Acidification and endosome-like compartments in the presynaptic terminals of frog retinal photoreceptors

Summary By using the acidotropic vital dye, Acridine Orange, we have found that the presynaptic terminals of rod and cone photoreceptors in retinas ofRana pipiens maintain a low pH relative to the surrounding medium through an energy dependent mechanism. When this pH is raised, by exposing the retinas to weak bases like ammonium chloride, the terminals exhibit large, membrane-delimited compartments, many of which accumulate endocytic tracers. This effect is partly reversed when the weak bases are removed. We infer that among the acidified structures within the terminals are endocytic compartments with at least some of the characteristics of the endosomes that participate in receptor-mediated endocytosis in other cell types. One role of these structures in the terminals may be in the recycling of synaptic vesicles.     

Kinetics of synaptic transfer from rods and cones to horizontal cells in the salamander retina

We examined synaptic transmission between rods or cones and horizontal cells, using perforated patch recording techniques in salamander retinal slices. Experimental conditions were established under which horizontal cells received nearly pure rod or pure cone input. The response-intensity relation for both photoreceptors and horizontal cells was described by a Michaelis-Menten function with an exponent close to 1. A dynamic model was developed for the transduction from photoreceptor voltage to postsynaptic current. The basic model assumes that: (i) photoreceptor light-evoked voltage controls Ca2+ entry according to a Boltzmann relation; (ii) the rate of glutamate release depends linearly on the voltage-gated Ca2+ current (ICa) in the synaptic terminal; (iii) glutamate concentration in the synaptic cleft reflects the balance of release and reuptake in which reuptake obeys first order kinetics; (iv) the binding of glutamate to its receptor and channel gating are fast compared with glutamate kinetics in the synaptic cleft. The good fit to the model confirms that these are the key features of synaptic transmission from rods and cones. The model accommodated changes in kinetics induced by the glutamate uptake blocker, dihydrokainate. The match between model and response was not improved by including an estimate of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptor desensitization or by making glutamate uptake voltage dependent.     

Top-down control is not lost in the attentional blink: evidence from intact endogenous cuing

The attentional blink (AB) refers to the finding that performance on the second of two targets (T1 and T2) is impaired when the targets are presented at a target onset asynchrony (TOA) of less than 500 ms. One account of the AB assumes that the processing load of T1 leads to a loss of top-down control over stimulus selection. The present study tested this account by examining whether an endogenous spatial cue that indicates the location of a following T2 can facilitate T2 report even when the cue and T2 occur within the time window of the AB. Results from three experiments showed that endogenous cuing had a significant effect on T2 report, both during and outside of the AB; this cuing effect was modulated by both the cue-target onset asynchrony and by cue validity, while it was invariant to the AB. These results suggest that top-down control over target selection is not lost during the AB.     

Ion channels in presynaptic nerve terminals and control of transmitter release.

The primary function of the presynaptic nerve terminal is to release transmitter quanta and thus activate the postsynaptic target cell. In almost every step leading to the release of transmitter quanta, there is a substantial involvement of ion channels. In this review, the multitude of ion channels in the presynaptic terminal are surveyed. There are at least 12 different major categories of ion channels representing several tens of different ion channel types; the number of different ion channel molecules at presynaptic nerve terminals is many hundreds. We describe the different ion channel molecules at the surface membrane and inside the nerve terminal in the context of their possible role in the process of transmitter release. Frequently, a number of different ion channel molecules, with the same basic function, are present at the same nerve terminal. This is especially evident in the cases of calcium channels and potassium channels. This abundance of ion channels allows for a physiological and pharmacological fine tuning of the process of transmitter release and thus of synaptic transmission.     

