Inhibition of acetylcholine-mediated effects by borneol

We previously reported that the aqueous extract from a medicinal plant Dryobalanops aromatica specifically inhibits the nicotinic acetylcholine receptor (nAChR) (Oh et al. Pharmacol Res 2000;42(6):559-64). Here, the effect of borneol, the main constituent of D. aromatica, on nAChR activity was investigated in bovine adrenal chromaffin cells. Borneol inhibited a nAChR agonist 1,1-dimethyl-4-phenylpiperazinium iodide (DMPP)-induced calcium increase with a half maximal inhibitory concentration (IC(50)) of 56+/-9 microM. In contrast, borneol did not affect the calcium increases induced by high K+, veratridine, and bradykinin. The sodium increase induced by DMPP was also inhibited by borneol with similar potency (49+/-12 microM), suggesting that the activity of nAChRs is inhibited by borneol. Borneol inhibited DMPP-induced secretion of [3H]norepinephrine with an IC(50) of 70+/-12 microM. Carbon-fiber amperometry also confirmed the inhibition of DMPP-induced exocytosis by borneol in single chromaffin cells. [3H]nicotine binding, however, was not affected by borneol. The inhibitory effect by borneol is more potent than the effect by lidocaine, a commonly used local anesthetic. The data suggest that borneol specifically inhibits the nAChR-mediated effects in a noncompetitive way.     

Inhibition of nicotinic receptor-mediated catecholamine secretion by dryobalanops aromatica in bovine adrenal chromaffin cells

Effect of the aqueous extract from a medicinal plant Dryobalanops aromatica(Dipterocarpaceae) on catecholamine secretion was investigated in bovine adrenal chromaffin cells. The aqueous extract inhibited [(3)H]norepinephrine ([(3)H]NE) secretion induced by 1,1-dimethyl-4-phenylpiperazinium iodide (DMPP), a nicotinic acetylcholine receptor (nAChR) agonist, with a half-maximal inhibitory concentration (IC(50)) of 8.4 +/- 1.7 microgml(-1). Increases in cytosolic calcium ([Ca(2+)](i)) and sodium ([Na(+)](i)) induced by DMPP were also inhibited by the extract. However, the binding of [(3)H]nicotine to nAChRs was not affected by the addition of the extract in receptor binding competition analysis, suggesting that active components in the extract and nicotine do not share the binding site in the nAChR. On the other hand, [Ca(2+)](i)increases induced by high K(+), ionomycin, bradykinin, angiotensin II, and thapsigargin were not inhibited by the extract. The data suggest that the extract from D. aromatica specifically inhibits catecholamine secretion by blocking nAChR in a noncompetitive manner.     

Inhibition of nicotinic acetylcholine receptors and calcium channels by clozapine in bovine adrenal chromaffin cells

The effects of clozapine on the activities of nicotinic acetylcholine receptors (nAChRs) and voltage-sensitive calcium channels (VSCCs) were investigated and compared with those of chlorpromazine (CPZ) in bovine adrenal chromaffin cells. [(3)H]Norepinephrine ([(3)H]NE) secretion induced by activation of nAChRs was inhibited by clozapine and CPZ with half-maximal inhibitory concentrations (IC(50)) of 10.4 +/- 1.1 and 3.9 +/- 0.2 microM, respectively. Both cytosolic calcium increase and inward current in the absence of extracellular calcium induced by nicotinic stimulation were also inhibited by clozapine and CPZ, but the greater inhibition was achieved by CPZ. In addition, [(3)H]nicotine binding to chromaffin cells was inhibited by clozapine and CPZ with IC(50) values of approximately 19 and 2 microM, respectively. On the other hand, [(3)H]NE secretion induced by high K(+) was inhibited by clozapine and CPZ with similar IC(50) values of 15.5 +/- 3.8 and 17.1 +/- 3.9 microM, respectively. Our results suggest that clozapine, as well as CPZ, inhibits nAChRs and VSCCs, thereby causing inhibition of catecholamine secretion, and that clozapine is much less potent than CPZ in inhibiting nAChRs.     

