Complex In Turtles Research Articles

Mediation of Vagal Cardioinhibitory Responses by Glutamatergic Receptors in the Caudal Medulla of Turtles

Our previous studies showed that electrical stimulation of the nuclei ambiguous (NA) or dorsomotor nuclei of the vagus (DMV) complex in the brain stem of spontaneously breathing pond turtles (Cyclemys fiavomarginata), anesthetized with chloralose (4 mg/100 g) and urethane (40 mg/100 g), produced a marked slowing or even cessation of the heart rate, and resulted in an immediate fall of blood pressure. Results of the present study further demonstrated that the cardioinhibitory responses could also be elicited by microinjection of monosodium glutamate (0.2-20 nl, 50 mM) into the NA/DMV complex in turtles. A two-barrel glass micropipette held in a manipulator was connected to a pneumatic pressure pump for microinjection. The glutamate-induced cardioinhibitory responses could be significantly reduced in a dose-dependent manner by pretreatment with AP-5 (a NMDA receptor antagonist, at 1-8 nmole) or CNQX (a non-NMDA receptor antagonist; at 0.1-0.8 nmole) 20 min before glutamate administration. Histochemical verification by injecting horseradish peroxidase into the cervical vagus nerves revealed that retrogradely labeled glutamatergic neurons in the NA/DMV complex were observed. These results suggest that glutamatergic receptors in the caudal medulla may mediate vagal cardioinhibitory responses in the turtle.

The Thalamus of Reptiles and Mammals: Similarities and Differences

Certain aspects of thalamic organization in reptiles and mammals are reviewed. Features shared by the dorsal thalamus of reptiles and that of mammals include projection to the telencephalon, specific and non-specific non-telencephalic afferents, and input from the thalamic reticular nucleus. Differences between the dorsal thalamus of reptiles and that of mammals are the absence of reciprocal telencephalic efferents to the dorsal thalamus and lack of local circuit neurons in reptiles (with the exception of the dorsal geniculate complex in turtles) and their presence in mammals. A thalamic reticular nucleus is present in both reptiles and mammals. In both of these classes of vertebrates, this neuronal aggregate surrounds the dorsal thalamus along its lateral surface, projects to the dorsal thalamus, and is organized into sectors. In one group of reptiles, Caiman crocodilus, the sole reptilian group in which immunocytochemical features have been investigated in detail, the reticular nucleus contains at least three neuronal subpopulations: neurons immunoreactive for glutamic acid decarboxylase (GAD); neurons immunoreactive for parvalbumin; and cells that are not immunoreactive for parvalbumin or, probably, GAD. On the other hand, the reticular nucleus of mammals contains a single population of neurons immunoreactive for GAD, gamma amino butyric acid, and parvalbumin.

Effect of ambient temperature on serotonin, norepinephrine, and epinephrine contents in the pineal-paraphyseal complex of the soft-shelled turtle (Lissemys punctata punctata).

The aim of the present investigation was to ascertain the influence of ambient temperature on serotonin, norepinephrine, and epinephrine contents in the pineal-paraphyseal complex of soft-shelled turtles. Turtles were exposed to low (10 degrees) and high (32 degrees) temperatures for 7 days in an incubator with the same photoperiod (12L:12D). Control animals were also kept in the same incubator at 25 degrees for similar duration with the same photoperiod. The findings revealed that ambient temperatures significantly altered serotonin, norepinephrine, and epinephrine content in the pineal-paraphyseal complex of turtles. The results thus indicate that ambient temperature has some influence on pineal serotonin, norepinephrine, and epinephrine contents in soft-shelled turtles.

Organization of retinogeniculate projections in turtles of the genera Pseudemys and Chrysemys.

