the activity of noradrenergic neurons in the cortex might be relevant to the anxiolytic and antidepressant effects of reboxetine.

Corticotropin-releasing hormone receptor blockade fails to alter stress-evoked catecholamine release in prefrontal cortex of control or chronically stressed rats

Although it is well documented that stress can increase the activity of central dopamine and norepinephrine neurons, little is known about the role of other neurotransmitters in modulating this response. Previous studies have implicated corticotropin-releasing hormone in modulating stress-evoked changes in the activity of locus coeruleus neurons. The present study examines whether corticotropin-releasing hormone contributes to stress-evoked increases in extracellular norepinephrine and dopamine in rat medial prefrontal cortex, as monitored by in vivo microdialysis. As noted previously, 30 min of tail-shock increased extracellular levels of norepinephrine and dopamine in the medial prefrontal cortex of naïve rats, and this was enhanced in rats previously exposed to chronic cold ( approximately 5 degrees C for 2-3 weeks). Previous intraventricular administration of a corticotropin-releasing hormone antagonist (D-Phe-corticotropin-releasing hormone; 3 and 9 microg) did not alter the tail-shock evoked in increase in extracellular levels of norepinephrine and dopamine in either naïve or chronically cold-exposed rats. Intraventricular administration of 3 microg of D-Phe-corticotropin-releasing hormone attenuated the increase in extracellular norepinephrine induced by co-administration of 3 microg of corticotropin-releasing hormone, confirming the efficacy of this compound. Results of the present study suggest that endogenous corticotropin-releasing hormone does not play a role in modulating the release of norepinephrine and dopamine occurring in response to acute tail-shock or the expression of a potentiated response to tail-shock in rats exposed chronically to cold.

Inhibition by venlafaxine of the increase in norepinephrine output in rat prefrontal cortex elicited by acute stress or by the anxiogenic drug FG 7142

Venlafaxine is an antidepressant drug that inhibits the reuptake of serotonin and norepinephrine with different efficacies. The effects of repeated administration of this drug on the increase in the extracellular concentration of norepinephrine in the prefrontal cortex, induced by stress or by the anxiogenic drug FG 7142, were studied in freely moving rats. Exposure to foot-shock stress induced a marked increase (+120%) in the extracellular norepinephrine concentration in the prefrontal cortex of control rats. Long-term administration of venlafaxine (10 mg/kg i.p., once a day for 21 days) reduced the effect of stress on norepinephrine output by 75%. This effect of venlafaxine persisted for at least 5 days after discontinuation of drug treatment. Acute administration of FG 7142 (20 mg/kg i.p.), a benzodiazepine receptor inverse agonist, increased norepinephrine output (+90%) in control rats. Chronic treatment with venlafaxine prevented the effect of FG 7142. In contrast, the acute administration of this antidepressant had no effect on the stress- or FG 7142-induced increase in norepinephrine output. These plastic changes in the sensitivity of norepinephrine neurones to foot-shock stress and to an anxiogenic drug may reveal an important neuronal mechanism for the physiological regulation of emotional state. Furthermore, this mechanism might be relevant to the anxiolytic and antidepressant effects of venlafaxine.

Chronic treatment with imipramine or mirtazapine antagonizes stress- and FG7142-induced increase in cortical norepinephrine output in freely moving rats

The effect of repeated administration of imipramine or mirtazapine, two antidepressant drugs with different mechanisms of action, was studied on the stress-induced increase in the extracellular concentration of norepinephrine in the prefrontal cortex of freely moving rats. Exposure to footshock in control rats induced a marked increase in extracellular norepinephrine concentrations in the prefrontal cortex (+120%). Long-term administration with imipramine or mirtazapine (10 mg/kg, i.p., twice or once a day, respectively, for 14 days) reduced (+50%) the effect of stress on basal norepinephrine output. Acute administration of FG7142 (30 mg/kg, i.p.), an anxiogenic benzodiazepine receptor inverse agonist, induced a marked increase in norepinephrine output (+90%) in control rats. In rats chronically treated with imipramine or mirtazapine this effect was completely antagonized. On the contrary, acute administration of these antidepressant drugs failed to reduce stress- and FG7142-induced increase in norepinephrine output. The plastic changes in the sensitivity of norepinephrine neurons to footshock stress and drug-induced anxiogenic stimuli may reveal a new important neuronal mechanism involved in the long-term modulation of emotional state. This action might be relevant for the anxiolytic and antidepressant effect of antidepressant drugs.

Prenatal opiate exposure: long-term CNS consequences in the stress system of the offspring

Our data show that prenatal morphine exposure induces long-term alterations in adult brain and behavior in both male and female rats, and these alterations are sex-specific. It is also evident that the alterations are not restricted to a single brain site or to a single neurotransmitter or neuromodulator. Moreover, there are long-term alterations in both the norepinephrine (NE) and opioid systems in several brain regions involved in stress responses and in the maintenance of homeostatic balance between the external environment, the brain and the rest of the body. Thus, this short paper reviews the prenatal morphine exposure data and highlights gaps in stress response to drug vulnerability/predisposition as an adult.

