Bullfrog Dorsal Root Ganglion Research Articles

Three kinds of current in response to substance P in bullfrog DRG neurons

In response to SP applied externally, neurons freshly isolated from bullfrog dorsal root ganglion (DRG) showed three kinds of current ( I SP), i.e. slow, fast and moderately activating I SPs. All the three kinds of I SP were inward currents, and were completely blocked by either peptide antagonist of SP receptor spantide or non-peptide antagonist of SP receptor WIN51708. The slow activating I SP showed slow kinetic features. Replacement of NaCl in external solution by NMDG had no effect on this kind of I SP, while Ba 2+ abolished it almost completely, thus the ionic mechanism underlying slow activating I SP was deduced to be the closure of K + channels. The fast activating I SP in bullfrog DRG neurons, just as in rat DRG neurons, was proved to be caused by the opening of Na + preferring non-selective cation channel, for it was abolished almost completely by replacement of NaCl in external solution with equimolar NMDG. The moderately activating I SP was similar to the fast activating I SP in current configuration, however, its kinetic characteristics lay between those of fast and slow activating I SPs. Either NMDG or Ba 2+ suppressed this kind of I SP partially. Therefore the moderately activating I SP might be mediated by non-selective cation channel. We used repatch technique to explore the intracellular mechanism underlying the three kinds of I SP and found that the three kinds of I SP were caused by the activity of either G-protein coupled channel (slow activating I SP) or directly opened channel (fast activating I SP) or both (moderately activating I SP ).

Novel mechanism of inhibition by the P2 receptor antagonist PPADS of ATP-activated current in dorsal root ganglion neurons.

The antagonist pyridoxal-phosphate-6-azophenyl-2',4'-disulfonic acid (PPADS) has been proposed to selectively antagonize the actions of ATP at P2X receptors. Whole cell patch-clamp recording techniques therefore were used to characterize PPADS inhibition of ATP-activated current in bullfrog dorsal root ganglion (DRG) neurons. PPADS, 0.5-10 microM, inhibited ATP-activated current in a concentration-dependent manner with an IC(50) of 2.5 +/- 0.03 microM. PPADS produced a gradual decline of ATP-activated current to a steady state, but this was not an indication of use dependence as the gradual declining component could be eliminated by exposure to PPADS before ATP application. In addition, ATP-activated current recovered completely from inhibition by PPADS in the absence of agonist. The slow onset of inhibition by PPADS was not apparently due to an action at an intracellular site as inclusion of 10 microM PPADS in the recording pipette neither affected the ATP response nor did it alter inhibition of the ATP response when 2.5 microM PPADS was applied externally. PPADS, 2.5 microM, decreased the maximal response to ATP by 51% without changing its EC(50). PPADS inhibition of ATP-activated current was independent of membrane potential between -80 and +40 mV and did not involve a shift in the reversal potential of the current. The magnitude of PPADS inhibition of ATP-activated current was dependent on the duration of the prior exposure to PPADS. The time constants of both onset and offset of PPADS inhibition of ATP-activated current did not differ significantly with changes in ATP concentration from 1 to 5 microM. Recovery of ATP-activated current from PPADS inhibition also exhibited a slow phase that was not accelerated by the presence of agonist and was dependent on the concentration of PPADS. The apparent dissociation rate of PPADS from unliganded ATP-gated ion channels was much greater than the rate of the slow phase of recovery of ATP-activated current from PPADS inhibition. The results suggest that PPADS can inhibit P2X receptor function in a complex noncompetitive manner. PPADS produces a long-lasting inhibition that does not appear to result from open channel block but rather from an action at an allosteric site apparently accessible from the extracellular environment that involves a greatly reduced rate of dissociation from liganded versus unliganded ATP-gated ion channels.

Ethanol-induced inhibition of a neuronal P2X purinoceptor by an allosteric mechanism.

Ethanol inhibits a neuronal P2X purinoceptor by shifting the ATP concentration-response curve to the right in an apparently competitive manner. However, the underlying mechanism has not been determined. We investigated the effects of ethanol on the activation and deactivation time constants for ATP-activated current in bullfrog dorsal root ganglion neurones. Ethanol decreased the time constant of deactivation of ATP-gated ion channels without affecting the time constant of activation. The observations are not consistent with a competitive mechanism of inhibition by ethanol, but may be explained by an allosteric action of ethanol to decrease apparent agonist affinity. This represents a novel mechanism of action of ethanol on a neurotransmitter-gated ion channel.

