The effect of dibutyryl cAMP on tetrodotoxin-sensitive and -resistant voltage-gated sodium currents in rat dorsal root ganglion neurons and the consequences for their sensitivity to lidocaine

Tetrodotoxin-sensitive (TTXS) sodium currents in dorsal root ganglia (DRG) neurons were enhanced by DcAMP applied acutely or by pre-treatment. Pre-treatment increased peak TTXS by 28%. This compared to the increase of tetrodotoxin-resistant sodium currents (TTXR) of 123%. In both cases the increase was associated with a hyperpolarizing shift in activation potentials. Slow inactivation was slower for both TTXR and TTXS in DcAMP treated neurons but rates of recovery from inactivation were not altered. Lidocaine blocked TTX-R with an IC(50) of 0.51+/-0.15mM (n=9) which was reduced to 0.14+/-0.05mM (n=8, P<0.05) in DcAMP treated cells. The sensitivity of TTX-S currents to lidocaine was not altered by DcAMP (control EC(50)=0.89+/-0.16mM, n=9; DcAMP EC(50)=0.73+/-0.19mM, n=6). It is concluded that TTXS currents in DRG are, like TTX-R currents, enhanced by cAMP but whilst the pharmacology of TTXR channels with respect to lidocaine is altered, that to TTXS channels is not.

The use of the rat isolated vagus nerve for functional measurements of the effect of drugs in vitro

In this article we describe how to dissect, set up and use the rat isolated vagus nerve in a 'grease gap' apparatus which provides a simple and practical method for measuring the effects of drugs on the membrane potential of axons in the nerve in vitro. Some discussion of the origins and development of the technique as well as the strengths and disadvantages of the preparation as a neuropharmacological tool are included. The vagus nerve conducts action potentials in at least three distinct types of axons that can be measured extracellularly as compound action potentials and distinguished on the basis of their conduction velocity and excitability. Activity in myelinated A fibres and unmyelinated C fibres can be measured separately easily. The axons express receptors for a wide range of putative neurotransmitter agents including 5-HT, GABA and ATP as well as other agents such as capsaicin, anandamide, bradykinin and prostanoids. Responses to all of these chemicals can be measured as a depolarization of the nerve fibres. The vagus nerve is an important target for a wide range of drugs and the isolated preparation provides a fairly simple preparation for studying their effects. The isolated vagus nerve is also a convenient system in which the effects of drugs that have been discovered using heterologous expression systems can be assayed on receptors and ion channels that are expressed in a native neural system.

Differential sensitivity to tetrodotoxin and lack of effect of prostaglandin E2 on the pharmacology and physiology of propagated action potentials

1. We have studied the effects of prostaglandin E(2) (PGE(2)) on action potential propagation in the isolated, desheathed vagus and saphenous nerves of rats using an extracellular grease gap recording method. 2. PGE(2) evoked a small depolarization of vagus nerves but had no effect on the stimulation threshold, size or latency of either the A wave (corresponding to conduction in A fibres) or the C wave (corresponding to conduction in C fibres) of the compound action potential (CAP) recorded from either vagus or saphenous nerves. 3. Lidocaine (0.01 - 10 mM) reduced all components of the CAP of both vagus and saphenous nerves. PGE(2) had no significant effect on the sensitivity of any component of the CAP to lidocaine. 4. Tetrodotoxin (TTX, 10 microM) blocked completely both the A wave and the C wave of the CAP in either vagus or saphenous nerves. 5. In saphenous nerve preparations the A wave was blocked by lower concentrations of TTX than the C wave or any component of the CAP in vagus nerve preparations which suggests that somatosensory A fibres express a different sub-type of TTX-sensitive voltage-gated sodium channel (VGSC) than somatosensory C-fibres or visceral sensory fibres. 6. Chemical activation of VGSCs with veratridine (10 or 50 microM) induced a depolarization in either nerve. The depolarization induced by 50 microM veratridine was blocked by 10 microM TTX. 7. Although TTX-insensitive VGSCs are expressed by some vagal and some somatosensory neurones they do not appear to be expressed functionally in the axons.