Depression as a spreading adjustment disorder of monoaminergic neurons: a case for primary implication of the locus coeruleus

A model for the pathophysiology of depression is discussed in the context of other existing theories. The classic monoamine theory of depression suggests that a deficit in monoamine neurotransmitters in the synaptic cleft is the primary cause of depression. More recent elaborations of the classic theory also implicitly include this postulate, other theories of depression frequently prefer to depart from the monoamine-based model altogether. We suggest that the primary defect emerges in the regulation of firing rates in brainstem monoaminergic neurons, which brings about a decrease in the tonic release of neurotransmitters in their projection areas, an increase in postsynaptic sensitivity, and concomitantly, exaggerated responses to acute increases in the presynaptic firing rate and transmitter release. It is proposed that the initial defect involves, in particular, the noradrenergic innervation from the locus coeruleus (LC). Dysregulation of the LC projection activities may lead in turn to dysregulation of serotonergic and dopaminergic neurotransmission. Failure of the LC function could explain the basic impairments in the processing of novel information, intensive processing of irrational beliefs, and anxiety. Concomitant impairments in the serotonergic neurotransmission may contribute to the mood changes and reduction in the mesotelencephalic dopaminergic activity to loss of motivation, and anhedonia. Dysregulation of CRF and other neuropeptides such as neuropeptide Y, galanin and substance P may reinforce the LC dysfunction and thus further weaken the adaptivity to stressful stimuli.

Clonidine immediately after immobilization stress prevents long-lasting locomotion reduction in the rat

1. Stress-induced behavioral change in the rat has been utilized as an animal model of anxiety disorder. The authors examined the effect of early intervention by noradrenergic inhibition on stress-induced long-lasting locomotion reduction. 2. Clonidine, an alpha2 agonist, was administered immediately after a single session of 8 min immobilization stress in a restraining box, followed by locomotion measurement on day 1, day 7, and day 14 after the stress session. 3. In the saline-treated control group, locomotion on day 1, day 7, and day 14 after the 8 min stress session was significantly reduced to about 80% in comparison with that before the stress. This finding confirmed the previous report that a single stressful event could lead to long-lasting behavioral changes. When clonidine was administered, locomotion reduction was not observed on any post-stress day. 4. The results suggest that early intervention by noradrenergic inhibition to stressful events may have a preventive effect on subsequent behavioral change which may be considered as an animal model of post-traumatic stress disorder.

Pharmacology and behavioral pharmacology of the mesocortical dopamine system

The prefrontal cortex (PFC) has long been known to be involved in the mediation of complex behavioral responses. Considerable research efforts are directed towards refining the knowledge about the function of this brain area and the role it plays in cognitive performance and behavioral output. In the first part, this review provides, from a pharmacological perspective, an overview of anatomical, electrophysiological and neurochemical aspects of the function of the PFC, with an emphasis on the mesocortical dopamine system. Anatomy of the mesocortical system, basic physiological and pharmacological properties of neurotransmission within the PFC, and interactions between dopamine and glutamate as well as other transmitters within the mesocorticolimbic circuit are included. The coverage of these data is largely restricted to what is relevant for the second part of the review which focuses on behavioral studies that have examined the role of the PFC in a variety of phenomena, behaviors and paradigms. These include reward and addiction, locomotor activity and sensitization, learning, cognition, and schizophrenia. Although the focus of this review is on the mesocortical dopamine system, given the intricate interactions of dopamine with other transmitter systems within the PFC and the importance of the PFC as a source of glutamate in subcortical areas, these aspects are also covered in some detail where appropriate. Naturally, a topic as complex as this cannot be covered comprehensively in its entirety. Therefore this review is largely limited to data derived from studies using rats, and it is also specifically restricted to data concerning the medial PFC (mPFC). Since in several fields of research the findings concerning the function or role of the mPFC are relatively inconsistent, the question is addressed whether these inconsistencies might, at least in part, be related to the anatomical and functional heterogeneity of this brain area.

Effect of noise exposure on rat cardiac peripheral benzodiazepine receptors

Noise is an environmental physical agent, which is regarded as a stressful stimulus: impairment and modifications in biological functions are reported, after loud noise exposure, at several levels in human and animal organs and apparatuses, as well as in the endocrine, cardiovascular and nervous system. In the present study equilibrium binding parameters of peripheral benzodiazepine receptors (PBRs) labelled by the specific radioligand [3H]PK 11195, were evaluated in cardiac tissue of rats submitted to 6 or 12 h noise exposure and of rats treated "in vivo" with PBR ligands such as PK 11195, Ro54864, diazepam and then noise-exposed. Results revealed a statistically significant decrease in the maximum number of binding sites (Bmax) of [3H]PK 11195 in atrial membranes of 6 or 12 h noise exposed rats, compared with sham-exposed animals, without any change in the dissociation constant (Kd). The "in vivo" PBR ligand pre-treatment counteracted the noise-induced modifications of PBR density. As PBRs are mainly located on mitochondria we also investigated whether noise exposure can affect the [3H]PK 11195 binding parameters in isolated cardiac mitochondrial fractions. Results indicated a significant Bmax value decrease in right atrial mitochondrial fractions of rats 6 or 12 h noise-exposed. Furthermore, as PBR has been suggested to be a supramolecular complex that might coincide with the not-yet-established structure of the mitochondrial permeability transition (MPT)-pore, the status of the MPT-pore in isolated heart mitochondria was investigated in noise- and sham-exposed rats. The loss of absorbance associated with the calcium-induced MPT-pore opening was greater in mitochondria isolated from hearts of 6 h noise- than those of sham-exposed rats. In conclusion, these findings represent a further instance for PBR density decrease in response to a stressful stimulus, like noise; in addition they revealed that "in vivo" administration of PBR ligands significantly prevents this decrease. Finally, our data also suggest the involvement of MPT in the response of an organism to noise stress.