Organization of retinal projections to the dorsal lateral geniculate complex in turtles has been studied by means of light and electron microscopic axon tracing techniques. Orthograde degeneration studies with Fink-Heimer methods following restricted retinal lesions show the entire retina has a topologically organized projection to the contralateral dorsal lateral geniculate complex. The nasotemporal axis of the retina projects along the rostrocaudal axis of the geniculate complex; the dorsoventral axis of the retina projects along the dorsoventral axis of the geniculate complex. The projection to the ipsilateral dorsal lateral geniculate complex originates from the ventral, temporal and nasal edges of the retina. The nasotemporal axis of the ipsilateral retina projects along the rostrocaudal axis of the geniculate complex. It was not possible to determine the orientation of the dorsoventral axis of the ipsilateral retina on the geniculate complex. Light microscopic autoradiographic tracing experiments and electron microscopic degeneration experiments show the retinogeniculate projection has a laminar organization. Retinogeniculate terminals are found in both the neuropile and cell plate throughout all three subnuclei of the dorsal lateral geniculate complex but have a distinctive distribution in each subnucleus. In the subnucleus ovalis, they are frequent in both the neuropile and cell plate which forms the rostral pole of the complex. In the dorsal subnucleus, they are most prevalent in the outer part of the neuropile layer, less frequent in the inner part of the neuropile, and rare in the cell plate. In the ventral subnucleus, they are frequent in the outer part of the neuropile but are also common in the inner part of the neuropile and cell plate. These observations point to several principles of geniculate organization in turtles. First, the complex receives projections from the entire contralateral retina and a segment of the ipsilateral retina. It thus has monocular and binocular segments that together receive a topologically organized representation of the binocular visual space and the contralateral monocular visual space. Second, the three geniculate subnuclei receive information from different, specialized regions of the retina and visual space. Subnucleus ovalis receives information from the frontal binocular visual field. The ventral subnucleus receives information from the caudal binocular field. The dorsal subnucleus receives input from the contralateral monocular field. Third, there is a lamination of retinal inputs in the geniculate complex which differs in character within the three subnuclei.(ABSTRACT TRUNCATED AT 400 WORDS)

Ultrastructure of the dorsal lateral geniculate complex in turtles of the genera Pseudemys and Chrysemys.

The ultrastructure of the dorsal lateral geniculate complex in the turtles Pseudemys scripta elegans and Chrysemys picta belli has been studied. The majority of neurons have somata situated in a cell plate that forms the medial face of the complex. Relatively few synaptic contacts occur on the somata of cell plate neurons, but three types of axon terminals contact their proximal dendrites as they course through the cell plate. The most frequent are terminals with clear, round synaptic vesicles that form asymmetric junctional complexes. A second type has both clear and round, and dense-cored vesicles and forms asymmetric junctional complexes. The third type has clear, pleomorphic vesicles and forms symmetric junctional complexes. The same three types of terminals are presynaptic to the dendrites of cell plate neurons as they proceed through the inner half of the geniculate neuropile that is interposed between the cell plate and optic tract. These dendrites branch into varicose, finger-like distal dendrites within the outer half of the neuropile. The finger-like processes contain clear, pleomorphic vesicles and are linked with each other via dendrodendritic synapses with symmetric junctional complexes. The distal dendrites are postsynaptic to axon terminals with clear, round synaptic vesicles and asymmetric junctional complexes. Cell plate neurons thus comprise three regions, each with a distinct synaptic organization. The first region is formed by the somata of cell plate neurons and receives few synaptic contacts. The second region is formed by the proximal dendrites both in the cell plate and the inner half of the neuropile. It receives a moderate contingent of axondendritic terminals. The last region is formed by the distal dendrites of the cell plate neurons. They participate in dendrodendritic synapses and are postsynaptic to axon terminals with clear, round synaptic vesicles.

Morphology of retinogeniculate terminals in the turtle, Pseudemys scripta elegans.

The dorsal lateral geniculate complex in turtles receives a bilateral, topographic projection from the retina and projects to the telencephalon. This study examined the morphology of individual retinogeniculate terminals that were filled with horseradish peroxidase by injections in the optic tract or optic tectum. A large number of retinogeniculate terminals were successfully filled and detailed drawings were prepared of 87 terminals. Terminals were classified into three types based on the size and number of varicosities in the terminal, and (if a terminal formed a spatially restricted arbor) the volume of the arbor. Type I retinogeniculate terminals form spatially restricted, large-volume arbors with a low density of large varicosities. Type II retinogeniculate terminals form small volume arbors with a high density of small varicosities. Type III retinogeniculate terminals, in contrast to types I and II, do not form spatially restricted arbors. Rather, they consist of sparsely branched axons that parallel the optic tract and contain scattered en passant varicosities. Plots of the distribution of different terminal types throughout the geniculate complex show that all three terminal types occur throughout the rostrocaudal and mediolateral extents of the complex. However, each terminal type has a preferential distribution with type II terminals being concentrated in the outer half of the neuropile, type I terminals in the inner half of the neuropile, and type III terminals in the cell plate. All three types can arise from axons that continue caudally to terminate in the tectum. These findings raise the possibility that various classes of retinal ganglion cells differ in their mode of termination within the geniculate complex, but the precise relation between the three types of retinogeniculate terminals and the classes of ganglion cells remains to be determined.