Indirect actions of bradykinin on neonatal rat dorsal root ganglion neurones: a role for non-neuronal cells as nociceptors

1. In this study we have investigated the action of bradykinin (Bk) on cultured neonatal rat dorsal root ganglion (DRG) cells, with the aim of elucidating whether the neuronal response to Bk is influenced by association with non-neuronal satellite cells. 2. Bradykinin (100 nM) evoked an inward current (I(Bk)) in 51 of 58 voltage clamped DRG neurones (holding potential (V(h)) = -80 mV) that were in contact with non-neuronal satellite cells. 3. Bradykinin failed to evoke an inward current in isolated DRG neurones (V(h) = -80 mV) that were not in contact with non-neuronal satellite cells (n = 41). 4. The lack of neuronal response to Bk was not influenced by time in culture. Bradykinin failed to evoke a response in isolated neurones through 1-5 days in culture. By contrast neurones in contact with satellite cells responded to Bk throughout the same time period. 5. Failure of isolated neurones to respond to Bk was not due to the replating procedure or to selective subcellular distribution of receptors/ion channels to the processes rather than the somata of neurones. 6. Using Indo-1 AM microfluorimetry Bk (100 nM) was demonstrated to evoke an intracellular Ca(2+) increase (Ca(Bk)) in DRG neurones in contact with non-neuronal satellite cells and in isolated neurones. 7. These data suggest that the inward current response to Bk requires contact between DRG neurones and non-neuronal satellite cells. This implies an indirect mechanism of action for Bk via the non-neuronal cells, which may perform a nociceptive role. However, Bk can also act directly on the neurones, since it evokes Ca(Bk) in isolated neurones. The relationship between Ca(Bk) and the Bk-induced inward current is unknown at present.

Bradykinin evokes a Ca2+-activated chloride current in non-neuronal cells isolated from neonatal rat dorsal root ganglia

We have studied the effect of bradykinin (Bk) on fibroblast-like satellite (FLS) cells isolated from cultures of neonatal rat dorsal root ganglia (DRG). In voltage-clamped FLS cells Bk evoked an inward current response that was concentration dependent with a half-maximal concentration of 2 nM. In indo-1 AM-loaded FLS cells Bk evoked a rise in intracellular Ca2+ that was concentration dependent with a half-maximal concentration of 1 nM. The FLS cells still produced an inward current in response to Bk in the absence of extracellular Ca2+ but the response was inhibited if the intracellular concentration of EGTA was increased from 0.5 to 5 mM, which suggests that the inward current was dependent on the release and subsequent rise of intracellular Ca2+. The reversal potential of the Bk-induced inward current was consistent with the current being due to an increase in Cl- conductance and shifted in a Nernstian manner when the intracellular Cl- concentration was reduced. The inward current response to Bk was blocked by the B2 receptor antagonist HOE-140, which indicates that the response was due to activation of B2 receptors. The data suggest that Bk evokes a rise in intracellular Ca2+ and activation of a Ca2+-activated Cl- conductance in the FLS cells and raise the possibility that FLS cells contribute to the pro-inflammatory effects of Bk in DRG.

One-way cross-desensitization between P2X purinoceptors and vanilloid receptors in adult rat dorsal root ganglion neurones

1. Capsaicin and ATP can activate ligand-gated cation channels in nociceptive rat dorsal root ganglion (DRG) neurones. We have studied cross-desensitization between these two agents in rat isolated DRG neurones using the whole-cell voltage-clamp technique. 2. ATP (10 microM) activated an inward current in DRG neurones at a holding potential of -60 mV. ATP evoked 'fast' responses that underwent rapid activation and desensitization, 'slow' responses that activated and desensitized more slowly, or responses that displayed a mixture of these two characteristics. The time course of the response to ATP was not related obviously to capsaicin sensitivity. 3. Prior application of capsaicin (0.5 microM) increased the proportion of cells displaying only fast responses to ATP (10 microM) suggesting that cross-desensitization had occurred between capsaicin and the slow component of the ATP response. Prior desensitization to ATP had no apparent effect on the inward current response to capsaicin (0.5 microM). 4. Cross-desensitization between capsaicin and ATP was Ca2+ dependent. 5. Changing the membrane holding potential (Vh) to +40 mV for brief period before applying ATP at -60 mV had a similar effect to capsaicin, i.e. the proportion of cells displaying only fast responses to ATP was increased significantly. This effect of depolarization was not Ca2+ dependent. 6. The heterogenity of responses to ATP is probably due to co-expression of homomeric P2X3 receptors and heteromeric receptors comprising P2X3 subunits with other P2X subunits. We propose that the change in time course of the ATP response produced by prior desensitization to capsaicin is due to selective cross-desensitization with the heteromeric P2X receptors.

