Characterization of noradrenaline release in the locus coeruleus of freely moving awake rats by in vivo microdialysis

Rationale The origin and regulation of noradrenaline (NA) in the locus coeruleus (LC) is unknown.Objectives The neurochemical features of NA overflow (nerve impulse dependence, neurotransmitter synthesis, vesicle storage, reuptake, ₂-adrenoceptor-mediated regulation) were characterized in the LC.Methods Brain microdialysis was performed in awake rats. Dialysates were analyzed for NA.Results NA in the LC decreased via local infusion of Ca²⁺-free medium (–42±5%) or the sodium channel blocker tetrodotoxine (TTX) (–47±8%) but increased (333±40%) via KCl-induced depolarization. The tyrosine hydroxylase (TH) inhibitor -methyl-p-tyrosine (250 mg kg^–1, i.p.) and the vesicle depletory drug reserpine (5 mg kg^–1, i.p.) decreased NA. Therefore, extracellular NA in the LC satisfies the criteria for an impulse flow-dependent vesicular exocytosis of neuronal origin. Local perfusion of the ₂-adrenoceptor agonist clonidine (0.1–100 M) decreased NA (E_max=–79±5%) in the LC, whereas the opposite effect (E_max=268±53%) was observed with the _2A-adrenoceptor antagonist BRL44408 (0.1–100 M). This suggests a tonic modulation of NA release through local _2A-adrenoceptors. The selective NA reuptake inhibitor desipramine (DMI) (0.1–100 M) administered into the LC increased NA in the LC (E_max=223±40%) and simultaneously decreased NA in the cingulate cortex, confirming the modulation exerted by NA in the LC on firing activity of noradrenergic cells and on the subsequent NA release in noradrenergic terminals.Conclusion Synaptic processes underlying NA release in the LC are similar to those in noradrenergic terminal areas. NA in the LC could represent local somatodendritic release, but also the presence of neurotransmitter release from collateral axon terminals.     

Influence of histamine depletion on learning and memory recollection in rats

To clarify the role of endogenous histamine in learning and memory, the effect of -fluoromethylhistidine on active avoidance response in rats was studied. -Fluoromethylhistidine (20–100 mg/kg or 10–50 µg) significantly (P<0.05 orP<0.01) prolonged the response latency in active avoidance response when administered by either intraperitoneal or intracerebroventricular injection. These effects were dose-related and long lasting. A prolongation of the response latency induced by an intraperitoneal injection of -fluoromethylhistidine (100 mg/kg) was antagonized by intracerebroventricular injection of histamine (10 and 20 ng) in a dose-dependent manner. In addition, the acquisition of this response was retarded by a consecutive intracerebroventricular injection of -fluoromethylhistidine (50 µg), whereas histamine (100 ng) facilitated the response acquisition when administered by the same route. Both intraperitoneal (100 mg/kg) and intracerebroventricular injection of -fluoromethylhistidine (50 µg) significantly (P<0.05 orP<0.01) decreased the brain histamine content, especially in the hippocampus and hypothalamus. When -fluoromethylhistidine (50 µg) was injected intracerebroventricularly, there is a high correlation between a prolongation of the response latency and a decrease in histamine content of these brain areas. Based on these findings, it was concluded that an intimate relation may exist between a prolongation of response latency in the active avoidance response and a decrease in the brain histamine content; endogenous histamine may play an important role in learning and memory recollection in rats.     

In vivo studies on alpha-adrenergic receptor subtypes in human veins

Summary We studied in vivo responsiveness of venous ₁ and ₂-adrenoceptors, measuring the diameter changes in superficial veins in response to -adrenergic agonists and antagonists in healthy human volunteers. The dorsal hand vein technique was used because it permits complete dose-response studies of venous constriction without confounding reflex alterations.Local infusions of all agonists studied induced dose-dependent contraction of the hand vein; the maximal effects (Emax) were: norepinephrine (88% ± 10%), methox amine (97% ± 5%), phenylephrine (95% ± 6%), clonidine (54% ± 12%), and azepexole (68% ± 26%). Clonidine reduced the norepinephrine-induced venoconstriction by 11% ± 10%. Oral doses of 1 mg prazosin antagonized the venoconstriction induced by norepinephrine, methoxamine, and clonidine, but not by azepexole. Yohimbineantagonism was observed against all agonists studied. Inhibition by yohimbine of clonidine-induced venoconstriction was irreversible over 60–180 min.Results show that the in vivo effects on veins of -adrenergic agonists are in good agreement with results from in vitro experiments. Agonists with ₁- and ₂-adrenoceptor subtype selectivity cause venoconstriction in vivo, but ₂-receptor mediated constriction is intrinsically weaker. Clonidine acts as a partial antagonist against norepinephrine, presumably on postsynaptic ₂-receptors. At high doses, ₂-adrenoceptor subtype selectivity of clonidine and yohimbine appear to be partially lost in vivo. Send offprint requests to H. G. Eichler at the above address     

