Postnatal ontogeny of the transcription factor Lmx1b in the mouse central nervous system.

The expression profile of Lim homeodomain transcription factor Lmx1b in the mouse brain was investigated at different postnatal stages by immunohistochemistry and in situ hybridization. At postnatal day (P) 7, many Lmx1b-expressing neurons were found in the posterior hypothalamic area, supramammillary nucleus, ventral premammillary nucleus, and subthalamic nucleus. In the midbrain, numerous Lmx1b-expressing neurons were present in the substantia nigra pars compacta and ventral tegmental area. In the hindbrain, Lmx1b-expressing neurons were primarily observed in the raphe nuclei, parabrachial nuclei, principal sensory trigeminal nucleus, nucleus of the solitary tract, and laminae I-II of the medullary dorsal horn as well as spinal dorsal horn. Although expression levels diminished as postnatal life progressed, persistent expression throughout the first year of life was observed in many of these regions. In contrast, Lmx1b was present in a few brain regions (e.g., principal sensory trigeminal nucleus) only in early life with expression expiring by P60. Lmx1b was observed in dopaminergic neurons in the midbrain and serotonergic neurons in the hindbrain, as determined by double labeling with specific markers. In addition, we found that Lmx1b-expressing neurons are not GABAergic, and Lmx1b was colocalized with Tlx3 in the parabrachial nuclei, principal sensory trigeminal nucleus, nucleus of the solitary tract. as well as the medullary and spinal dorsal horns, suggesting that Lmx1b-expressing cells in these areas are excitatory neurons. Our data suggest that Lmx1b is involved in the postnatal maturation of certain types of neurons and maintenance of their normal functions in the adult brain.

Pontine stimulation overcomes developmental limitations in the neural mechanisms of eyeblink conditioning.

Pontine neuronal activation during auditory stimuli increases ontogenetically between postnatal days (P) P17 and P24 in rats. Pontine neurons are an essential component of the conditioned stimulus (CS) pathway for eyeblink conditioning, providing mossy fiber input to the cerebellum. Here we examined whether the developmental limitation in pontine responsiveness to a CS in P17 rats could be overcome by direct stimulation of the CS pathway. Eyeblink conditioning was established in infant rats on P17-P18 and P24-P25 using pontine stimulation as a CS. There were no significant age-related differences in the rate or level of conditioning. Eyeblink conditioned responses established with the stimulation CS were abolished by inactivation of the ipsilateral cerebellar nuclei and overlying cortex in both age groups. The findings suggest that developmental changes in the CS pathway play an important role in the ontogeny of eyeblink conditioning.

Developmental expression of the group III metabotropic glutamate receptor mGluR4a in the medial nucleus of the trapezoid body of the rat.

A preembedding immunocytochemical method for light microscopy was used to study the postnatal development of expression of the group III metabotropic glutamate receptor mGluR4a in the medial nucleus of the trapezoid body (MNTB) of the rat. Immunoreactivity for mGluR4a was localized in axonal endings wrapping the principal globular neurons in MNTB, known as calyces of Held. The percentage of calyces of Held immunoreactive for mGluR4a increased progressively from postnatal day 3 (PND3), showing the highest density of labeled calyces by PND9. From this postnatal age on, a gradual reduction in the number of mGluR4a-immunopositive calyces of Held was observed, reaching the lowest level of labeled profiles in adult tissue. The developmental expression of mGluR4a in calyces of Held correlates well with previous studies in young animals showing a modulation of synaptic neurotransmission by group III mGluRs in these giant excitatory synapses made on MNTB principal neurons. All these observations together suggest that the expression of mGluR4a mainly between PND7 and PND12 might be relevant to the maturation and modulation of synaptic transmission at the calyces of Held.

Axonal growth and target selection during development: retinal projections to the ventrobasal complex and other "nonvisual" structures in neonatal Syrian hamsters.

In newborn hamsters, there is a direct retinal projection to the ventrobasal complex, the principal thalamic somatosensory nucleus. The projection decreases precipitously between the second and third postnatal days. A few retinofugal axons remain dorsally along the lateral border of the nucleus on day 4, and none are present thereafter. In neonatal hamsters, retinofugal axons project to additional "nonvisual" nuclei including the periventricular and anterior nuclei of the hypothalamus, zona incerta, substantia nigra, inferior colliculus, pons, and mesencephalic tegmentum. Some of these connections remain in adult hamsters, although in apparently reduced density or relative volume, while others disappear. The contribution of transient connections to the normal morphological or functional development of the brain remains to be clarified. The combined results of this and other studies show that the normally transient retino-ventrobasal projection is a substrate for abnormal connections in that it can be permanently stabilized by appropriate neurosurgery on the day of birth.

Histamine synthesis in the developing rat brain: evidence for a multiple compartmentation.

Subcellular fractionation techniques, radio-labeling by the 3H-precursor and pharmacological approach applied to the developing rat indicate the presence of at least two types of histamine-containing cells in brain. The presence of the histamine synthesizing enzyme in neurons is suggested by its developmental pattern: there is a 4- to 5-fold increase in enzyme activity from birth to adulthood, with a time-course paralleling the synaptogenesis in whole brain as well as in the 4 regions studied (medulla-pons, midbrain, hypothalamus and forebrain). As is the case for different transmitter synthesizing enzymes such as tyrosine hydroxylase, there is a shift in the subcellular distribution of histidine decarboxylase (H.D.) activity from the soluble fraction at birth to the synaptosomal fraction in the adult brain. On the other hand, several lines of evidence indicates that a portion of histamine is localized, at least in the noenatal rat brain, in mast cells: (a) the high level of histamine in the neonatal rat brain is, like in peripheral mast-cells, associated with a low enzyme activity; (b) the half-life of [3H]histidine injected s.c. at birth was about 4 days, a value close to that found in skin (a tissue rich in mast cells), but contrasting with that in adult brain (less than 1 h); (c) after subcellular fractionation, the endogenously formed [3H]histamine was recovered in the crude nuclear fraction as was the amine from peritoneal mast cells added to the brain homogenate; (d) the mast cell degranulators, compound 48/80 and polymyxin B, induce a small but significant release of the amine from incubated neonatal brain slices. Thus it appears that cerebral histamine is localized in at least two cell types. Its presence in neurons is compatible with a neurotransmitter function and its release from mast cells might represent some primitive form of cell-to-cell communication.