Effect of K+ channel blockers on the alpha 2-adrenoceptor-coupled regulation of electrically evoked noradrenaline release in hippocampus

Molecular mechanisms underlying the modulation of exocytotic noradrenaline release via presynaptic receptors

The release of noradrenaline from nerve terminals is modulated by a variety of presynaptic receptors. These receptors belong to one of the following three receptor superfamilies: transmitter-gated ion channels, G protein-coupled receptors (GPCR), and membrane receptors with intracellular enzymatic activities. For representatives of each of these three superfamilies, receptor activation has been reported to cause either an enhancement or a reduction of noradrenaline release. As these receptor classes display greatly diverging structures and functions, a multitude of different molecular mechanisms are involved in the regulation of noradrenaline release via presynaptic receptors. This review gives a short overview of the presynaptic receptors on noradrenergic nerve terminals and summarizes the events involved in vesicle exocytosis in order to finally delineate the most important signaling cascades that mediate the modulation via presynaptic receptors. In addition, the interactions between the various presynaptic receptors are described and the underlying molecular mechanisms are elucidated. Together, these presynaptic signaling mechanisms form a sophisticated network that precisely adapts the amount of noradrenaline being released to a given situation.

Neurochemistry and pharmacology of the major hippocampal transmitter systems: synaptic and nonsynaptic interactions

Hippocampus plays a crucial role in important brain functions (e.g. memory, learning) thus in the past two decades this brain region became a major objective of neuroscience research. During this period large number of anatomical, neurochemical and electrophysiological data have been accumulated. While excellent reviews have been published on the anatomy and electrophysiology of hippocampal formation, the neurochemistry of this area has not been thoroughly surveyed. Therefore the aim of this review is to summarize the neurochemical and pharmacological data on the release of the major neurotransmitters found in the hippocampal region: glutamate (GLU), gamma-amino butyric acid (GABA), acetylcholine (ACh), noradrenaline (NA) and serotonin (5-HT). In addition, this review analyzes the synaptic and nonsynaptic interactions between hippocampal neuronal elements and overviews how auto- and heteroreceptors are involved in the presynaptic modulation of transmitter release. The presented data clearly show that transmitters released from axon terminals without synaptic contact play an important role in the fine tuning of communication between neurons within a neuronal circuit.

Toxin I, but not 4-aminopyridine, blocks the late inhibitory component of the dorsal root reflex in an isolated preparation of rat spinal cord

Bursts of spontaneous antidromic dorsal root action potentials, and evoked dorsal root reflexes (DRR), have been recorded from lumbar roots of isolated spinal cord preparations of rats weighing 70-90 g. The pattern of dorsal root activity was similar to that reported for isolated cord preparations from hamsters, but the frequency of spontaneous dorsal root activity was approximately 10 times slower in the rat. Toxin I and 4-aminopyridine (4-AP) both increased the frequency of spontaneous dorsal root activity. The threshold concentration of 4-AP was 1 microM, with EC50 at 20 microM. Insufficient Toxin I was available to reach a maximal response, but the threshold concentration for producing an increase in spontaneous activity was 0.1 microM, and the curve appeared to be parallel to that of 4-AP. The patterns of spontaneous dorsal root activity in the presence of 4-AP and Toxin I differed. In 4-AP bursts of large amplitude action potentials were followed by periods of depressed activity lasting up to 500 ms, whereas in Toxin I bursts of large amplitude action potentials caused no change in the continuously firing small amplitude action potentials. DRR evoked by stimulation of adjacent dorsal roots also showed differences in the presence of 4-AP and Toxin I. In 4-AP the excitatory phase of the reflex was followed by a period of depressed activity lasting up to 500 ms. This was was reduced or absent in the presence of Toxin I. Paired pulse stimulation confirmed the presence of inhibition in 4-AP, and its reduction in Toxin I. Examination of the pattern of spontaneous dorsal root activity following dorsal root stimulation showed strong oscillatory activity in the presence of 4-AP, but little such activity in the presence of Toxin I. It was concluded that the actions of 4-AP and Toxin I on the isolated preparation of rat spinal cord are similar in that both cause an increase in the spontaneous dorsal root firing rate, but that Toxin I also blocks the period of inhibition which follows bursts of large amplitude action potentials.

