Excitation of brain stem neurons by noradrenaline and histamine

Norepinephrine homogeneously inhibits alpha-amino-3-hydroxyl-5-methyl-4-isoxazole-propionate- (AMPAR-) mediated currents in all layers of the temporal cortex of the rat

The primary auditory cortex is subject to the modulation of numerous neurotransmitters including norepinephrine (NE), which has been shown to decrease cellular excitability by yet unclear mechanisms. We investigated the possibility that NE directly affects excitatory glutamatergic synapses. We found that bath applications of NE (20 microM) decreased glutamatergic excitatory post-synaptic currents (EPSCs) in all cortical layers. Changes in the kinetics of synaptic EPSCs, invariance of pair pulse ratio and of the coefficient-of-variation, together with the decrease of responses to pressure-application of AMPA (500 microM), indicated the postsynaptic nature of the adrenergic effect. Pharmacological experiments suggested that the NE-induced depression of EPSCs is caused by the activation of alpha1 adrenoceptors, PLC, and a Ca(2+)-independent PKC. We speculate that the decrease in temporal cortex excitability might promote a posterior-to-anterior shift in cortical activation together with a decrease in spontaneous background activity, resulting eventually in more effective sensory processing.

Estradiol modulation of phenylephrine-induced excitatory responses in ventromedial hypothalamic neurons of female rats

Estrogens act within the ventromedial nucleus of the hypothalamus (VMN) to facilitate lordosis behavior. Estradiol treatment in vivo induces alpha(1b)-adrenoreceptor mRNA and increases the density of alpha(1B)-adrenoreceptor binding in the hypothalamus. Activation of hypothalamic alpha(1)-adrenoceptors also facilitates estrogen-dependent lordosis. To investigate the cellular mechanisms of adrenergic effects on VMN neurons, whole-cell patch-clamp recordings were carried out on hypothalamic slices from control and estradiol-treated female rats. In control slices, bath application of the alpha(1)-agonist phenylephrine (PHE; 10 microM) depolarized 10 of 25 neurons (40%), hyperpolarized three neurons (12%), and had no effect on 12 neurons (48%). The depolarization was associated with decreased membrane conductance, and this current had a reversal potential close to the K(+) equilibrium potential. The alpha(1b)-receptor antagonist chloroethylclonidine (10 microM) blocked the depolarization produced by PHE in all cells. From estradiol-treated rats, significantly more neurons in slices depolarized (71%) and fewer neurons showed no response (17%) to PHE. PHE-induced depolarizations were significantly attenuated with 4-aminopyridine (5 mM) but unaffected by tetraethylammonium chloride (20 mM) or blockers of Na(+) and Ca(2+) channels. These data indicate that alpha(1)-adrenoceptors depolarize VMN neurons by reducing membrane conductance for K(+). Estradiol amplifies alpha(1b)-adrenergic signaling by increasing the proportion of VMN neurons that respond to stimulation of alpha(1b)-adrenergic receptors, which is expected in turn to promote lordosis.

Histamine-induced prolonged depolarization in rat supraoptic neurons: G-protein-mediated, Ca(2+)-independent suppression of K+ leakage conductance

Ionic mechanisms responsible for histamine-induced prolonged depolarization in supraoptic nucleus neurons were investigated using whole-cell patch recordings in horizontally prepared brain slices from adult male rats. Bath application of histamine (1-10 microM) in control medium induced membrane depolarization in nine of 12 phasically firing, putative vasopressin cells, but not in continuous firing, putative oxytocin cells (none of five cells). Depolarization, usually accompanied by increased firing rate, started within 20 s after histamine reached the slices, lasting for 3-13 min, after which they repolarized, and this was repeatable upon washout. Chelation of intracellular Ca2+ with 11 mM 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetra-acetate and perfusion of slices with Ca(2+)-free medium blocked neither histamine-induced membrane depolarizations nor increased firing rates in 24 of 30 cells recorded. Depolarizations were always associated with decreases in membrane conductance. Following treatment with promethazine (H1 receptor antagonist) in six cells excited previously by histamine, subsequent application induced neither membrane depolarization nor increased firing. H1 receptor agonists mimicked histamine-induced depolarization (four of six cells) but the H2 receptor agonist, dimaprit (10 microM), had no effect (all of nine cells). In medium containing 0 mM Ca2+, 2 mM Co2+ and 1-2 microM tetrodotoxin, with internal Ca2+ chelation, bath application of histamine induced an apparent inward current in 15 of 20 supraoptic neurons tested. The peak of inward current evoked by 1-10 microM histamine at holding potentials around -50 mV varied from 10 to 50 pA (27.3 +/- 0.3 pA, mean +/- S.E.M.). Ramp voltage tests revealed that this inward current decreased as membrane potential was hyperpolarized and had a reversal potential of -90.1 +/- 3.8 mV (n = 10). Subtraction of current obtained before from that during histamine application revealed a current that was linear against membrane potential. Increasing external K+ concentration or introduction of K+ channel blockers in the medium attenuated or abolished histamine-induced inward current at membrane potentials close to -50 mV. When external Cl- concentration was reduced, histamine-induced inward current was still seen in five of seven supraoptic cells tested. Neither inward current nor change in conductance was observed following bath application of histamine in 11 of 12 neurons recorded using patch pipettes containing guanosine 5'-O-(2-thiodiphosphate), and in seven of eight neurons using pipettes containing guanosine 5'-O-(3-thiotriphosphate). These results suggest that histamine depolarizes supraoptic neurons, at least in part, by inhibiting a K+ leakage current mediated by H1 receptors linked to GTP-binding proteins and Ca(2+)-independent pathways. This study provides initial evidence for the second messengers regulating K+ leakage current.

