Heather L. Eisthen

Why Are Olfactory Systems of Different Animals So Similar

As we learn more about the neurobiology of olfaction, it is becoming increasingly clear that olfactory systems of animals in disparate phyla possess many striking features in common. Why? Do these features provide clues about the ways the nervous system processes olfactory information? This might be the case if these commonalities are convergent adaptations that serve similar functions, but similar features can be present in disparate animals for other reasons. For example, similar features may be present because of inheritance from a common ancestor (homology), may represent responses to similar constraints, or may be superficial or reflect strategies used by researchers studying the system. In this paper, I examine four examples of features of olfactory systems in members of different phyla: the presence of odorant binding proteins in the fluid overlying olfactory receptor neurons; the use of G protein-coupled receptors as odorant receptors; the use of a two-step pathway in the transduction of odorant signals; and the presence of glomerular neuropils in the first central target of the axons of olfactory receptor cells. I analyze data from nematodes, arthropods, molluscs, and vertebrates to investigate the phylogenetic distribution of these features, and to try to explain the presence of these features in disparate animals. Phylogenetic analyses indicate that these features are not homologous across phyla. Although these features are often interpreted as convergent adaptations, I find that alternative explanations are difficult to dismiss. In many cases, it seems that olfactory system features that are similar across phyla may reflect both responses to similar constraints and adaptations for similar tasks.

Silver lampreys (Ichthyomyzon unicuspis) lack a gonadotropin-releasing hormone- and FMRFamide-immunoreactive terminal nerve

The terminal nerve is a ganglionated cranial nerve with peripheral processes that enter the nasal cavity and centrally directed processes that enter the forebrain. Members of all classes of gnathostomes have been found to possess a terminal nerve, some components of which demonstrate immunoreactivity to the peptides Phe‐Met‐Arg‐Phe‐NH2 (FMRFamide) and gonadotropin‐releasing hormone (GnRH). To explore the possibility that lampreys possess a terminal nerve, we examined the distribution of these peptides in the silver lamprey, Ichthyomyzon unicuspis, by using antisera to FMRFamide and to four forms of GnRH. We found cells with FMRFamide‐like immunoreactivity in the preoptic area and the isthmal gray region of the mesencephalon, and found labeled fibers throughout the preoptic‐infundibular region. Occasional labeled fibers were scattered through many regions of the brain, including the optic nerve and olfactory bulb; however, unlike species that possess a terminal nerve, lampreys have no immunoreactive cells or fibers in the olfactory nerve or nasal epithelia. In addition, we observed GnRH‐immunoreactive cell bodies in the preoptic area of all animals and in the ventral hypothalamus of one individual. Most of the labeled fibers extended ventrally to the hypothalamus, with other fibers extending throughout the striatum and hypothalamic‐neurohypophyseal region. A few fibers in other regions, including the optic nerve, were also labeled; we detected no immunoreactivity in the olfactory bulb, olfactory nerve, or nasal epithelia. The use of different GnRH antisera resulted in remarkably similar patterns of labeling of both cells and fibers. In summary, we did not observe either GnRH or FMRFamide‐like immunoreactivity in the olfactory regions that represent the typical path of terminal nerve fibers, nor were we able to locate a terminal nerve ganglion. We conclude that lampreys may lack a terminal nerve, and that the previously described fiber bundle extending from the nasal sac to the ventral forebrain may constitute an extra‐bulbar olfactory pathway.

Terminal-Nerve-Derived Neuropeptide Y Modulates Physiological Responses in the Olfactory Epithelium of Hungry Axolotls (Ambystoma mexicanum)

The vertebrate brain actively regulates incoming sensory information, effectively filtering input and focusing attention toward environmental stimuli that are most relevant to the animals behavioral context or physiological state. Such centrifugal modulation has been shown to play an important role in processing in the retina and cochlea, but has received relatively little attention in olfaction. The terminal nerve, a cranial nerve that extends underneath the lamina propria surrounding the olfactory epithelium, displays anatomical and neurochemical characteristics that suggest that it modulates activity in the olfactory epithelium. Using immunocytochemical techniques, we demonstrate that neuropeptide Y (NPY) is abundantly present in the terminal nerve in the axolotl (Ambystoma mexicanum), an aquatic salamander. Because NPY plays an important role in regulating appetite and hunger in many vertebrates, we investigated the possibility that NPY modulates activity in the olfactory epithelium in relation to the animals hunger level. We therefore characterized the full-length NPY gene from axolotls to enable synthesis of authentic axolotl NPY for use in electrophysiological experiments. We find that axolotl NPY modulates olfactory epithelial responses evoked by l-glutamic acid, a food-related odorant, but only in hungry animals. Similarly, whole-cell patch-clamp recordings demonstrate that bath application of axolotl NPY enhances the magnitude of a tetrodotoxin-sensitive inward current, but only in hungry animals. These results suggest that expression or activity of NPY receptors in the olfactory epithelium may change with hunger level, and that terminal nerve-derived peptides modulate activity in the olfactory epithelium in response to an animals changing behavioral and physiological circumstances.

