Evelyn K. Lambe

Differential Postnatal Development of Catecholamine and Serotonin Inputs to Identified Neurons in Prefrontal Cortex of Rhesus Monkey

The monoaminergic innervation of cerebral cortex has long been implicated in its development. Methods now exist to examine catecholamine and serotonin inputs to identified neurons in the cerebral cortex. We have quantified such inputs on pyramidal and nonpyramidal cells in prefrontal cortex of rhesus monkeys ranging in age from 2 weeks to 10 years. Individual layer III neurons were filled with Lucifer yellow and double-immunostained with axons containing either tyrosine hydroxylase (TH) or 5-hydroxytryptamine (5-HT). The filled cells were reconstructed, and putative appositions between the axons and dendritic spines and shafts were quantified at high magnification using light microscopy. The density of catecholamine appositions on pyramidal neurons matures slowly, reaching only half the adult level by 6 months of age and thereafter rising gradually to adult levels by 2 years of age. By contrast, the density of serotonin appositions on pyramidal cells reaches the adult level before the second week after birth. The average adult pyramidal neuron in layer III of area 9m receives three times stronger input from catecholaminergic than from serotoninergic axons. The overall density of both inputs to interneurons does not appear to change during postnatal development. Selective changes in the TH innervation of pyramidal cells against a backdrop of constant TH innervation of interneurons suggest that the balance between excitation and inhibition may change developmentally in the prefrontal cortex. By contrast, 5-HT innervation of both types of neurons remains relatively constant over the age range studied.

Cerebral gray matter and white matter volume deficits in adolescent girls with anorexia nervosa.

OBJECTIVESnThis study was undertaken to determine whether the increased cerebrospinal fluid (CSF) volumes found in anorexia nervosa (AN) are the result of differences in gray matter or white matter volumes or both.nnnMETHODSnThirteen adolescent girls with AN who were receiving inpatient care at a tertiary-care university childrens hospital and eight healthy female control subjects were studied by using magnetic resonance imaging. Images were processed by means of software developed to classify all pixels as either CSF, gray matter, or white matter. Pixels of each class were then summed across all sections.nnnRESULTSnThe AN group had larger total CSF volumes in association with deficits in both total gray matter and total white matter volumes. Lowest reported body mass index was inversely correlated with total CSF volume and positively correlated with total gray matter volume. Urinary free cortisol levels were positively correlated with total CSF volume and inversely correlated with central gray matter volume.nnnCONCLUSIONSnThese findings add support to the view that the brain abnormalities found in AN are in large part the result of the effects of the illness. The extent to which these differences in gray matter and white matter volumes are reversible with recovery remains to be established.

Hypocretin (Orexin) Induces Calcium Transients in Single Spines Postsynaptic to Identified Thalamocortical Boutons in Prefrontal Slice

In vivo, thalamocortical axons are susceptible to the generation of terminal spikes which antidromically promote bursting in the thalamus. Although neurotransmitters could elicit such ectopic action potentials at thalamocortical boutons, this hypothesis has never been confirmed. Prefrontal cortex is the cortical area most implicated in arousal and is innervated by thalamic neurons that are unusual since they burst rhythmically during waking. We show that a neurotransmitter critical for alertness, hypocretin (orexin), directly excites prefrontal thalamocortical synapses in acute slice. This TTX-sensitive activation of thalamic axons was demonstrated electrophysiologically and by two-photon sampling of calcium transients at single spines in apposition to thalamic boutons anterogradely labeled in vivo. Spines receiving these long-range projections constituted a unique population in terms of the presynaptic excitatory action of hypocretin. By this mechanism, the hypocretin projection to prefrontal cortex may play a larger role in prefrontal or executive aspects of alertness and attention than previously anticipated.

Somatodendritic autoreceptor regulation of serotonergic neurons: dependence on l-tryptophan and tryptophan hydroxylase-activating kinases

The somatodendritic 5‐HT1A autoreceptor has been considered a major determinant of the output of the serotonin (5‐HT) neuronal system. However, recent studies in brain slices from the dorsal raphe nucleus have questioned the relevance of 5‐HT autoinhibition under physiological conditions. In the present study, we found that the difficulty in demonstrating 5‐HT tonic autoinhibition in slice results from in vitro conditions that are unfavorable for sustaining 5‐HT synthesis. Robust, tonic 5‐HT1A autoinhibition can be restored by reinstating in vivo 5‐HT synthesizing conditions with the initial 5‐HT precursor l‐tryptophan and the tryptophan hydroxylase co‐factor tetrahydrobiopterin (BH4). The presence of tonic autoinhibition under these conditions was revealed by the disinhibitory effect of a low concentration of the 5‐HT1A antagonist WAY 100635. Neurons showing an autoinhibitory response to l‐tryptophan were confirmed immunohistochemically to be serotonergic. Once conditions for tonic autoinhibition had been established in raphe slice, we were able to show that 5‐HT autoinhibition is critically regulated by the tryptophan hydroxylase‐activating kinases calcium/calmodulin protein kinase II (CaMKII) and protein kinase A (PKA). In addition, at physiological concentrations of l‐tryptophan, there was an augmentation of 5‐HT1A receptor‐mediated autoinhibition when the firing of 5‐HT cells activated with increasing concentrations of the α1 adrenoceptor agonist phenylephrine. Increased calcium influx at higher firing rates, by activating tryptophan hydroxylase via CaMKII and PKA, can work together with tryptophan to enhance negative feedback control of the output of the serotonergic system.

