Ca2+ and Cl(-)-dependence of the turtle olfactory response to odorants and forskolin

In the turtle olfactory system, large responses to odorants appeared after application of cAMP of forskolin at high concentrations to the isolated olfactory receptor neurons or the olfactory epithelium, suggesting that a cAMP-independent pathway greatly contributes to the generation of odor responses. We measured the effects of the mucosal Ca2+ and Cl- concentrations and a Ca(2+)-activated Cl(-)-channel blocker, 4-acetamide-4'-(isothiocyano) stilbene-2,2'-disulphonic acid (SITS), upon olfactory bulbar responses to explore the contribution of the Ca(2+)-activated Cl(-)-channels to cAMP-dependent and independent pathways. Elimination of mucosal Ca2+ by addition of 2 mM EGTA to the stimulating solution partially inhibited the cAMP-independent responses to 0.1 mM citralva but did not affect those to 0.1 mM lilial or 0.1 mM l-carvone. Substitution of mucosal Cl- with gluconate slightly enhanced the total response to 0.1 mM citralva but practically did not affect the responses to other odorants tested. SITS (2 mM) partially inhibited the responses to 50 microM forskolin and 0.1 mM citralva but did not affect the cAMP-independent response to 0.1 mM citralva in Ca(2+)-free Ringer's solution. These results suggested that the Ca(2+)-activated Cl- channels do not contribute to the generation of the cAMP-independent responses, but that they partially contribute to the generation of the cAMP-dependent responses in the turtle.

The rodent granular convoluted tubule cell--an update

The cells of the granular convoluted tubule (GCT) of the rodent submandibular gland (SMG) are under complex developmental and multihormonal regulation. Recent findings indicate that GCT cells also synthesize transforming growth factor alpha (TGF-alpha), hepatocyte growth factor, erythroid differentiation factor, endothelin, and insulin-like growth factor, as well as several novel androgen-dependent proteins of unknown function. The GCTs of hypophysectomized mice provide a convenient model to study multihormonal regulation of gene expression. The GCT system of the rodent SMG also is a fruitful model for study of hormone receptors.

Regulation of Na+,K+-ATPase in submandibular glands of hypophysectomized male mice by steroid and thyroid hormones

The effects of thyroid hormone, androgen, glucocorticoid, and mineralocorticoid on Na+,K+-ATPase activity and on levels of its alpha-subunit protein (alpha 1 isoform) in mouse submandibular gland (SMG) were studied by enzyme assay for ouabain-sensitive ATP hydrolysis, by quantitative densitometric scanning of Western blots, and by immunohistochemistry. To define the specific regulatory effects of various pituitary-dependent hormones on expression of Na+,K+-ATPase in the SMG, we treated hypophysectomized (hypox) male mice with triiodo-L-thyronine (T3), 5 alpha-dihydrotestosterone (DHT), dexamethasone (Dex), and aldosterone (Ald), injected singly or in combination. Na+,K+-ATPase was confined to the duct system of the SMG. In intact mice there was a gender difference in SMG Na+,K+-ATPase, with levels of the enzyme's activity and of its alpha 1-subunit being less in the glands of males. In males, hypophysectomy caused a rise in levels of Na+,K+-ATPase activity and in levels of the alpha 1-subunit protein of this enzyme, and in intensity of immunocytochemical staining for this subunit but there were no such changes in the SMG of hypox females. Changes caused by hormonal replacement to hypox males in Na+,K-ATPase activity, levels of its alpha 1-subunit, or the intensity of immunocytochemical staining for this subunit were complex. Ald had no effect. T3 or dexamethasone, given alone, induced Na+,K+-ATPase activity above control values (hypox males) and increased levels of its alpha 1-subunit protein and immunohistochemical staining for this subunit. By contrast, DHT did not cause a decline in any of these parameters. However, when treatment with T3 was combined with administration of Dex or DHT, enzymatic activity of Na+,K+-ATPase decreased but levels of the alpha 1-subunit protein and immunohistochemical staining for this subunit increased. Therefore, inductions of the alpha 1-subunit of this enzyme are not always correlated with increases in levels of activity of Na+,K+-ATPase, and we propose that both enzymatic and immunochemical analyses are essential for evaluation of hormonal regulation of Na+,K+-ATPase in salivary gland and in other tissues.

