Effects of alpha 2-adrenergic agonists and antagonists on photoreceptor membrane currents

GABA-, histamine-, and FMRFamide-immunoreactivity in the visual, vestibular and central nervous systems of Hermissenda crassicornis

Hermissenda crassicornis is a model for studying the molecular and cellular basis for classical conditioning, based on its ability to associate light with vestibular stimulation. We used confocal microscopy to map histamine (HA), FMRF-amide, and γ-aminobutyric acid (GABA) immunoreactivity in the central nervous system (CNS), eyes, optic ganglia and statocysts of the nudibranchs. For HA immunoreactivity, we documented both consistently and variably labeled CNS structures across individuals. We also noted minor differences in GABA immunoreactivity in the CNS compared to previous work on Hermissenda. Contrary to expectations, we found no evidence for GABA inside the visual or vestibular systems. Instead, we found only FMRFamide- and HA immunoreactivity (FMRFamide: 4 optic ganglion cells, 4-5 hair cells; HA: 3 optic ganglion cells, 8 hair cells). Overall, our results can act as basis for comparisons of nervous systems across nudibranchs, and suggest further exploration of intraspecific plasticity versus evolutionary changes in gastropod nervous systems.

Norepinephrine modulates excitability of neonatal rat optic nerves through calcium-mediated mechanisms

We report that norepinephrine markedly increases excitability of neonatal rat optic nerves. To investigate the mechanisms of the norepinephrine-induced excitability increase, we studied isolated optic nerves from 42 neonatal (< three days old) and five adult (> three months old) Long-Evan's hooded rats. Norepinephrine (10(-6), 10(-5) and 10(-4) M) rapidly and reversibly increased the amplitude (mean +/- S.D.: 3.5 +/- 1.7%, 12.1 +/- 2.8% and 35.6 +/- 8.4%) of compound action potentials elicited by submaximal stimulation of neonatal optic nerves. The beta-1 adrenoceptor antagonist atenolol (10(-5) M) blocked the norepinephrine-induced increase in excitability but the alpha antagonist phentolamine (10(-5) M) did not. The beta agonist isoproterenol (10(-5) and 10(-4) M) increased response amplitudes (8.7 +/- 4.1% and 25.8 +/- 4.6%) but the alpha-1 agonist methoxamine and alpha-2 agonist clonidine did not. The beta antagonist propranolol blocked the isoproterenol effect. Replacing Ca2+ with Mg2+ or adding 0.8 mM of Cd2+ reversibly blocked the norepinephrine effects. Extracellular K+ concentrations did not change in optic nerves during norepinephrine application. Blockade of K+ channels with apamin (10(-6) M) or tetraethylammonium (10(-3) M) did not prevent the excitatory effects of norepinephrine. Adult rat optic nerves were insensitive to both norepinephrine (10(-4) M) and isoproterenol (10(-4) M). Our results indicate that norepinephrine increases neonatal optic axonal excitability through Ca(2+)-dependent mechanisms. The data suggest that the adrenoceptors are situated on the axons, that the excitability changes are not due to changes in extracellular K+ concentration or K+ channels sensitive to apamin or tetraethylammonium. The sensitivity of rat optic nerves to norepinephrine declined with age. Axonal adrenoceptors may play a role in optic axonal development and injury.

Serotonin increases the frequency of light-elicited discrete waves in type-B photoreceptors of Hermissenda

Serotonin (5-HT) produces both short- and long-term enhancement of generator potentials in identified type B-photoreceptors of Hermissenda. The actions of 5-HT have been shown to modulate both light- and voltage-dependent conductances. To further examine light-dependent processes we studied spontaneous and light-elicited discrete waves in the presence of 5-HT. Serotonin significantly increased the frequency of light-elicited discrete waves without initially increasing spontaneous discrete waves recorded in the dark. The increase in discrete wave frequency produced by 5-HT could not be described by a simple Poisson process, since the observed interval distribution after 5-HT application deviated significantly from the predicted exponential interval distribution.