Effects of pentagastrin and carbachol on the gastric histamine level in α-fluoromethylhistidine-treated mice and rats

Summary The gastric mucosal histamine level in mice increased by about 80% and 100% after fasting for 24 and 48 h, respectively. In non-fasted mice, -fluoromethylhistidine (-FMH), a specific histidine decarboxylase inhibitor, significantly decreased the histamine level, the reduction amounting to 35% and 49%, 2 h and 4 h after treatment, respectively. In mice fasted for 24 h, a significant decrease of 42% was observed 4 h after treatment. However, in mice fasted for 48 h, no significant decrease was seen even 4 h after -FMH treatment. Therefore, the histamine-releasing effect of re-feeding and drugs on the gastric mucosa was examined in vivo, using animals fasted for 48 h and subsequently treated with -FMH. Food given simultaneously with -FMH to 48-h fasted mice significantly decreased the histamine level 4 h later. Pentagastrin and carbachol administered alone (0.25–2.0 mg/kg, i.p.) had no significant effect on the histamine level. However, the combined treatment with these drugs significantly decreased the histamine level. In rats fasted for 48 h and treated with -FMH, pentagastrin (0.25 and 0.5 mg/kg, i.p.) but not carbachol (0.125 – 0.5 mg/kg, i. p.) caused a significant decrease in the mucosal histamine level. In contrast to mice, the effect of the combined treatment with pentagastrin and carbachol was not synergistic in rats. These findings suggest that gastrin acts synergistically with acetylcholine in the histamine release from the gastric mucosa in mice, whereas such synergism may not occur in rats. Send offprint requests to K. Saeki     

Subclassification of presynaptic α2-adrenoceptors: α2D-autoreceptors and α2D-adrenoceptors modulating release of acetylcholine in guinea-pig ileum

The study was designed to classify in terms of _2A, _2B, _2C and _2D the presynaptic ₂-autoreceptors, as well as the ₂-receptors modulating the release of acetylcholine, in the myenteric plexus-longitudinal muscle (MPLM) preparation of the guinea-pig ileum. A set of antagonists was chosen that was able to discriminate between the four subtypes. Small pieces of the MPLM preparation were preincubated with ³H-noradrenaline or ³H-choline and then superfused and stimulated electrically.The stimulation periods used (3H-noradrenaline: 3 trains of 20 pulses, 50 Hz, train interval 60 s; ³H-choline: single trains of 30 pulses, 0.2 Hz) did not lead to ₂-autoinhibition or inhibition of ³H-acetylcholine release by endogenous noradrenaline. The ₂-selective agonist 5-bromo6-(2-imidazolin-2-ylamino)-quinoxaline (UK 14,304) reduced the evoked overflow of tritium in both ³H-noradrenaline and ³H-choline experiments. Most (³H-noradrenaline) or all (³H-choline) of the 10 antagonists shifted the concentration-inhibition curves of UK 14,304 to the right. pK_d values of the antagonists were calculated from the shifts. pK_d values from ³H-noradrenaline experiments correlated with pK_d values from ³H-choline experiments (r = 0.981).It is concluded that ₂-autoreceptors and ₂-heteroreceptors modulating the release of acetylcholine in the MPLM preparation are of the same subtype. Comparison with antagonist affinities for prototypic native ₂ binding sites, binding sites in cells transfected with ₂ subtype genes, and previously classified presynaptic ₂-adrenoceptors — all taken from the literature — indicates that both are _2D. The results are consonant with the hypothesis that at least the majority of ₂-autoreceptors belong to the _2A/D branch of the ₂-adrenoceptor tree, across mammalian or at least across rodent and lagomorph species. The same may hold true for ₂-adrenoceptors on non-noradrenergic neurones.     