Inhibition of ATP‐activated current by zinc in dorsal root ganglion neurones of bullfrog

1. The effect of Zn2+ on ATP-activated current was studied in bullfrog dorsal root ganglion (DRG) neurones using the whole-cell patch-clamp technique. 2. Zn2+ (2-800 microM) inhibited current activated by submaximal concentrations of ATP. The Zn2+ concentration that produced 50% inhibition (IC50) of current activated by 2.5 microM ATP was 61 +/- 9.8 microM. When ATP concentrations were adjusted to account for chelation of Zn2+, the IC50 of Zn2+ was 86 +/- 18 microM. 3. The inhibitory action of Zn2+ on ATP-gated channels did not appear to be due to a decrease in the concentration of one or more species of ATP. 4. Zn2+ inhibition of ATP-activated current was independent of membrane potential between -80 and +40 mV, and did not involve a shift in the reversal potential of the current. 5. Zn2+ (100 microM) shifted the ATP concentration-response curve to the right in a parallel manner, increasing the EC50 for ATP from 2.5 +/- 0.5 microM to 5.5 +/- 0.4 microM. 6. Zn2+ decreased the time constant of deactivation of ATP-gated ion channels without affecting the time constant of activation or desensitization. 7. Dithiothreitol (DTT) reversed Zn2+ inhibition of ATP-activated current. 8. 2-Methylthio ATP, alpha,beta-methylene ATP and ADP activated current with EC50 values of 2.4 +/- 0.3. 50.1 +/- 5.8 and 303.1 +/- 53.9 microM, respectively. Adenosine, AMP or beta,gamma-methylene ATP did not evoke detectable current. 9. Reactive Blue 2 and pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid inhibited ATP-activated current. 10. The results suggest that Zn2+ can inhibit P2X purinoceptor function by decreasing the affinity of the binding site for ATP. These observations provide the first evidence for this action of Zn2+ on a neurotransmitter-gated ion channel. Furthermore, the receptor-channel in these neurones appears to be a novel member of the P2X purinoceptor class.

Baclofen reduces GABAA receptor responses in acutely dissociated neurons of bullfrog dorsal root ganglia

The effect of baclofen on the function of the γ-aminobutyric acidA (GABAA) receptor was examined in acutely dissociated neurons of bullfrog dorsal root ganglia (DRG) by using the whole-cell coltage-clamp method. Baclofen (0.1–100 μM) depressed the inward currents produced by GABA (100 μM) and muscimol (100 μM). Baclofen shifted the concentration-response curve for GABA (1 μM-1 mM) downward. Baclofen decreased the maximum response (Vmax) to GABA without changing the apparent dissociation constant (Kd), suggesting a noncompetitive antagonism. The effect of baclofen on the GABA current was blocked by antagonists for the GABAB receptor; the rank order of potency was P-[3-Aminopropyl]-P-diethoxymethylphosphinic acid (CGP 55845A) ≫ 3-N[1-(S)-(3,4-dichlorophenyl)ethyl]amino-2-(S)-hydroxypropyl-P-benzylphosphinic acid (CGP 35348) > saclofen ≫ phaclofen. Baclofen produced an irreversible depression of the GABA current in neurons dialyzed with an internal solution containing guanosine (5′-O-(3-thiotriphosphate) (GTPγS, 100 μM). Intracellular guanosine 5′-O-(2-thiodiphosphate) (GDPβS, 100 μM) blocked the inhibitory effect of baclofen on the GABA current. Forskolin (10 μM) and dibutyryl N6, 2′-O-dibutyryladenosine 3′:5′-cyclic monophophate (db-cyclic AMP) (200 μM) depressed the GABA current. N-(2-aminoethyl)-5-isoquinolinesulfonamide (H-9, 40 μM) and N-(2-guanidinoethyl)-5-isoquinolinesulfonamide (HA-1004, 50 μM), protein kinase A (PKA) inhibitors, reduced the depressant effect of baclofen on the GABA current. The baclofen-induced depression of the GABA current was blocked by PKI(5–24), a specific PKA inhibitor, but not by PKC(19–36), a specific protein kinase C(PKC) inhibitor. We suggest that GABAB receptors regulate the GABAA receptor function through a G-protein linked to the adenylyl cyclase-PKA pathway in bullfrog DRG neurons. Synapse 26:165–174, 1997. © 1997 Wiley-Liss, Inc.

Baclofen reduces GABAA receptor responses in acutely dissociated neurons of bullfrog dorsal root ganglia.

