Development of functional thalamocortical synapses studied with current source-density analysis in whole forebrain slices in the rat

Perinatal subplate neuron injury: implications for cortical development and plasticity

Perinatal brain injury may result in widespread deficits in visual, motor and cognitive systems suggesting disrupted brain development. Neurosensory and cognitive impairment are observed at increasing frequency with decreasing gestational ages, suggesting a unique vulnerability of the developing brain. The peak of human subplate neuron development coincides with the gestational ages of highest vulnerability to perinatal brain injury in the premature infant. At the same time, human thalamocortical connections are forming and being refined by activity-dependent mechanisms during critical periods. Subplate neurons are the first cortical neurons to mature and are selectively vulnerable to early hypoxic-ischemic brain injury in animal models. Timing of subplate neuron death determines the resulting defect in thalamocortical development: very early excitotoxic subplate neuron death results in failure of thalamocortical innervation, while later subplate neuron death interferes with the refinement of thalamocortical connections into mature circuits. We suggest that subplate neuron injury may be a central component of perinatal brain injury resulting in specific neurodevelopmental consequences.

Transient microcircuits formed by subplate neurons and their role in functional development of thalamocortical connections

Subplate neurons are a transient population of neurons in the brain forming one of the first functional cortical circuits. Past experiments have demonstrated their importance in growth of thalamocortical afferents into the cortical plate and later segregation of thalamocortical afferents. Recently, subplate neurons have been shown to be required for the functional maturation of both thalamocortical connections and mature visual responses in visual cortex. These findings suggest that thalamocortical afferents might not segregate properly in the absence of subplate neurons because the thalamocortical synapse does not mature. Subplate neurons are unique in that they form a circuit that appears to promote synaptic scaling and maturation. Although the precise contribution of subplate neurons within the context of cortical development is unknown, they might play an early role in providing thalamic input to cortex that then interacts with learning rules governing synaptic strengthening at the thalamocortical synapse. Because they appear to play multiple key roles at different stages of development, subplate neurons might also play a role in the pathology of developmental disorders, such as epilepsy and schizophrenia.

Postnatal development of the afferent projections from the dorsomedial thalamic nucleus to the frontal cortex in mice

The postnatal development of mediodorsal thalamic projections to the dorsomedial frontal cortex of mice was assessed by means of the retrograde peroxidase-colloidal gold complex tract tracing system. The tracer was injected into the dorsomedial frontal cortex from the day of birth (P0) to 60 days of postnatal age (P60). Since birth, a dense retrograde labeling has been found in the mediodorsal nucleus, which increased progressively from P4 to P8 and began to decrease at P10 until P13 (67.37% vs. the maximal average, P4). After P16, the mean average remains stable up to P60.

Prenatal development of neural excitation in rat thalamocortical projections studied by optical recording

To elucidate the formation of early thalamocortical synapses we recorded optical images with voltage-sensitive dyes from the cerebral cortex of prenatal rats by selective thalamic stimulation of thalamocortical slice preparations. At embryonic day (E) 17, thalamic stimulation elicited excitation that rapidly propagated through the internal capsule to the cortex. These responses lasted less than 15 ms, and were not affected by the application of glutamate receptor antagonists, suggesting that they might reflect presynaptic fiber responses. At E18, long-lasting (more than 300 ms) responses appeared in the internal capsule and in subplate. By E19, long-lasting responses increased in the cortical subplate. By E21, shortly before birth, the deep cortical layers were also activated in addition to the subplate. These long-lasting responses seen in the internal capsule and subplate were blocked by the antagonist perfusion, but the first spike-like responses still remained. The laminar location of the responses was confirmed in the same slices by Nissl staining and subplate cells were labeled by birthdating with bromodeoxyuridine at E13. Our results demonstrate that there is a few days delay between the arrival of thalamocortical axons at the subplate at E16 and the appearance of functional thalamocortical synaptic transmission at E19. Since thalamocortical connections are already functional within the subplate and in the deep cortical plate at embryonic ages, prenatal thalamocortical synaptic connections could influence cortical circuit formation before birth.

Normal development of embryonic thalamocortical connectivity in the absence of evoked synaptic activity

This study is concerned with the role of impulse activity and synaptic transmission in early thalamocortical development. Disruption of the gene encoding SNAP-25, a component of the soluble N-ethylmaleimide-sensitive factor attachment protein (SNAP) receptor complex required for regulated neuroexocytosis, eliminates evoked but not spontaneous neurotransmitter release (Washbourne et al., 2002). The Snap25 null mutant mouse provides an opportunity to test whether synaptic activity is required for prenatal neural development. We found that evoked release is not needed for at least the gross formation of the embryonic forebrain, because the major features of the diencephalon and telencephalon were normal in the null mutant mouse. However, half of the homozygous mutants showed undulation of the cortical plate, which in the most severely affected brains was accompanied by a marked reduction of calbindin-immunoreactive neurons. Carbocyanine dye tracing of the thalamocortical fiber pathway revealed normal growth kinetics and fasciculation patterns between embryonic days 17.5 and 19. As in normal mice, mutant thalamocortical axons reach the cortex, accumulate below the cortical plate, and then start to extend side-branches in the subplate and deep cortical plate. Multiple carbocyanine dye placements in the cortical convexity revealed normal overall topography of both early thalamocortical and corticofugal projections. Electrophysiological recordings from thalamocortical slices confirmed that thalamic axons were capable of conducting action potentials to the cortex. Thus, our data suggest that axonal growth and early topographic arrangement of these fiber pathways do not rely on activity-dependent mechanisms requiring evoked neurotransmitter release. Intercellular communication mediated by constitutive secretion of transmitters or growth factors, however, might play a part.