A study of the voltage dependence of capsaicin-activated membrane currents in rat sensory neurones before and after acute desensitization

1. Responses to capsaicin in isolated sensory neurones have been shown to desensitize in a Ca2+- and voltage-dependent manner. We have studied desensitization of capsaicin-activated currents in cultured adult rat dorsal root ganglion (DRG) neurones over a range of membrane potentials using whole-cell patch-clamp techniques. 2. Acute desensitization of responses to capsaicin (0.5 microM) was significantly less when the holding potential (Vh) was +40 mV rather than -60 mV. This was not due only to reduced Ca2+ entry as the response to capsaicin was desensitized by the same amount whether prior exposure to capsaicin was at -60 or +40 mV. The I-V relationship for capsaicin-induced current, determined using a voltage step protocol, was outwardly rectifying and during the acute phase of desensitization the degree of outward rectification increased. 3. Acute desensitization and the increase in outward rectification that accompanied desensitization were inhibited when cells were dialysed with the rapid Ca2+ chelator BAPTA. Addition of a pseudosubstrate inhibitor of the Ca2+-calmodulin-dependent enzyme calcineurin (CI, 100 microM) prevented the increase in outward rectification although it did not cause a significant decrease of acute desensitization. 4. Removal of external Ca2+ or Mg2+ did not reverse the increase in outward rectification of capsaicin-activated current after Ca2+-dependent desensitization had occurred. This indicates that a voltage-dependent block of the capsaicin-activated ion channel by Ca2+ or Mg2+ was not responsible for the observed changes in the properties of the capsaicin-activated conductance.

Molecular characterisation of cloned bradykinin B1 receptors from rat and human

This report describes the characterisation of cloned rat and human bradykinin B1 receptors in African green monkey kidney fibroblast (Cos-7) cells. A ligand binding assay with [3H]des-Arg10-kallidin was used to compare their pharmacology with respect to known bradykinin B1 and B2 receptor ligands. In addition, the pharmacology of T-kinin and its' derivative des-Arg11-T-kinin was investigated. The cloned rat receptor had a similar pharmacology to that of the recently described mouse receptor and differs from that described for the human receptor. The rat receptor had a higher affinity for des-Arg11-T-kinin than the human receptor. These differences in pharmacological properties may relate to the presence of T-kinin, bradykinin and their des-Arg derivatives as the major physiological peptides in rat and the predominance of kallidin and its derivatives in human. We confirm that the rat bradykinin B1 receptor gene is organised in a two exon structure and differs from the human gene which has a three exon structure and we further examine the inducible expression of this gene in a wide range of tissues using Northern blotting.

A comparison of the effects of capsaicin with inhibitory nerve stimulation in the rat anococcygeus muscle in vitro

Capsaicin was used to test whether centrifugal activation of sensory fibres in the rat anococcygeus muscle can contribute to non-adrenergic non-cholinergic (NANC) relaxation of the muscle. In a solution containing 0.5 mM Ca2+ and in the presence of carbachol (10 microM) capsaicin evoked a fast concentration-dependent relaxation of the muscle that was usually followed by a smaller, slower, relaxant response. The fast relaxant response was reduced when extracellular Ca2+ was raised to 2.5 mM, desensitized after a single application of capsaicin and was blocked by tetrodotoxin (1 microM) or ruthenium red (10 microM). The fast response was greatly reduced by haemoglobin, by cold storage of the muscles or by N-monomethyl-L-arginine (100 microM) in the absence but not in the presence of L-arginine (100 microM). It is concluded that centrifugal activation of sensory fibres evokes a nitric oxide-mediated relaxation of the anococcygeus muscles that probably contributes to electrically evoked NANC relaxation.

Capsazepine block of voltage-activated calcium channels in adult rat dorsal root ganglion neurones in culture

1. We have found that capsazepine, a competitive antagonist at the vanilloid (capsaicin) receptor, blocks voltage-activated calcium currents in sensory neurones. 2. The block of calcium current was slow to develop with a half time of about one minute at 100 microM and lasted for the duration of the experiment. The rate of block of calcium current was strongly concentration-dependent. 3. The EC50 for the blocking effect at 0 mV was 7.7 +/- 1.4 microM after 6 min exposure to capsazepine. The EC50 at equilibrium was estimated to be 1.4 +/- 0.2 microM. 4. The block of calcium current showed some voltage-dependence but there was no indication of any selectivity of action for a calcium channel subtype. The characteristics of the blocking action of capsazepine on the residual current of cells which were pretreated with either omega-conotoxin or nimodipine were similar to control. 5. The data suggest that capsazepine, in addition to its competitive antagonism of vanilloid receptors, has a non-specific blocking action on voltage-activated calcium channels which should be taken into account when interpreting the effects of this substance on intact preparations in vitro or in vivo.

