Ultrastructure of the Frog Pars Intermedia in Relation to Hypothalamic Control of Hormone Release

Pituitary intermediate lobe melanocyte-stimulating hormone (MSH) is responsible for normal skin darkening in amphibians. Light-background adapted frogs (Rana pipiens) injected with naloxone and placed on black backgrounds maintain melanophore indices and pituitary cytology characteristic of light-background adaptation. In vitro and in vivo experiments showed that naloxone hydrochloride did not have a direct effect on skin melanophores or on the neurointer-mediate lobe. These data suggest that naloxone acts at the level of the central nervous system to inhibit the mechanism(s) responsible for release of MSH when light-background adapted frogs are placed on a dark background. Release of MSH, known to be tonically inhibited by the hypothalamus, may be modulated by opiate receptor-dependent mechanisms.

Section: Significantly (P<005) Less Than the Control Group (Fig Ib)mentioning

confidence: 84%

“…3B) showed some areas of expanded rough endoplasmic retic ulum, suggesting new peptide synthesis such as that ob served after normal long-term dark adaptation [13,14]. Cells of the pituitary pars intermedia ( fig.…”

Section: Significantly (P<005) Less Than the Control Group (Fig Ib)mentioning

confidence: 97%

Naloxone Prevents Dark-Background Adaptation in Amphibians

Mennin

Saland

1980

“…In fact, this apparent duality may have a structural counterpart in the regional innervation of the intermediate lobe. Several investi gators have confirmed that nerve terminals found in the intermediate lobe appear to be distributed so as to form two more-or-Iess distinct regions in a sagittal section of the lobe [Saland, 1968;Nakai and Gorbman, 1969;Doerr-Schott and Follenius, 1970;Hopkins, 1971], In the region of the intermediate lobe bordering the pars nervosa both 'adrenergic type' and 'neurosecretory type' terminals are found, while away from this border (more ventrally) nerve terminals appear to be almost exclusively of the 'adrenergic type'. The majority of electrode placements in this study were undoubtedly in the latter region, oc cupied primarily by adrenergic terminals.…”

Section: Discussionmentioning

confidence: 98%

Light-Evoked Neural Activity in the Intermediate Lobe of the Pituitary of <i>Rana pipiens</i>: Electrophysiological Evidence for a Neural Pathway Linking the Eyes With the Pars Intermedia

Dawson¹,

Ralph²

1974

Neural activity was recorded from the pars inter-media of Rana pipiens whose heads were fastened in a moldedplastic holder equipped with fiber optic light guides for the selective illumination of the lateral eyes and the pineal region. The major findings of this study are as follows: Illumination of the eyes influences the discharge of some neurons which terminate in the pars intermedia; Single units which responded to illumination of the lateral eyes were ‘on-type’, i.e. they responded to the onset of illumination with a burst of impulses and discharged at an increased average rate throughout the period of illumination; ‘On-type’ units were binocular, responding to illumination of either the right eye or the left eye individually; The latencies of responses to light suggest that a neural pathway links the retinae of the lateral eyes with the pars intermedia; and Illumination of the pineal region had no detectable effect upon neural activity in the pars intermedia and no effect of illumination could be detected in blinded animals. The results of this study indicate that a neural pathway exists connecting the lateral eyes with the intermediate lobe of the frog pituitary. These observations are consistent with the hypothesis of Hogben and Slome [1936] that the lateral eyes provide the photic information which regulates the output of the pars intermedia, and they provide the most compelling evidence yet obtained for a retinohypophysial neural pathway.

“…Color changes in amphibians arc said to be regulated principally by MSH produced by the pars intermedia; submicroscopic analyses of the pars intermedia cells have revealed that MSH is synthesized on the rough-surfaced ER, condensed in the Golgi region, and stored in the form of secretory granules in the cytoplasm [Ito , 1968;Saland, 1968; I m a i, 1969|.…”

Section: Discussionmentioning

confidence: 99%

Changes in Skin Color and Fine Structure of the Intermediate Pituitary Gland of the Frog, <i>Rana nigromaculata</i>, after Extirpation of the Median Eminence

Ito¹

1971

The pars intermedia was investigated by electron microscopy, with relation to changes in skin color, in frogs (Rana nigromaculata) that were first adapted to a white background, then subjected to extirpantion of the median eminence, and subsequently kept on the white background for various periods. It was found that: 1. Following extirpation, the animals begin to darken on a white background and to lose their background adaptability. After 2–3 h, the extirpated animals become intensely dark and remain so for at least 2 weeks. From 3 or 4 weeks on, such animals begin to adapt to their background again and slowly blanch, although they do not lighten at 6 months as fully as do intact animals at the end of that period. 2. The pars intermedia cells at the irreversible, dark stage of the extirpated animals show a progressive decrease in the number of secretory granules in the cytoplasm, followed by an incease in the amount of rough-surfaced endoplasmic reticulum. In the 2-day animals, there is a marked diminution of the secretory granules and a slight expansion of the rough-surfaced endoplasmic cisternae. At this stage, degenerative changes appear in the ordinary nerve endings that make synaptic contact with the pars intermedia cells. 3. The pars intermedia cells at the adaptive, light stage of the extirpated animals indicate the successive accumulation of secretory granules in the cytoplasm, followed by the reduction of rough-surfaced endoplasmic reticulum. The appearance of the cells of the 6-month animals seems very similar to that seen in the preoperative, white-background-adapted animal. At this stage, the degenerated synapses rapidly progress toward normal. These findings were discussed with particular respect to the hypothalamic neural inhibition of the pars intermedia, leading to the conclusion that this inhibition appears to operate first on the release mechanism of melanophore-stimulating hormone (MSH) in the gland through the ordinary, possibly adrenergic, nerve fibers originating in the hypothalamus. Furthermore, it was suggested that the synthesis of MSH is regulated by a feedback mechanism activated by the hormone stored in the gland.