By use of several prostacyclin analogs and an in vitro autoradiographic technique, we have found a novel subtype of the prostacyclin receptor, one having different binding properties compared with those of the known prostacyclin receptor in the rat brain. Isocarbacyclin, which is a potent agonist for the known prostacyclin receptor, had high affinity for the novel subtype (dissociation constant (K d ) of 7.8 nM). However, iloprost, which is usually used as a stable prostacyclin analog, showed low affinity binding ( 3 H]iloprost also had high affinity in these regions, and the binding specificity was similar to that for the known prostacyclin receptor. Hemilesion studies of striatal neurons lesioned by kainate or of dopaminergic afferents lesioned by 6-hydroxydopamine revealed that the binding sites of the novel subtype exist on neuronal cells in the striatum, but not on the presynaptic terminal of afferents or on glial cells. Electrophysiological studies carried out in the CA1 region of the hippocampus revealed that prostacyclin analogs have a facilitatory effect on the excitatory transmission through the novel prostacyclin receptor. The widespread expression of the prostacyclin receptor in the central nervous system suggests that prostacyclin has important roles in neuronal activity. Prostaglandins (PGs)1 and thromboxane are formed from arachidonic acid by cyclooxygenase and the respective synthase for each PG, such as PGD synthase and PGI 2 synthase. These prostanoids exert a variety of functions through their specific membrane receptors coupled to G proteins not only in the peripheral organs, but also in the central nervous system. For example, PGD 2 acts as a sleep inducer (1, 2), produces hypothermia (3), inhibits luteinizing hormone-releasing hormone release (4), and is involved in biphasic modulation of pain sensation (5) and modification of olfaction (6). PGE 2 has hyperthermic (7), sedative (8), anticonvulsive (9, 10), and antidiuretic effects (11) and induces wakefulness (12, 13), stimulates luteinizing hormone-releasing hormone release (14), modifies pain (5, 9), and regulates food intake (11). Also, PGF 2␣ has an antidiuretic effect (15) and an inhibitory effect on oxytocin release (16). These three PGs are reported to be involved in the modulation of neurotransmitter release as well (17). Prostacyclin, discovered in 1976, is known to be a potent vasodilator and also to have an inhibitory effect on platelet aggregation (18). In the brain, however, it is not clear whether prostacyclin has specific functions or not. It seems that the chemical instability of prostacyclin and lack of knowledge about the prostacyclin receptor in the brain have hampered the investigation.Recently, we reported the presence and distribution of the prostacyclin receptor in the central nervous system by use of [ 3 H]iloprost, a stable prostacyclin analog, and an in vitro autoradiographic technique (19). High density binding was observed in the nucleus of the solitary tract (NTS) and in the spinal trigeminal nucleus in the lowe...

Cooperative regulation of inosiol-1,4,5-trisphosphate receptors (IP(3)Rs) by Ca(2+) and IP(3) has been increasingly recognized, although its functional significance is not clear. The present experiments first confirmed that depolarization-induced Ca(2+) influx triggers an outward current in visual cortex pyramidal cells in normal medium, which was mediated by apamin-sensitive, small-conductance Ca(2+)-dependent K(+) channels (SK channels). With IP(3)-mobilizing neurotransmitters bath-applied, a delayed outward current was evoked in addition to the initial outward current and was mediated again by SK channels. Calcium turnover underlying this biphasic SK channel activation was investigated. By voltage-clamp recording, Ca(2+) influx through voltage-dependent Ca(2+) channels (VDCCs) was shown to be responsible for activating the initial SK current, whereas the IP(3)R blocker heparin abolished the delayed component. High-speed Ca(2+) imaging revealed that a biphasic Ca(2+) elevation indeed underlays this dual activation of SK channels. The first Ca(2+) elevation originated from VDCCs, whereas the delayed phase was attributed to calcium release from IP(3)Rs. Such enhanced SK currents, activated dually by incoming and released calcium, were shown to intensify spike-frequency adaptation. We propose that spike-induced calcium release from IP(3)Rs leads to SK channel activation, thereby fine tuning membrane excitability in central neurons.

Structural modification of the prostacyclin agonist isocarbacyclin resulted in the stable, highly selective ligand 1, which displays high binding affinity for a prostacyclin receptor in the central nervous system. The binding profile of 1 thus contrasts markedly with those of cicaprost and iloprost.

The coordinated activity of large numbers of adjacent parallel fiber synapses elevate calcium concentration locally in small regions of Purkinje cell dendrites. Such activity has also been reported to produce long-term depression of parallel fiber synaptic transmission. We have examined the relationship between these two events by combining patch clamp measurements of parallel fiber synaptic transmission with confocal microscopic imaging of the local calcium signals. We find that patterns of parallel fiber activity capable of evoking long-term depression invariably cause increases in Purkinje cell calcium concentration that are very spatially restricted. These results suggest that one function of the local dendritic calcium signals is to induce long-term depression of parallel fiber synapses.

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