D1 Dopamine Receptors in the Amygdala Exhibit Unique Properties

Dopamine D1-like receptor antagonism in amygdala impairs the acquisition of glucose-conditioned flavor preference in rats

This study examined the role of dopamine within the amygdala (AMY) in flavor preference learning induced by post-oral glucose. In Experiment 1, rats were trained with a flavor [conditioned stimulus (CS+)] paired with intragastric (IG) infusions of 8% glucose and a different flavor (CS-) paired with IG water infusions. The CS+ preference was evaluated in two-bottle tests following bilateral injection of the dopamine D1-like receptor antagonist, SCH23390 (SCH), into the AMY at total doses of 0, 12, 24 and 48 nmol. SCH produced dose-dependent reductions in CS+ intake but did not block the CS+ preference except at the two highest doses, which also greatly suppressed the CS intakes. In Experiment 2, new rats were injected daily in the AMY with either saline or SCH (12 nmol), prior to training sessions with CS+/IG glucose and CS-/IG water. In the two-bottle tests, SCH rats, unlike the control rats, failed to prefer the CS+ (55 vs. 81%). In Experiments 3 and 4, new rats were trained as in Experiment 2, except that brain injections were in the basolateral and central nuclei of the AMY, respectively. SCH rats learned to prefer the CS+ to the CS-, although their preference was weaker than that displayed by the control rats (Experiment 3: 59 vs. 80%; Experiment 4: 73 vs. 88%). These results show an essential role for D1-like receptor activation in the AMY in the acquisition of flavor preference learning induced by the post-oral reinforcing properties of glucose. A distributed network mediating flavor-nutrient incentive learning is discussed.

Low affinity binding of the classical D1 antagonist SCH23390 in rodent brain: potential interaction with A2A and D2-like receptors

Whereas structurally dissimilar D(1) antagonists competing for [(3)H]-SCH23390 binding recognize primarily one site in striatum, two distinct affinity states are observed in both amygdala and hippocampus. The binding profile of SCH23390 is similar in both of these regions, with the high affinity site (K(D) approximately 0.4 nM) consistent with D(1)/D(5) receptors. The appearance of the low affinity site (K(D) approximately 300 nM) is dependent upon the absence of MgCl(2), but independent of D(1) expression (i.e., still present in D(1) knockout mice). Although the density of high affinity state receptor is lower in hippocampus or amygdala of D(1) knockout mice, some residual binding remains, consistent with the known expression of D(5) receptors in these regions. Remarkably, in hippocampus, the affinity of the low affinity site is shifted rightward in the presence of the D(2) antagonist domperidone and is largely absent in the hippocampus of D(2) knockout animals. Additionally, this site is also shifted rightward in the presence of the A(2A) ligands SCH58261, CSC, or NECA, or in the absence of A(2A) receptors. The affinity of SCH23390 for this low affinity site is greater than seen for SCH23390 binding to D(2) receptors in heterologous expression systems, consistent with the hypothesis that both D(2) and A(2A) receptors are involved in the low affinity binding site. Therefore, we suggest that the heteromerization of D(2) and A(2A) receptors reported previously in vitro also may occur in the brain of both rats and mice.

Amygdaloid D1 receptors are not linked to stimulation of adenylate cyclase

In contrast to the classic signal transduction of D1 dopamine receptors in striatum or molecular expression systems, it has been reported that D1 receptor agonists do not stimulate adenylate cyclase in homogenates of microdissected nuclei of the amygdaloid complex. This article examines this phenomenon in detail to determine if lack of cAMP signaling in the amygdaloid complex is an experimental artifact, or an indication of a marked difference from the well-studied basal ganglia terminal fields. Thus, whereas dopamine agonists failed to increase cAMP synthesis in the amygdala, forskolin, guanine nucleotides, or Mg2+ were able to stimulate adenylate cyclase activity in the same preparations. Under several different conditions, caudate preparations responded more robustly than amygdaloid preparations, while amygdala homogenates exhibited higher basal production of cAMP. Whereas the beta-adrenergic agonist isoproterenol was able to stimulate cAMP efflux in membranes from both the caudate and amygdala under a variety of tested conditions, neither dopamine nor fenoldopam (D1 agonist) could stimulate adenylate cyclase in the amygdala. Additionally, while manipulation of Ca2+ and calmodulin affected the differential actions of dopamine in the caudate, no change in these parameters restored sensitivity to dopamine in the amygdala. Together, these data challenge the commonly accepted notion that cAMP is a mandatory signaling pathway for D1 receptors. Because it is now proven that G protein-coupled receptors can signal promiscuously, elucidation of the non-cAMP-dependent signaling mechanisms resulting from D1 activation is clearly critical in understanding how this important receptor functions in situ.