Mechanisms of antihistamine-induced sedation in the human brain: H1 receptor activation reduces a background leakage potassium current

Antihistamines, more formally termed H1 receptor antagonists, are well known to exert sedative effects in humans, yet their locus and mechanism of action in the human brain remains unknown. To better understand this phenomenon, the effects of histamine upon human cortical neurons were studied using intracellular recordings in brain slices maintained in vitro. Bath application of 50 microM histamine induced a depolarization which could be attributed to reduction of a background voltage-independent "leakage" potassium current: the depolarization was associated with an increase in apparent input resistance, under voltage clamp its reversal potential approximated the potassium reversal potential, and the histamine-induced current exhibited little voltage dependence. The pharmacology of the histamine-induced depolarization of human cortical neurons was studied by use of both agonists and antagonists. Depolarizing responses were blocked by the H1 antagonist mepyramine, but not by the H2 antagonist cimetidine nor the H3 antagonist thioperamide. The H3 receptor agonist R-alpha-methyl-histamine did not mimic the effects of histamine. Thus, histamine depolarizes human cortical neurons via action at an H1 receptor. These effects of neuronal histamine upon cortical neurons are likely to affect synaptic transmission in several ways. The depolarization per se should increase the likelihood that excitatory synaptic potentials will evoke an action potential. The increase in whole-cell input resistance evoked by H1 receptor activation should make the cell more electrotonically compact, thereby altering its integrative properties. We hypothesize that these mechanisms would allow histamine, acting at cortical H1 receptors, to enhance behavioral arousal. During waking when histamine release is highest, blockade of H1 receptors by systemically administered H1 receptor antagonists would be sedating.

REM-enhancing effects of the adrenergic antagonist idazoxan infused into the medial pontine reticular formation of the freely moving cat

Adrenergic input to the medial pontine reticular formation (mPRF) has been postulated to play an important role in behavioral state control. Previous work has shown that direct microinjections of the alpha 2 agonist clonidine and of norepinephrine into the mPRF suppress the REM phase of sleep. Here, we report that the alpha 2 antagonist idazoxan (IZ) enhances REM-like sleep states in the cat when microinfused into the rostral part of the mPRF at sites where carbachol produced a marked increase in REM. Significant increases in the absolute number of min in REM (70%) were observed over the entire 5 h of recordings and an especially robust increase occurred in the first hour (1000%). Other effects of IZ included increased REM duration and decreased latency to REM onset. These results represent the most marked REM-enhancing effects thus far reported for any antagonist at any brain site.

Aminergic and cholinergic afferents to REM sleep induction regions of the pontine reticular formation in the rat

Microinjection of cholinergic agonists in a dorsolateral part of the mesopontine tegmentum has been shown to induce a rapid eye movement (REM) sleep-like state. Physiological evidence indicates that not only acetylcholine but also various amine transmitters, including those implicated in behavioral state regulation, affect neuronal activity in this region of the pontine reticular formation. In the present study, sources of select aminergic and cholinergic inputs to this REM sleep induction zone were identified and quantitatively analyzed by using fluorescence retrograde tracing combined with immunofluorescence in the rat. In addition to previously demonstrated cholinergic projections from the pedunculopontine and laterodorsal tegmental nuclei, the REM sleep induction zone received various aminergic inputs that originated in widely distributed regions of the brainstem and hypothalamus. Serotoninergic afferents represented a mean of 44% of all aminergic/cholinergic source neurons projecting to the REM sleep induction zone, which was comparable to the mean percentage of 39% represented by cholinergic afferent neurons. The serotoninergic afferents originated from the raphe nuclei at all brainstem levels, with heavier projections from the pontine than from the medullary raphe nuclei. Unexpectedly, an additional major serotoninergic input was provided by serotoninergic neurons in the nucleus prosupralemniscus (B9). Noradrenergic afferent neurons represented a mean of 14% of all aminergic/cholinergic source neurons, which was only about one-third of the mean percentage of either cholinergic or serotoninergic source neurons. These noradrenergic projection neurons were located not only in the locus ceruleus (8%) but also in the lateral tegmentum, including the A5 (4%) and A7 (2%) cell groups. Histaminergic neurons in the tuberomammillary hypothalamic nucleus represented a minor group of afferent neurons (3%), and a still smaller input came from adrenergic C1 neurons. The pattern of these transmitter-specific afferent connections appeared to be similar regardless of the longitudinal level within the REM sleep induction zone. The present results are consistent with previous behavioral and physiological evidence for a role of the pontine REM sleep induction zone in triggering REM sleep. The regulation of REM sleep induction would be best understood in terms of a state-dependent interplay of cholinergic, serotoninergic, and other inputs all acting convergently upon neurons in the REM sleep-inducing region of the pontine reticular formation.