strychnine and 1-3-dipropyl-8-cyclopentylxanthine

It is not well-studied how the ubiquitous neuromodulator adenosine (ADO) affects mammalian locomotor network activities. We analyzed this here with focus on roles of 8-cyclopentyl-1,3-dipropylxanthine (DPCPX)-sensitive A(1)-type ADO receptors. For this, we recorded field potentials from ventral lumbar nerve roots and electrically stimulated dorsal roots in isolated newborn rat spinal cords. At ≥ 25μM, bath-applied ADO slowed synchronous bursting upon blockade of anion-channel-mediated synaptic inhibition by bicuculline (20 μM) plus strychnine (1 μM) and this depression was countered by DPCPX (1 μM) as tested at 100 μM ADO. ADO abolished this disinhibited rhythm at ≥ 500 μM. Contrary, the single electrical pulse-evoked dorsal root reflex, which was enhanced in bicuculline/strychnine-containing solution, persisted at all ADO doses (5 μM-2 mM). In control solution, ≥ 500 μM ADO depressed this reflex and pulse train-evoked bouts of alternating fictive locomotion; this inhibition was reversed by 1 μM DPCPX. ADO (5 μM-2 mM) did not depress, but stabilize alternating fictive locomotion evoked by serotonin (10 μM) plus N-methyl-d-aspartate (4-5 μM). Addition of DPCPX (1μM) to control solution did not change either the dorsal root reflex or rhythmic activities indicating lack of endogenous A(1) receptor activity. Our findings show A(1) receptor involvement in ADO depression of the dorsal root reflex, electrically evoked fictive locomotion and spontaneous disinhibited lumbar motor bursting. Contrary, chemically evoked fictive locomotion and the enhanced dorsal root reflex in disinhibited lumbar locomotor networks are resistant to ADO. Because ADO effects in standard solution occurred at doses that are notably higher than those occurring in vivo, we hypothesize that newborn rat locomotor networks are rather insensitive to this neuromodulator.

Topics: Adenosine; Adenosine A1 Receptor Antagonists; Animals; Animals, Newborn; Bicuculline; Data Interpretation, Statistical; Electric Stimulation; Electrophysiological Phenomena; GABA Antagonists; Locomotion; Medulla Oblongata; Nerve Net; Rats; Rats, Sprague-Dawley; Rats, Wistar; Receptor, Adenosine A1; Spinal Cord; Spinal Nerve Roots; Strychnine; Xanthines

The pre-Bötzinger complex (preBötC) is a central pattern generator within the ventrolateral medulla oblongata's ventral respiratory group that is important for the generation of respiratory rhythm. Activation of adenosine A(1) receptors (A(1)R) depresses preBötC rhythmogenesis. Although it remains unclear whether A(1)R activation is important for organisms in a normal metabolic state, A(1)R activation is important to the response of the preBötC to metabolic stress, such as hypoxia. This study examined mechanisms linking A(1)R activation to depression of preBötC rhythmogenesis in medullary slice and island preparations from neonatal mice.. Converting medullary slices to islands by cutting away much of the medullary tissue adjacent to the preBötC decreased the amplitude of action potential bursts generated by a population of neurons within the preBötC (recorded with an extracellular electrode, and integrated using a hardware integrator), without noticeably affecting burst frequency. The A(1)R agonist N6-Cyclopentyladenosine (NCPA) reduced population burst frequency in slices by ca. 33% and in islands by ca. 30%. As in normal (drug-free) artificial cerebrospinal fluid (aCSF), NCPA decreased burst frequency in slices when GABA(A)ergic or GABA(A)ergic and glycinergic transmission were blocked, and in islands when GABA(A)ergic transmission was antagonized. Converting slices to island preparations decreased synaptic input to inspiratory neurons. NCPA further decreased the frequency of synaptic inputs to neurons in island preparations and lowered the input resistance of inspiratory neurons, even when chemical communication between neurons and other cells was impeded.. Together these data support the suggestion that depression of preBötC activity by A(1)R activation involves both decreased neuronal excitability and diminished inter-neuronal communication.

Topics: Adenosine; Adenosine A1 Receptor Agonists; Adenosine A1 Receptor Antagonists; Animals; Animals, Newborn; Bicuculline; Electrophysiology; Evoked Potentials; Female; In Vitro Techniques; Male; Medulla Oblongata; Mice; Neural Inhibition; Neurons; Patch-Clamp Techniques; Pyridazines; Receptor, Adenosine A1; Respiratory Center; Respiratory Mechanics; Strychnine; Synaptic Transmission; Xanthines