The mechanisms of enhancement and inhibition of field stimulation responses of guinea-pig vas deferens by prostacyclin analogues

1. In the guinea-pig isolated vas deferens preparation bathed in Tyrode's solution, the prostacyclin analogues, cicaprost, TEI-9063, iloprost, taprostene and benzodioxane-prostacyclin, enhanced twitch responses to submaximal electrical field stimulation (20%-EFS). The high potency of cicaprost (EC150 = 1.3 nM) and the relative potencies of the analogues (equi-effective molar ratios = 1.0, 0.85, 1.6, 17 and 82, respectively) suggest the involvement of a prostacyclin (IP-) receptor. 2. Maximum enhancement induced by cicaprost in 2.5 mM K+ Krebs-Henseleit solution was similar to that in Tyrode solution (2.7 mM K+), but was progressively reduced as the K+ concentration was increased to 3.9, 5.9 and 11.9 mM. There was also a greater tendency for the other prostacyclin analogues to inhibit EFS responses in 5.9 mM standard K+ Krebs-Henseleit solution; this may be attributed to their agonist actions on presynaptic EP3-receptors resulting in inhibition of transmitter release. 3. The EFS enhancing action of cicaprost was not affected by the alpha1-adrenoceptor antagonist prazosin (100 and 1000 nM). Cicaprost (20 and 200 nM) did not affect contractile responses of the vas deferens to either ATP (5 microM) or alpha,beta-methylene ATP (1 microM) in the presence of tetrodotoxin (TTX, 100 nM). In addition, enhancement by cicaprost of responses to higher concentrations of ATP (30 and 300 microM) in the absence of TTX, as shown previously by others, was not seen. Prostaglandin E2 (PGE2, 10 nM) and another prostacyclin analogue TEI-3356 (20 nM) enhanced purinoceptor agonist responses. Unexpectedly, TTX (0.1 and 1 microM) partially inhibited contractions elicited by 10-1000 microM ATP; contractions elicited by 1-3 microM ATP were unaffected. Further studies are required to establish whether a pre- or post-synaptic mechanism is involved. 4. In a separate series of experiments, cicaprost (5-250 nM), TEI-9063 (3-300 nM), 4-aminopyridine (10-100 microM) and tetraethylammonium (100-1000 microM) enhanced both 20%-EFS responses and the accompanying overflow of noradrenaline to a similar extent. In further experiments with the EP1-receptor antagonist AH 6809, TEI-3356 (1.0-100 nM) and the EP3-receptor agonist, sulprostone (0.1-1.0 nM) inhibited both maximal EFS responses and noradrenaline overflow, thus confirming previous reports of the high activity of TEI-3356 at the EP3-receptor. Cicaprost had no significant effect on noradrenaline overflow at 10 and 100 nM, but produced a modest inhibition at 640 nM. 5. In conclusion, our studies show that prostacyclin analogues (particularly TEI-3356) can inhibit EFS responses of the guinea-pig vas deferens by acting as agonists at presynaptic EP3-receptors. Prostacyclin analogues (particularly cicaprost and TEI-9063) can also enhance EFS responses through activation of IP-receptors. The mechanism of the enhancement has not been rigorously established but from our results we favour a presynaptic action to increase transmitter release.

