Intra-accumbens injection of a dopamine aptamer abates MK-801-induced cognitive dysfunction in a model of schizophrenia

Abstract

Systemic administration of the noncompetitive NMDA-receptor antagonist, MK-801, has been proposed to model cognitive deficits similar to those seen in patients with schizophrenia. The present work investigated the ability of a dopamine-binding DNA aptamer to regulate these MK-801-induced cognitive deficits when injected into the nucleus accumbens. Rats were trained to bar press for chocolate pellet rewards then randomly assigned to receive an intra-accumbens injection of a DNA aptamer (200 nM; n = 7), tris buffer (n = 6) or a randomized DNA oligonucleotide (n = 7). Animals were then treated systemically with MK-801 (0.1 mg/kg) and tested for their ability to extinguish their bar pressing response. Two control groups were also included that did not receive MK-801. Data revealed that injection of Tris buffer or the random oligonucleotide sequence into the nucleus accumbens prior to treatment with MK-801 did not reduce the MK-801-induced extinction deficit. Animals continued to press at a high rate over the entire course of the extinction session. Injection of the dopamine aptamer reversed this MK-801-induced elevation in lever pressing to levels as seen in rats not treated with MK-801. Tests for activity showed that the aptamer did not impair locomotor activity. Results demonstrate the in vivo utility of DNA aptamers as tools to investigate neurobiological processes in preclinical animal models of mental health disease.

Figure 1. Aptamer sequence.

Schematic of the aptamer sequence used in this experiment (top). The two complementary regions shown in red and the conserved bases shown in blue are thought to form the target binding site. Also shown is the sequence of the random oligonucleotide control (bottom).

Figure 2. Intra-accumbens aptamer injection reverses the MK-801-induced elevation in extinction pressing.

A) Cumulative correct lever responses per 5-min interval over the 30-min extinction session. B) Cumulative number of incorrect lever presses per 5-min interval over the 30-min extinction session. Groups pretreated with intra-accumbens vehicle (0 nM/ MK; n = 7) or the random oligonucleotide sequence (Random/ MK; n = 7) and systemically with 0.1 mg/ kg MK-801 showed elevated correct lever pressing throughout the entire 30-min extinction session compared to animals pretreated with 200 nM dose of the aptamer (200 nM/ MK; n = 7) and both groups treated systemically with saline (0 nM/ saline; n = 5 and 200 nM/ saline; n = 5). Correct extinction pressing was not significantly different between the 200 nM/ MK group and the 0 nM/ saline or the 200 nM/ saline groups. *, p<0.05; numbers in parentheses = animals per group. No significant differences were detected when measuring incorrect lever presses during the extinction session. Data expressed as mean ± SEM.

Figure 3. Horizontal activity was not affected by intra-accumbens injection of the aptamer.

Locomotor activity was assessed in an elevated cross maze for 30 min. Activity was measured as total distance in meters traveled (A), average speed in meters/ sec (B) and total arm entries (C). No main effect of group was detected. Arm entries in all MK-801-injected groups (0 nM/ MK, 200 nM/ MK and Random/ MK; n = 5 per group) were significantly different from the 200 nM/ saline-treated group (n = 5) (*, p<0.05). The saline treated groups were not significantly different on any measure of activity. Data are expressed as mean ± SEM.

Figure 4. Representative histological sections from nucleus accumbens show reduced pTH staining in aptamer-treated rats.

A) Quantification of the phosphorylated tyrosine hydroxylase (pTH) to tyrosine hydroxylase (TH) ratio in the nucleus accumbens shell region (AcbShell) shows a lower ratio in the 200 nM/ MK group (** p<0.01). B) Quantification of TH staining in the AcbShell shows similar levels between the 0 nM/ MK group and 200 nM/ MK group. C) Quantification of pTH staining in the AcbShell shows lower levels of staining in the 200 nM/ MK group compared to the 0 nM/ MK group (** p<0.01). n = 3/ group. Abbreviations: ACA, anterior commisure; AcbCore, nucleus accumbens core; AcbShell, nucleus accumbens shell. Scale bars = 500 µm. Magnifications shown at top of 0 nM images. Data expressed as mean ± SEM.