Evidence for a frontal-organ homologue in the pineal complex of the salamander, Hynobius dunni

Hideki Takahama

doi:10.1007/bf00318564

Neural elements of the pineal complex of the frog, rena esculenta, I: Centrally projecting neurons

Visual Neuroscience ◽

10.1017/s0952523800005150 ◽

1990 ◽

Vol 4 (05) ◽

pp. 389-397 ◽

Cited By ~ 3

Author(s):

Peter Ekström ◽

Hilmar Meissl

Keyword(s):

Pineal Organ ◽

Photoreceptor Cells ◽

Rana Esculenta ◽

Projection Neurons ◽

Pineal Complex ◽

Bipolar Neuron ◽

Neural Organization ◽

Hand Functions ◽

Frontal Organ ◽

Pineal Tract

AbstractThe pineal complex of anuran &hibians is a directly photosensory organ, encompassing both an extracranial portion, the frontal organ, and an intracranial portion, the pineal organ proper. The projection neurons of the frontal organ respond differentially according to the wavelengths of the light stimuli. The pineal organ, on the other hand, functions mainly as a luminosity meter. Most of its centrally projecting neurons respond to all increases in ambient illumination with decreases in spontaneous firing of action potentials, although some neural units in the pineal organ may respond according to wavelength. This difference in responses to light stimulation may be reflected in the neural organization of the two parts of the pineal complex. In the present study, we have analyzed the morphology of the projection neurons of the frontal and pineal organs of the frog,Rana esculenta, by backfilling of the neurons with horseradish peroxidase through their cut axons. In the pineal organ, several types of centrally projecting neurons were observed: peripherally situated unipolar and multipolar neurons, the dendrites of which extend into a superficial axon plexus that surrounds the pineal epithelium; smaller unipolar, bipolar, or multipolar neurons situated close to the central pineal tract; and radially oriented bipolar neurons, with short dendritic processes oriented towards the lumen of the pineal organ. This latter type was strongly reminiscent of photoreceptor cells. The centrally projecting neurons of the frontal organ were multipolar, and situated in the ventral part of the organ. One photoreceptor-like bipolar neuron was observed in one frontal organ. The neurons of the frontal organ did not form a superficial plexus of neurites. This difference may relate to the different ratio of chromaticity/luminosity units in the frontal and pineal organs.

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Neural elements in the pineal complex of the frog, Rana esculenta, II: GABA-immunoreactive neurons and FMRFamide-immunoreactive efferent axons

Visual Neuroscience ◽

10.1017/s0952523800005162 ◽

1990 ◽

Vol 4 (05) ◽

pp. 399-412 ◽

Cited By ~ 17

Author(s):

P. Ekström ◽

T. östholm ◽

H. Meissl ◽

A. Bruun ◽

J.G. Richards ◽

...

Keyword(s):

Pineal Organ ◽

Sensory System ◽

Rana Esculenta ◽

Posterior Commissure ◽

Major Pathway ◽

Pineal Complex ◽

Frontal Organ ◽

Pineal Nerve ◽

Rostral Portion ◽

Immunoreactive Axons

AbstractThe photosensory pineal complex of anurans comprises an extracranial part, the frontal organ, and an intracranial part, the pineal organ proper. Although the pineal organ functions mainly as a luminosity detector, the frontal organ monitor the relative proportions of short and intermediate/long wavelengths in the ambient illumination. The major pathway of information processing in the pineal and frontal organs is the photoreceptor to ganglion cell synapse. It is not known whether interneurons form part of the neural circuitry. In the present study, we demonstrate GABA-immunoreactive (GABA-IR) neurons in the pineal and frontal organs of the frog,Rana esculenta. No GABA-IR axons were observed in the pineal nerve between the frontal and pineal organs, or in the pineal tract that connects the pineal complex with the brain. The GABA-IR neurons differed in morphology from centrally projecting neurons visualized by retrograde labeling with horseradish peroxidase. Thus, we suggest that the GABA-IR neurons in the pineal and frontal organs represent local interneurons.Axons of central origin, immunoreactive with a sensitive antiserum against the tetrapeptide Phe-Met-Phe-Arg-NH2(FMRFamide), were observed in the intracranial portion of the photosensory pineal organ. The immunoreactive axons enter the caudal pole of the pineal organ via the posterior commissure. The largest density of axons was observed in the caudal part, while fewer axons were detected in the rostral portion. The uneven distribution of the FMRFamide-immunoreactive axons may be related to the distribution of different types of intrapineal neurons. FMRFamide-immunoreactive varicose axons were observed in the extracranial frontal organ. A central innervation of the pineal organ, previously known exclusively from amniotes, is probably notper selinked with the evolutionary transition of the pineal organ from a directly photosensory organ to a neuroendocrine organ. It could rather represent a centrifugal input to a sensory system which has been retained when the directly sensory functions have changed, during phylogency, to neuroendocrine functions.

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