The role of afferents to the locus coeruleus in the handling stress-induced increase in the release of noradrenaline in the medial prefrontal cortex: a dual-probe microdialysis study in the rat brain

This study was aimed to identify the neuronal pathways that mediate the handling stress-induced increase in the release of noradrenaline in the medial prefrontal cortex of the rat brain. For that purpose a microdialysis probe was implanted in the vicinity of the locus coeruleus and a second probe was placed in the ipsilateral medial prefrontal cortex. Receptor specific antagonists acting on the alpha(2)-adrenoceptor (50 microM idazoxan), GABA(A) (50 microM bicuculline), GABA(B) (100 microM (3, 4-Dichlorophenyl)methyl]propyl](diethoxymethyl) phosphonic acid; CGP 52432), acetylcholine (10 microM atropine), corticotropin releasing factor (CRF) (100 microM butyl-ethyl-[2,5-dimethyl-7-(2,4, 6-trimethyl-phenyl)-7H-pyrrolo[2,3-d]pyrimidin-4-yl]-amine; CP-154, 526), NMDA glutamate (300 microM (+/-)-3(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid; CPP) and non-NMDA glutamate receptors (500 microM 6,7-dinitroquinoxaline-2, 3-dione; DNQX) were infused into the locus coeruleus by retrograde dialysis, whereas extracellular noradrenaline was recorded in the ipsilateral medial prefrontal cortex. During infusion of the various compounds rats were gently handled for 10 min. Infusion of idazoxan potentiates the handling-induced increase in the release of noradrenaline in the medial prefrontal cortex. The infusions of, atropine, bicuculline, CGP 52432 and DNQX were without effect on the handling response. Infusion of the NMDA receptor antagonist CPP or the non-peptide CRF receptor antagonist CP-154,526 suppressed the stimulation of noradrenaline during stress. It is concluded that alpha(2)-adrenoceptors, NMDA glutamate receptors and CRF receptors modify the handling stress response of locus coeruleus neurones. The data suggest no major role for glutamatergic, GABAergic, or cholinergic afferents to the locus coeruleus in mediating the stress response.

Differential effect of immobilization stress on in vivo synthesis rate of monoamines in medial prefrontal cortex and nucleus accumbens of conscious rats

We have used microdialysis to measure the in vivo hydroxylation level of tyrosine and tryptophan in the medial prefrontal cortex and nucleus accumbens of conscious rats that were subjected to immobilization. The brain was perfused with an inhibitor of aromatic L-amino acid decarboxylase, 3-hydroxybenzylhydrazine, and the amount of 3,4-dihydroxyphenylalanine (DOPA) and 5-hydroxytryptophan (5-HTP) accumulating in the dialysate was measured as an index of the in vivo hydroxylation rate of tyrosine and tryptophan. One hour of immobilization caused a significant increase in extracellular DOPAin the medial prefrontal cortex but not nucleus accumbens. The same manipulation produced a significant and more prolonged elevation in extracellular 5-HTP in the nucleus accumbens as well as medial prefrontal cortex. The observed profile of stress-induced 5-HTP response was comparable in two brain regions. The results suggest that in vivo catecholamine synthesis is heterogenous, whereas in vivo serotonin synthesis is homogenous, with respect to responsiveness to stress in the medial prefrontal cortex and nucleus accumbens.

Tyrosine improves cognitive performance and reduces blood pressure in cadets after one week of a combat training course

The effects of the amino acid tyrosine on cognitive task performance were studied on a group of 21 cadets during a demanding military combat training course. In addition, the effects on mood, blood pressure and the norepinephrine metabolite MHPG were determined. Ten subjects received five daily doses of a protein-rich drink containing 2 g tyrosine, and 11 subjects received a carbohydrate rich drink with the same amount of calories (255 kcal). Assessments were made both immediately prior to the combat course and on the 6th day of the course. The group supplied with the tyrosine-rich drink performed better on a memory and a tracking task than the group supplied with the carbohydrate-rich drink. In addition, the supplementation of tyrosine decreased systolic blood pressure. No effects on mood were found. These findings suggest that supplementation with tyrosine may, under operational circumstances characterized by psychosocial and physical stress, reduce the effects of stress and fatigue on cognitive task performance.

Alteration of dopamine metabolites in CSF and behavioral impairments induced by neonatal hippocampal lesions

Alterations of monoamine metabolites in CSF and behavioral abnormalities were studied in rats with neonatal hippocampal lesions and controls. Lesions of the ventral hippocampus were produced bilaterally by ibotenic acid on postnatal day 7. Lesion-induced neurochemical alterations and behavioral impairments were examined concurrently when rats were 12 weeks old. CSF from the cisterna magna was sampled repeatedly from freely moving rats. The levels of free 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), and 5-hydroxyindoleacetic acid (5-HIAA) in CSF were determined. An exposure to a novel environment induced hyperexploratory behavior and elevated the level of free DOPAC in CSF in lesioned rats. Although a swim stress increased the levels of free DOPAC and 5-HIAA in CSF in both control and lesioned groups, rats with hippocampal lesions had a further elevation of free DOPAC in CSF and greater spontaneous activity relative to controls shortly after stress. Amphetamine (1.5 mg/kg, i.p.) induced hyperlocomotion in lesioned rats compared to controls. For the control group, the levels of the three monoamine metabolites in CSF were not significantly influenced by amphetamine. However, for the lesioned group, the level of DOPAC significantly decreased compared to preinjection of amphetamine. The results indicate that neonatal hippocampal lesion-induced impairments can be manifested by behavioral and neurochemical abnormalities. Alterations of monoamine metabolites in CSF may be determined quantitatively and used as indices for monitoring lesion-impaired monoaminergic function in the central nervous system.

