Pre- and postsynaptic actions of noradrenaline and clonidine on myenteric neurons

Norepinephrine Modulates Pyramidal Cell Synaptic Properties in the Anterior Piriform Cortex of Mice: Age-Dependent Effects of β-adrenoceptors

Early odor preference learning in rodents occurs within a sensitive period [≤postnatal day (P)10–12], during which pups show a heightened ability to form an odor preference when a novel odor is paired with a tactile stimulation (e.g., stroking). Norepinephrine (NE) release from the locus coeruleus during stroking mediates this learning. However, in older pups, stroking loses its ability to induce learning. The cellular and circuitry mechanisms underpinning the sensitive period for odor preference learning is not well understood. We first established the sensitive period learning model in mice – odor paired with stroking induced odor preference in P8 but not P14 mice. This learning was dependent on NE-β-adrenoceptors as it was prevented by propranolol injection prior to training. We then tested whether there are developmental changes in pyramidal cell excitability and NE responsiveness in the anterior piriform cortex (aPC) in mouse pups. Although significant differences of pyramidal cell intrinsic properties were found in two age groups (P8–11 and P14+), NE at two concentrations (0.1 and 10 μM) did not alter intrinsic properties in either group. In contrast, in P8–11 pups, NE at 0.1 μM presynaptically decreased miniature IPSC and increased miniature EPSC frequencies. These effects were reversed with a higher dose of NE (10 μM), suggesting involvement of different adrenoceptor subtypes. In P14+ pups, NE at higher doses (1 and 10 μM) acted both pre- and postsynaptically to promote inhibition. These results suggest that enhanced synaptic excitation and reduced inhibition by NE in the aPC network may underlie the sensitive period.

Catecholaminergic neurons in rat dorsal motor nucleus of vagus project selectively to gastric corpus

Nitric oxide synthase-immunoreactive (NOS-IR) neurons in the rat caudal dorsal motor nucleus of the vagus (DMV) project selectively to the gastric fundus and may be involved in vagal reflexes controlling gastric distension. This study aimed to identify the gastric projections of tyrosine hydroxylase-immunoreactive (TH-IR) DMV neurons, whether such neurons colocalize NOS-IR, and if they are activated after esophageal distension. Gastric-projecting neurons were identified after injection of retrograde tracers into the muscle wall of the gastric fundus, corpus, or antrum/pylorus before removal and processing of the brain stems for TH- and NOS-IR. A significantly higher proportion of corpus- compared with fundus- and antrum/pylorus-projecting neurons were TH-IR (14% compared with 4% and 2%, respectively, P < 0.05). Colocalization of NOS- and TH-IR was never observed in gastric-projecting neurons. In rats tested for c-Fos activation after intermittent esophageal balloon distension, no colocalization with TH-IR was observed in DMV neurons. These findings suggest that TH-IR neurons in the caudal DMV project mainly to the gastric corpus, constitute a subpopulation distinct from that of nitrergic vagal neurons, and are not activated on esophageal distension.

Mechanisms underlying presynaptic inhibition through alpha 2-adrenoceptors in guinea-pig submucosal neurones

1. Intracellular recordings were made from submucosal neurones of the guinea-pig ileum. The actions of noradrenaline, somatostatin and [Met5]enkephalin on nicotinic synaptic potentials (EPSPs) were studied. 2. In one series of experiments, agonists were applied by superfusion; noradrenaline (0.1-20 microM) decreased EPSP amplitude by 95-100% in all neurones. Similar application of somatostatin (1-100 nM) inhibited EPSPs in about half the neurones by a maximum of 40%. [Met5]enkephalin (0.1-10 microM) did not alter EPSPs. Idazoxan and yohimbine competitively antagonized the action of noradrenaline with dissociation equilibrium constants of 20 and 30 nM respectively. 3. In another series of experiments, noradrenaline and somatostatin were applied locally from a pipette so that they reached presynaptic terminals but not the cell bodies or axons of the presynaptic cell: noradrenaline inhibited EPSPs by 90% in all neurones but somatostatin had no effect. When applied locally to the cell bodies giving rise to the presynaptic fibres, both agonists inhibited EPSPs in half the neurones by 40%. 4. When noradrenaline was applied locally to presynaptic terminals, the latency to onset of noradrenaline to inhibit EPSPs was 45-160 ms; cadmium applied similarly depressed EPSPs in 5-50 ms. 5. Pertussis toxin pre-treatment only partially blocked presynaptic inhibition caused by noradrenaline but abolished the reduction of EPSP amplitude by somatostatin. 6. It is concluded that noradrenaline and somatostatin reduce the amplitude of the fast EPSP because they hyperpolarize cell bodies and prevent action potential initiation. Noradrenaline, but not somatostatin, has an additional action to inhibit acetylcholine release by acting at nerve terminal receptors. 7. The presynaptic inhibitory action of noradrenaline results from activation of alpha 2-adrenoceptors at nerve terminals but the mechanism(s) by which these presynaptic receptors act cannot be explained adequately by either activation of a potassium conductance and/or inhibition of a calcium conductance.