Cladinose analogues of sixteen-membered macrolide antibiotics. I. Synthesis of 4-O-alkyl-L-cladinose analogues via glycosylation

The synthesis and biological evaluation of sixteen-membered macrolides possessing a 4-O-alkyl-alpha-L-cladinosyl moiety as the neutral sugar are described. The nine novel derivatives have been synthesized by glycosylation with 1-thio sugars. The most active derivative of them showed prolonged antibacterial activity in rat plasma in vitro and improved pharmacokinetics.

Effects of phorbol ester on expression of CNTF-mRNA in cultured astrocytes from rat olfactory bulb

Ciliary neurotrophic factor (CNTF) is a neuropoietic cytokine which has various functions, such as survival promoting effect on both peripheral and central neurons, promotion of cholinergic differentiation, and participation in differentiation of Type-2 astrocytes (reviewed in ref. [30]). However, the regulatory mechanism of the CNTF expression is largely unknown. In this study, we analyzed the effects of phorbol 12-myristate 13-acetate (PMA), an activator of PKC, on the expression of CNTF-mRNA in cultured astrocytes from neonatal rat olfactory bulb. PMA induced a transient decrease of CNTF-mRNA levels which was followed by a persistent increase of the mRNA up to 4-fold of the control level at 24 h after the addition of the compound. Both the PMA-induced decrease and increase of the CNTF-mRNA levels were canceled by treatment with cycloheximide, an inhibitor of protein synthesis, suggesting that protein synthesis-dependent mechanisms participate in both the PMA-induced decrease and increase of CNTF-mRNA levels. On the other hand, PMA induced expressions of mRNAs of several subunit members of the AP-1 complex, such as c-fos, c-jun and jun-B. Furthermore, dexamethasone, a synthetic glucocorticoid which is known to inhibit the AP-1 complex-mediated transcription [14,27,36], canceled the PMA-induced decrease of the CNTF-mRNA levels. These results suggested that the AP-1 complex participates in the regulatory mechanism of the CNTF expression in the cultured astrocytes treated with PMA.

Characteristics of phosphatidic acid-containing lipoproteins which selectively inhibit bitter taste: high affinity to frog tongue surface and hydrophobic model membranes

In previous studies (Katsuragi and Kurihara (1993) Nature 365,213--214; Katsuragi et al. (1995) Pharm. Res. 12,658--662) we showed that a lipoprotein composed of phosphatidic acid (PA) and beta-lactoglobulin (LG) selectively suppressed the taste responses to bitter substances without affecting those to other taste stimuli in the frog and man, while complexes composed of other lipids except for phosphatidylserine and LG had little inhibitory activity. In the present study, we found that the lipoproteins having inhibitory activity are adsorbed on the frog tongue surface, while those having no inhibitory activity are not adsorbed. We also examined adsorption of the lipoproteins on model lipid membranes coated on a quartz-crystal microbalance by measuring changes in its frequency. The lipoproteins having inhibitory activity were well adsorbed on the hydrophobic lipid membranes, while the lipoproteins having no inhibitory activity were little adsorbed on the membranes. It seems that receptor sites for bitter substances on the taste cell membranes are hydrophobic and those for other taste stimuli such as salts, acids and sugars are hydrophilic. Hence, the binding of PA-LG to hydrophobic sites of the receptor membranes will lead to selective inhibition of bitterness.

