Serotonin-activated adenylate cyclase and the possible role of cyclic AMP in modulation of buccal muscle contraction in Aplysia

Amiloride inhibits contraction and serotonin modulation of Aplysia muscle

1. Serotonin (5-HT) potentiates acetylcholine (ACh)-elicited contractions of Aplysia buccal muscles. Serotonin potentiation was significantly reduced by 0.03 mM, 0.1 mM, and 0.3 mM amiloride. 2. Unpotentiated ACh-elicited contractions were significantly reduced by 0.1 mM and 0.3 mM amiloride. 3. Amiloride reduced ACh-elicited depolarization. The reduction in contraction caused by 0.3 mM amiloride (to 16% of control) was larger than could be explained by the reduction in depolarization (86% of control). 4. Amiloride had no effect on tension in skinned muscle fibers, indicating that amiloride probably did not have a direct effect on contractile mechanisms. 5. Potentiation of contraction produced by zero sodium (Tris substituted, 0 Na-Tris) medium could be abolished by 0.3 mM amiloride. 6. Zero Na-Tris increased 45Ca influx 2.7-fold. In the presence of 0.3 mM amiloride, 0 Na-Tris increased 45Ca influx only 1.4-fold. 7. Amiloride (0.3 mM) reduced the elevation of muscle cAMP caused by 10(-6) M 5-HT by 60%. Zero Na-Tris did not cause a change in muscle cAMP.

Skinned muscle fibers of Aplysia: potentiation of contraction by "background" calcium and lack of effect of cyclic AMP

1. Aplysia buccal muscle E1 can be skinned with saponin in a low ionic strength medium. Pulses of calcium, which were ineffective at causing contraction in intact fibers, elicited contraction in skinned fibers. 2. Tension in skinned fibers increased at [Ca2+] greater than 10(-7) M and was maximal at 6 x 10(-7) M. 10(-5) M [Ca2+] caused irreversible damage to the fibers. 3. Fibers did not exhibit "catch", i.e. they relaxed quickly upon removal of calcium. 4. Optimal pH for tension was 7.0. 5. Contractile responses to calcium pulses were increased by raising "background" [Ca2+] to 10(-7) M. 6. Cyclic AMP (10(-4) and 10(-3) M) had no effect on tension.

Inward rectifier K channels in renal epithelioid cells (MDCK) activated by serotonin

The present study has been performed to test for the effect of intracellular calcium and of serotonin on the channel activity in patches from subconfluent MDCK-cells. In inside-out patches, inwardly rectifying potassium-selective channels are observed with open probabilities of 0.01 +/- 0.01, 0.24 +/- 0.03 and 0.39 +/- 0.07, at 100 nmol/liter, 1 mumol/liter or 10 mumol/liter calcium activity, respectively. The single-channel slope conductance is 34 +/- 2 pS, if the potential difference across the patch (Vp) is zero, and approaches 59 +/- 1 pS, if Vp is -50 mV, cell negative. In the cell-attached mode, little channel activity is observed prior to application of serotonin (open probability = 0.03 +/- 0.03). If 1 mumol/liter serotonin is added to the bath perfusate, the open probability increases rapidly to a peak value of 0.34 +/- 0.04 within 8 sec. In continued presence of the hormone, the open probability declines to approach 0.06 +/- 0.02 within 30 sec. At zero potential difference between pipette and reference in the bath (i.e., the potential difference across the patch is equal to the potential difference across the cell membrane), the single-channel conductance is 59 +/- 4 pS. In conclusion, inwardly rectifying potassium channels have been identified in the cell membrane of subconfluent MDCK-cells, which are activated to a similar extent by increase of intracellular calcium activity to 1 mumol/liter and by extracellular application of 1 mumol/liter serotonin.

Effects of serotonin on electrical properties of Madin-Darby canine kidney cells

The present study has been performed to test for the influence of serotonin on the potential difference across the cell membrane (PD) of Madin-Darby canine kidney (MDCK)-cells. Under control conditions PD averages -48.6 +/- 0.6 mV (n = 98). Increasing extracellular potassium concentration from 5.4 to 10 and 20 mmol/l depolarizes the cell membrane by +6.3 +/- 0.6 mV (n = 6) and +14.1 +/- 1.0 mV (n = 12), respectively. The cell membrane is transiently hyperpolarized to -67.8 +/- 0.8 mV (n = 63) by 1 mumol/l serotonin. In the presence of serotonin, increasing extracellular potassium concentration from 5.4 to 20 mmol/l depolarizes the cell membrane by +26.4 +/- 1.0 mV (n = 11). 1 mmol/l barium depolarizes the cell membrane by +15.7 +/- 1.3 mV (n = 17) and abolishes the effect of step increases of extracellular potassium concentration from 5.4 to 10 mmol/l. In the presence of barium, serotonin leads to a transient hyperpolarization by -26.3 +/- 1.0 mV (n = 16). During this transient hyperpolarization, the cell membrane is sensitive to extracellular potassium concentration despite the continued presence of barium. 10 mumol/l methysergide hyperpolarize the cell membrane by -7.2 +/- 2.0 mV (n = 6). In the presence of 10 mumol/l methysergide, the effect of serotonin is virtually abolished (+0.4 +/- 0.9 mV, n = 6). 1 mumol/l ketanserin, a 5-HT2 receptor blocking agent, ICS 205-930, a 5-HT3 receptor blocking agent, and phentolamine, an unspecific alpha-receptor blocking agent, do not significantly modify the effect of serotonin. In the nominal absence of extracellular calcium, the effect of serotonin is markedly reduced.(ABSTRACT TRUNCATED AT 250 WORDS)

LY165163 and 8-OH-DPAT have agonist effects on a serotonin responsive muscle of Aplysia

Serotonin enhances acetylcholine-elicited contractions of buccal muscle E1 of Aplysia. To characterize the serotonin receptor, actions of ligands of specific serotonin receptor subtypes were examined. Ketanserin, an antagonist at 5-HT2 receptors, had little effect. Two 5-HT1A ligands, LY165163 and 8-OH-DPAT, mimicked the effect of serotonin, although with slower kinetics. This suggests that the peripheral serotonin receptor in Aplysia more closely resembles 5-HT1 than 5-HT2 receptors.