Chlorpromazine-induced inhibition of catecholamine secretion by a differential blockade of nicotinic receptors and L-type Ca2+ channels in rat pheochromocytoma cells.

We investigated the effect of chlorpromazine (CPZ), a phenothiazine neuroleptic, on catecholamine secretion in rat pheochromocytoma (PC12) cells. CPZ inhibited [3H]norepinephrine ([3H]NE) secretion induced by 1,1-dimethyl-4-phenylpiperazinium iodide (DMPP), an agonist of nicotinic acetylcholine receptors (nAChRs) with an IC50 value of 1.0 +/- 0.2 microM. The DMPP-induced rise in cytosolic free Ca2+ concentration [Ca2+]i was inhibited by CPZ with an IC50 of 1.9 +/- 0.1 microM. The DMPP-induced increase in cytosolic free Na+ concentration [Na+]i was also inhibited by CPZ with a similar potency. Furthermore, the binding of [3H]nicotine to PC12 cells was inhibited by CPZ with an IC50 value of 2.7 +/- 0.6 microM, suggesting that the nAChRs themselves are inhibited by CPZ. In addition, both 70 mM K+-induced [3H]NE secretion and [Ca2+]i increase were inhibited by CPZ with IC50 of 7.9 +/- 1.1 and 6.2 +/- 0.3 microM, respectively. Experiments with Ca2+ channel antagonists suggest that L-type Ca2+ channels are mainly responsible for the inhibition. We conclude that CPZ inhibits catecholamine secretion by blocking nAChRs and L-type Ca2+ channels, with the former being more sensitive to CPZ.     

Noncompetitive inhibition by camphor of nicotinic acetylcholine receptors

The effect of camphor, a monoterpenoid, on catecholamine secretion was investigated in bovine adrenal chromaffin cells. Camphor inhibited [3H]norepinephrine ([3H]NE) secretion induced by a nicotinic acetylcholine receptor (nAChR) agonist, 1,1-dimethyl-4-phenylpiperazinium iodide (DMPP), with a half-maximal inhibitory concentration (IC50) of 70 +/- 12 microM. In addition, camphor inhibited the rise in cytosolic calcium ([Ca2+]i) and sodium ([Na+]i) induced by DMPP with IC50 values of 88 +/- 32 and 19 +/- 2 microM, respectively, suggesting that the activity of nAChRs is also inhibited by camphor. On the other hand, binding of [3H]nicotine to nAChRs was not affected by camphor. [Ca2+]i increases induced by high K+, veratridine, and bradykinin were not affected by camphor. The data suggest that camphor specifically inhibits catecholamine secretion by blocking nAChRs without affecting agonist binding.     

Effects of neuronal nicotinic acetylcholine receptor allosteric modulators in animal behavior studies

Nicotinic acetylcholine receptors (nAChRs) are ligand-gated cation-conducting transmembrane channels from the cys-loop receptor superfamily. The neuronal subtypes of these receptors (e.g. the α7 and α4β2 subtypes) are involved in neurobehavioral processes such as anxiety, the central processing of pain, food intake, nicotine seeking behavior, and a number of cognitive functions like learning and memory. Neuronal nAChR dysfunction is involved in the pathophysiology of many neurological disorders, and behavioral studies in animals are useful models to assess the effects of compounds that act on these receptors. Allosteric modulators are ligands that bind to the receptors at sites other than the orthosteric site where acetylcholine, the endogenous agonist for the nAChRs, binds. While conventional ligands for the neuronal nAChRs have been studied for their behavioral effects in animals, allosteric modulators for these receptors have only recently gained attention, and research on their behavioral effects is growing rapidly. Here we will discuss the behavioral effects of allosteric modulators of the neuronal nAChRs.     