PGE2 modulates the tetrodotoxin-resistant sodium current in neonatal rat dorsal root ganglion neurones via the cyclic AMP-protein kinase A cascade

1. In current-clamp recordings, 1 microM prostaglandin E2 (PGE2) increased the excitability of neonatal rat dorsal root ganglion neurones. The current threshold for firing was reduced, and the response to a constant suprathreshold stimulation was modified such that a single evoked action potential was converted to a train of action potentials. The excitatory action of PGE2 was still apparent when action potentials were evoked in the presence of 500 nM tetrodotoxin. 2. In voltage-clamp experiments 1 microM PGE2 frequently increased the magnitude of the peak currents recorded, and caused a hyperpolarizing shift (of approximately 6 mV) in the activation curve for the tetrodotoxin-resistant sodium current (TTX-R INa). In some cells, the hyperpolarizing shift in the activation curve was accompanied by a decrease in peak conductance. PGE2 also caused a hyperpolarizing shift in the steady-state inactivation curve for the sodium current. 3. Extracellular application of the cAMP analogue dibutyryl cAMP (dbcAMP) at a concentration of 1 mM produced effects on both the current-voltage relationship and the steady-state inactivation curve for the TTX-R INa which were indistinguishable from those observed with PGE2. Prior exposure of the neurones to dbcAMP occluded the effect of a subsequent treatment with PGE2. 4. Forskolin (10 microM), a direct activator of adenylate cyclase, mimicked the effects of PGE2 and dbcAMP on TTX-R INa. The inactive congener of forskolin, 1, 9-dideoxyforskolin (10 microM), reduced the amplitude of TTX-R INa, but did not evoke a hyperpolarizing shift in the activation curve. 5. Intracellular perfusion of the neurones with an inhibitor of protein kinase A inhibited the effect of PGE2 on TTX-R INa. 6. PGE2 also reduced the amplitude of voltage-gated potassium currents (IK), which will contribute to the excitatory action. The mechanisms underlying the changes in IK have yet to be elucidated. 7. We propose that the PGE2-mediated increase in excitability in sensory neurones may be due, at least in part, to the cAMP-protein kinase A-dependent modulation of the tetrodotoxin-resistant sodium channel.

Modulation by calcineurin of 5-HT3 receptor function in NG108-15 neuroblastoma x glioma cells

1. We have investigated the mechanism of regulation of 5-HT3 receptor channel sensitivity in voltage-clamped (-80 mV) NG108-15 neuroblastoma cells. 2. The 5-HT-induced inward current activated rapidly. The fast onset was followed by a biphasic decay which was characterized by two time constants, tau 1 (1.1 +/- 0.21s) and tau 2 (8.9 +/- 1.6s), respectively. Brief applications of 5-HT, applied at 2 min intervals, induced a decrease in the amplitude of the 5-HT3 receptor-mediated peak inward currents. 3. Buffering of intracellular calcium with the calcium chelator BAPTA (10 mM) instead of EGTA (10 mM) attenuated the 5-HT-induced loss of responsiveness of 5-HT3 receptors. Omission of calcium from the extracellular medium yielded a similar attenuation of loss of responsiveness. 4. Inclusion of the protein kinase inhibitor, staurosporine (1 microM) or of okadaic acid (1 microM), an inhibitor of protein phosphatases 1 and 2A, in the intracellular buffer solution did not affect 5-HT3 receptor sensitivity. 5. Injection of cyclosporin A-cyclophilin A complex (20 nM), which potently inhibits calcineurin, did not affect the time constants of the biphasic decay of the 5-HT response tau 1 (1.4 +/- 0.28s) and tau 2 (11.3 +/- 1.7s). The complex, however, prevented the loss of 5-HT3, receptor responsiveness upon repeated application of 5-HT. A similar, but weaker effect was observed after intracellular application of the autoinhibitory peptide domain of calcineurin (1 microM). 6. The recovery of desensitized 5-HT3 receptors upon a second application of 5-HT (1 microM) showed a half-life time (tau 1/2) of 2.6 +/- 0.12 min in control cells which was reduced to 1.6 +/- 0.09 min in cells treated with cyclosporin A-cyclophilin A (20 nM) complex. 7. We conclude that calcineurin does not affect the fast decay of the 5-HT3 receptor response but may be involved in a slower process which regulates channel activity.