The effect of baclofen on the function of the gamma-aminobutyric acidA (GABAA) receptor was examined in acutely dissociated neurons of bullfrog dorsal root ganaglia (DRG) by using the whole-cell voltage-clamp method. Baclofen (0.1-100 microM) depressed the inward currents produced by GABA (100 microM) and muscimol (100 microM). Baclofen shifted the concentration-response curve for GABA (1 microM-1 mM) downward. Baclofen decreased the maximum response (Vmax) to GABA without changing the apparent dissociation constant (Kd), suggesting a noncompetitive antagonism. The effect of baclofen on the GABA current was blocked by antagonists for the GABAB receptor; the rank order of potency was P-[3-Aminopropyl]-P-diethoxymethylphosphinic acid (CGP 55845A) > > 3-N[1-(S)-(3,4-dichlorophenyl)ethyl]amino-2-(S)-hydroxypropyl-P- benzyl-phosphinic acid (CGP 35348) > saclofen > > phaclofen. Baclofen produced an irreversible depression of the GABA current in neurons dialyzed with an internal solution containing guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S, 100 microM). Intracellular guanosine 5'-O-(2-thiodiphosphate) (GDP beta S, 100 microM) blocked the inhibitory effect of baclofen on the GABA current. Forskolin (10 microM) and dibutyryl N6, 2'-O-dibutyryladenosine 3':5'-cyclic monophophate (db-cyclic AMP) (200 microM) depressed the GABA current. N-(2-aminoethyl)-5-isoquinolinesulfonamide (H-9, 40 microM) and N-(2-guanidinoethyl)-5-isoquinolinesulfonamide (HA-1004, 50 microM), protein kinase A (PKA) inhibitors, reduced the depressant effect of baclofen on the GABA current. The baclofen-induced depression of the GABA current was blocked by PKI(5-24), a specific PKA inhibitor, but not by PKC(19-36), a specific protein kinase C (PKC) inhibitor. We suggest that GABAB receptors regulate the GABAA receptor function through a G-protein linked to the adenylyl cyclase-PKA pathway in bullfrog DRG neurons.

Opioid peptides modulate GABA A receptor responses in neurons of bullfrog dorsal root ganglia

Effects of enkephalin and selective opioid-receptor agonists on GABA-induced current were examined in dissociated neurons of bullfrog dorsal root ganglia (DRG) by using whole-cell patch-clamp method. Leucine-(Leu)-enkephalin and methionine-(Met)-enkephalin depressed GABA A receptor-mediated currents. DPDPE, DAMGO and dynorphin-A (Dyn-A) also depressed the inward current produced by GABA: the order of agonist potency was DPDPE ≥ DAMGO > Dyn-A. Naloxone blocked the inhibitory effects of ekephalins and other opioid agonists on the GABA current. Naltrindole (NTI), a δ-receptor antagonist, prevented the DPDPE-induced depression of the GABA current. β-Funaltrexamine (β-FNA), a μ-receptor antagonist, reduced the DAMGO-induced depression of GABA currents. Nor-binaltorphimine (nor-BNI), a κ-receptor antagonist, reduced the effects of Dyn-A in depressing the GABA current The results suggest that enkephalin down-regulates GABA A receptor function through mainly δ- and μ-opioid receptors in bullfrog DRG neurons.

Enhancement of ATP-activated current by protons in dorsal root ganglion neurons.

The effect of pH on ATP-activated current in bullfrog dorsal root ganglion neurons was studied using the whole-cell patch-clamp technique. ATP-activated current amplitude was highly dependent upon extracellular pH. An acid pH increased, whereas alkaline pH decreased, ATP-activated current amplitude. The half-maximal pH (EC50) for potentiation of 2.5 micro;M ATP-activated current was 7.2. Acidification alone did not activate detectable current and, at an acid pH, ATP-activated current was abolished by suramin. Proton-induced enhancement of ATP-activated current was not sensitive to membrane potential between -80 and +40 mV, and did not involve a shift in reversal potential. Lowering pH from 7.2 to 6.5 or elevating pH from 7.2 to 8.0 shifted the ATP concentration/response curve to the left or right, respectively, without changing the maximal response to ATP. Protons increased the time constant of deactivation without affecting the time constant of activation or desensitization of ATP-activated current. Alteration of patch-pipette (intracellular) pH did not affect the enhancement of ATP-activated current by extracellular protons. Diethylpyrocarbonate (DEP), dithiothreitol (DTT), 5, 5'-dithio-bis-(2-nitro-benzoic acid) (DTNB), or N-ethylmaleimide (NEM) did not affect enhancement of ATP-activated current by protons. The results suggest that extracellular protons, at physiological concentrations, can regulate the function of P2X purinoceptors by modulating the affinity of the ATP-binding site.

