Michael H. Hofmann

Functional subdivisions of the olfactory system correlate with lectin-binding properties inXenopus

Soybean agglutinin (SBA) is known to selectively label a portion of neurons in amphibian and mammalian primary olfactory systems. Hitherto, no other distinctive features have been found to correlate with the two neuronal populations. Investigating SBA-HRP binding in olfactory mucosa and CNS of Xenopus, we noted that labelled and unlabelled structures can readily be assigned to different olfactory subsystems. The SBA negative one is utilized to detect air-borne odors, whereas major SBA-positive structures serve a role in the perception of water dissolved molecules. Some labelled fibers by-pass the olfactory bulb, traverse the telencephalon and innervate prosencephalic structures. They are considered to be aberrant olfactory nerve fibers, rather than being part of the terminal nerve.

The Extrabulbar Olfactory Pathway: Primary Olfactory Fibers Bypassing the Olfactory Bulb in Bony Fishes?

Recent evidence has revealed that some primary olfactory fibers bypass the olfactory bulb and terminate in tel- and/or diencephalic areas (extrabulbar olfactory pathway, EBOP). We investigated the projections of this system in different fishes by means of soybean agglutinin binding studies. In all species in which primary olfactory fibers were labelled, fiber bundles can be traced beyond the olfactory bulb. These run with the medial forebrain bundle and terminate at different targets, depending on the species. In the teleosts Macrognathus, Mogurnda, and Hemichromis, EBOP fibers can be traced into the ventral telencephalon, pars ventralis, pars supracommissuralis and/or into the preoptic nucleus. In most nonteleosts studied (Polypterus, Chalamoichthys, Amia), the EBOP also innervates diencephalic targets. An exception is Acipenser, which displays an innervation pattern similar to that in teleosts. Comparison with results obtained by other techniques suggests that the EBOP consists of primary olfactory fibers, which project not only to the olfactory bulb but also to various other targets in the prosencephalon of anamniotic vertebrates.

Peripheral origin of olfactory nerve fibers by-passing the olfactory bulb in Xenopus laevis

After DiI injections into the diencephalon of Xenopus, two types of retrogradely labelled cells were found in the nasal area: (i) receptor cells in the olfactory epithelium and (ii) a small cell group located between the main olfactory epithelium and the vomeronasal system. These results reveal an extensive extrabulbar olfactory projection of olfactory receptor cells. Fibers of these cells do not terminate in the olfactory bulb but innervate targets in the diencephalon directly. The other type of retrogradely labelled cells, apparently, are not part of any epithelium. They resemble similar cell groups which have previously been regarded as part of the nervus terminalis system in other vertebrates.

Central projections of the nervus terminalis in four species of Amphibians

The central projections of the nervus terminalis were investigated in two anuran and two urodele species by means of horseradish peroxidase injections into one nasal cavity. In anurans, the nervus terminalis projects to the medial septum, to the preoptic nucleus, to the nucleus of the anterior commissure and to the hypothalamus. In addition to these structures, the dorsal thalamus, the infundibulum and the mesencephalic tegmentum are innervated in urodeles. The structure containing the highest density of terminals in the amphibians investigated is the hypothalamus. In one anuran and one urodele species, the contralateral hypothalamus is primarily innervated, whereas in the other two species the majority of fibers remain ipsilateral. A comparison with other vertebrates shows that the terminalis system in urodeles has the greatest diversity of connections. Anurans, in contrast, lack some connections that are present in urodeles and fishes. These findings have implications for a possible relation of the nervus terminalis to an aquatic habitat.

Interval-specific event related potentials to omitted stimuli in the electrosensory pathway in elasmobranchs: an elementary form of expectation.

