The hippocampal input to the nucleus accumbens was studied by correlative electrophysiological and anatomical techniques in acutely prepared rabbits. Field and extracellular unitary potentials were recorded in the nucleus accumbens following ipsilateral fimbria stimulation. Analysis of the components of the field response was based on the relevant correlations with extracellular unitary activity. The cellular types that are the recipients of the hippocampal projection were determined by combined intracellular horseradish peroxidase (HRP) and Golgi analyses. The distribution of the hippocampal input was determined by combined field potential and current source density analyses. It was found that the ipsilateral fimbria projection was distributed to the dorsal two-thirds of the nucleus, with the projection being heaviest in the more caudal portions of the nucleus. The negative (N) component of the field response was studied by correlating its behavior with the appropriate extracellular unitary recordings. It was concluded that the N-component represented an envelope of monosynaptically activated action potentials. The positive (P) component of the field response throughout the nucleus accumbens was studied pharmacologically with the iontophoretic administration of bicuculline. The P-components, in both the dorsal and ventral regions of the nucleus, were diminished by bicuculline application, indicating that this potential results from the activation of gamma-aminobutyric acid (GABA) mechanisms. The cell populations that are the targets for the hippocampal projections were studied by the technique of intracellular staining with HRP. These results were correlated with the findings of a Golgi analysis. Two distinct cell types were found to respond in a monosynaptic manner to ipsilateral fimbria stimulation. The most common of the two were the small-to medium-sized spiny neurons, and they were distributed throughout the nucleus. These cells have a spherical dendritic arrangement. The second, and most distinctive, of the cell types were the large aspiny neurons. These cells were distributed medially and caudally in the nucleus. Two of the outstanding features of these cells were the expanse of their dendritic domains and the fact that axons originated from relatively remote portions of the dendrites.
Cognitive ERPs and EEG spectral differences were compared in three groups of children: nonreferred controls, those with a dominant hyperactivity/impulsivity factor (ADHD-Im), and those with a dominant inattentive factor (ADHD-Ia). The results from the ERP analyses indicated that the P250, P350, and P500 components differed between the groups. The most marked differences were seen with respect to the amplitude of the P500 components. In addition, the topographic foci of the P500 components for the CON and ADHD-Im groups were symmetrical, but the ADHD-Ia group featured P250 and P350 components that were biased away from the right hemisphere. Nevertheless, the P500 was found to be an effective discriminator between the groups. The combined spectral and ERP results suggest that the attention disordered children have difficulty adjusting their level of physiological arousal, and are defective with respect to controlled (or effortful) processing.
The action of dopamine was studied in the nucleus accumbens of acutely prepared rabbits. Dopamine was applied iontophoretically to those cells and cell populations that responded in a monosynaptic excitatory manner to ipsilateral fimbrial stimulation. This strategy was adopted to isolate the effects of dopamine on postsynaptic receptors thus avoiding the bias resulting from activation of presynaptic dopamine receptors on dopaminergic afferents. Dopamine was found to have a suppressive effect on the excitatory (N) component of the field response and on driven extracellular unitary discharges. The specificity of dopamine's effect with receptors was indicated by the facts that fluphenazine effectively antagonized dopamine's effect, whereas bicuculline did not. The effect of dopamine was dependent on the rate of fimbrial stimulation. Dopamine has a marked suppressive effect on the fimbria-induced response at 0.5 Hz of stimulation but not at 6.0 Hz. This frequency specificity could not be linked directly to a cyclic adenosine 3',5'-cyclic monophosphate (cyclic AMP) mechanism because the iontophoresis cyclic AMP and dibutyryl cyclic AMP had suppressive effects at both 0.5 and 6.0 Hz rates of stimulation. It is suggested that dopamine acts in the nucleus accumbens to increase the "signal-to-noise" ratio. This might be a form of "contrast enhancement" of an incoming hippocampal message.
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