Morphine and specific changes in the sensitivity of noradrenergic receptors within the "limbic" part of the feline caudate nucleus: a behaviour study

Altered Spatiotemporal Dynamics of Cortical Activation to Tactile Stimuli in Somatosensory Area 3b and Area 1 of Monkeys after Spinal Cord Injury

Reactivation of deafferented cortex plays a key role in mediating the recovery of lost functions, although the precise mechanism is not fully understood. This study simultaneously characterized the dynamic spatiotemporal features of tactile responses in areas 3b and 1 before and 6–8 weeks after partial dorsal column lesion (DCL), and examined how the reactivation relates to the recovery of simple hand use in squirrel monkeys. A combination of high spatiotemporal resolution functional intrinsic optical imaging, microelectrode mapping, behavioral assessment, and tracer histology methods were used. Compared with the normal cortex, we found that the responses of deafferented areas 3b and 1 to 3 s of continuous 8 Hz tactile stimulation of a single digit were significantly weaker and more transient. This finding indicates a loss of response to sustained tactile stimuli. The activation area enlarged for areas 3b and 1 in both directions along digit representation (medial–lateral) and across areas (anterior–posterior). All subjects showed behavioral deficits in a food reaching-grasping-retrieving task within the first 5 weeks after DCL, but recovered at the time when optical images were acquired. Summarily, we showed that these populations of cortical neurons responded to peripheral tactile inputs, albeit in significantly altered manners in each area, several weeks after deafferentation. We propose that compromised ascending driven inputs, impaired lateral inhibition, and local integration of input signals may account for the altered spatiotemporal dynamics of the reactivated areas 3b and 1 cortices. Further investigation with large sample sizes is needed to fully characterize the effects of deafferentation on area 1 activation size.

Dopamine, schizophrenia, mania, and depression: Toward a unified hypothesis of cortico-striatopallido-thalamic function

Considerable evidence from preclinical and clinical investigations implicates disturbances of brain dopamine (DA) function in the pathophysiology of several psychiatric and neurologic disorders. We describe a neural model that may help organize theseindependent experimental observations. Cortical regions classically associated with the limbic system interact with infracortical structures, including the nucleus accumbens, ventral pallidum, and dorsomedial nucleus of the thalamus. In our model, overactivity in forebrain DA systems results in the loss of lateral inhibitory interactions in the nucleus accumbens, causing disinhibition of pallidothalamic efferents; this in turn causes rapid changes and a loss of focused corticothalamic activity in cortical regions controlling cognitive and emotional processes. These effects might be manifested clinically by some symptoms of psychoses. Underactivity of forebrain DA results in excess lateral inhibition in the nucleus accumbens, causing tonic inhibition of pallidothalamic efferents; this perpetuates tonic corticothalamic activity and prevents the initiation of new activity in other critical cortical regions. These effects might be manifested clinically by some symptoms of depression. This model parallels existing explanations for the etiology of several movement disorders, and may lead to testable inferences regarding the neural substrates of specific psychopathologies.

Mesolimbic noradrenaline: specificity, stability and dose-dependency of individual-specific responses to mesolimbic injections of alpha-noradrenergic agonists

The ability of intra-accumbens phenylephrine (PE) and oxymetazoline to potentiate the 'explosive motor behaviour' (EMB) elicited from the deeper layers of the superior colliculus by picrotoxin injections was examined in Wistar rats. Using a dose of intracollicular picrotoxin that was 10 ng lower than the threshold dose for generating EMB, evidence was obtained for the selective potentiation of EMB by alpha-noradrenergic agonists. The PE-induced potentiation was prevented by phentolamine given 48 h prior to PE. Damage caused by multiple injections of intra-accumbens PE prevented the PE-induced effect. It is concluded that the PE-induced effect is accumbens- and noradrenaline-specific. When rats were injected with intervals of 48 h or more, two types of rats could be discerned: responders, i.e. rats that consistently displayed EMB during all trials, and non-responders, i.e. rats that never displayed EMB. When the intertrial interval was 24 h, responders became temporary non-responders, and vice versa. This temporary change in sensitivity was found to be a drug-induced after-effect. The dose-dependency of the PE-induced after-effect in responders differed significantly from that in non-responders. It is concluded that rats belonging to the same strain are nevertheless marked by an individual-specific, neurochemical state within the nucleus accumbens. It is tentatively suggested that responders are marked by noradrenergic neurones with a low firing rate in contrast to non-responders which are marked by noradrenergic neurones with a high firing rate. Finally, evidence was obtained that ergometrine and (3,4-dihydroxyphenylamino)-2-imidazoline (DPI) act simultaneously at dopaminergic sites which are involved in the control of locomotor activity in a familiar environment and at alpha-noradrenergic sites which are involved in the control of EMB elicited from the superior colliculus.

Effect of excitotoxin lesions in the medical prefrontal cortex on cortical and subcortical catecholamine turnover in the rat

Catecholamine turnover in brain areas innervated by dopaminergic neurons was examined 2, 6, and 12 days after bilateral, N-methyl-D-aspartate lesions confined to the rat medial prefrontal cortex. The lesion produced a significant regional increase in the concentration of 3,4-dihydroxyphenylethylamine (DA, dopamine) in both the medial prefrontal cortex and the ventral tegmental area. DA concentrations were increased in the nucleus accumbens on day 6 (128% of control), in the ventral tegmental area on day 2 (130% of control), and in the medial prefrontal cortex on days 2 (145% of control) and 6 (127% of control). The only significant changes in the concentration of 3,4-dihydroxyphenylacetic acid (DOPAC) (197% of control), and in the ratio DOPAC/DA (163% of control) were found in the medial prefrontal cortex on day 6 post-lesion. All parameters had returned to control levels by day 12. DA depletion after the administration of alpha-methyl-p-tyrosine (AMPT) was not significantly different between excitotoxin-lesioned and sham animals on day 6 in all brain regions. Noradrenaline (NA) and 3,4-dihydroxyphenylethyleneglycol concentrations and their ratios, and the depletion of noradrenaline after AMPT were also determined, and the lesion resulted in a significant regional increase in NA in both the nucleus accumbens and the ventral tegmental area. An elevation of NA (147% of control) in the nucleus accumbens was found on day 12. Since the excitotoxin lesion destroys corticofugal efferents from medial prefrontal cortex to the nucleus accumbens, the anterior corpus striatum and the ventral tegmental area, our results provide no evidence for a role of these cortical projections in the regulation of subcortical DA metabolism.(ABSTRACT TRUNCATED AT 250 WORDS)