Inhibition of calcineurin inhibits the desensitization of capsaicin-evoked currents in cultured dorsal root ganglion neurones from adult rats

Capsaicin activates a non-specific cation conductance in mammalian sensory neurones. If capsaicin is applied continuously or repeatedly then there is a progressive decline in responsiveness. We have studied the mechanism of this desensitization using electrophysiological methods in cultured dorsal root ganglion neurones from adult rats. The rate of desensitization of capsaicin-induced responses is partly dependent on the extracellular calcium concentration and is slower when extracellular calcium is reduced. Desensitization is strongly inhibited by intracellular application of the calcium chelator 1,2-bis(2-aminophenoxy)ethane-N, N, N',N'-tetraacetic acid (BAPTA). These data suggest that desensitization is due to a rapid rise in intracellular calcium levels which occurs when capsaicin-sensitive ion channels are activated. Desensitization is not reduced by the non-specific protein kinase inhibitors H7 or staurosporine or by okadaic acid, a selective inhibitor of protein phosphatases 1 and 2A. Desensitization is greatly reduced by cyclosporin A complexed to cyclophilin, which is a specific inhibitor of protein phoshatase 2B (calcineurin). A mechanism for desensitization of capsaicin responsiveness is proposed whereby the evoked rise in calcium activates calcineurin leading to dephosphorylation and desensitization of the capsaicin-sensitive ion channels.

Expression of tetrodotoxin-resistant sodium channels in capsaicin-sensitive dorsal root ganglion neurons of adult rats

We have used conventional whole-cell electrophysiological recording techniques to measure the level of expression of tetrodotoxin-sensitive (TTX-S) and tetrodotoxin-resistant (TTX-R) sodium currents in the capsaicin-sensitive and capsaicin-insensitive subpopulations of dorsal root ganglion neurons. We find that there are significantly higher levels of TTX-R expressed in the capsaicin-sensitive group but no difference in the levels of TTX-S. Since capsaicin is a selective stimulant of polymodal nociceptors, these data raise the possibility that TTX-R sodium channels may be an important determinant of nociceptor phenotype.

Capsaicin causes prolonged inhibition of voltage-activated calcium currents in adult rat dorsal root ganglion neurons in culture

The effect of capsaicin on voltage-activated calcium currents was investigated in voltage-clamped somata of cultured adult rat dorsal root ganglion neurons. About half the neurons studied were sensitive to capsaicin, which induced an inward current at negative membrane potentials accompanied by an increase in membrane conductance. In the sensitive neurons capsaicin inhibited voltage-activated calcium current to an extent that depended on the size and duration of the capsaicin-induced inward current. Calcium channels were protected from the long-lasting inhibitory action of capsaicin by substituting extracellular Ca with Mg or Ba when capsaicin was applied, which suggests that the inhibition was dependent on the presence of extracellular Ca. Substituting Ca with Co did not prevent the prolonged block of calcium channels. It is concluded that the inhibition of voltage-activated calcium currents by capsaicin is secondary to increased intracellular Ca levels due to calcium entry through capsaicin-activated cation-specific ion channels in the plasma membrane. Long-lasting inhibition of voltage-activated calcium channels may contribute to the mechanism of the analgesic and anti-inflammatory effects of capsaicin through inhibition of neurotransmitter release from central and peripheral terminals of primary afferent nociceptive neurons.

Noradrenaline-Induced Inhibition of Voltage-Sensitive Calcium Currents in NG108-15 Hybrid Cells

The effect of noradrenaline (NA) on voltage-sensitive calcium currents in neuroblastoma x glioma hybrid (NG108-15) cells has been studied using a whole-cell clamp technique. NA inhibited calcium current. The EC50 for NA induced inhibition was 177 nM. The NA receptor involved had a similar pharmacological profile to alpha2 adrenoreceptors but did not respond to clonidine. NA inhibited calcium current by an amount which dependend on both membrane potential and current amplitude. When current-dependent inactivation of calcium current was maximal, NA was without effect. The data are consistent with a mechanism where NA enhances current-dependent inactivation.

