Proceedings of The Physiological Society

Europhysiology 2018 (London, UK) (2018) Proc Physiol Soc 41, PCB266

Poster Communications

Investigation of NMDA receptor modulation by adenosine A2A and D2 dopamine receptor activation in rat substantia nigra dopaminergic neurones.

R. Al-Hosni1, F. Cherchi2, X. Cai3, W. Lei3, Z. Huang3, E. Coppi2, A. Gibb1

1. Pharmacology, UCL, London, United Kingdom. 2. Neurofarba, University of Florence, Florence, Italy. 3. Department of Molecular and Cellular Pharmacology, Peking University, Beijing, China.

Substantia nigra (SN) dopaminergic neurons degenerate in Parkinson's Disease (PD). Ropinorole, a D2 receptor agonist is currently used in PD therapy and D2 receptor activation has been shown to decrease NMDA responses in hippocampus and striatum. SN dopaminergic neurons express D2 autoreceptors and somatodendritically released dopamine mediates a negative feedback via D2 receptors on dopaminergic neuron activity. Via cAMP signalling, Gas-coupled adenosine A2A receptors (A2ARs) have the potential to counteract modulation by Gai -coupled D2Rs. Because cAMP regulates protein kinase A, an enzyme that phosphorylates NMDARs and increases NMDAR responses, these mechanisms have the potential to regulate NMDAR signaling. Here we tested the effects of D2R activation with ropinirole, and A2AR activation with CGS-21680. Whole-cell patch clamp recordings were made from DAergic neurons in the SNc of acute midbrain slices from neonatal (P7, P21 and P28) rats. Dopaminergic neurons were identified by the presence of a prominent hyperpolarisation-activated inward current (In P7 rats, amplitude, 173 ± 34 pA; activation time constant, 797 ± 113 ms; mean ± S.E.M., n = 11) in response to a voltage step from -60 to -120 mV. In each cell, two successive responses to 20 mM NMDA with 10 mM glycine were recorded. In P7 rats, control experiments gave NMDA responses of 919 ± 89 pA (1st response) and 881 ± 113 pA, n = 11 (2nd response) while in P28 rats the control NMDA responses were 790 ± 110 pA (1st response) and 800 ± 93 pA, n = 10 respectively. In P7 or P28 rats, upon application of 200 nM ropinirole, the steady state NMDA current was not significantly changed suggesting D2R activation may not modulate NMDARs in neonatal rat SNc. The A2AR agonist, CGS-21680 (1 or 10 mM) was applied in absence or presence of ropinirole to P28 dopaminergic neurons to determine the effect of activating this Gas - coupled receptor which could be in a D2R-A2AR heteromeric complex. 1 mM CGS-21680 gave a 46 ± 16% increase in NMDA response (P = 0.05, paired t-test; n = 12) while in the combined presence of 10 mM CGS-21680 and 200 nM ropinirole, the NMDA current increased by 59 ± 15% (P = 0.002, paired t-test; n = 16). These data suggest D2Rs do not modulate dopaminergic neuron NMDA receptors, at least under the conditions of these experiments. It is possible a D2 effect might be observed in older rats. Furthermore, limitations involved in the whole-cell patch-clamp configuration, such as dilution of cellular components could affect the result. In contrast, activation of A2A receptors enhanced the NMDA response.

Where applicable, experiments conform with Society ethical requirements