Nonclassical mechanisms of progesterone action in the brain: I. Protein kinase C activation in the hypothalamus of female rats.

The modulation of gene regulation by progesterone (P) and its classical intracellular regulation by progestin receptors in the brain, resulting in alterations in physiology and behavior has been well studied. The mechanisms mediating the short latency effects of P are less well understood. Recent studies have revealed rapid nonclassical signaling action of P involving the activation of intracellular signaling pathways. We explored the involvement of protein kinase C (PKC) in P-induced rapid signaling in the ventromedial nucleus of the hypothalamus (VMN) and preoptic area (POA) of the rat brain. Both the Ca2+-independent (basal) PKC activity representing the activation of PKC by the in vivo treatments and the Ca+2-dependent (total) PKC activity assayed in the presence of exogenous cofactors in vitro were determined. A comparison of the two activities demonstrated the strength and temporal status of PKC regulation by steroid hormones in vivo. P treatment resulted in a rapid increase in basal PKC activity in the VMN but not the POA. Estradiol benzoate priming augmented P-initiated increase in PKC basal activity in both the VMN and POA. These increases were inhibited by intracerebroventricular administration of a PKC inhibitor administered 30 min prior to P. The total PKC activity remained unchanged demonstrating maximal PKC activation within 30 min in the VMN. In contrast, P regulation in the POA significantly attenuated total PKC activity +/- estradiol benzoate priming. These rapid changes in P-initiated PKC activity were not due to changes in PKC protein levels or phosphorylation status.

MYELINATION IN UNDERNOURISHED RAT BRAIN

Several interactive parameters of protein-calorie malnutrition imposed during postnatal ontogeny on the myelination of rat brain wre investigated. Postnatal starvation depresses the rate of myelin protein synthesis to approximately the same extent in all major brain regions examined (cerebral cortex, cerebellum, striatum, hippocampus, hypothalamus, midbrain and medulla), indicating a relatively uniform reduction in myelination throughout the brain. Early starvation from birth through 8 days, as well as starvation occurring late, from 14 to 30 days, produced no lasting deficit in myelin accumulation. Starvation from birth through 14 days or from birth through 20 days produces lasting, significant myelin deficits in all brain regions when examined following ad libitum feeding to 60 days of age. These data, in combination with the metabolic studies of myelin synthesis, show that severe starvation occurring during the 2nd and 3rd weeks of postnatal life produces an immediate reduction in myelin synthesis, and that the subsequent deficit in myelin accumulation is irreversible by nutritional rehabilitation. With respect to the relative severity of nutritional restriction occurring during this "critical" interval of brain ontogeny, additional studies showed that mild undernourishment (producing less than 20 percent growth lag) produces no myelin deficit. There appears to be a threshold effect such that undernutrition producing a growth lag of between 20 to 30 percent first produces a measurable deficit. Increasingly severe regimens of nutritional restriction which produce approximately 30, 40 and 50 percent body weight lags result in initial myelin deficits of 25, 55 and 60 percent, respectively. Initial myelin deficits do not recover following nutritional rehabilitation, although myelin continues to increase in both normal and all undernourished populations. At the cellular level, severe postnatal nutritional restriction appears to depress both the initial synthesis of myelin precursor proteins (as demonstrated for proteolipid protein) as well as their subsequent assembly into myelin membrane. All of the findings of the present studies are consistent with a hypothetical model of undernutrition-induced brain hypomyelination in which the primary defect consists of a failure of oligodendroglia to myelinate a substantial percentage of axons, resulting in a greatly decreased ratio of myelinated to unmyelinated axons. ^

Function of histaminergic retinopetal axons in rat and primate retinas

Mammalian retinas receive input from histaminergic neurons in the posterior hypothalamus. These neurons are most active during the waking state of the animal, but their role in retinal information processing is not known. To determine the function of these retinopetal axons, their targets in the rat and monkey retina were identified. Using antibodies to three histamine receptors, HR1, HR2, and HR3, the immunolabeling was analyzed by confocal and electron microscopy. These experiments showed that mammalian retinas possess histamine receptors. In macaques and baboons, diurnal species, HR3 receptors were found at the apex of ON-bipolar cell dendrites in cone pedicles and rod spherules, sclerad to the other neurotransmitter receptors that have been localized there. In addition, HR1 histamine receptors were localized to large puncta in the inner plexiform layer, a subset of ganglion cells and retinal blood vessels. In rats, a nocturnal species, the localization of histamine receptors in the retina was markedly different. Most HR1 receptors were localized to dopaminergic amacrine cells and on elements in the rod spherule. To determine how histaminergic retinopetal axons contribute to retinal information processing, responses of retinal ganglion cells to histamine were analyzed. The effects of histamine on the maintained and light-evoked activity of retinal ganglion cells were analyzed. In monkeys, histamine and the HR3 agonist, methylhistamine, increased or decreased the maintained activity of most ganglion cells, but a few did not respond. The responses of a subset of ganglion cells to light stimuli were decreased by histamine, a finding suggesting that histaminergic retinopetal axons contribute to light adaptation during the day. In rats, histamine nearly always increased the maintained activity and produced both increases and decreases in the light responses. The effects of histamine on maintained activity of ganglion cells in the rat can be partially attributed to HR1-mediated changes in the activity of dopaminergic amacrine cells, at night. Together, these experiments provide the first indication of the function of retinopetal axons in mammalian retinas. ^