Monoaminergic fluorescence in frog skin

In vitro relation between preganglionic sympathetic stimulation and activity of cutaneous glands in the bullfrog

1. Activation of cutaneous glands was studied by measuring changes in transepithelial potentiation (TEP) after pre- and postganglionic sympathetic stimulation in the bullfrog, Rana catesbeiana. 2. In normal Ringer solution, TEP was 20-90 mV with the basolateral (inside) surface positive. Single shocks to the preganglionic B pathway decreased TEP by up to 3 mV. Cutaneous depolarizations had a latency of 1.2 s, a rise time of 2.5 s, and decayed with an exponential time constant of 15 s. Similar depolarizations were evoked by postganglionic stimulation. 3. Cutaneous depolarizations summed during repetitive stimulation and > 0.05 Hz. For trains of three stimuli, peak amplitude increased with frequency and saturated at 2 Hz. In some preparations, longer trains evoked polyphasic changes in TEP. Preganglionically evoked cutaneous responses were abolished by (+)-tubocurarine. Postganglionically evoked cutaneous depolarizations were antagonized by phentolamine, but not propranolol. 4. Repetitive preganglionic stimulation of the C pathway (> 100 at 20 Hz) evoked little change in TEP and did not modulate depolarizations evoked through the B pathway. In nicotine, peptidergic cotransmission was enhanced in the ganglia, and repetitive C pathway stimulation evoked cutaneous depolarizations whose time course mirrored that of the postganglionic peptidergic after-discharge. The after-discharge and associated cutaneous depolarization were blocked by a luteinizing hormone-releasing hormone antagonist. 5. The results show cutaneous glands are selectively innervated by B neurones and respond to low levels of neural activity. Asynchronous postganglionic firing mediated by peptidergic cotransmission can provide a basis for heterosynaptic interactions between the B and C pathways.

Biosynthesis of frog skin mucins: cysteine-rich shuffled modules, polydispersities and genetic polymorphism

1. Frog integumentary mucins (FIM-A.1, FIM-B.1 and FIM-C.1) consist of typical threonine-rich highly O-glycosylated (semi)repetitive domains, and cysteine-rich modules, i.e. the P-domain, the short consensus repeat and a region with high similarity to the C-terminal end of von Willebrand factor (designated here CC29-motif). 2. These modules are thought to be involved in protein-protein interactions and they have been observed in a variety of extracellular proteins. In FIMs, these modules may be involved in oligomerization processes leading to an entangled mucin network. 3. Polydispersities have been detected in FIM-B.1 and FIM-C.1 within single individuals. Multiple transcripts are probably generated by alternative splicing of a huge array of different (semi)repetitive cassettes encoding the threonine-rich domains. 4. Furthermore, genetic polymorphism is observed between different individuals, probably due to allelic variations in the number of (semi)repetitive cassettes.

Role of pars nervosa of the hypophysis in amphibian water economy: a re-assessment

1. Responses in renal function and in water permeability of skin and bladder to wet and dry environments are accomplished within the range of normal hydration of the amphibian organism. 2. Urine production is discontinued at moderate dehydration. 3. Strong dehydration is needed to raise plasma arginine vasotocin (AVT). 4. Surgical interference with hypophysial function may repress water balance responses because of pars distalis dysfunction, with no clear effect of elimination of pars nervosa function. 5. Antidiuretic hormones, along with adrenergic agonists, may be potent stimulators of the water permeability of membranes of variable permeability, such as skin of terrestrial anurans. 6. AVT does not play a key role in amphibian water economy, but may exert a modulatory role in the control of renal function, secondary to nervous control.

6-Hydroxydopamine produces a weight increase in toads

A large increase in body weight occurred after injection of 6-hydroxydopamine (6-OHDA, 100 mg kg-1) into the iliac lymph sac of toads. The body weight increase was apparently due to retention of fluid and only occurred when toads could not avoid contact of their ventral skin with water. Lower doses of 6-OHDA (25 or 50 mg kg-1) did not cause a weight increase although they were sufficient to eliminate fluorescence from peripheral adrenergic nerves. A peripheral injection of 6-OHDA did not affect the appearance of central aminergic nerves. Removal of the pituitary and median eminence did not affect the weight increase. Although the primary action of 6-OHDA in upsetting water balance in toads has not been revealed by this study, it is clearly not related to the chemical sympathectomy of adrenergic nerves.

Sympathetic modulation of mechanoreceptor sensitivity in frog skin

1. Sympathetic effects on the mechanical sensitivity of frog cutaneous mechanoreceptors were examined in vivo. 2. Functionally identified units were tested with repetitive mechanical stimuli of threshold intensity during electrical stimulation of the sympathetic trunk. 3. Sympathetic activity resulted in increased sensitivity for three classes of afferents; slowly adapting compression receptors, slowly adapting stroke receptors, and rapidly adapting stroke receptors. Decreased sensitivity was produced in the fourth class, rapidly adapting compression receptors. 4. Preliminary tets of several possible modes of sympathetic influence indicated that blood flow changes, changes in probe-skin coupling and changes in tissue compliance could not account for the observed changes in receptor sensitivity. Na+ and Cl- ions, secreted by cutaneous mucous glands were found to be possible contributors to the decreased sensitivity of rapidly adapting compression receptors. Direct neurotransmitter action on the receptors, a likely mechanism of sympathetic action, was not tested. 5. The data indicate that systematic changes in cutaneous sensibility occur with modest changes in sympathetic efferent activity. Possible mechanisms of these sympathetic effects are discussed.

In vitro studies of frog mucous glands

The ionic outflow, mainly consisting of Na+ and Cl-, from the mucous glands in an excised nerve-skin preparation of frog has been determined by recording the conductance changes occurring in a fluid layer covering a small area of the skin surface. In the main series of experiments the glands were activated by stimulation of sympathetic nerve fibers in the skin nerve. The relationship between the ionic outflow and the number of nerve volleys was studied over a wide range. The outflow per impulse was found to be fairly constant during the first tens of impulses but diminished gradually with increasing number of stimuli up to a certain maximum value--varying in different preparations--after which the outflow ceased completely. During the initial phase of stimulation the outflow is most likely caused by an ejection of performed secretion due to the contractions of the glandular myoepithelium. The continued outflow in the later stages of the stimulation periods must be due to production of new secretion. Since the glandular epithelium is devoid of nerve terminals a nervous control of the ionic secretion can only be explained by an indirect influence mediated either by transmitter diffusion from the myoepithelial nerve endings or by a close electric coupling between the contractile and the secretory gland cells. Adrenaline and noradrenaline induce ionic outflows which like those evoked by nerve stimulation are inhibited by the beta-adrenoreceptor blocker propranolol, alpha-adrenoreceptor blockers being without effect. A serendipitous finding of tonus changes in the frog skin during nerve stimulation is also described.