Imidazoline receptors in rat liver cells: a novel receptor or a subtype of alpha 2-adrenoceptors?

An imidazoline/guanidine receptor has been characterized in rat liver cells. Binding of [3H]idazoxan, a selective benzodioxan antagonist, to imidazoline receptor on intact fresh hepatocytes (Bmax = 801 +/- 23 fmol/mg protein, Kd = 11 +/- 0.8 nM) and to liver membranes (Bmax = 400 +/- 38 fmol/mg protein, Kd = 10 +/- 2 nM) was saturable at 4 degrees C within 3.5 h and at 30 degrees C within 30 min, respectively. Rat lung membranes had more imidazoline sites (Bmax = 578 +/- 30 fmol/mg protein, Kd = 14 +/- 1.4 nM) than alpha 2-adrenoceptors (Bmax = 175.0 +/- 20.0 fmol/mg protein, Kd = 4.8 +/- 2.0 nM). We also screened other tissues for imidazoline sites; the ratio of adrenoceptors to total sites labeled with [3H]idazoxan displaced by cirazoline was lower in rat lung compared to rat brain and human platelets. The imidazoline receptor has common pharmacological properties with alpha 2-adrenoceptors, although it is not a subtype of the adrenoceptor, since it bound neither the endogenous agonists norepinephrine and epinephrine, nor the selective alpha 2-antagonists yohimbine and phentolamine. All guanidine type alpha 2-adrenoceptor drugs (e.g. guanbenz, guanoxan) and imidazolines (e.g., UK-14,304, naphazoline) competed with high affinity for the liver imidazoline receptor. The lack of effect by Gpp(NH)p, a non-hydrolysable GTP analogue, on the affinity of guanidine- and imidazoline-type ligands for liver imidazoline receptors suggests that the mode of action of these drugs at imidazoline receptors is different than at conventional alpha 2-adrenoceptors. Ionic changes were considered as a possible mechanism underlying the alpha 2-adrenoceptor effects in various cells. Opening of K+ channels by alpha 2-adrenoceptors agonists is a pathway which might be shared by imidazoline-type agonists at imidazoline sites. Indeed, 4-aminopyridine, a K+ channel blocker, inhibited the specific binding of [3H]idazoxan to liver cells with an IC50 of 0.34 +/- 0.07 mM a concentration which is effective in blocking K+ channels in neuronal cells. Similarly, Cs+ and NH4+ effectively interfered with [3H]idazoxan binding, suggesting a possible coupling of imidazoline sites to K+ gating. The endogenous ligand clonidine-displacing substance (CDS), which was isolated from bovine brain and which binds to alpha 2-adrenoceptors in brain membranes and human platelets competed with idazoxan at rat liver imidazoline receptors.(ABSTRACT TRUNCATED AT 400 WORDS)

"Upstream" regulation of the release probability in sympathetic nerve varicosities

The results appear to support the following tentative working hypothesis. (1) Nerve impulse-induced transmitter release from sympathetic nerve varicosities is monoquantal and highly intermittent (probability range: 0-0.03). (2) Nerve impulses invade varicosities as all-or-none, Na+ channel-dependent action potentials; invasion failure may be rare. (3) The release probability is not controlled by properties (amplitude or duration) of the invading action potential or the resulting Ca2+ current, but by the availability of an as yet unidentified permissive factor. (4) The permissive factor is actively transported intra-axonally, probably in association with organelles (LDVs?). (5) The activation and/or transport of the permissive factor are controlled "upstream" of the varicosity; they depend on Ca2+ influx through channels insensitive to nifedipine (hence, not of L-type) but blocked by Cd2+ and apparently opened by slight depolarization of the resting membrane, in this respect behaving more as T- than N-type channels. (6) A high resting K+ efflux "upstream" of the varicosity restricts the availability of the permissive factor; it is the main mechanism maintaining the (economically necessary) low release probability. (7) Prejunctional agonists do not inhibit transmitter secretion by causing a conduction block or by reducing the action potential-induced Ca2+ influx into the varicosity itself, but by depressing the Ca2(+)-dependent activation and/or transport of the permissive factor; they act at least in part via receptors "upstream" of the varicosity. (8) This hypothesis for regulation of the release probability in sympathetic nerves may apply, at least in part, to other neurons as well.

