Release of neurotransmitters in the locus coeruleus.

In the past 15 years the release of neurotransmitters and their metabolites in the locus coeruleus (LC) has been studied by using three approaches: microdialysis; push-pull superfusion; and voltammetry. These sophisticated techniques, which render it possible to follow the time course and magnitude of neurochemical changes in anaesthetized and conscious animals, have permitted great strides towards understanding neurotransmission in the LC. It appears that noradrenaline, known to be released in distant terminal fields, is also released in the somatodendritic area of LC neurons in response to drugs and physiological stimuli. Furthermore, determination of in vivo release enables the identification of functionally important neurotransmitter systems involved in relaying and integrating information reaching the LC via afferent neurons. As outlined in this review, the release rates of glutamate, aspartate, gamma-aminobutyric acid, glycine, 5-hydroxytryptamine and catecholamines, are modified in particular by arousing and stressful stimuli, pain, changes in cardiovascular homeostasis, as well as during opioid withdrawal or the sleep-wake-cycle. Profound interactions also occur between some of the neurotransmitters released during these situations. It appears that individual stimuli produce distinct neurochemical changes which contribute to the regulation of neuronal LC activity. Stimuli that activate LC neurons, such as pain, fall of blood pressure, noise, opiate withdrawal, do not produce a uniform response but modality-specific release patterns of excitatory and inhibitory neurotransmitters within the LC. From these studies and from existing neuroanatomical and electrophysiological data our knowledge of how neurotransmitters work in concert to regulate the functional state of LC noradrenergic perikarya in physiological and pathophysiological conditions is just emerging.

Nitric oxide as modulator of neuronal function.

The gas NO is a messenger that modulates neuronal function. The use of NO donors and NO synthase inhibitors as pharmacological tools revealed that this free radical is probably implicated in the regulation of excitability and firing, in long-term potentiation and long-term depression, as well as in memory processes. Moreover, NO modulates neurotransmitter release. In vivo and in vitro studies have shown that, in all brain structures investigated, endogenous NO modulates the release of several neurotransmitters, such as acetylcholine, catecholamines, excitatory and inhibitory amino acids, serotonin, histamine, and adenosine. In most cases, enhanced NO level in the tissue increases the release of neurotransmitters, although decreasing effects have also been observed. Cyclic 3'-5' guanosine monophosphate and glutamate mediate the modulation of transmitter release by NO. Recent observations suggest that the release of some transmitters is dually influenced by NO. Thus, besides modulation by presynaptically located auto- and heteroreceptors, NO released from nitrergic neurons seems to play a universal role in modulating the release of transmitters in the brain.

Involvement of biogenic amines and amino acids in the central regulation of cardiovascular homeostasis.

Biogenic amines and amino acids have been implicated in central cardiovascular homeostasis. Initially, drugs were injected into the brain and their effects on blood pressure were investigated. Other approaches allowed endogenous neurotransmitters released in the extracellular space of brain structures involved in cardiovascular regulation to be identified. As Nicolas Singewald and Athineos Philippu outline, even slight disturbances in blood pressure and/or isovolaemia lead to marked changes in the release rates of biogenic amines and amino acids in various brain structures. Blood pressure homeostasis is maintained with the participation of several brain regions and neurotransmitters which possess the same or opposing functions when released from CNS neurones.

Induction of GTP cyclohydrolase I by bacterial lipopolysaccharide in the rat.

A 2- to 3-fold increase of GTP cyclohydrolase I (E.C. 3.5.4.16), the key enzyme of tetrahydrobiopterin biosynthesis from GTP, was observed in cerebellum, remaining brain, liver, spleen, and adrenal gland of rats treated with a single dose of lipopolysaccharide (LPS). This led to increased biopterin levels in tissues but not in plasma. Parallel induction of nitric oxide (NO) synthase was indicated by a 10- to 100-fold increase of plasma nitrate levels 6 and 12 hours after injection of LPS. Furthermore, systolic blood pressure was reduced significantly by 23%. Our results demonstrate induction of tetrahydrobiopterin biosynthesis after LPS treatment in vivo.

