Zdravko Petanjek

Extraordinary neoteny of synaptic spines in the human prefrontal cortex

The major mechanism for generating diversity of neuronal connections beyond their genetic determination is the activity-dependent stabilization and selective elimination of the initially overproduced synapses [Changeux JP, Danchin A (1976) Nature 264:705–712]. The largest number of supranumerary synapses has been recorded in the cerebral cortex of human and nonhuman primates. It is generally accepted that synaptic pruning in the cerebral cortex, including prefrontal areas, occurs at puberty and is completed during early adolescence [Huttenlocher PR, et al. (1979) Brain Res 163:195–205]. In the present study we analyzed synaptic spine density on the dendrites of layer IIIC cortico–cortical and layer V cortico–subcortical projecting pyramidal neurons in a large sample of human prefrontal cortices in subjects ranging in age from newborn to 91 y. We confirm that dendritic spine density in childhood exceeds adult values by two- to threefold and begins to decrease during puberty. However, we also obtained evidence that overproduction and developmental remodeling, including substantial elimination of synaptic spines, continues beyond adolescence and throughout the third decade of life before stabilizing at the adult level. Such an extraordinarily long phase of developmental reorganization of cortical neuronal circuitry has implications for understanding the effect of environmental impact on the development of human cognitive and emotional capacities as well as the late onset of human-specific neuropsychiatric disorders.

Loss of interneurons innervating pyramidal cell dendrites and axon initial segments in the CA1 region of the hippocampus following pilocarpine-induced seizures

In the pilocarpine model of chronic limbic seizures, vulnerability of GABAergic interneurons to excitotoxic damage has been reported in the hippocampal CA1 region. However, little is known about the specific types of interneurons that degenerate in this region. In order to characterize these interneurons, we performed quantitative analyses of the different populations of GABAergic neurons labeled for their peptide or calcium‐binding protein content. Our data demonstrate that the decrease in the number of GAD mRNA‐containing neurons in the stratum oriens of CA1 in pilocarpine‐treated rats involved two subpopulations of GABAergic interneurons: interneurons labeled for somatostatin only (O‐LM and bistratified cells) and interneurons labeled for parvalbumin only (basket and axo‐axonic cells). Stratum oriens interneurons labeled for somatostatin/calbindin or somatostatin/parvalbumin were preserved. The decrease in number of somatostatin‐ and parvalbumin‐containing neurons was observed as early as 72 hours after the sustained seizures induced by pilocarpine injection. Many degenerating cell bodies in the stratum oriens and degenerating axon terminals in the stratum lacunosum‐moleculare were observed at 1 and 2 weeks after injection. In addition, the synaptic coverage of the axon initial segment of CA1 pyramidal cells was significantly decreased in pilocarpine‐treated animals. These results indicate that the loss of somatostatin‐containing neurons corresponds preferentially to the degeneration of interneurons with an axon projecting to stratum lacunosum‐moleculare (O‐LM cells) and suggest that the death of these neurons is mainly responsible for the deficit of dendritic inhibition reported in this region. We demonstrate that the loss of parvalbumin‐containing neurons corresponds to the death of axo‐axonic cells, suggesting that perisomatic inhibition and mechanisms controlling action potential generation are also impaired in this model. J. Comp. Neurol. 459:407–425, 2003.

Ontogenesis of goal-directed behavior: anatomo-functional considerations

Recent neuroanatomical and neurophysiological studies in man have revealed ontogenetic events which coincide with broadly defined phases of behavioral and cognitive development. During the early fetal period, early produced neurons make initial synapses which form the basis for the earliest electrical activity of the human brain. The overall immaturity of neuronal connections, in particularly in cortical areas, correlates with the absence of any behavioral pattern or goal-directed movements. In the late fetus and preterm infant, transient accumulation of major afferent pathways, the presence of transient layers (subplate zone) and transient pattern of transmitter-related organization form the neurological basis of cortical electric responses as well as transient behavioral states and sleep patterns. Parallel to the profound structural and chemical reorganization of the human cerebrum during the first 6 postnatal months there is a disappearance of transient behavioral and motor patterns. The previously close spatio-temporal correlation between these events becomes progressively looser. The overproduction of circuitry elements during the subsequent period peaks in associative cortex between 1 and 2 years of age, corresponding to the emergence of skilled actions and cognitive functions. After the elimination of some circuitry elements after the second year of life, the prolonged maturation of goal-directed behavior and the protracted emergence of different cognitive functions correlates with the development plateau of synapse production which can be seen up to 16 years of age. Parallel to the prolonged maturation of postsynaptic elements, there are well defined maturational changes in the chemical properties of associative pyramidal neurons of cortical layer III. These findings correspond to the prolonged maturation of movement-related brain macropotentials as well as other cognition-related potentials, where the last prominent changes were seen after 10 years of age. Although the coincidence of the developmental events does not necessarily mean a causal relationship, the combination of structural and physiological data opens new vistas for the further investigation of the neurobiological basis of goal-directed movement and cognitive behavior.