Effects of a histamine synthesis inhibitor and antihistamines on the sexual behavior of female rats

Intraventricular administration of -hydrazinohistidine, a histamine synthesis inhibitor, at different doses and times before testing produced a significant decrease of lordotic responses and sexual receptivity in ovariectomized estrogen plus progesterone-primed female rats. The H₁-antihistamines pyrilamine and chlorfeniramine and the H₂-antihistamine metiamide, injected in the lateral ventricle, significantly decreased the lordosis quotient but did not modify receptivity; antihistamine-injected rats showed no soliciting behavior. Exploratory activity was decreased by both -hydrazinohistidine and metiamide but not by the H₁-antihistamines. It is concluded that treatments which either deplete histamine or block their receptors can alter female copulatory responsiveness. The mechanism of this antihistamine effect appears to be unrelated to that of other side effects, such as motor impairment, sedation, or local anesthesia.     

In vivo continuous estimation of 3H-dopamine synthesis and release in the cat caudate nucleus

Summary Synthesis and release of ³H-DA were examined during the continuous superfusion of a limited area of the ventricular surface of the cat caudate nucleus with l-3,5-³H-tyrosine, using a cup technique. ³H–H₂O, an index of the conversion of l-3,5-³H-tyrosine into ³H-Dopa, and ²H-DA were estimated in serial superfusate fractions. Alpha-methyl-paratyrosine (-MpT) (10^–4 M) inhibited rapidly and simultaneously both ³H–H₂O formation and ³H-DA release. Transection of the nigro-striatal dopaminergic pathway immediately blocked ³H-DA release but ³H–H₂O levels were reduced by 30% one hour later.     

Lysergic acid diethylamide antagonizes shaking induced in rats by five chemically different compounds

Thyrotropin-releasing hormone (TRH), sodium valproate, AG-3-5 (1-[2-hydroxyphenyl]-4-[3-nitrophenyl]-1,2,3,6-tetrahydropyrimidine-2-one), RX336-M (7,8-dihydro-5, 6-dimethylcyclohex-5-eno-1,2,8,14 codeinone), and Sgd 8473 (-[(4-chlorobenzylideneamino)-oxy]-isobutyric acid) each induced repetitive shaking of the body of rats after intraperitoneal injection. This action of the five diverse chemicals appears to be subserved by a common pharmacological component, because pretreatment with d-lysergic acid diethylamide (0.03–1.0 mg kg^-1, s.c.) attenuated the shaking behavior in a dose-related manner, and cross tolerance was found between RX 336-M and TRH, sodium valproate, and AG-3-5.     

In vivo and in vitro interactions of TCDD and other ligands of theAh-receptor: effect on embryonic and fetal tissues

Interactions of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), and other ligands of theAh-receptor, had been studied in vivo, in pregnant NMRI mice, and in vitro, in the fetal thymus organ culture system. Benzo(a)pyrene (BP) increased TCDD-induced fetolethality, whereas it did not affect the rate of cleft palate formation. This may indicate that the mechanism of TCDD-induced fetal death is different from that of TCDD-induced cleft palate. 2,3,7,8-Tetrachlorodibenzofuran (TCDBF) and 3,3,4,4-tetrachloroazoxybenzene (TCAOB) were added together to the thymus organ culture, each at a concentration that caused about 25–50% lymphoid development inhibition. Such treatment resulted in an additive effect of about 75%. Similarly, when the slightly toxic -naphthoflavone (BN) was added together with TCDD to the same culture system, it caused a significant increase in the lymphoid inhibitory effect of the latter compound. These may all suggest a common mechanism of action for TCDD and other ligands, which may involve a direct interaction with the receptors present in the thymus.Abbreviations BN -naphthoflavone - BP Benzo(a)pyrene - PAH Polycyclic aromatic hydrocarbons - TCAOB 3,3,4,4-tetrachloroazoxybenzene - TCDBF 2,3,7,8-tetrachlorodibenzofuran - TCDD 2,3,7,8-tetrachlorodibenzo-p-dioxin     

Depression of exploratory activity by clonidine in rats as a model for the detection of relative pre- and postsynaptic central noradrenergic receptor selectivity of α-adrenolytic drugs

