Ventral Pallidum

d-amino acid oxidase (d-AAO) is a peroxisomal flavoenzyme, the physiological substrate and the precise function of which are still unclear. We have investigated D-AAO distribution in rat brain, by immunocytochemistry, with an affinity-purified polyclonal antibody. Immunoreactivity occurred in both neuronal and glial cells, albeit at different densities. Glial immunostaning was strongest in the caudal brainstem and cerebellar cortex, particularly in astrocytes, Golgi-Bergmann glia, and tanycytes. Hindbrain neurons were generally more immunoreactive than those in the forebrain. Immunopositive forebrain cell populations included mitral cells in the olfactory bulb, cortical and hippocampal neurons, ventral pallidum, and septal, reticular thalamic, and paraventricular hypothalamic nuclei. Within the positive regions, not all the neuronal populations were equally immunoreactive; for example, in the thalamus, only the reticular and anterodorsal nuclei showed intense labelling. In the hindbrain, immunopositivity was virtually ubiquitous, and was especially strong in the reticular formation, pontine, ventral and dorsal cochlear, vestibular, cranial motor nuclei, deep cerebellar nuclei, and the cerebellar cortex, especially in Golgi and Purkinje cells.

The distribution of electrical self-stimulation (ESS) foci within the ventral pallidum (VP) was mapped using moveable electrodes in rats. The function relating ESS bar-pressing rate to the frequency of cathodal rectangular pulses (0.4 mA and 0.1 ms) was obtained for several positions of a moveable electrode in the VP and in the various adjacent to VP nuclei. The rate-frequency functions were fitted to a sigmoid model to obtain the asymptotic rate and threshold frequency. ESS was found in almost all (98%) VP sites tested and to a lesser degree (66%) in the surrounding areas (namely globus pallidus and caudate). Depending on the VP site, maximum rates varied from 14 to 85 bar presses/min, whereas threshold frequencies varied from 10.2 to 36.4 pulses/train; no correlation between these two aspects of ESS was found. Extra-pallidal areas contained less low-frequency threshold sites compared to VP. The lowest threshold found in the VP was slightly higher than that usually obtained for the most rewarding brain areas (VTA, dorsal raphé, LH, amygdala), which suggests that the VP represents an important structure for reward. Furthermore the threshold frequencies were found to decline along the rostrocaudal axis of the VP which supports the view that the VP is heterogeneous in regard to reward related functions.

Neurons that utilize dopamine (DA) as a neurotransmitter have attracted great interest because of their involvement in the behav- ioral, endocrine and descending control of major brain functions. DA is known to exert its physiological effects on target neurons through D1-like (D1 and D5) and D2-like (D2, D3 and D4) receptor subtypes. To better understand the DA modulation of brain

Manganese-enhanced MRI (MEMRI) is a newly developed noninvasive imaging technique of brain activities. The signal intensity of MEMRI reflects cumulative activities of the neurons. To validate the use of MEMRI technique to investigate the neural mechanisms of learning and memory, we tried to map brain areas involved in the retrieval of conditioned taste aversion (CTA) memory. CTAs were established to saccharin (conditioned stimulus: CS) by pairing its ingestion with an i.p. injection of LiCl (unconditioned stimulus: US). LiCl solutions (as a robust aversion chemical) of 0.15 M were injected i.p. 15 min after drinking the saccharine solution (CS). After the two times conditionings, these rats showed a robust aversion to the saccharine solution (CS). Rats of the control group were injected saline i.p. instead of LiCl solutions. The MRI signal intensities at the gustatory cortex (GC), the core subregion of the nucleus accumbens (NAcC), the shell subregion of the nucleus accumbens (NAcSh), the ventral pallidum (VP), the central nucleus of amygdala (CeA), the lateral hypothalamus (LH), and the basolateral nucleus of amygdala (BLA) of the conditioned group were higher than those of the control group. There were no significant differences between the conditioned and the control groups in the intensities for other regions, such as the striatum area, motor cortex, cingulate cortex, interstitial nucleus of the posterior limb of the anterior commissure and hippocampus. These indicate that the GC, NAcC, NAcSh, VP, CeA, LH and BLA have important roles in the memory retrieval of CTA.

Sensory processing disturbances, as measured in the P50/sensory gating paradigm, have been linked to aberrant auditory information processing and sensory overload in schizophrenic patients. In this paradigm, the response to the second of paired-click stimuli is attenuated by an inhibitory effect of the first stimulus. Sensory gating has been observed in most healthy human subjects and normal laboratory rats. Because mesolimbic dopamine has been implicated in other filtering disturbances such as prepulse inhibition of the acoustic startle response and given the fact that amphetamine and apomorphine have been shown to disrupt gating, this study was performed to investigate the role of mesolimbic dopamine in sensory gating. The dopamine D2 receptor agonist quinpirole (10 microg/0.5 microl) was injected bilaterally in nucleus accumbens core and shell and effects on cortical and hippocampal sensory gating were investigated. Also, effects of the dopamine D2 receptor antagonist haloperidol (0.1 mg/kg, subcutaneously) as pretreatment were studied. First, quinpirole significantly reduced both the amplitude to the first click and gating as measured in the cortex and in the hippocampus. There was a tendency for the quinpirole effects on hippocampal gating to be more pronounced in rats injected in the shell. Secondly, haloperidol did not antagonize effects of quinpirole on hippocampal parameters, whereas haloperidol pretreatment fully antagonized quinpirole effects on cortical parameters. In conclusion, gating can be significantly reduced when a dopamine agonist is specifically targeted at mesolimbic dopamine D2 receptors. However, an important consideration is that the dopaminergic effects in the present study on gating are predominantly mediated by the effects on the amplitude to the first click. This has also been suggested for systemic amphetamine injections in rats and schizophrenic patients. This casts doubt on whether dopamine receptor activation affects the putative inhibitory process between the first and the second stimulus.