What defines the nervus terminalis? Neurochemical, developmental, and anatomical criteria

Publisher Summary This chapter provides an overview of the nervus terminalis. The nervus terminalis or terminal nerve is a diffusely organized system of neurons that lie within the nasal cavity and rostral forebrain of all jawed vertebrates, including humans. Its most significant feature is that some of its neurons contain the reproductive neuropeptide, gonadotropin-releasing hormone (GnRH). External to the forebrain, the cell bodies of the nervus terminalis are typically embedded within chemosensory nerves in the nasal cavity or they may be congregated in compact ganglia, commonly in the region of the olfactory bulbs. The nervus terminalis was first described as an “uberzahliger Nerv.” The GnRH neurons that come to lie along the nervus terminalis in the nasal cavity and rostra1 forebrain, as well as those that reside in the preoptic/hypothalamic areas, migrate out of the olfactory placodal region during development. These neurons migrate along a path on the nasal septum where the mature nervus terminalis eventually lies. Some GnRH neurons of the nervus terminalis migrate into the ventral forebrain but many remain in the nasal cavity and in peripheral autonomic ganglia associated with the nasal cavity.

The goldfish knows: Olfactory receptor cell morphology predicts receptor gene expression

Two related but distinct nasal chemosensory systems, the olfactory and vomeronasal systems, are present in vertebrates as diverse as garter snakes, laboratory mice, and clawed frogs. The olfactory epithelium is located inside the nasal cavity, and the axons of its sensory receptor neurons project to the main olfactory bulb, at the rostral pole of the telencephalon. In contrast, the vomeronasal epithelium is located in a separate organ located between the nasal and oral cavities, or in a diverticulum off the nasal cavity. The axons of the vomeronasal receptor neurons terminate in the accessory olfactory bulb, a histologically distinct structure that is usually dorsal or caudal to the main olfactory bulb. A distinct vomeronasal system is generally present in tetrapods, the group of vertebrates that includes amphibians, reptiles, and mammals. Although the origin of tetrapods led to the evolution of terrestriality in vertebrates, it is clear that the vomeronasal system did not arise as an adaptation to terrestrial life (Eisthen, 1992, 1997). All fishes examined to date lack both a discrete vomeronasal organ and an accessory olfactory bulb. Intriguing new data by Hansen, Anderson, and Finger presented in this issue of the journal (Hansen et al., 2004) demonstrate that in goldfish the olfactory sensory neurons that bear cilia, like mammalian olfactory receptor neurons, express receptor genes similar to those that characterize the mammalian olfactory epithelium. A second class of olfactory sensory neurons that bear microvilli, and therefore resemble mammalian vomeronasal receptor neurons, expresses receptor genes that characterize the mammalian vomeronasal epithelium. Does this mean that goldfish have a vomeronasal system? The authors interpret their data cautiously, pointing out some of the parallels between the organization of the olfactory system in teleost fishes and the olfactory and vomeronasal systems in tetrapods without jumping to conclusions. Because their work will cause many readers to question the evolutionary implications of the data, it is worth taking the time to consider the issues involved. WHAT DOES THE VOMERONASAL SYSTEM DO?

Anatomy and forebrain projections of the olfactory and vomeronasal organs in axolotls (ambystoma mexicanum)

We examined the anatomy of the nasal cavity and forebrain in the axolotl (Ambystoma mexicanum) to determine whether the olfactory and vomeronasal systems are present in this neoteni

The role of the terminal nerve and GnRH in olfactory system neuromodulation.

