Sensing effectors make sense

Tension sensitivity of the heart pacemaker neurons in the isopod crustacean Ligia pallasii

In the crustacean neurogenic heart, the cardiac ganglion (CG) acts as a peripherally located central pattern generator (CPG) by producing rhythmic motor output that initiates the heartbeat. In the isopod Ligia, the CG consists of six electrically coupled neurons that all function both as endogenous oscillators and as glutamatergic motoneurons innervating heart muscle. In the present study, we present several lines of evidence to suggest that the CG neurons are sensitive to passive stretch and active tension of the heart muscle. Stretching the heart wall caused a sustained decrease in the burst frequency of the CG neuron. Releasing from the stretch caused a rebound increase in burst frequency above the control rate. A brief stretch (200-300 ms duration) caused either phase advance or phase delay of the following CG bursts, depending on the timing at which the stretch was applied. Repeated brief stretches could entrain the CG bursts to either higher or lower frequencies than the free-run burst frequency. Intracellular recording from one of the CG neurons revealed that it exhibited hyperpolarization during the stretch. The stretch-induced hyperpolarization was followed by a burst discharge upon release from the stretch. With increased stretch amplitude, the amplitude of hyperpolarizing response increased and the timing of the following burst was advanced. When the myogenic activity of the heart muscle was pharmacologically isolated from the ganglionic drive by applying a glutamatergic antagonist, Joro spider toxin (JSTX), the spontaneous muscle contraction caused a hyperpolarizing deflection in the CG neuron. Under specific conditions made by JSTX and tetrodotoxin, the CG burst became entrained to the myogenic rhythm. These results suggest that the Ligia CG neurons have tension sensitivity in addition to their pacemaker and motoneuronal functions. Such multifunctional neurons may form a single neuron reflex arc inside the heart.

Integration of sodium and osmosensory signals in vasopressin neurons

Vasopressin (antidiuretic hormone) release has been thought to be controlled by interacting osmoreceptors and Na(+)-detectors for > 20 years. Only recently, however, have molecular and cellular advances revealed how changes in the external concentration of Na+ and osmolality are detected during acute and chronic osmotic perturbations. In rat vasopressin-containing neurons, local osmosensitivity is conferred by intrinsic stretch-inactivated cation channels and by taurine release from surrounding glia. Na+ detection is accomplished by acute regulation of the permeability of stretch-inactivated channels and by changes in Na+ channel gene expression. These features provide a first glimpse of the integrative processes at work in a central osmoregulatory reflex.

Indirectly gated Cl(-)-dependent Cl(-) channels sense physiological changes of extracellular chloride in the leech

The maintenance of ion homeostasis requires adequate ion sensors. In leeches, 34 nephridial nerve cells (NNCs) monitor the Cl(-) concentration of the blood. After a blood meal, the Cl(-) concentration of leech blood triples and is gradually restored to its normal value within 48 h after feeding. As previously shown in voltage-clamp experiments, the Cl(-) sensitivity of the NNCs relies on a persistent depolarizing Cl(-) current that is turned off by an increase of the extracellular Cl(-) concentration. The activation of this Cl(-)-dependent Cl(-) current is independent of voltage and of extra- and intracellular Ca(2+). The transduction mechanism is now characterized on the single-channel level. The NNC's sensitivity to Cl(-) is mediated by a slowly gating Cl(-)-dependent Cl(-) channel with a mean conductance of 50 pS in the cell-attached configuration. Gating of the Cl(-) channel is independent of voltage, and channel activity is independent of extra- and intracellular Ca(2+). Channel activity and the macroscopic current are reversibly blocked by bumetanide. In outside-out patches, changes of the extracellular Cl(-) concentration do not affect channel activity, indicating that channel gating is not via direct interaction of extracellular Cl(-) with the channel. As shown by recordings in the cell-attached configuration, the activity of the channels under the patch is instead governed by the Cl(-) concentration sensed by the rest of the cell. We postulate a membrane-bound Cl(-)-sensing receptor, which-on the increase of the extracellular Cl(-) concentration-closes the Cl(-) channel via a yet unidentified signaling pathway.