Gradient of Expression of Dopamine D2 Receptors Along the Dorso-Ventral Axis of the Hippocampus

Abstract

Dopamine D2-like receptors (D2R) play an important role in the regulation of hippocampal neuronal excitability and contribute to the regulation of synaptic plasticity, the encoding of hippocampus-dependent memories and the regulation of affective state. In line with this, D2R are targeted in the treatment of psychosis and affective disorders. It has been proposed that the dorso-ventral axis of the hippocampus can be functionally delineated into the dorsal pole that predominantly processes spatial information and the ventral pole that mainly addresses hippocampal processing of emotional and affective state. Although dopaminergic control of hippocampal information processing has been the focus of a multitude of studies, very little is known about the precise distribution of D2R both within anatomically defined sublayers of the hippocampus and along its dorsoventral axis, that could in turn yield insights as to the functional significance of this receptor in supporting hippocampal processing of spatial and affective information. Here, we used an immunohistochemical approach to precisely scrutinize the protein expression of D2R both within the cellular and dendritic layers of the hippocampal subfields, and along the dorso-ventral hippocampal axis. In general, we detected significantly higher levels of protein expression of D2R in the ventral, compared to the dorsal poles with regard to the CA1, CA2, CA3 and dentate gyrus (DG) regions. Effects were very consistent: the molecular layer, granule cell layer and polymorphic layer of the DG exhibited higher D2R levels in the ventral compared to dorsal hippocampus. D2R levels were also significantly higher in the ventral Stratum oriens, Stratum radiatum, and Stratum lacunosum-moleculare layers of the CA1 and CA3 regions. The apical dendrites of the ventral CA2 region also exhibited higher D2R expression compared to the dorsal pole. Taken together, our study suggests that the higher D2R expression levels of the ventral hippocampus may contribute to reported gradients in the degree of expression of synaptic plasticity along the dorso-ventral hippocampal axis, and may support behavioral information processing by the ventral hippocampus.

Keywords: dopamine receptor; dorso-ventral axis; hippocampus; immunohistochemistry; rodent.

Figure 1

Verification of antibody specificity. Example of a western blot showing binding specificity of the dopamine D2-like receptor (D2R) antibody used in the immunohistochemistry experiments. The antibody labeled a band of ca. 55 kDa, corresponding to the reported kDa weight of the target receptor as reported by others (Gemechu et al., 2018).

Figure 2

Illustration of hippocampal separation into longitudinal and transverse axes. (A) A drawing of the rat brain with horizontally sectioned hippocampus is presented. Representative images of immunohistochemically stained sections against D2R of the dorsal (a), intermediate (b) and ventral hippocampus (c) from the same animal are shown. Scale bar is 500 μm. Squares on each image correspond to zoom-in fragments on the panel (C). (B) Schematic representation of the hippocampal transverse axis and laminar separation (left). Immunohistochemically stained section with marked regions taken for the analysis is depicted (right). Abbreviations correspond to: ml, molecular layer of the DG; gcl, granule cell layer of the DG; pl, polymorphic layer of the DG; so, Stratum oriens of CA3/CA2/CA1; pcl, pyramidal cell layer of CA3/CA2/CA1; sr, Stratum radiatum of CA3/CA2/CA1; and slm, Stratum lacunosum-moleculare of CA3/CA2/CA1. (C) A closer view of subsections of the dentate gyrus (DG) and CA regions from the dorsal (a), intermediate (b) and ventral (c) hippocampal sections, including the laminar delineation, is shown.

Figure 3

In the DG, the dorsal component exhibits the lowest D2R expression. The molecular layer (ml) (A) and granule cell layer (gcl) of the ventral DG (B) exhibit higher D2R levels compared to the dorsal DG. The polymorphic layer (pl) (C) of the dorsal DG shows the lowest receptor expression as opposed to the ventro-intermediate DG. Values expressed in arbitrary units (a.u.). Error bars indicate standard error of the mean (SEM). *p < 0.05 or **p < 0.01. ml, molecular layer; gcl, granule cell layer; pl, polymorphic layer.

Figure 4

In the CA3 region, the ventral component shows the highest D2R expression. All layers of the ventral CA3 region, namely the Stratum oriens (so) (A), pyramidal cell layer (pcl) (B), Stratum radiatum (sr) (C), and Stratum lacunosum-moleculare (slm) (D) display high D2R levels compared to the dorso-intermediate CA3. Values expressed in arbitrary units (a.u.). Error bars indicate SEM. *p < 0.05, **p < 0.01, ***p < 0.001. so, Stratum oriens; pcl, pyramidal cell layer; sr, Stratum radiatum; and slm, Stratum lacunosum-moleculare.

Figure 5

In the CA2 region, D2R expression is highest in the ventral apical dendrites. The D2R is equally expressed in the Stratum oriens (so) (A) and pyramidal cell layer (pcl) (B) across the longitudinal axis of the CA2 region. Receptor protein expression is highest in the ventral Stratum radiatum (sr) (C) and Stratum lacunosum-moleculare (slm) (D) as opposed to dorso-intermediate CA2. Values expressed in arbitrary units (a.u.). Error bars indicate SEM. *p < 0.05, **p < 0.01, ***p < 0.001. so, Stratum oriens; pcl, pyramidal cell layer; sr, Stratum radiatum; and slm, Stratum lacunosum-moleculare.

Figure 6

D2R expression is higher in the ventral CA1 region. Proximal dendrites (A,C) of the ventral relative to the dorsal CA1 display high D2R protein levels. D2R show similar expression levels throughout the pyramidal cell layer (pcl) layer of the CA1 region (B). Distal dendrites of the ventral CA1 express higher D2R levels compared to the dorso-intermediate parts. (D) A gradient of expression of D2R occurs in the Stratum lacunosum moleculare. Expression is highest in the ventral, and lowest in the dorsal SLM. Values expressed in arbitrary units (a.u.). Error bars indicate SEM. *p < 0.05 or ***p < 0.001. so, Stratum oriens; pcl, pyramidal cell layer; sr, Stratum radiatum; and slm, Stratum lacunosum-moleculare.