Bimodal regulation of pancreatic exocrine function in vitro by somatostatin-28

The action of natural and synthetic somatostatin-(1--28), [Nle8]somatostatin-(1--28), somatostatin-(15--28), and somatostatin-(1--14) was examined in dispersed acini from guinea pig pancreas. At high concentrations, the 28-amino acid form of somatostatin increased amylase release, outflux of 45Ca, cellular cGMP, and to a lesser extent cellular cAMP. The increase in amylase release was suppressed by dibutyryl cGMP but was not modified by theophylline or atropine. Binding of 125I-labeled [Thr28, Nle31] cholecystokinin-(25--33) was inhibited by [Nle8]somatostatin-(1--28). These effects required the entire 28-amino acid peptide and appeared to result from occupation of cholecystokinin receptors. It is postulated that they involve interactions between the C-terminal and the N-terminal sequences of the molecule with the participation of the amino acid in position 8. At low concentrations, natural and synthetic forms of somatostatin-(1--28) and somatostatin-(15--28) inhibited secretin- and vasoactive intestinal peptide (VIP)-stimulated increases in cellular cAMP concentration. No difference was found between the potency of somatostatin peptides, indicating that the tetradecapeptide somatostatin-(15--28) is sufficient to exert an inhibitory action on secretin- and VIP-stimulated cellular cAMP concentration. By contrast, the somatostatin fragment S-(1--14) was inactive on pancreatic cellular function.     

Somatostatin Receptor type 2 Mediates Bombesin-Induced Inhibition of Gastric Acid Secretion in Mice

Studies in isolated mouse stomach showed that bombesin releases somatostatin. We characterized the effects of exogenous bombesin on gastric acid secretion in mice and determined the involvement of somatostatin and somatostatin receptor type 2 (SSTR2) by using somatostatin immunoneutralization, the SSTR2 antagonist, PRL-2903, and SSTR2 knockout mice. Gastric acid secretion was monitored under basal and pentagastrin-, histamine- or bethanechol-stimulated conditions in urethane-anaesthetized mice. Bombesin (10–40 μg kg⁻¹ h⁻¹) and somatostatin-14 (20 μg kg⁻¹ h⁻¹) were infused i.v. 10 and 30 min after PRL-2903 or somatostatin antibody pretreatment, respectively. Urethane-anaesthetized wild-type mice had low basal acid secretion (0.12 ± 0.01 μmol (10 min)⁻¹) compared with SSTR2 knockout mice (1.43 ± 0.10 μmol (10 min)⁻¹). Somatostatin antibody and PRL-2903 increased basal secretion in wild-type mice but not in SSTR2 knockout animals. In wild-type mice, bombesin inhibited secretagogue-stimulated acid secretion in a dose-dependent manner, and somatostatin-14 inhibited pentagastrin-stimulated secretion. In wild-type mice pretreated with somatostatin antibody or PRL-2903 and in SSTR2 knockout mice, bombesin and somatostatin-14 i.v. infusion did not alter the increased gastric acid secretion. These results indicate that, in mice, bombesin inhibits gastric acid secretion through the release of somatostatin and the activation of SSTR2. These observations strengthen the important role of SSTR2 in mediating somatostatin inhibitory actions on gastric acid secretion.     

Effects of somatostatin-14 on the in vitro release of [3H]GABA from slices of rat caudatoputamen

