Depression by neuropeptide Y of noradrenergic inhibitory postsynaptic potentials of locus coeruleus neurones.

Intracellular recordings were performed in a pontine slice preparation of the rat brain containing the locus coeruleus (LC). The spontaneous firing of action potentials was prevented by passing continuous hyperpolarizing current via the recording electrode. Focal electrical stimulation evoked a synaptic depolarization (PSP) followed by a hyperpolarization (IPSP). Neuropeptide Y (NPY; 0.1 mumol/l) inhibited the IPSP only. Pressure ejection of noradrenaline produced hyperpolarization which was potentiated in the presence of NPY (0.1 mumol/l). Hence, NPY appears to inhibit the release of noradrenaline from dendrites or recurrent axon collaterals of LC neurones.

Inhibitory adenosine A1-receptors on rat locus coeruleus neurones. An intracellular electrophysiological study.

Intracellular recordings were performed in a pontine slice preparation of the rat brain containing the locus coeruleus (LC). Adenosine (100, 300 mumol/l) and its structural analogues, namely (-)-N6-(R-phenylisopropyl)-adenosine (R-PIA; 3-30 mumol/l) and S-PIA (10, 30 mumol/l), as well as 5'-N-ethylcarboxamido-adenosine (NECA; 3-30 mumol/l) inhibited the firing rate of spontaneous action potentials and produced hyperpolarization; their rank order of potency was R-PIA congruent to NECA greater than S-PIA greater than adenosine. When applied by superfusion, all agonists strongly desensitized the LC cells; the hyperpolarization never surmounted 6 mV. Upon pressure ejection of adenosine 10 mmol/l from a micropipette positioned close to an LC neurone, the membrane potential was raised by 14 mV and the apparent input resistance decreased by 20%. When the membrane potential was hyperpolarized by current injection to a similar extent as adenosine did, the fall in input resistance was only 7%. The adenosine uptake inhibitor S-(p-nitrobenzyl)-6-thioguanosine (NBTG) 30 mumol/l decreased the frequency of action potentials alone; on simultaneous bath-application with adenosine 300 mumol/l it potentiated the hyperpolarization caused by the purine derivative. 8-Cyclopentyl-1,3-dipropylxanthine (CPDPX) 0.1 mumol/l had no effect on its own, but it antagonized both R-PIA 30 mumol/l and NBTG 30 mumol/l. A higher concentration of CPDPX (1 mumol/l) facilitated the spontaneous firing. In conclusion, both exogenous and endogenous adenosine activates somatic and/or dendritic A1-receptors of LC neurones leading to an enhancement of potassium conductance and thereby to a decreased firing rate and a hyperpolarization.

Interaction between neuropeptide Y and noradrenaline on central catecholamine neurons.

Despite their widespread occurrence in the central nervous system, interactions between co-localized transmitters and their receptors remain poorly understood. Noradrenergic neurons of the nucleus locus coeruleus contain the peptide co-transmitter neuropeptide Y (refs 1,2). In locus coeruleus cells, stimulation of alpha2-adrenoceptors 3,4 or opioid mu-receptors 5,6 increases a potassium conductance and thereby leads to hyperpolarization and inhibition of spontaneous firing. Coupling between these receptors and the inward rectifying K+ channels involves a pertussis toxin-sensitive GTP-binding protein (Gi or Go)7. Here we investigate whether the neuropeptide Y and alpha2-receptors of locus coeruleus neurons interact with one another. When administered alone, neuropeptide Y reduces the discharge of action potentials, probably by increasing the permeability of the membrane to potassium ions through the activation of a G protein; this effect is reduced in the presence of alpha2-adrenoceptor antagonists. Moreover, the peptide selectively increases the hyperpolarizing effect of alpha2-agonists, but does not enhance responses to opioid mu-agonists. We suggest that noradrenaline and its co-transmitter neuropeptide Y stimulate separate receptors, which influence each other in a specific way.

Effects of purinoceptor agonists on electrophysiological properties of rat mesencephalic trigeminal neurones in vitro.

Intracellular recordings were performed in a midpontine slice preparation of the rat brain containing the mesencephalic trigeminal nucleus (MTN). In spite of the previous demonstration of an adenosine deaminase-containing plexus terminating on this nucleus, adenosine, adenosine 5'-triphosphate (ATP), alpha,beta-methylene ATP (alpha,beta-meATP) and 2-methylthio ATP all failed to influence the membrane potential or input resistance of the MTN cells. Moreover, there was no apparent change in the shape of action potentials in the presence of these drugs, and the accomodation of the firing rate to depolarizing pulses was not affected either.

Presynaptic P1-purinoceptors in jejunal branches of the rabbit mesenteric artery and their possible function.

