Non-selective attention and nitric oxide in putative animal models of Attention-Deficit Hyperactivity Disorder

Abstract

Non-selective attention (NSA) to environmental stimuli has been measured in putative animal models of Attention-Deficit Hyperactivity Disorder (ADHD), such as the Spontaneously Hypertensive (SHR) and the Naples High-Excitability (NHE) rat lines. A series of experiments has been carried out on male juvenile SHR and Wistar–Kyoto (WKY) controls (experiment 1) and on the NHE and two controls, i.e. the Naples Low-Excitability (NLE) and a random-bred (NRB) line (experiment 2). It was done under basal conditions or following a single injection of the nitric oxide synthase (NOS) inhibitor l-nitro-arginine-methylester (l-NAME; 0.1–10 mg/kg, i.p.), or vehicle, 30 min before testing on day 1 and vehicle alone before testing on days 2 and 3 in SHR/WKY (experiment 3) and the Naples lines (experiment 4). The behavior in a Làt maze during three consecutive 10-min exposures at 24-h intervals was monitored by a CCD video camera and analyzed off-line for frequency and duration of rearings on hindlimbs per 1-min blocks. The results demonstrated that both SHR and NHE rats showed a higher frequency of rearings of shorter duration than controls. With time of testing, the duration of rearings tended to increase in the WKY but not the SHR. In the Naples lines the duration tended to increase in all but mostly in the NHE rats. The acute inhibition of NOS by l-NAME significantly increased the duration of rearing episodes both in SHR and NHE rats only at 10 mg/kg in the second part of the testing period. Therefore, NSA, as indexed by the duration of rearings, is defective in both hyperactivity models against different genetic backgrounds. In addition, this impairment is dependent upon nitric oxide (NO), which appears to play a significant role in these processes.

Introduction

Attention-Deficit Hyperactivity Disorder (ADHD) is characterized by attentional problems accompanied or not by compulsiveness, hyperkinesis, restlessness, disturbances in timing, and affects mostly male children in 4:1 ratio (DSM-IV) 1, 2.

Several variants have been described with prevailing attentional or activity problems [3]. A high genetic contribution, estimated at 30–50%, has been suggested by family studies and the allelic polymorphism of the gene coding for the dopamine (DA) receptor D-4 and the DA transporter (DAT) [4]protein [5], which displays in ADHD children a higher frequency number of variable number tandem repeat (VNTR) sequences coding for DAT than expected [6].

Functional imaging studies and magnetic resonance-based morphometry [7]have revealed an asymmetry in the cortico-striato-pallidal system with atrophy on the right side of the brain.

In order to investigate the neural substrates responsible for the main aspects of ADHD in children, model systems have been used such as the Spontaneously Hypertensive (SHR) 8, 9, the Wistar–Kyoto hyperactive rat (WKHA) [10], and the Naples High-Excitability (NHE) [11]rat lines. In fact, these lines are all hyperactive, though to a different degree [12]and in different contexts 13, 9. So far only the SHR show sustained attention deficits in behavioral paradigms, thus featuring the main symptoms of the human syndrome [8]. However, given the heterogeneity of ADHD with several variants, different animal models are useful (i) to feature its different aspects, and (ii) to validate the relevance of the putative neural substrates across different genetic backgrounds, independent of such confounding variables as arterial hypertension.

A multiple and widely distributed network system is thought to control attention processes 14, 15, 16. Attentive processes are measured in laboratory animals in different behavioral settings and paradigms 17, 16, which require extensive training. While selective attention towards environmental stimuli has been extensively studied, non-selective attention (NSA) and its relevance to the former are not yet ascertained.

Recently, we have focused upon NSA by monitoring the duration of rearing episodes in a novelty situation, a variable familiar to all observers of animal behavior, which, however, has received little systematic approach. Duration of rearings is of extreme interest for the screening of drugs, which affect attentive processes, and which should be followed by a careful and thorough investigation in complex behavioral settings. In addition, while selective attention requires manipulation of the motivational level by food- or water-deprivation, NSA can be measured at different motivational levels in the absence of such manipulations (Aspide et al., in preparation; [18]).

The aim of the studies reported here was to investigate NSA in putative animal models of ADHD, i.e. the juvenile SHR and the NHE/NLE rat lines. On the one hand, the juvenile prehypertensive SHR is homologous to the WKHA (see above). On the other hand, the NHE and NLE rats, which are hyperactive and hypoactive, respectively, allow the study of NSA at different activity levels. Therefore, the Naples lines should complement the studies on SHR and WKHA. Moreover, a series of pharmacological experiments has been carried out to manipulate the brain systems which might underlie the process of NSA. Consequently, drugs were used which are known to interfere with synaptic transmission, such as methylphenidate, which blocks the re-uptake of biogenic amines, and DA, in particular, into the synaptic terminal (MP) [19]or interfere with extra-synaptic transmission such as inhibitors of the enzyme nitric oxide synthase (NOS), which forms nitric oxide (NO) from the amino acid l-arginine by activation of glutamate receptors of the NMDA type in a Ca²⁺-dependent manner [20].