Fine structure and organization of the infrared receptor relay, the lateral descending nucleus of the trigeminal nerve in pit vipers

Hunting with heat: thermosensory-driven foraging in mosquitoes, snakes and beetles

Animals commonly use thermosensation, the detection of temperature and its variation, for defensive purposes: to maintain appropriate body temperature and to avoid tissue damage. However, some animals also use thermosensation to go on the offensive: to hunt for food. The emergence of heat-dependent foraging behavior has been accompanied by the evolution of diverse thermosensory organs of often exquisite thermosensitivity. These organs detect the heat energy emitted from food sources that range from nearby humans to trees burning in a forest kilometers away. Here, we examine the biophysical considerations, anatomical specializations and molecular mechanisms that underlie heat-driven foraging. We focus on three groups of animals that each meet the challenge of detecting heat from potential food sources in different ways: (1) disease-spreading vector mosquitoes, which seek blood meals from warm-bodied hosts at close range, using warming-inhibited thermosensory neurons responsive to conductive and convective heat flow; (2) snakes (vipers, pythons and boas), which seek warm-blooded prey from ten or more centimeters away, using warmth-activated thermosensory neurons housed in an organ specialized to harvest infrared radiation; and (3) fire beetles, which maximize their offspring's feeding opportunities by seeking forest fires from kilometers away, using mechanosensory neurons housed in an organ specialized to convert infrared radiation into mechanosensory stimuli. These examples highlight the diverse ways in which animals exploit the heat emanating from potential food sources, whether this heat reflects ongoing metabolic activity or a recent lightning strike, to secure a nutritious meal for themselves or for their offspring.

Neuronal Substrates for Infrared Contrast Enhancement and Motion Detection in Rattlesnakes

Pit vipers detect infrared (IR) radiation with loreal pit organs [1] that are connected to the hindbrain by trigeminal nerve fibers [2-4]. The pattern of central afferent termination forms a topographical representation of the sensory periphery within the nucleus of the lateral descending trigeminal tract (LTTD) [4-7]. All LTTD neurons project to another specialized, ipsilateral hindbrain area, the nucleus reticularis caloris (RC) [8-11], before IR signals are integrated with visual signals in the optic tectum [12, 13]. Pit-organ-innervating afferent fibers provoke in individual LTTD neurons a direct, robust spike activity upon peripheral activation [7, 14]. This discharge is truncated by an indirect, delayed synaptic inhibition from afferent fibers of adjacent sensory areas through parallel microcircuitry that converges with afferent fibers onto the same target neurons [7]. Here, we determined the impact of this interaction on IR contrast enhancement and/or motion detection in LTTD and RC neurons using isolated whole-brain preparations of rattlesnakes with intact pit organs. Simulated and real IR source motion provoked weak directional tuning of the discharge in LTTD neurons and RC neurons expressed a strong, motion-direction-differentiating activity. The hierarchically increasing motion sensitivity potentially derives from a direction-specific inhibition or spike frequency adaptation of LTTD neuronal discharge that becomes further pronounced by convergent projections onto individual RC neurons. The emerging signaling pattern complies with contrast enhancement (LTTD) and extraction of movement-related signals (RC), thereby forming a motion detection mechanism that encodes moving IR sources relative to the ambient temperature [14].

Synaptic convergence of afferent inputs in primary infrared-sensitive nucleus (LTTD) neurons of rattlesnakes (Crotalinae) as the origin for sensory contrast enhancement

Pitvipers have a specialized sensory system in the upper jaw to detect infrared (IR) radiation. The bilateral pit organs resemble simple pinhole cameras that map IR objects onto the sensory epithelium as blurred representations of the environment. Trigeminal afferents transmit information about changing temperature patterns as neuronal spike discharge in a topographic manner to the hindbrain nucleus of the lateral descending trigeminal tract (LTTD). A presumed, yet so far unknown neuronal connectivity within this central nucleus exerts a synaptic computation that constrains the relatively large receptive field of primary afferent fibers. Here, we used intracellular recordings of LTTD neurons in isolated rattlesnake brains to decipher the spatio-temporal pattern of excitatory and inhibitory responses following electrical stimulation of single and multiple peripheral pit organ-innervating nerve branches. The responses of individual neurons consisted of complex spike sequences that derived from spatially and temporally specific interactions between excitatory and inhibitory synaptic inputs from the same as well as from adjacent peripheral nerve terminal areas. This pattern complies with a central excitation that is flanked by a delayed lateral inhibition, thereby enhancing the contrast of IR sensory input, functionally reminiscent of the computations for contrast enhancement in the peripheral visual system.

