Paul R. Manger

Sleep in the platypus

We have conducted the first study of sleep in the platypus Ornithorhynchus anatinus. Periods of quiet sleep, characterized by raised arousal thresholds, elevated electroencephalogram amplitude and motor and autonomic quiescence, occupied 6-8 h/day. The platypus also had rapid eye movement sleep as defined by atonia with rapid eye movements, twitching and the electrocardiogram pattern of rapid eye movement. However, this state occurred while the electroencephalogram was moderate or high in voltage, as in non-rapid eye movement sleep in adult and marsupial mammals. This suggests that the low-voltage electroencephalogram is a more recently evolved feature of mammalian rapid eye movement sleep. Rapid eye movement sleep occupied 5.8-8 h/day in the platypus, more than in any other animal. Our findings indicate that rapid eye movement sleep may have been present in large amounts in the first mammals and suggest that it may have evolved in pre-mammalian reptiles.

Ultrastructure, number, distribution and innervation of electroreceptors and mechanoreceptors in the bill skin of the platypus, Ornithorhynchus anatinus.

The platypus is presently the only mammal demonstrated to use electroreception to obtain food. The electroreceptive system of the platypus is far more complex than that of its close relative the echidna. This paper presents an anatomical study of the basis of electroreception in the platypus. The innervation of the bill by the trigeminal nerve is described, as are three sensory structures, associated with food gathering, within the bill skin. There are 40,000 mucous gland electroreceptors found in the bill skin of the platypus. The papillary portion of each of these sensory mucous glands is modified to accommodate electrosensory nerve terminals. In contrast to fish electroreceptors, the electrosensory terminals of the platypus are not associated with a sensory cell. These mucous gland electroreceptors are arranged in a series of parasagittal stripes on the bill. This array suggests a basis for the ability of the platypus to quickly and accurately locate the origin of an electrical stimulus. A push-rod mechanoreceptor, similar in morphology to Eimers organ of the mole, and bill-tip organs in birds, was also found in the bill skin. The slightly differing morphology of these mechanoreceptors when compared to their avian and talpid counterparts suggests that this is another example of convergent evolution, with the common need to provide a solution to increasing tactile sensitivity on bare rhinarial skin. These push-rods are found to be most dense around the labial margins of the bill, with a marked decrease in density towards the middle and caudal portions of the bill. The distribution of the push-rods is similar to the distribution of the third sensory structure found on the bill, the sensory serous gland. Although less numerous than the mechanoreceptors (46,500 mechanoreceptors compared with 13,500 sensory serous glands), these sensory serous glands have a similar distribution and similar changes in density. These concurrent distributions argue for some functional correlation of these two sensory structures. The papillary region of the serous gland is modified in a manner similar to that of the mucous gland electroreceptor to accommodate sensory input. The sensory terminals of the serous glands are very similar to those of the mucous gland electroreceptors, and so it is presumed that these sensory serous glands are a type of electroreceptor that might be involved in detection of electrical signals at close quarters where the mechanoreceptors are also engaged.

The distribution and morphological characteristics of serotonergic cells in the brain of monotremes

The distribution and cellular morphology of serotonergic neurons in the brain of two species of monotremes are described. Three clusters of serotonergic neurons were found: a hypothalamic cluster, a cluster in the rostral brainstem and a cluster in the caudal brainstem. Those in the hypothalamus consisted of two groups, the periventricular hypothalamic organ and the infundibular recess, that were intimately associated with the ependymal wall of the third ventricle. Within the rostral brainstem cluster, three distinct divisions were found: the dorsal raphe nucleus (with four subdivisions), the median raphe nucleus and the cells of the supralemniscal region. The dorsal raphe was within and adjacent to the periaqueductal gray matter, the median raphe was associated with the midline ventral to the dorsal raphe, and the cells of the supralemniscal region were in the tegmentum lateral to the median raphe and ventral to the dorsal raphe. The caudal cluster consisted of three divisions: the raphe obscurus nucleus, the raphe pallidus nucleus and the raphe magnus nucleus. The raphe obscurus nucleus was associated with the dorsal midline at the caudal-most part of the medulla oblongata. The raphe pallidus nucleus was found at the ventral midline of the medulla around the inferior olive. Raphe magnus was associated with the midline of the medulla and was found rostral to both the raphe obscurus and raphe pallidus. The results of our study are compared in an evolutionary context with those reported for other mammals and reptiles.

