Regulation of cyclic AMP level by progesterone in ovariectomized rat neocortex.

Exposure of neocortical slices to progesterone, without prior treatment with estrogen, augmented forskolin-induced cyclic AMP within 15 min. 30 nM progesterone produced approximately 1/2 the maximal effect but as little as 10 nM progesterone produced a detectable increase in cyclic AMP. When forskolin was replaced by dideoxyforskolin, an analog that does not directly stimulate adenylyl cyclase but shares many of its other actions, progesterone did not augment cyclic AMP. Progesterone also failed to affect increased cyclic AMP that followed exposure to norepinephrine or isoproterenol. The effect of progesterone upon cyclic AMP was also evident when tetrodotoxin was added to block voltage-dependent sodium channels, suggesting that intercellular communication that is dependent upon action potentials was not necessary. The effect of progesterone was at least partially blocked by antagonists of GABAA receptor action, suggesting the involvement of GABAA or GABAA-like receptors. The effect of progesterone was also not homogeneous over the neo cortex. While forskolin-stimulated cyclic AMP was augmented by progesterone in the parietal and occipital regions, it was suppressed in the frontal region. These results are envisioned as a progesterone action upon a small and perhaps compartmentalized component of the cellular cyclic AMP system, an effect that is made detectable in our whole-tissue assay by the well known ability of forskolin to potentiate many hormonal effects upon cyclic AMP.

Regulation of gamma-aminobutyric acidB (GABAB) receptors in cerebral cortex during the estrous cycle.

We examined binding of the GABAB receptor agonist baclofen to brain synaptic membranes as a function of the natural variations in gonadal steroids that occur during the estrous cycle of the adult rat. We found that the binding of baclofen to neocortical membranes varied systematically as a function of the estrous cycle, with the lowest binding occurring during the estrus stage. Binding to archicortical (hippocampal) and hypothalamic preparations also varied with the estrous cycle, except that the lowest level of binding in these latter cases occurred during the diestrus stage. The variation of [3H]baclofen binding during the estrous cycle was different with respect to the binding of [3H]muscimol, an agonist for GABAA receptors, and [3H]8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), an agonist for serotonin 5-HT1A receptors that shares similar G proteins and effectors with GABAB receptors. Saturation binding studies of cortical GABAB receptors showed that apparent receptor density (Bmax) rather than affinity (Kd) best accounted for the change in binding during the estrous cycle in that Bmax, like total specific binding, was at a minimum during the estrus stage. The robust regulation of GABAB receptors in neocortex was unexpected and its functional significance is at present unknown. However, the correlation of the menstrual cycle with mood and other behavioral changes, and the correlations of the estrous and menstrual cycles with seizure susceptibility, may somehow depend upon hormonal regulation of transmitter systems such as the one we have observed here.

Sodium channel density in hypomyelinated brain increased by myelin basic protein gene deletion.

Trophic control over the expression and membrane distribution of voltage-dependent ion channels is one of the principal organizing events underlying the maturation of excitable cells. The myelin sheath is a major structural determinant of regional ion channel topography in central axons, but the exact molecular signals that mediate local interactions between the oligodendrocyte and axolemma are not known. We have found that large caliber fibre pathways in the brain of the mutant mouse shiverer (shi, gene on chromosome 18), whose developmental fate of myelination is averted by deletion of five exons in the myelin basic protein gene, have a striking excess of sodium channels. As cytoplasmic membranes of shiverer oligodendroglia still adhere to axons, the evidence indicates that myelin basic protein or a myelin basic protein-dependent glial transmembrane signal associated with compact myelin formation, rather than a simple glial-axon contact inhibition or an intrinsic genetic program of neuronal differentiation, could be critical in downregulating sodium channel density in axons. Here we use the shiverer mutant to show that mature central nervous system projection neurons with large caliber unmyelinated fibres sustain functional excitability by increasing sodium channel density. This axon plasticity, triggered by the absence of a single glial protein, contributes to the unexpectedly mild degree of neurological impairment in the mutant brain without myelin, and may be a potentially inducible mechanism determining the recovery of function from dysmyelinating disease.

Androgen-induced sexual dimorphism in high affinity dopamine binding in the brain transcends the hypothalamic-limbic region.

1 High affinity binding of [3H]-dopamine and [3H]-5-hydroxytryptamine ([3H]-5-HT) was measured in membrane fractions prepared from cerebral cortex, amygdala, hypothalamus, thalamus and brain stem of rats of either sex and of rats which had been either neonatally castrated or androgenized. 2 Binding was measured in rats of 8, 20 and 30 days old as well as in adults. 3 [3H]-dopamine bound with approximately 30 nM affinity ahd [3H]-5-HT with approximately 10 nM affinity to all areas of the brain tested. The relative inhibitory effects of haloperidol, apomorphine, cis-flupenthixol, unlabelled dopamine, noradrenaline, spiroperone, (+)-butaclamol, fluphenazine, pimozide and 5-HT on [3H]-dopamine binding in the cerebral cortex was consistent with receptor status for the binding components there as were the relative inhibitory effects of methysergide, dopamine, fluoxetine and ouabain on [3H]-5-HT binding in the fore brain. 4 Neither [3H]-dopamine nor [3H]-5-HT binding varied with the state of the sexual cycle in females. 5 There were no sexual differences in [3H]-5-HT binding in any of the brain areas tested nor was it affected by neonatal androgenization or neonatal castration. 6 [3H]-dopamine binding was greater in the cerebral cortex and amygdala of male than of female rats. These differences could be mimicked artificially by neonatal castration of males (female type development) or neonatal androgenization of females (male type development). Sexual dimorphism did not become overt until 20 days of age and did not extend to hypothalamus, thalamus or brain stem. 7 It is concluded that neonatal sex differences in exposure to steroid hormones has permanent effects on the number of dopamine binding sites in the cerebral cortex and is suggested that this sexual dimorphism extends to the amygdala.