Slices (300 microns) of rat caudatoputamen were incubated in Krebs-Henseleit medium and loaded with [3H]glutamine, part of which was converted to [3H]GABA. This conversion takes place only in GABA-neurons most of which probably contribute to the striatonigral pathway. After a 24 min equilibration period, release of radioactivity was stimulated with veratridine (3.1-4 mumol/l) or K+ (15-25 mmol/l) in the absence or presence of somatostatin-14. From the radioactivity released [3H]GABA was separated by cationic exchange chromatography and measured. Somatostatin-14 affected the release of [3H]GABA in a manner which depended on its concentration as well as on the extent of stimulus-evoked release. Somatostatin-14 (1 nmol/l) enhanced the moderate release (2-4% of tissue content) elicited by veratridine (3.1 mumol/l) or K+ (20 mmol/l), but had no effect on the more pronounced release (5-8% of tissue content) elicited by veratridine (4 mumol/l) or K+ (25 mmol/l). Somatostatin-14 (10 nmol/l) had no effect on the moderate release of [3H]GABA, but diminished the pronounced one. Further experiments provided evidence that the somatostatin-14-induced enhancement was not brought about by a direct action on GABA-neurons but was probably indirect, i.e. mediated by other striatal neurons. In contrast, the diminution of the release of [3H]GABA caused by somatostatin-14 may be due to its direct action on releasing neurons. Two antisera against somatostatin lowered the pronounced release indicating that endogenous somatostatin may also enhance the release of [3H]GABA. In addition, endogenous somatostatin seems also to be able to diminish the release under certain experimental conditions.(ABSTRACT TRUNCATED AT 250 WORDS)     

Somatostatin is increased in the dorsal root ganglia of adjuvant-inflamed rat

To determine whether biosynthesis of somatostatin is enhanced in the primary sensory neurons by inflammatory pain, we examined the effects of adjuvant inoculation on the content of immunoreactive somatostatin, mainly composed of somatostatin-14 and somatostatin-28, in the dorsal root ganglia and the spinal cord of the rat. The adjuvant inoculation, which produced long-lasting inflammation and hyperalgesia, increased the content of immunoreactive somatostatin, especially somatostatin-14, in the dorsal root ganglia at L4-L6 levels with no change in the dorsal and ventral horns of lumbar enlargement. Such an increase was enhanced by an intrathecal injection of colchicine (0.2 mg) that inhibits axonal flow of somatostatin. Chronic administration of the anti-inflammatory analgesic, sodium diclofenac (3 mg.kg-1.d-1), abolished an adjuvant-induced increase in the content of immunoreactive somatostatin in the dorsal root ganglia. These results suggest that the turnover (biosynthesis and axonal flow) of somatostatin in the primary sensory neurons is enhanced in the presence of persisting inflammatory pain, and support the idea that somatostatin-containing primary afferents are involved in the transmission of pain in the spinal dorsal horn.     

Presynaptic muscarinic (M3) receptors reduce excitatory transmission in dopamine neurons of the rat mesencephalon.

The effects of carbachol (0.01-30 microM) and muscarine (10-30 microM) on the excitatory synaptic potentials were studied using conventional intracellular recordings from dopaminergic neurons in rat mesencephalic slices. Both muscarinic agonists reversibly reduced the excitatory synaptic potentials, evoked by local electrical stimulation. The EC50 for carbachol was determined to be 4.5 microM. The maximal degree of the excitatory synaptic potentials suppression caused by carbachol and muscarine was around 40% of control. This suppression was completely blocked by the non-specific muscarinic antagonist atropine (1 microM) and the selective M3 antagonist 4-diphenylacetoxy-N-methylpiperidine methiodide (1 microM). Other antagonists, preferentially acting at M1, M2 and M4 receptors, were not effective. Furthermore, the acetylcholinesterase inhibitor, physostigmine (50 microM), decreased the amplitude of the excitatory synaptic potentials, indicating that ambient acetylcholine can depress this potential. Direct depolarizing responses to glutamate were not changed by muscarine. In addition, muscarine facilitated the second excitatory synaptic potentials during a paired-pulse protocol. Thus, the effect of the muscarinic agonists is attributable to a presynaptic locus of action. The action of muscarine was not mediated by an N-ethylmaleimide-sensitive G-protein since it was not modified by a treatment of the slices with this agent. The calcium channels blockers, omega-conotoxin GIVA, omega-agatoxin IVA and omega-conotoxin MVIIC did not affect the action of muscarine on the excitatory synaptic potentials. When the potassium currents were reduced by extracellular barium and 4-aminopyridine, the muscarinic agonists still depressed the excitatory synaptic potentials. Our data indicate that presynaptically located M3 receptors modulate the excitatory transmission to midbrain dopaminergic neurons via a N-ethylmaleimide-insensitive G-protein which activates mechanisms neither linked to N-, P-, Q-type calcium channels nor to barium- and 4-aminopyridine-sensitive potassium channels.     

