omega-agatoxin-iva and 1-3-dipropyl-8-cyclopentylxanthine

R-type calcium channels in postsynaptic spines signal through functional calcium microdomains to regulate a calcium/calmodulin-sensitive potassium channel that in turn regulates postsynaptic hippocampal long-term potentiation (LTP). Here, we ask whether R-type calcium channels in presynaptic terminals also signal through calcium microdomains to control presynaptic LTP. We focus on presynaptic LTP at parallel fiber to Purkinje cell synapses in the cerebellum (PF-LTP), which is mediated by calcium/calmodulin-stimulated adenylyl cyclases. Although most presynaptic calcium influx is through N-type and P/Q-type calcium channels, blocking these channels does not disrupt PF-LTP, but blocking R-type calcium channels does. Moreover, global calcium signaling cannot account for the calcium dependence of PF-LTP because R-type channels contribute modestly to overall calcium entry. These findings indicate that, within presynaptic terminals, R-type calcium channels produce calcium microdomains that evoke presynaptic LTP at moderate frequencies that do not greatly increase global calcium levels.

Topics: Adenosine A1 Receptor Antagonists; Analysis of Variance; Animals; Animals, Newborn; Calcium; Calcium Channel Blockers; Calcium Channels, R-Type; Calcium Signaling; Cerebellum; Dose-Response Relationship, Drug; Electric Stimulation; Excitatory Amino Acid Antagonists; GABA Antagonists; In Vitro Techniques; Long-Term Potentiation; Membrane Microdomains; Neural Pathways; Nickel; omega-Agatoxin IVA; omega-Conotoxin GVIA; Patch-Clamp Techniques; Phosphinic Acids; Piperidines; Presynaptic Terminals; Propanolamines; Purkinje Cells; Pyrazoles; Quinoxalines; Rats; Rats, Sprague-Dawley; Sodium Channel Blockers; Spider Venoms; Tetrodotoxin; Xanthines

Presynaptic adenosine receptors inhibit transmitter release at many synapses and are known to exist on retinotectal terminals. In this paper we show that adenosine decreases retinotectal field potentials by approximately 30% and investigate the mechanism. First, as judged by the effects of specific calcium channel blockers, retinotectal transmission was mediated almost exclusively by N-type calcium channels, which are known to be modulated by adenosine A1 receptors. Transmission was completely blocked by either omega-Conotoxin GVIA (-100%, N-type blocker) or omega-Conotoxin MVIIC (-99%, N-, P- and Q-type blocker) and was not significantly affected by omega-Agatoxin IVA [+1.7 +/- 9. 3% (SE), P-,Q-type blocker], but was augmented slightly by nifedipine(+9.3 +/- 2.1%, L-type blocker). Second, the adenosine inhibition was presynaptic, as indicated by a 43% increase in paired-pulse facilitation. Third, the selective A1 agonist cyclohexyl adenosine (CHA) at 50 nM caused a 21% decrease in amplitude and the selective A2 agonist N6-[2-(3, 5-dimethoxyphenyl)-2-(2-methylphenyl)-ethyl]adenosine (DPMA) at 100 nM caused a 24% increase. Fourth, the selective A1 antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) alone produced an increase in the field potential, suggesting a tonic inhibition mediated by endogenous adenosine. Fifth, pertussis toxin eliminated adenosine inhibition implicating Gi or Go protein coupling. Sixth, C-kinase activation eliminated the A1-mediated inhibition. In regenerating projections, adenosine also caused a decrease in transmission (-30 +/- 12%), but after induction of long-term potentiation (LTP) via trains of stimuli or via treatment with the phosphatase inhibitor okadaic acid, the adenosine response was converted to an augmentation. Because LTP is associated with C-kinase activation, this is consistent with C-kinase uncoupling the A1 receptor from inhibiting N-type Ca2+ channels. This uncovers the A2-mediated augmentation as demonstrated in normals with DPMA. Such an effect could account in part for the LTP of immature synapses and the change from rapidly fatiguing to robust synaptic transmission.

Topics: Adenosine; Animals; Calcium; Enzyme Activation; Evoked Potentials, Visual; Goldfish; Long-Term Potentiation; Nerve Crush; Nerve Regeneration; Nifedipine; omega-Agatoxin IVA; omega-Conotoxin GVIA; omega-Conotoxins; Optic Nerve; Optic Nerve Injuries; Peptides; Pertussis Toxin; Protein Kinase C; Receptors, Purinergic P1; Retina; Spider Venoms; Superior Colliculi; Synaptic Transmission; Virulence Factors, Bordetella; Visual Pathways; Xanthines

In the hippocampus, the neuromodulatory role of adenosine depends on a balance between inhibitory A1 responses and facilitatory A2A responses. Since the presynaptic effects of hippocampal inhibitory A1 adenosine receptors are mostly mediated by inhibition of Ca2+ channels, we now investigated whether presynaptic facilitatory A2A adenosine receptors would modulate calcium influx in the hippocampus. The mixed A1/A2 agonist, 2-chloroadenosine (CADO; 1 microM) inhibited veratridine (20 microM)-evoked 45Ca2+ influx into hippocampal synaptosomes of the CA1 or CA3 areas by 24.2 +/- 4.5% and 17.2 +/- 5.8%, respectively. In the presence of the A, antagonist, 1,3-dipropyl-8-cyclopentylxanthine (DPCPX; 100 nM), the inhibitory effect of CADO (1 microM) on 45Ca2+ influx was prevented in CA1 synaptosomes, but was converted into a facilitatory effect (14.2 +/- 6.7%) in CA3 synaptosomes. The A2A agonist, CGS 21680 (3-30 nM) facilitated 45Ca2+ influx in CA3 synaptosomes, with a maximum increase of 22.9 +/- 3.9% at 10 nM, and was virtually devoid of effect in CA1 synaptosomes. This facilitatory effect of CGS 21680 (10 nM) in CA3 synaptosomes was prevented by the A2A antagonist 8-(3-chlorostyryl)caffeine (CSC; 200 nM), but not by the A1 antagonist, DPCPX (20 or 100 nM). The facilitatory effect of CGS 21680 on 45Ca2+ uptake by CA3 synaptosomes was prevented by the class A calcium channel blocker, omega-agatoxin-IVA (200 nM). These results indicate that presynaptic adenosine A2A receptors facilitate calcium influx in the CA3 but not the CA1 area of the rat hippocampus through activation of class A calcium channels.

Topics: 2-Chloroadenosine; Adenosine; Animals; Caffeine; Calcium Channel Blockers; Calcium Channels; Calcium Radioisotopes; Hippocampus; Male; omega-Agatoxin IVA; Phenethylamines; Purinergic P1 Receptor Agonists; Rats; Rats, Wistar; Receptors, Purinergic P1; Spider Venoms; Synaptosomes; Veratridine; Xanthines