Ronald A. Browning

Enhanced recognition memory following vagus nerve stimulation in human subjects.

Neuromodulators associated with arousal modulate learning and memory, but most of these substances do not freely enter the brain from the periphery. In rodents, these neuromodulators act in part by initiating neural messages that travel via the vagus nerve to the brain, and electrical stimulation of the vagus enhances memory. We now extend that finding to human verbal learning. We examined word-recognition memory in patients enrolled in a clinical study evaluating the capacity of vagus nerve stimulation to control epilepsy. Stimulation administered after learning significantly enhanced retention. These findings confirm in humans the hypothesis that vagus nerve activation modulates memory formation similarly to arousal.

Locus Coeruleus Lesions Suppress the Seizure‐Attenuating Effects of Vagus Nerve Stimulation

Summary: Purpose: Although vagus nerve stimulation (VNS) is now marketed throughout most of the world as a treatment for drug‐resistant epilepsy, the therapeutic mechanism of action of VNS‐induced seizure suppression has not yet been established. Elucidation of this mechanism is an important first step in the development of strategies to improve VNS efficacy. Because the locus coeruleus (LC) has been implicated in the antinociceptive effects of VNS, we chemically lesioned the LC in the present study to determine if it is a critical structure involved in the anticonvulsant mechanisms of VNS.

Modification of electroshock and pentylenetetrazol seizure patterns in rats affer precollicular transections

Rats with precollicular transections leaving the brain stem intact were subjected to electroshock or pentylenetetrazol-induced convulsions. Animals with transections failed to display the face and forelimb clonus (with or without rearing) typically seen in sham-operated rats subjected to pentylenetetrazol- or minimal electroshock-induced convulsions elicited with corneal electrodes. Moreover, rats with transections exhibited all other components of pentylenetetrazol- and electroshock-induced seizures. These findings support the hypothesis that tonic and some types of clonic convulsions can occur independently of seizure discharge in forebrain structures, whereas clonus restricted to the face and forelimbs depends on seizure discharge emanating from structures within the forebrain for expression.

Behavioral and neurobiological changes in C57BL/6 mice exposed to cuprizone.

C57BL/6 mice were given 0.2% cuprizone (CPZ) for 2 to 6 weeks while controls ate the same diet without CPZ. At various time points the animals were subjected to behavioral tests and their brains were analyzed. Mice exposed to CPZ for 2 and 3 weeks displayed more climbing behavior and lower prepulse inhibition, suggesting an increase in central nervous system activity and impaired sensorimotor gating. In addition, they showed lower activities of monoamine oxidase and dopamine beta hydroxylase in the hippocampus and prefrontal cortex, and had higher dopamine but lower norepinephrine levels in the prefrontal cortex. Mice exposed to CPZ for 4 to 6 weeks had less social interaction, which is an animal correlate of social withdrawal of patients with schizophrenia. Also, these CPZ-exposed mice showed evident brain demyelination, myelin break down, and loss of oligodendrocytes. At all time points the CPZ-exposed mice spent more time in the open arms of an elevated plus maze and exhibited spatial working memory impairment. These data are in line with evidence from human studies suggesting a putative role of white matter abnormality in the pathophysiology of schizophrenia.

Behavioral and neurobiological changes in C57BL/6 mouse exposed to cuprizone: effects of antipsychotics.

Recent human studies suggest a role for altered oligodendrocytes in the pathophysiology of schizophrenia. Our recent animal study has reported some schizophrenia-like behaviors in mice exposed to cuprizone (Xu et al., 2009), a copper chelator that has been shown to selectively damage the white matter. This study was to explore mechanisms underlying the behavioral changes in cuprizone-exposed mice and to examine effects of the antipsychotics haloperidol, clozapine and quetiapine on the changes in the mice. Mice given cuprizone for 14 days showed a deficit in the prepulse inhibition of acoustic startle response and higher dopamine in the prefrontal cortex (PFC), which changes were not seen in mice given cuprizone plus antipsychotics. Mice given cuprizone for 21 days showed lower spontaneous alternations in Y-maze, which was not seen in mice treated with cuprizone plus the antipsychotics. Mice given cuprizone for 28 days displayed less social interactions, which was not seen in mice given cuprizone plus clozapine/quetiapine, but was seen in mice given cuprizone plus haloperidol. Mice given cuprizone for 42 days showed myelin sheath loss and lower myelin basic protein in PFC, caudate putamen, and hippocampus. The white matter damage in PFC was attenuated in mice given cuprizone plus clozapine/haloperidol. But the white matter damage in caudate putamen and hippocampus was only attenuated by clozapine and quetiapine, not by haloperidol. These results help us to understand the behavioral changes and provide experimental evidence for the protective effects of antipsychotics on white matter damage in cuprizone-exposed mice.