Effects of somatostatin on dopamine sensitive adenylate cyclase activity in the caudate-putamen of the rat

Summary The effect of somatostatin-14 (SRIF) on dopamine-sensitive adenylate cyclase in caudateputamen pellets was studied in naive female rats, and in rats with chemical lesions of the nigrostriatal dopaminergic tract produced by injecion of 6-hydroxydopamine, or of the caudate-putamen itself produced by injection of kainic acid 3 week earlier. In unlesioned rats somatostatin at a concentration of 10^–7 moles/1 inhibited adenylate cyclase activation by submaximal concentrations of dopamine, increasing the apparent K_m but not altering E_max. In 6-hydroxydopamine lesioned rats somatostatin no longer influenced adenylate cyclase activity, whereas in kainic acid lesioned rats somatostatin still increased the apparent K_m for dopamine activation. The effect of somatostatin in untreated and lesioned rats is compatible with a partial competitive antagonism to dopamine. Although the data from the lesioned rats present preliminary results, the dose response characteristics and the effects in lesioned animals suggest a more complex interaction, possibly by binding of somatostatin to an inhibitory subunit of regulatory adenylate cyclase components.     

Properties of Ca2+ currents in acutely dissociated neurons of the chick ciliary ganglion: inhibition by somatostatin-14 and somatostatin-28.

Whole-cell voltage-clamp recordings were made from acutely dissociated neurons obtained from the embryonic chick ciliary ganglion. Recording pipettes were filled with salines containing 120 mM CsCl or 120 mM tetraethylammonium-Cl. Application of depolarizing voltage commands evoked L-type Ca2+ currents and, at voltages positive to 0 mV, an unidentified cationic conductance. The unidentified cationic conductances made the Ca2+ currents appear to undergo voltage-dependent inactivation and made a large contribution to tail currents present during repolarizing voltage steps. Ca2+ Ca2+ currents showed little or no sign of inactivation and did not reverse at voltages up to +60 mV. Application of somatostatin-14 or somatostatin-28 produced a reversible inhibition of Ca2+ currents in virtually all cells, regardless of size. Somatostatin-28 (1-14) was inactive. The effects of somatostatin-14 and somatostatin-28 were attenuated by pretreatment with pertussis toxin, suggesting a role for G-proteins in mediating the response. Somatostatin-14 and somatostatin-28 had no effect on voltage-dependent K+ currents. The results suggest that somatostatin peptides modulate the motor output of the chick ciliary ganglion.     

Pre- and postsynaptic modulation of glycinergic and gabaergic transmission by muscarinic receptors on rat hypoglossal motoneurons in vitro

The motor output of hypoglossal motoneurons to tongue muscles takes place in concert with the respiratory rhythm and is determined by the balance between excitatory glutamatergic transmission and inhibitory transmission mediated by glycine or GABA. The relative contribution by these transmitters is a phasic phenomenon modulated by other transmitters. We examined how metabotropic muscarinic receptors, widely expressed in the brainstem where they excite cranial motor nuclei, might influence synaptic activity mediated by GABA or glycine. For this purpose, using thin slices of the neonatal rat brainstem, we recorded (under whole-cell patch clamp) glycinergic or GABAergic responses from visually identified hypoglossal motoneurons after pharmacological block of glutamatergic transmission. Muscarine inhibited spontaneous and electrically induced events mediated by GABA or glycine. The amplitude of glycinergic miniature inhibitory postsynaptic currents was slightly reduced by muscarine, while GABAergic miniature inhibitory postsynaptic currents were unaffected. Motoneuron currents induced by focally applied GABA and glycine were depressed by muscarine with stronger reduction in glycine-mediated responses. Histochemical observations indicated the presence of M1, M2 and M5 subtypes of muscarinic receptors in the neonatal hypoglossal nucleus. These results suggest that muscarine potently depressed inhibitory neurotransmission on brainstem motoneurons, and that this action was exerted via preterminal and extrasynaptic receptors. Since the large reduction in inhibitory neurotransmission may contribute to overall excitation of brainstem motoneurons by muscarinic receptors, these data might help to understand the central components of action of antimuscarinic agents in preanesthetic medication or against motion sickness.     