1. Excitatory junction potentials (EJPs) evoked by nerve stimulation with fifteen pulses at 1 Hz were recorded from smooth muscle cells of the rabbit isolated mesenteric artery. The effects of P1-purinoceptor agonists and antagonists, as well as of substances which interfere with the inactivation of endogenous adenosine, were tested. 2. Adenosine and its analogues depressed the EJPs in the train in a concentration-dependent manner. The percentage inhibition of the first EJP and that of the later ones was similar; some early EJPs, however, were inhibited more markedly. The rank order of potency of the agonists was (-)-N6-(R-phenylisopropyl)-adenosine (R-PIA) congruent to 5'-N-ethylcarboxamidoadenosine (NECA) greater than (+)-N6-(S-phenylisopropyl)-adenosine (S-PIA) greater than adenosine. The respective IC40 values (the concentrations producing 40% inhibition of the first EJP in the train) were 0.018, 0.028, 0.83 and 4.7 mumol/l. 3. Three methylxanthines, namely 8-phenyltheophylline (1, 10 mumol/l), 8-cyclopentyltheophylline (0.1, 1 mumol/l) and 8-(p-sulphophenyl)-theophylline (100 mumol/l), antagonized the effect of R-PIA (0.1 mumol/l). When given alone they also enhanced the amplitudes of all EJPs in the train. The percentage facilitation of the first EJP and that of the later ones was similar. Some early EJPs, however, were potentiated more markedly. 8-Phenyltheophylline was less potent than 8-cyclopentyltheophylline both in preventing the action of R-PIA and in enhancing the EJPs. A concentration (100 mumol/l) of 8-(p-sulphophenyl)-theophylline, which strongly antagonized the R-PIA effect, produced only a moderate facilitation of EJPs. 4. S-(p-nitrobenzyl)-6-thioguanosine (10 mumol/l) both depressed the EJPs, and enhanced the inhibitory effect of adenosine. Adenosine deaminase (10 micrograms/ml) caused some potentiation of EJPs; this action was prevented by a concentration (10 mumol/l) of deoxycoformycin, which had no effect of its own. AH21-132 (10 mumol/l) enhanced all EJPs in the train. 5. None of the above substances influenced the resting membrane potential of the smooth muscle cells. In addition, R-PIA (0.1 mumol/l) did not change the depolarization induced by noradrenaline (3 mumol/l). 6. We suggest that the axon terminals of postganglionic sympathetic neurones in the rabbit mesenteric artery possess P1-purinoceptors of the A1-type. The activation of these presynaptic receptors by endogenous adenosine may inhibit the release of the main neuroeffector transmitter, which is probably ATP.(ABSTRACT TRUNCATED AT 400 WORDS)

Identification of the neuroeffector transmitter in jejunal branches of the rabbit mesenteric artery.

Vasoconstriction or excitatory junction potentials (e.j.ps) evoked by nerve stimulation (15 field pulses at 2 Hz every 3 min) were recorded in rabbit isolated jejunal arteries. The resting diameter of the arteries and its decrease in response to stimulation was measured by a photoelectric method. Vasoconstriction was insensitive to prazosin 0.1 or 1 mumol/l. Yohimbine 1 mumol/l considerably enhanced, whereas alpha,beta-methylene ATP (alpha,beta-meATP) 1 mumol/l abolished the contractile response. In order to test the effect of exogenously applied transmitter candidates, noradrenaline (0.1-1 mumol/l) and ATP (10-30 mumol/l) were added in concentrations which evoked a vasoconstriction comparable to that induced by electrical stimulation. The action of noradrenaline was prevented by prazosin 0.1 mumol/l, but was unaffected by both yohimbine 1 mumol/l and alpha,beta-meATP 1 mumol/l. Alpha,beta-meATP 1 mumol/l depressed the effect of ATP. The e.j.ps evoked by a train of 15 pulses showed facilitation up to the third response and thereafter depression; a partial summation was also observed. Prazosin 0.1 mumol/l did not change the e.j.p. amplitudes. By contrast, when yohimbine 0.1 or 1 mumol/l was added to the prazosin-containing medium, both the late e.j.ps in the train and the summation were enhanced in a concentration-dependent manner. Alpha,beta-meATP 1 mumol/l almost abolished the e.j.ps. In conclusion, in rabbit jejunal arteries, stimulation of postganglionic sympathetic nerves may release noradrenaline together with ATP which is probably the sole neuroeffector transmitter under our conditions. Transmitter release seems to be modulated by the activation of presynaptic alpha 2-adrenoceptors. Under the stimulation conditions of the present experiments the released transmitter does not activate postsynaptic alpha 1-adrenoceptors.