Effects of NMDA receptor antagonists on the binding of agonistic and antagonistic radiotracers to striatal dopamine D2 receptors

Introduction Ketamine and phencyclidine (PCP) act as noncompetitive antagonists to the N-methyl-D-aspartate (NMDA) receptor. Ketamine and PCP both bind to the PCP recognition site in the Ca2+ channel domain, and exert psychotomimetic and anesthetic effects at relatively low and high doses, respectively. In our previous monkey PET study we noticed that ketamine and xylazine anesthesia increased the binding potential of the dopamine D2 receptor agonist [11C]MNPA (Tokunaga et al., J Neurosci 2009). This could be in line with the view that NMDA receptor antagonists produce psychotomimetic effects through alterations of the dopaminergic neurotransmission. However, mechanisms by which NMDA receptor antagonists modulate D2 receptor binding remain unclear. In the present study, we have investigated the effect of ketamine (at an anesthetic dose) and PCP (at a psychotomimetic dose) on the binding of agonistic and antagonistic radioligands to striatal D2 receptors. In addition, we have pursued the neurochemical basis of this crosstalk between NMDA and D2 receptors.Methods Male Sprague-Dawley rats weighing 300-400 g were operated in order to fixate their heads during the PET measurement under a conscious state. PET scans of rats were performed with a microPET FOCUS220 system for 90 min after administration of agonistic ([11C]MNPA) and antagonistic ([11C]raclopride) radiotracers. Anatomical regions of interests (ROIs) were manually defined on the striatum and cerebellum in PET images coregistered with MR images using PMOD® software. Binding potential (BPND) values were obtained using the multilinear reference tissue model (MRTM), with cerebellum used as reference region. Rats were also treated with ketamine (90 mg/kg/h) and PCP (3 mg/kg/h) starting 30 min before the scan start using repeated intravenous boluses. Microdialysis studies were performed on conscious rats which were pretreated with ketamine and PCP, similar as in the PET measurements. Dialysate samples were collected from the striatum every 10 minutes, and DA contents were measured by HPLC with an ECD system. Autoradiographic analysis of coronal brain sections was performed using brains obtained from untreated and treated rats. Striatal [11C]raclopride binding was densitometrically quantified.Results and Discussion During ketamine anesthesia, striatal BPND values for both agonistic and antagonistic radioligands were increased by approximately 20%, while ketamine-induced changes of DA contents were barely detectableby microdialysis in the rat striatum. In vitro autoradiography studies demonstrate a ketamine-induced increase in the striatal Bmax for [11C]raclopride, but no change in Kd. The PCP-induced increase of BPND for [11C]MNPA was however significantly larger than that for [11C]raclopride.Conclusion Ketamine did not modify dopamine levels as measured by microdialysis. The similar increase in agonistic and antagonistic radioligand binding indicated that ketamine, at an anesthetic dose, does not alter the functional states of D2 receptors (i.e. high-affinity and low-affinity states). The autoradiography results confirm that ketamine enhances radioligand binding in vivo due to an increase in Bmax. In contrast, PCP-enhanced radioligand binding may result from a change in the functional states of D2 receptors, which could be an effect characteristic of PCP receptor ligands at a psychotomimetic dose.The 9th International Symposium on Functional Neuroreceptor Mapping of the Living Brain (NRM12