Stabilization of the binding of phosphatidylinositol bisphosphate (PIP(2)) to G protein-coupled inward rectifier K+ (GIRK) channels is essential for their activation, whereas hydrolysis of PIP(2) by phospholipase C (PLC) inhibits channel activity. Apparently inconsistent with this mechanism, we found that the commonly used PLC inhibitor, U73122 (1 microM), produced a significant reduction in the amplitude of baclofen (20 microM)-evoked GIRK currents in whole-cell recordings from acutely isolated rat neocortical pyramidal cells. Also, U73122 reduced the percentage of baclofen-responsive neurons from 100% (n=40) to 56% (n=25). Since NCDC (100 microM), a PLC inhibitor of another molecular class, displayed no effect on GIRK current amplitude or responsiveness (100%, n=6), inhibition of PLC is unlikely to account for the effects of U73122 in our preparation. Lending further support to this notion, the structurally closely related compound, U73343, which does not inhibit PLC, proved to be even more efficient in suppressing GIRK current as compared to U73122. In neurons, in which GIRK channels were irreversibly activated by GTPgammaS (n=10), the depressant action of U71322 was fully preserved. These findings hint at a direct interaction of U73122 with the GIRK channel or a closely associated protein. Caution is therefore warranted when employing this compound to examine the role of PLC and PIP(2) in the regulation of GIRK channel activity.

Whole cell recordings from acutely isolated rat neocortical pyramidal cells were performed to study the kinetics and the mechanisms of short-term desensitization of G-protein-activated, inwardly rectifying K+ (GIRK) currents during prolonged application (5 min) of baclofen, adenosine, or serotonin. Most commonly, desensitization of GIRK currents was characterized by a biphasic time course with average time constants for fast and slow desensitization in the range of 8 and 120 s, respectively. The time constants were independent of the agonist used to evoke the current. The biphasic time course was preserved in perforated-patch recordings, indicating that neither component of desensitization is attributable to cell dialysis. Desensitization of GIRK currents displayed a strong heterologous component in that application of a second agonist substantially reduced the responsiveness to a test agonist. Fast desensitization, but not slow desensitization, was lost in cells loaded with GDP, suggesting that the hydrolysis cycle of G proteins might underlie the initial, rapid current decline. Hydrolysis of phosphatidylinositol biphosphate is an unlikely candidate underlying short-term desensitization, because both components of desensitization were preserved in the presence of the phospholipase C inhibitor U73122. We conclude that short-term desensitization does neither result from receptor downregulation nor from altered channel gating but might involve modifications of the G-protein-dependent pathway that serves to translate receptor activation into channel opening.

We performed whole-cell recordings from acutely isolated pyramidal cell somata of rat neocortex to measure and compare G protein-activated, inwardly rectifying K+ (GIRK) currents induced by adenosine, serotonin and baclofen at different postnatal stages (postnatal days 3–19). In about two thirds of neurons, baclofen-induced GIRK currents were already detected at postnatal days 3 and 4 (P3–P4) and almost all neurons between P5 and P19 were responsive. This robust response suggests that postsynaptic effects of baclofen occur much earlier than previously thought. Sensitivity to adenosine was around 70% during the first two postnatal weeks. Given the late maturation of functional synaptic inhibition in neocortex, we propose that phasic and/or tonic activation of GIRK current by baclofen and adenosine might serve as a mechanism to control neuronal excitability during early postnatal development. In marked contrast to the pronounced early sensitivity to baclofen and adenosine, only 20% of the neurons displayed a GIRK current response to serotonin during the first postnatal week. After that, about half of the neurons tested positive for serotonin. GIRK current densities for baclofen and adenosine attained a maximum at the end of the second postnatal week, whereas the serotonin-induced current showed a linear increase during the second and third week of life. Set in relationship with previous data on the postnatal expression of receptor protein and GIRK channel mRNA, our findings suggest that the maturation of GIRK current responses is determined predominantly by the different postnatal patterns of receptor expression.

The auditory organs of the tettigoniid are located just below the femoral tibial joint in the forelegs. Structurally each auditory organ consists of a tonotopically organized crista acustica and intermediate organ and associated sound conducting structures; an acoustic trachea and two lateral tympanic membranes located at the level of the receptor complex. The receptor cells and associated satellite structures are located in a channel filled with hemolymph fluid. The vibratory response characteristics of the tympanic membranes generated by sound stimulation over the frequency range 2-40 kHz have been studied using laser vibrometry. The acoustic trachea was found to be the principal structure through which sound energy reached the tympana. The velocity of propagation down the trachea was observed to be independent of the frequency and appreciably lower than the velocity of sound in free space. Structurally the tympana are found to be partially in contact with the air in the trachea and with the hemolymph in the channel containing the receptor cells. The two tympana were found to oscillate in phase, with a broad band frequency response, have linear coherent response characteristics and small time constant. Higher modes of vibration were not observed. Measurements of the pattern of vibration of the tympana showed that these structures vibrate as hinged flaps rather than vibrating stretched membranes. These findings, together with the morphology of the organ and physiological data from the receptor cells, suggest the possibility of an impedance matching function for the tympana in the transmission of acoustic energy to the receptor cells in the tettigoniid ear.

