Neuronal responses to extracellularly applied cyclic AMP:Role of the adenosine receptor

Peripheral vibration causes an adenosine-mediated postsynaptic inhibitory potential in dorsal horn neurons of the cat spinal cord

We have previously reported a vibration-induced, adenosine-mediated inhibition of nociceptive dorsal horn neurons in the cat spinal cord. The present study was conducted to investigate the mechanisms of this inhibition. In vivo intracellular recording was obtained from dorsal horn neurons in the lower lumbar segments of the anaesthetized cat. Vibration (80-250 Hz for 2-3 s every 15-20 s) was applied to the glabrous skin of the toes of the hind foot using a feedback-controlled mechanical stimulator. In 32 of 43 neurons tested, vibration produced a pronounced hyperpolarization of the membrane potential. This hyperpolarization peaked at -10 mV and decayed throughout the period of the application of vibration. It was associated with a decrease in membrane resistance, had a reversal potential negative to the resting membrane potential and was Cl(-)-independent, suggesting that it was due to an increase in a K+ conductance, properties typical of the response to adenosine. This inhibitory postsynaptic potential was unaffected by intravenous administration of bicuculline, strychnine and naloxone but was blocked by iontophoretic administration of 8-sulphophenyltheophylline, a P1-purinergic receptor antagonist. These results confirm our previous finding that vibration-induced inhibition of nociceptive dorsal horn neurons is mediated via the release of an endogenous purine compound and further suggests that this inhibition involves a postsynaptic inhibitory mechanism.

The adenosine agonist N6-R-phenylisopropyladenosine (R-PIA) stimulates feeding in rats

Administration of adenosine and agonists of the adenosine receptors to rats results in hypoactivity, hypothermia, muscle relaxation and antinociception. In the present study, we found that the adenosine ligand, N6-R-phenylisopropyladenosine (R-PIA), increased food intake in rats at a time in the day when rats normally eat very little food or none at all. Feeding was not reliably stimulated upon the first exposure to R-PIA, but was clearly increased following repeated administration of this agonist. Other adenosine agonists, namely 2-chloradenosine and 5'N-ethylcarboxamide adenosine, failed to alter feeding after a single injection or after repeated exposure. The adenosine antagonist, caffeine, did not block R-PIA's effect on food intake, whereas the opioid antagonist, naloxone, blocked R-PIA-induced eating. These data suggest that R-PIA stimulates feeding independent of the A1 or A2 adenosine receptors.

Tachykinins enhance the depression of spinal nociceptive neurons caused by cutaneously applied vibration in the cat

The present investigation was prompted by previous studies in our laboratory which have indicated that tachykinins enhance depressant effects of purines and that the purine adenosine mediates a vibration-induced depression of nociceptive dorsal horn neurons. Extracellular recordings were made from single nociceptive neurons in the lower lumbar segments of anaesthetized cats. Vibration (80 Hz; 2.5-3.5 s every 20-25 s) was applied to the hindlimb using a feedback-controlled mechanical stimulator. The tachykinins physalaemin, substance P and neurokinin A were administered by iontophoresis. Physalaemin was tested on vibration-induced responses of 29 neurons; each neuron was excited by this tachykinin. To control for possible changes in the response to vibration caused by the excitation per se, statistical comparisons were made of the vibration-induced responses during excitation by tachykinins and during excitation by glutamate. In 16 cases the magnitude of the vibration-induced depression was significantly greater during the excitation caused by physalaemin. With the remaining neurons the response to vibration failed to differ during the excitation by physalaemin compared with the excitation by glutamate. In four of the 16 cases subthreshold applications of vibration caused depression after administration of physalaemin. The P1-purinergic (adenosine) antagonist, caffeine, was administered in three cases where vibration caused depression only with application of physalaemin. In each of these cases the depression was reversibly blocked by caffeine (10 or 40 mg kg-1 i.v.). The magnitude of vibration-induced depression was significantly increased during excitation by neurokinin A (5/14 neurons) or by substance P (1/9 neurons). From the results of the present study we suggest that tachykinins enhance the vibration-induced depression. This enhancement may be due to enhanced depression by adenosine.(ABSTRACT TRUNCATED AT 250 WORDS)

Purine-induced depression of dorsal horn neurons in the cat spinal cord: enhancement by tachykinins