Cotinine and nicotine inhibit each other's calcium responses in bovine chromaffin cells

Cotinine is the major metabolite of nicotine. It has some biological activity, but its pathophysiological effects are largely unclear. We studied whether cotinine initiates calcium transients or affects those induced by nicotine. In bovine adrenal chromaffin cells labeled with the fluorescent calcium indicator Fura 2, cotinine (0. 32-3.2 mM) concentration-dependently increased the intracellular Ca(2+) concentration ([Ca(2+)](i)). The effect was abolished by omitting extracellular Ca(2+) during the stimulations. Also nicotinic receptor channel blockers hexamethonium (10 microM-1 mM) and chlorisondamine (100 microM), as well as a competitive nicotinic receptor antagonist dihydro-beta-erythroidine (10-100 microM), inhibited the response. Cotinine (0.32-3.2 mM) preincubation for 2 min inhibited both the nicotine-induced and the cotinine-induced increases in [Ca(2+)](i). Also nicotine (3.2-10 microM) inhibited the cotinine-induced increase in [Ca(2+)](i). Tetrodotoxin (1 microM) and thapsigargin (1 microM) pretreatments did not affect the responses to cotinine, while 300 nM nimodipine partially inhibited the cotinine-induced increase in [Ca(2+)](i). The results indicate that cotinine has nicotine-like effects on chromaffin cells. It may also desensitize the nicotinic cholinergic receptors, possibly by acting as a low-affinity agonist at these receptors.     

Stimulus secretion coupling in cultured chromaffin cells. Dependency on external sodium and on dihydropyridine-sensitive calcium channels

Stimulus secretion in response to acetylcholine (or nicotine) in adrenal chromaffin cells involves calcium influx and is tetrodotoxin insensitive. The mechanism by which activation of the nicotinic acetylcholine receptor (AChR) leads to calcium influx is not clear, and the requirement for external sodium is controversial. We report that when bovine chromaffin cells are continuously perfused in the absence of sodium, secretion of catecholamines in response to 3 X 10(-5) M nicotine is not impaired. Also, secretion stimulated by this concentration of nicotine is not potently and stereospecifically inhibited by the (+)- and (-)-isomers of nicardipine. At a concentration of (+)-nicardipine (10(-7) M) which inhibits most of the release stimulated by 50 mM potassium, the response to 3 X 10(-5) M nicotine remains; however, the dose-response curve to nicotine is shifted slightly to the left. The results are discussed with respect to models of stimulus secretion coupling in bovine adrenal chromaffin cells.     

L-type calcium channel ligands block nicotine-induced signaling to CREB by inhibiting nicotinic receptors

Nicotinic acetylcholine receptors (nAChRs) are inhibited by several drugs that are commonly thought to be specific for L-type calcium channels (LTCCs). In neurons, LTCCs are activated by nicotine-induced depolarization to engage downstream signaling events; however, the role of LTCC drug interactions with nAChRs in signaling has not been examined in detail. We investigated the effects of LTCC ligands on nAChR currents and downstream signaling in rat superior cervical ganglion (SCG) neurons. We found that 10microM nicotine and 40mM K(+) both reversibly depolarize SCG neurons to -20mV, sufficient to activate LTCCs and downstream signaling, including induction of nuclear phospho-CREB (pCREB); this induction was blocked by LTCC antagonists. Interestingly, the effects of LTCC antagonists on nicotine-induced signaling to CREB are not mediated by their actions on LTCCs, but rather via inhibition of nAChRs, which prevents nicotine-induced depolarization. We show that this effect is sufficient to block pCREB induction in neurons expressing an antagonist-insensitive LTCC. Taken together, our data show that, at concentrations typically used to block LTCCs, these antagonists inhibit nAChR currents and downstream signaling. These findings serve as a caution in attributing a role for LTCCs when using these drugs experimentally or therapeutically.     

