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Receptors located in the facial pit organ of certain species of snake signal the presence of prey. Infrared radiation is an effective stimulus suggesting that these receptors may be low-threshold temperature receptors. We recorded from the nerve innervating the pit organ of snakes belonging to the family of Crotalinae while stimulating the receptive area with well-defined optical stimuli. The objective was to determine the sensitivity of these receptors to a wide range (0.400–10.6 μm) of optical stimuli to determine if a temperature-sensitive or photosensitive protein initiated signal transduction. We found that receptors in the pit organ exhibited a unique broad response to a wide range of electromagnetic radiation ranging from the near UV to the infrared. The spectral tuning of these receptors parallels closely the absorption spectra of water and oxyhemoglobin, the predominant chromophore in tissue. Our results support the hypothesis that these are receptors activated by minute temperature changes induced by direct absorption of optical radiation in the thin pit organ membrane.

Inositol 1,4,5-trisphosphate (InsP3) mobilizes intracellular stored Ca2+ by binding to specific receptors that are similar to the ryanodine receptor of skeletal and cardiac muscle. We have immunolocalized the InsP3 receptor to the inner nuclear layer and limiting membranes of the catfish retina. Immunocytochemistry on dissociated retinal cells further localized the receptor in the horizontal, bipolar and Müller glial cells. Immunostaining of the rat retina localized the InsP3 receptor to the plexiform layers. These data show a different distribution of InsP3 receptor in the catfish retina compared to that of other vertebrates, that may be suggestive of a different functional role for this receptor in different species.

Rattlesnakes, copperheads, and other pit vipers have highly sensitive heat detectors known as pit organs, which are used to sense and strike at prey. However, it is not currently known how temperature change triggers cellular and molecular events that activate neurons supplying the pit organ. We dissociated and cultured neurons from the trigeminal ganglia (TG) innervating the pit organs of the Western Diamondback rattlesnake ( Crotalus atrox) and the copperhead ( Agkistrodon contortix) to investigate electrophysiological responses to thermal stimuli. Whole cell voltage-clamp recordings indicated that 75% of the TG neurons from C. atrox and 74% of the TG neurons from A. contortix showed a unique temperature-activated inward current ( IΔT). We also found an IΔT-like current in 15% of TG neurons from the common garter snake, a species that does not have a specialized heat-sensing organ. A steep rise in the current-temperature relationship of IΔTstarted just below 18°C, and cooling temperature-responsive TG neurons from 20°C resulted in an outward current, suggesting that IΔTis on at relatively low temperatures. Ion substitution and Ca2+imaging experiments indicated that IΔTis primarily a monovalent cation current. IΔTwas not sensitive to capsaicin or amiloride, suggesting that the current did not show similar pharmacology to other mammalian heat-sensitive membrane proteins. Our findings indicate that a novel temperature-sensitive conductance with unique ion permeability and low-temperature threshold is expressed in TG neurons and may be involved in highly sensitive heat detection in snakes.

Transient amacrine cells were identified by their intracellularly recorded response to flashes of light. These cells typically respond with a transient depolarization, often followed by a steady-state response during the stimulus. When two electrodes were placed in different transient amacrine cells, current of either polarity passed through one electrode produced a steady-state voltage change that was recorded by the electrode in the nearby cell. Following identification of the physiological type, transient amacrine cells were injected with horseradish peroxidase and the tissue was processed for light and electron microscopy. Both conventional chemical synaptic junctions and gap junctions were found to connect amacrine cells.

Microelectrodes filled with horseradish peroxidase (HRP) were used to label single identified giant axons in the isolated lamprey spinal cord. Subsequent to the iontophoretic injection of HRP, the spinal cord was stimulated at repetition rates of 20-30/s and the activity in labelled axons monitored. Immediately following failure of the action potential, the spinal cord was fixed by immersion and processed for light and electron microscopy. Electron micrographs were taken of synaptic contacts made by the labelled axons. Several quantitative measures were made from each synapse using a digitizing tablet interfaced with a digital computer. These measures included vesicle number (VN), vesicle area (VA), length of differentiated membrane (DM), vesicle density (VD = VN/VA), vesicle frequency (VF = VN/DM), and a relative measure of the amount of vesicle membrane added to the axolemma during the stimulation period, the curvature ratio (CR). Measures from 106 stimulated synapses were compared with 134 synapses from injected but unstimulated giant axons. The results from these experiments suggest that measurable ultrastructural changes occur during transmitter release at identified C.N.S. synapses, which are consistent with the hypothesis of synaptic vesicle recycling.