The neurokinins, physalaemin, substance P, neurokinin A and bradykinin, were tested on the responses of single spinal neurons to the purines, adenosine 5'-triphosphate (ATP) and adenosine 5'-monophosphate and to GABA. Experiments were done on anaesthetized cats, recording extracellularly from functionally identified sensory neurons in the lumbar dorsal horn. All compounds were administered by iontophoresis. Neurokinins caused a slow, prolonged excitation which outlasted the period of application. Physalaemin was tested on responses to ATP in 24 units. In each case application of ATP caused either depression, excitation or a biphasic response when the application was not pre-conditioned by ejection of physalaemin. For 11 units, with ATP applications subthreshold to alter the on-going firing rate, such applications caused depression when they were preceded by administration of physalaemin. Three units were tested with ATP applications which caused the excitatory response; when the applications of ATP were preceded by ejection of physalaemin, there was then a depressant component in the response. In these 14 cases, the magnitude of the depression or of the depressant component of the response, was measured using currents which failed to produce depression in the absence of physalaemin ejection; the mean magnitude of this depression was 34.7 +/- 1.6% (+/- S.E.M.). With the 10 remaining units, responses to ATP were unaffected by application of physalaemin. The early components of the biphasic and excitatory responses were unaffected by physalaemin and hence it appeared to have a differential effect, enhancing only the depressant effects of ATP. The enhancement of depression was reversible, lasting up to 30 min following a single ejection. Neither control current nor glutamate mimicked the effect of physalaemin in the responses to application of ATP. The depressant response to adenosine 5'-monophosphate was also enhanced by physalaemin: ejections of adenosine 5'-monophosphate subthreshold to affect the on-going firing rate caused depression after physalaemin application in 3 of 8 units (average depression: 35.0 +/- 3.3%). On the other hand, depression induced by GABA was unaffected by physalaemin in every case (n = 8); in 4 of these cases GABA was tested on units for which purine-induced depression was enhanced by physalaemin. Thus, physalaemin preferentially affected depressant responses to ATP and to adenosine 5'-monophosphate.(ABSTRACT TRUNCATED AT 400 WORDS)

Evidence that adenosine mediates the depression of spinal dorsal horn neurons induced by peripheral vibration in the cat

Nociceptive neurons in the dorsal horn of the cat spinal cord are depressed by vibration applied to the ipsilateral hind limb. The present study investigated the pharmacological properties of this depression because of the possibility that it represents the neural basis at the spinal level for the analgesic effects of vibration in humans. Experiments were done in cats anesthetized with sodium pentobarbital and acutely spinalized at the first lumbar level. Extracellular recordings were made from nociceptive neurons in the lower lumbar segments. The depression of these neurons induced by vibration to the hindlimb was attenuated by administration of the P1-purinergic (adenosine) receptor antagonist, caffeine (20-60 mg/kg i.v.); the maximum attenuation was 100%. Effects of caffeine began within 2 min after the start of injection (1-3 min injection period), were greatest in the 10 min period after the end of injection and lasted for up to 2 hr. Importantly, another P1-purinergic receptor antagonist, which does not cross the blood-brain barrier, 8-sulphophenyltheophylline (8-16 mg/kg), had no effect on the depression when given intravenously (n = 5); however, when administered by iontophoresis 8-sulphophenyltheophylline blocked the depression in 2 of 6 units. Dipyridamole (1.0-2.0 mg/kg i.v.), an inhibitor of adenosine uptake, potentiated the depression in 2 of 5 cases. These results prompt us to suggest that depression induced by vibration may be mediated by adenosine via the activation of P1-purinergic receptors. On the other hand, the GABAA antagonist, bicuculline, failed to attenuate vibration-induced depression when administered either intravenously (0.2-0.4 mg/kg; n = 5) or by iontophoresis (n = 10) and the glycine antagonist, strychnine (0.2-0.6 mg/kg; n = 3) and the opiate antagonist, naloxone (0.1-0.4 mg/kg; n = 4) were similarly ineffective. These findings suggest that vibration-induced depression of these units occurs without involvement of bicuculline-sensitive GABA receptors, strychnine-sensitive glycine receptors and naloxone-sensitive opiate receptors. In view of the fact that vibration-induced depression is evoked synaptically, this study is the first to demonstrate in the central nervous system a synaptic response which is mediated by adenosine. In addition, we suggest that the analgesic effects of vibration in humans may be mediated at the spinal level by activation of P1-purinergic receptors.