Effects of antipsychotics, vitamin E, and MK-801 on dopamine dynamics in the rat brain following discontinuation of cocaine

Cocaine, 10 mg/kg, I.P., twice daily, was given to rats for 1 week. At 1 and 4 weeks following discontinuation of cocaine, the initial rate of 3,4-dihydroxyphenylacetic acid (DOPAC) formation was assessed. The initial rate of DOPAC formation was found to be decreased in the frontal and cingulate cortices at 1 week, but was only decreased in the frontal cortex at 4 weeks. When administered in conjunction with cocaine, haloperidol, clozapine, and vitamin E, but not MK-801, were found to prevent cocaine's effects. In addition to the potential value these findings have for further understanding cocaine abuse, it is proposed that the alteration in dopamine metabolism produced by cocaine, and the ability of haloperidol, clozapine and vitamin E to decrease cocaine's effects, model some biochemical aspects of schizophrenia.

Forced swim test-induced neurochemical endocrine, and immune changes in the rat

The forced swim test (FST) is a behavioral paradigm that is widely used as a screening test for antidepressant activity in rodents. The objectives of the present study were to characterize the corticosterone and immune responses and in addition to examine neurotransmitter levels, in five brain regions at intervals (15, 30, 60, 90, and 120 min) following the second exposure to the FST. There was a significant but transient reduction in noradrenaline and 5-HT concentrations, in the hypothalamus 15 min post-FST exposure. 5-HT turnover in the frontal cortex and amygdala was significantly increased between 20-120 min post-FST exposure. The FST elicited a robust corticosterone response that peaked significantly at 30 min and had almost returned to baseline 120 min after exposure. There was a significant reduction in total white blood cell count 120 min after the FST, which was accompanied by a significantly reduced percentage of lymphocytes 90 and 120 min post-FST exposure. In addition, there was a significant but transient suppression of both PHA and Con A-induced lymphocyte proliferation 15 min following FST exposure. This study demonstrates that there are neurochemical changes that are coincident with the endocrine and immune changes associated with FST exposure in rats. Furthermore, this model could be used to examine the effects of manipulation of this stress response by antidepressant drugs. Such an investigation could add to our understanding of the interactions between antidepressants, stress and the neuroendocrine and immune systems.

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.

Temporal dynamics of glutamate efflux in the prefrontal cortex and in the hippocampus following repeated stress: effects of pretreatment with saline or diazepam

Acute stress has been associated with activation of glutamate efflux in forebrain structures. The present study sought to characterize the extracellular dynamics of glutamate in response to acute and repeated stress in the prefrontal cortex and hippocampus in rats. One-minute sampling of extracellular glutamate levels was performed during repeated tail-pinch stress. Animals were stressed three times, beginning at approximately 10.30 a.m. and continuing at 2.5-h intervals. In the prefrontal cortex, the initial 10-min tail pinch produced a robust increase in extracellular levels of glutamate. This increase was apparent immediately (i.e. 1 min) after the start of the stress procedure. The second tail pinch produced a smaller increase in glutamate levels while the third tail pinch did not significantly increase these levels. In the hippocampus, the initial stress response was smaller in magnitude than that observed in the prefrontal cortex. Furthermore, responses to subsequent tail pinches were similar to that seen following the first tail pinch. Treatment with diazepam (3 mg/kg/i.p.) 30 min before the first stress session abolished the stress response in the prefrontal cortex and hippocampus. However, in the prefrontal cortex, the second tail pinch (performed approximately 3 h after diazepam administration) produced a robust increase in glutamate efflux. In contrast, in the hippocampus of diazepam-treated rats, the second tail pinch produced a small delayed response. Pretreatment with saline resulted in non-significant responses to all three tail pinches in the prefrontal cortex. The present study suggests that: (i) stress produces a rapid increase in glutamate efflux in the prefrontal cortex and hippocampus, (ii) repeated stress reveals tolerance in the glutamatergic response in the prefrontal cortex, (iii) saline and diazepam pretreatment reduce the stress-induced efflux of glutamate in the prefrontal cortex, and (iv) exposure to diazepam may prevent the prefrontal cortex from adapting its response to the subsequent stressor. These finding are consistent with the role of the prefrontal cortex as a region which may regulate reactions to aversive stimuli.

Increased levels of extracellular noradrenaline in the frontal cortex of rats exposed to naturalistic environmental stimuli: modulation by acute systemic administration of diazepam or buspirone

In vivo microdialysis was used to investigate the effects of an IP injection of diazepam or buspirone (each at 3 mg/kg) on spontaneous efflux of noradrenaline in rat frontal cortex, and on changes in efflux induced by naturalistic stress. After drug administration, rats either remained in their home cages or were transferred individually to a novel cage, 1 h later. The novel cage was brightly lit (1500 lux) and contained another, unfamiliar rat. After transfer to the novel cage, noradrenaline efflux was lower in diazepam-injected rats than in their vehicle-injected counterparts. However, in both cases, stress caused a significant increase in efflux and the net increase was not affected by diazepam. Similarly, buspirone, which increased spontaneous efflux of noradrenaline, did not affect the net increase in efflux during stress. Neither compound modified locomotor activity in the novel cage. This suggests that any changes in noradrenaline efflux are unrelated to drug effects on non-specific arousal. It is concluded that generically unrelated anxiolytic agents can have different effects on spontaneous efflux of noradrenaline but do not modify the noradrenergic response to naturalistic stimuli.

Depression as a spreading neuronal adjustment disorder

The outlines of a theory of the pathophysiology of depression are presented. The classic monoamine theory of depression as well as its more recent elaborations suggests that a deficit in monoamine neurotransmitters in the synaptic cleft is the primary cause of depression. We suggest that the primary defect emerges in the regulation of firing rates in brainstem monoaminergic neurons, which brings about a decrease in the tonic release of neurotransmitters in their projection areas, an increase in postsynaptic sensitivity and, concomitantly, exaggerated responses to acute increases in presynaptic firing rate and transmitter release. We propose that the initial defect involves, in particular, the noradrenergic innervation from the locus coeruleus, which in turn leads to dysregulation of 5-HT-ergic and dopaminergic neurotransmission.