Origins of Cortical GABAergic Neurons in the Cynomolgus Monkey

In human most cortical γ-aminobutyric acidergic (GABAergic) neurons are produced in the proliferative zones of the dorsal telencephalon in contrast to rodents. We report that in cynomolgus monkey fetuses cortical GABAergic neurons are generated in the proliferative zones of the dorsal telencephalon, in addition to the proliferative region of the ventral telencephalon, the ganglionic eminence (GE), however, with a temporal delay. GABAergic neuron progenitors labeled for Mash1 and GAD65 were present mainly in the GE at embryonic days (E) 47–55, and in the entire dorsal telencephalon at E64–75. These progenitors within the dorsal telencephalon are generated locally rather than in the GE. The ventral and dorsal lineages of cortical GABAergic neurons display different laminar distribution. Early generated GABAergic neurons from the GE mostly populate the marginal zone and subplate, whereas cortical plate GABAergic neurons originate from both ventral and dorsal telencephalon. A differential regulation of the two GABA synthesizing enzymes (GAD65 and GAD67) parallels GABAergic neuron differentiation. GAD65 is preferentially expressed in GABAergic progenitors and migrating neurons, GAD67 in morphologically differentiated neurons. Therefore, the dorsal telencephalic origin of cortical GABAergic neurons is not human-specific but appears as a former event in the ascent of evolution that could provide GABAergic neurons to an expending neocortex.

In vivo blockade of neural activity alters dendritic development of neonatal CA1 pyramidal cells

During development, neural activity has been proposed to promote neuronal growth. During the first postnatal week, the hippocampus is characterized by an oscillating neural network activity and a rapid neuronal growth. In the present study we tested in vivo, by injecting tetanus toxin into the hippocampus of P1 rats, whether this neural activity indeed promotes growth of pyramidal cells. We have previously shown that tetanus toxin injection leads to a strong reduction in the frequency of spontaneous GABA and glutamatergic synaptic currents, and to a complete blockade of the early neural network activity during the first postnatal week. Morphology of neurobiotin‐filled CA1 pyramidal cells was analyzed at the end of the first postnatal week (P6–10). In activity‐reduced neurons, the total length of basal dendritic tree was three times less than control. The number, but not the length, of basal dendritic branches was affected. The growth impairment was restricted to the basal dendrites. The apical dendrite, the axons, or the soma grew normally during activity deprivation. Thus, the in vivo neural activity in the neonate hippocampus seems to promote neuronal growth by initiating novel branches.

High bifurcation of common carotid artery, anomalous origin of ascending pharyngeal artery and anomalous branching pattern of external carotid artery

We present a rare case of combined high bifurcation of the common carotid artery, anomalous origin of the ascending pharyngeal artery and unusual branching pattern of the external carotid artery. The right common carotid artery bifurcated at the level between the second and the third cervical vertebrae, giving rise to the ascending pharyngeal artery just below the bifurcation. The right external carotid artery branched directly at its origin into the superior thyroid, lingual and occipital arteries and the distal part of the external carotid artery. The latter gave rise to the right facial artery and finally bifurcated into the maxillary and superficial temporal arteries. The right posterior auricular artery arose from the right occipital artery. The finding was unilateral and other vascular anomalies were not observed. The embryogenesis of such a combination of anomalies is not clear, but the anatomic consequences may have important clinical implications.

