The Osteichthyes comprise at least three groups of bony fishes which are generally believed to share a common ancestor: the Crossopterygii, the Dipnoi, and the Actinopterygii. The intergroup relationships have long been disputed by paleontologists, and there is no consensus at the present time. It is usually argued either that the Crossopterygii and the Dipnoi are more closely related to one another than either is to the Actinopterygii (Romer, 1966; Gardiner, 1973) or that the three have been distinct groups since their earliest appearance (Schaeffer, 1969; Miles in Moy-Thomas, 1971). Jarvik (1968), however, has favored a closer affinity between the Crossopterygii and the Actinopterygii than between either of these groups and the Dipnoi.

New Observations on the Organization and Evolution of the Telencephalon of Actinopterygian Fishes

The study of diverse animal groups allows us to discern the evolution of the neurobiology of nociception. Nociception functions as an important alarm system alerting the individual to potential and actual tissue damage. All animals possess nociceptors, and, in some animal groups, it has been demonstrated that there are consistent physiological mechanisms underpinning the nociceptive system. This review considers the comparative biology of nociception and pain from an evolutionary perspective.

Comparative Physiology of Nociception and Pain.

Afferents to the dorsal and ventral striatum of the bullfrog (Rana catesbeiana) were revealed by horseradish peroxidase (HRP) histochemistry. Anterograde tracing techniques (autoradiography and anterograde HRP transport) were then used to confirm the projections and to describe their terminal fields within the striatum. The major input arises from the ipsi-lateral lateral anterior and central thalamic nuclei, which receive tectal (Rubinson, ′68) and toral (Neary, ′74) input, respectively. These projections terminate in a dense, homogeneous field within the striatal neuropil adjacent to the cell plates of both striatal divisions. A second heavy input arises from the anterior entopeduncular nucleus bilaterally, with axons from the contralateral side crossing in the anterior commissure. This input terminates in both striatal divisions but is heaviest ventrally, Sparser inputs are present from the ipsilateral lateral and medial amygdalar nuclei, the preoptic area (mainly from the very caudal suprachiasmatic division), the posterior tuberculum, and the ipsilateral superficial isthmal reticular nucleus of the tegmentum. All these afferents, with the possible exception of the preoptic input, ascend to the striatum via the lateral forebrain bundle, and all innervate both striatal divisions. The preoptic input terminates within the cell plates, as well as subpially in a pattern similar to that described for tubercular input (Neary and Wilczynski, ′77). The tegmental input is very sparse and is most apparent superficially. Afferents from pallial telencephalic areas are not present, suggesting that although an-urans receive striatal input from the diencephalon and mesencephalon, they do not possess a homolog of the mammalian corticostriatal system. Further, the extremely heavy input from the middle dorsal thalamic zone suggests that a major function of the anuran striatum involves processing sensory information from the midbrain roof.

Connections of the bullfrog striatum: afferent organization.

Recent studies dealing with the investigation of the afferent and efferent connections of the basal ganglia of amphibians have revealed many similarities with basal ganglia structures of amniotes. In a further step, the chemoarchitecture of basal ganglia of the frog Rana perezi has been investigated. For use as main markers of amphibian basal ganglia structures, antibodies against tyrosine hydroxylase, substance P, and enkephalin were selected. Moreover, the distributions of nitric oxide synthase (nicotinamide adenine dinucleotide phosphate-diaphorase histochemistry), calretinin, dopamine-β-hydroxylase, choline acetyltransferase, mesotocin, vasotocin, somatostatin, neuropeptide Y, neuropeptide FF, and serotonin were studied to corroborate a comparison with both basal ganglia and amygdaloid structures of amniotes. On the basis of connections and chemoarchitecture, a striatum proper, nucleus accumbens, dorsal and ventral pallidum, bed nucleus of the stria terminalis, and amygdaloid complex have been identified. Accordingly, a new terminology is proposed that is in line with our current understanding of basal ganglia organization in amphibians. J. Comp. Neurol. 392:285–312, 1998. © 1998 Wiley-Liss, Inc.

Basal ganglia organization in amphibians: chemoarchitecture.

From the dawn of comparative neuroanatomy the central nervous system of the lamprey has attracted the attention of a great number of workers. Owing t o its alleged primitiveness and simplicity’ it was expected that analysis of the brain and spinal cord of these forms would yield the “prototype”2 i.e., the natural starting point for the study of the development and differentation of the vertebrate neuraxis. Hence, many so-called evolutionary trends, like for example the progressive migration, differentiation, segregation, and specialization of neurons and neuron groups, were considered as having their point of departure in the brain of the lamprey or in lamprey-like brains. Unfortunately, many of these trends have been described in a straightforwardly phylogenetic setting without any clear justification for doing so. In fact, all of these trends are naturally based on the study of a single or a few related features and should be considered on that account as typologic rather than phylogenetic. Lampreys have a long, independent phylogenetic history behind them and n o recent group of vertebrates, let alone the entire vertebrate kingdom, can be said t o have arisen from them. However, it cannot be denied that the lampreys retain more features of the presumed ancestral craniates than d o any other members of the group,3 and therefore deserve especial attention. The first part of the present paper will survey the structural organization of the brain and spinal cord of the lamprey. This survey is based on a study of the literature and o n our own Nisslstained and silver-impregnated material of Lampetra fluviatilis. In the second part of this paper the central nervous system of the lamprey will be placed in a comparative perspective. Some of the “trends” referred to above will be review ed. It should be noted that our present-day knowledge of the central nervous system of the lamprey is based on the study of many different species. It stands t o reason that these species differ sufficiently t o be distinguished by the taxonomist. However, neuroanatomically there are no appreciable differences among them; hence, I feel justified in using in the present treatise the term “lamprey” without any further specification.*

The brain of the lamprey in a comparative perspective.

Until recently, the frog telencephalon, like those of other nonmammalian vertebrates, was believed to be dominated by the olfactory system. This misconception may originally have been derived from the relatively large size of the olfactory bulbs and tracts. It was thought that all regions of the telencephalon received olfactory afferents. Domination of the telencephalon by olfaction left little room for telencephalic representation of other sensory modalities: a case in point is vision. The optic tectum was considered to be most important part of the visual system of nonmammalian vertebrates; diencephalic and telencephalic structures were presumed to be little involved in this modality. Herrick (1948) conceived of the amphibian dorsal thalamus as a nucleus sensitivus integrating multimodal afferents and lacking functional subdivisions.

Organization of the “Nonolfactory” Telencephalon

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Organization of the “Nonolfactory” Telencephalon