Altered responsiveness of adenosine 3',5'-monophosphate-generating systems in rat cortical slices after lesions of the medial forebran bundle

Locus coeruleus-induced modulation of forebrain electroencephalographic (EEG) state in halothane-anesthetized rat

The effects of reversible enhancement or suppression of locus coeruleus (LC) neuronal discharge activity on forebrain electroencephalographic (EEG) activity have been previously examined in two series of experiments in halothane-anesthetized rats. Unilateral enhancement of LC activity increased EEG measures of arousal in frontal cortex and hippocampus. The EEG effects of LC activation were blocked by intracerebroventricular pretreatment with the noradrenergic beta-antagonist, propranolol. Bilateral, but not unilateral, suppression of LC activity substantially increased EEG measures of sedation/anesthesia in cortex and hippocampus. In all experiments: a) EEG responses were only observed following changes in LC activity levels; b) onset of EEG responses closely followed changes in LC neuronal activity; c) recovery of EEG responses closely followed the recovery of LC neuronal activity. The present report integrates these previous results and considers their implications for the hypothesis that the LC may be an important modulator of behavioral state and/or state-dependent processes. Together, the two series of experiments yield complementary observations that have implications for LC function that are not apparent when considering each series in isolation.

Alpha- and beta-adrenergic receptors of the rat cerebral cortex and cerebral microvessels in aging, and their response to denervation

We studied the effects of aging and norepinephrine depletion 2 weeks after unilateral locus ceruleus lesion on alpha 1-, alpha 2- and beta-adrenergic receptors by ligand binding methods in the ipsilateral and contralateral cerebral cortex of Fischer-344 rats. We also studied the effects of aging and noradrenergic denervation on beta-adrenergic receptors in isolated cerebral microvessels. We found that specific [125I]HEAT binding to alpha 1-adrenergic receptors was not affected by aging or by norepinephrine depletion. Although aging also had no effect on the density or affinity of [3H]UK-14,304 and [125I]pindolol binding to alpha 2- and beta-adrenergic receptors, the density of receptors increased significantly in all age groups after noradrenergic denervation. beta-Adrenergic receptors of cerebral microvessels also were unaffected by aging, but increased their density after noradrenergic denervation at all ages. In all instances, there were no significant effects on the affinity of ligand binding. We conclude that aging does not affect the density or the affinity of adrenergic receptors in the cerebral cortex of Fischer-344 rats, nor does it affect the response of these receptors to norepinephrine depletion.

Histamine stimulation of cyclic AMP accumulation in astrocyte-enriched and neuronal primary cultures from rat brain

Histamine stimulates cyclic AMP accumulation in astrocyte-enriched and neuronal primary cultures from rat brain in the presence of the phosphodiesterase inhibitor isobutylmethylxanthine. The response in the astrocyte cultures (Emax = 304 +/- 44% over basal, EC50 = 43 +/- 5 microM) was much higher than in neuronal cultures (Emax = 24 +/- 2%, EC50 = 14 +/- 7 microM). The histamine effect in astrocytes was competitively inhibited by the H2 antagonists cimetidine (Ki = 1.1 +/- 0.2 microM) and ranitidine (Ki = 46 +/- 10 nM) but was insensitive to the H1 antagonist mepyramine (1 microM). The two selective H2 agonists impromidine and dimaprit behaved as partial agonists and showed relative potencies (139 and 0.5, respectively) consistent with an interaction with H2 receptors. The more selective H1 agonist 2-thiazolylethylamine (0.01-1 mM) did not potentiate the response to impromidine (10 microM). Thus, in contrast to what is generally observed in intact cell preparations from brain, the histamine-induced cyclic AMP accumulation in astroglial cells is mediated solely by H2 receptors. The small effect shown in neuronal cultures also appears to be mediated by H2 receptors.

DSP-4-induced depletion of brain norepinephrine produces opposite effects on exploratory behavior 3 and 14 days after treatment

Exploratory behavior of a complex novel environment was examined 3 and 14 days following treatment with the noradrenergic-selective neurotoxin, DSP-4. This toxin significantly decreased norepinephrine concentrations in neocortex and hippocampus but not hypothalamus. DSP-4 significantly increased exploratory behavior in animals tested 3 days after treatment. In contrast, exploratory behavior was decreased in animals tested 14 days after treatment. The effect of DSP-4 at 3 days is similar to treatments that act to inhibit noradrenergic function such as administration of the alpha 1-antagonist, prazosin, or the alpha 2-agonist, clonidine. The effect of DSP-4 at 14 days resembles that observed following treatment with the alpha 1-agonist, phenylephrine, or the alpha 2-antagonist, idazoxan. These data provide additional support for a role of noradrenergic systems in exploratory behavior. The simplest explanation for the time dependent effects of DSP-4 on exploratory behavior is the occurrence of the slow development of a supersensitivity of cerebral systems affected by norepinephrine.