Noradrenergic mechanisms in stress and anxiety: I. Preclinical studies

There is considerable preclinical evidence for a relationship between noradrenergic brain systems and behaviors associated with stress and anxiety. The majority of noradrenergic neurons are located in the locus coeruleus (pons), with projections throughout the cerebral cortex and multiple subcortical areas, including hippocampus, amygdala, thalamus, and hypothalamus. This neuroanatomical formation of the noradrenergic system makes it well suited to rapidly and globally modulate brain function in response to changes in the environment, as occurs during the presentation of stressors. Stress exposure is associated with an increase in firing of the locus coeruleus and with associated increased release and turnover of norepinephrine in brain regions which receive noradrenergic innervation. Increased firing of the locus coeruleus is also associated with behavioral manifestations of fear, such as arched back and piloerection in the cat. Exposure to chronic stress results in long-term alterations in locus coeruleus firing and norepinephrine release in target brain regions of the locus coeruleus. Norepinephrine is also involved in neural mechanisms such as sensitization and fear conditioning, which are associated with stress. These findings are relevant to an understanding of psychiatric disorders, such as panic disorder and post-traumatic stress disorder (PTSD), the symptoms of which have been hypothesized to be related to alterations in noradrenergic function.

Medial prefrontal cortical D1 receptor modulation of the meso-accumbens dopamine response to stress: an electrochemical study in freely-behaving rats

Voltammetry was used to study the role of prefrontal cortex (PFC) dopamine (DA) in modulating the nucleus accumbens (NAcc) DA response to stress. Signal increases elicited in NAcc by 15 min of restraint were monitored in freely-behaving rats following intra-PFC microinjections of D1 and D2 receptor-selective drugs. The exact site of injection was first determined by assessing the electrochemical response to stress at two dorsal-ventral levels of PFC. Consistent with previous reports, a pronounced stress response was observed ventrally at sites within the infralimbic PFC but not dorsally within the superficial layers of PFC. When microinjected into the infralimbic PFC, the D1 receptor antagonist SCH 23390 significantly enhanced the NAcc stress response. While the D1 receptor agonist SKF 38393 tended to decrease the NAcc stress response, it failed to do so reliably. Neither sulpiride (D2 receptor antagonist) nor quinpirole (D2 receptor agonist) had a significant effect. Finally, systemic administration of the selective DA uptake inhibitor GBR 12909 dose-dependently potentiated stress-induced signal increases in NAcc and in PFC, indicating that the electrochemical responses to stress in both regions were due primarily to increases in extracellular DA levels. Together, these data add to other evidence indicating that the PFC exerts an inhibitory influence on subcortical DA transmission. Specifically, the present results suggest that the NAcc DA response to stress is dampened by the concurrent activation of meso-PFC DA neurons and that this action is mediated, at least in part, by D1 receptors in PFC.

Release of excitatory and inhibitory amino acids from the locus coeruleus of conscious rats by cardiovascular stimuli and various forms of acute stress

The release of amino acids in the locus coeruleus (LC) of conscious, freely moving rats was studied in time periods of 3 min by use of push-pull superfusion under basal conditions and during application of various experimental stimuli known to influence the activity of the LC-noradrenergic system. Tail pinch for 3 min led immediately to a pronounced tetrodotoxin-sensitive increase in the release rates of the excitatory amino acids (EAA) glutamate (Glu) and aspartate (Asp) and to moderate increases in GABA and taurine (Tau) outflow. Immobilization stress for 9 min elevated the release of the EAA Glu and Asp, as well as that of the inhibitory amino acid GABA to a similar extent. A fall of blood pressure (BP) by nitroprusside or haemorrhage slightly enhanced the release rates of Glu and Asp. Noradrenaline-induced rise in BP, as well as hypervolaemia increased the release rate of GABA, but did not influence the release rates of Glu, Asp, Tau and arginine (Arg). The results provide direct evidence that the amino acid release pattern in the LC of conscious rats differs in response to various stimuli, according to the modality of the stimulus. A functional significance of excitatory and inhibitory amino acids in the regulation of LC activity during stress and haemodynamic changes is suggested.

Brain microdialysis and its application for the study of animal behaviour

Microdialysis is a sampling method that is used to determine the extracellular concentration of neurotransmitters in the brain. The method can be applied to conscious and unrestrained animals and is very suitable for the study of the chemistry of endogenous behaviour. This article reviews the contribution that microdialysis made to our understanding of the chemistry of behaviour. Methodological and practical considerations such as the implantation time and the use of guide cannulas are reviewed. The question whether neurotransmitters and related metabolites in dialysates reflect true synaptic release is critically discussed. There is much evidence that dopamine, noradrenaline, acetylcholine and serotonin in dialysates are related to neurotransmission, but there is serious doubt whether this is the case with amino acid transmitters such as GABA, glutamate and aspartate. Until now far over 100 papers appeared that used microdialysis in behavioural studies. Behavioural activation, the sleep-awake cycle and diurnal rhythms were subject of several of these studies. Various workers have described neurochemical changes in the brain that are related to feeding. Other studies were concerned with sexual behaviour and the sexual cycle in females. Parturition, maternal behaviour and offspring recognition have been studied in a series of microdialysis studies carried out in sheep. An overview is given of the microdialysis studies that were carried out to understand the biochemistry of stress. In this respect dopamine and noradrenaline have received much attention. A great number of microdialysis studies dealt with the role of dopamine in self-stimulation, reward and aversive emotions. It is concluded that microdialysis is at presently the most versatile and practical method to study the chemistry of behaviour and it is to be expected that it will soon be a routine methodology in behavioural research. Finally, perspectives and possible future developments of the methods are discussed.

