Early developmental changes in intracellular Ca²⁺ stores in rat brain

Developmental changes in intracellular Ca²⁺ stores in brain was studied by examining: (1) IP₃- and cADPR-induced increase in [Ca²⁺](i) in synaptosomes; (2) Ca²⁺-ATPase activity and ATP-dependent ⁴⁵Ca²⁺ uptake into Ca²⁺ store in ER microsomes; (3) TG-induced inhibition of Ca²⁺- ATPase activity and ATP-dependent ⁴⁵Ca²⁺ uptake into Ca²⁺ store in ER microsomes; and (4) gene expression of Ca²⁺-ATPase pump in neurons obtained from brains of the new-born and the 3-week-old rats. IP₃ (EC₅₀ 310 ± 8 nM, 200% maximum increase in [Ca²⁺](i)) and cADPR (EC₅₀ 25 ± 3 nM, greater than 170% maximum increase in [Ca²⁺](i)) both were potent agonist of Ca²⁺ release from internal stores in synaptosomes obtained from the 3- week-old rats. However, IP₃ (EC₅₀ 250 ± 10 nM, 175 maximum increase in [Ca²⁺](i)) was a potent, but cADPR (EC₅₀ 300±20 nM, 75% maximum increase) was a poor agonist of Ca²⁺ release from intracellular stores in synaptosomes obtained from the new-born rats. [³H]IP₃, [³²p]cADPR and [³H]Ry binding in the new-born samples were significantly less than that in the 3-week-old samples. [³H]Ry binding to its receptor was more sensitive to cADPR in microsomes from the 3-week-old rats than those from the new-born rats. Microsomes from the new-born rats exhibited TG-sensitive (IC₅₀ 30 ± 4 nM) and TG-insensitive forms of Ca²⁺-ATPase, while microsomes from the 3- week-old rats exhibited only the TG-sensitive form of Ca²⁺-ATPase (5±1 nM IC₅₀). Microsomes from the 3-week-old rats were more sensitive to TG but less sensitive to IP₃, while microsomes from the new-born rats were more sensitive to IP₃ but less sensitive to TG. The lower TG sensitivity of the new-born Ca²⁺ store may be because they poorly express a 45 amino acid C- terminal tail of Ca²⁺-ATPase that contains the TG regulatory sites. This site is adequately expressed in the older brain. This suggests that: (1) the new-born brain contains fully operational IP₃ pathway but poorly developed cADPR pathway, while the older brain contains both IP₃ and cADPR pathways; and (2) a developmental switch occurs in the new-born Ca²⁺-ATPase as a function of maturity.