Influence of excitatory amino acids on basal and sensory stimuli-induced release of 5-HT in the locus coeruleus.

1. The interactions between 5-hydroxytryptaminergic neurones and excitatory amino acid utilizing neurones were studied in the locus coeruleus of conscious, freely moving rats. The locus coeruleus was superfused with artificial cerebrospinal fluid through a push-pull cannula and 5-hydroxytryptamine (5-HT) was determined in the superfusate that was continuously collected in time periods of 10 min. 2. Superfusion of the locus coeruleus with the NMDA receptor antagonist AP5 (10 microM), kynurenic acid (1 mM), or the AMPA/kainate receptor antagonist DNQX (10 microM) reduced the 5-HT release in the locus coeruleus. 3. Superfusion with the agonists NMDA (50 microM), kainic acid (50 microM) or AMPA (10 microM) enhanced the release rate of 5-HT. AP5 (10 microM) blocked the stimulant effect of NMDA, while tetrodotoxin (1 microM) failed to influence the NMDA-induced release of 5-HT. In the presence of 10 microM DNQX, the releasing effect of 50 microM kainic acid was abolished. 4. Pain elicited by tail pinch, as well as noise-induced stress, increased the release of 5-HT. Superfusion of the locus coeruleus with 10 microM AP5 reduced the tail pinch-induced 5-HT release. AP5 (10 microM) did not affect the noise-induced release of 5-HT which was reduced, when the locus coeruleus was superfused simultaneously with this concentration of AP5 and 1 microM kynurenic acid. DNQX (10 mM) failed to influence the release of 5-HT induced by tail pinch or noise. 5. The findings suggest that 5-hydroxytryptaminergic neurones of the locus coeruleus are tonically modulated by excitatory amino acids via NMDA and AMPA/kainate receptors. The release of 5-HT elicited by tail pinch and noise is mediated to a considerable extent through endogenous excitatory amino acids acting on NMDA receptors, while AMPA/kainate receptors are not involved in this process.

Disturbances in blood pressure homeostasis modify GABA release in the locus coeruleus.

The locus coeruleus (LC) of anaesthetized rats was superfused with artificial cerebrospinal fluid through a push-pull cannula and the release of the amino acids gamma-aminobutyric acid (GABA), taurine and arginine was determined in the superfusate. Increases in blood pressure (BP) induced either by intravenous infusions of noradrenaline and phenylephrine or by blood injection enhanced the release of GABA in the LC. Decreases in BP elicited by intravenous infusion of sodium nitroprusside or by haemorrhage decreased the GABA release rate. The BP changes did not influence the release rates of taurine and arginine. These findings demonstrate that GABA release in the LC is modified by cardiovascular impulses and suggest that GABAergic neurones modulate LC activity in response to disturbances in BP homeostasis.

Noradrenaline release in the locus coeruleus of conscious rats is triggered by drugs, stress and blood pressure changes.

The in vivo release of noradrenaline (NA) in the locus coeruleus (LC) of conscious rats was enhanced by local superfusion of pargyline, idazoxan, bicuculline, AMPA as well as by experimentally induced hypotension. Noise stress considerably enhanced NA release in the LC and this response was promoted after local alpha2-adrenoceptor blockade by idazoxan. Air jet stress and noise stress elicited comparable increases in NA release in the LC and the simultaneously superfused amygdala. The NA responses in both areas did not change during a second exposure to each of the stressors. It is concluded that NA release at the somatodendritic level of LC neurons is triggered by high LC activity and most likely serves to limit LC activation to excitatory stimuli by feedback inhibition via alpha2-adrenoceptors.

Influence of NOS inhibitors on changes in ACH release and NO level in the brain elicited by amphetamine neurotoxicity.