Effects of the benzodiazepine antagonist flumazenil in PTSD

OBJECTIVE

Evidence from preclinical and clinical studies suggests a role for alterations in the benzodiazepine/GABAA receptor complex in stress and anxiety. Flumazenil is a relatively pure benzodiazepine/GABAA antagonist with limited intrinsic activity. In panic disorder patients, but not healthy controls, flumazenil has been demonstrated to provoke panic attacks.

METHOD

Vietnam combat veterans with PTSD (n = 14) received 90-second intravenous infusions of flumazenil 2 mg or placebo in a double-blind, crossover study design. PTSD symptomology was assessed using the PTSD Symptom Scale, and anxiety symptoms were measured with visual analogue rating scales.

RESULTS

There was no significant difference in PTSD and anxiety symptoms between administration of flumazenil and placebo.

CONCLUSION

Flumazenil administration does not produce an increase in anxiety and PTSD symptoms in patients with PTSD. This suggests that PTSD and panic disorder are dissimilar in terms of benzodiazepine/GABAA system function.

Idazoxan-induced reductions in cortical glucose use are accompanied by an increase in noradrenaline release: complementary [14C]2-deoxyglucose and microdialysis studies

The autoradiographic [14C]2-deoxyglucose procedure was used to map function-related alterations in local cerebral glucose use following acute administration of the alpha 2-adrenoceptor antagonist, idazoxan (0.3-3 mg kg-1 s.c.). The most prominent feature of the results obtained was the significant reduction in glucose use in certain locus coeruleus projection areas. Thus, in various cortical, hippocampal and thalamic regions, as well as structures involved in auditory and visual function, idazoxan administration was associated with a 13-20% decrease in glucose use. In a complementary microdialysis study, the effect of idazoxan on extracellular noradrenaline levels in the frontal cortex of rats, manipulated in the same fashion as during the [14C]2-deoxyglucose procedure (i.e. following the application of surgery and partial restraint), was examined. Both surgery and restraint were associated with a modest but significant increase in basal noradrenaline release (+31% and +26%, respectively). Subsequent administration of idazoxan (3 mg kg-1 s.c.) evoked a further increase in noradrenaline release, the magnitude of which was the same as that observed following its administration to freely-moving rats (+113%). These combined data suggest that idazoxan-induced reductions in cerebral glucose use, at least in the frontal cortex, may occur as a consequence of the increase in noradrenaline release. In addition, it appears that surgery and partial restraint do not alter alpha 2-adrenoceptor tone in the frontal cortex.

Brain transcription factor gene expression, neurotransmitter levels, and novelty response behaviors: alterations during rat amphetamine withdrawal and following chronic injection stress

Transcription factors are known to act as gene expression regulators, possibly linking extracellular stimuli to long-term modifications at the neuronal level. Such modifications may potentially underlie chronic psychostimulant- and stress-induced behavioral alterations. This study illustrates how a 2 week, twice daily 7.5 mg/kg d-amphetamine or saline regimen alters rat brain regional expression of transcription factor genes, including c-fos, fos-B, jun-B, c-jun, and zif 268, and seeks potential correlations between those changes and alterations in neurotransmitter levels and behavioral novelty responses. Amphetamine withdrawal-induced decreases in transcription factor mRNA levels, assessed using Northern blot analysis, appear most prominent in prefrontal cortex, begin approximately 12 h after the last injection, and largely recover to control levels by 54 h. Prefrontal cortical and striatal dopamine content, assessed using HPLC, decrease and recover over a similar time course. Behavioral "stereotypy time" manifest by animals exposed to a novel environment, a measure sensitive to psychostimulant withdrawal, also decreases beginning 12 h after the last injection, is still significantly reduced at 54 h, and recovers at 72 h. Chronic saline injections are followed by a consistent decrease in transcription factor gene expression, observed 6 h after the last injection, followed by a "rebound" increase at 12 h. These changes are accompanied by dramatic, mostly biphasic alterations in prefrontal cortical biogenic amines and by a short-lived increase in striatal dopamine turnover. At the same time, rats display much longer-lasting decreases in locomotor responses when exposed to a novel environment, with recovery occurring only 54 h after the last injection. The delayed recovery of behavioral responses to novelty is consistent with potential involvement of changes in transcription factor-mediated gene expression in neurochemical mechanisms underlying psychostimulant withdrawal and chronic injection stress-induced behavioral alterations.

Increased dopamine and norepinephrine release in medial prefrontal cortex induced by acute and chronic stress: effects of diazepam

We have examined the effects of diazepam on the stress-induced increase in extracellular dopamine and norepinephrine in the medial prefrontal cortex using in vivo microdialysis. In naive rats, acute tail pressure (30 min) elicited an increase in the concentrations of dopamine and norepinephrine in extracellular fluid of medial prefrontal cortex (+54 and +50%, respectively). Diazepam (2.5 mg/kg, i.p.) decreased the basal concentration of extracellular dopamine and norepinephrine. Diazepam also attenuated the stress-evoked increase in the absolute concentrations of extracellular dopamine (+17%), but did not alter the stress-induced increase in norepinephrine (+41%). However, when the drug-induced decrease in basal dopamine and norepinephrine concentration was taken into account, the stress-induced net increase in dopamine above the new baseline was equivalent to that obtained in vehicle pretreated rats, whereas the net increase in norepinephrine was almost twice that obtained in control subjects. In rats previously exposed to chronic cold (three to four weeks at 5 degrees C), tail pressure again produced an increase in the concentrations of dopamine and norepinephrine in the medial prefrontal cortex (+42% and +92%, respectively). However, in these chronically stressed rats, diazepam no longer decreased basal dopamine or norepinephrine in extracellular fluid, nor did it affect the stress-induced increase in the concentrations of these catecholamines. These data indicate that diazepam has complex effects on the extracellular concentrations of dopamine and norepinephrine which vary depending upon whether the rat is undisturbed or stressed during the period of drug exposure as well as the rat's prior history of exposure to stress. Moreover, these data raise questions regarding the role of catecholamines in the mechanism by which diazepam exerts its anxiolytic properties.