Neurokinin-1 (NK1) receptors mediate tachykinin-induced depression of GABA current in bullfrog sensory neurons.

Receptors responsible for the tachykinin-induced depression of gamma-aminobutyric acid-A (GABAA) receptors in neurons of bullfrog dorsal root ganglia (DRG) were investigated by using whole-cell voltage-clamp techniques. Substance P (10 nM-1 microM) depressed the inward current produced by GABA (GABA current) in a concentration-dependent manner. Substance P did not change the reversal potential of the GABA current. Neurokinin A also depressed the GABA current with potency similar to those of substance P. [D-Arg1, D-Trp7,9, Leu11]substance P (spantide, 1 microM) shifted the concentration-inhibition curve of substance P to the right. Spantide (1 microM) increased the IC50 from 56 nM to 210 nM. Lineweaver-Burk plot, a reciprocal plot of the concentration-inhibition curve, showed that spantide did not change the maximum response (Vmax) to substance P but increased the apparent dissociation constant (Kd). CP99994 (1 microM), an neurokinin-1 (NK1) receptor antagonist, inhibited the substance P-induced depression of the GABA current. These results suggest that the tachykinin-induced depression of the GABAA receptor sensitivity is mediated by NK1 receptors in neurons of bullfrog DRG.

Substance P suppresses GABAA receptor function via protein kinase C in primary sensory neurones of bullfrogs.

1. The effects of substance P (SP) and related tachykinins on the function of gamma-aminobutyric acid-A (GABAA) receptors were examined in acutely dissociated neurones of bullfrog dorsal root ganglia (DRG) by using whole-cell voltage-clamp techniques. 2. Application of SP (10 nM to 1 microM) depressed inward currents produced by GABAA receptor activation (IGABA). Neurokinin A (NKA) and neurokinin B (NKB) also depressed IGABA; the rank order of agonist potency was SP > NKA > NKB. Spantide ([D-Arg1, D-Trp7,9,Leu11]SP) and L-703,606, NK1 receptor antagonists, blocked the SP-induced depression of IGABA. 3. SP irreversibly depressed IGABA, when neurones were intracellularly dialysed with GTP gamma S. Intracellular application of GDP beta S prevented the SP-induced depression of IGABA. Pertussis toxin (PTX) did not block the inhibitory effect of SP on IGABA. 4. The depression of IGABA produced by SP was inhibited by H-7 and PKC(19-36), protein kinase C (PKC) inhibitors, but not by H-9 and HA-1004, protein kinase A inhibitors. IGABA was suppressed by application of sn-1,2-dioctanoyl glycerol (DOG), a PKC activator. 5. It is concluded that activation of neurokinin-1 (NK1) receptors downregulates the function of the GABAA receptor of primary sensory neurones through a PTX-insensitive G-protein. PKC may be involved in the transduction pathway of the tachykinin-induced inhibition of the GABAA receptor.

Tetraethylammonium-sensitive calcium-sensitive potassium current in a subclass of the bullfrog dorsal root ganglion cells

Bullfrog dorsal root ganglion (DRG) cells were classified into three types, As, Ar and C, according to their electrophysiological properties. Actions of tetraethylammonium (TEA; 100 μM−100 mM) on As-type cells were examined using current- and voltage-clamp methods; TEA caused a membrane depolarization or an inward current, associated with a decrease in membrane conductance. These TEA-induced responses reversed in polarity at −85 to −90 mV, and the change in reversal potential followed the Nernst equation as extracellular K + concentration was changed. The TEA-induced responses were reversibly inhibited by Ca ++ -free/high-Mg ++ solutions and inorganic Ca blockers. It is concluded that bullfrog DRG As-type cells might be also endowed with Ca-sensitive K channels which may be open at rest and blocked by TEA.

Tetraethylammonium-sensitive calcium-sensitive potassium current in a subclass of the bullfrog dorsal root ganglion cells

Bullfrog dorsal root ganglion (DRG) cells were classified into three types, As, Ar and C, according to their electrophysiological properties. Actions of tetraethylammonium (TEA; 100 μM−100 mM) on As-type cells were examined using current- and voltage-clamp methods; TEA caused a membrane depolarization or an inward current, associated with a decrease in membrane conductance. These TEA-induced responses reversed in polarity at −85 to −90 mV, and the change in reversal potential followed the Nernst equation as extracellular K + concentration was changed. The TEA-induced responses were reversibly inhibited by Ca ++-free/high-Mg ++ solutions and inorganic Ca blockers. It is concluded that bullfrog DRG As-type cells might be also endowed with Ca-sensitive K channels which may be open at rest and blocked by TEA.