Organotopic organization of the primary Infrared Sensitive Nucleus (LTTD) in the western diamondback rattlesnake (Crotalus atrox)

Pit vipers (Crotalinae) have a specific sensory system that detects infrared radiation with bilateral pit organs in the upper jaw. Each pit organ consists of a thin membrane, innervated by three trigeminal nerve branches that project to a specific nucleus in the dorsal hindbrain. The known topographic organization of infrared signals in the optic tectum prompted us to test the implementation of spatiotopically aligned sensory maps through hierarchical neuronal levels from the peripheral epithelium to the first central site in the hindbrain, the nucleus of the lateral descending trigeminal tract (LTTD). The spatial organization of the anatomical connections was revealed in a novel in vitro whole-brain preparation of the western diamondback rattlesnake (Crotalus atrox) that allowed specific application of multiple neuronal tracers to identified pit-organ-supplying trigeminal nerve branches. After adequate survival times, the entire peripheral and central projections of fibers within the pit membrane and the LTTD became visible. This approach revealed a morphological partition of the pit membrane into three well-defined sensory areas with largely separated innervations by the three main branches. The peripheral segregation of infrared afferents in the sensory epithelium was matched by a differential termination of the afferents within different areas of the LTTD, with little overlap. This result demonstrates a topographic organizational principle of the snake infrared system that is implemented by maintaining spatially aligned representations of environmental infrared cues on the sensory epithelium through specific neuronal projections at the level of the first central processing stage, comparable to the visual system.

Reptiles: a new model for brain evo-devo research

Vertebrate brains exhibit vast amounts of anatomical diversity. In particular, the elaborate and complex nervous system of amniotes is correlated with the size of their behavioral repertoire. However, the evolutionary mechanisms underlying species-specific brain morphogenesis remain elusive. In this review we introduce reptiles as a new model organism for understanding brain evolution. These animal groups inherited ancestral traits of brain architectures. We will describe several unique aspects of the reptilian nervous system with a special focus on the telencephalon, and discuss the genetic mechanisms underlying reptile-specific brain morphology. The establishment of experimental evo-devo approaches to studying reptiles will help to shed light on the origin of the amniote brains.

Cyto- and chemoarchitecture of the sensory trigeminal nuclei of the echidna, platypus and rat

We have examined the cyto- and chemoarchitecture of the trigeminal nuclei of two monotremes using Nissl staining, enzyme reactivity for cytochrome oxidase, immunoreactivity for calcium binding proteins and non-phosphorylated neurofilament (SMI-32 antibody) and lectin histochemistry (Griffonia simplicifolia isolectin B4). The principal trigeminal nucleus and the oralis and interpolaris spinal trigeminal nuclei were substantially larger in the platypus than in either the echidna or rat, but the caudalis subnucleus was similar in size in both monotremes and the rat. The numerical density of Nissl stained neurons was higher in the principal, oralis and interpolaris nuclei of the platypus relative to the echidna, but similar to that in the rat. Neuropil immunoreactivity for parvalbumin was particularly intense in the principal trigeminal, oralis and interpolaris subnuclei of the platypus, but the numerical density of parvalbumin immunoreactive neurons was not particularly high in these nuclei of the platypus. Neuropil immunoreactivity for calbindin and calretinin was relatively weak in both monotremes, although calretinin immunoreactive somata made up a large proportion of neurons in the principal, oralis and interpolaris subnuclei of the echidna. Distribution of calretinin immunoreactivity and Griffonia simplicifolia B4 isolectin reactivity suggested that the caudalis subnucleus of the echidna does not have a clearly defined gelatinosus region. Our findings indicate that the trigeminal nuclei of the echidna do not appear to be highly specialized, but that the principal, oralis and interpolaris subnuclei of the platypus trigeminal complex are highly differentiated, presumably for processing of tactile and electrosensory information from the bill.