Summary The effects of various -adrenoceptorblocking drugs on the depression of exploratory activity (ambulation and rearing) induced by 0.1 mg/kg i.p. clonidine were investigated in the rat. In parallel experiments, the effects of the same drugs on pre- and postsynaptic -receptors were determined in vitro (field-stimulated cortex slices and isolated vas deferens of the rat, respectively). Tolazoline, esproquine, yohimbine and piperoxan distinctly antagonized the inhibition of exploration produced by clonidine. All these drugs were found to possess relatively higher selectivity for the presynaptic -receptors, as judged by the ratios of the concentrations inducing a 50% increase in field-stimulated ³H-noradrenaline-overflow and the concentrations required to shift the EC₅₀ for the antagonism of noradrenaline-induced contractions of the vas deferens to the right by a factor of 2 (pA₂, ratio <1): In contrast, phentolamine and phenoxybenzamine which showed preferential postsynaptic -receptor blocking activity (ratio>1), potentiated rather than antagonized the effects of clonidine. Mianserin, although preferentially blocking the postsynaptic receptors, had no effect on clonidine-induced hypoactivity up to the high dose of 100 mg/kg i.p., probably because of its additional NA-uptake-inhibiting properties. The antagonism of clonidine by the selective presynaptic -receptor blockers was observed within a limited dose-range. Increasing the doses above an optimal level, which varied from one compound to another, resulted in a decrease in the effect. It is suggested that this phenomenon reflects the counter-balancing postsynaptic -adrenoceptor blockade occuring at higher concentrations of these drugs. In general, the results show a fairly good correlation between antagonism of clonidine in vivo and preferential blockade of presynaptic -receptors in vitro. Clonidine-induced suppression of exploration therefore seems to be a valuable model for the investigation of drug interactions with -adrenergic receptors in the central nervous system.These results were presented in part at the Spring Meeting of German Pharmacological Society, Mainz, 16–18 March, 1977     

Characterization of histamine H3 receptors inhibiting 5-HT release from porcine enterochromaffin cells: Further evidence for H3 receptor heterogeneity

The nature of the histamine receptor mediating inhibition of 5-HT release was investigated in strips of the porcine small intestine by investigating the effects of histamine ligands on the overflow of endogenous 5-HT and its metabolite 5-hydroxyindoleacetic acid (5-HIAA). The overflow was measured by HPLC, combined with electrochemical detection and represents calcium-sensitive 5-HT release from enterochromaffin cells, as reported previously. The histamine H₃ receptor selective agonists (R)--methyl-histamine and imetit inhibited the overflow of 5-HT maximally by 50–60%, with EC₅₀ values of 48 and 3.2 nmol/l, respectively. Effects on 5-HT overflow were always accompanied by similar effects on the overflow of 5-HIAA. Thioperamide (100 nmol/l) shifted the concentration response curve of (R)--methyl-histamine to the right (pK_B value 8.38). The inhibitory effect of 1 mol/l (R)--methyl-histamine was antagonized in a concentration-dependent manner by thioperamide (IC₅₀: 65 nmol/l) and dimaprit (IC₅₀: 8.6 mol/l); however, the effect of (R)--methyl-histamine was weakly antagonized by burimamide (by 38% at 100 mol/l) and not significantly affected by other H₃ receptor antagonists, such as impromidine, betahistine and phenylbutanoyl-histamine (each up to 100 mol/l). In conclusion, H₃ receptors mediating inhibition of 5-HT release from porcine enterochromaffin cells have a particular pharmacological profile indicating that heterogeneity of H₃ receptors may exist. The data suggest that histamine H₃ receptors modulating 5-HT release in pig small intestine do not belong to either H_3A or H_3B receptors as defined in rat tissue. Correspondence to: K. Racke at the above address     

Transforming Growth Factor-α (TGF-α) in a Semisolid Dosage Form: Preservative and Vehicle Selection

The selection of an ideal semisolid vehicle for growth factors presents a challenge. Some antimicrobial agents are known to delay wound healing. The objective of this investigation was to identify appropriate preservatives and vehicles for TGF-. Criteria for acceptance are noninterference with the mitogenic activity of TGF- as well as adequate product preservation. Vehicles considered were o/w creams, ointments, and a gel. Combinations of six preservatives were tested. Selection was determined using both microbial preservative challenge and TGF- mitogenic assay. In the former, 10 species of microorganisms were inoculated into formulation samples. At selected time intervals, it was determined whether colonies decreased, increased, or remained constant. In the mitogenic assay, samples of either preservatives or formulation prototypes were introduced to TGF--stimulated fibroblast cell cultures. Mitogenesis was determined by measuring ³H-dThd uptake into newly synthesized DNA. As preservatives, sorbic acid and quaternium-15 appear to satisfy both selection criteria. A thermosetting gel appears most promising as vehicle.     