Noradrenaline- and Enkephalin-Induced Inhibition of Voltage-Sensitive Calcium Currents in NG108-15 Hybrid Cells

Voltage-sensitive calcium currents were recorded from chemically differentiated neuroblastoma x glioma hybrid (NG108-15) cells using the whole-cell clamp technique. Both noradrenaline and [D-Ala2, D-Leu5] enkephalin (DADLE) reversibly depressed the amplitude of the calcium current by up to 30%. The response to noradrenaline occluded that to DADLE suggesting that both agonists depress the same fraction of current. The response to DADLE but not that to noradrenaline desensitized rapidly. Cells responded normally to noradrenaline when desensitized to the opioid. Responses to either agonist were absent in cells pre-incubated with pertussis toxin. In addition the response to noradrenaline became irreversible in cells dialysed internally with a non-hydrolysable analogue of GTP. The response to noradrenaline was not affected by treatment of the cells with either membrane-permeable analogues of cAMP or a combination of forskolin and isobutylmethylxanthine. It is concluded that both noradrenaline and DADLE depress the same fraction of voltage-dependent calcium current in NG108-15 cells; that the responses are mediated by a pertussis-sensitive GTP-binding protein but are not secondary to a reduction in the intracellular concentration of cAMP; and that desensitization of the opioid response occurs at a site linked intimately to the opioid receptor rather than at a common site in the transduction pathway between receptor activation and reduction in the calcium channel current.

Gadolinium selectively blocks a component of calcium current in rodent neuroblastoma x glioma hybrid (NG108-15) cells

1. The effect of the lanthanide cation Gd3+ on voltage-dependent calcium currents in neuroblastoma x glioma (NG108-15) cells has been studied using a whole-cell clamp technique. 2. Gd3+ reduced the amplitude of calcium currents. The amount of inhibition produced by Gd3+ was concentration dependent between about 0.5 and 5 microM and reached a maximum at about 10-20 microM. 3. A proportion of the total calcium current was resistant to blockade by Gd3+. 4. Gd3+-resistant calcium current consisted of two components: a rapidly inactivating, 'fast' component which was activated at potentials more positive than about -45 mV, and a long-lasting, 'slow' component which was activated at potentials more positive than about -10 mV. 5. It was possible to isolate the slow component, in the presence of Gd3+, by selectively inactivating the fast component with a brief depolarizing pre-pulse. The fast and slow components of current probably reflect the activity of two subpopulations of calcium channels which are resistant to block by Gd3+. 6. In control conditions inactivation of calcium current could be described by the sum of a fast (tau congruent to 40 ms at +10 mV) and a slow (tau congruent to 800 ms at +10 mV) exponential decay plus a constant. Gd3+ selectively blocked the slowly decaying current. 7. The current blocked by Gd3+ was activated at potentials more positive than about -35 mV and decayed monoexponentially (tau congruent to 800 ms at +10 mV). 8. It is concluded that under the experimental conditions used in the present study calcium currents recorded in NG108-15 cells are made up of at least three components which reflect the activity of three distinct subpopulations of calcium channels, one of which is selectively blocked by Gd3+.

Type I collagen permits invasive behaviour by retinal pigmented epithelial cells in vitro

Epithelial cells cultured on type I collagen gels adopt a typical apical—basal polarity and undergo differentiation. We have compared the behaviour of chick embryo retinal pigmented epithelial (RPE) cells on collagen and on plastic with and without gelatin coats. RPE cell proliferation was similar on all three substrata, and post-confluent cultures exhibited multilayering. On plastic and gelatin-coated plastic, dome formation, typical of transporting epithelia, occurred. On type I collagen gels, however, dome formation did not occur, but rather invasion of the gel matrix by cords of epithelial cells took place. In contrast, invasive behaviour of the cells was markedly reduced on type IV coated collagen gels, particularly in the presence of laminin. These results illustrate the prominent role of the extracellular matrix on phenotypic expression by RPE cells and may represent a more general phenomenon.

Excitatory action of BAYK8644 on hippocampal pyramidal neurones of the guinea pig in vitro

BAYK8944, a calcium 'agonist' caused a slow depolarization of a proportion of CA1 hippocampal pyramidal cells and increased their input resistance. In voltage-clamped CA1 neurones BAYK8644 caused a decrease in depolarization-activated outward current. Reduction of depolarization-activated outward current was also produced by the Ca ionophore A23187. It is proposed that BAYK8644 and A23187 may cause suppression of outward current secondary to a prolonged increase in cytosolic Ca activity.

