Comparative regulation of potassium and amphetamine induced release of 3H-norepinephrine from rat brain via presynaptic mechanisms

Enduring vulnerability to transient reinstatement of hemiplegia by prazosin after traumatic brain injury

A single dose of an alpha1-noradrenergic antagonist transiently reinstates hemiplegia after recovery from brain injury, which suggests that noradrenaline (NA) is required to maintain recovery. No systematic studies have determined the postinjury duration of this vulnerability. This study used a within-subject, dose-response design to determine whether prazosin (PRAZ), an alpha1-NA antagonist, or propranolol (PROP), a beta-NA antagonist, would continue to reinstate hemiplegia over time after recovery from weight-drop traumatic brain injury (TBI). PRAZ transiently reinstated hemiplegia as measured by beam walk (BW) score in a dose-dependent manner, with the same degree of symptom reinstatement at 1, 3, 6, and 12 months post-TBI. Between-animal variability in reinstatement of hemiplegia by PRAZ was predicted by severity of deficits in BW ability 24 h after TBI. In contrast, PRAZ did not reinstate tactile placing deficits at 1 month post-TBI suggesting a different mechanism of maintaining recovery for each task. Reinstatement of symptoms are not due to sedation. Only TBI rats receiving PRAZ, not high, sedating doses of PROP or saline (SAL), showed return of hemiplegia. These data indicate that vulnerability to transient reinstatement of hemiplegia on some tasks endures long after functional recovery from TBI.

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.

The lack of involvement of central nervous system in the antiarrhythmic effects of prostaglandins E2, F2 alpha, and I2 in cat

The role of the central nervous system (CNS) in the antiarrhythmic effects of prostaglandins (PGs) E2, F2 alpha, and I2 was studied by administering each agent into the left lateral cerebral ventricle (i.c.v. administration) of chloralose-anaesthetized cats. The cardiac arrhythmias were produced by intravenous (i.v.) infusion of ouabain (1 microgram/kg/min). The PGs E2, F2 alpha and I2 on i.c.v. administration in the dose range of 1 ng to 10 micrograms failed to inhibit ouabain-induced cardiac arrhythmias. However, when infused i.v., PGE2 (1 microgram/kg/min), PGF2 alpha (5 micrograms/kg/min), and PGI2 (2 micrograms/kg/min) effectively suppressed these arrhythmias. The standard antiarrhythmic drug propranolol (0.5-8.0 mg) on i.c.v. administration also significantly reduced the ouabain-induced cardiac arrhythmias. It is suggested that the CNS is not the site of action of PGs E2, F2 alpha, and I2 in antagonising the ouabain-induced cardiotoxicity in cats.

Evidence that amphetamine and Na+ gradient reversal increase striatal synaptosomal dopamine synthesis through carrier-mediated efflux of dopamine

Amphetamine (AMPH) releases dopamine (DA) from striatal synaptosomes and concomitantly increases DA synthesis. Since AMPH may release DA through carrier-mediated diffusion via reversal of the DA uptake system, the increase in DA synthesis might depend on a functioning uptake carrier. Consistent with such a mechanism, the uptake inhibitors nomifensine (NMF) and benztropine (BZT) completely prevented the AMPH-induced increase in DA synthesis at concentrations known to inhibit DA uptake. Changes in the Na+ gradient across the synaptosomal membrane also promote DA release, since DA and Na+ are cotransported by the neuronal uptake carrier. Incubation of synaptosomes in medium containing decreasing Na+ increased DA synthesis inversely proportional to Na+ over the range 128 to 20 mM. Similarly, incubations in the presence of 10(-4) M ouabain to inhibit Na+, K+-ATPase and allow intracellular accumulation of Na+ also increased DA synthesis. These changes in DA synthesis could also be prevented by BZT and were non-additive with the AMPH-induced increase in DA synthesis. However, a concentration of ouabain (10(-6) M) which by itself did not increase DA synthesis, and does not promote DA release, potentiated the AMPH-induced increase in DA synthesis. Further, the increased DA synthesis promoted by all three manipulations was only marginally dependent on the presence of Ca2+ in the incubation medium. However, at 5 and 10 mM Na+, a second component of increased DA synthesis was observed which was insensitive to BZT, but was prevented by Ca2+ removal. These results suggest that the increase in DA synthesis, and presumably DA release promoted by AMPH, lowered Na+, and ouabain, depend on the availability of the DA carrier at the internal face of the neuronal membrane and the intracellular content of Na+. The second component of increased DA synthesis which is evident at 5 and 10 mM Na+ is discussed in terms of a possible Ca2+-mediated change in DA synthesis which is independent of the DA carrier.