Dyslexia, gender, and brain imaging

Future brain imaging studies of dyslexia should have a sufficient number of males and females to detect possible gender differences in the neurological underpinning of this disorder. Detailed knowledge about such differences may clarify our understanding of the structural and functional impairments which lead to the phonological deficits that characterize dyslexia. Functional brain imaging studies have shown that males and females exhibit different patterns of brain activation during phonological processing. Further differences between the brains of males and females have been suggested by studies of normal brain development, morphology, and functional activation during reading. Animal studies have shown that lesions, similar to those seen in postmortem studies of dyslexia, affect rapid auditory processing in males, but not in females. The large body of research on gender differences in brain development, functional organization, and activation during reading tasks urges separation of males and females in dyslexia research in order to minimize variance and to detect subtle, but functionally-relevant, differences. Well-controlled studies, with large numbers of male and female dyslexics, may produce more sensitive and accurate identification of the neurological substrates of dyslexia.

MRI correlates of treatment response in first episode psychosis

It is not known whether the magnitude of the structural brain abnormalities that underlie schizophrenia is a determinant of the extent to which patients respond to antipsychotic medication. This study was undertaken in order to explore this relationship. Twenty-six patients receiving treatment for a first episode of psychosis were involved in both a study measuring treatment response and a magnetic resonance imaging (MRI) study. In the treatment study, haloperidol dose was increased weekly beginning at 2 mg/day until patients showed evidence of a response or extrapyramidal symptoms. MRI scans were analyzed using a computerized volumetric approach to yield estimates of cerebrospinal fluid (CSF), gray-matter and white-matter volumes. Improvement in positive and negative symptoms after 1 week of treatment was significantly correlated with cortical gray-matter volumes. Those patients who were maintained on 2 mg/day of haloperidol had greater cortical gray-matter volume than those who were treated with higher doses. The severity of structural brain abnormalities at the onset of psychosis may contribute to individual variation in response to antipsychotic medication. It remains to be determined whether the degree to which particular domains of symptomatology can improve is related to the severity of structural brain pathology in specific brain regions.

Prefrontal cortical network activity: Opposite effects of psychedelic hallucinogens and D1/D5 dopamine receptor activation

The fine-tuning of network activity provides a modulating influence on how information is processed and interpreted in the brain. Here, we use brain slices of rat prefrontal cortex to study how recurrent network activity is affected by neuromodulators known to alter normal cortical function. We previously determined that glutamate spillover and stimulation of extrasynaptic N-methyl-d-aspartic acid (NMDA) receptors are required to support hallucinogen-induced cortical network activity. Since microdialysis studies suggest that psychedelic hallucinogens and dopamine D1/D5 receptor agonists have opposite effects on extracellular glutamate in prefrontal cortex, we hypothesized that these two families of psychoactive drugs would have opposite effects on cortical network activity. We found that network activity can be enhanced by 2,5-dimethoxy-4-iodoamphetamine (DOI) (a psychedelic hallucinogen that is a partial agonist of 5-HT(2A/2C) receptors) and suppressed by the selective D1/D5 agonist SKF 38393. This suppression could be mimicked by direct activation of adenylyl cyclase with forskolin or by addition of a cAMP analog. These findings are consistent with previous work showing that activation of adenylyl cyclase can upregulate neuronal glutamate transporters, thereby decreasing synaptic spillover of glutamate. Consistent with this hypothesis, a low concentration of the glutamate transporter inhibitor threo-beta-benzoylaspartic acid (TBOA) restored electrically-evoked recurrent activity in the presence of a selective D1/D5 agonist, whereas recurrent activity in the presence of a low level of the GABA(A) antagonist bicuculline was not resistant to suppression by the D1/D5 agonist. The tempering of network UP states by D1/D5 receptor activation may have implications for the proposed use of D1/D5 agonists in the treatment of schizophrenia.

Electrophysiology of 5-HT 2A Receptors and Relevance for Hallucinogen and Atypical Antipsychotic Drug Actions

The feature that distinguishes psychedelic agents from some other classes of psychotomimetic drug is their capacity reliably to induce states of altered perception, thought, and feeling without producing marked disorientation or delirium. There is an accumulation of evidence that the 5-hydroxytryptamine2A (5-HT2A) subtype of serotonin (5-HT) receptor mediates the actions of psychedelic hallucinogens. The majority of 5-HT2A receptors are found in the cerebral cortex. Here, we explore the electrophysiology of 5-HT2A receptors in the prefrontal cortex and their relevance for normal brain function and for hallucinogen and atypical antipsychotic drug actions.

Effects Of Hypocretin/Orexin on the Thalamocortical Activating System

Hypocretin can excite the thalamocortical arousal pathway at two levels. It can selectively depolarize neurons in the midline-intralaminar nuclei of the thalamus, but it can also directly excite their axon terminals within prefrontal cortex. Since these two regions appear to receive projections from different populations of hypocretin neurons, further understanding of what governs hypocretin release in these two areas will yield greater insight into thalamocortical involvement in arousal and attention.