The insular formations of the dolphin brain: quantitative cytoarchitectonic studies of the insular component of the limbic lobe.

The large insula of the bottlenose dolphin consists of radial gyri arising, in fanlike fashion, from the transverse insular gyrus, and is covered completely by the frontal, parietal, and temporal opercula . On cytoarchitectonic grounds, the dolphin insula is divided into anterior, middle, and posterior sectors that may be the equivalent of the three similar sectors present in the primate insula. Rostrocaudally, these sectors become increasingly more homogeneous and less laminated. Within each sector progressive differentiation occurs in the direction of the circular sulcus. A transitional cortex, the peripaleocortex in the transverse insular gyrus, is interposed between the prepiriform and the periamygdalar cortex and the proisocortex of the insula proper. This peripaleocortex consists of outer and inner cellular strata separated by a hypocellular lamina dissecans. The outer cell stratum is continuous with layers II and III of the insular proisocortex ; the more prominent inner stratum is continuous with proisocortical layers V and VI; the intervening lamina dissecans becomes partially filled, mostly with modified pyramidal cells of medium size that may constitute an incipient layer IV. A band of myelinated fibers corresponding to the external band of Baillarger is found within the lamina dissecans. The anterior insular sector is characterized by distinct lamination and a well-defined, ribbonlike layer Va. In the middle sector, the cortex is internodense and lamination is less clear. The posterior sector is even less laminated and tends to be externodense . Within each sector, lamination becomes clearer in the direction of the circular sulcus. Furthermore, the rostrocaudal architectonic changes suggest a possible transition from a motor-type to a sensory-type cortex. Beyond the insula, the architecture of the opercular cortices reflect, in turn, the influences of the insular sectors.

The limbic lobe of the dolphin brain: a quantitative cytoarchitectonic study.

In these cytoarchitectonic studies of the cortical limbic formations of the bottlenose dolphin and other whale brains we have carried out quantitative analyses of the entire limbic lobe, including all of its sectors: supracallosal, retrosplenial and temporal. The limbic lobe proper has been examined as well as transitional areas between the limbic lobe and the archicortical and paleocortical formations and the extralimbic neocortices, including the entorhinal area and presubiculum. Analyses include total cortical thickness, thickness of individual cortical layers, overall cortical cell densities and glia/neuron ratios, individual laminar counts and glia/neuron ratios and neuron size. Comparisons have been made between these parameters in the brains of the dolphin (Tursiops truncatus), beluga whale (Delphinapterus leucas) and humpback whale (Megaptera novaeangliae). Cortical neuron density values (cells per mm3) in these three species of whales and in the human brain have been compared with similar data given in the literature for elephant, fin whale (Balaenoptera physalus) and human brains. Our values reflect the inverse relationship between brain size and neuron density. Thus, the dolphin shows approximately 13,000 neurons/mm3 in its limbic cortex, compared to 12,000 in the beluga whale and 8,000 in the humpback whale. Further, the data provide the first quantitative accounts on a layer by layer basis of the limbic cortices in the whale brain. In the dolphin, the anterior limbic cortices have a much lower cell density than the posterior limbic area. However, in the humpback whale these two cortices have similar neuron densities. In the temporal region, the entorhinal area is well differentiated into many architectonic subdivisions in the dolphin though not to the extent described in the primate brains. Our findings in the three whale species are discussed in terms of their possible significance and provide quantitative data for future comparative studies with other mammalian species.

The anatomy of the brain of the bottlenose dolphin (Tursiops truncatus). Rhinic lobe (Rhinencephalon): The archicortex.

The hippocampal formation or archicortical division of the rhinecephalon of the bottlenose dolphin, Tursiops truncatus, is described from the standpoint of its gross topographic relations and cytoarchitecture. A feature of the dolphin brain, which lacks olfactory bulbs and peduncles, is the striking reduction of the archicortical relative to the paleocortical formations. The small, poorly developed archicortex covered by massive epihippocampal portions of the hemispheres (parietal and temporal lobes), appears greatly reduced relative to the large, well developed olfactory lobes which are covered by small epistriatal portions of the hemispheres (orbital lobes). The archicortex exhibits three junctional zones with the paleocortex, two laterally in the unci and one anteriorly in the septal area. Despite the small size of the hippocampal formations, the general topographic disposition of its cytoarchitectonic areas and their cellular organization in Tursiops have many features that are similar to those in other placental mammals. The archicortex is subdivisible into four major sectors: temporal, retrosplenial, supracallosal and subcallosal. With the exception of the temporal sector, cytoarchitectonic areas of the other sectors are variously attenuated and poorly differentiated, particularly the dentate area and the hippocampal areas H5 and H4. Here, the dentate area and hippocampal areas H5 and H4 which are present along the paradentate bank of the hippocampal sulcus, extend to the level of the oblique sulcus of the parahippocampal gyrus and then disappear. Hippocampal areas H3, H2 and H1 are also clear in the floor and along the parahippocampal bank of the hippocampal sulcus in the temporal sector. These areas are less definable as they extend beyond the oblique sulcus into the retrosplenial sector and are difficult to recognize as distinct areas in the supracallosal and subcallosal sectors of the archicortex. The archicortex is demarcated bilaterally from limbic formations in the border of the hemisphere by segments of the rhinic cleft which are very clear. Equally clear is the cytoarchitectonic demarcation of the archicortex from the neocortex in the border (limbus) of each hemisphere, i.e., where the subiculum abuts against the presubiculum. The subicular area, best expressed in the temporal sector, extends anteriorly over the corpus callosum to the subcallosal gyrus and, throughout its extent from the uncal to the septal junction, is clearly demarcated from limbic neocortex by a transition zone characterized by archicortical cells merging with cells in the deep layer of the bordering neocortex. Overall, the archicortical formations of the dolphin and other whale brains we have examined exhibit many regional peculiarities that we have described, both grossly and architectonically, with emphasis on the comparative anatomical approach.

Classical conditioning of hippocampal theta patterns in the rat.

Electrical stimulation (88 HZ) of the lateral hypothalamus elicited a sustained theta response at hippocampal recording sites of rats immobilized with succinylcholine. By pairing this unconditioned stimulus with a 10-sec presentation of a light, conditioned theta responses were demonstrated in as few as 40 trials. Spectral analysis of hippocampal bioelectric patterns during acquisition, extinction, and reconditioning indicated that the earliest change as a result of conditioning is a loss of power in EEG frequency below 8 HZ, followed by the development of a peak at 8 HZ with further conditioning. Extinction was associated with an increase in power in the frequencies below 8 HZ. When the conditioned rats were tested in the absence of the neuromuscular blocking agent, the conditioned stimulus elicited a theta response that was associated with slow motor activity on 70% of the trials.

Relationship between hippocampal theta activity and running speed in the rat.

The frequency of occurrence and amplitude of hippocampal theta waves induced by forced locomotion is proportional to speed of movement on a treadmill. Although induction of hippocampal theta waves is related to the initiation of movement, it is not dependent upon proprioceptive feedback because it persists in the resting animal after a bout of running. It is possible to obtain cortical theta waves in the absence of hippocampal theta activity.

Reversal of morphine tolerance after medial thalamic lesions in the rat.

Tolerance, manifested by a diminished electroencephalographic response at cortical and subcortical recording sites, was found in rats subjected to repeated systemic injections of morphine sulfate. Reversal of tolerance to morphine resulted from destruction of the medial thalamus.