Regulation of alpha and beta components of noradrenergic cyclic AMP response in cortical slices

The cyclic AMP response to catecholamines in the rat cerebral cortex is mediated by both beta- and alpha-adrenoceptors. The beta-receptors cause a direct activation of adenylate cyclase whereas the alpha alpha-receptors play a modulatory role and act by potentiating the response to beta stimulation. The present study investigated whether the functions of these two types of cyclic AMP-linked receptors are regulated differently by various physiological factors known to affect adrenoceptor function. It was found that treatments that affect central noradrenergic neuronal function including repeated administration of desmethylimipramine or lesion of central noradrenergic pathways produced selective changes in the cAMP response to beta-receptor stimulation whereas treatments that affect adrenocortical function including ACTH of corticosterone administration and hypophysectomy or adrenalectomy produced selective changes in the potentiation response to alpha-receptor stimulation. The change in the alpha potentiation effect caused by corticosterone was found to be abolished in the presence of prazosin indicating that the hormone affects alpha 1-adrenoceptor function. The results support the hypothesis that the beta response in the cortex is under the control of the noradrenergic system while the alpha potentiation response is under the control of the adrenocortical system.

Sustained elevation of hippocampal cyclic 3'-5' adenosine monophosphate levels after medial septal lesions

The cyclic 3'-5' adenosine monophosphate (cyclic AMP) content of the rat hippocampal formation doubles during the week following a medial septal lesion and remains elevated for at least 1 month, the longest time period studied. This elevation in cyclic AMP does not result from sympathetic ingrowth, as neither superior cervical ganglion stimulation nor ganglionectomy influences hippocampal cyclic AMP content after lesions. Interruption of the cholinergic septohippocampal pathway in the fornix did not elevate hippocampal cyclic AMP content. Further, treatment of septal-lesioned animals with oxotremorine or of normal animals with atropine did not influence hippocampal cyclic AMP content. Finally, neither locus ceruleus lesions nor treatment with propranolol affected hippocampal cyclic AMP content. We believe this to be the first report of a sustained elevation in hippocampal cyclic AMP content. Like other long-term events, it is likely to have profound effects on hippocampal function and represents a remarkable brain adaptation to remote injury.

Activation of cyclic AMP-generating systems in brain membranes and slices by the diterpene forskolin: augmentation of receptor-mediated responses

The diterpene forskolin markedly activates adenylate cyclase in membranes from various rat brain regions and elicits marked accumulations of radioactive cyclic AMP in adenine-labeled slices from cerebral cortex, cerebellum, hippocampus, striatum, superior colliculi, hypothalamus, thalamus, and medulla-pons. In cerebral cortical slices, forskolin has half-maximal effects at 20-30 microM on cyclic AMP levels, both alone and in the presence of the phosphodiesterase inhibitor ZK 62771. The presence of a very low dose of forskolin (1 microM) can augment the response of brain cyclic AMP-generating systems to norepinephrine, isoproterenol, histamine, serotonin, dopamine, adenosine, prostaglandin E2, and vasoactive intestinal peptide. Forskolin does not augment responses to combinations of histamine-norepinephrine adenosine-norepinephrine, or histamine-adenosine. For norepinephrine and isoproterenol in rat cerebral cortical slices and for histamine in guinea pig cerebral cortical slices, the presence of 1 microM-forskolin augments the apparent efficacy of the amine, whereas for adenosine, prostaglandin E2, and vasoactive intestinal peptide, the major effect of 1 microM-forskolin is to increase the apparent potency of the stimulatory agent. In rat striatal slices, forskolin reveals a significant response of cyclic AMP systems to dopamine and augments the dopamine-elicited activation of adenylate cyclase in rat striatal membranes. The activation of cyclic AMP systems by forskolin is rapid and reversible, and appears to involve both direct activation of adenylate cyclase and facilitation and/or enhancement of receptor-mediated activation of the enzyme.