Presynaptic beta-adrenoceptors

The existence of facilitatory presynaptic beta-adrenoceptors has been shown in approximately 30 tissues of 6 different species including human. A positive feed back loop for further release of the transmitter appears to be activated by an endogenous agonist, epinephrine, taken up and released as a cotransmitter with norepinephrine rather than norepinephrine itself released from peripheral noradrenergic nerve terminals. Presynaptic beta-adrenoceptors are mainly of a beta 2-subtype. Some beta 1-subtype receptors are also suggested. There coexist presynaptic beta 1- and beta 2-adrenoceptors in cat and rat hypothalamus. Higher sensitivity of peripheral presynaptic beta-adrenoceptors to isoproterenol may be implicated in the early development of hypertension in SHR. Epinephrine taken up and released initiates the development of hypertension in rats via activation of these receptors. Increased activation of these receptors by epinephrine may play a role in the development of essential hypertension. The antihypertensive action of beta-antagonists may be in part due to blockade of these facilitatory presynaptic beta-adrenoceptors.

Altered norepinephrine uptake in neuronal cultures from spontaneously hypertensive rat brain

Uptake of [3H]norepinephrine (NE) has been characterized and compared in neuronal cultures prepared from the brains of 1-day-old normotensive (Wistar-Kyoto, WKY) and spontaneously hypertensive (SH) rats. In cultures from both strains total [3H]NE uptake consisted of a sodium-dependent portion and a sodium-independent portion. The sodium-dependent [3H]NE uptake was inhibited by NE uptake blockers such as maprotiline or desmethylimipramine (both at 0.5-100 microM). This sodium-dependent, NE uptake blocker-sensitive portion of the uptake was also stereospecific, preferring the l-isomer of NE. In contrast, the sodium-independent uptake was not sensitive to maprotiline or desmethylimipramine. Autoradiograms of cultures incubated with [3H]NE showed label concentrated in certain, but not all, neurites and in a few neuronal cell bodies. Cultures incubated with label in sodium-free buffer did not show any such localization of grains but instead showed a diffuse pattern. Incubation of neuronal WKY or SH brain cultures with various concentrations of l-[3H]NE and unlabeled l-NE in the presence or absence of sodium enabled the construction of saturation curves for sodium-dependent uptake in each culture type. In WKY cultures, Km and maximal velocity of uptake (Vmax) values of 0.37-0.45 microM and 0.58-0.69 pmol X mg protein-1 X min-1, respectively, were obtained for sodium-dependent uptake. In contrast, the Km and Vmax values for [3H]NE uptake in SH neuronal cultures were 1.4 microM and 1.31 pmol X mg protein-1 X min-1, respectively. Kinetic analyses of the results show that in SH neuronal cultures the [3H]NE uptake sites are of lower affinity but higher capacity compared with those in WKY neuronal cultures.

Indomethacin potentiates the operant behavior suppressant and rectal temperature lowering effects of low doses of d-amphetamine in rats

Prostaglandins (PGs) may be important in modulating the actions of d-amphetamine. To test this hypothesis male rats were pretreated with indomethacin to inhibit PG synthesis before d-amphetamine was injected. Indomethacin (5 mg/kg, IP) was found to be without direct effect upon fixed-ratio behavior or rectal temperature, but significantly enhanced the capacity of low doses of d-amphetamine to suppress behavior and lower temperature. These effects were not due to partial food deprivation or to an increase in the concentration of d-amphetamine in brain. It is concluded that one or more of the PGs modulate the central actions of d-amphetamine, perhaps by modifying the release and/or reuptake of catecholamines, or through a postsynaptic action.