The distribution and morphological characteristics of cholinergic cells in the brain of monotremes as revealed by ChAT immunohistochemistry.

The present study employs choline acetyltransferase (ChAT) immunohistochemistry to identify the cholinergic neuronal population in the central nervous system of the monotremes. Two of the three extant species of monotreme were studied: the platypus (Ornithorhynchus anatinus) and the short-beaked echidna (Tachyglossus aculeatus). The distribution of cholinergic cells in the brain of these two species was virtually identical. Distinct groups of cholinergic cells were observed in the striatum, basal forebrain, habenula, pontomesencephalon, cranial nerve motor nuclei, and spinal cord. In contrast to other tetrapods studied with this technique, we failed to find evidence for cholinergic cells in the hypothalamus, the parabigeminal nucleus (or nucleus isthmus), or the cerebral cortex. The lack of hypothalamic cholinergic neurons creates a hiatus in the continuous antero-posterior aggregation of cholinergic neurons seen in other tetrapods. This hiatus might be functionally related to the phenomenology of monotreme sleep and to the ontogeny of sleep in mammals, as juvenile placental mammals exhibit a similar combination of sleep elements to that found in adult monotremes.

The distribution and morphological characteristics of catecholaminergic cells in the brain of monotremes as revealed by tyrosine hydroxylase immunohistochemistry

The present study describes the distribution and cellular morphology of catecholaminergic neurons in the CNS of two species of monotreme, the platypus (Ornithorhynchus anatinus) and the short-beaked echidna (Tachyglossus aculeatus). Tyrosine hydroxylase immunohistochemistry was used to visualize these neurons. The standard A1–A17, C1–C3 nomenclature was used for expediency, but the neuroanatomical names of the various nuclei have also been given. Monotremes exhibit catecholaminergic neurons in the diencephalon (A11, A12, A13, A14, A15), midbrain (A8, A9, A10), rostral rhombencephalon (A5, A6, A7), and medulla (A1, A2, C1, C2). The subdivisions of these neurons are in general agreement with those of other mammals, and indeed other amniotes. Apart from minor differences, those being a lack of A4, A3, and C3 groups, the catecholaminergic system of monotremes is very similar to that of other mammals. Catecholaminergic neurons outside these nuclei, such as those reported for other mammals, were not numerous with occasional cells observed in the striatum. It seems unlikely that differences in the sleep phenomenology of monotremes, as compared to other mammals, can be explained by these differences. The similarity of this system across mammalian and amniote species underlines the evolutionary conservatism of the catecholaminergic system.

The locus coeruleus complex of the bottlenose dolphin (Tursiops truncatus) as revealed by tyrosine hydroxylase immunohistochemistry

Using tyrosine hydroxylase immunohistochemistry we examined the structure of the pontine, or rostral rhombencephalic, catecholaminergic cells groups, which may be collectively termed the locus coeruleus complex (LC), in the bottlenose dolphin. The present study is the first to describe the LC in a cetacean species and, at 1.3u2003kg, represents the largest non‐human brain to date in which the LC has been investigated. We identified four catecholaminergic cell groups in the dorsal pontine tegementum and peri‐aqueductal gray matter: A6 dorsal (locus coeruleus), A6 ventral (locus coeruleus alpha), A7 (subcoeruleus), and A5 (fifth arcuate nucleus). No patterns of cellular distribution, nuclear subdivision, or cellular morphology indicate specialization of the LC, which might have been anticipated because of the large absolute brain size and unihemispheric sleep phenomenology of cetaceans.

Ultrastructure and Distribution of Epidermal Sensory Receptors in the Beak of the Echidna, Tachyglossus aculeatus

Within the rostral one centimetre of the Echidna beak, three specialised receptors were found: a mucous sensory gland, a rod-like structure, and an innervated epidermal pit. The mucous sensory gland consists of a dermal mucous gland and a modified epidermal portion. Bulbous nerve terminals, similar to those reported for the Platypus, were found within the modified epidermal portion of the mucous gland. The rod-like structure contains four types of nerve terminals: Merkel cells, Paciniform corpuscles, and a central and a peripheral vesicle chain receptor. Apart from minor differences, the rod-like structure is similar to that previously reported for the Platypus. Preliminary results are presented for a third structure: an innervated epidermal pit. Topographical and ultrastructural analyses are used in the context of functional interpretation.

