Levels of amino acid neurotransmitters during mouse olfactory bulb neurogenesis and in histotypic olfactory bulb cultures.

The developmental changes in the levels of amino acid neurotransmitters were analyzed by high pressure liquid chromatography during mouse olfactory bulb neurogenesis, from embryonic day (E)13 until the young adult age, between postnatal days (P)30 and P40. During the embryonic period, high levels of glutamate, aspartate and GABA were observed, with the values of GABA about 2-fold higher than those of glutamate and aspartate. At P0, the production of these neurotransmitters experienced birth stress as shown by a significant 2-fold reduction in their levels. During the first two postnatal weeks, a progressive increase in the glutamate content was detected diminishing slightly in the adult stage. The aspartate concentrations showed a maximal value at P3 and then decreased gradually until the second postnatal week; in the young adult age, its concentration was comparable with that of glutamate. The postnatal GABA contents increased progressively from birth to maturity, showing maximal levels at P3, P11 and in the adult. Throughout the studied developmental period, the concentration of glycine remained relatively low. With regard to taurine, very low concentrations were detected during the prenatal period but after birth, the taurine content gradually increased with age, and in the adult animal, its concentration was comparable with those of GABA and glutamate. Our data demonstrate the predominance of GABA and glutamate during olfactory bulb synaptogenesis, however, in the adult animal, both glutamate and aspartate exert the same influence in the excitatory synaptic transmission; in the adult inhibitory synaptic transmission, taurine appears to play an important neuromodulatory or neurotransmitter role as that of GABA. To determine the intrinsic neurotransmitter production, primary histotypic olfactory bulb cultures were prepared from mice at P10. The comparative analysis of in vitro neurotransmitter contents with those in in situ adult animal showed higher levels of endogenously produced glutamate, glycine and GABA in the olfactory bulb than the extrinsic ones coming from olfactory nerve axons and higher olfactory brain centers. On the other hand, most of aspartate and taurine neurotransmitters apparently come from extrinsically located neurons.

Levels of amino acid neurotransmitters during mouse cerebellar neurogenesis and in histotypic cerebellar cultures.

The variation in the levels of excitatory (glutamate and aspartate) and inhibitory (GABA, glycine and taurine) neurotransmitters during neurogenesis in mouse cerebellum, from embryonic day (E) 15 until the young adult stage, was analyzed by high-pressure liquid chromatography. Between E15 and E21, high contents of GABA, glutamate and aspartate were detected, with the GABA levels approximately 2- to 3-fold higher than those of glutamate and aspartate. After birth, the levels of GABA remained high during the first 2 postnatal weeks and then reached a plateau comparable to adult values by the third week. The levels of glutamate and aspartate increased gradually from birth to the young adult stage, showing peak values at postnatal days (P) 3 and P11. Glycine and taurine were present at relatively low concentrations during the prenatal period, then rose significantly by about 4-fold after birth; their levels decreased by the end of the first postnatal week but increased gradually thereafter until reaching adult values by the third week. To determine the endogenous neurotransmitter production in the cerebellar cortex, primary histotypic cerebellar cultures prepared at P10 were analyzed and the in vitro transmitter contents were compared with the adult in situ values. The cultures showed about the same levels of glutamate and aspartate; however, their concentrations were lower by half than in vivo, suggesting that both play an equally important role in the excitatory neurotransmission of the cerebellar cortex internal circuitry pathways and that in mature cerebellum, about 50% of the excitatory synaptic inputs derive from the afferent climbing and mossy fibers. The in vitro GABA and glycine contents were comparable with the in vivo levels, whereas the taurine concentrations were about 5-fold lower in vitro than in vivo. These findings indicate that most of the GABA and glycine transmitters are produced intrinsically while a high proportion of taurine in the cerebellum comes from extracerebellar afferents. This study provides data on the changing levels of the amino acid neurotransmitters in developing mouse cerebellum and the relative proportions of neurotransmitter contents that are produced by intrinsic neurons in contrast to those derived from extrinsic afferent fibers.