Muscarinic depression of synaptic transmission at the hippocampal mossy fiber synapse

1. The action of muscarine was studied in the CA3 region of the rat hippocampal slice with single-electrode voltage-clamp techniques. 2. Bath application of 1 or 10 microM muscarine produced an increase in the input resistance of these cells and reduced the slow afterhyperpolarization (sAHP) response. Changes in input resistance were more pronounced around the resting potential of the cell (-50 to -60 mV), but in many cells an effect was also seen at -80 mV. These effects were absent when cesium chloride-containing microelectrodes were used. 3. At 1 microM, muscarine had little effect on synaptic transmission, causing a 0 +/- 7% (mean +/- SE, n = 19) change in excitatory postsynaptic potential (EPSP) and decreasing the excitatory postsynaptic current (EPSC) by 11 +/- 6% (n = 14); neither change was statistically significant. 4. In contrast, 10 microM muscarine produced a reliable depression of both the EPSP and EPSC. This effect was independent of the electrolyte used: with KCl the EPSP was depressed 23 +/- 4% (n = 5) and the EPSC 35 +/- 5% (n = 4); for CsCl the EPSP was depressed 23 +/- 10% (n = 7) and the EPSC 34 +/- 5% (n = 7). 5. Muscarine did not alter the reversal potential of the synaptic current but merely produced a decrease in slope conductance (37 +/- 5%, n = 6). 6. Muscarine did not significantly alter the shape of the EPSC waveform. This was assessed by comparing the 10-90% rise time and the half decay time of the current before and after muscarine.(ABSTRACT TRUNCATED AT 250 WORDS)     

Postsynaptic action mechanism of somatostatin on the membrane excitability in spinal substantia gelatinosa neurons of juvenile rats

We used tight-seal, whole-cell recording in juvenile rat spinal slices to investigate the action of somatostatin on substantia gelatinosa neurons. Bath application of somatostatin caused a robust and repeatable hyperpolarization or outward current in substantia gelatinosa neurons. Somatostatin inhibited spontaneous action potentials in subpopulation of substantia gelatinosa neurons. The amplitude of dorsal root-evoked excitatory postsynaptic currents and the frequency of spontaneous excitatory postsynaptic currents were not affected by somatostatin. The current induced by somatostatin developed almost instantaneously and did not show any time-dependent inactivation. The current-voltage relationship exhibited inward rectification. The conductance of somatostatin-sensitive current increased with the concentration of external K(+). The reversal potentials in different external K(+) concentrations were close to the K(+) equilibrium potentials. The effect of somatostatin was dose-dependent, with an EC(50) of 113 nM. The somatostatin-sensitive current was blocked by low concentration of extracellular Ba(2+) but not by glibenclamide, an inhibitor of ATP-sensitive K(+) channels. Hyperpolarization-activated cation current in a subpopulation of substantia gelatinosa neurons was not affected by somatostatin. In neurons recorded with an internal solution containing GTPgammaS, somatostatin induced outward current and hyperpolarization that did not reverse on washing. When the spontaneous induction of outward current with GTPgammaS was greatest, somatostatin did not induce any outward currents. Furthermore, intracellular dialysis of GDPbetaS, a G-protein antagonist, abolished the effect of somatostatin. In addition, SST-sensitive neurons were fewer in slices incubated with pertussis toxin than in adjacent control slices incubated without pertussis toxin.These results suggest that somatostatin decreases the postsynaptic membrane excitability of substantia gelatinosa neurons by a pertussis toxin-sensitive G-protein-mediated activation of an inwardly rectifying K(+) conductance.     