We studied the possible role of neurotoxicity in the d,l-amphetamine (AMPH)-induced release of acetylcholine (ACH) in the nucleus accumbens (Nac) and the involvement of endogenous NO in this process. For determination of ACH release the Nac was superfused using the push-pull-technique. NO was directly measured using the electron paramagnetic resonance technique. Repeated administration of AMPH increased ACH release by about 400%. N-nitro-L-arginine (L-NNA) and 7-nitroindazole (7-NI) nearly abolished the AMPH-induced increase in ACH release. AMPH increased NO as well as lipid peroxidation (LPO) products in the cortex. L-NNA and 7-NI substantially diminished NO increase. AMPH-evoked LPO was only slightly reduced by these compounds. It is concluded that AMPH enhances ACH release through increased NO synthesis and induces neurotoxicity via NO and by LPO independent NO generation.

Importance of histamine in modulatory processes, locomotion and memory.

Acetylcholine modulates histaminergic transmission via M(1) receptors. On the other hand, cholinergic transmission is modulated by neighbouring histaminergic neurons via H(1), H(2) and H(3) receptors. Dopaminergic and GABAergic neurons are also involved in these modulatory mechanisms. Furthermore, the release of histamine is modulated by glutamatergic neurons and nitric oxide of neuronal origin. The release of histamine in the brain oscillates according to circadian, slow ultradian and fast ultradian rhythms. Ultradian fluctuations have also been observed in the theta- and delta-frequency bands of the EEG spectral power. Simultaneous recordings of histamine outflow and EEG in the hypothalamus revealed that the ultradian histamine release rhythm coincides temporally with ultradian fluctuations in the EEG spectral power. Histamine receptor ligands used in pharmacotherapy, like H(1) and H(2) antagonists, modify the frequency of the EEG fluctuations. Brain histamine seems to be involved in memory processes, since inhibition of histamine synthesis deteriorates, while H(3) antagonists, histamine and histidine improve short-term memory. The latter finding may open new horizons in pharmacological treatment of memory disorders.

Role of histaminergic and cholinergic transmission in cognitive processes.

Mutual modulatory and functional interactions exist between the histaminergic and cholinergic systems in the brain. The activity of histaminergic neurons is permanently modulated by neighboring cholinergic neurons via muscarinic M(1) receptors, cholinergic transmission by histaminergic neurons through H(1), H(2), H(3A) and H(3B) receptors. In the nucleus accumbens, glutamatergic neurons originating from the hippocampus modulate cholinergic transmission in a direct way via stimulation of NMDA receptors located on cholinergic neurons. Additionally, glutamatergic neurons of the hippocampus modulate the activity of cholinergic neurons in an indirect way by stimulating histaminergic neurons within the nucleus accumbens. Reciprocal regulatory influences and neurotransmission are subjected to the global modulatory influence of nitric oxide. Both histaminergic and cholinergic systems in the nucleus accumbens are implicated in the response to aversive stimuli. Memory acquisition is associated with activation of cholinergic transmission in the nucleus accumbens, while stimulation of histaminergic neurons facilitates memory in a way that is independent of the cholinergic system. Hence, both histaminergic and cholinergic transmission within the nucleus accumbens and interactions between the two systems seem to play a predominant role in cognition.

Histaminergic neurons facilitate social memory in rats.

The social memory test was used so as to investigate whether brain histamine is involved in short-term memory. Histamine injected intracerebroventricularly (i.c.v.) decreased investigation time of a juvenile rat by an adult rat. A similar effect was elicited by i.c.v. administration of histidine. Compared with the control animals, rat pretreatment with alpha-fluoromethylhistidine (FMH), which inhibits neuronal synthesis of histamine, prolonged recognition time. The H3-receptor agonist immepip also prolonged investigation time, while the H3-antagonist thioperamide exerted the opposite effect. Treatment with histidine increased, while treatment with FMH decreased histamine levels in various brain regions. It is concluded that histamine released from histaminergic neurons facilitates short-term memory.

Presynaptic regulation of the release of catecholamines in the cat hypothalamus.