Interaction of 1,4-dihydropyridines with somatic Ca currents in hippocampal CA1 neurones of the guinea pig in vitro

Two distinct Ca currents may be recorded from guinea pig hippocampal CA1 neuronal somata: a non-inactivating (persistent) current and a rapidly inactivating (transient) current. Nimodipine, a 'calcium antagonist', reduced both Ca currents though the transient current was somewhat less sensitive than the persistent current. BAY K 8644, a 'calcium agonist' enhanced the persistent current and shifted the activation threshold for this current to more negative potentials. BAY K 8644 also enhanced the transient Ca current though the effect was very variable. The results demonstrate that Ca channels in hippocampal CA1 neurones possess a dihydropyridine receptor in common with Ca channels in other tissues.

Chick embryonic pigmented retina is one of the group of epithelioid tissues that lack cytokeratins and desmosomes and have intermediate filaments composed of vimentin

Using sodium dodecyl sulphate/polyacrylamide gels to analyse detergent-insoluble residues, and indirect immunofluorescence, we have found that the major protein of intermediate filaments in cultures and freshly explanted fragments of chick embryonic retinal pigment epithelium (RPE) is vimentin. Moreover, these cells also fail to stain with antibodies against cytokeratins and most components of true desmosomes (maculae adhaerentes). Staining with anti-vinculin antibody suggests that the principal intercellular junction is the zonula adherens. Thus although RPE is an epithelium according to all other criteria, it belongs to a group of tissues (including vascular endothelium, iris and lens-forming epithelium) that have intermediate filaments composed of vimentin and possess neither cytokeratins nor desmosomes. That a tissue can be fully epithelial by other criteria, whilst lacking these components, is in agreement with other work, which has shown a lack of effect of micro-injection of antibodies to cytokeratin, and of the suppression of desmosome formation, on epithelial organization in culture. Although our observations were made solely on chick embryonic tissue, we suggest that published ultrastructural studies are consistent with the possibility that RPE of other species, including human, may lack true desmosomes.

The action of cholinomimetic substances on impulse conduction in the habenulointerpeduncular pathway of the rat in vitro

The effects of some cholinomimetic substances and their antagonists on the peak height of compound action potentials recorded from the terminal region of the habenulointerpeduncular pathway have been studied using a rat brain slice preparation. Carbachol and acetylcholine (ACh) depressed the peak height of the compound action potential and increased the latency to peak. The nicotinic agonists nicotine and dimethylphenylpiperazinium depressed the peak height of the compound action potential while muscarine and glutamate had no effect. The depressant effect of carbachol was blocked by the nicotinic antagonists hexamethonium, mecamylamine and d-tubocurarine but not by atropine. Physostigmine enhanced the effects of ACh and, to a lesser extent, carbachol. In the presence of physostigmine, carbachol or ACh initiated a spontaneous oscillation of the amplitude of the compound action potential which was Ca2+ dependent and was blocked by mecamylamine. It is concluded that depression of the amplitude of the compound action potential is due to activation of presynaptic nicotinic receptors. The results are discussed with reference to possible cholinergic mechanisms in the habenulointerpeduncular pathway.

Chemical transmission in the rat interpeduncular nucleus in vitro

We have used a rat brain-slice preparation to study the effects of some cholinomimetic and amino acid agonists and antagonists on the discharge frequency of neurones in the interpeduncular nucleus (i.p.n.), and on the response of these neurones to electrical stimulation of their main excitatory input, the fasciculus retroflexus of Meynert (f.r.m.). A high proportion of i.p.n. neurones were excited by carbachol, acetylcholine (ACh) and muscarine, but methacholine was less effective. The amino acids L-glutamate and L-aspartate were highly effective stimulants of i.p.n. neurones. The responses to ACh or carbachol were greatly reduced by the nicotinic blocking agents hexamethonium, d-tubocurarine and mecamylamine but only slightly reduced by atropine. The response to muscarine was abolished by low doses of atropine. Alpha-Bungarotoxin did not block the response of i.p.n. neurones to f.r.m. stimulation or to cholinomimetic agonists. The response of i.p.n. neurones to f.r.m. stimulation was not appreciably affected by high doses of nicotinic antagonists or atropine nor was there any enhancement of the response by physostigmine. The amino acid antagonists gamma-D-glutamylglycine (gamma DGG) and 2-amino phosphonovalerate (2-APV) were effective blockers of the response to f.r.m. stimulation and preferentially reduced responses to aspartate while having little effect on responses to glutamate or cholinomimetic agonists. It is concluded that ACh is an unlikely candidate for transmitter in this pathway despite abundant neurochemical evidence in its favour. It is more likely that the transmitter is an excitatory amino acid, probably an aspartate-like substance.