On the transmitter function of 5-hydroxytryptamine at excitatory and inhibitory monosynaptic junctions

1. Two symmetrical giant neurones located in the cerebral ganglion of Aplysia californica contain 4-6 p-mole 5-hydroxytryptamine (5-HT) and are able to synthesize it (Weinreich, McCaman, McCaman & Vaughn, 1973; Eisenstadt, Goldman, Kandel, Koike, Koester & Schwartz, 1973). Stimulation of each of these neurones evokes excitatory and inhibitory potentials in various cells of the ipsilateral buccal ganglion. In nine buccal neurones it evokes excitatory potentials, in other three, ;classical' inhibitory potentials and in one neurone an ;atypical' inhibitory potential.2. The connexion between the giant cerebral neurone and the cells receiving either an excitatory or a ;classical' inhibitory input from it are monosynaptic. TEA injection into the cerebral giant neurone, which prolongs the presynaptic spike, causes a gradual increase of both the excitatory and the inhibitory potentials. On the other hand, high Ca(2+) media, which block polysynaptic pathways, do not suppress these synaptic potentials.3. The iontophoretic application of 5-HT to the buccal neurones receiving excitatory input from the giant cerebral neurones evokes depolarizations showing the pharmacological properties of both A- and A'-responses to 5-HT (see preceding paper). Antagonists which block only the A-receptors (curare, 7-methyltryptamine, LSD 25) block partially the synaptic depolarizing potentials. Bufotenine, which blocks both the A- and A'-receptors, completely blocks the excitatory potentials. Thus, the post-synaptic membrane of these buccal neurones appears to be endowed with both A- and A'-receptors to 5-HT.4. The ;classical' inhibitory potentials elicited in three buccal neurones are hyperpolarizations which reverse at - 80 mV and are due to an increase in K(+)-conductance. The iontophoretic application of 5-HT to these post-synaptic neurones evokes hyperpolarizing B-responses which are also generated by an increase in K(+)-conductance. Antagonists which block the B-responses (bufotenine, methoxygramine) also block the inhibitory potentials.5. The ;atypical' inhibitory potential evoked in one buccal neurone consists in an hyperpolarization which increases in amplitude with cell hyperpolarization. Iontophoretic application of 5-HT to this buccal cell evokes an hyperpolarizing beta-response which also increases in amplitude with cell polarization and results from a decrease in both Na(+)- and K(+)- conductances. The monosynaptic character of the ;atypical' inhibitory potential is not yet fully proven.6. It can be concluded that the excitatory and inhibitory synaptic effects evoked in the buccal neurones by the stimulation of the 5-HT-containing-giant cerebral neurones are very likely mediated by 5-HT.     

Effects of excitatory modulation on intrinsic properties of turtle motoneurons

The purpose of this study was to quantify the effects of excitatory modulation on the intrinsic properties of motoneurons (MNs) in slices of spinal cord taken from the adult turtle. Responses were noted following application of an excitatory modulator: serotonin (5-HT), muscarine, trans-1-amino-1,3-cyclopentane dicarboxylic acid (tACPD), or all three combined. A sample of 44 MNs was divided into 2 groups, on the basis of whether MNs did (28/44) or did not (16/44) demonstrate a nifedipine-sensitive acceleration of discharge during a 2-s, intracellularly injected stimulus pulse. Such acceleration indicates the development of a plateau potential (PP). Excitatory modulation lowered the MNs' resting potential, increased input resistance, decreased rheobase, reduced several afterhyperpolarization values, and shifted the conventional, one-phase stimulus current-spike frequency (I-f) relation to the left. For both MN groups, the relative efficacy of excitatory modulation on both non-PP and PP MNs was generally in the following order: combined application > 5-HT approximately muscarine > tACPD. In many instances, the effects of modulation differed significantly for non-PP versus PP MNs, the most pronounced being in their I-f relation. To describe this difference, it was necessary to measure a two-phase relation. In PP MNs, excitatory modulation considerably increased the slope of the first (initial) phase and flattened the second (later) phase of this relation. The latter result bore similarities to that obtained in a previous study, which addressed MN firing behavior during fictive locomotion of the high-decerebrate cat.     