Aminophylline differentiates between the depressant effects of morphine on the spinal nociceptive reflex and on the spinal ascending activity evoked from afferent C fibres

The action of aminophylline on anti-nociceptive effects of morphine in rats was tested on the tail-flick response to noxious heat and on the activity evoked in ascending axons of the spinal cord by stimulation of nociceptive afferents. The depression of the tail-flick response produced by an intraperitoneal (i.p.) injection of morphine 2 mg/kg in intact and spinal rats was abolished by an i.p. injection of aminophylline 25 mg/kg. The activity evoked in ascending axons of spinal rats by electrical stimulation of afferent C fibres of the sural nerve was depressed by an intravenous (i.v.) injection of morphine 2 mg/kg. Aminophylline 25 mg/kg injected i.v. after morphine produced a slight and transient increase in the ascending activity immediately after its administration but did not abolish the depressant effect of morphine. Naloxone 0.2 mg/kg administered after aminophylline antagonized the depressant effect of morphine on the ascending activity. It is suggested that morphine exerts its depressant effect on the two nociceptive responses (the motor and the sensory response) by different mechanisms, one being sensitive to aminophylline, the other being relatively resistant to the action of the purine derivative.

Methylxanthines modulate adenosine release from slices of cerebral cortex

Using slices of mouse or rat cerebral cortex incubated with [3H]adenosine or [3H]adenine and/or [14C]GABA we have examined factors affecting the release of these compounds, and especially the influence of methylxanthines. Although release of purines and GABA could be induced by ouabain (10(-4) M), or p-hydroxymercuribenzoate (5 x 10(-4) M) no release was produced by ethacrynic acid (10(-3) or 10(-4) M) phenytoin (10(-3) M), noradrenaline or SC 13504. Release is probably not therefore related to (Na+, K+) ATPase or Mg2+-ATPase inhibition. At concentrations of 10(-3) and 10(-4) M, caffeine, theophylline, aminophylline and isobutyl-methylxanthine (IBMX) markedly depressed the release of purines evoked by ouabain. Non-xanthine inhibition of phosphodiesterase had much weaker though statistically significant effects. The methylxanthines had no significant effect on GABA release. It is suggested that the results can be explained on the basis of a positive feedback system in which released adenosine activates membranal adenylate cyclase, and the increased concentration of cyclic AMP which results form or origin of much of the adenosine released subsequently. However, we cannot exclude the existence of an intracellular receptor for methylxanthines which causes directly the inhibition of purine release.

4-Aminopyridine blockade of neuronal depressant responses to adenosine triphosphate

1 Adenosine, adenosine monophosphate and adenosine triphosphate (ATP) depressed the firing rate of neurones in the rat cerebral cortex when applied by microinontophoresis. 2 4-Aminopyridine, also applied iontophoretically blocked the depressant effects of the purines, without affecting responses to gamma-aminobutyric acid (GABA). This blockade was effected against purine depressions of both spontaneous and glutamate-evoked activity, suggesting that the interaction occurred postsynaptically.

Characteristics of the release of adenosine from slices of rat cerebral cortex

1. The characteristics of the release of adenosine have been examined from slices of rat cerebral cortex after incubation with [3H]adenine or [3H]adenosine. 2. Increasing the potassium concentration of the extracellular medium to 36 or 54 mM did not evoke any release, but release was observed in the first post-potassium sample. This occurred whether potassium was present for 2 or 10 min. 3. Calcium-free solutions or verapamil prevented the post-potassium release of tritium. Tetraethylammonium bromide and 4-aminopyridine had no effect. 4. Ouabain (100 microM) induced the release of tritium, and did not prevent an additional increment of release after potassium stimulation. Ouabain induced release did not occur in calcium-free or sodium-free media, but was increased in low calcium (0.1 mM) medium. 5. It is concluded that the characteristics of adenosine release are unlike those of conventional neurotransmitters. It is suggested that the release is associated with the influx of sodium and calcium ions through sodium channels.

Blockade of striatal neurone responses to morphine by aminophylline: evidence for adenosine mediation of opiate action

1 The responses of cortical and striatal neurones to morphine and adenosine applied iontophoretically have been studied in the male rat. 2 The majority of cells (57%) within the corpus striatum were profoundly inhibited, and a smaller proportion (18%) excited by morphine. Adenosine depressed the firing rate of 30/44 cells in the striatum, excitation never being observed. In contrast, the responses of cortical cells to morphine were typically weak and required longer ejection pulses to effect comparable changes in firing rate. 3 Aminophylline applied iontophoretically, as an anion, proved able to antagonize reversibly both morphine and adenosine in parallel. 4 On a number of cells, gamma-aminobutyric acid (GABA) was used as a control depressant but aminophylline did not appear to reduce these responses. 5 The responses to morphine (both inhibitory and excitatory) were not easily antagonized by naloxone. Typically, excitatory reponses were easier to antagonize than the inhibitory ones. 6 It is concluded that a consequence of the interaction of morphine with its receptors may be the release of adenosine which subsequently produces the inhibition observed with morphine.