Central projections and somatotopic organisation of trigeminal primary afferents in pigeon (Columba livia)

Injections of cholera toxin B-chain conjugated to horseradish peroxidase into individual peripheral branches of the trigeminal nerve or into the trigeminal ganglion showed that an ascending trigeminal tract (TTA) terminated in distinct ventral and dorsal divisions of the principal sensory nucleus (PrVv and PrVd, respectively), and a descending tract (TTD) terminated within pars oralis, pars interpolaris, and pars caudalis divisions of the nucleus of TTD (nTTD) and within the dorsal horn of the first six cervical spinal segments. In PrVd, mandibular, ophthalmic, and maxillary projections were predominantly located dorsally, ventrally, and medially, respectively. In nTTD, mandibular projections lay dorsomedially, ophthalmic projections lay ventrolaterally, and maxillary projections lay in between. At caudal medullary and spinal levels, mandibular projections were situated medially, ophthalmic projections were situated laterally, and maxillary projections were situated centrally. The terminations within the dorsal horn were most dense in laminae III and IV and were least dense in lamina II, with laminae III-IV also receiving topographically organised contralateral projections. Extratrigeminal projections were mainly to the external cuneate nucleus by way of a lateral descending trigeminal tract (lTTD; Dubbeldam and Karten [1978] J. Comp. Neurol. 180:661-678) and to the region of the tract of Lissauer and lamina I of the dorsal horn. Other projections were to a region medial to the apex of pars interpolaris, to the nuclei ventrolateralis anterior (Vla) and presulcalis anterior (Pas) of the solitary complex, and sparsely to the lateral reticular formation (plexus of Horsley) ventral to TTD. No projections were seen to the trigeminal motor nuclei or to the cerebellum.

Calcitonin gene-related peptide immunoreactivity in the trigeminal ganglion of Trimeresurus flavoviridis

Crotaline snakes, which have infrared-sensitive pit organs, provide a good model for linking neuron morphology with sensory modality. In the trigeminal ganglion of the habu, Trimeresurus flavoviridis, cells positive for calcitonin gene-related peptide-like (CGRP) immunoreactivity were found to be of two types, darkly stained and lightly stained. They were pseudo-unipolar, having an axon divided into stem, peripheral branch, and central branch, all of which were 1 micron or less in diameter. Other, CGRP-negative cells in the ganglion were also pseudo-unipolar, but much larger. In configuration, some of the positive cells were similar to the neurons with A-delta fibers, and others to the neurons with C fibers that have been reported by other workers. On the basis of their distribution and density, and physiological studies by other workers, the CGRP-positive cells were judged to be not part of the infrared-receptive system, but to be involved in the transmission of nociception in small fibers.

Selective labeling of [3H]2-deoxy-D-glucose in the snake trigeminal system: basal and infrared-stimulated conditions

[3H]2-Deoxy-D-glucose (2-DG) and high-resolution autoradiography were employed to investigate labeling patterns of the trigeminal and infrared sensory system in a crotaline snake, the pit viper (Trimeresurus flavoviridis). Following intracardiac injection of 9.25 MBq [3H]2-DG, neurons in the nucleus of the lateral descending trigeminal tract (LTTD), nucleus reticularis caloris (RC), nucleus trigemini mesencephalicus, nucleus trigemini motorius, and trigeminal ganglia were labeled in various degrees after the pit organ had been removed (basal condition). This revealed that a higher rate of glucose utilization occurred in these nuclei than in the common sensory trigeminal nuclei, which lacked labeling entirely. When a pit was stimulated periodically with an infrared stimulus for 45 min, the difference in percentage of labeled cells was ipsilaterally increased by 12.84% in large cells of the LTTD and by 7.55% in the RC, as compared with the contralateral, basal-condition side. These slight changes indicate a small increase of glucose consumption during infrared reception. On the other hand, the small cells in the LTTD showed labeling that did not change with stimulation, suggesting that 2-DG uptake in inhibitory interneurons is relatively constant.