Nicotine at concentrations found in cigarette smokers activates and desensitizes nicotinic acetylcholine receptors in CA1 interneurons of rat hippocampus

Behavioral effects of cigarette smoking are attributed to the interactions of nicotine with brain nicotinic acetylcholine receptors (nAChRs). However, the mechanisms by which nAChR function in developing and mature brain is affected by a smoker's level of nicotine (50–500 nM) remain unclear. Thus, the objective of this study was to determine the concentration- and time-dependent effects of nicotine on 7 and 4β2 nAChRs, the two major brain subtypes, natively expressed in CA1 interneurons of rat hippocampal slices. Only at concentrations ≥5 μM did nicotine (applied for 6–60 s) elicit action potentials or measurable whole-cell currents (EC₅₀=158 μM) in stratum radiatum interneurons that express 7 nAChRs. Continuous exposure for 10–15 min of the neurons to nicotine (0.5–2.5 μM) inhibited 7 nAChR-mediated currents (IC₅₀=640 nM) evoked by choline (10 mM). Nicotine (≥0.125 μM) applied to the neurons for 1–5 min induced slowly desensitizing whole-cell currents (EC₅₀=3.2 μM) in stratum lacunosum moleculare interneurons; this effect was mediated by 4β2 nAChRs. Also via activation of 4β2 nAChRs, nicotine (0.125–0.5 μM) increased the frequency and amplitude of GABAergic postsynaptic currents (PSCs) in stratum radiatum interneurons. However, exposure of the neurons for 10–15 min to nicotine (0.25–0.5 μM) resulted in desensitization of 4β2 nAChRs. It is suggested that nanomolar concentrations of nicotine after acute intake suppress inhibitory inputs to pyramidal cells through a disinhibitory mechanism involving activation of 4β2 nAChRs and desensitization of 7 nAChRs, and after chronic intake leads to up-regulation of both receptor subtypes via desensitization. These findings have direct implications to the actions of nicotine in cigarette smokers.     

The role of neuronal nicotinic acetylcholine receptors in acute and chronic neurodegeneration

In the last five years there has been a rapid explosion of publications reporting that neuronal nicotinic acetylcholine receptors (nAChRs) play a role in neurodegenerative disorders. Furthermore, there is a well-established loss of nAChRs in post-mortem brains from patients with Alzheimer's disease, Parkinson's disease and a range of other disorders. In the present review we discuss the evidence that nicotine and subtype selective nAChR ligands can provide neuroprotection in in vitro cell culture systems and in in vivo studies in animal models of such disorders. Whilst in vitro data pertaining to a protective effect of nicotine against nigral neurotoxins like MPTP is less robust, most studies agree that nicotine is protective against glutamate and beta-amyloid toxicity in various culture systems. This effect appears to be mediated by alpha7 subtype nAChRs since the protection is blocked by alpha-bungarotoxin and is mimicked by alpha7 selective agonists. In vivo studies indicate that alpha7 receptors play a critical role in protection from cholinergic lesions and enhancing cognitive function. The exact subtype involved in the neuroprotectant effects seen in animal models of Parkinson's disease is not clear, but in general broad spectrum nAChR agonists appear to provide protection, while alpha4beta2 receptors appear to mediate symptomatic improvements. Evidence favouring a protectant effect of nicotine against acute degenerative conditions is less strong, though some protection has been observed with nicotine pre-treatment in global ischaemia models. A variety of cellular mechanisms ranging from the production of growth factors through to inactivation of toxins and antioxidant actions of nicotine have been proposed to underlie the nAChR-mediated neuroprotection in vitro and in vivo. In summary, although the lack of subtype selective ligands has hampered progress, it is clear that in the future neuronal nAChR agonists could provide functional improvements and slow or halt the progress of several crippling degenerative diseases.     

α7 nicotinic acetylcholine receptor mediated neuroprotection in Parkinson's disease

Parkinson's disease (PD) is characterized by relatively selective degeneration of dopaminergic neurons in the substantia nigra and loss of dopamine in the striatum. More than 50 epidemiological studies confirmed the low incidence of PD in smokers. Examining the distribution of subtypes of nicotinic acetylcholine receptors (nAChRs) in dopaminergic neurons of nigrostriatal system and its change in PD patients is quite important to elucidate possible neuroprotective cascade triggered by nicotine. Evidences of nAChR-mediated protection against neurotoxicity induced by rotenone, 6- hydroxydopamine (6-OHDA), and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) are briefly reviewed. In rotenone- and 6-OHDA-induced PD models, nAChR-mediated neuroprotection was blocked not only by α4β2 but also by α7 nAChR antagonists. The survival signal transduction, α7 nAChR-Src family-PI3K-Akt/PKB cascade and subsequent upregulation of Bcl-2, would lead to neuroprotection. These findings suggest that nAChR-mediated neuroprotection is achieved through subtypes of nAChRs and common signal cascades. An early diagnosis and protective therapy with specific nAChR modulations could be effective in delaying the progression of PD.     