Neurochemical and immunocytochemical studies of serotonin in the Hermissenda central nervous system

In the preceding paper we showed that serotonin mimics the effects of associative conditioning on Type B photoreceptors of Hermissenda. Here we show that serotonin is present in the Hermissenda central nervous system, and that it is released in a calcium-dependent manner. A large number of serotonergic cell bodies are present in the cerebropleural ganglia (CPG). Cell bodies in the CPG are located in three small, symmetrical clusters. One cluster (2-3 cells) located at the anterior end of the CPG contains a large cell that projects to the buccal ganglion. A second cluster (2-3 cells) is located posterior to the first and near the midline of the nervous system. A third cluster (1-2 cells) was located slightly more posteriorly than the second. Type B photoreceptors and the S/E optic ganglion cell project to a region of the CPG neuropil which is heavily innervated by serotonin immunoreactive fine processes. Since serotonin mimicked the effects of associative conditioning on Type B cells, and is present in an appropriate region of the CPG neuropil, serotonin may mediate some effects of associative conditioning on Type B cells. Results supporting this hypothesis are presented in an accompanying paper.

Serotonin involvement during in vitro conditioning of Hermissenda

To determine if serotonin may be involved in associative conditioning-produced changes in the excitability and photoresponses of Type B photoreceptors, isolated nervous systems were exposed to an in vitro conditioning procedure in the presence or absence of drugs that alter normal serotonergic neurotransmission. Pairings of light and intracellular depolarization of a caudal hair cell (in vitro conditioning) produced a pairing-specific depolarization of Type B photoreceptors that was accompanied by an increase in resting input resistance. Treatment of nervous systems with pharmacological agents which disrupt 5-HT neurotransmission attenuated membrane potential and input resistance changes of Type B photoreceptors. These drugs included serotonin uptake inhibitors (imipramine, fluoxetine), a receptor antagonist (bufotenine), and a neurotoxin (5,7-dihydroxytryptamine; 5, 7-DHT). Yohimbine, an alpha 2-receptor antagonist, was without effect. These results, and those in the accompanying papers, suggest that serotonin modulates Type B photoreceptor excitability during associative conditioning.

Serotonin modulation of Hermissenda type B photoreceptor light responses and ionic currents: implications for mechanisms underlying associative learning

Type B photoreceptors in the eyes of the nudibranch mollusc Hermissenda have previously been shown to exhibit enhanced steady-state depolarizing light responses, increases in steady-state light-induced impulse frequency and a decrease in resting membrane conductance on days following exposure to repeated pairings of light and rotation. These training-produced changes in the electrophysiological properties of B cells have been attributed to reductions in a voltage-dependent K+ current (IA), a calcium-activated K+ current (IK-Ca), and enhancement of a voltage-dependent Ca2+ current (ICa). Current- and voltage-clamp analysis of the effects of serotonin (5-HT) upon B cells revealed that 5-HT mimicked many of the effects of associative training. 5-HT enhanced the steady-state light response and decreased resting membrane conductance of B cells. Voltage-clamp analysis revealed five distinct effects of 5-HT upon the voltage-, calcium-, and light-dependent ionic conductances of B photoreceptors. In the dark-adapted cell, 5-HT enhanced but then suppressed IA, IK-Ca, and enhanced ICa. The presentation of 5-HT in combination with light accelerated the reduction of K+ currents. 5-HT also transiently reduced the fast component of light-induced inward current (INa-light), and enhanced a slower component of light-induced inward current. In an accompanying paper, 5-HT is shown to be localized within the cerebropleural neuropil where terminals presynaptic to B photoreceptors are located. Disruption of serotonergic neurotransmission by a variety of drugs is also shown to reduce the pairing-specific differences in Type B photoreceptor cumulative depolarization and resting membrane conductance decreases that are produced by in vitro conditioning. Collectively, the results indicate that 5-HT plays an important role in mediating the conductance changes produced in Type B photoreceptors by associative training.

A spatial-temporal model of cell activation

A spatial-temporal model of calcium messenger function is proposed to account for sustained cellular responses to sustained stimuli, as well as for the persistent enhancement of cell responsiveness after removal of a stimulus, that is, cellular memory. According to this model, spatial separation of calcium function contributes to temporal separation of distinct phases of the cellular response. At different cellular sites, within successive temporal domains, the calcium messenger is generated by different mechanisms and has distinct molecular targets. In particular, prolonged cell activation is brought about by the interaction of calcium with another spatially confined messenger, diacylglycerol, to cause the association of protein kinase C with the plasma membrane. Activity of the membrane-associated protein kinase C is controlled by the rate of calcium cycling across the plasma membrane. In some instances, a single stimulus induces both protein kinase C activation and calcium cycling and thus causes prolonged activation; but in others, a close temporal association of distinct stimuli brings about cell activation via interaction of these intracellular messengers. Persistent enhancement of cell responsiveness after removal of stimuli is suggested to be due to the continued association, or anchoring, of protein kinase C to the membrane.