Corticotropin-releasing factor administered into the locus coeruleus, but not the parabrachial nucleus, stimulates norepinephrine release in the prefrontal cortex

Previous studies have indicated that intracerebroventricular application of corticotropin-releasing factor (CRF) activates noradrenergic neurons in the brain stem locus coeruleus (LC) and norepinephrine (NE) metabolism in several brain regions. To assess whether CRF has direct effects on LC noradrenergic neurons, CRF was infused into the LC and concentrations of NE and its metabolites were measured in microdialysates collected from the medial prefrontal cortex (PFM). Infusion of 100 ng of CRF into the LC significantly increased dialysate concentrations of NE and of its catabolite MHPG in the ipsilateral PFM, whereas no significant changes were observed following infusion of artificial CSF. No response was observed when the infusions of CRF occurred outside of the LC, including those in the parabrachial nucleus. Although CRF administered into the LC slightly increased dialysate concentrations of NE in the contralateral PFM, this effect was not statistically significant. The effect of CRF injected into the LC on dialysate NE was prevented by combination with a 10-fold excess of the CRF antagonist alpha-helical CRF9-41, indicating some specificity in the response. These results are consistent with anatomical and electrophysiological evidence suggesting that CRF may directly activate noradrenergic neurons in or close to the LC.

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".

Regional extracellular norepinephrine responses to amphetamine and cocaine and effects of clonidine pretreatment

Microdialysis in behaving animals was used to characterize the hippocampus (HP) and prefrontal cortex (PFC) norepinephrine (NE) responses to amphetamine (AMPH) and cocaine (COC). NE exhibited regionally similar dose- and time-dependent increases to each drug. However, peak NE concentrations were approximately 2-fold greater at behaviorally similar doses of AMPH compared with COC. To examine the role of noradrenergic impulse flow in the mechanism(s) by which these stimulants enhance extracellular NE, groups of animals were pretreated with the alpha 2 autoreceptor agonist, clonidine (CLON), prior to stimulant administration. CLON (50 micrograms/kg) administration completely blocked the NE response to both 20 and 30 mg/kg COC. By contrast, CLON decreased the NE response to 0.5 mg/kg AMPH by 75%, but became progressively less effective on the response as the dose was increased to 1.75 and 5.0 mg/kg. CLON also had no effect on the caudate dopamine responses to either AMPH or COC, consistent with the presumed specificity of this drug for alpha 2 receptors and suggesting the absence of any significant pharmacokinetic interactions. These results indicate that COC acts an uptake blocker at NE-containing neurons and suggest that AMPH increases extracellular NE through two consequences of its interaction with the neuronal transport carrier: (1) reuptake blockade which predominates at lower doses; and (2) release which becomes more prevalent at higher doses. Behavioral analyses revealed effects of CLON which varied as a function of stimulant and dose.

Effect of tyrosine on cognitive function and blood pressure under stress

The effects of tyrosine on mood, performance, heart rate and blood pressure of 16 healthy young subjects were assessed. Subjects were tested on two separate days, one test session after ingestion of 100 mg/kg tyrosine and the other test session after placebo, in random order. While performing a number of stress sensitive tasks, subjects were exposed to a stressor consisting of 90 dB noise. Tyrosine was found to improve the performance on two cognitive tasks, which were performed 1 h after administration of the medication and which could be characterized as highly sensitive to stress. In addition, tyrosine decreased diastolic blood pressure 15 min after ingestion, while 1 h after ingestion diastolic blood pressure was the same with tyrosine and placebo. No effects on mood, systolic blood pressure and heart rate were found.

Stress, antidepressant drugs, and the locus coeruleus

This review presents a synthesis of a large body of seemingly inconsistent literature on the role of the locus coeruleus-norepinephrine (LC-NE) system and the corticotropin-releasing hormone (CRH)-median eminence system in mediating the CNS effects of stress and the therapeutic effects of antidepressant drugs. The clinical implications of these findings for the etiology and treatment of stress-related psychiatric disorders such as depression will be discussed.

Comparison of the effects of trimebutine and YM114 (KAE-393), a novel 5-HT3 receptor antagonist, on stress-induced defecation