Primate-Specific Origins and Migration of Cortical GABAergic Neurons

Gamma-aminobutyric-acidergic (GABAergic) cells form a very heterogeneous population of neurons that play a crucial role in the coordination and integration of cortical functions. Their number and diversity increase through mammalian brain evolution. Does evolution use the same or different developmental rules to provide the increased population of cortical GABAergic neurons? In rodents, these neurons are not generated in the pallial proliferative zones as glutamatergic principal neurons. They are produced almost exclusively by the subpallial proliferative zones, the ganglionic eminence (GE) and migrate tangentially to reach their target cortical layers. The GE is organized in molecularly different subdomains that produce different subpopulations of cortical GABAergic neurons. In humans and non-human primates, in addition to the GE, cortical GABAergic neurons are also abundantly generated by the proliferative zones of the dorsal telencephalon. Neurogenesis in ventral and dorsal telencephalon occurs with distinct temporal profiles. These dorsal and ventral lineages give rise to different populations of GABAergic neurons. Early-generated GABAergic neurons originate from the GE and mostly migrate to the marginal zone and the subplate. Later-generated GABAergic neurons, originating from both proliferative sites, populate the cortical plate. Interestingly, the pool of GABAergic progenitors in dorsal telencephalon produces mainly calretinin neurons, a population known to be significantly increased and to display specific features in primates. We conclude that the development of cortical GABAergic neurons have exclusive features in primates that need to be considered in order to understand pathological mechanisms leading to some neurological and psychiatric diseases.

Perinatal growth of prefrontal layer III pyramids in down syndrome

We analyzed the dendritic differentiation of layer IIIc pyramidal neurons of prefrontal cortex (prospective area 9) in the brains of a premature infant and a 2.5-month-old infant with Down syndrome and two age-matched control subjects during the peak period of dendritic growth and differentiation. Our quantitative analysis supports qualitative observation and revealed no significant differences in the tempo and mode of dendritic differentiation between normal and Down syndrome cases. Thus we have concluded that the children with Down syndrome from our study begin their lives with morphologically normal layer III pyramidal neurons. Our findings suggest that pathologic changes of key prefrontal input-output neuronal elements begin to develop in Down syndrome after 2.5 months of postnatal age.

Epigenetic regulation of fetal brain development and neurocognitive outcome

Twenty-five years ago, the neurodevelopmental hypothesis of schizophrenia opened the way for analyzing potential prenatal influences on late postnatal pathogenetic events (1). Since that time, it has been well-established that hypoxia, infection, and other noxious factors can cause fetal brain injury and intrauterine growth restriction and that prenatal brain lesions can cause alterations in the postnatal brain development (2). A study in PNAS (3) suggests that even the subtle normal variations of intrauterine environment may lead to recognizable differences in postnatal brain structure and cognitive functions.

Spatio-temporal extension in site of origin for cortical calretinin neurons in primates

The vast majority of cortical GABAergic neurons can be defined by parvalbumin, somatostatin or calretinin expression. In most mammalians, parvalbumin and somatostatin interneurons have constant proportions, each representing 5–7% of the total neuron number. In contrast, there is a threefold increase in the proportion of calretinin interneurons, which do not exceed 4% in rodents and reach 12% in higher order areas of primate cerebral cortex. In rodents, almost all parvalbumin and somatostatin interneurons originate from the medial part of the subpallial proliferative structure, the ganglionic eminence (GE), while almost all calretinin interneurons originate from its caudal part. The spatial pattern of cortical GABAergic neurons origin from the GE is preserved in the monkey and human brain. However, it could be expected that the evolution is changing developmental rules to enable considerable expansion of calretinin interneuron population. During the early fetal period in primates, cortical GABAergic neurons are almost entirely generated in the subpallium, as in rodents. Already at that time, the primate caudal ganglionic eminence (CGE) shows a relative increase in size and production of calretinin interneurons. During the second trimester of gestation, that is the main neurogenetic stage in primates without clear correlates found in rodents, the pallial production of cortical GABAergic neurons together with the extended persistence of the GE is observed. We propose that the CGE could be the main source of calretinin interneurons for the posterior and lateral cortical regions, but not for the frontal cortex. The associative granular frontal cortex represents around one third of the cortical surface and contains almost half of cortical calretinin interneurons. The majority of calretinin interneurons destined for the frontal cortex could be generated in the pallium, especially in the newly evolved outer subventricular zone that becomes the main pool of cortical progenitors.