Irreversible receptor inactivation reveals differences in dopamine receptor reserve between A9 and A10 dopamine systems: an electrophysiological analysis

Partial receptor inactivation was used as a tool to examine whether differences in receptor reserve exist between the dopamine receptor populations which mediate responses of substantia nigra (A9) and ventral tegmental area (A10) dopamine neurons to dopamine agonist drugs. The irreversible receptor inactivator, N -ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ), was administered to rats intraperitonealy at a dose of 6 mg/kg (in an ethanol-water vehicle). Approximately 24 h after EEDQ treatments, extracellular, single-unit recording experiments were carried out. In the first series of experiments, dose-response curves were constructed for the inhibition of A9 and A10 dopamine cell firing by intravenous administration of the potent dopamine agonist, R-(−)- N-n -propylonorapomorphine (NPA). For the A9 dopamine cell group, EEDQ pretreatments caused a 3-fold rightward shift in the NPA dose-response curve (ED 50 s, 0.3 vs 0.8 μg/kg for vehicle- and EEDQ-treated rats, respectively), but there was no change in the maximum attainable response (>95% inhibition of cell firing). For A10 neurons, the same EEDQ treatments produced a greater rightward shift in the dose-response curve to NPA (ED 50 s, 0.6 vs 5.4 μg/kg for vehic and EEDQ-treated rats), and also depressed the maximum response by about 25% relative to the control (vehicle) curve. The dose-response curves from each region were subjected to Furchgott analysis to determine relative receptor occupancy-response relationships for NPA. For the A9 system, a steep, hyperbolic occupancy-response plot revealed that a 50% inhibitory response required only 4% receptor occupancy, while complete (>95%) inhibition of cell firing required about 30% occupancy. This suggests about a 70% receptor reserve for this agonist in inhibiting A9 dopamine cell firing. The occupancy-response curve for A10 cells was less steep with 50% and maximal (>95%) responses occurring when 11 and 70% of receptors were occupied by the agonist, indicating only about a 30% reserve for A10 cell responses to NPA. While the level of ‘spare’ receptors differed substantially between the two areas, calculatedpseduo-K A values were similar (7.7 μg/kg for A9 cells and 5.5 μg/kg for A10 cells), suggesting no regional differences in receptor affinity. To explore where the differences in receptor reserve might reside, a secind series of studies evaluated the effects of iontophoretically applied dopamine and NPA on both cell groups in vehicle- and EEDQ-treated rats. EEDQ (6 mg/kg, i.p.) treatments lessened the ability of both iontophoresed agonists to inhibit firing in both cell groups, but caused equivalent parallel shifts to the right in the current-response curves to both dopamine and NPA for both cell groups. This finding suggests that the apparent difference in receptor reserve between A9 and A10 systems is probably not attributable to differences at the level of the somatic autoreceptors reachable by iontophoresed drugs, but may be due to differences at more distal dendritic autoreceptors or at other sites in the circuitry which may indirectly mediate midbrain neuronal responses to systemically administered agonists.