Adenosine inhibition of gamma-aminobutyric acid release from slices of rat cerebral cortex

1 The effect of purine compounds on the potassium-evoked release of 14C-labelled gamma-aminobutyric acid (GABA) has been studied in 400 micrometers slices of rat cerebral cortex in vitro. 2 Adenosine and adenosine 5' monophosphate (AMP) inhibited the release of GABA at 10(-5) to 10(-3) M. Adenosine triphosphate (ATP) produced a significant inhibition of release only at 10(-3) M. 3 Theophylline 10(-4) or 10(-3) M reduced the inhibitory effect of adenosine, but did not change basal release of GABA. 4 Dipyridamole 10(-5) M itself reduced evoked GABA release, but did not prevent the inhibitory effect of adenosine, implying that adenosine was acting at an extracellularly directed receptor. 5 Calcium removal or antagonism by verapamil reduced the evoked release of GABA, but adenosine did not produce any further reduction of the calcium-independent release. This may indicate that the inhibitory effect of adenosine on GABA release results from interference with calcium influx or availability within the terminals.

Adenine nucleotides and synaptic transmission in the in vitro rat hippocampus

1 The effects of adenosine and various derivatives were examined in the in vitro hippocampal slice preparation from rat.2 The amplitudes of extracellularly recorded field potentials from the CA1 region were depressed by adenosine, and this effect could be antagonized by methylxanthines. Because presynaptic field potentials were unaffected by adenosine, while the field e.p.s.p. was depressed, adenosine would appear to act at a synaptic site to depress transmission.3 Adenosine deaminase, which breaks down adenosine to inosine, increased the amplitude of synaptic responses, while hexobendine, which blocks reuptake of adenosine, had a depressant effect. This strongly suggests that the endogenous release of adenosine from the hippocampal slice preparation is sufficient to exert a tonic inhibitory influence on the amplitude of synaptic responses.4 Cyclic adenosine 3',5'-monophosphate (cyclic AMP) and its dibutyryl derivative had depressant effects on the amplitude of field responses which were blocked by theophylline, suggesting that they are able to act at the extracellular adenosine receptor. (-)-Isoprenaline (which raises tissue cyclic AMP levels), and the 8-p-chlorophenylthio derivative of cyclic AMP both increased the amplitude of population spike responses, and these effects were not blocked by theophylline, suggesting that the physiological effects of adenosine are not mediated via a cyclic AMP-dependent mechanism.5 Since adenosine is not the transmitter at this CA1 pyramidal cell synapse, but is apparently present in the extracellular compartment in sufficient concentrations to affect the synaptic physiology of this region, this provides strong evidence in favour of the concept of a neuromodulatory role for adenosine in the central nervous system.

Antidepressant drugs potentiate suppression by adenosine of neuronal firing in rat cerebral cortex

Adenosine and AMP (5'-adenosine monophosphate) applied by microiontophoresis produced depression of neuronal firing rates in cerebral cortex. A number of antidepressant drugs including examples which are known not to affect noradrenaline uptake systems, potentiated the depressant purine effects. Noradrenaline responses were unaffected or reduced. Purines may therefore be important in the mechanism of action of antidepressant drugs.

Antagonism by clonidine of neuronal depressant responses to adenosine, adenosine-5'-monophosphate and adenosine triphosphate

1. Adenosine and its nucleotides adenosine-5'-monophosphate (AMP) and adenosine triphosphate (ATP) have been applied by microiontophoresis to neurones in the cerebral cortex of rats anesthetized with urethane. The firing rate of most neurones was depressed, though two cells were encountered which showed biphasic responses to ATP consisting of an initial excitation succeeded by depression. 2. The application of clonidine with iontophoretic currents of less than 25 nA resulted in blockade of the depressant responses to the purines, without affecting responses to noradrenaline, 5-hydroxytryptamine or gamma-aminobutyric acid (GABA). At much higher doses of clonidine, direct depression of cell firing occurred and occasional interaction with noradrenaline was noted. 3. In the case of the biphasic responses to ATP, clonidine seemed to block only the depressant phase. Reduction of the excitatory component paralleled changes of background firing. 4. It is concluded that, in common with some other 2-substituted imidazoline derivatives, clonidine possesses the ability to block responses to purine compounds.