The posterior hypothalamus of cats was superfused through a push-pull cannula and the release of endogenous catecholamines was determined in the superfusate. Superfusion with yohimbine, isoprenaline, salbutamol or tazolol increased, while superfusion with propranolol decreased, the release of all three catecholamines. Transection of the brain caudal to the hypothalamus inhibited 'resting' and drug-induced release. It is concluded that alpha- and beta-adrenoceptors of the hypothalamus are involved in the regulation of the release of catecholamines.

Release of glutamate and GABA in the amygdala of conscious rats by acute stress and baroreceptor activation: differences between SHR and WKY rats.

To reveal the functional importance of amino acid neurotransmission in the amygdala (AMY) of conscious spontaneously hypertensive rats (SHR) and Wistar-Kyoto (WKY) rats, the in vivo release of glutamate (GLU) and GABA in this brain structure was studied using the push-pull superfusion technique. Basal GLU and GABA release rates in the AMY were comparable in SHR and WKY rats, although arterial blood pressure (BP) in SHR (152+/-6 mmHg) was higher than in WKY rats (102+/-4 mmHg). Neuronal depolarization by superfusion with veratridine enhanced the release of GLU and GABA to a similar extent in both rat strains. On the other hand, exposure to noise stress (95 dB) for 3 min led to a tetrodotoxin-sensitive increase in GLU release in the AMY of SHR, but not WKY rats. The concurrent pressor response to noise was enhanced in SHR as compared to WKY rats. A rise in BP induced by intravenous infusion of phenylephrine for 9 min had no effect on amino acid release in the AMY of both strains. The data suggest an exaggerated stress response of glutamatergic neurons in the AMY of SHR as compared with WKY rats, which might be of significance for the strain differences in the cardiovascular and behavioural responses to stress. The results also show that, in both rat strains, glutamatergic and GABAergic neurons in the AMY are not modulated by baroreceptor activation. Moreover, hypertension in adult SHR does not seem to be linked to a disturbed synaptic regulation of glutamatergic or GABAergic transmission in the AMY.

Conditioned fear and inescapable shock modify the release of serotonin in the locus coeruleus.

The aim of the present study was to investigate the importance of the serotonergic transmission in the locus coeruleus (LC) to conditioned fear. Rats were conditioned to fear by exposing them to noise signal (N), light signal (L) and electric foot shock (S) for 4 days. Control rats were exposed to the same events without receiving S. The LC was superfused with artificial cerebrospinal fluid (aCSF) through a push-pull cannula, and the release of 5-hydroxytryptamine (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) was determined in the superfusate. Motility, blood pressure (BP) and heart rate (HR) were telemetrically recorded. (1) The process of moving animals from their home cage into the grid-floor chamber transiently increased the release rate of 5-HT and the outflow of 5-HIAA in control and naive rats. In conditioned rats, 5-HT release was similarly increased during transfer but was permanently decreased in the grid-floor chamber. Control rats showed phases of enhanced motility in the chamber, while conditioned animals displayed continuous immobility. In naive rats, enhanced motility persisted in the novel environment. (2) Exposure of rats to N+L+S increased the release of 5-HT and the outflow of 5-HIAA to the same extent in conditioned and naive rats. These changes were associated with elevated motility, rise in BP and tachycardia. (3) In conditioned subjects, exposure to N+L in the fifth day led to a pronounced and sustained decrease in the release rate of 5-HT and to tachycardia, while no effects were observed in control rats or naive rats. The findings suggest that conditioned fear attenuates serotonergic neurotransmission within the LC. Telemetric recording of HR proves to be a valuable index for fear and stress processes.

Nitric oxide modulates the release of serotonin in the rat hypothalamus.