Neurophysiological consequences of presynaptic receptor activation: changes in noradrenergic terminal excitability

Experiments were carried out to explore the view that activation of presynaptic receptors on the terminals of noradrenergic neurons is accompanied by alterations in their excitability to direct electrical stimulation. Antidromic action potentials evoked from frontal cortex of urethane anesthetized rats were recorded extracellularly from nucleus locus coeruleus. The threshold current necessary to evoke antidromic action potentials varied as a result of infusion of adrenergic agonists and antagonists into frontal cortex within 50 micrometer of the stimulating electrode. Local infusion of the alpha-adrenergic agonist clonidine produced a marked decrease in terminal excitability, while the alpha-antagonist phentolamine produced an increase in terminal excitability and was shown to reverse the effect of the agonist. Infusion of the beta-adrenergic agonist isoproterenol was without effect, although the beta-antagonist propranolol resulted in a decrease in terminal excitability. Infusions of potassium increased excitability of locus coeruleus terminals. Terminal excitability was seen to vary inversely with the rate of spontaneous or high frequency stimulation-induced firing of locus coeruleus neurons. From these observations, it may be inferred that activation or blockade of alpha-adrenergic presynaptic receptors results in changes in polarization and/or conductance of the noradrenergic synaptic endings. These results are discussed with respect to phenomena associated with the possible presynaptic regulation of neurotransmitter release.

Differences in presynaptic alpha-blockade, noradrenaline uptake inhibition, and potential antidepressant activity between (+)- and (-)mianserin

The effect of racemic mianserin on K+-evoked tritium release from rat brain cortex slices previously incubated with 3H-L-noradrenaline was studied. Racemic mianserin (10(-9)--10(-5) M) increased stimulation-induced release dose-dependently. As methysergide, metiamide, and cyproheptadine failed to do so, it was concluded that this effect was probably not caused by the antihistamine or antiserotonin activity of racemic mianserin, but due to its alpha-adrenolytic effect. Evaluation of the effects of the enantiomers (+)(S)mianserin and (-)(R)mianserin showed that the alpha-adrenolytic effect resided in the (+)isomer, whereas the (-)isomer was inactive at a concentration of 10(-6) M. Inhibition of noradrenaline into rat hypothalamic synaptosomes also showed stereospecificity in that (+)mianserin was about 300-times more active than(-)mianserin. Inhibition of rat muricidal behavior, a test for potential antidepressant activity, showed a similar dissociation in the effects of the two enantiomers, in that (+)mianserin was active, whereas (-)mianserin was not.

Regulation of noradrenaline overflow in rat cerebral cortex by prostaglandin E2

1 The effects of prostaglandin E2 (PGE2), PGF2 alpha and PGI2 and of inhibitors of prostaglandin synthesis and action, on the K+-evoked [3H]-noradrenaline ([3H]-NA) overflow from rat cerebral cortex slices have been investigated. 2 PGE2 reduced, while indomethacin (a prostaglandin synthesis inhibitor) or SC 19220 (a prostaglandin receptor antagonist) increased, the evoked overflow compared with controls. 3 The inhibition of [3H]-NA overflow by PGE2 was dose-dependently antagonized by SC 19220. 4 The results indicate that PGE2 modulates NA release in rat cerebral cortex in vitro.

Regulation of a presynaptic adenylate cyclse from bovine cerebellum by beta-adrenergic receptors

Intact crude synaptosome from bovine cerebellum contain, in addition to an externally accessible (postsynaptic) adenylate cyclase, an enzyme with its catalytic center oriented towards the inside of the synaptosome (presynaptic adenylate cyclase). This is demonstrated by the unmasking of latent adenylate cyclase activity by Triton X-100. Furthermore, intact crude synaptosomes can synthesize cyclic AMP from adenine. This synthesis takes place inside the synaptosomes as the postsynaptic adenylate cyclase is inactive in the Krebs-Ringer buffer. Presynaptic adenylate cyclase activity is not influenced by depolarization, as shown by [3H]adenine pulse-labeling, but is stimulated by (-)-norepinephrine and (-)-isoproterenol. (+/-)-Propranolol inhibits this stimulation whereas phentolamine has no effect, suggesting the presence of a beta-adrenergic receptor-coupled presynaptic adenylate cyclase.