The effectiveness of bicuculline as an antagonist of GABA and visually evoked inhibition in the cat''s striate cortex.

1. The iontophoretic application of the alkaloid bicuculline to neurones in area 17 of the cat's visual cortex effectively antagonized the inhibitory action of iontophoretically applied GABA in fifty-four out of sixty-two neurones examined. It had little or no effect on the inhibitory action of iontophoretically applied glycine. 2. At the stage that the iontophoretic application of bicuculline blocked the inhibitory action of GABA it also reduced or blocked visually evoked inhibitory influences acting on forty-three of the fifty-four cells. This effect on visually evoked inhibition was not reproduced by simply raising the neural spontaneous activity with iontophoretically applied glutamate. 3. For those seven neurones where the iontophoresis of bicuculline failed to block the inhibitory action of iontophoretically applied GABA it also failed to produce any change in visually evoked inhibition. 4. In all cases where a visually evoked inhibition of a cells resting discharge was reduced by the iontophoretic application of bicuculline, the inhibitory response was replaced by an excitatory response. The application of bicuculline also revealed excitatory responses to certain of the visual stimuli that previously appeared to exert neither inhibitory nor excitatory effects on a cell, and often where cells normally exhibited small excitatory responses it produced large increases in the magnitude of the evoked response. 5. These results indicate that the normal responses of the neurones examined in the present work, to the particular visual stimuli used, reflect an interaction between simultaneously evoked excitatory and inhibitory inputs. It is suggested that the iontophoretic application of bicuculline by blocking or reducing the inhibitory input moves the balance between the inputs in favour of the excitatory input. 6. The present results support the view that GABA is an inhibitory transmitter in the visual cortex.     

Inhibitory and excitatory effects of dopamine on Aplysia neurones

1. Electrophoretic application of dopamine (DA) on Aplysia neurones elicits both excitatory and inhibitory effects, which in many cases are observed in the same neurone, and often result in a biphasic response.2. The DA receptors are localized predominantly on the axons. Desensitization, which occurs after repeated injections or with bath application of DA, is more marked for excitatory responses.3. Tubocurarine and strychnine block the DA excitatory responses without affecting the inhibitory ones, which can be selectively blocked by ergot derivatives. It is concluded that the excitatory and inhibitory effects are mediated by two distinct receptors.4. The two DA receptors can be pharmacologically separated from the three ACh receptors described in the same nervous system.5. In some neurones the dopamine inhibitory responses can be inverted by artificial hyperpolarization of the membrane at the potassium equilibrium potential, E(K), indicating that dopamine causes a selective increase in potassium permeability.6. In other neurones the reversal potential of dopamine inhibitory responses is at a more depolarized level than E(K), but can be brought to E(K) by pharmacological agents known to block the receptors mediating the excitatory effects of DA.7. In still other neurones, the hyperpolarization induced by DA cannot be inverted in normal conditions, but a reversal can be induced by ouabain or by the substitution of external sodium by lithium. These results are discussed in terms of an hypothesis in which dopamine increases the potassium permeability of a limited region of the axonal membrane.8. It is concluded that a selective increase in potassium permeability probably accounts for all dopamine inhibitory effects in the neurones studied.     

Evidence for two somatostatin-14 receptor types in rat brain cortex

The binding characteristics of a stable somatostatin analogue, SMS 201-995, have been evaluated in a somatostatin receptor binding assay using [125I-Tyr11]somatostatin, and cortical, pituitary or pancreatic membranes. High affinity binding sites for somatostatin-14 and for SMS 201-995 were identified in rat cortex (Kd = 0.60 nM) as well as in pituitary (Kd = 0.74 nM) and pancreatic beta-cells (Kd = 0.18 nM). In the cortex, in contrary to the two other tissues, SMS 201-995 only labels a part (approximately 75%) of the receptors recognized by somatostatin-14. This suggests the presence in the cortex of more than one population of somatostatin receptors. In conclusion, SMS 201-995 appears as a valuable tool for differentiating the two somatostatin receptor types.     