N-Methyl- d-aspartate depresses GABA A receptor-mediated currents in neurons of bullfrog dorsal root ganglia

The effect of N-methyl- d-aspartate (NMDA) on the function of GABA A receptors was examined in acutely dissociated neurons of bullfrog dorsal root ganglia (DRG) by using whole-cell patch clamp methods. The application of NMDA (100 μM) to DRG neurons suppressed the inward currents produced by GABA (100 μM) and muscimol (100 μM). dl-2-Amino-5-phosphonovaleric acid (APV; 100 μM) antagonized the NMDA-induced depression of the GABA current in a competitive manner. Kainic acid (100 μM) did not depress the GABA current. NMDA induced no depression of the GABA current in a nominally Ca 2+-free solution containing 5 mM Mg 2+. These data indicate that the activation of the NMDA receptor inhibits the GABA A receptor-mediated current via a Ca 2+-dependent process.

Voltage-gated calcium currents in whole-cell patch-clamped bullfrog dorsal root ganglion cells: effects of cell size and intracellular solutions

Acutely dissociated bullfrog dorsal root ganglion (DRG) cells could be divided into two classes by measurement of cell capacitance. A bimodal distribution of cell capacitance was found and a value of 75 pF was used to divide frog DRG cells into ‘small’ and ‘large’ types. Two distinct voltage-activated Ca 2+ currents were evoked in both classes of cells: a rapidly inactivating, low-voltage-activated current and a slowly-inactivating, high-voltage-activated current. When the recording pipette contained CsCl, greater peak inward current values and densities were seen in large cells compared to small cells. No significant differences were observed in the distribution of low-and high-voltage-activated currents in small and large cells. Replacement of pipette solutions containing CsCl with solutions containing equimolar concentrations of Cs glutamate, l-arginine Cl, or N-methyl- d-glucamine significantly increased both the reversal potential and the maximum amplitude of the Ca 2+ currents in both small and large DRG cells. These increases indicate that internal substitutions with organic ions suppresses outward currents more effectively than does CsCl. In contrast to findings with CsCl, when organic ions were used in the pipette solution a significantly higher proportion of low-threshold Ca 2+ channels was observed in small cells compared to large cells. These observations indicate that when organic solutions were used internally, significant differences in the proportion of low-threshold to high-threshold Ca 2+ channels were observed in small and large cells. The composition of the internal solution is a critical variable when determining the type and amount of inward Ca 2+ current in different types of neurons.

Tachykinins cause inward current through NK 1 receptors in bullfrog sensory neurons

The effects of tachykinins on primary afferent neurons of bullfrog dorsal root ganglia (DRG) were examined by using whole-cell patch-clamp methods. Neurokinin A (NKA) caused inward current ( I NKA) in a concentration-dependent manner. Concentration-response curve showed that the EC 50 for NKA was 6 nM. The I NKA showed strong tachyphylaxis, when NKA was continuously applied for more than 1 min. Substance P (SP) also produced inward current with potency similar to that of NKA. Neurokinin B (NKB) was less effective in producing the inward current. The order of agonist potency was NKA = SP ⪢ NKB. Spantide ([D-Arg 1, D-Trp 7,9, Leu 11]SP), non-selective peptide antagonist at tachykinin receptors, reduced the tachykinin-induced current. CP-99,994, a selective non-peptide antagonist for neurokinin-1 (NK 1) receptor, inhibited the inward currents produced by NKA and SP. The I NKA was associated with decrease in K + conductance. NKA suppressed both a voltage-dependent K + current, the M-current ( I M), and a voltage-independent background K + current, I K(B). Intracellular dialysis with GTPγS (100 nM) or GDPβS (100 μM) depressed the I NKA. Pre-treatment of DRG neurons with pertussis toxin (PTX) did not prevent the I NKA. Depletion of intracellular ATP depressed the I NKA. These results suggest that the tachykinin-induced inward current is mediated through the NK 1 receptor which mainly couples to PTX-insensitive G-protein in bullfrog primary afferent neurons.