Lipoprotein that selectively inhibits taste nerve responses to bitter substances

The development of a specific inhibitor for bitter taste has been widely required in the fields of taste physiology and pharmaceutical sciences, but no inhibitor has been available. We found that lipoproteins, PA-LG composed of phosphatidic acid (PA) and beta-lactoglobulin (LG) and PA-LA composed of PA and alpha-lactalbumin (LA) reversibly suppressed the responses of the frog glossopharyngeal nerve to the bitter substances. The frog tongue was treated with PA-LG solution for 10 min and then stimulated by a stimulus dissolved in water. The responses to the bitter substances such as quinine hydrochloride, papaverine hydrochloride, caffeine and L-leucine were completely suppressed by PA-LG, while those to the salt type bitter substances such as CsCl, MgCl2 and tetraethylammonium chloride were not suppressed. The responses to NaCl, galactose, acetic acid and L-alanine were unchanged or only slightly increased. The results suggested that binding of PA-LG to the hydrophobic region of the receptor membranes leads to suppression of the responses to the bitter substances. It was pointed out that PA-LG is useful not only for elucidating the receptor mechanisms of bitter substances, but also can be safely used to mask the bitter taste of foods and drugs, since PA, LG and LA are prepared from foods (soybean and milk).

Experimental free-space optical network for massively parallel computers

A free-space optical interconnection scheme is described for massively parallel processors based on the interconnection-cached network architecture. The optical network operates in a circuit-switching mode. Combined with a packet-switching operation among the circuit-switched optical channels, a high-bandwidth, low-latency network for massively parallel processing results. The design and assembly of a 64-channel experimental prototype is discussed, and operational results are presented.

Odor discrimination of "IP3-" and "cAMP-increasing" odorants in the turtle olfactory bulb

The ability of the turtle olfactory system to discriminate between various odorants that increase levels of adenosine 3',5'-cyclic monophosphate (cAMP) and inositol trisphosphate (IP3) in the olfactory bulb was examined by the cross-adaptation technique and analyzed by multidimensional scaling. The mean values of the degree of discrimination among the IP3-increasing odorants were higher than those among the cAMP-increasing odorants, and were similar to those between cAMP- and IP3-increasing odorants, suggesting that the features of the receptors of cAMP-increasing odorants are different from those which respond to IP3-increasing odorants. Analysis by multidimensional scaling suggested that differences in second messenger pathways are not related to detecting odor quality in the turtle olfactory system.

Olfactory responses of a euryhaline fish, the rainbow trout: adaptation of olfactory receptors to sea water and salt-dependence of their responses to amino acids

Salmonid fishes are able to survive in both fresh water and sea water. Concentrations of NaCl in fresh water and sea water are 0.5 and 493 mmol l-1, respectively, and, hence, salt concentrations in the medium at the olfactory epithelium are greatly changed when the fish migrate between fresh water and sea water. We used the rainbow trout, which is a salmonid fish, to examine the adaptation mechanisms of the olfactory receptors to high concentrations of salts in sea water. Application of sea water to the olfactory epithelium elicited only a very small response in the olfactory nerve, but 500 mmol l-1 NaCl elicited a large response which did not adapt to a spontaneous level with time. It is considered that the olfactory nerve becomes fatigued when the olfactory epithelium is exposed to 500 mmol l-1 NaCl for long periods. We found that the presence of 10 mmol l-1 Ca2+ in sea water inhibited the response to 500 mmol l-1 NaCl, suggesting that Ca2+ in sea water is essential for adaptation of the olfactory receptors to sea water. In the second part of the study, we examined whether the olfactory nerve responses to amino acids, potent stimulants for fish, were altered between fresh water and sea water. The magnitudes of the responses to the six amino acids examined were similar in artificial pond water and artificial sea water, indicating that a large change in NaCl concentration between fresh and sea water does not affect the olfactory nerve responses to amino acids. We used fish reared in fresh water and fish acclimated to sea water and obtained similar results. It was concluded that the tolerance of the olfactory receptors for large changes in osmotic pressure is not acquired while fish are maintained in fresh or sea water, but that the receptors of these euryhaline fish naturally provide the tolerance.