The serotonergic and noradrenergic systems of the hippocampus: their interactions and the effects of antidepressant treatments

Previous reviews have well illustrated how antidepressant treatments can differentially alter several neurotransmitter systems in various brain areas. This review focuses on the effects of distinct classes of antidepressant treatments on the serotonergic and the noradrenergic systems of the hippocampus, which is one of the brain limbic areas thought to be relevant in depression: it illustrates the complexity of action of these treatments in a single brain area. First, the basic elements (receptors, second messengers, ion channels, ...) of the serotonergic and noradrenergic systems of the hippocampus are revisited and compared. Second, the extensive interactions occurring between the serotonergic and the noradrenergic systems of the brain are described. Finally, issues concerning the short- and long-term effects of antidepressant treatments on these systems are broadly discussed. Although there are some contradictions, the bulk of data suggests that antidepressant treatments work in the hippocampus by increasing and decreasing, respectively, serotonergic and noradrenergic neurotransmission. This hypothesis is discussed in the context of the purported function of the hippocampus in the formation of memory traces and emotion-related behaviors.

Prejunctional actions of K+ channel blockers in rat vas deferens

In studies of isometric contractions in prostatic portions of rat vas deferens evoked by single pulse electrical stimulation, the K+ channel blockers 4-aminopyridine, tetraethylammonium and charybdotoxin, but not apamin, significantly reduced the prejunctional inhibitory potency and the maximum inhibitory effect of the alpha 2-adrenoceptor agonist xylazine. The protein kinase C activator phorbol dibutyrate had similar effects to 4-aminopyridine against xylazine. However, 4-aminopyridine, tetraethylammonium, charybdotoxin and phorbol dibutyrate, but not apamin, significantly increased the magnitude of the isometric contraction to a single stimulus. 4-Aminopyridine and phorbol dibutyrate significantly reduced, while tetraethylammonium did not affect, isometric contractions to noradrenaline, and 4-aminopyridine failed to affect contractions to alpha,beta-methylene-ATP, so that the effects of these agents on the isometric contraction to a single stimulus were presumably by a prejunctional action. The Ca2+ entry facilitator Bay K 8644 (1,4-dihydro-2,6-dimethyl-5-nitro-4-[2-(trifluoromethyl)-phenyl]-3-pyrid ine carboxylic acid methylester) increased stimulation-evoked contractions by a postjunctional action and reduced the inhibitory effects of xylazine. When the isometric contraction following 4-aminopyridine was reduced by decreasing the stimulation voltage or by reducing the Ca2+ concentration from 2.5 to 0.9 mM, 4-aminopyridine significantly reduced the potency of xylazine. However, tetraethylammonium and Bay K 8644 failed to affect the inhibitory potency of xylazine in low Ca2+. It is concluded that the K+ channel blocker 4-aminopyridine reduces the prejunctional inhibitory potency of xylazine, and this action is independent of increased neurotransmitter release. These results suggest that prejunctional alpha 2-adrenoceptor-mediated inhibition in rat vas deferens involves K+ channels sensitive to block by 4-aminopyridine.

Differential effects of potassium channel blockers on extracellular concentrations of dopamine and 5-HT in the striatum of conscious rats

1. The selective Ca(2+)-activated K+ channel blocker apamin increased extracellular 5-hydroxytryptamine (5-HT) concentrations in the striatum when administered through the microdialysis probe at doses of 0.1 mM and 1 mM. Extracellular dopamine concentrations increased only at the highest dose administered (1 mM). 2. Mast cell degranulating peptide (MCDP), which blocks the dendrotoxin sensitive delayed rectifier (DR) current, increased extracellular concentrations of dopamine at dose of 10 microM-100 microM but had no effect on 5-HT. 3. The non selective K+ channel blocker tetraethylammonium (TEA) induced a dose-dependent (1 mM-10 mM) increase in extracellular dopamine concentrations and an increase in 5-HT which showed little or no dose-dependency. 4. 4-Aminopyridine (4-AP), a blocker with some similar characteristics to MCDP, increased extracellular dopamine concentrations at doses of 10 microM-1 mM, but had no effect on 5-HT. 5. These findings suggest that dopamine release may be modulated by DR-like current and/or A-current K+ channels. However, in view of the similar effects of MCDP and 4-AP at the concentrations used it is more likely that the dendrotoxin-sensitive DR-like current is involved. In contrast, 5-HT release appears to be modulated by Ca(2+)-activated K+ channels.