Abnormalities in Brain Serotonin Concentration, High‐Affinity Uptake, and Tryptophan Hydroxylase Activity in Severe‐Seizure Genetically Epilepsy‐Prone Rats

Summary: We characterized the nature of the deficit in brain serotonin (5‐HT) exhibited by genetically epilepsy‐prone rats (GEPR‐9s) by regionally assessing three markers for 5‐HT terminals/neurons (5‐HT content, 5‐HT uptake into the P2synaptosomal fraction, and tryptophan hydroxylase activity) in GEPR‐9s and nonepileptic control rats. As compared with controls, GEPR‐9s had reduced brain 5‐HT concentration, synaptosomal 5‐HT uptake, and tryptophan hydroxylase activity (measured in vivo and in vitro) in most regions of the forebrain and in selected regions of brainstem. Analysis of kinetic constants for synaptosomal [3H]5‐HT uptake and in vitro tryptophan hydroxylase activity showed that the decrements in these parameters exhibited by GEPR‐9s resulted from reductions in Vmax rather than changes in Km. In general, the reduction in each of the presynaptic markers for 5‐HT terminals/neurons was similar in both magnitude and in their regional distribution in the GEPR‐9 brain. An exception to this was noted in the midbrain tegmentum of GEPR‐9s, which displayed a significant reduction in tryptophan hydroxylase activity without showing alterations in 5‐HT concentration or in high‐affinity 5‐HT uptake. The present findings support the hypothesis that there is a widespread reduction in the number of serotonergic terminals/neurons in GEPR‐9 brain.

CHANGES IN SEIZURE SUSCEPTIBILITY AFTER INTRACEREBRAL TREATMENT WITH 5,7-DIHYDROXYTRYPTAMINE: ROLE OF SEROTONERGIC NEURONS*

The possible importance of serotonin [5-hydroxytryptamine (5-HT)I as a modulator of seizure susceptibility was suggested more than 20 years ago when Chen et al.1 observed seizure-facilitating effects of reserpine in mice subjected to electroshockor pentylenetetrazol (PTZ) induced convulsions. Since then, many investigators (for reviews, see References 2-5) employing several species have provided evidence that the seizure-facilitating effects of reserpine are due to the depletion of both catecholamines and 5-HT. Most attempts to elucidate the putative role of 5-HT in the convulsive process have involved pharmacologic manipulations of this amine. In these studies, seizure susceptibility has usually been examined after drug treatments designed either to increase or decrease the availability of 5-HT at central 5-HT receptors. Several laboratories have shown that treatments believed to increase the synaptic concentration of 5-HT reduce the susceptibility to experimentally induced seizures. For example, the systemic administration of 5-hydroxytryptophan (5-HTP), the precursor of 5-HT, in conjunction with a monoamine oxidase inhibitor e-4 or a peripheral decarboxylase inhibitor: has been shown to either reduce seizure susceptibility or antagonize the reserpine-induced facilitation of seizures.1o. l1

Region-specific susceptibilities to cuprizone-induced lesions in the mouse forebrain: Implications for the pathophysiology of schizophrenia.