Intracellular dialysis of cyclic nucleotides induces inward currents in turtle vomeronasal receptor neurons

Turtle vomeronasal receptor neurons in slice preparations were studied using the patch-clamp technique in the whole-cell and cell-attached configurations. The mean resting potential was -48, and the response to an injected current step consisted of either a single spike or a train of spikes. An injected current of 3-30 pA was required to depolarize the neuron to spike threshold near -50 mV. Voltage-clamped vomeronasal receptor neurons displayed transient inward currents followed by sustained outward currents in response to depolarizing voltage steps. In cell-attached recordings, 10 microM forskolin added to the bath caused a transient increase of spike rate. Intracellular application of cAMP evoked ann inward current in a dose-dependent manner from the neurons voltage clamped at -70 mV; 0.1 mM cAMP was sufficient to elicit an inward current in the neurons. The magnitude of the response to cAMP reached a plateau at 1 mM with an average peak amplitude of 176 +/- 34 pA. Intracellular application of 1 mM cGMP also evoked an inward current with an average peak amplitude of 227 +/- 61 pA. The reversal potentials of the induced components were estimated to be 10 +/- 7 mV for cAMP and -4 +/- 16 mV for cGMP. The reversal potential of the cAMP-induced current in external Cl(-)-free solution was similar to that in normal Ringer's solution, suggesting that Cl- current is not significantly involved in the current. The present results represent the first evidence of cyclic nucleotide-activated conductance in the vomeronasal receptor membranes.

ATP- and EGF-stimulated phosphatidulinositol synthesis by two different pathways, phospholipase D and diacylglycerol kinase, in A-431 epidermoid carcinoma cells

The [(3)H]inositol incorporation into the membrane fraction of A-431 human epidermoid carcinoma cells was markedly increased by stimulation of the cells with either epidermal growth factor (EGF), ATP, bradykinin, or a calcium ionophore A23187 in the presence of 1 mM extracellular calcium ions; most incorporated [(3)H]inositol was found to have accumulated as phosphatidylinositol (PI). The EGF- and ATP-stimulated PI synthesis was inhibited by two protein kinase C inhibitors, staurosporine and 1-(5-isoquinolinesulfonyl)-2-methylpiperazine dihydrochloride (H-7), and an intracellular calcium chelator, 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid acetoxymethyl ester (BAPTA/AM), but not by the calmodulin antagonist N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide hydrochloride (W-7). Pretreatment of cells with pertussis toxin (IAP, islet-activating protein) inhibited the PI synthesis, [Ca(2+)]i elevation, and inositol trisphosphate (IP(3)) production by ATP, suggesting that the phospholipase C(PLC) system coupled with IAP-sensitive G protein is involved in the ATP-stimulated PI synthesis. On the other hand, the ATP stimulation increased the release of [(3)H]choline and [(32)P)phosphatidic acid (PA) from radiolabeled cells, and such release was not inhibited by IAP. In the presence of n-butyl alcohol, which prevents the production of PA by generation of phosphatidylbutanol, the ATP-stimulated PI synthesis was reduced. Because n-butyl alcohol did not inhibit IP(3) production and [Ca(2+)]i elevation, this fact suggests that the lAP-insensitive PLD system is involved in the ATP-stimulated PI synthesis. In A-431 cells, the stimulation of P(2)-purinergic receptors appears to activate the IAP-sensitive PLC system and IAP-insensitive PLD system, both of which are essential for the stimulation of PI synthesis. The present results imply the general prospect that ligand stimulation, which mobilizes second messengers and consumes their precursors, simultaneously provokes the pathway to synthesize and salvage the second messenger precursors as well.