The effects of acetylcholine and dopamine on carotid chemosensory activity in the rabbit

1. Intracarotid (I.C.) injection of either acetylcholine (ACh) or dopamine inhibited spontaneous chemosensory activity recorded from the peripheral cut end of the sinus nerve in the anaesthetized rabbit. 2. High doses of ACh (greater than or equal to 50 micrograms I.C.) evoked a slight increase in discharge which preceded the inhibition. This excitation was attributable to a nicotinic action of the drug since it was abolished by mecamylamine. 3. The muscarinic agonist bethanechol inhibited chemoreceptor activity, an effect which was blocked by high doses of atropine, as was the inhibition caused by ACh. Dopamine-induced inhibition was unaffected by atropine. 4. Atropine, in doses sufficient to abolish the vasodepressor effect of ACh, only slightly reduced the inhibitory action of ACh on the chemoreceptors. Also, the vasodilators sodium nitrite and sodium nitroprusside did not appreciably alter chemosensory discharge. It seems unlikely, therefore, that the inhibitory response to ACh is secondary to vascular changes. 5. The inhibitory response to dopamine, but not that to ACh, was blocked by the dopamine antagonist alpha-flupenthixol. This implies that inhibition of chemosensory activity evoked by exogenous ACh was not secondary to dopamine release. 6. The implications of the results are discussed, particularly with regard to the possible physiological role of ACh as a modulator of carotid chemosensory activity.

Inhibitory action of dopamine on cat carotid chemoreceptors

1. The influence of some drugs which affect the dopaminergic system was studied on chemosensory responses to dopamine (DA), acetylcholine (ACh), sodium cyanide NaCN) and hypoxia during experiments on pentobarbitone anaesthetized cats in which chemoreceptor activity was recorded from the peripheral end of a sectioned sinus nerve. 2. Spontaneous chemosensory activity was inhibited in a dose-dependent manner by DA (0.5-5 microgram, I.A.). Higher doses (10-50 microgram) caused a delayed increase in discharge and were associated with inconsistent inhibitory responses. 3. The DA antagonist alpha-flupenthixol (0.2 mg/kg, I.A.) blocked the inhibitory response to DA without affecting either the spontaneous discharge frequency or the response to ACh. The effect of NaCN was potentiated, and during hypoxia chemoreceptor activity increased more rapidly, although the maximum frequency attained was not appreciably different from control values. Similar results were obtained with haloperidol (0.5 and 1.0 mg/kg, I.V.). 4. Higher doses of alpha-flupenthixol (0.5-1.0 mg/kg, I.A.) increased spontaneous chemoreceptor activity, but this was regarded as a non-specific effect of the drug since at these doses the inhibitory effect of 5-hydroxytryptamine (5-HT) was also abolished. 5. The animals were exposed to alternate periods of hypoxia and hyperoxia following administration of the tyrosine hydroxylase inhibitor alpha-methyl p-tyrosine (AMPT, 0.2-10 mg/kg, I.A.). The inhibitory response previously evoked by amphetamine was abolished, and electron microscopic studies showed a great reduction in the number of dense-cored granules, both of which suggested that DA levels in the carotid body had been substantially reduced. Responses to NaCN and hypoxia were slightly potentiated following AMPT, but neither spontaneous activity nor the response to ACh was affected. 6. Apomorphine (0.05-0.2 mg/kg, I.A.) inhibited the chemoreceptor discharge for up to 45 min, an effect which was antagonized by alpha-flupenthixol (0.2 mg/kg, I.A.), implying it resulted from DA receptor stimulation. Although responses to NaCN, hypoxia and higher doses of ACh were reduced following administration of apomorphine, the reduction was not very marked. 7. These results are not compatible with the theory of Osborne & Butler (1975), that in normoxia DA is tonically released in the carotid body and suppresses spontaneous chemosensory activity. 8. It is concluded that DA modulates chemosensory activity by influencing the rate of increase in discharge, without affecting maximum discharge frequency. The mechanism whereby DA is released in response to increased chemosensory activity remains to be established.