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Most neurons in the central nervous system and peripheral nervous system do not express water transporting protein, aquaporin (AQP). In the present study, we have demonstrated the presence of AQP1 immunoreactivity in a particular neuronal subtype in the enteric nervous system (ENS) of the rat ileum. AQP1-immunoreactive (IR) neurons simultaneously expressed a neuronal marker HuC/D. Moderate numbers of AQP1-IR neuronal somata were found in the myenteric plexus, and a very few were found in the submucosal plexus. AQP1-IR neurons can be classified as Dogiel type I cells, which have several short processes and a single long process. Many AQP1-IR fibers were found both in the myenteric and submucosal plexi. Many AQP1-IR varicose fibers were closely associated with neuronal somata in the ganglia, whereas other AQP1-IR fibers penetrated into the muscle layers. These results suggest that AQP1-IR neurons probably play a significant role within the ENS to control gut functions.

Changes in parvalbumin-like immunoreactivity were studied during ontogenetic development of the central auditory system of the Mongolian gerbil ( Meriones unguiculatus ). The nucleus of the trapezoid body contained cells that could be stained on the day of birth while in the superior olivary complex, prominent staining of cell somata was only found at P15 (postnatal day 15). Neurons located in the ventral nucleus, in the dorsal nucleus of the lateral lemniscus and in the inferior colliculus developed parvalbumin immunoreactivity mostly between P11–P15 (ventral nucleus), P15–P19 (dorsal nucleus) and P15–P19 (inferior colliculus), respectively. The accumulation of PV correlates reasonably well with the functional maturation of the neurons in these areas.

D-Amino acids are thought not to occur in mammalian tissues. However, previous studies reported D-serine was present only in astrocytes in the rat brain. In the present study, it was indicated by a highly sensitive immunocytochemical method with a D-serine specific antibody that D-serine was contained not only in astrocytes but also in some neurons, such as pyramidal neurons in the cerebral cortex, and neurons in the nucleus of the trapezoid body. Some amacrine cells also showed strong immunoreactivity for D-serine in the eyes which were injected with colchicine into the corpus vitreum.

Recent studies have shown that free d-serine, an agonist for the N-methyl-d-aspartate receptor, is present in the forebrain of rats. In the present study, we raised antibodies against d-serine and examined the distribution of both endogenous and intracerebroventricularly administered d-serine in the rat forebrain by an immunocytochemical method. d-Serine-like immunoreactive cells were found in glial cells in the brains of the rats injected with d-serine into the lateral ventricle. No immunoreactive cells were seen in the brains of untreated rats. The results suggest that glial cells may accumulate and catabolized d-serine to regulate the concentration of this neuroactive amino acid in the forebrain.

Cerebellar cortical neurons immunoreactive to anti-γ-aminobutyric acid (GABA) antibody were examined in the rat. In addition to the Purkinje, Golgi, basket and stellate cells, spindle-shaped cells lying just below the Purkinje cell layer were found to be strongly immunoreactive to the antibody. By the combination of immunofluorescence and hematoxylin stainings, these GABA-positive cells were shown to be the Lugaro cells. Unlike the immunopositive small Golgi cell, the pale cell was not immunoreactive to the antibody.

β-Bungarotoxin (BuTx)-binding cells were immunocytochemically examined in the developing rat cerebellum. The tissue was incubated with BuTx and then immunostained with antiserum against its toxoid. On postnatal day 6, only Golgi cells were positive for immunoreaction. Immunoreactive Golgi cells were reduced in number on day 15 and disappeared on day 25. On day 15, Purkinje cells were strongly stained, while some basket and stellate cells stained weakly. On day 25 and in adult, basket and stellate cells were more immunoreactive than Purkinje cells. Thus, age-dependent and selective binding of BuTx was restricted to γ-aminobutyric acid (GABA)ergic neurons.