Somatostatin and somatostatin receptors in the pig pineal gland during postnatal development: an immunocytochemical study

An immunohistochemical study of the pineal gland of the domestic pig was carried out using rabbit antisera raised against synthetic peptide fragments corresponding to different amino acid sequences of the prosomatostatin, the somatostatin-14, and the somatostatin-28 molecule. The study was supplemented by immunohistochemical staining with rabbit antisera raised against five subtypes of somatostatin receptors. The pineal glands were taken from the newborn, 21-day-old and 7-month-old pigs. Immunoreactive nerve fibers and cells were observed in the pineal gland with all the antisera against somatostatin and prosomatostatin. The nerve fibers were located throughout the pineal gland-in the capsule, connective septa, and parenchyma-with the highest density in proximo-ventral part of the gland. The somatostatin positive fibers were also found in the habenular and posterior commissurae areas. Somatostatin-immunoreactive cell bodies were observed mostly in the central part of the gland. These results point to the existence of two somatostatin sources in the pig pineal gland: 1) nerve fibers, probably of central origin; and 2) cells that may represent intrapineal neurons or specialised pinealocytes. A clear difference in the immunoreactivity between newborn, 21-day-old, and 7-month-old pigs was found. Generally, the density of nerve fibers was lower in adult than young animals. The number of the cells also decreased with age. By using the antisera against the five somatostatin receptors, only sst3 - receptor immunoreactivity could be detected. The receptor-immunoreactivity was confined to varicose and smooth fibers and some cells. The sst(3)-receptor positive structures were localised in all parts of the gland and their number was higher in younger pigs.     

Somatostatin receptors in differentiated ovarian tumors.

The presence of somatostatin receptors was investigated in 57 primary human ovarian tumors using in vitro receptor autoradiography with three different somatostatin radioligands, 125I-[Tyr11]-somatostatin-14, 125I-[Leu8, D-Trp22, Tyr25]-somatostatin-28, or 125I-[Tyr3]-SMS 201-995. Three cases, all belonging to epithelial tumors, were receptor positive; specifically 1 of 42 adenocarcinomas, 1 of 3 borderline malignancies, and 1 of 2 cystadenomas. Four other epithelial tumors (3 fibroadenomas, 1 Brenner tumor), 4 sex cord-stromal tumors (2 fibrothecomas, 2 granulosa cell tumors), and 2 germ cell tumors (1 dysgerminoma, 1 teratoma) were receptor negative. In the positive cases, the somatostatin receptors were localized on epithelial cells exclusively, were of high affinity (KD = 4.6 nmol/l [nanomolar]), and specific for somatostatin analogs. These receptors bound somatostatin-14 and somatostatin-28 radioligands with a higher affinity than the octapeptide [Tyr3]-SMS 201-995. Healthy ovarian tissue had no somatostatin receptors. A subpopulation of relatively well-differentiated ovarian tumors, therefore, was identified pathobiochemically on the basis of its somatostatin receptor content. This small group of somatostatin receptor-positive tumors may be a target for in vivo diagnostic imaging with somatostatin ligands.     

Metabotropic glutamate and muscarinic cholinergic receptor-mediated preferential inhibition of N-methyl-D-aspartate component of transmissions in rat ventral tegmental area