Animals must regulate their sensory responsiveness appropriately with respect to their internal and external environments, which is accomplished in part via centrifugal modulatory pathways. In the olfactory sensory system, responsiveness is regulated by neuromodulators released from centrifugal fibers into the olfactory epithelium and bulb. Among the modulators known to modulate neural activity of the olfactory system, one of the best understood is gonadotropin-releasing hormone (GnRH). This is because GnRH derives mainly from the terminal nerve (TN), and the TN-GnRH system has been suggested to function as a neuromodulator in wide areas of the brain, including the olfactory bulb. In the present article we examine the modulatory roles of the TN and GnRH in the olfactory epithelium and bulb as a model for understanding the ways in which olfactory responses can be tuned to the internal and external environments.

Discrimination of conspecific sex and reproductive condition using chemical cues in axolotls (Ambystoma mexicanum)

Chemosensory cues play an important role in the daily lives of salamanders, mediating foraging, conspecific recognition, and territorial advertising. We investigated the behavioral effects of conspecific whole-body odorants in axolotls, Ambystoma mexicanum, a salamander species that is fully aquatic. We found that males increased general activity when exposed to female odorants, but that activity levels in females were not affected by conspecific odorants. Although males showed no difference in courtship displays across testing conditions, females performed courtship displays only in response to male odorants. We also found that electro-olfactogram responses from the olfactory and vomeronasal epithelia were larger in response to whole-body odorants from the opposite sex than from the same sex. In males, odorants from gravid and recently spawned females evoked different electro-olfactogram responses at some locations in the olfactory and vomeronasal epithelia; in general, however, few consistent differences between the olfactory and vomeronasal epithelia were observed. Finally, post hoc analyses indicate that experience with opposite-sex conspecifics affects some behavioral and electrophysiological responses. Overall, our data indicate that chemical cues from conspecifics affect general activity and courtship behavior in axolotls, and that both the olfactory and vomeronasal systems may be involved in discriminating the sex and reproductive condition of conspecifics.

Behavioral responses of male guinea pigs to conspecific chemical signals following neonatal vomeronasal organ removal

Following vomeronasal organ removal or sham surgery at 4-7 days of age, male guinea pigs were tested for responsiveness to conspecific chemical uses as infants and again as adults. In the first experiment, vocalizations in response to soiled home cage bedding and male bedding were monitored twice prior to surgery and twice weekly for four weeks. Home cage cues elicited more vocalizations than did male bedding in both groups; however, there was no effect of vomeronasal organ removal. When tested as adults in a second experiment, animals without vomeronasal organs exhibited depressed investigative responsiveness and vocalizations to female genital smears. The data from the first experiment fail to indicate a role for the vomeronasal organ in infantile response to conspecific odor. However, the second experiment demonstrates that adult responses to similar odors are substantially depressed by an absence of the vomeronasal organ.

Convergence: Obstacle or Opportunity?

Evolutionary convergence is a remarkable phenome-non that spans all levels of biological organization. Exam-ples are ubiquitous, and often involve correlated changesin molecules, morphology, physiology, and behavior.Birds and mammals convergently evolved the ability toregulate their body temperature, an adaptation that in-volves a suite of changes in physiology and anatomy, andhas enabled both groups to expand into niches not avail-able to many exothermic animals. Bats, birds, and someinsects have acquired flapping flight independently, andagain some of the underlying changes and resulting abili-ties are remarkably similar. Some bats, dolphins, and tele-ost fishes send out signals that produce echoes used tolocalize objects in the environment. What can we learnabout neurobiology and behavior by investigating suchphenomena?Light-detecting eyes have evolved repeatedly in differ-ent animal phyla. Many elements of eye structure are con-vergent; for example, lenses have evolved repeatedly, pre-sumably because the ability to form an image can beadaptive for many organisms inhabiting a variety ofniches. Similarly, the ability to discriminate differentwavelengths of light reflected by objects in the environ-ment (color vision) has evolved repeatedly, perhaps as anadaptive trait that allows for improved selection of fooditems or more elaborate intraspecific communication [Pi-chaud et al., 1999]. The repeated origin of these featuresprovides opportunities for us to test hypotheses concern-ing the pressures that have given rise to these features, andthe function of each. Are these examples of convergencedue to functional demands, mechanical constraints relat-ed to the physics of light, or the repeated deployment of anetwork of developmental genes? Are these three explana-tions of convergence mutually exclusive? The papers con-tained in this volume explore these questions throughdetailed analysis of a wide range of examples.In organizing this workshop and compiling this vol-ume, our goal is to urge comparative neurobiologists toconsider the heuristic value of evolutionary convergence(homoplasy) in helping us understand the diversity ofstructure and function in the nervous system; that is, wewant to encourage our colleagues to view convergence asan