Substance P-like immunoreactive fibers in the trigeminal sensory nuclei of the pit viper, Trimeresurus flavoviridis

Substance P-like immunoreactive nerve fibres were located in the trigeminal sensory system of the infrared-sensitive snake, Trimeresurus flavoviridis, using the immunohistochemical method. There are two trigeminal sensory systems in the medulla of this animal: the descending nucleus and the lateral descending nucleus. The descending nucleus is equivalent to the trigeminal spinal nucleus in other vertebrates, and the lateral descending nucleus is a special trigeminal sensory nucleus belonging to the infrared sensory system. In the present study we determined that the lateral descending nucleus is completely ensheathed by large numbers of substance P-like immunoreactive fibers. The distribution of these fibers seems to be similar to that of the thin vagal unmyelinated fibers, rather than to that of the thick trigeminal myelinated fibers. More substance P-like immunoreactive nerve fibers were observed in the lateral descending tract than in the descending tract. Almost no dense substance P-like immunoreactive fibers were found in these tracts rostral to the lateral descending nucleus or rostral to the subnucleus caudalis of the descending nucleus. The substance P-like immunoreactive fibers in the lateral descending tract extended to those of Lissauer's tract of the spinal cord, and the substance P-like immunoreactive fibers surrounding the Lissauer's tract were similar in appearance to those of the lateral descending nucleus. This nucleus seems to have developed from the elements existing in Lissauer's tract, and also to have a similar modulating function. The primary nucleus of the infrared sensory system is the most substance P-like immunoreactive nucleus in the trigeminal sensory system of this animal. Even in the trigeminal sensory system, substance P-like immunoreactive fibers seem not to be related solely to the nociceptive sensation.

2-Deoxyglucose labelling of the infrared sensory system in the rattlesnake, Crotalus viridis

Infrared (IR) responsive nuclei in the rattlesnake Crotalus viridis were identified by using 14C-2-deoxyglucose (2DG) and autoradiography. Following 2DG intracardial injection, the IR-sensitive pit organ was stimulated periodically with an IR stimulus for 5 hours. The nucleus of the lateral descending trigeminal tract (LTTD, the primary IR sensory nucleus) was labelled heavily with 2DG. Labelling was bilateral, but somewhat heavier ipsilateral to the stimulated pit organ. The nucleus reticularis caloris (RC, the secondary nucleus of the IR system) was lightly labelled ipsilaterally. The middle laminae of the contralateral optic tectum (which contain IR-responsive units) were distinctly labelled; the corresponding layers of the ipsilateral tectum were lightly labelled. A subcerebellar nucleus not known to be part of the IR system was heavily labelled bilaterally. No consistent labelling was found in the diencephalon or telencephalon. Since units in the LTTD do not respond to stimulation of the contralateral pit yet the LTTD is labelled with 2DG when there is contralateral pit stimulation, several controls were carried out. Unilateral injection of 3H-proline into LTTD revealed no projection to the contralateral LTTD. In a monocularly, visually stimulated animal with both pits occluded, the LTTD still showed heavy but equal 2DG labelling bilaterally. In addition, the outer layers of the contralateral optic tectum were heavily labelled. No 2DG labelling of the LTTD was obtained when branches of the trigeminal nerve innervating the LTTD were previously cut. These results suggest that much of the 2DG labelling in the LTTD is due to spontaneous ongoing activity from the pit organ rather than from IR evoked activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Fine structure and organization of the infrared receptor relays: lateral descending nucleus of V in Boidae and nucleus reticularis caloris in the rattlesnake

The morphology of the lateral descending nucleus of V (LTTD) in three species of Boidae and nucleus reticularis caloris (RC) of the rattlesnake have been studied with the light and electron microscope. First- and second-order relays in the infrared receptor pathway to the tectum are contained within LTTD in the Boidae, whereas in the rattlesnakes the secondary relay to tectipetal neurons is in nucleus RC. The lateral descending nucleus in the boids contains small and large neurons. The larger cells project to the optic tectum and morphologically are quite similar to those of nucleus RC. It has been determined at the ultrastructural level that LTTD of the three species of boids studied have very similar morphology and organization. A marginal neuropil, located near the lateral descending tract, consists of terminals of thin unmyelinated axons in synaptic contact with thin dendrites. Deeper within the nucleus primary afferent terminals containing clear spherical vesicles form synaptic clusters with dendrites and are post-synaptic to other axon terminals containing pleomorphic vesicles. The large, tectipetal neurons are postsynaptic to two morphological types of synapses, one with clear spherical vesicles, asymmetric membranes, and subsynaptic web and the other with flattened vesicles and symmetrical membranes. Similar synapses are also present on the cells of nucleus RC in the rattlesnake. There is a close similarity in function, structure, and synaptic organization between the LTTD and boids and the LTTD plus nucleus RC of pit vipers, suggesting similar or identical evolutionary origin.