In Polysarcus denticauda, a phaneropterine bushcricket with extremely thick uncovered tympana and an aberrant morphology of the cristae acusticae of the complex tibial organs, the electrophysiology of the auditory-vibratory ventral cord neurons ascending to the brain was investigated. Although the receptor organs in this species have some extraordinary response properties, the central auditory-vibratory neurons could be basically classified into the same functional types of S-, V-, and VS-neurons previously described for other bushcricket species. However, in some details the responses of most of the S-and VS-neurons are different. The S-neurons are generally more tonic and the VS-neurons give smaller responses to airborne sound stimulation than do the neurons belonging to the same functional types in most other bushcricket species.Adult males of P. denticauda use an extremely complicated and variable proclamation song for intraspecific acoustic communication. The song, which is often broadcast for a long time, can be divided into three phases on the basis of its time-amplitude structure. The syllable sequence exhibits some constant and other highly variable parameters. The basic, repeated element of the proclamation song is a pair of syllables; both syllables in the pair differ in duration and amplitude as well as in repetition rate between the three phases.When P. denticauda was stimulated with the conspecific male song, the responses of most S-neurons were found to copy the time patterns relatively well. Some of the V-and VS-neurons also responded relatively well to the syllables when vibrational signals were presented simultaneously. However, under these conditions the time structure was not copied, as it was by the S-neurons.In comparative investigations, the responses of central neurons of Decticus albifrons and Tettigonia viridissima, two species with more simply constructed songs having either low or high repetition rates, reflect the parameters of the song of P. denticauda less well than do the central neurons of P. denticauda. Therefore, some physiological adaptations to this complex male proclamation song seem to be inherent in the reactions of these central neurons in P. denticauda. (C) 1997 Wiley-Liss, Inc.

The acoustic-vibratory communication plays an important role within the life cycle of Tettigoniidae. It serves for intraspecific contacts between males and females in aggregation patches supporting especially reproduction, but also population dynamics. Communication needs specific signals which are exchanged between sender and receiver animals. The signals and also the acoustic behaviour of the animals have to be adapted to the generally noisy properties of the transmission channel of the biotope.

The structure of the complex tibial organs in the fore-, mid- and hindlegs of the bushcricket Polysarcus denticauda (Tettigoniidae, Phaneropterinae) is described comparatively. As is common for bushcrickets, in each leg the tibial organs consist of the subgenual and intermediate organs and the crista acustica. Only in the forelegs are sound-transmitting structures present. They consist of the spiracle, acoustic trachea, and two tympana; the latter are not protected by tympanal covers. The tympana in P. denticauda are extremely thick, not only bordering the two tracheal branches to the outside but also forming the outer wall of the hemolymph channel. The morphology of the tracheae in the mid- and hindlegs is significantly different, causing structural differences, especially in dimensions of the hemolymph channel. The number of scolopidia of the crista acustica of the foreleg is extremely high for a bushcricket. Approximately 50 receptor cells were found, about half of them being located in the distal quarter of the long axis of this organ. Some of the receptors are positioned in parallel on the dorsal wall of the anterior tracheal branch. The number, morphology and dimensions of the scolopidia within the crista acustica of the mid- and hindlegs differ significantly from those of the forelegs, decreasing in both legs to eight and seven receptor cells, respectively. Although the dimensions of the subgenual and intermediate organs are considerably larger in the mid- and hindlegs, the number of receptor cells is approximately the same in the different legs, being somewhat higher in both receptor organs than in those of many other bushcricket species studied previously.

The complex tibial organs of the bushcricket Polysarcus denticauda (Phaneropterinae) have some exceptional morphological features. In the forelegs, these organs have an extremely thick uncovered tympanum and about 50 receptor cells in the crista acustica. In the mid- and hindlegs, the cristae are extraordinarily reduced, with only seven or eight receptor cells. Physiological investigation of the receptor organs reveals that, in spite of the thick tympana, the auditory receptor cells of the forelegs have surprisingly low threshold values; they are as sensitive as the receptor cells of other bushcrickets, with very thin tympana. The high sensitivity is valid for the frequency range from at least 3-4 kHz up to 20 kHz. However, receptor cells tuned to frequencies above 20 K-Iz are less sensitive, suggesting a lack of discriminatory ability. This may be caused by the crowded arrangement of receptors at the distal part of the crista acustica. The frequency range of the conspecific proclamation song is therefore picked up only in the lower part of its power spectrum (10-20 kHz). The sensitive, low-frequency detection may originate from the broad-band sound transmission of the elaborate acoustic trachea of the forelegs, with a cut-off frequency at 4.5 kHz. The bimodal vibratory-auditory receptor cells of the tibial organs in the mid- and hindlegs are very sensitive to vibration, especially in the midlegs. They have significantly lower thresholds than receptors in the forelegs which are tuned to the same frequencies. This response property seems to have its origin within the specific structure of the organs in the mid- and hindlegs. (C) 1996 Wiley-Liss, Inc.

The morphology and the physiology of the auditory receptor organs in the forelegs of the tettigoniid species Gampsocleis gratiosa were investigated in detail. According to the tonotopic organization of the receptor cells of the crista acoustica in bushcrickets the threshold curves of all receptors found in the organs were correlated with the morphology of the auditory receptor cells. This was done by arranging them in relation to their frequency tuning with the receptors along the chain of cells in both organs starting proximally with the extreme low frequency cell up to the highest ultrasonic cells at the distal end of the crista acoustica.This investigation reveals that the auditory receptor organs of Gampsocleis gratiosa possess a broad banded hearing threshold curve including very low frequency parts.Also the conspecific stridulatory song is very broad banded with extraordinarily low frequency components. Therefore the sender and the receiver of this species-specific communication system seem to be well adapted to each other. (C) 1993 Wiley-Liss, Inc.