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Sixteen rats were initially exposed for 50 sessions to either a fixed-ratio 40 or an interresponse-time-greater-than-l1-second food reinforcement schedule, then shifted to a fixedinterval 15-second food reinforcement schedule. Animals with fixed-ratio 40 histories lever pressed at much higher rates under the fixed-interval schedule than did animals with interresponse-time-greater-than-11-second histories. This difference persisted across 93 sessions of fixed-interval exposure. The effects of d-amphetamine were assessed after 15 and 59 sessions of fixed-interval exposure. On both occasions, the low-rate responding of animals with interresponse-time-greater-than-l1-second histories was typically increased by all doses of the drug, while the high-rate responding of animals with fixed-ratio 40 histories was typically decreased by all doses of the drug. These results suggest that control response rate under the fixed-interval schedule, which may be affected by a history of responding under another schedule, is the primary determinant of the relative effects of d-amphetamine.Key words: operant history, fixed-interval schedule, fixed-ratio schedule, IRT greaterthan-t schedule, rate dependency, d-amphetamine, lever press, ratsThe effects of most behaviorally-active drugs on a given behavior depend critically on the steady-state characteristics of that behavior in the absence of the drug (Kelleher and Morse, 1968; Thompson and Schuster, 1968). Of particular importance is the rate of occurrence of the behavior. The effects produced by a number of compounds are so strongly influenced by response rate that they are termed rate-dependent (Dews, 1958;Dews and Wenger, 1977;Kelleher and Morse, 1968). d-Amphetamine prototypically exemplifies rate-dependency: with few exceptions, low doses of this drug decrease high-rate behaviors while increasing low-rate behaviors (e.g., Clark and Steele, 1966;Dews, 1958;Poling, Urbain, and Thompson, 1977;Smith, 1964).A number of factors may influence the rate of occurrence of a particular behavior. Rein-

Brains of nonmammalian vertebrates typically contain multiple forms of gonadotropin-releasing hormone (GnRH). Until recently, only the mammalian form of GnRH (mGnRH) had been isolated in placental mammals. Biochemical and histological data show that both mGnRH and chicken-II GnRH (cGnRH-II) are present in a primitive placental mammal, the musk shrew (Suncus murinus). Similar to the case in nonmammalian species, in the musk shrew, neurons that express cGnRH-II are located in a discrete cluster in the midbrain. We have used a combination of radioimmunoassay and immunocytochemistry, analyzed at the light level and with electron microscopy, to describe the distribution of cGnRH-II cell bodies and fibers in the musk shrew brain. All cGnRH-II-immunoreactive (ir) neurons reside in the midbrain, and this area contains the greatest concentration of cGnRH-II peptide in the brain. At the light and electron micrographic levels, we have identified synaptic terminals containing dense core vesicles that are immunoreactive for cGnRH-II in the medial habenula. Radioimmunoassay reveals that this region contains the second greatest concentration of cGnRH-II in the brain. Widely scattered cGnRH-II-ir fibers are present throughout the forebrain, particularly in the medial septum, hypothalamus, and midbrain central gray. Scant cGnRH-II fibers are present in the median eminence, arcuate nucleus, and infundibular stem, and only low concentrations of the peptide are detected in these areas. Finally, intravenous administration of mGnRH is ten times more effective than cGnRH-II in promoting ovulation.(ABSTRACT TRUNCATED AT 250 WORDS)

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The distribution of cells and fibers immunoreactive (ir) for either chicken gonadotropin-releasing hormone I (cGnRH I; [Gln8]GnRH) or II ([His5,Trp7,Tyr8]GnRH) was determined in brains of turkey hens to reveal whether these peptides occur in separate neuronal systems. ir-cGnRH I cells were located: along the medial aspect of the ventriculus lateralis, nucleus accumbens, and bed nucleus of the stria terminalis; ventral to the tractus septomesencephalicus and extending medially to the third ventricle, and caudally into the lateral hypothalamic area; and in a diffuse band extending from the nucleus preopticus medialis to the nucleus dorsomedialis anterior thalami. cGnRH I fibers were evident in these areas in addition to the hippocampus, nucleus subhabenularis medialis, nucleus ventromedialis hypothalami, and median eminence. Two groups of ir-cGnRH II cells were observed: a magnocellular group lying between the substantia grisea centralis and the nucleus ruber; and a parvicellular group lying medial to the nucleus of the basal optic root and extending into the lateral hypothalamic area. ir-cGnRH II fibers were prominent in limbic structures (cortex piriformis, lateral to nucleus taeniae, hippocampus); olfactory areas (tuberculum olfactorium, nucleus subhabenularis lateralis, nucleus septalis lateralis); areas that in other avian species have steroid-concentrating cells or receptors (medial edge of lobus parolfactorius, nucleus septalis medialis, nucleus periventricularis magnocellularis, nucleus dorsomedialis posterior thalami); and areas containing ir-GnRH I cells or fibers but not in median eminence. These results suggest that cGnRH I and II occur in separate neuronal systems and that cGnRH II does not directly promote pituitary gonadotropin secretion.