Cuprizone (CPZ) is a neurotoxic agent acting as a copper chelator. In our recent study, C57BL/6 mice given dietary CPZ (0.2%) showed impairments in spatial working memory, social interaction, and prepulse inhibition. These abnormalities are reminiscent of certain schizophrenia symptoms and are not likely due to damage in the whole brain or in any single white matter tract/brain region. We hypothesized that white matter damage resulting from CPZ-treatment may be site-specific rather than universal. We examined the forebrains of C57BL/6 mice given the CPZ-containing diet and compared them with those of controls. We assessed CPZ-induced demyelination in main white matter tracts of the forebrain, evaluated myelin break down in the neuropil of the main olfactory bulb (MOB), cerebral cortex (CTX), caudate putamen (CP), hippocampus (HP), thalamus (TH), and hypothalamus (HY), and counted the number of myelin sheath forming oligodendrocytes (OLs) in CTX, CP, TH, and HY. Obvious demyelination was observed in the corpus callosum, external capsule, CP, and dorsal hippocampal commissure whereas other tracts seemed to be unaffected. The neuropil of CTX, HP and MOB showed myelin break down, which was mild in TH and HY. The number of OLs was decreased in all the above regions of CPZ-treated mice although the degree of OL loss was not consistent across regions. The data provide further support for white matter abnormalities contributing to schizophrenia-like behaviors in mice.

Effect of 5,7-dihydroxytryptamine on audiogenic seizures in genetically epilepsy-prone rats

To further assess the role of 5-HT in the modulation of audiogenic seizures (AGS) in the Genetically Epilepsy-Prone Rat (GEPR), changes in AGS severity after widespread chronic depletion of brain 5-HT by intracerebroventricular administration of 5,7-dihydroxytryptamine (5,7-DHT) were examined in moderate seizure GEPRs (GEPR-3s). Following treatment with 5,7-DHT (150 micrograms/30 microliters), a significant increase in seizure severity was observed at 2, 3 and 4 weeks as compared to vehicle-injected controls. The increase in seizure severity was evidenced by a significant increase in the incidence of tonic convulsions in 5,7-DHT treated animals (53% in treated animals compared to 0% in vehicle treated controls) over the testing period. Interestingly, the latency to wild running was increased in 5,7-DHT treated GEPRs, suggesting that depletion of brain 5-HT may slow initiation of AGS. Neurochemical analysis revealed marked depletion of 5-HT in the cortex (-96%), hippocampus (-94%), thalamus (-80%), hypothalamus (-62%), midbrain (-51%) and pons-medulla (-52%) in animals that received 5,7-DHT. However, no significant reductions in brain norepinephrine content were observed in any of the regions assayed due to the pretreatment of all animals with protriptyline. The present findings lend further support for an inhibitory action of brain 5-HT on audiogenic seizures in GEPRs.

Seizures and proto-oncogene expression of fos in the brain of adult genetically epilepsy-prone rats

The mechanisms and brain circuitry that render genetically epilepsy-prone rats (GEPRs) susceptible to acoustically induced seizures are not completely known. The present study explores the neuroanatomy of acoustically induced seizures by immunohistochemical analysis of the proto-oncoprotein fos after intense acoustic stimulation (AS) with and without seizures. Acoustic stimulation induced tonic convulsions in GEPR-9s, but not in control rats. Locations of brain nuclei showing fos-like immunoreactive (FLI) neurons following AS with and without seizures were mapped. Semiquantitative methods were used to compare FLI neuron numerical densities in AS control rats and GEPRs. Many brain areas exhibited profound FLI in AS control rats and GEPRs. Unexpectedly, the cochlear nuclei and the central nucleus of the inferior colliculi (ICc), both of which are requisite for AGS initiation, exhibited a diminished fos expression in animals having seizures compared to AS controls. In contrast, GEPRs displayed a significant increase in FLI neurons within the dorsal cortex of the IC (ICd) compared to AS controls. This finding may suggest a seizure-related amplification of the auditory signal between the ICc and the ICd. Other nuclei, known to be involved in auditory transmission (i.e., superior olivary complex; trapezoid nucleus; dorsal nucleus of the lateral lemniscus, DNLL), did not show differential FLI densities between seizure and AS control animals. In contrast, seizure-induced FLI was observed in many nonauditory brain nuclei. Of particular interest was the identification of an intensely labeled nucleus in the GEPR. This nucleus resides in the most posterior and dorsal-lateral part of the pedunculopontine tegmental nucleus-pars compacta (PPTn-pc) immediately adjacent to the DNLL and extends posteriorly into the superior lateral subnucleus of the lateral parabrachial area (SLPBn). Therefore, we have tentatively termed this nucleus the PPSLPBn. The PPSLPBn lies in a region previously described as a mesencephalic locomotor region and a suspected functional involvement of this nucleus in display of seizure activity is under investigation. Other brain stem nuclei showing differential fos expression between GEPRs and AS control rats are also described.