Susan L. Erickson

Postnatal development of prefrontal inhibitory circuits and the pathophysiology of cognitive dysfunction in schizophrenia

Abstract: The typical appearance of the clinical features of schizophrenia during late adolescence or early adulthood suggests that adolescence‐related neurodevelopmental events may contribute to the pathophysiology of this disorder. Here the role that GABA‐mediated inhibition in the dorsal lateral prefrontal cortex (DLPFC) plays in regulating working memory, a core cognitive process that matures late and that is disturbed in schizophrenia, is reviewed. Recent studies are summarized that demonstrate (1) that certain pre‐ and postsynaptic markers of GABA neurotransmission in the monkey DLPFC exhibit striking changes during adolescence, and (2) that these same markers are markedly altered in the DLPFC of subjects with schizophrenia. The implications of these findings for treatment and prevention strategies are discussed.

Postnatal development of parvalbumin- and GABA transporter-immunoreactive axon terminals in monkey prefrontal cortex

In the primate prefrontal cortex, the axon terminals of the chandelier class of inhibitory local circuit neurons have a distinctive time course of postnatal development. In this study, we sought to determine whether the axon terminals of other classes of local circuit neurons are also refined during postnatal development. We examined postnatal changes in the density of punctate structures immunoreactive for the calcium binding protein parvalbumin, which identifies a subset of gamma‐aminobutyric acid (GABA) ‐containing terminals, in the prefrontal cortex of 35 rhesus monkeys ranging in age from newborn to adult. In area 46, the density of parvalbumin‐ immunoreactive puncta in the superficial and middle layers was extremely low in the newborn animals, then increased more than 10‐fold to adult levels, which were achieved by 3 to 4 years of age. In layer V, a band of labeled puncta present in the newborn animals also increased in density until 3 to 4 years of age. Developmental changes of parvalbumin‐immunoreactive puncta in area 9 were similar to those in area 46. In contrast, the density of punctate structures labeled with an antibody against a GABA membrane transporter (GAT‐1) did not change across development, suggesting that the number of GABAergic terminals is stable over time, but that the level of parvalbumin protein within the terminals varies. The time course of the observed changes in these parvalbumin‐labeled terminals is markedly different from that of parvalbumin‐immunoreactive chandelier cell terminal clusters. These findings suggest that morphologically specialized classes of inhibitory interneurons assume prominence within the prefrontal cortical network at different stages of postnatal development. J. Comp. Neurol. 448:186–202, 2002.

Cortical connections of the lateral mediodorsal thalamus in cynomolgus monkeys

The prefrontal cortex has been defined as that cortical territory that has “essential or sustaining” connections with the mediodorsal (MD) nucleus of the thalamus (Rose and Woolsey [1948] Publ Assoc Res Nerv Ment Dis 27:210–232). However, recent studies in the monkey have documented projections from MD to the more caudal, agranular regions of the frontal cortex, suggesting that the connections of MD may be characterized by a breadth of distribution and diversity of functional roles too great to be useful as a unifying and defining feature for a specific cortical territory. In this study, we placed tracer injections in the lateral divisions of MD in cynomolgus monkeys (Macaca fascicularis) to assess the relative proportions of connections devoted to diverse regions of the frontal cortex (FC). Three different patterns of label were observed in the cortex, associated with different locations within lateral MD. We have designated these as the ventrolateral MD‐arcuate FC circuit, having most label in areas 8 and 6; the caudoventral MD‐dorsomedial FC circuit, having most label in areas 24 and presupplementary motor area (SMA); and the anterodorsal MD‐anterior FC circuit, with the most label in areas 9, 46, 12, and 10. Only two of the nine cases injected in lateral MD were predominantly connected with the anterior FC. Thus, particular locales within lateral MD are connected with multiple, functionally diverse cortical regions, including several not classically recognized as “prefrontal” areas. This divergence may distinguish MD‐frontocortical and reciprocal corticothalamic pathways from the largely segregated pathways arising from the other thalamic nuclei that are interconnected with the frontal cortex, such as those from the ventrolateral nuclear group. J. Comp. Neurol. 473:107–127, 2004.

Subcortical afferents to the lateral mediodorsal thalamus in cynomolgus monkeys.

