Autonomic control of the swimbladder.

The swimbladder of teleost fishes is the primary organ for controlling whole-body density, and thus buoyancy. The volume of gas in the swimbladder is adjusted to bring the organism to near neutral buoyancy at a particular depth. Swimbladder morphology varies widely among teleosts, but all species are capable of inflating and deflating this organ under reflex control by the autonomic nervous system, to achieve neutral buoyancy. Here we review the control of effectors within the swimbladder, including acid-secreting cells, vasculature and musculature, that are involved in determining gas volume. This control system is complex. It incorporates the "classical" efferent elements of the autonomic nervous system, the spinal autonomic and cranial autonomic limbs and their neurotransmitters (typically noradrenaline (NA)/adrenaline (ADR), and acetylcholine, respectively), but also non-adrenergic, non-cholinergic neurotransmitters such as peptides, purines and nitric oxide. The detailed patterns of autonomic innervation of swimbladder effectors are not well understood, nor are the relationships of terminals releasing non-adrenergic, non-cholinergic neurotransmitters onto these effectors. Furthermore, in most cases the complement of postjunctional receptor subtypes activated by adrenergic, cholinergic and other neurotransmitters, and the biological effects of these neurochemicals, have not been completely established. In order to clarify some of these issues and to provide insight into basic principles underlying autonomic control of swimbladder function, we propose the zebrafish as a potentially useful model teleost.