The origin of cholecystokinin terminals in the basal forebrain of the rat: evidence from immunofluorescence and retrograde tracing

The origins of cholecystokinin (CCK) fibers in the olfactory tubercle, nucleus accumbens and amygdala of the basal forebrain of the albino rat were studied with combined immunofluorescence and fluorescent retrograde tracing techniques. In each case, the majority of the CCK innervation arises topographically from subpopulations of neurons in the substantia nigra-ventral tegmental area of the midbrain. This ascending CCK input to the forebrain appears to exceed the amount of descending CCK input from the cortex. In this regard, the CCK innervation of limbic structures is quite different from that of the neostriatum. It has been reported that the CCK innervation of the neostriatum is derived primarily from piriform cortex as a descending corticostriatal projection. Limbic structures, on the other hand, are primarily innervated by ascending CCK, as well as local circuit, projections.     

Immunohistochemical evidence for synaptic release of glutamate from orexin terminals in the locus coeruleus

Orexin (Orx or hypocretin) is critically important for maintaining wakefulness, since in its absence, narcolepsy with cataplexy occurs. In this role, Orx-containing neurons can exert their influence upon multiple targets through the brain by release of Orx but possibly also by release of other neurotransmitters. Indeed, evidence was previously presented to suggest that Orx terminals could utilize glutamate (Glu) in addition to Orx as a neurotransmitter. Using fluorescence and confocal laser scanning microscopy, we investigated whether Orx varicosities contain the presynaptic markers for synaptic release of Glu or GABA and come into contact with postsynaptic markers for excitatory synapses within the locus coeruleus of the rat brain. We found that a proportion of the Orx+ varicosities were immunostained for the vesicular transporter for Glu, VGluT2. None were immunostained for vesicular glutamate transporter 1 (VGluT1) or VGluT3 or for the vesicular transporter for GABA, vesicular GABA transporter (VGAT). Among the Orx+ varicosities, 4% of all and 28% of large varicosities contained VGluT2. A similar proportion of the large Orx+ varicosities contained synaptophysin (Syp), a presynaptic marker for synaptic vesicles. Orx+ varicosities also contacted elements immunostained for postsynaptic density protein-95 (PSD)-95, a postsynaptic marker for glutamatergic synapses. We thus conclude that synaptic release of Glu occurs from Orx terminals within the locus coeruleus and can thus be important for the engagement of noradrenergic neurons in stimulating and maintaining arousal.     

Monosynaptic connections between identified A and B photoreceptors and interneurons in Hermissenda: evidence for labeled-lines

The cellular and synaptic organization of the eye of the nudibranch mollusk Hermissenda is well-documented. The five photoreceptors within each eye are mutally inhibitory and can be classified into two types: A and B based on electrophysiological and anatomical criteria. Two of the three type B and two type A photoreceptors can be further identified according to their medial or lateral positions within each eye. In addition to reciprocal synaptic connections between photoreceptors, photoreceptors also project to second-order neurons in the cerebropleural ganglion. The second-order neurons receive convergent synaptic input from two additional sensory pathways; however, it has not been previously established if lateral A, lateral B, or medial B photoreceptors converge onto the same second-order neurons. To determine the specific synaptic organization of these components of the visual system, we have examined monosynaptic connections between identified lateral and medial type A and B photoreceptors and second-order cerebropleural (CP) interneurons. We found that monosynaptic connections between identified lateral A and lateral and medial B photoreceptors and CP interneurons follow a labeled-line principle. Illumination of the eyes or extrinsic depolarizing current applied to identified photoreceptors evoked excitatory and inhibitory postsynaptic potentials (EPSPs and IPSPs, respectively) in different CP interneurons. The PSPs in CP interneurons followed one-for-one spikes in the photoreceptors and could be elicited in artificial seawater solutions containing high divalent cations. Identified photoreceptors projected to more than one CP interneuron and expressed both excitatory and inhibitory connections with the different CP interneurons. In examples where a monosynaptic connection between a lateral B photoreceptor and a CP interneuron was identified, lateral A, medial A, or medial B photoreceptors did not project to the same CP interneuron. Moreover, when connections between medial B and CP interneurons were identified, lateral A, medial A, and lateral B connections were not found to project to the same CP interneuron. Similar results were obtained for a lateral A and CP interneuron connection. These results indicate that divergent labeled-lines exist between specific photoreceptors and second-order CP interneurons and potential convergence of synaptic input from primary and secondary elements of the visual system must occur at sites that are postsynaptic to the CP interneurons.     