Regulation of calcium influx into buccal muscles of Aplysia by acetylcholine and serotonin

Acetylcholine (ACh) causes contraction of Aplysia buccal muscles E1 and I5, and serotonin (5-hydroxytryptamine, 5-HT) enhances ACh-elicited contractions of these muscles. Possible roles of calcium influx in mediating these responses were examined by studying influx of 45Ca++. 5-HT increased calcium influx into both I5 and E1. Maximal influx occurred at 10(-6) M 5-HT and the increased influx could be sustained in the presence of 5-HT for at least 10 min. ACh also caused calcium influx, and calcium influx increased approximately in proportion to log[ACh] from 10(-5) M to 10(-3) M ACh. 5-HT and ACh probably bring about calcium influx by different mechanisms since the effect of ACh was additive to a maximal 5-HT response, and 10(-4) M hexamethonium bromide inhibited the increased influx caused by ACh but did not affect influx caused by 5-HT. Cyclic AMP analogues and forskolin neither caused an increase in calcium influx nor an increase in the influx caused by ACh. The data support a model in which ACh-elicited contractions of I5 and E1 are due primarily to calcium entry across the extracellular membrane, and 5-HT can "load" an intracellular site by a mechanism different from that activated by ACh. The data do not support a role for cyclic AMP in mediating the calcium influx response to 5-HT.

Extracellular calcium dependence of contracture and modulation by serotonin in buccal muscle E1 of Aplysia

Serotonin [5-hydroxytryptamine (5-HT)] enhances acetyl choline (ACh)-elicited contractures of Aplysia buccal muscles E1 and I5 . The possible role of external calcium in regulating the magnitude of ACh contracture in the presence and absence of 5-HT was investigated. Superfusion of E1 with zero calcium medium caused ACh contractures to fail within one to two minutes. Recovery of ACh contracture upon restoring normal medium occurred within two to four minutes. In the absence of 5-HT, ACh contracture decreased proportionally to external [Ca++] in the concentration range of 0-10 mM; however, the amount of enhancement of ACh contracture following 5-HT treatment did not decrease with external [Ca++] as long as [Ca++] was above a threshold concentration that varied from preparation to preparation. For most preparations, the enhancement of ACh contracture by 5-HT was dependent on the presence of external calcium during 5-HT treatment. Calcium influx into muscles E1 and I5 increased approximately two and a half fold in the presence of 10(-6) M 5-HT. A model in which 5-HT brings about calcium "loading" of an ACh releasable intracellular storage site is discussed.

Serotonin and forskolin enhance both magnitude of contraction and relaxation rate of Aplysia dorsal extrinsic muscle independently of acetylcholine receptor

Hexamethonium bromide (Hex. Br.) blocks acetylcholine (ACh) elicited contractions but not electrically elicited contractions of isolated preparations of Aplysia californica dorsal extrinsic muscle. Serotonin (5-hydroxytryptamine, 5-HT) enhances both magnitude and relaxation rate of ACh and electrically elicited contractions. In the presence of Hex. Br., 5-HT still exhibits its modulatory effects on electrically elicited contractions. Forskolin enhances both magnitude and relaxation rate of ACh and electrically elicited contractions. Forskolin (10(-5) M) increases the cyclic AMP content of the accessory radula closer and dorsal extrinsic muscles.

The pharmacology of molluscan neurons

It is commonly accepted that the basic physiological properties of the neurons as well as the nature of transmitter substances have remained relatively unchanged through evolution, while brain size and neuron number have greatly increased. Among invertebrates the molluscs, due to the large size of their neurons and lesser complexity of the neural networks controlling specific behavior, have proved to be especially useful for studying elementary properties of single neurons, network organization as well as various forms of learning and memory. The study of putative neurotransmitters has indicated that molluscs use the same low molecular-weight substances and peptides or their metabolites and cyclic nucleotides as transmitters and second messengers as the other species of various phyla. At the same time the receptors of neurotransmitters were found to have certain characteristic properties in the molluscs. The large molluscan neurons have permitted the isolation of individual identifiable nerve cells, and the subsequent analysis of quantities of the transmitters and their metabolic enzymes. These studies have demonstrated that single neurons frequently can contain more than one putative neurotransmitter. It can be expected that this model will contribute to an understanding of the role of multiple transmitters within a single neuron assuring the plasticity of the nervous system. The cellular mechanisms of plasticity have been demonstrated first in molluscan nervous systems. It was proved in identified Aplysia neurons that the same transmitter (ACh) can be released from an interneuron onto two or more follower neurons and can excite one and inhibit another or evoke a biphasic response on a third type of cell. The biphasic response of the molluscan neurons to neurotransmitters was the first demonstration of the plastic synaptic changes. The discovery of individual neurons with their groups of follower cells acting as chemical units has provided an insight into the organization of various behavioral acts. Study of the gastropod molluscs has also shown that the giant serotonergic cells can act as peripheral modulator neurons, as well as interneurons, and in this way they can affect their target organs at more than one level. The molluscan studies have provided more information on transmitter receptors as it was shown that molluscan neurons have at least six different 5HT receptors, three Ach receptors which can be separated pharmacologically. This type of study has led to the discovery of numerous new antagonists and poisons.(ABSTRACT TRUNCATED AT 400 WORDS)