Properties of Electrosensory Neurons in the Cortex of the Platypus (Ornithorhynchus anatinus): Implications for Processing of Electrosensory Stimuli

Electroreceptor organs and mechanoreceptor organs located in the bill skin of the platypus are used by the animal to locate prey items, underwater, with eyes and ears closed. The precise manner in how these senses aid the platypus to locate food is not yet known. In this study we provide data on the activity of cortical neurons in the bill representation of SI when stimulated electrically, mechanically, and concurrently. Within the SI bill representation, there are alternating stripes of cortex that represent purely mechanical inputs, and combined electrical and mechanical inputs. Generally, the bimodal units responded more vigorously to electrical stimulation and had very small dynamic ranges, usually saturating within 20 µV cm-1 of threshold. Latencies to electrical or mechanical stimulation were around 25 ms, but were significantly reduced for concurrent stimulation. Combined with the previously reported observation that the receptive fields of bimodal neurons within cortical modules were the same, and that thresholds varied considerably, the observation of limited dynamic range suggests a mechanism for localization of stimuli. A model is developed wherein the relative activation of a module is dependent on its proximity to the source.

Histological observations on presumed electroreceptors and mechanoreceptors in the beak skin of the long-beaked echidna, Zaglossus bruijnii.

Sensory receptors in the rostral portion of the beak skin of a single specimen of the rare long–beaked echidna, Zaglossus bruijnii, are described. Mucous glands which have been modified to accommodate sensory innervation, similar to those seen in Ornithorhynchus, are found in the rostral 2 cm of the beak skin, anterior to the maxillofacial foramen, at a density of approximately 12/mm2. The papillary epidermal portion of the gland ducts are walled by concentric layers of keratinocytes, and each duct is innervated by 10–15 myelinated nerve terminals. The mucous gland receptors in Zaglossus are intermediate in structure between those of Ornithorhynchus and Tachyglossus, but are similar enough to the former to suggest that electroreception may play a major role in the sensory experience of Zaglossus. Push–rod mechanoreceptors also occur throughout the same region of beak skin, and appear similar to those described for Tachyglossus.

Somatotopic organization and cortical projections of the ventrobasal complex of the flying fox: an "inverted" wing representation in the thalamus.

The present study investigates the somatotopic representation in the somatosensory thalamus of a megachiropteran bat. Using standard microelectrode mapping techniques, representational maps were generated for the ventrobasal (Vb) and posterior (Po) thalamic complexes of the Grey-headed flying fox. Anatomical tracing from neocortical injections provided additional data confirming the somatotopy found physiologically. A full representation of the body surface innervated by the trigeminal and spinal nerves was found. However, in contrast with other mammals, the representations of the forelimb and adjacent thoracic trunk within the thalamus were inverted. This means that the distal portions of the wing membrane and the tips of the digits were represented dorsally in Vb, and the thoracic trunk was represented ventrally. In Po the digit tips were represented in the ventral most portion and the thoracic trunk in the dorsal portion of the nucleus.These results are discussed in relation to similarities of megachiropteran somatosensory thalamic nuclei to those of other mammalian species and in relation to the formation of thalamic somatotopic maps and fiber sorting.The present study investigates the somatotopic representation in the somatosensory thalamus of a megachiropteran bat. Using standard microelectrode mapping techniques, representational maps were generated for the ventrobasal (Vb) and posterior (Po) thalamic complexes of the Grey-headed flying fox. Anatomical tracing from neocortical injections provided additional data confirming the somatotopy found physiologically. A full representation of the body surface innervated by the trigeminal and spinal nerves was found. However, in contrast with other mammals, the representations of the forelimb and adjacent thoracic trunk within the thalamus were inverted. This means that the distal portions of the wing membrane and the tips of the digits were represented dorsally in Vb, and the thoracic trunk was represented ventrally. In Po the digit tips were represented in the ventral most portion and the thoracic trunk in the dorsal portion of the nucleus. These results are discussed in relation to similarities of megachiropteran somatosensory thalamic nuclei to those of other mammalian species and in relation to the formation of thalamic somatotopic maps and fiber sorting.