Distribution and time course of appearance of "dark" neurons and EEG activity after amygdaloid kainate lesion

Ventromedial prefrontal cortex is involved in preference and hedonic evaluation of tastes

The ventromedial prefrontal cortex (vmPFC) of rats has reciprocal connections with the gustatory and the hedonic impact coding structures. The main goal of the present study was to investigate the involvement of local neurons of vmPFC and their catecholaminergic innervations in taste preference and taste reactivity test. Therefore, kainate or 6-hydroxydopamine (6-OHDA) lesions were performed in the vmPFC by iontophoretic method. In the first experiment, taste preference was tested to 250 mM and 500 mM glucose solutions over water in two-bottle choice test. In the second experiment, taste reactivity was examined to 4 concentrations of glucose solutions (250 mM, 500 mM, 750 mM and 1000 mM) and 4 concentrations of quinine solutions (0.125 mM, 0.25 mM, 1.25 mM and 2.5 mM). Our results showed, that kainate microlesion of vmPFC did not modify the preference of 250 mM and 500 mM glucose solutions in two-bottle choice test. In contrast, 6-OHDA microlesion of vmPFC resulted in increased preference to the higher concentration of glucose (500 mM) solution over water. Results of taste reactivity test showed that kainate lesion resulted in more ingestive and less rejective responses to 750 mM glucose solution and elevated rejectivity to the higher concentrations (1.25 mM and 2.5 mM) of quinine solutions. 6-OHDA lesion of vmPFC increased the number of ingestive responses to highly concentrated (500 mM, 750 mM and 1000 mM) glucose solutions and decreased the number of ingestive responses to the lower concentration (0.125 mM) of quinine solution. The present data provide evidence for the important role of vmPFC neurons and catecholaminergic innervation of the vmPFC in the regulation of hedonic evaluation of tastes and in the hedonic consummatory behavior.

Iontophoretic microlesions with kainate or 6-hydroxidopamine in ventromedial prefrontal cortex result in deficit in conditioned taste avoidance to palatable tastants

Effects of kainate or 6-hydroxidopamine (6-OHDA) lesions in the ventromedial prefrontal cortex (vmPFC) on taste-related learning and memory processes were examined. Neurotoxins were applied by iontophoretic method to minimize the extent of lesion and the side effects. Acquisition and retention of conditioned taste avoidance (CTA) was tested to different taste stimuli (0.05 M NaCl, 0.01 M saccharin, 0.01 M citrate and 0.00025 M quinine). In the first experiment, palatability index of taste solutions with these concentrations has been determined as strongly palatable (NaCl, saccharin), weakly palatable (citrate) and weakly unpalatable (quinine) taste stimuli. In two other experiments vmPFC lesions were performed before CTA (acquisition) or after CTA (retrieval). Our results showed that both kainate and 6-OHDA microlesions of vmPFC resulted in deficit of CTA acquisition (to NaCl, saccharin and citrate) and retrieval (to NaCl and saccharin). Deficits were specific to palatable tastants, particularly those that are strongly palatable, and did not occur for unpalatable stimulus. The present data provide evidence for the important role of vmPFC neurons and catecholaminergic innervation of the vmPFC in taste related learning and memory processes.

Generalization of seizures parallels the formation of "dark" neurons in the hippocampus and pontine reticular formation after focal-cortical application of 4-aminopyridine (4-AP) in the rat

Distribution and time course of the occurrence of "dark" neurons were compared with the EEG activity and behavior of rats during 4-aminopyridine (4-AP) induced epileptic seizures. A crystal of the K(+) channel blocker 4-AP (0.5 mg/kg) was placed onto the exposed parieto-occipital cortex of Halothane-anesthetized rats for 40 min. Thereafter, the anesthesia was discontinued and the behavioral signs of the epileptic seizure activity were observed. The presence of "dark" neurons was demonstrated by the sensitive silver method of Gallyas in rats sacrificed at 0, 3 and 6 h after the end of the 4-AP crystal application. The EEG activity was recorded in the rats with longer survival times. The EEG analysis revealed the generalization of the epileptic seizures. We found that the formation of "dark" neurons in the hippocampus and the pontine reticular formation paralleled the generalization of the seizures.

Neurotoxic regimens of methamphetamine induce persistent expression of phospho-c-Jun in somatosensory cortex and substantia nigra

Repeated systemic administration of moderate doses of methamphetamine (mAMPH) can result in neuronal damage. In addition to the prominent damage of forebrain dopamine and serotonin terminals, mAMPH also injures certain non-monoaminergic neuronal somata in the cerebral cortex. In previous studies, we have localized the damaged neurons to the "whisker barrels" in primary somatosensory cortex, reported the time course of their appearance, and found that sensory inputs from the mystacial vibrissae appear to play a crucial role in the mechanism of their injury by mAMPH. One common feature of these studies is that they used a single marker for neuronal injury, the fluorochrome dye Fluoro-Jade, which stains neurons injured by disparate mechanisms. Here we compare mAMPH-induced damage to somatosensory cortical neurons as assessed by Fluoro-Jade and immunohistochemical staining for phospho-c-Jun. A neurotoxic regimen of mAMPH induced phospho-c-Jun-positive neurons in both cortical whisker barrels and the substantia nigra. Neurons in the barrel cortex can be sufficiently damaged by mAMPH that they become Fluoro-Jade-positive within 2 hr after the final mAMPH injection. By contrast, phospho-c-Jun immunoreactivity does not appear until 12-24 hr after mAMPH. As reported in an earlier study, unilateral removal of vibrissae prior to mAMPH treatment affords partial protection from injury in the hemisphere contralateral to the vibrissotomy. The vibrissotomized animals show similar decreases in Fluoro-Jade staining and phospho-c-Jun immunoreactivity in the protected hemisphere. Since phospho-c-Jun indicates activation of Jun N-terminal kinase pathways, which have been implicated in apoptosis, we conclude that phospho-c-Jun provides a useful new marker for mAMPH-induced damage to cortical neurons.

Changes in brain serotonin turnover, body and head shakes in kainic acid-treated rats

Kainic acid induces seizures and neurotoxicity in rats, produces changes in brain serotonin (5-HT), dopamine and noradrenaline metabolites among other changes in neurotransmitters. In this work, we investigated the changes in 5-HT turnover in brain regions from 84 rats intraperitoneally injected with kainic acid and a specific behavioural change, the body and head shakes, exerted by this neurotoxin in the presence of 5-HT receptor antagonists. Kainic acid produced an increase in 5-hydroxyindoleacetic acid levels in frontal cortex (212%; 180%), striatum (177%; 116%), amygdala (202%; 337%) and hippocampus (43%; 70 %) at 2 and 24 hr as compared with controls, respectively. Serotonin turnover was increased in amygdala (157%) and frontal cortex (169%) at 2 hr; whereas 24 hr after kainic acid administration, increases were observed in amygdala (207%), and frontal cortex (178%). Kainic acid also produced an increase in the frequency of head and body shakes when administered alone or together with pargyline, a monoamine oxidase inhibitor; whereas the administration of 5-HT receptor antagonists such as ketanserin and methiothepin, decreased this behaviour 54% and 50% as compared with kainic acid alone, respectively. These results suggest an active participation of 5-HT neurotransmission on the excitotoxic action of kainic acid in the brain.