Identification of novel guanylyl cyclases from chemosensory tissues of rat and cattle

A number of studies have shown that cGMP may play some roles in chemosensory transduction. To identify the structure of guanylyl cyclase in chemosensory tissues, cDNA fragments encoding guanylyl cyclase catalytic domain were amplified from rat and bovine olfactory and tongue epithelium using degenerate oligonucleotide primers and reverse transcription-polymerase chain reaction (RT-PCR). Three novel clones, two membrane type guanylyl cyclases (RAT GC-1, BOV GC-3) and one soluble type guanylyl cyclase (RAT GC-2) were identified. RAT GC-1 was distributed over various rat tissues in addition to these chemosensory organs. BOV GC-3 was similar to but distinct from recent cloned olfactory-specific guanylyl cyclase. RAT GC-2 was identified as rat homologue of alpha 2 subunit of the soluble guanylyl cyclase.

Involvement of direct phosphorylation in the regulation of the rat parotid Na(+)-K(+)-2Cl- cotransporter

We identify a 175-kDa membrane phosphoprotein (pp175) in rat parotid acini whose properties correlate well with the Na(+)-K(+)-2Cl- cotransporter previously characterized functionally and biochemically in this tissue. pp175 was the only phosphoprotein immunoprecipitated by an anti-Na(+)-K(+)-2Cl- cotransporter antibody and the only membrane protein whose phosphorylation state was conspicuously altered after a brief (45-s) exposure of acini to the beta-adrenergic agonist isoproterenol. Phosphopeptide mapping provided evidence for three phosphorylation sites on pp175, only one of which was labeled in response to isoproterenol treatment. The half-maximal effect of isoproterenol on phosphorylation of pp175 (approximately 20 nM) was in excellent agreement with its previously demonstrated up-regulatory effect on cotransport activity. Increased phosphorylation of pp175 was also seen following acinar treatment with a permeant cAMP analogue and with forskolin, conditions that have likewise been shown to up-regulate the cotransporter. Combined with earlier results from our laboratory, these data provide strong evidence that the up-regulation of the cotransporter by these agents is due to direct phosphorylation mediated by protein kinase A. AlF(-)4 treatment, which results in an up-regulation of cotransport activity comparable with that observed with isoproterenol (approximately 6-fold), caused a similar increase in phosphorylation of pp175. However, hypertonic shrinkage and treatment with the protein phosphatase inhibitor calyculin A, which also up-regulate the cotransporter (approximately 3-fold and approximately 6-fold, respectively) caused no change in the phosphorylation level. Furthermore, although acinar treatment with the muscarinic agonist carbachol results in a dramatic up-regulation of cotransport activity and a concomitant phosphorylation of pp175, no phosphorylation of pp175 was seen with the Ca(2+)-mobilizing agent thapsigargin, which is able to fully mimic the up-regulatory effect of carbachol on transport activity. Taken together, these results indicate that direct phosphorylation is only one of the mechanisms involved in secretagogue-induced regulation of the rat parotid Na(+)-K(+)-2Cl- cotransporter.

Effects of protein kinase inhibitors and protein phosphatase inhibitors on cyclic AMP-dependent down-regulation of vesicular monoamine transport in pheochromocytoma PC12 cells

Cyclic AMP down-regulates vesicular monoamine transport in PC12 cells and thereby decreased catecholamine reuptake from the extracellular fluid. We examined the effects of protein kinase inhibitors and protein phosphatase inhibitors on this cAMP action. Treatment of cells with a protein kinase inhibitor, K252a, increased vesicular amine transport and cellular amine uptake, thereby antagonizing the regulatory action of cAMP. In contrast, a protein phosphatase inhibitor, okadaic acid, had the opposite effect on the amine transport, i.e. it enhanced the cAMP action. These results suggest the involvement of a protein phosphorylation process in the cAMP-dependent modulation of vesicular monoamine transport.

Odor responses after complete desensitization of the cAMP-dependent pathway in turtle olfactory cells

The degree of contribution of the cAMP signal transduction pathway to odor responses was examined by recording current responses from isolated turtle olfactory cells under the whole-cell voltage clamp conditions. The cAMP signal transduction pathway was desensitized by dialyzing 1 mM cAMP and 0.5 mM IBMX from the patch pipette into the cells. Extracellular application of 3 mM cpt-cAMP, a membrane-permeable cAMP analogue elicited no response, indicating that the cAMP pathway was completely desensitized. Application of an odorant cocktail induced a large inward current under these conditions, suggesting that the cAMP-independent signal transduction contributes significantly to generation of odor responses in the turtle.