Inositol monophosphate esterase inhibition by lithium in normal human studies using neuroendocrine tests. Part II. HGH, PRL and TSH responses to clonidine

1. Recent preclinical researches gave evidence that the actions of many neurotransmitters and drugs are mediated through the activation of the turnover of phosphatidylinositol 4,5 biphosphate (PIP2). Lithium (Li) a specific inhibitor of the enzyme inositol monophosphate esterase reduces the turn over of PIP2 and decreases the cell responses mediated via the cleavage of PIP2. 2. The authors studied the influence of a therapeutic single dose of Li on the response of human growth hormone (HGH) prolactin (PRL) and thyrotropin (TSH) to clonidine (C), an alpha adrenergic agonist which stimulates the turn over of PIP2. 3. Li reduces the HGH response to C in 3, increases in 1 and provokes no change in 2 subjects. The basal secretion of PRL and TSH were not modified. 4. These findings could support the hypothesis that some of the action of Li in humans is mediated through the inhibition of the enzyme inositol monophosphate esterase. Further studies are needed in order to determine the importance of this action.

Alpha 1-adrenoreceptor-mediated increase in acetylcholine release in brain slices during morphine tolerance

Norepinephrine, clonidine, and phenylephrine increased the electrically evoked release of endogenous acetylcholine in cortical slices taken from morphine-tolerant guinea pigs. This effect was alpha 1-adrenoreceptor mediated and was opposite to the alpha 2-adrenoreceptor-mediated inhibition of acetylcholine release, normally elicited by norepinephrine and clonidine. In the presence of prazosin, clonidine recovered its normal inhibitory properties, suggesting that morphine tolerance induced the appearance of an alpha 1-adrenoreceptor-mediated response that overshadowed, but did not cancel, the still present alpha 2-adrenoreceptor inhibitory control. The attempt to prove the presence of alpha-adrenoreceptors on the nerve endings by testing the effect of norepinephrine in synaptosomal preparations (preloaded with [3H]choline and depolarized with KCl and veratridine) was unsuccessful. Therefore the problem of the exact location of this excitatory input remains to be solved. These results confirm previous findings reporting the increase in cortical acetylcholine release induced by the alpha-adrenoreceptor agonists in morphine-tolerant, freely moving guinea pigs and demonstrate that opiate tolerance inverts the direction of the noradrenergic modulation even in the isolated intracortical cholinergic structures.

Inhibition of Ca-spikes in rat preganglionic cervical sympathetic nerves by sympathomimetic amines

1. Propagated Ca-spikes were recorded from isolated cervical sympathetic nerve trunks of the rat when bathed in a solution containing 5 mM Ca2+, 0.5 or 1 microM tetrodotoxin (to block Na currents) and 1 mM 4-aminopyridine (to reduce K currents). 2. Spikes persisted when external Ca2+ was replaced with Sr2+ or Ba2+, but were blocked by the addition of the following inorganic Ca-channel blockers (in descending order of potency): Cd2+ greater than La3+ greater than Ni2+ greater than Co2+ greater than Mn2+ greater than Mg2+. 3. Ca-spike amplitude was reduced by up to 90% by (-)-noradrenaline (IC50 1.5 microM). The following sympathomimetic amines imitated this effect (in descending order of potency): clonidine greater than or equal to (-)-adrenaline greater than or equal to [(-)-noradrenaline] greater than or equal to dopamine greater than (-)-phenylephrine greater than or equal to (+/-)-amidephrine. 4. Ca-spike inhibition by (-)-noradrenaline was antagonized by phentolamine (pA2 6.5). Yohimbine was about 10 times weaker than phentolamine; (+/-)-propranolol (1 microM) and prazosin (10 microM) had no clear effect. 5. (-)-Noradrenaline reduced the amplitude of the compound action potential recorded from the superior cervical sympathetic ganglion following supramaximal preganglionic trunk stimulation when recorded in normal Krebs solution and hyperpolarized the ganglion with respect to the post-ganglionic trunk. Depression of the transmitted ganglionic action potential was antagonized by phentolamine (5 microM) but not by yohimbine (1 microM); in contrast 1 microM yohimbine completely prevented the ganglionic hyperpolarization. (-)-Noradrenaline did not hyperpolarize the preganglionic cervical sympathetic nerve trunk under these recording conditions. 6. It is suggested that inhibition of transmitter release from sympathetic preganglionic fibres produced by noradrenaline results from a depression of the voltage-gated Ca current in the fibres and/or their terminals, and that this action is mediated by an alpha-adrenoceptor which does not fully conform to either alpha 1 or alpha 2 subtypes.