YM114 (KAE-393), (R)-5-[(2,3-dihydro-1-indolyl)carbonyl]-4,5,6,7- tetrahydro-1H-benzimidazole hydrochloride, is a derivative of YM060, a potent 5-HT3 receptor antagonist. We investigated the effects of YM114 on 5-HT3 receptors, both in vitro and in vivo, and on bowel dysfunction induced by restraint stress, 5-HT and thyrotropin-releasing hormone (TRH), and compared them with the effect of trimebutine. YM114 dose dependently inhibited the reduction in heart rate induced by 5-HT (30 micrograms/kg i.v.) in rats (ED50 = 0.31 micrograms/kg i.v.), and the potency of YM114 was almost the same as that of the racemate. The S-form of YM114 also inhibited 5-HT-induced bradycardia, but 1350 times less potent than the R-form. YM114 and its S-form inhibited [3H]GR65630 binding to N1E-115 cell membranes in a concentration-dependent manner with Ki values of 0.341 and 616 nM, respectively, showing the isomeric activity ratio (R-/S-form) of YM114 to be much greater (1800). YM114 antagonized 5-HT-induced depolarization of the nodose ganglion (EC50 = 1.39 nM). Trimebutine (1 mg/kg i.v.) failed to inhibit 5-HT-induced bradycardia, implying that it does not possess 5-HT3 receptor antagonistic activity. YM114 significantly and dose dependently prevented restraint stress-, 5-HT- and TRH-induced increases in fecal pellet output, and restraint stress- and 5-HT-induced diarrhea in rats and mice (ED50 = 6.9, 72.5, 154.6, 9.7 and 52.4 micrograms/kg p.o., respectively). Trimebutine significantly prevented stress- and 5-HT-induced diarrhea (ED50 = 29.4 and 87.3 mg/kg p.o., respectively), but only partially affected stress-, 5-HT- and TRH-induced increases in fecal pellet output. Thus, YM114 is a potent and stereoselective 5-HT3 receptor antagonist with much greater protective effects against stress-induced defecation than trimebutine.

Involvement of alpha 2-receptors in the analgesia induced by transient forebrain ischemia in rats

Transient forebrain ischemia induced in rats by the four-vessel occlusion method produced analgesic effects in the hotplate test that persisted for 2 weeks. Ischemia-induced analgesia was attenuated by low doses of alpha 2-agonist clonidine (0.01-0.10 mg/kg, IP) and enhanced by low doses of alpha 2-antagonists yohimbine (1-2 mg/kg, IP) and idazoxan (0.25-1.00 mg/kg, IP) administration 7 days after ischemia. Ischemia-induced analgesia was not affected by methysergide, naloxone, propranolol, or phenoxybenzamine administered 7 days after ischemia, when motor control and arousal level of rats recovered to normal conditions. The enhanced response to yohimbine was antagonized by pretreatment with clonidine (0.75 mg/kg, IP) and naloxone (10 mg/kg, IP), suggesting the involvement of endogenous opioid peptides. The enhanced response to yohimbine was still present 2 months after ischemia, when preischemic hotplate threshold was restored. As alpha 2-agonists reduce and alpha 2-antagonists increase the outflow of central noradrenaline, it is suggested that activation of central noradrenergic systems is involved in the mediation of ischemia-induced analgesia.

Tianeptine raises dopamine and blocks stress-induced noradrenaline release in the rat frontal cortex

The effects of various doses of tianeptine on extracellular concentrations of dopamine were studied in the frontal cortex and nucleus accumbens of the rat. At 5 and 10 mg/kg intraperitoneally, tianeptine raised extracellular dopamine in the nucleus accumbens but only the higher dose caused a significant increase in the frontal cortex. At 10 mg/kg tianeptine significantly raised the concentrations of dihydroxyphenylacetic acid and homovanillic acid in both brain regions. In another experiment, 10 and 20 mg/kg tianeptine did not modify the extracellular concentrations of noradrenaline in the frontal cortex but dose dependently blocked the increase in extracellular noradrenaline caused by restraint stress.

Benzodiazepines reverse the anti-immobility effect of antidepressants in the forced swimming test in mice

The present study provides evidence that, in mice subjected to the forced swimming test, the anti-immobility effect of the tricyclic antidepressants, desipramine and imipramine (16-32 mg/kg) was antagonized by the acute co-administration of a benzodiazepine, diazepam (0.25-2 mg/kg) and lorazepam (0.125 mg/kg). This effect cannot be accounted for by variations in plasma and/or brain levels of each compound since brain and plasma concentrations of desipramine and plasma levels of diazepam and desmethyldiazepam, measured immediately after the swimming test, were not significantly modified by the co-administration. Diazepam (2 mg/kg) also counteracted the reduction of time spent immobile induced by the MAO inhibitors, toloxatone (256 mg/kg) and selegiline (4 mg/kg) and the 5-HT1A receptor agonist, 8-OH-DPAT (1 mg/kg), but not by the psychostimulant, caffeine (32 mg/kg). The sedative neuroleptic, thioridazine (4 mg/kg) was also found to reverse the anti-immobility effect of desipramine whereas the non-benzodiazepine anxiolytics, alpidem (8 mg/kg) and buspirone (0.5 mg/kg) did not. These results indicate that the observed interactions were unlikely to be accounted for by a reduction of the stressful aspect of the situation whereas the participation of some motor or sedative component could not be totally ruled out.

Extraneuronal noradrenaline in the prefrontal cortex of morphine-dependent rats: tolerance and withdrawal mechanisms

The changes in extracellular concentrations of noradrenaline (NA) in the prefrontal cortex of morphine-dependent rats were studied by microdialysis following an acute morphine challenge and during naloxone-precipitated withdrawal. Animals were implanted with morphine- or placebo-containing pellets for 5 days. In control rats a challenge dose of morphine (5 mg/kg s.c.) induced a maximum decrease in NA output of about 45% of pre-drug levels. In contrast, morphine challenge had no effect on extraneuronal NA concentrations in morphine-implanted animals. In control animals, naloxone (2 mg/kg i.p.) produced no behavioral effect nor changed NA levels. However, in morphine-dependent animals naloxone suddenly increased extraneuronal NA by 175% of baseline dialysate levels in the first sample after the injection and precipitated a morphine-withdrawal symptomatology that paralleled the changes in NA output. Thus, chronic morphine treatment in rats results in the development of tolerance to the acute inhibitory effect of morphine on extraneuronal NA and is associated with a stimulation of prefrontocortical NA output during naloxone-precipitated withdrawal.