To investigate the effect of nitric oxide (NO) on the release of serotonin and its main metabolite, 5-hydroxyindoleacetic acid (5-HIAA), the posterior hypothalamus of the conscious rat was superfused through a push-pull cannula with drugs which either liberate NO, or inhibit NO synthase (NOS). The NO donors, linsidomine, diethylamine/nitric oxide (DEA/NO), S-nitroso-N-acetylpenicillamine (SNAP), S-nitroso-glutathione (SNOG) and sodium nitroprusside influenced the release of serotonin in a biphasic way. Low concentrations of drugs diminished, while higher concentrations of these compounds enhanced the outflow of serotonin. The NOS inhibitors N(G)-methyl-L-arginine methyl ester (L-NAME) and 7-nitroindazole (7-NINA) enhanced the serotonin release. A high concentration of L-NAME slightly diminished the outflow of serotonin. Inhibition of the guanylyl cyclase by oxodiazolo[4, 3]quinoxaline-one (ODQ) abolished the changes in serotonin outflow induced by both low and high concentrations of linsidomine. The extracellular concentration of the 5-HIAA was not influenced by the compounds used. These data suggest that endogenous NO modulates the release of serotonin in a biphasic and cGMP-dependent way.

Nitric oxide releases acetylcholine in the basal forebrain.

In conscious rats, the basal forebrain was superfused through a push-pull cannula and the release of acetylcholine was determined in the superfusate. Superfusion with the nitric oxide (NO) synthase inhibitor, NG-nitro-L-arginine, diminished the release of acetylcholine. Subsequent superfusion with the NO donor, 3-morpholino-sydnonimine, enhanced the release of the neurotransmitter. It is concluded that endogenous NO enhances the release of acetylcholine from its neurons.

Adenosine release in the ventral striatum of the rat is modulated by endogenous nitric oxide.

The influence of nitric oxide (NO) on adenosine release was investigated by the push-pull technique in the ventral striatum of the urethane-anaesthetized rat. Superfusion with the NO donor diethylamine-NO enhanced, whereas superfusion with the NO synthase inhibitor L-NG-nitroarginine methyl ester decreased the output of adenosine. The effect of L-NG-nitroarginine methyl ester was abolished by L-arginine methyl ester. These findings indicate that, in the ventral striatum of the rat, NO modulates adenosine release.

Hypotension alters the release of catecholamines in the hypothalamus of the conscious rabbit.

The posterior hypothalamic nucleus of conscious, freely moving rabbits was superfused with CSF through a cannula. Intravenous injection of nitroprusside elicited a fall of the arterial blood pressure and increased the rates of release of endogenous catecholamines in the posterior hypothalamic nucleus, while noradrenaline increased the blood pressure but did not change the release of catecholamines. It is concluded that hypotension leads to a counteracting increase in the release of catecholamines in the posterior hypothalamus.

Melatonin facilitates short-term memory.

The olfactory social memory test, based on the recognition of a juvenile rat by a male adult rat, was used to investigate whether melatonin influences memory. Intracerebroventricular (i.c.v.) injection of 1.1 nmol melatonin shortened recognition time, while the melatonin ML1 receptor antagonist luzindole (1 nmol) exerted the opposite effect. The facilitating influence of melatonin was abolished in the presence of 0.5 nmol luzindole. The findings suggest that endogenous melatonin facilitates short-term memory.

Taurine release in the hypothalamus is altered by blood pressure changes and neuroactive drugs.

The release of endogenous taurine was determined in the posterior hypothalamus of the conscious, freely moving rat by using the push-pull superfusion technique. At the start of superfusion, the outflow of endogenous taurine declined rapidly with a half-time of 7.9 min and reached a steady state after approximately 1 h. Thereafter, the release rate was constant and amounted to 2.6 +/- 0.3 pmol/min. During depolarization either with K+ (50 or 90 mM) or veratridine (1 or 10 microM), taurine outflow was increased in a concentration-dependent way. Hypothalamic superfusion with tetrodotoxin (1 microM) elicited a sustained decrease in the taurine release to 60% of the control values. Intravenous infusion of noradrenaline led to a rise in blood pressure (45 mm Hg) and enhanced the release of taurine in the hypothalamus. A fall of blood pressure (30 mm Hg) caused by an intravenous infusion of nitroprusside diminished taurine outflow. The results suggest that a considerable amount of the taurine detected is released from hypothalamic neurons. Changes in the release rate of taurine by experimentally induced alterations of blood pressure indicate that, in the posterior hypothalamus, the amino acid might play an important role as a neurotransmitter or neuromodulator possessing a hypotensive function.