Selective inhibition of bitter taste of various drugs by lipoprotein

Previously, we demonstrated that lipoprotein composed of phosphatidic acid (PA) and beta-lactoglobulin (LG) selectively and reversibly suppress the frog taste nerve response to bitter substances. In the present study, we examined the effects of various lipoproteins on the taste sensation to various stimuli in humans by a psychophysical method. Among various lipoproteins composed of different of lipids and proteins, the lipoproteins composed of PA and proteins were most effective in suppressing bitter taste. The lipoproteins composed of PA and LG, bovine serum albumin, ovalbumin, alpha-lactoalbumin or casein similarly suppressed effects on sensation of bitter taste. Using PA-LG, the effects on taste sensation to various stimuli were examined. The bitter taste of all twelve substances examined was inhibited, while saltiness of NaCl and sweetness of sucrose were not inhibited. The inhibition of bitter taste was completely reversible. Masking of the target sites for bitter substances on the taste receptor membranes with PA-LG seems to contribute to the inhibition of bitter taste. Direct binding of the bitter substances to PA-LG in the medium also contributes to the inhibition of bitter taste of certain substances. Among various drugs, basic and hydrophobic substances such as quinine, denatortium and propranolol have low taste thresholds and are said to be the most bitter. PA-LG most effectively suppressed the bitter taste of such substances. PA originates from soybeans and the proteins used except for bovine serum albumin originate from milk or eggs, and hence the lipoproteins can be safely used to mask the bitter taste of drugs.

Effects of adenylyl cyclase-linked neuropeptides on the expression of ciliary neurotrophic factor-mRNA in cultured astrocytes

Ciliary neurotrophic factor (CNTF) is a molecule which has profound effects on various neural cell types. In the central nervous system, expression of CNTF-mRNA is highly concentrated in olfactory bulb. In the present study, we examined the regulatory mechanism of CNTF-mRNA expression in cultured astrocytes from newborn rat brain. Cultured astrocytes from new born rat brain expressed CNTF-mRNA at levels comparable to the level in olfactory bulb in vivo. Treatment of the astrocytes with forskolin, an activator of adenylyl cyclase, led to a decrease of CNTF-mRNA level. The effect of forskolin was mimicked by cAMP-linked agonists, such as VIP, PACAP, isoproterenol and dopamine. Cycloheximide, an inhibitor of protein synthesis, did not abolish the forskolin-induced decrease of CNTF-mRNA. Measurement of the half-life of CNTF-mRNA in the presence of actinomycin D, an inhibitor of transcription, indicated that the degradation of CNTF-mRNA is not destabilized by the forskolin-treatment. These data taken together suggest that the cAMP-induced suppression of CNTF-mRNA is mainly caused by the inhibition of CNTF gene transcription.

Intracellular injection of inositol 1,4,5-trisphosphate increases a conductance in membranes of turtle vomeronasal receptor neurons in the slice preparation

Inositol 1,4,5-trisphosphate (IP3) was injected into turtle vomeronasal receptor neurons in the slice preparation under a whole-cell patch clamp, and the evoked current was measured. Application of 0.1 mM IP3 evoked a prolonged, inward current (52 of 98 neurons) with an average peak amplitude of 89.9 +/- 10.9 pA. The reversal potential of the response induced by IP3 was estimated to be -32.3 +/- 1.5 mV (6 neurons). Bathing the neurons in 10 microM ruthenium red solution greatly reduced the IP3 evoked inward current to 18.0 +/- 4.6 pA (5 neurons). This is the first study to demonstrate that the membranes of the turtle vomeronasal neurons carry IP3-activated conductance.