The noradrenaline receptor coupled adenylate cyclase system in brain. Lack of modification by changes in the availability of serotonin

The present studies were undertaken to ascertain whether or not an alteration in the availability of serotonin (5HT) can modify central noradrenergic function at the level of the noradrenaline (NA) receptor coupled adenylate cyclase system in brain. The chronic but not acute administration of the 5HT uptake inhibitors amitriptyline and chlorimipramine reduced the sensitivity of the cyclic AMP generating system to NA in the limbic forebrain. This subsensitivity was linked to a decrease in the Bmax value of beta-adrenergic binding sites without appreciable changes in the Kd values, as assessed by specific 3H-dihydroalprenolol binding. The specific 5HT uptake inhibitor fluoxetine did not change either the responsiveness of the cyclic AMP generating system to NA or the density of beta-adrenergic receptor sites. Raphé lesions which selectively reduced the level of 5HT also did not cause any changes in the neurohormonal responsiveness or the density of beta-adrenergic receptor sites. In contrast, medial forebrain bundle lesions which reduced the levels of both 5HT and catecholamines (NA and dopamine) in the forebrain, increased the responsiveness of the cyclic AMP generating system to NA. It can thus be concluded that a selective change in the availability of 5HT per se does not modify noradrenergic receptor function at the level of the NA receptor coupled adenylate cyclase system. The subsensitivity of the noradrenergic receptor system developed following amitriptyline and chlorimipramine may in all likelihood be due to the in vivo conversion to the secondary amines, nortriptyline and desmethylchlorimipramine respectively. These secondary amine metabolites are potent inhibitors of the NA reuptake and consequently could be responsible for the demonstrated in vivo down-regulation of central adrenergic receptor function (homospecific down-regulation).

Neurotransmitter receptor localizations: brain lesion induced alterations in benzodiazepine, GABA, beta-adrenergic and histamine H1-receptor binding

Selective neuronal lesions have been utilized in efforts to localize binding sites in rat brain for beta-adrenergic, gamma-aminobutyric acid (GABA), histamine H1 and benzodiazepine receptors. The various receptors respond differentially to lesions both in extent of change and in time course. After kainate lesions in the corpus striatum, benzodiazepine receptors are depleted up to 45% at 45--78 days but are unaffected after 7 days. By contrast striatal GABA receptors are increased at 7 days but depleted at later times. Thus both striatal benzodiazepine and GABA receptors appear to be associated at least in part with intrinsic neurons. In the cerebellum both benzodiazepine and GABA receptors are reduced in kainate treated rats and in Nervous mice, mutants which lack Purkinje cells. The most pronounced dissimilarity between benzodiazepine and GABA receptors occurs in Weaver mice, which selectively lack granule cells and display a 60% reduction in GABA receptors but a 40% augmentation in benzodiazepine receptors. A major portion of cerebellar GABA receptors, therefore, appear to be localized to granule cells. Striatal beta-adrenergic receptors are reduced following intrastriatal kainate injections but are unaffected by cerebral cortex ablation, suggesting an association with intrinsic neurons but not with axon terminals of the corticostriate pathway. While intraventricular injections of 6-hydroxydopamine enhance [3H]dihydroalprenolol binding to beta-adrenergic receptors in the cerebral cortex and hippocampus, such binding is not augmented in the corpus striatum, brain stem, midbrain or thalamus-hypothalamus by this treatment. Moreover, medial forebrain bundle lesions, which destroy ascending adrenergic neurons, fail to alter cerebral cortical or striatal beta-adrenergic receptors. Thus denervation-elicited increases in beta-adrenergic receptors vary with brain region and the type of denervating lesion. Histamine H1-receptors are the most resistant of all to neuronal lesions. In the corpus striatum [3H]mepyramine binding is unaffected by cerebral cortex ablation, nigral injections of 6-hydroxydopamine or brain stem hemisection. In the hippocampus, medial forebrain bundle lesions, intrahippocampal kainate injection, and fimbria and fornix transection largely fail to alter [3H]mepyramine binding. Accordingly, a major portion of these receptors may be associated with nonneuronal elements such as glia or blood vessels.

Cyclic nucleotides in nervous tissue

A review of the literature emphasizes that cyclic nucleotides play a key role in regulation of cell growth, differentiation and metabolism in diverse tissues and, in addition, are closely involved in neural tissue function. The role of cAMP as a second messenger is discussed.

Effect of stress on norepinephrine-stimulated cyclic AMP formation in brain slices

Experiments were conducted to determine if stressful procedures, which increase brain norepinephrine (NE) release in rats, lower the responsiveness of central noradrenergic receptors as measured by the catecholamine (CA)-induced cAMP accumulation in hypothalamic and cerebral cortical slices. No conclusive evidence of subsensitivity was found after either acute or chronic electric footshock or continuous restraint. Failure to find a significant reduction after stress may have resulted from several methodological problems. These include (a) the inhibition of phosphodiesterase activity with isobutylmethylxanthine, which may have obscured possible adaptive changes in cAMP degradation and/or adenosine-dependent adrenergic receptors; (b) a low initial responsiveness to NE in these animals as suggested by the greater case in inducing supersensitivity with reserpine than subsensitivity with amphetamine; and (c) the use as a test agent of exogenous NE which may stimulate a far broader population of receptive sites in brain slices than are activated during stress by the local release of endogenous NE.