The relationship between amphetamine antagonism and depletion of brain catecholamines by alpha-methyl-p-tyrosine in rats

Summary The time-course and the dose-response relationship for the antagonistic effect of alpha-methyl-p-tyrosine methyl ester HCl H 44/68 (-MT) on damphetamine (10.6 moles/kg) induced increase in motor activity was studied. The effect of amphetamine was gradually reduced from 30–60 min to a minimum at 1–4 h after the administration of 0.407 mmoles/kg of -MT. From (4-) 8 h the amphetamine response started to reappear and the original response was restored completely at 16 h after -MT. The dose-response curve showed, that between 0.051–0.41 mmoles/kg of -MT, given 1 h before amphetamine, there was a gradual reduction of the amphetamine response; doses above 0.41 mmoles/kg did not cause any further effect.The antiamphetamine action of -MT was compared with its time-and dose-dependent effects of inhibition of synthesis and reduction of stores of brain catecholamines. It was found, that the antiamphetamine action was more closely correlated with the reduction of the levels of brain dopamine, than with the brain noradrenaline levels. Further, the inhibition of catecholamine synthesis per se did not appear to be a sufficient condition for -MT induced antagonism of amphetamine These finding support the view that amphetamine is dependent on a substantial portion of the brain pool of dopamine and possibly noradrenaline rather than on very small, newly synthesized pools of these neurotransmitters.     

Effects of compound 48/80 on mast cells,histamine and cyclic AMP in isolated superior cervical ganglia

Summary Superior cervical ganglia of the rat contain mast cells which are sensitive to degranulation by compound 48/80. The granulation process is shown to the independent of the ATP content of the ganglion. Compound 48/80 released histamine into the incubation medium, thereby decreasing the histamine content of the ganglia. Moreover, the release of ³H-noradrenaline was accelerated by the compound. Histamine and adrenaline induced a rapid accumulation of cyclic AMP in the ganglia. This effect of the amines was specifically blocked by diphenhydramine or propranolol with an ID₅₀ of 1.5×10^–9 M and 2.2×10^–7 M, respectively.In contrast to other findings with isolated mast cell preparations, compound 48/80 induced a rapid and marked accumulation of cyclic AMP in intact ganglia and an enhanced release of cyclic AMP into the incubation fluid. Diphenhydramine prevented the accumulation in the tissue but only partly inhibited the enhanced appearance of cyclic AMP in the medium. The accumulation of the cyclic nucleotide in the tissue was partly blocked by propranolol, suggesting an additional action of compound 48/80 on cyclic AMP through catecholamines.The cyclic nucleotide phosphodiesterase activity in homogenates of superior cervical ganglia was completely inhibited by compound 48/80 at 7 g/ml when low cyclic AMP concentrations were used.In addition to cyclic AMP release, rapid and marked efflux of ATP into the medium was observed during incubations with compound 48/80. The lactate dehydrogenase activity in the incubation medium was significantly enhanced with incubation periods of 40 to 60 min indicating rather slowly occurring toxic damage to cell membranes by compound 48/80.This work was supported by the Deutsche Forschungsgemeinschaft (SFB 70).     

Greater vasodepressor sensitivity to nicardipine in spontaneously hypertensive rats (SHR) compared to normotensive rats