Inhibition of store-operated calcium entry-mediated superoxide generation by histamine trifluoromethyltoluide independent of histamine receptors

Store-operated calcium entry (SOCE) plays an important role in shaping the Ca(2+) response of various tissues and cell types. In this report, we show that thapsigargin (TG)-induced SOCE was inhibited by the histamine receptor agonist, histamine-trifluoromethyltoluide (HTMT), in U937 and HL-60 human promyelocytes. Preincubation of HTMT resulted in a significant inhibition of subsequent TG-induced Ca(2+) elevation without affecting Ca(2+) release from intracellular stores. HTMT also inhibited TG-induced Ca(2+) current and Ba(2+)/Mn(2+) influx in a concentration-dependent manner. In contrast with HTMT, other H1 histamine receptor agonists, histamine, 2-methylhistamine and 2-thiazolylethylamine, did not affect TG-induced SOCE. In addition, HTMT also attenuated TG-induced cytosolic superoxide generation. Taken together, our data clearly suggest that the anti-inflammatory effect of HTMT may occur through direct inhibition of SOCE.     

GABAB receptors modulate catecholamine secretion in chromaffin cells by a mechanism involving cyclic AMP formation.

1. The function of gamma-aminobutyric acidB (GABAB) receptors in modulation of catecholamine secretion by chromaffin cells and the possible mechanism involved in this action have been examined. 2. The GABAB agonists (-)-baclofen and 3-aminopropylphosphinic acid (3-APPA) were found to induce a dose-dependent increase of basal catecholamine secretion. The EC50s were 151 +/- 35 microM and 225 +/- 58 microM for baclofen and 3-APPA, respectively. This stimulatory effect was specific since it could be blocked by 0.5 mM of the specific GABAB antagonist CGP-35348. 3. In contrast, preincubation of chromaffin cells with the GABAB agonists was found to inhibit, in a dose-dependent manner, the catecholamine secretion evoked by 10 microM nicotine and 200 microM muscimol. 4. The effects of GABAB agonists on both basal and evoked catecholamine secretion were found to be accompanied by parallel changes in intracellular calcium concentration ([Ca2+]i). GABAB agonists produced a dose-dependent increase in [Ca2+]i which was partially blocked by CGP 35348, but they produced a strong inhibition of the [Ca2+]i increase induced by nicotine and muscimol. 5. The GABAB agonists also produced a dose-dependent increase in intracellular cyclic AMP levels, there being a direct correlation between both increase in catecholamine secretion and in intracellular cyclic AMP levels. 6. The pretreatment of chromaffin cells with pertussis toxin doubled the catecholamine secretion and increased by four times the intracellular cyclic AMP levels evoked by GABAB agonists. 7. The possible involvement of adenylate cyclase in the mechanism of GABAA receptor modulation of catecholamine secretion is discussed.     

Inhibition of adrenomedullary catecholamine release by propranolol isomers and clonidine involving mechanisms unrelated to adrenoceptors.