Synaptic junctions located on the dendrites of lamprey (Petromyzon marinus) reticulospinal neurons labelled with intracellularly-injected horseradish peroxidase were studied. The normal ultrastructure of the synaptic junctions was defined and several quantitative measures made from each junction in order to test the hypothesis that distally-located synapses are ultrastructurally different from those located at proximal dendritic sites. A total of 820 contacts from one neuron and 279 from a second neuron ranging from 20 to 340 microns from the soma were quantified. The vast majority of the presynaptic endings contained round, clear-cored vesicles and formed an asymmetrical membrane differentiation with the postsynaptic dendrite. A small fraction of the population contained flattened or pleomorphic vesicles and these synapses were equally distributed with respect to distance from the soma. Many of the terminals contained a few large dark- and clear-cored vesicles. Four quantitative measures of each synaptic contact were made. These included vesicle number, length of differentiated membrane, vesicle area and terminal area. Four ratios relating the different quantitative measures were also calculated. Each ratio or measurement from the synaptic junctions was plotted as a function of distance from the soma to determine if differences existed at any distance. It was found that synaptic junctions are uniformly similar and that distal junctions did not differ significantly (P greater than 0.05) from those at proximal dendritic sites. It is concluded that if distal synapses do compensate for their remote location they do this is some other way, possibly by increasing the number of synaptic contacts made by each presynaptic axon.

Horizontal cells enzymatically isolated from catfish retina were exposed to the putative neurotransmitters aspartate (Asp) or N-methyl- D -aspartate (NMDA). Under voltage clamp conditions, inward currents were recorded when the holding potential was more negative than zero and outward currents were recorded when the membrane potential was more positive than zero. The current voltage curve was highly non-linear in the range of membrane potential between −30 and −100 mV. This non-linearity was largely removed in zero magnesium solution. 2-Amino-phosphonovaleric acid selectively blocked Asp and NMDA responses. These response characteristics are consistent with the presence of NMDA receptors in these cells.

Synaptic structures between receptors and horizontal and bipolar cells in the outer plexiform layer (OPL) of Golgi-impregnated catfish retina were examined by conventional electron microscopy of serial ultrathin sections and by high-voltage electron microscopy (HVEM) of thick sections. Cone terminals contained multiple synaptic ribbons and rod terminals contained single synaptic ribbons. This observation was used to identify these two types of photoreceptors. The cone horizontal cell, located in the most distal part of the inner nuclear layer (INL), invaginated only cone terminals, whereas the rod horizontal cell, located in the proximal part of the INL, invaginated only rod terminals. Both lateral elements of the triad in the rod terminal originated from a single rod horizontal cell whereas the same structures in the cone terminal were often derived from several cone horizontal cells. Golgi-impregnated catfish bipolar cells were classified into two types based on the differences in their axonal arborization as described by Famiglietti et al. ('77). Axonal endings of type a bipolar cells were located in the distal part, sublamina a, of the inner plexiform layer (IPL), and axonal endings of type b cells were located in the proximal part, sublamina b, of the IPL. Dendrites from type a bipolar cells made direct contact with the synaptic ribbons in both rod and cone terminals whereas those from type b cells made indirect contact with the ribbons in both rod and cone terminals, but rare direct contact with the ribbon in rod terminals were also seen. In addition, bipolar cells made basal junctions or superficial contacts in both rod and cone terminals. The "lateral" processes of bipolar cells invaginating rods penetrated between the rod terminal and rod horizontal cell processes, and made basal junctions with both rod terminals and rod horizontal cells. There was no definitive morphological feature that could be associated with sign-conserving and sign-inverting signal transmission.