Summary Differences in the degree of attenuation by the calcium entry blocker, nicardipine, of the pressor responses to -1 (phenylephrine) and -2 (UK 14.304) adrenoceptor agonists was investigated in pentobarbital-anesthetized, normotensive Sprague-Dawley (SD) or Wistar Kyoto (WKY) rats, and spontaneously hypertensive rats (SHR), treated with the ganglionic blocking agent, pentolinium. Following administration of the ganglionic blocking agent, pentolinium, nicardipine produced a significant fall in blood pressure in SHR but not in SD or WKY rats. Nicardipine had no effect on the basal blood pressure of pithed SHR. In rats treated with the ganglionic blocking agent, pentolinium, nicardipine produced parallel shifts to the right in the doseresponse curves for phenylephrine but had no effect on maximal responses to phenylephrine. The decrease in the ED₅₀ of phenylephrine was greater in the SHR than in normotensive rats. Nicardipine produced a decrease in both the ED so and the maximal response to the -2 adrenoceptor agonist, UK 14.304. The decrease in the maximal response was greater in SHR than in WKY normotensive rats but the change in ED₅₀ for UK 14.304 was greater in WKY than in SHR. SD normotensive rats gave intermediate results. We conclude that the inhibition of -adrenoceptor-mediated pressor responses by nicardipine is generally more pronounced in SHR than in normotensive rats. This suggests that hypertension may be accompanied by an increase in the sensitivity of peripheral resistance beds to calcium entry blockers. Send offprint requests to: J. Atkinson     

Comparison of effects of phencyclidine and methamphetamine on body temperature in mice: a possible role for histamine neurons in thermoregulation

Summary We compared the effects of phencyclidine (PCP) and methamphetamine on body temperature and brain histamine turnover in mice. Methamphetamine at 5 and 10 mg/kg produced dose-related increases in rectal temperature, whereas PCP given in the same doses had no significant effect. In mice pretreated with -fluoromethylhistidine, an inhibitor of histidine decarboxylase, PCP produced a marked hyperthermia. PCP markedly accelerated brain histamine turnover, as measured by the accumulation of telemethylhistamine, a predominant metabolite of brain histamine, following administration of pargyline. Methamphetamine had no significant effect on the histamine dynamics. These results suggest involvement of brain histaminergic neurons in the action of PCP but not methamphetamine, and the presence of a histaminergic thermoregulatory mechanism.     

Responses to catecholamines of the rat isolated uterus throughout the natural oestrous cycle

Summary The effects of noradrenaline (NA), adrenaline (Adr) and isoprenaline (Iso) on rat isolated uterus were studied, throughout the natural oestrous cycle. The -inhibitory effects of the catecholamines were measured as a percentage inhibition of a standard acetylcholine (ACh)-induced contraction. Iso produced approximately 80% maximum inhibition of the standard ACh-contraction in all 4 stages of the oestrous cycle. Adr and NA produced 80% maximum inhibition in oestrus only, and 50–60% maximum inhibition in proestrus, metoestrus and dioestrus. The differences in the degree of inhibition produced by the catecholamines were overcome (i.e. the maximum inhibition produced by Adr and NA was increased to become equal to that produced by Iso) when experiments were repeated in the presence of the uptake inhibitors desmethylimipramine (DMI) and normetanephrine (NMN), but not in the presence of an -adrenoceptor antagonist (azapetine 10^–7 M). Variations in the uptake of³H-Iso and³H-NA were observed in the different stages of the oestrous cycle. Small -adrenoceptor mediated motor responses to NA and Adr were observed only in the presence of a -antagonist (propranolol 10^–5 M), in uteri from rats in oestrus, metoestrus and dioestrus, but not proestrus. It is concluded that in the rat isolated uterus, -inhibitory receptors predominate throughout the natural oestrous cycle, although the existence of -excitatory receptors has been shown in 3 of the 4 stages.     

Activation of histamine H3 receptors produces presynaptic inhibition of neurally evoked cat nictitating membrane responses in vivo

Summary This study was undertaken in order to determine the potential role of prejunctional histamine H₃ receptors in an in vivo adrenergic model system. Frequency-dependent nictitating membrane responses were elicited by sympathetic nerve stimulation in anesthetized cats. Systemic administration of the selective histamine H₃ receptor agonist, (R)--methylhistamine (RMeHA) produced a dose-related depression of amplitude of the evoked nictitating membrane responses with a threshold of about 10 g/kg and maximal effect (50% depression at the lowest frequency; 0.5 Hz) seen at 100–300 gg/kg. Responses obtained with low frequency stimulation were more sensitive to depression by RaMeHA than were responses evoked with higher frequencies of stimulation. Larger doses of RaMeHA given to the same animals, failed to produce additional inhibition.RMeHA depressed the amplitude of nictitating membrane responses evoked by either pre- or postganglionic nerve stimulation to an equivalent degree. This depressant action of RMeHA was antagonized by pretreatment with the specific histamine H₃ antagonist, thioperamide (3 mg/kg), but not by combined pretreatment with histamine H₁ and H₂ blockers chlorpheniramine (300 Ftg/kg) and cimetidine (5 mg/kg). Intravenous administration of adrenaline (1–30 wg/kg) also produced graded nictitating membrane responses that were not altered by subsequent administration of RMeHA.These results suggest that histamine H₃ receptors are involved in the modulation of neurally evoked noradrenaline release in the cat nictitating membrane by an inhibitory presynaptic action. The most likely site of drug action appears to be at the neuroeffector junction as no appreciable ganglionic effect of RMeHA was observed in this in vivo model system. Send offprint requests to M. C. Koss at the above address     