Delta-opioid receptor (DOPr) activation fails to produce cellular physiological responses in many brain regions, including the periaqueductal gray (PAG), despite neural expression of high densities of the receptor. Previous histochemical studies have demonstrated that a variety of stimuli, including chronic morphine treatment, induce the translocation of DOPr from intracellular pools to the surface membrane of CNS neurons. PAG neurons in slices taken from untreated mice exhibited mu-opioid receptor (MOPr) but not DOPr-mediated presynaptic inhibition of GABAergic synaptic currents. In contrast, after 5-6 d of chronic morphine treatment, DOPr stimulation inhibited synaptic GABA release onto most neurons. Shorter exposure to morphine in vitro (upto 4 h) or in vivo (18 h) did not induce functional DOPr responses. DOPr-mediated presynaptic inhibition could not be induced in slices from untreated animals by increasing synaptic activity in vitro using high extracellular potassium concentrations or activation of protein kinase A. Induction of functional DOPr signaling by chronic morphine required MOPr expression, because no DOPr receptor responses were observed in MOPr knock-out mice. DOPr agonists also had no effect on miniature IPSCs in beta-arrestin-2 knock-out mice after chronic morphine. These results suggest that induction of DOPr-mediated actions in PAG by chronic morphine requires prolonged MOPr stimulation and expression of beta-arrestin-2.

Topics: 6-Cyano-7-nitroquinoxaline-2,3-dione; Animals; Arrestins; beta-Arrestin 2; beta-Arrestins; Dose-Response Relationship, Drug; Drug Administration Schedule; Drug Interactions; Electric Stimulation; Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; Enkephalin, Leucine; Excitatory Amino Acid Antagonists; G Protein-Coupled Inwardly-Rectifying Potassium Channels; gamma-Aminobutyric Acid; Glycine Agents; In Vitro Techniques; Isoquinolines; Male; Membrane Potentials; Mice; Mice, Inbred C57BL; Mice, Knockout; Morphine; Narcotic Antagonists; Narcotics; Neurons; Oligopeptides; Patch-Clamp Techniques; Periaqueductal Gray; Protein Kinase Inhibitors; Receptors, Opioid, delta; Receptors, Opioid, mu; Strychnine; Sulfonamides; Synaptic Transmission; Time Factors; Xanthines

Intrathecal (i.t.) strychnine produces localized allodynia in the rat without peripheral or central nerve injury. Intrathecal CPA (A1-selective agonist) and CGS-21680 (A2-selective agonist) dose-dependently inhibited strychnine-allodynia but with a 50-fold difference in potency (0.02-0.07 vs. 2.7-3.1 microgram, respectively). The anti-allodynic effect of CPA and CGS was completely blocked by pretreatment with the A1-selective antagonist, DPCPX (10 microgram i.t. ), but unaffected by the A2-selective antagonist, CSC (2 microgram i.t. ). The results indicate that spinal A1-, but not A2-, receptors modulate abnormal somatosensory input in the strychnine model, and suggest a difference in spinal purinergic modulation in injury vs. non-injury models of allodynia.

Topics: Adenosine; Animals; Blood Pressure; Caffeine; Heart Rate; Injections, Spinal; Male; Pain; Phenethylamines; Purinergic P1 Receptor Agonists; Purinergic P1 Receptor Antagonists; Rats; Rats, Sprague-Dawley; Receptors, Purinergic P1; Spinal Cord; Strychnine; Xanthines

1. An eye-cup preparation in anaesthetized rabbits was used to examine opioid modulation of acetylcholine (ACh) release from cholinergic neurones in the retina. 2. The mu-opioid receptor agonist, [D-Ala2, MePhe4, Gly-ol5]-enkephalin (DAMGO), when applied locally to the retina at concentrations between 1-30 microM significantly increased the light-evoked release of ACh. The effect of DAMGO was completely blocked by the selective mu-receptor antagonist CTOP but the kappa-receptor antagonist nor-binaltorphimine (norBNI) did not affect the action of DAMGO on ACh release indicating that the opioid produced its effect by activation of mu-receptors (the rabbit retina has negligible delta-receptors). 3. Blockade with bicuculline and strychnine of GABAergic and glycinergic inputs to the cholinergic neurones did not affect the action of DAMGO on ACh release. Also DAMGO did not reduce the potassium-evoked release of either GABA or glycine from rat isolated retinas. 4. Exposure of the rabbit retina to a combination of an A1-adenosine receptor antagonist, 8-cyclopentyl-1,3 dipropylxanthine (DPCPX), and adenosine deaminase did not affect the enhancing action of DAMGO on the light-evoked release of ACh. 5. When the retina in the rabbit eye-cup was exposed to kainate, the release of ACh was increased by approximately three times the resting release. In the presence of DAMGO the kainate-evoked release of ACh was enhanced by 44%. 6. These experiments show that activation of mu-opioid receptors by DAMGO increases the release of ACh elicited by physiological stimulation (flickering light). Since we could find no evidence thatDAMGO reduces inhibitory inputs to the cholinergic neurones, it seems that the enhancing action ofDAMGO on the light-evoked release of ACh involves a direct excitatory effect rather than disinhibition.This conclusion is supported by the enhancing action of DAMGO on the kainate-evoked release of ACh because kainate is thought to act directly on the cholinergic neurones.

Topics: Acetylcholine; Animals; Bicuculline; Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; Enkephalins; gamma-Aminobutyric Acid; Glycine; Kainic Acid; Male; Rabbits; Rats; Receptors, Opioid, mu; Retina; Strychnine; Xanthines