The mediodorsal (MD) nucleus of the thalamus has long been known to provide the principal source of subcortical input to the primate prefrontal cortex, as well as to other areas of the frontal lobe that are thought to contribute to higher-order cognitive functions. In this study, we used injections of retrograde tracers in the lateral portion of the monkey MD to assess the locations of labeled cells in subcortical structures. Three main patterns were identified in the distribution of subcortical connections. We found that the claustrum, superior colliculus and ventral midbrain regions were heavily labeled in the cases with injections in caudoventral MD. In these cases, labeled cells were also found in either the periaqueductal gray or zona incerta, depending on the specific case. In one case with an injection in anterodorsal MD, labeled cells were most numerous in the structures of the ventral midbrain, especially the ventral tegmental area. Finally, the claustrum and superior colliculus contained the largest percentage of labeled subcortical cells in cases with injections in ventrolateral MD. These three patterns of subcortical label corresponded to three equally distinctive trends in the distribution of MD connections with the cortex in these same cases [J Comp Neurol 473 (2004) 107]. Very few labeled cells were found in other areas such as the amygdala, globus pallidus and deep cerebellar nuclei, suggesting that pathways leading from these structures to dorsolateral and dorsomedial frontal cortices are not likely to include the lateral divisions of MD. In concert, these findings show that particular locales within lateral MD receive distinct profiles of subcortical afferents, and project into specific neocortical domains, suggesting that these different sites within lateral MD may participate in functionally distinct circuits of information processing.

Subbarrel domains in rat somatosensory (S1) cortex.

We used cytochrome oxidase (CO) histochemistry in conjunction with other histological methods to investigate the histochemoarchitecture of barrel hollows in rat somatosensory cortex. We found that individual large barrels in the posteromedial barrel subfield encompass two or three discrete subbarrel domains. Detailed analysis revealed, further, that subbarrel domains are relatively consistent in size, each having average dimensions that approximate those of large barrels in mouse S1. Unexpectedly, subbarrel domains are organized into a few distinct, repeated patterns. The small barrels in rat anterolateral barrel subfield and all barrel hollows in mouse S1 appear to consist of single CO domains. Subbarrel domains revealed here by CO are columnar entities that correspond with cyto‐ and myeloarchitectonic inhomogeneities within the barrels and are enriched in thalamocortical axon terminals. The present findings together with existing data indicate that barrels in rat posteromedial barrel subfield are structurally and functionally heterogeneous. J. Comp. Neurol. 490:414–426, 2005.

Dopamine innervation of monkey entorhinal cortex: Postsynaptic targets of tyrosine hydroxylase-immunoreactive terminals

Dopamine (DA) has been demonstrated to play an important role in regulating cortical activity in both neocortical and periallocortical regions. However, marked differences between these two types of cortices in the laminar pattern of DA axons, the types and distribution of DA receptors, and the postnatal development of the DA innervation suggest that DA may have region‐specific effects. Such regional specialization may also include the types of cortical cells apposed to DA terminals. In neocortical regions, such as the prefrontal and motor cortices, the majority of structures apposed to DA terminals appear to be the dendritic spines and shafts of pyramidal cells, and a minority are dendrites immunoreactive for gamma‐amino butyric acid (GABA). However, the identity of the neural elements apposed to DA terminals in the entorhinal cortex, a periallocortical region, is unknown. In this study, we used immunocytochemical techniques and antibodies against tyrosine hydroxylase (TH) and GABA, visualized with preembedding immunoperoxidase and immunogold‐silver labels, respectively, to examine DA terminals and their targets with electron microscopy. In the superficial layers of the monkey entorhinal cortex, TH‐immunoreactive (IR) terminals varied greatly in size and formed thin, symmetric synapses. The majority of dendritic structures apposed to these TH‐terminals were not GABA‐IR, and included both dendritic shafts (64%) and spines (14%). A minority (22%) of the apposed dendrites were GABA‐IR. A similar distribution of targets was observed for the subset of TH‐IR terminals with identifiable synaptic specializations. In addition, the proportions of GABA‐labeled and unlabeled dendrites apposed to TH terminals did not differ from those previously reported for monkey prefrontal cortex. These findings indicate that DA terminals provide direct input to both excitatory and inhibitory cells in the monkey entorhinal cortex and suggest that the effects of DA are mediated through a set of targets that are common to both neo‐ and periallocortex. Synapse 36:47–56, 2000.

Dopamine innervation of the monkey mediodorsal thalamus : Location of projection neurons and ultrastructural characteristics of axon terminals

Dopamine (DA) axons and receptors have recently been identified in the primate thalamus, including the mediodorsal thalamic nucleus (MD). In order to determine whether the DA innervation of the primate MD shares the anatomical features of the mesocortical or nigrostriatal DA projections, we performed tract-tracing and immunocytochemistry studies in macaque monkeys (Macaca fascicularis) to identify the location of the DA neurons that project to MD and immuno-electron microscopy to determine the distribution of the dopamine transporter (DAT) in axons within the MD. Similar to the mesocortical projection, retrogradely-labeled, tyrosine hydroxylase-containing neurons were present in dorsal tier ventral mesencephalic nuclei, such as the ventral tegmental area and the dorsal portion of the substantia nigra pars compacta. In contrast, no dual-labeled neurons were present in the ventral tier nuclei, the primary origin of the nigrostriatal DA pathway. In addition, like the DA projection to the prefrontal cortex, DAT immunoreactivity was predominantly localized to the pre-terminal portion of axons in the MD, and was infrequently found in association with synaptic vesicles, in contrast to nigrostriatal DA axons. These findings indicate that the DA projection to the MD shares anatomical features with the mesocortical DA system, suggesting that the functional properties of DA neurotransmission in the MD might be more similar to those in the cortex than in the striatum.