1 Transmural electrical stimulation (10 Hz, 40 V, 1 ms for 60s) increased total catecholamine secretion from perfused cat adrenal glands; this response was enhanced by neostigmine and inhibited by mecamylamine, suggesting that release of acetylcholine from splanchnic nerve terminals was stimulating nicotinic receptors and enhancing catecholamine secretion. 2 Isoprenaline, (-)-propranolol and (+)-propranolol (10(-7)-10(-5)M) inhibited the electrically-evoked secretory response by 40-70%; similar reductions were obtained with clonidine and yohimbine. Neither, (+)-propranolol nor (-)-propranolol inhibited K-evoked secretion from cat adrenals; in contrast, nimodipine potently inhibited it (IC50 = 24 nM). 3 Either, racemic propranolol or the (+)- or (-)-isomers (1-10 microM) equally inhibited [3H]-noradrenaline release evoked by nicotine or acetylcholine from cultured bovine adrenal chromaffin cells; clonidine (10 microM) inhibited secretion by 50% and yohimbine or isoprenaline did not affect it. 4 The results indicate that adrenomedullary catecholamine release evoked by splanchnic nerve stimulation is not modulated by alpha- or beta-adrenoceptors and suggest that propranolol may inhibit secretion by blocking ion fluxes through the acetylcholine receptor ionophore. Clonidine may inhibit secretion by this same mechanism, and/or by interfering with some intracellular event in the secretory mechanism.     

Interactions of atropine with heterologously expressed and native alpha 3 subunit-containing nicotinic acetylcholine receptors

(1) Atropine, a classical muscarinic antagonist, has been reported previously to inhibit neuronal nicotinic acetylcholine receptors (nAChRs). In the present study, the action of atropine has been examined on alpha3beta4 receptors expressed heterologously in Xenopus oocytes and native nAChRs in medial habenula neurons. (2) At concentrations of atropine often used to inhibit muscarinic receptors (1 micro M), responses induced by near-maximal nicotine concentrations (100 micro M) at negative holding potentials (-65 mV) are inhibited (14-30%) in a reversible manner in both alpha4 and alpha3 subunit-containing heteromeric nAChRs. Half-maximal effective concentrations (IC(50) values) for atropine inhibition are similar for the four classes of heteromeric receptors studied (4-13 micro M). (3) For alpha3beta4 nAChRs in oocytes, inhibition by atropine (10 micro M) is not overcome at higher concentrations of agonist, and is increased with membrane hyperpolarization. These results are consistent with non-competitive antagonism--possibly ion channel block. (4) At low concentrations of both nicotine (10 micro M) and atropine (<10 micro M), potentiation ( approximately 25%) of alpha3beta4 nAChR responses in oocytes is observed. The relative balance between potentiation and inhibition is dependent upon membrane potential. (5) In rat medial habenula (MHb) neurons, atropine (0.3-3.0 micro M) inhibited nicotine-induced responses in both a concentration and membrane potential-dependent manner (at -40 mV, IC(50)=4 micro M), similar to the effects on alpha3beta4-nAChRs in oocytes. However, unlike heterologously expressed receptors, potentiation was barely detectable at depolarized membrane potentials using low concentrations of nicotine (3-10 micro M). Conversely, the weak agonist, choline (1-3 mM) was observed to augment responses of MHb nAChRs.     

Amphetamine enhances Ca2+ entry and catecholamine release via nicotinic receptor activation in bovine adrenal chromaffin cells

Amphetamine, a psychostimulant, has been shown to act as a channel blocker of muscle nicotinic receptors and to induce a Ca(2+)-dependent secretion from adrenal chromaffin cells. In this study, the relationship between amphetamine and nicotinic receptors was studied using bovine adrenal chromaffin cells as a model system. Our results show that D-amphetamine sulfate alone induced an increase in the cytosolic Ca(2+) concentration ([Ca(2+)](c)) and [3H]norepinephrine release in a dose-dependent and extracellular Ca(2+)-dependent manner. Two common nicotinic receptor antagonists, hexamethonium and mecamylamine, suppressed the D-amphetamine sulfate-induced [Ca(2+)](c) rise and [3H]norepinephrine release. In addition, D-amphetamine sulfate inhibited the 1,1-dimethyl-4-phenyl-piperazinium iodide (DMPP)-induced [Ca(2+)](c) rise and [3H]norepinephrine release, but not the high K(+)- or veratridine-induced [Ca(2+)](c) increase and [3H]norepinephrine release. Antagonists, including alpha-bungarotoxin and choline, that are more specific for alpha7 nicotinic receptors were capable of inhibiting the D-amphetamine sulfate-induced [Ca(2+)](c) rise, while D-amphetamine sulfate was found to be capable of inhibiting the [Ca(2+)](c) rise induced by the alpha7-nicotinic receptor agonists, epibatidine and choline. Moreover, D-amphetamine sulfate dose-dependently suppressed [3H]nicotine binding to chromaffin cells. We, therefore, conclude that D-amphetamine sulfate acts as a nicotinic receptor agonist to induce [Ca(2+)](c) increase and [3H]norepinephrine release in bovine adrenal chromaffin cells.     