Proliferative vitreoretinopathy in Coats' disease. Clinicohistopathological case report

We report the clinicohistopathological findings of the proliferative vitreoretinopathy in an eye with Coats' disease using the tissue obtained during surgery. A 28-year-old man, who had experienced poor vision in his right eye for a period of 4 years, was referred to our hospital. Examination revealed an extensive yellow exudate in the subretinal space and a tractional retinal detachment. Prominent teleangiectatic retinal vessels were also temporally present. Evident epiretinal membranes were present in the postequatorial area. We performed a scleral buckling, vitrectomy, membrane peeling, endophotocoagulation and silicone oil tamponade. A histological examination revealed that the epiretinal membrane consisted of collagen fibers, glial proliferation, foam cells and lymphocytes. The foam cell in the epiretinal membrane is a characteristic finding in an eye with Coats' disease.

High sensitivity of the turtle olfactory system to nonvolatile substances: comparison of response properties with those in gustatory systems

The olfactory responses of the aquatic turtles, Geocylemys reevesii whose nostrils are closed underwater to nonvolatile substances were measured by recording the olfactory bulbar responses. Various salts, acids and bitter substances elicited large responses, while sugars and amino acids did not elicit the responses. The thresholds for the salts were much lower than those of corresponding salts in the rat gustatory system. The responses to the salts were partially suppressed by amiloride. Various acids induced large responses and the magnitudes greatly depended on the anion species. The thresholds for the bitter substances were much lower than those of corresponding substances in the taste systems. Similar to the responses in taste systems, both electrostatic and hydrophobic interactions contribute to the binding of the substances to the receptor membranes. Similar to the taste systems, the response to quinine hydrochloride showed a sharp temperature dependence having a peak around 25 degrees C, while the responses to odorants did not show such peak. The present results suggest that the olfactory system has similar abilities to respond to salts, acids and bitter substances to those in gustatory systems and that the high sensitivity of the olfactory system to chemical stimuli is not only attributable to the second messenger amplification system, but also to the basic property of the receptor membrane independent of the amplification system.

Retinoic acid induces BDNF responsiveness of sympathetic neurons by alteration of Trk neurotrophin receptor expression

The expression of high affinity neurotrophin receptors (TrkA, TrkB, and TrkC) determines the survival response of different populations of neurons to specific members of the neurotrophin family, including nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), and neurotrophin-3 (NT-3). However, the mechanism which controls the expression of neurotrophin receptors during neuronal development is largely unknown. Here we show that the treatment of the cultured sympathetic neurons from newborn rat superior cervical ganglia (SCG) with retinoic acid (RA), a derivative of vitamin A, suppressed the expression of trkA mRNA and induced the expression of trkB mRNA. Expression of the functional TrkB receptor was confirmed by the emergence of trophic dependence of these neurons on BDNF in the absence of NGF. Differential regulation of trk mRNAs by RA provides a possible model for the establishment of neurotrophin dependence of peripheral neurons.

Receptor mechanisms of bitter substances

The receptor mechanism of bitter substances was discussed from the following points of views. (a) Both electrostatic and hydrophobic interactions of bitter substances with taste receptor membranes contribute to reception of bitter substances having a positive charge. (b) In the frog, the responses to bitter substances are easily adapted. The presence of Ca ion in the medium prolongs the responses. (c) Bitter substances elicit electrical responses in nongustatory cells such as neuroblastoma cells and olfactory cells, suggesting that bitter substances induce the response by nonreceptor-mediated mechanism. (d) There is also a possibility that receptors for some bitter substances are G-protein coupled. We cloned G-protein coupled receptors from bovine taste tissues. (e) A specific inhibitor of bitter taste has been desired in pharmaceutical and food sciences, but it has not been available. We found that a lipoprotein made of phosphatidic acid and beta-lactoglobulin selectively inhibits the responses to bitter substances in the frog and humans. Binding of the lipoprotein to the receptor sites for bitter substances leads to suppression of the response.