Gamma-aminobutyric acid A receptor function is modulated by cyclic GMP

Gamma-aminobutyric acid (GABA) is the main inhibitory neurotransmitter in the vertebrate nervous system. Modulatory effects of intracellular ATP on the GABA response in isolated bullfrog dorsal root ganglion neurons were examined using whole-cell voltage clamp. Investigation of the plausible mechanisms ATP might utilize to regulate the GABA response led to the discovery that intracellular cyclic GMP may play an important role in modulating inhibitory neurotransmission. This modulatory effect of cyclic GMP is likely to be mediated via a cyclic GMP-dependent protein kinase.

Neurokinin A depolarizes neurons of bullfrog dorsal root ganglia by suppressing K+ conductances.

The effect of neurokinin A (NKA) on neurons of bullfrog dorsal root ganglia (DRG) in primary culture was examined by using whole-cell patch-clamp methods. Application of NKA (1 microM) depolarized the DRG neurons, resulting in spontaneous firing of action potentials. Under voltage-clamp condition, NKA (3 nM-1 microM) caused an inward current (INKA) associated with decreased membrane conductance. The INKA reversed its polarity at the equilibrium potential for K+. The INKA was blocked by extracellular Ba2+ (1 mM) but not by nominally 0 mM Ca2+, tetraethylammonium (40 mM), 4-aminopyridine (2 mM) or apamin (50 nM). Intracellular Cs+ blocked the INKA. NKA depressed a voltage-dependent non-inactivating K+ current, the M-current (IM), at potentials more positive than -55mV. NKA reduced the maximum M-conductance (GM) without changing the kinetics of M-channels. NKA also depressed a voltage- and time-independent background K+ current, IK(B). It is concluded that the INKA is produced by suppression of both IM and IK(B) in bullfrog primary afferent neurons.

Substance P produces an inward current by suppressing voltage-dependent and -independent K + currents in bullfrog primary afferent neurons

A whole-cell patch-clamp study was carried out to examine the effect of substance P (SP) on the excitability of neurons in bullfrog dorsal root ganglia (DRG). SP (3 nM to 1 μM) produced an inward current associated with decreased membrane conductance at voltage range between −10 and −130 mV. Neurokinin A (NKA) and neurokinin B (NKB) also produced the inward current in DRG cells; the rank order of agonist potency was NKA = SP ⪢ NKB. An antagonist for SP receptors, [D-Arg 1, D-Trp 7,9, Leu 11]SP, did not prevent the response to SP. SP (3 nM to 1 μM) suppressed the voltage-dependent non-inactivating K + current, the M-current ( I M) by reducing the maximum M-conductance. A voltage-independent background K + current, I K(B), could be recorded at a hyperpolarizing voltage (≤−60 mV) from DRG neurons. SP (3 nM to 1 μM) produced the inward current associated with decreased I K(B) at a holding potential more negative than −60 mV. The SP-induced inward current reversed its polarity at the equilibrium potential for K ions. Intracellular dialysis with Cs + blocked the SP-induced responses. Depletion of intracellular ATP reduced SP-induced inward current. These results suggest that the SP-induced inward current was due to suppression of both the I M and I K(B) that are regulated by intracellular activity of ATP in bullfrog DRG neurons.

Chemosensitivity of C-cells in bullfrog dorsal root ganglia to substance P and adenosine 5′-triphosphate

Dissociated bullfrog dorsal root ganglion cells were voltage clamped in the whole-cell configuration. In small C-cells having 20 μm as averaged diameter, substance-P (0.1-1 μM) inhibited an M-type potassium current while ATP (1–10 μM) activated a sodium-potassium current. In large A-cells (≈ 65 μm in diameter) in which ATP has been shown to inhibit M-current, substance P (0.1-1 μM) also inhibited this potassium current without activating the sodium-potassium current. Results provided evidence for the distinction between A- and C-cells in terms of their chemosensitivity.

Intracellular ATP changes the voltage-dependence of delayed rectifier potassium current in bullfrog primary afferent neurons

Dissociated bullfrog dorsal root ganglion cells were voltage-clamped in the whole-cell configuration to study the steady-state activation and inactivation curves for a delayed rectifier potassium current. The 50%-activation of the current occurred +15 mV when measured with ATP (5 mM) in the pipette solution as opposed to −11 mV with 5′-adenylylimidodiphosphate (AMP-PNP, 5 mM) and −15 mV with adenosine 5′- O-(3-thiotriphosphate) (5 mM). The 50%-inactivation of the current occured at −6 mV with ATP but at −31 mM with AMP-PNP. The results suggest that intracellular ATP modulates voltage-dependence of the delayed rectifier in amphibian afferent neurons.