Primary vagal nerve projections to the lateral descending trigeminal nucleus in boidae (Python molurus and Boa constrictor)

The primary vagal axons and terminals within the lateral descending trigeminal tract (dlv) and nucleus (DLV) of two species of Boidae are studied following HRP injections of the vagal nerve. Labeled fibers and terminals are found in the tail portion of the dlv and DLV, partly forming a neuropil at its margin. The labeled thin fibers and neuropil seem to correspond to the C-fibers and marginal neuropil of Crotalinae.

Infrared sensory neurons in the trigeminal ganglia of crotaline snakes: transganglionic HRP transport

Trigeminal neurons were labeled by inserting HRP into holes cut in the pit receptor membranes of a crotaline snake, Agkistrodon blomhoffi brevicaudus. Neurons were labeled in the ophthalmic ganglion and the maxillary division of the maxillo-mandibular ganglion, and the HRP was further transported across the ganglia and through the lateral descending trigeminal tract (dlv) to label axon terminals exclusively in the dlv nucleus (DLV). In 6 successful preparations, 7.1-19.3% of totals of 5568-5986 cells in the maxillary division of the ganglion were labeled, but none at all were labeled in the mandibular division. Only a few or none at all were labeled in the ophthalmic ganglion. Cells in the two ganglia ranged in size from 10 to 55 micrometers, but large cells (greater than or equal to 40 micrometers) were scarce (4.9% of the total population). All HRP-labeled neurons fell in the median range of 20-39 micrometers. We concluded that these ganglion cells were infrared neurons, and were therefore the origin of the A delta fibers in the pit membrane. There were no HRP-labeled neurons above or below this range, in spite of the fact that smaller cells (less than or equal to 19 micrometers) made up 35.8% of the total population. In normal Nissl preparations we found both light- and dark-staining cells, but the size range of neither corresponded to the size range of infrared neurons.

The ascending projection of the nucleus of the lateral descending trigeminal tract: a nucleus in the infrared system of the rattlesnake, Crotalus viridis

The efferent projections of the nucleus of the lateral descending trigeminal tract (LTTD) in the rattlesnake (Crotalus viridis) were studied by anterograde tracing techniques. The LTTD, a brainstem trigeminal nucleus, is the sole projection site of the infrared-sensitive trigeminal fibers that innervate the pit organs in these snakes. The efferent fibers exit from the ventromedial edge of the LTTD and course medially and caudally toward the central grey area of the medulla. Upon reaching the central region of the medulla these fibers turn and move laterally and rostrally, eventually forming a tract on the ventrolateral surface of the brainstem. Embedded in this tract and slightly overlapping the LTTD in the rostrocaudal axis, is a population of large (20-45 micrometer) multipolar neurons that forms the nucleus reticularis caloris. Heavy terminal and preterminal degeneration in this area indicates that many of the efferent fibers of the LTTD terminate in this nucleus. A small bundle of degenerating fibers turn dorsally from the ventrolateral tract and ascend to terminate in a nucleus associated with the cerebellum, the lateral tegmental nucleus. No projection was found to any other nuclei or areas in the brain. This study demonstrates that the infrared-sensitive snakes, along with developing peripheral specializations (the pit organs), have developed specialized nuclei to handle this additional sensory information. The direct projection from the LTTD to the nucleus reticularis caloris provides a pathway linking the infrared-sensitive neurons of the LTTD with neurons of the same modality in the optic tectum. The second LTTD projection, to the lateral tegmental nucleus, suggests a connection between the infrared system and the cerebellum in these animals.