The role of cyclic nucleotides in the CNS

On the basis of the information presented in this review, it is difficult to reach any firm decision regarding the role of cyclic AMP (or cyclic GMP) in synaptic transmission in the brain. While it is clear that cyclic nucleotide levels can be altered by the exposure of neural tissues to various neurotransmitters, it would be premature to claim that these nucleotides are, or are not, essential to the transmission process in the pre-or post-synaptic components of the synapse. In future experiments with cyclic AMP it will be necessary to consider more critically whether the extracellularly applied nucleotide merely provides a source of adenosine and is thus activating an extracellularly located adenosine receptor, or whether it is actually reaching the hypothetical sites at which it might act as a second messenger. The application of cyclic AMP by intrcellular injection techniques should minimize this particular problem, although possibly at the expense of new diffulties. Prio blockade of the adenosine receptor with agents such as theophylline or adenine xylofuranoside may also assist in the categorization of responses to extracellularly applied cyclic AMP as being a result either of activation of the adenosine receptor or of some other mechanism. Utimately, the developement of highly specific inhibitor for adenylate cyclase should provide a firm basis from which to draw conclusions about the role of cyclic AMP in synaptic transmission. Similar considerations apply to the action of cyclic GMP and the role of its synthesizing enzyme, guanylate cyclase. The use of phosphodiesterase inhibitors in studies on cyclic nucleotides must also be approached with caution. The diverse actions of many of these compounds, which include calcium mobilization and block of adenosine uptake, could account for many of the results that have been reported in the literature.

Central actions of histamine: microelectrophoretic studies

Responses of neurones in different regions of the rat and cat brain to micro-electrophoretically applied histamine and some related substances including antagonists are reported. Histamine excites most neurones in the hypothalamus but depresses the vast majority of neurones in other structures. Depressant actions of histamine and some inhibitions in the cortex and hippocampus of the rat are antagonized by metiamide, an H2-receptor antagonist. The results lend strong support to the idea of histaminergic transmission in the mammalian brain.

Strain differences in responsiveness of norepinephrine-sensitive adenosine 3',5'-monophosphate-generating systems in rat brain slices after intraventricular administration of 6-hydroxydopamine

The development of hyperresponsiveness in cyclic AMP-generating systems has been investigated in brain slices of Sprague-Dawley and F-344 rats following intraventricular administration of 6-hydroxydopamine. Hyperresponsiveness to adrenergic agonists in cerebral cortical slices of Sprague-Dawley rats pertained during the period 5-25 days after treatment with 6-hydroxydopamine. In contrast, hyperresponsiveness did not develop in cerebral cortical slices of F-344 rats. Reductions in norepinephrine levels of the cerebral cortex and hypothalamus following 6-hydroxydopamine treatment were comparable in the two rat strains. A hyperresponsiveness to norepinephrine and isoproterenol failed to develop in mesencephalic slices of either strain 12-14 days after treatment with 6-hydroxydopamine. The accumulation of cyclic AMP elicited by norepinephrine in cortical slices of F-344 rat is normally about 50% greater than the accumulation elicited in slices from Sprague-Dawley rats. However, after 6-hydroxydopamine treatment, there was no significant difference in the accumulations of cyclic AMP elicited in cortical slices from the two rat strains. These data indicate there may be a limit to the responsiveness of catecholamine-sensitive cyclic AMP generating systems which can develop following alterations in synaptic input.

Cyclic AMP and alpha-receptor-mediated modulation of noradrenaling release from rat brain slices

Oxymetazoline (an alpha-receptor agonist) reduced and phentolamine (an alpha-receptor antagonist) increased depolarization-induced release (potassium or electrical field stimulation) of 3H-noradrenaline (NA) from superfused neocortical slices in a dose-dependent way. Inhibition of phosphodiesterase also reduced NA release; this effect could be reversed by phentolamine. Phosphodiesterase inhibition potentiated the effect of oxymetazoline. It is suggested that stimulation of presynaptic alpha-receptors may reduce NA release up to about 60% and that increased cyclic AMP formation might be involved in this modulation.

Altered brain cyclic AMP-responses in rats reared in enriched or impoverished environments

The accumulation of radioactive cyclic AMP elicited by various neurohormones has been examined in adenine-labeled telencephalon slices from rats raised in enriched or impoverished environments. Basal levels of cyclic AMP and responses of the brain slice cyclic AMP-generating systems to norepinephrine, isoproterenol and adenosine did not differ between the two group of rats, while responses to prostaglandin E1 were significantly greater with the impoverished group and responses to histamine appeared to be greater with the enriched group.