Development of synapsin I and synapsin II in intraocular hippocampal transplants

Synapsin I in Intraocular Hippocampal Transplants during Maturation and Aging: Effects of Brainstem Cografts

The role of target innervation for maintenance of synaptic proteins in the hippocampal formation during aging was investigated. Fetal CA1 tissue and brainstem tissue containing the nucleus locus coeruleus was dissected from albino rats and grafted sequentially into the anterior chamber of the eye of adult rat recipients. Synapsin protein distribution and levels were evaluated by immunohistochemistry and quantitative immunolabeling in single hippocampal grafts or brainstem-hippocampal double grafts at 6,12, or 24 mo postgrafting. The synapsin levels in 6-mo-old single hippocampal transplants were significantly lower than those in situ, and remained at these lower levels at 12 and 24 mo. On the contrary, synapsin levels were close to normal in the hippocampal portion of double grafts in the 6- and the 12-mo-group. However, in the 24-mo-old double transplants the levels had declined significantly, approaching levels seen in single hippocampal grafts. The immunoblot results were supported by morphological observations with synapsin antibodies and immunohistochemistry. The present data demonstrate that hippocampal tissue maintained near normal synapsin levels when grafted together with brainstem tissue, as compared to the lower levels seen in single hippocampal grafts. This normalization of synapsin levels was, however, not seen in the aged hippocampal-brainstem double grafts.

Neuron-specific and developmental regulation of the synapsin II gene expression in transgenic mice

Synapsin II, a major phosphoprotein of synaptic vesicles, is believed to function in neurotransmitter release as well as in synapse formation. The expression of the synapsin II gene is neuron-specific, and correlates temporally with synaptogenesis. To understand the mechanisms by which the expression of the synapsin II gene is regulated in vivo, we generated transgenic mice carrying a 5.1-kb 5'-flanking sequence of the murine synapsin II gene fused to the firefly luciferase reporter gene. The synapsin II-luciferase transgene is specifically expressed in neural tissues, such as brain and spinal cord, but not in non-neural tissues. Throughout the brain, the expression of the transgene is widely distributed, and restricted only to neuronal cells. Moreover, the expression of the transgene is developmentally regulated, with a temporal profile similar to that of endogenous synapsin II expression. These results indicate that the 5.1-kb flanking sequence of the murine synapsin II gene contains cis-regulatory elements that are required for directing neuron-specific and synaptogenesis-regulated expression in vivo.

Collateral sprouting of central noradrenergic neurons during aging: histochemical and neurochemical studies in intraocular triple transplants

The sprouting capacity of aged noradrenergic neurons of the brain-stem nucleus locus coeruleus (LC) was examined using intraocular transplants of fetal tissues. Fetal hippocampal tissue (E18) and LC tissue (E15) were transplanted together as a double transplant into the anterior chamber of the eye of young adult Fischer 344 rats. The double transplants were allowed to mature for 14-18 months, after which an additional fetal hippocampal transplant was placed next to the LC graft. The triple transplants were monitored for overall growth and vascularization for an additional 2-6 months. Immunohistochemical examinations showed that both young (2-6 months old) and aged (16-24 months old) hippocampal cografts contained a plexus of thin varicose tyrosine hydroxylase (TH)-immunoreactive fibers extending throughout the grafted hippocampal tissues. However, the aged hippocampal grafts contained a denser uniform plexus of TH-positive fibers compared to the young transplants. Immunohistochemistry with synapsin antibodies demonstrated that both the young and the aged hippocampal transplants contained much higher densities of synaptic elements than the LC grafts. In vivo electrochemical measurements of potassium-evoked overflow of norepinephrine (NE) in the grafts showed that similar amounts of NE overflow were detected in both the young and the aged hippocampal grafts. HPLC-EC measurements of NE levels in the grafts revealed that there were similar amounts of NE in the young and the aged grafts, and the grafts did not contain serotonin or dopamine. In summary, the findings of the present study show that aged LC neurons are capable of undergoing collateral sprouting producing a functional NE neuronal system when introduced to an appropriate young target.

Expression of synapsin I gene in primary cultures of differentiating rat cortical neurons

Synapsin I is a neuron-specific protein which is present in two isoforms, Ia and Ib. In the last few years this protein has been demonstrated to play a central role in the regulation of neurotransmitter release and synaptic plasticity. In this paper the developmental expression of this protein has been investigated in primary neuronal cultures from fetal rat brain cortices. The presence of thyroid hormone in the culture medium stimulates an early expression of the protein without exerting any effect at the level of mRNA transcription and accumulation. These observations implicate a T3-dependent regulation of this neuron-specific gene at the level of mRNA translation.

Concurrent immediate early gene induction by epileptic seizures in heterotopic cortical grafts and neocortex

Cortical primordia of rat fetuses (gestation day 14) were stereotactically grafted into the rostral striatum of adult recipient rats. After 8 weeks, the transplants had developed into a highly differentiated population of mature neuroectodermal cells. Host rats were then subjected to 15 min of bicuculline-induced epileptic seizures or served as controls. Seizure-elicited immediate early gene (IEG) expression was investigated after various postictal survival times (up to 24 h), using immunocytochemistry with specific antisera against seven IEG encoded proteins (c-FOS, FOS B, c-JUN, JUN B, JUN D, KROX-24, KROX-20). Constitutive IEG expression in intra striatum grafted neocortical neurons was identical to that in the corresponding host neocortex. In particular, abundant KROX-24 and lack of c-JUN expression implies the establishment of synaptic contacts within the graft or with the host circuitry. Postictal expression kinetics of individual IEG encoded proteins within the transplants were strikingly similar to those seen in the neocortex in situ. c-FOS and KROX-24 were most rapidly induced, followed by c-JUN and JUN B, and a more delayed induction of FOS B, JUN D and KROX-20. Apart from a slightly prolonged c-FOS expression in grafts, individual transcription factors remained elevated for different time periods and showed a concurrent decline in transplants and in neocortex in situ. In conclusion, IEG induction in grafts closely paralleled that in the host neocortex but differed from the adjacent striatum which exhibited no c-JUN induction at any time point investigated. These results indicate that following an appropriate differentiation period, heterotopically grafted embryonic cortical neurons respond to extracellular stimuli with changes of gene expression that closely resemble the normal host cortex. This suggests development of a similar molecular phenotype, including proper acquisition and intracellular processing of information.