Caffeine and carbonyl cyanide m-chlorophenylhydrazone increased evoked and spontaneous release of luteinizing hormone-releasing hormone from intact presynaptic terminals.

In bullfrog sympathetic ganglia, the ryanodine-sensitive Ca2+ store and mitochondria modulate [Ca2+] within nerve terminals. We used caffeine (10 mM) and carbonyl cyanide m-chlorophenylhydrazone (10 microM) to assess how these Ca2+ stores affect release of a neuropeptide, luteinizing hormone-releasing hormone, from these nerve terminals. Release of luteinizing hormone-releasing hormone was evoked by electrical stimulation to presynaptic nerves and was monitored as a late slow excitatory postsynaptic potential in ganglionic neurons. Caffeine increased release of luteinizing hormone-releasing hormone similarly whether the release was evoked by 4 or 20 Hz stimulations (by 2.7 +/- 1.1- and 3.2 +/- 0.9-fold, mean +/- S.E.M., n = 27, respectively). Carbonyl cyanide m-chlorophenylhydrazone augmented release of luteinizing hormone-releasing hormone evoked by 4 Hz stimulation much more strongly (by 11.8 +/- 1.8-fold) than it increased the release evoked by 20 Hz stimulation (by 3.6 +/- 1.3-fold, n = 25). We detected spontaneous release of luteinizing hormone-releasing hormone as a slow hyperpolarization in response to a brief application of an antagonist to the receptors for luteinizing hormone-releasing hormone in 65% (34 of 52) and 39% (11 of 28) of the ganglionic B and C neurons, respectively. Caffeine increased spontaneous release of luteinizing hormone-releasing hormone by 2.3 +/- 0.7-fold (n = 6) whereas carbonyl cyanide m-chlorophenylhydrazone increased this release by 4.27- and 1.76-fold (n = 2). Facilitation of Ca2+ release from the intracellular store by caffeine and inhibition of mitochondrial Ca2+ removal by carbonyl cyanide m-chlorophenylhydrazone increased spontaneous as well as evoked release of luteinizing hormone-releasing hormone. Moreover, caffeine increments of evoked release did not depend on the firing frequency of the nerve whereas carbonyl cyanide m-chlorophenylhydrazone augmentations of evoked release strongly depended on the firing frequency.     

Depolarization-induced calcium channel facilitation in rod photoreceptors is independent of G proteins and phosphorylation

Depolarization-induced facilitation of L-type Ca channels in rod photoreceptors was investigated with nystatin-perforated and ruptured whole cell patch-clamp techniques in cells isolated from tiger salamander retina. Induction of facilitation was voltage dependent with a half-maximal effect seen at prepulse potentials near +31 mV. Reversal of facilitation was time dependent with fast (tau approximately 20 ms) and slow (tau approximately 1 s) components at -60 mV. Incubation of cells with pertussis toxin or intracellular administration of guanosine 5'-O-(3-thiotriphosphate) or guanosine 5'-O-(2-thiodiphosphate) had no effect on the degree to which facilitation could be evoked, implying the absence of a significant role for G proteins. Application of the phosphatase inhibitor okadaic acid or inclusion of ATP, to boost levels of phosphorylation, or inclusion of 5'adenylylimidophosphate or inhibitors of protein kinase in the pipette, to reduce levels of phosphorylation, had no effect on the development of facilitation, suggesting that phosphorylation has little or no role in this phenomenon. These results show that the L-type Ca channels in rod photoreceptors, which appear to be composed of alpha(1F)-like subunits, undergo voltage-dependent facilitation in a manner that differs from some other L-type Ca channels which undergo facilitation via phosphorylation or through G-protein-mediated inhibition.