On the mechanism of alpha-receptor mediated modulation of 3H-noradrenaline release from slices of rat brain neocortex

Summary Brain cortical slices were superfused with Krebs-Ringer media and the effects of oxymetazoline (an -adrenoceptor agonist) and phentolamine (an -antagonist) on depolarization-induced ³H-NA release were examined. Depolarization was effected by various K⁺-concentrations or by electrical pulses.The effects of the -receptor agents on stimulated ³H-NA overflow appeared to be dependent on the strength of the depolarizing stimulus. Thus, at low K⁺-concentrations (13 or 26 mM) oxymetazoline decreased and phentolamine increased the stimulated overflow, while at 56 mM K⁺ little or no modulation was found. The agents acting on -receptors modulated ³H-NA release in a dose-dependent way (5 · 10^–8–10^–5 M).The lack of modulation by oxymetazoline of ³H-NA release induced by 56 mM K⁺ seems not to be due to a high concentration of NA released into the synaptic cleft, since reduction of the endogenous NA level by pretreatment with -methyl-para-tyrosine did not reveal such modulation.However, oxymetazoline was found to decrease 56 mM K⁺-induced ³H-NA release effectively, if the Ca²⁺-concentration in the medium was lowered from 1.2 to 0.2 mM. This suggests that -receptor mediated modulation of release may occur as a result of a change in Ca²⁺-availability to the depolarization-secretion process. In addition, hyperpolarization of nerve endings might be involved in the modulatory process, as concluded from calculations of the (theoretical) trans-membrane potential at various K⁺-concentrations.Although the -receptors modulating NA release seem to be localized presynaptically, their precise location remains uncertain. Experiments with tetrodotoxin suggested that the -receptor mediated modulation does not operate via a local interneuronal loop.     

In the ventral tegmental area picrotoxin blocks FGIN 1-27-induced increases in sexual behavior of rats and hamsters

Rationale and objectives There are two types of benzodiazepine receptors, mitochondrial benzodiazepine receptors (MBRs), and -aminobutyric acid (GABA_A)/benzodiazepine receptor complexes (GBRs). MBR activation increases neurosteroidogenesis. Ventral tegmental area (VTA) infusions of the MBR agonist, FGIN 1-27, increase midbrain levels of the progesterone metabolite 5-pregnan-3-ol-20-one (3,5-THP) and lordosis of rats and hamsters. Activation of GBRs leads to membrane hyperpolarization. In the VTA, infusions of GBR agonists enhance 3,5-THP-facilitated lordosis. Thus, if, in the VTA, MBR-mediated increases in 3,5-THP enhance sexual responses via actions at GBRs, then blocking GBRs with picrotoxin will reduce FGIN 1-27-induced increases in sexual behavior of female rodents.Methods Ovariectomized rats and hamsters, with unilateral guide cannula to the VTA, received estradiol benzoate (10 g; EB) at hour 0. Hamsters also received progesterone (100 g) at hour 44. At hour 47.5, all animals were infused first with 10 ng or 20 ng picrotoxin or saline, vehicle to the VTA and, 30 min later, with 5 g/11.4 nM FGIN 1-27 or saline, vehicle. Ten minutes later, animals were tested for sex and motor behavior.Results Picrotoxin, but not vehicle, infusions blocked FGIN 1-27-mediated increases in lordosis of rats and hamsters, proceptivity of rats, and sexual responsiveness of hamsters. In addition, midbrain 3,5-THP levels were higher in animals that received VTA infusions of FGIN 1-27, compared to those infused with saline, vehicle.Conclusion In the VTA, GBRs are required for MBR-enhanced sexual behavior of EB-primed rats and EB- and progesterone-primed hamsters.