Functional pharmacology of H1 histamine receptors expressed in mouse preoptic/anterior hypothalamic neurons

BACKGROUND AND PURPOSE

Histamine H₁ receptors are highly expressed in hypothalamic neurons and mediate histaminergic modulation of several brain-controlled physiological functions, such as sleep, feeding and thermoregulation. In spite of the fact that the mouse is used as an experimental model for studying histaminergic signalling, the pharmacological characteristics of mouse H₁ receptors have not been studied. In particular, selective and potent H₁ receptor agonists have not been identified.

EXPERIMENTAL APPROACH

Ca²⁺ imaging using fura-2 fluorescence signals and whole-cell patch-clamp recordings were carried out in mouse preoptic/anterior hypothalamic neurons in culture.

KEY RESULTS

The H₁ receptor antagonists mepyramine and trans-triprolidine potently antagonized the activation by histamine of these receptors with IC₅₀ values of 0.02 and 0.2 μM respectively. All H₁ receptor agonists studied had relatively low potency at the H₁ receptors expressed by these neurons. Methylhistaprodifen and 2-(3-trifluoromethylphenyl)histamine had full-agonist activity with potencies similar to that of histamine. In contrast, 2-pyridylethylamine and betahistine showed only partial agonist activity and lower potency than histamine. The histamine receptor agonist, 6-[2-(4-imidazolyl)ethylamino]-N-(4-trifluoromethylphenyl)heptanecarboxamide (HTMT) had no agonist activity at the H₁ receptors H1 receptors expressed by mouse preoptic/anterior hypothalamic neurons but displayed antagonist activity.

CONCLUSIONS AND IMPLICATIONS

Methylhistaprodifen and 2-(3-trifluoromethylphenyl)histamine were identified as full agonists of mouse H₁ receptors. These results also indicated that histamine H₁ receptors in mice exhibited a pharmacological profile in terms of agonism, significantly different from those of H₁ receptors expressed in other species.     

Pentobarbitone inhibition of catecholamine secretion

1. The perfused isolated cow adrenal gland was used to investigate the effect of barbituric acid, phenobarbitone and pentobarbitone on catecholamine secretion.2. Pentobarbitone reduced catecholamine secretion induced by a number of drugs which cause exocytosis. The concentration of pentobarbitone which caused a 50% inhibition of catecholamine secretion was for acetylcholine 5.6 x 10(-5)M, for carbachol 6.3 x 10(-5)M, for histamine 1.6 x 10(-4)M, for (+)-amphetamine 4.4 x 10(-5)M and for potassium chloride 1.5 x 10(-4)M. The degree of inhibition by pentobarbitone was not dependent on the concentration of the secretagogue.3. Pentobarbitone (up to 10(-3)M) did not inhibit the catecholamine release that was induced by acetyldehyde or by calcium chloride; it inhibited slightly (34%) the catecholamine secretion induced by tyramine.4. Catecholamine release induced by carbachol was also inhibited by phenobarbitone (50% inhibition at 2.8 x 10(-4)M (n=7)) but was unaffected by barbituric acid.5. Pentobarbitone had no effect on spontaneous or on (+)-amphetamine- or tyramine-induced release of catecholamines from isolated chromaffin vesicles of cow adrenal medulla.6. It is concluded that pentobarbitone inhibits catecholamine release by preventing a configurational change in the structure of the membrane of the chromaffin cell which is a necessary link between receptor activation and catecholamine release.