Anesthetic treatment blocks synaptogenesis but not neuronal regeneration of cultured Lymnaea neurons.

Trauma and injury necessitate the use of various surgical interventions, yet such procedures themselves are invasive and often interrupt synaptic communications in the nervous system. Because anesthesia is required during surgery, it is important to determine whether long-term exposure of injured nervous tissue to anesthetics is detrimental to regeneration of neuronal processes and synaptic connections. In this study, using identified molluscan neurons, we provide direct evidence that the anesthetic propofol blocks cholinergic synaptic transmission between soma-soma paired Lymnaea neurons in a dose-dependent and reversible manner. These effects do not involve presynaptic secretory machinery, but rather postsynaptic acetylcholine receptors were affected by the anesthetic. Moreover, we discovered that long-term (18-24 h) anesthetic treatment of soma-soma paired neurons blocked synaptogenesis between these cells. However, after several hours of anesthetic washout, synapses developed between the neurons in a manner similar to that seen in vivo. Long-term anesthetic treatment of the identified neurons visceral dorsal 4 (VD4) and left pedal dorsal 1 (LPeD1) and the electrically coupled Pedal A cluster neurons (PeA) did not affect nerve regeneration in cell culture as the neurons continued to exhibit extensive neurite outgrowth. However, these sprouted neurons failed to develop chemical (VD4 and LPeD1) and electrical (PeA) synapses as observed in their control counterparts. After drug washout, appropriate synapses did reform between the cells, although this synaptogenesis required several days. Taken together, this study provides the first direct evidence that the clinically used anesthetic propofol does not affect nerve regeneration. However, the formation of both chemical and electrical synapses is severely compromised in the presence of this drug. This study emphasizes the importance of short-term anesthetic treatment, which may be critical for the restoration of synaptic connections between injured neurons.

Synapse number and synaptic efficacy are regulated by presynaptic cAMP and protein kinase A.

The mechanisms by which neurons regulate the number and strength of synapses during development and synaptic plasticity have not yet been defined fully. This lack of fundamental knowledge in the fields of neurodevelopment and synaptic plasticity can be attributed, in part, to compensatory mechanisms by which neurons accommodate for the loss of function in their synaptic partners. This is generally achieved either by scaling up neuronal transmitter release capabilities or by enhancing the postsynaptic responsiveness. Here, we demonstrate that regulation of synaptic strength and number between identified Lymnaea neurons visceral dorsal 4 (VD4, the presynaptic cell) and left pedal dorsal 1 (LPeD1, the postsynaptic cell) requires presynaptic activation of a cAMP-PKA-dependent signal. Experimental activation of the cAMP-PKA pathway resulted in reduced synaptic efficacy, whereas inhibition of the cAMP-PKA cascade permitted hyperinnervation and an overall enhancement of synaptic strength. Because synaptic transmission between VD4 and LPeD1 does not require a cAMP-PKA pathway, our data show that these messengers may play a novel role in regulating the synaptic efficacy during early synaptogenesis and plasticity.

Retinoic acid involvement in the reciprocal neurotrophic interactions between newt spinal cord and limb blastemas in vitro.

The purpose of this study was to investigate the reciprocal neurotrophic interaction between regenerating limb blastemas and spinal cord explants from the newt Notophthalmus viridescens. Axon outgrowth was measured from spinal cord explants in vitro to assess the neurotrophic activity of early to mid-bud stage blastemas after various treatments. When retinoic acid, a vitamin A metabolite, was added to the medium, it increased both the number and length of axons extending from spinal cord explants. Spinal cord explants co-cultured with blastemas that were previously treated with citral, an inhibitor of retinoic acid synthesis, extended significantly fewer axons than control co-cultures. Blastemas, which were denervated by surgical resection of the brachial plexus 48 h before co-culture, also exhibited a significantly weaker neurotrophic activity than did control innervated blastemas. These results are consistent with a reciprocal interaction between blastema mesenchyme and nerves and suggest either a stimulatory or synergistic role for endogenous retinoic acid in the blastema-derived trophic activity.