Multiunit activity and slow local field potentials show Omitted Stimulus Potentials (OSP) in the electrosensory system in rays (Platyrhinoidis triseriata, Urolophus halleri) after a missing stimulus in a 3 to >20 Hz train of μV pulses in the bath, at levels from the primary medullary nucleus to the telencephalon. A precursor can be seen in the afferent nerve. The OSP follows the due-time of the first omitted stimulus with a, usually, constant main peak latency, 30–50 ms in medullary dorsal nucleus, 60–100 ms in midbrain, 120–190 ms in telencephalon — as though the brain has an expectation specific to the interstimulus interval (ISI). The latency, form and components vary between nerve, medulla, mid-brain and forebrain. They include early fast waves, later slow waves and labile induced rhythms. Responsive loci are quite local. Besides ISI, which exerts a strong influence, many factors affect the OSP slightly, including train parameters and intensity, duration and polarity of the single stimulus pulses. Jitter of ISI does not reduce the OSP substantially, if the last interval equals the mean; the mean and the last interval have the main effect on both amplitude and latency.Taken together with our recent findings on visually evoked OSPs, we conclude that OSPs do not require higher brain levels or even the complexities of the retina. They appear in primary sensory nuclei and are then modified at midbrain and telencephalic levels. We propose that the initial processes are partly in the receptors and partly in the first central relay including a rapid increase of some depressing influence contributed by each stimulus. This influence comes to an ISI-specific equilibrium with the excitatory influence; withholding a stimulus and hence its depressing influence causes a rebound excitation with a specific latency.

Thyroxine influences neuronal connectivity in the adult frog brain.

In contrast to results of earlier investigations the influence of thyroxine on CNS connectivity is not restricted to circum-metamorphic stages in frogs. Neuroanatomical findings in adult Xenopus treated with thyroxine reveal a spread of the ipsilateral retino-tectal projection. Sprouting fibers establish a tectal innervation pattern similar to the one found in primitive fish. The question arises, whether thyroxine also has morphogenetic effects in the mature CNS of other species.

The valvula cerebelli of the spiny eel, Macrognathus aculeatus, receives primary lateral-line afferents from the rostrum of the upper jaw

SummaryIn the spiny eel, Macrognathus aculeatus, anterodorsal and (to a lesser degree) anteroventral lateralline nerves project massively to the granular layer of the valvula cerebelli, throughout its rostrocaudal extent. The posterior lateral-line nerve terminates in the corpus cerebelli. Thus, valvula and corpus cerebelli are supplied with mechanosensory input of different peripheral origins. An analysis of the taxonomic distribution of experimentally determined primary lateral-line input to the three parts of the teleostean cerebellum reveals that the eminentia granularis always receives such input, and that the corpus cerebelli is the recipient of primary lateral-line input in many teleosts. The valvula, however, receives primary lateral-line afferents in only two examined species. In M. aculeatus, the massive lateral-line input to the valvula probably originates in mechanoreceptors located in the elongated rostrum of the upper jaw, a characteristic feature of mastacembeloid fishes. This projection to the valvula may therefore represent a unique specialization that arose with the evolution of the peculiar rostrum.

Histochemical, Connectional and Cytoarchitectonic Evidence for a Secondary Reduction of the Pretectum in the European Eel, Anguilla anguilla: A Case of Parallel Evolution

There are at least three different patterns of pretectal organization in teleost fishes: a simple pattern observed in cyprinids, an elaborate pattern present in percomorphs, and an intermediately complex pattern seen in many other teleost groups. The taxonomic distribution of the pretectal patterns indicates that the simple and the elaborate patterns are both evolutionarily derived (apomorphic) from the primitive (plesiomorphic) intermediately complex one. In anguillids, the pretectal pattern observed cytoarchitectonically has an anatomical configuration similar to that of the simple pattern in cyprinids. The distribution of acetylcholinesterase positivity in the pretectum (namely acetylcholinesterase positivity in the parvo- and magnocellular superficial and posterior pretectal nuclei, and acetylcholinesterase negativity in the pretectal cell plate and the ovoid preglomerular cell aggregate), as well as the retinal projections (namely retinal terminals in the parvocellular superficial and central pretectal nuclei, and absence of such terminals in the magnocellular superficial and posterior pretectal nuclei and the pretectal cell plate), strongly supports the interpretation suggested by the cytoarchitectonic analysis. As anguillids (elopomorpha) and cyprinids (ostariophysi) are related only distantly, this secondary simplification in the pretectum likely occurred independently, i.e. this simplification represents a case of parallel reduction.

Contents Vol. 72, 2008

78 28th Annual Meeting of the J.B. Johnston Club and 20th Annual Karger Workshop Washington, D.C., November 13–14, 2008 36 Erratum