Subbarrel patterns of thalamocortical innervation in rat somatosensory cortical barrels: Organization and postnatal development

Barrel hollows in the posteromedial barrel subfield of adult rat somatosensory cortex typically encompass two or three metabolically and structurally distinct regions, termed subbarrels. We used immunohistochemical staining for vesicular glutamate transporter 2 and the neuronal serotonin transporter, in conjunction with cytochrome oxidase (CO) histochemistry, to investigate the distribution of thalamocortical (TC) axon terminals in relation to subbarrel domains. We found, first, that CO‐dark subbarrels are more intensely immunoreactive for thalamocortical terminals than the CO‐light clefts that separate them. Second, during the first postnatal week, immunoreactivity for markers of TC terminals is relatively homogeneous throughout the barrel hollow; subbarrel patterns of distribution only become recognizable between P‐8 and P‐10. These observations extend previous findings that subbarrels denote barrel regions enriched in synaptic contacts. The data also indicate that allocation of TC terminals into subbarrel domains does not occur immediately upon thalamic axon ingrowth. Instead, refinement of TC arbors into subbarrels is a gradual process, the outcome of which is not manifest until the second week of postnatal life. J. Comp. Neurol. 497:32–41, 2006.

Developmental Trajectories of Auditory Cortex Synaptic Structures and Gap-Prepulse Inhibition of Acoustic Startle Between Early Adolescence and Young Adulthood in Mice

Cortical excitatory and inhibitory synapses are disrupted in schizophrenia, the symptoms of which often emerge during adolescence, when cortical excitatory synapses undergo pruning. In auditory cortex, a brain region implicated in schizophrenia, little is known about the development of excitatory and inhibitory synapses between early adolescence and young adulthood, and how these changes impact auditory cortex function. We used immunohistochemistry and quantitative fluorescence microscopy to quantify dendritic spines and GAD65-expressing inhibitory boutons in auditory cortex of early adolescent, late adolescent, and young adult mice. Numbers of spines decreased between early adolescence and young adulthood, during which time responses increased in an auditory cortex-dependent sensory task, silent gap-prepulse inhibition of the acoustic startle reflex (gap-PPI). Within-bouton GAD65 protein and GAD65-expressing bouton numbers decreased between late adolescence and young adulthood, a delay in onset relative to spine and gap-PPI changes. In mice lacking the spine protein kalirin, there were no significant changes in spine number, within-bouton GAD65 protein, or gap-PPI between adolescence and young adulthood. These results illustrate developmental changes in auditory cortex spines, inhibitory boutons, and auditory cortex function between adolescence and young adulthood, and provide insights into how disrupted adolescent neurodevelopment could contribute to auditory cortex synapse pathology and auditory impairments.

Chronic desipramine treatment alters tyrosine hydroxylase but not norepinephrine transporter immunoreactivity in norepinephrine axons in the rat prefrontal cortex.

Pharmacological blockade of norepinephrine (NE) reuptake is clinically effective in treating several mental disorders. Drugs that bind to the NE transporter (NET) alter both protein levels and activity of NET and also the catecholamine synthetic enzyme tyrosine hydroxylase (TH). We examined the rat prefrontal cortex (PFC) by electron microscopy to determine whether the density and subcellular distribution of immunolabelling for NET and co-localization of NET with TH within individual NE axons were altered by chronic treatment with the selective NE uptake inhibitor desipramine (DMI). Following DMI treatment (21 d, 15 mg/kg.d), NET-immunoreactive (ir) axons were significantly less likely to co-localize TH. This finding is consistent with reports of reduced TH levels and activity in the locus coeruleus after chronic DMI and indicates a reduction of NE synthetic capacity in the PFC. Measures of NET expression and membrane localization, including the number of NET-ir profiles per tissue area sampled, the number of gold particles per NET-ir profile area, and the proportion of gold particles associated with the plasma membrane, were similar in DMI- and vehicle-treated rats. These findings were verified using two different antibodies directed against distinct epitopes of the NET protein. The results suggest that chronic DMI treatment does not reduce NET expression within individual NE axons in vivo or induce an overall translocation of NET protein away from the plasma membrane in the PFC as measured by ultrastructural immunogold labelling. Our findings encourage consideration of possible post-translational mechanisms for regulating NET activity in antidepressant-induced modulation of NE clearance.