Comparison between the pharmacology of dopamine receptors mediating the inhibition of cell firing in rat brain slices through the substantia nigra pars compacta and ventral tegmental area

Abstract

1 Electrophysiological recordings were made from presumed dopaminergic neurones in the substantia nigra pars compacta and ventral tegmental area of rat brain slices. The ability of selective dopamine receptor agonists to hyperpolarize neurones and inhibit cell firing, as well as the ability of dopamine receptor antagonists to block responses to quinpirole were compared. 2 Six dopamine receptor agonists were examined for their ability to hyperpolarize neurones within the substantia nigra pars compacta. Of these, the most potent ligand tested was naxagolide with an EC50 value of 20 nM and estimated maximum of 10 mV. The rank order of agonist potency was naxagolide> quinpirole>apomorphine>dopamine. 3 Quinpirole was more potent at inhibiting cell firing in the substantia nigra pars compacta (pIC5o = 7.65 ± 0.06, n = 35) than in the ventral tegmental area (pIC5o = 7.24 ± 0.06, n = 32; P<0.01, Student's t test). 7-Hydroxy-NN-di-n-propyl-2-aminotetralin (7-OH-DPAT), a putative D3 selective agonist, had a comparable potency to quinpirole in both the ventral tegmental area (pICv = 7.39 ± 0.26, n = 4), and substantia nigra pars compacta (pIC5o = 7.71 ± 0.20; n = 4). 4 The inhibition of cell firing by quinpirole was antagonized by haloperidol, S(-)-sulpiride, clozapine, and ritanserin. S(-)-sulpiride and haloperidol had the highest estimated affinities in the substantia nigra, with pA2 values of 8.97 (slope = 0.85) and 8.20 (slope = 2.09) respectively. The pA2 values for S(-)-sulpiride and haloperidol in the ventral tegmental area were 8.07 (slope = 0.87) and 8.11 (slope = 1.48) respectively. Clozapine had a lower functional affinity than S(-)-sulpiride and haloperidol in both the substantia nigra (pA2 = 6.47, slope = 1.19) and ventral tegmental area (pA2 = 6.53, slope 0.87). Ritanserin, a 5-HT2 receptor antagonist that also binds to D2.u. dopamine receptors, caused a slight but significant shift in the concentration-effect curve to quinpirole with an estimated pKA of 6.97 ± 0.13 (n =4) in the substantia nigra and pKA of 7.12 ± 0.22 (n =4) in the ventral tegmental area. 5 Comparison of these data with the binding affinity for cloned dopamine receptors demonstrates that the responses to quinpirole on dopaminergic neurones in both the A9 (substantia nigra) and A10 (ventral tegmental area) brain areas are consistent with the activation of predominantly D2, and not D3 or D4 dopamine receptors. Furthermore, the similarity in functional affinity of antagonists for these receptors suggest that the mesolimbic selectivity of atypical neuroleptics, like clozapine, may be a consequence of their actions on other receptors or their effects elsewhere in the brain.

Increased desensitization of dopamine D2 receptor-mediated response in the ventral tegmental area in the absence of adenosine A2A receptors

“…Bath application (15 min) of single concentrations of quinpirole ranged between 1 nM and 1 µM inhibited dopamine neuron firing rate concentration-dependently, in agreement with previous reports (Bowery et al, 1994;Mercuri et al, 1997;Centonze et al, 2002). Complete inhibition was achieved in both wild-type ( In addition, the baseline firing frequency and the pacemaker firing pattern of dopamine neurons were also similar (P > 0.05; Student's t-test) between wild-type (1.8 ± 0.2Hz, n = 22) and adenosine A 2A receptor knockout mice (1.8 ± 0.3Hz, n = 14).…”

“…Action potential waveforms were also unchanged, indicating unaltered basic electrophysiological characteristics in dopamine neurons (data not shown). The D 2 receptor antagonist sulpiride (10 µM) blocked the effects of quinpirole in both genotypes without altering baseline firing rate (data not shown), confirming that the effect of quinpirole was mediated by D 2 receptors (Bernardini et al, 1991;Bowery et al, 1994;Mercuri et al, 1997;Centonze et al, 2002), and that the baseline firing rate of dopamine neurons in vitro was not affected by D 2 receptors (Grace & Onn, 1989;Johnson & North, 1992). However, the maximal effects of quinpirole were different.…”

“…D 2 receptor desensitisation is not normally apparent when agonists are used to inhibit dopamine neuron firing in vitro (Fig. 1) (Bowery et al, 1994;Mercuri et al, 1997;Centonze et al, 2002) or in vivo (Bunney et al, 1973;White & Wang, 1984;Kalivas et al, 1993;Sibley et al, 1993;Adell & Artigas, 2004), in agreement with their role of providing essential feedback autoinhibition on dopamine neurons. However, desensitisation can be induced with high concentrations of agonists (Beckstead & Williams, 2007), in particular with accumulative dosing (Fig.…”

Electrophysiological Pharmacology of Mesencephalic Dopaminergic Neurons

“…It is this hyperpolarization which can reach about 12 mV in vitro in response to a maximal concentration of quinpirole (Bowery et al 1994) that is responsible for the inhibition of spontaneous activity seen after local or systemic administration of autoreceptor agonists. The potassium channel linked to the dopamine autoreceptor in situ appears to be the same one that is opened by activation of GABA B receptors since the autoreceptor-mediated potassium current is reversibly occluded by maximal stimulation of the GABA B receptor by baclofen (Lacey et al 1988).…”

Tyramine excites rat subthalamic neurons in vitro by a dopamine-dependent mechanism

“…However, in the present study, we found that 3 μM sulpiride reduced tyramineinduced inward currents by only about 36%. This is surprising, because others (Bowery et al, 1994;Calabresi et al, 1992) and we (Shen and Johnson, 2000) have shown that this concentration of sulpiride is maximally effective in blocking D 2 -like responses in brain slice preparations. Thus, it is possible that the dopamine receptor mediating the tyramine effect may possess an unconventional pharmacological profile.…”