Presynaptic inhibition is one of the major control mechanisms in the CNS. Our laboratory recently reported that presynaptic GABA(B) and adenosine A(1) receptors mediate a preferential inhibition on N-methyl-D-aspartate receptor-mediated excitatory postsynaptic currents recorded in rat midbrain dopamine neurons. Here we extended these findings to metabotropic glutamate and muscarinic cholinergic receptors. Intracellular voltage clamp recordings were made from dopamine neurons in rat ventral tegmental area in slice preparations. (+/-)-1-Aminocyclopentane-trans-1,3-dicarboxylic acid (agonist for groups I and II metabotropic glutamate receptors) and L(+)-2-amino-4-phosphonobutyric acid (L-AP4; agonist for group III metabotropic glutamate receptors) were significantly more potent for inhibiting N-methyl-D-aspartate receptor-mediated excitatory postsynaptic currents, as compared with inhibition of excitatory postsynaptic currents mediated by alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors. Such preferential inhibition of the N-methyl-D-aspartate component was also observed for muscarine (agonist for muscarinic cholinergic receptors). Inhibitory effects of (+/-)-1-aminocyclopentane-trans-1,3-dicarboxylic acid, L-AP4, and muscarine were blocked reversibly by their respective antagonists [(RS)-alpha-methyl-4-carboxyphenylglycine, (RS)-alpha-methyl-4-phosphonophenylglycine, and 1,1-dimethyl-4-diphenylacetoxypiperidinium iodide]. In addition, all three agonists increased the ratio of excitatory postsynaptic currents in paired-pulse studies and did not reduce currents induced by exogenous N-methyl-D-aspartate and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid. Interestingly, the glutamate release stimulator 4-aminopyridine (30 microM) and the glutamate uptake inhibitor L-anti-endo-3,4-methanopyrrolidine dicarboxylate (300 microM) preferentially increased the amplitude of N-methyl-D-aspartate excitatory postsynaptic currents.Thus, agonists for metabotropic glutamate and muscarinic cholinergic receptors act presynaptically to cause a preferential reduction in the N-methyl-D-aspartate component of excitatory synaptic transmissions. Together with the evidence for GABA(B) and adenosine A(1) receptor-mediated preferential inhibition of the N-methyl-D-aspartate component, the present results suggest that limiting glutamate spillover onto postsynaptic N-methyl-D-aspartate receptors may be a general rule for presynaptic modulation in midbrain dopamine neurons.     

Distribution of somatostatin-28 (1-12) immunoreactivity in the diencephalon and the brainstem of the dog

The term somatostatin refers to a family of peptides, mainly somatostatin-14, somatostatin-28 and somatostatin-28 (1-12), which are the cleavage products of a single 116 amino acid-long preprosomatostain molecule. The production of antibodies to these peptides allows their localization in a number of neuronal populations throughout the entire neuroaxis in many mammals. The dog has been pointed out as an extremely useful animal model for studying age-related cognitive dysfunction and other neuronal changes associated with aging in which somatostatin appears to be involved. However, only very scanty information is available with regard to the distribution of somatostatin in the brain of the dog. In the present work we have determined the pattern of the distribution of somatostatin-28 (1-12) immunoreactivity in the diencephalon and the brainstem of the dog. High to moderate densities of labeled perikarya were found in the anterior periventricular and arcuate hypothalamic nuclei, the reticular thalamic nucleus, in delimited parts of the nucleus of the brachium inferior colliculus, the retrorubral area, the dorsal raphe nucleus, the myelencephalic reticular formation and the dorsal motor nucleus of the vagus. Less dense population of somatostatin cells were localized in other diencephalic and brainstem nuclei. The distribution of labeled fibers was even broader as in addition to those above mentioned there were a number of areas that appeared devoid of labeled perikarya. Many of the findings were similar to those reported in earlier works while others underlined the existence of inconsistencies in the distribution pattern of this peptide in the brain of mammals.     

Substance P as a neuromodulator of cerebellar cholinergic systems

Substance P in low concentrations (10_-7 M) activates rat cerebellar neurons (in slices), while in high concentrations (10_-6 and 10_-5 M) this compound causes a biphasic response (excitation-inhibition). Substance P probably acts as the excitatory neurotransmitter in the cerebellum and produces modulatory effects (triggering and facilitation) on cerebellar cholinergic structures. Substance P reactivates cholinergic excitatory processes, while acetylcholine prevents substance P-induced inhibitory phase. The data suggest that the modulatory effects of substance P are realized via the feedback mechanisms.