Changes of pharmacological properties of (1S,3R)-ACPD-sensitive glutamate binding sites in developing mouse cerebellum

Neurotransmitters, neuropeptides and calcium binding proteins in developing human cerebellum: a review

Many endogenous neurochemicals that are known to have important functions in the mature central nervous system have also been found in the developing human cerebellum. Cholinergic neurons, as revealed by immunoreactivities towards choline acetyltransferase or acetylcholinesterase, appear early at 23 weeks of gestation in the cerebellar cortex and deep nuclei. Immunoreactivities gradually increase until the first postnatal month. Enkephalin is localized in the developing cerebellum, initially in the fibers of the cortex and deep nuclei at 16-20 weeks and then also in the Purkinje cells, granule cells, basket cells and Golgi cells at 23 weeks onward. Another neuropeptide, substance P, is localized mainly in the fibers of the dentate nucleus from 9 to 24 weeks but substance P immunoreactivity declines thereafter. GABA, an inhibitory neurotransmitter of the central nervous system, starts to appear at 16 weeks in the Purkinje cells, stellate cells, basket cells, mossy fibers and neurons of deep nuclei. GABA expression is gradually upregulated toward term forming networks of GABA-positive fibers and neurons. Catecholaminergic fibers and neurons are also detected in the cortex and deep nuclei at as early as 16 weeks. Calcium binding proteins, calbindin D28K and parvalbumin, make their first appearance in the cortex and deep nuclei at 14 weeks and then their expression decreases toward term, while calretinin appears later at 21 weeks but its expression increases with fetal age. The above findings suggest that many neurotransmitters, neuropeptides and calcium binding proteins (1) appear early during development of the cerebellum; (2) have specific temporal and spatial expression patterns; (3) may have functions other than those found in the mature neural systems; and (4) may be able to interact with each other during early development.

Multiple actions of 1S,3R-ACPD in modulating endogenous synaptic transmission to spinal respiratory motoneurons

To determine physiological roles of metabotropic glutamate receptors (mGluRs) affecting breathing, we examined the effects of (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD) on synaptic transmission and excitability of phrenic motoneurons (PMNs) in an in vitro neonatal rat brainstem/spinal cord preparation. The effects of 1S,3R-ACPD were multiple, including reduction of inspiratory-modulated synaptic currents and increase of neuronal excitability via an inward current (Iacpd) associated with a decrease of membrane conductance. The mechanism underlying synaptic depression was examined. We found that 1S,3R-ACPD reduced the frequency but not the amplitude of miniature excitatory postsynaptic currents. The current induced by exogenous AMPA was not significantly affected by 1S,3R-ACPD. These results suggest that 1S,3R-ACPD-induced reduction of inspiratory synaptic currents is mediated by presynaptic mGluRs. We also examined the ionic basis for Iacpd. We found that Iacpd had a reversal potential of approximately -100 mV, close to the estimated, EK+ (-95 mV). Elevating extracellular [K+] to 9 mM reduced the Iacpd reversal potential to -75 mV. The K+ channel blocker Ba2+ induced an inward current with a reversal potential at -93 mV associated with a decrease of membrane conductance, closely resembling the effect of 1S,3R-ACPD. Moreover, Ba2+, occluded 1S,3R-ACPD effects. In the presence of Ba2+, Iacpd and the 1S,3R-ACPD-induced decrease of membrane conductance were diminished. Our data indicate that the dominant component of Iacpd results from the blockade of a Ba(2+)-sensitive resting K+ conductance. We conclude that the activation of mGluRs affects the inspiratory-modulated activity of PMNs via distinct mechanisms at pre- and postsynaptic sites.