Molecular and neuroanatomical characterization of single neurons in the mouse medullary gigantocellular reticular nucleus

Medullary gigantocellular reticular nucleus (mGi) neurons have been ascribed a variety of behaviors, many of which may fall under the concepts of either arousal or motivation. Despite this, many details of the connectivity of mGi neurons, particularly in reference to those neurons with ascending axons, remain unknown. To provide a neuroanatomical and molecular characterization of these cells, with reference to arousal and level-setting systems, large medullary reticular neurons were characterized with retrograde dye techniques and with real-time reverse transcriptase PCR (RT-PCR) analyses of single-neuron mRNA expression in the mouse. We have shown that receptors consistent with participation in generalized arousal are expressed by single mGi neurons and that receptors from different families of arousal-related neurotransmitters are rarely coexpressed. Through retrograde labeling, we have shown that neurons with ascending axons and neurons with descending axons tend to form like-with-like clusters, a finding that is consistent across age and gender. In comparing the two groups of retrogradely labeled neurons in neonatal animals, those neurons with axons that ascend to the midbrain show markers for GABAergic or coincident GABAergic and glutamatergic function; in contrast, approximately 60% of the neurons with axons that descend to the spinal cord are glutamatergic. We discuss the mGi's relationship to the voluntary and emotional motor systems and speculate that neurons in the mGi may represent a mammalian analogue to Mauthner cells, with a separation of function for neurons with ascending and descending axons.

Steady-state methadone effect on generalized arousal in male and female mice

Methadone is widely used in treatment of short-acting opiate addiction. The on-off effects of opioids have been documented to have profound differences from steady-state opioids. The authors hypothesize that opioids play important roles in either generalized arousal (GA) or aversive state of arousal during opioid withdrawal. Both male and female C57BL6 mice received steady-state methadone (SSM) through osmotic pumps at 10 or 20 mg/kg/day, and GA was measured in voluntary motor activity, sensory responsivity, and contextual fear conditioning. SSM did not have any effect on those GA behaviors in either sex. Females had higher activity and less fear conditioning than males. The effects of SSM on stress-responsive orexin gene expression in the lateral hypothalamus (LH) and medial hypothalamus (MH, including perifornical and dorsomedial areas) were measured after the behavioral tests. Females showed significantly lower basal LH (but not MH) orexin mRNA levels than males. A panel of GA stressors increased LH orexin mRNA levels in females only; these increases were blunted by SSM at 20 mg/kg. In summary, SSM had no effect on GA behaviors. In females, SSM blunted the GA stress-induced LH orexin gene expression.

Voltage-dependent calcium channels in ventromedial hypothalamic neurones of postnatal rats: modulation by oestradiol and phenylephrine

Oestradiol actions in the hypothalamus play an important role in reproductive behaviour. Oestradiol treatment in vivo induces alpha(1b)-adrenoceptor mRNA and increases the density of alpha(1B)-adrenoceptor binding in the hypothalamus. Oestradiol is also known to modulate neuronal excitability, in some cases by modulating calcium channels. We assessed the effects of phenylephrine, an alpha(1)-adrenergic agonist, on low-voltage-activated (LVA) and high-voltage-activated (HVA) calcium channels in ventromedial hypothalamic (VMN) neurones from vehicle- and oestradiol-treated female rats. Whole-cell and gramicidin perforated-patch recordings were obtained, with barium as the charge carrier. In the absence of phenylephrine, oestradiol treatment increased the magnitude of LVA currents compared to controls, but had no effect on HVA currents. Phenylephrine enhanced HVA currents in a significantly greater proportion of neurones from oestradiol-treated rats (76%) than from vehicle-treated (41%) rats. The L-channel blocker nifedipine abolished this oestradiol effect on phenylephrine-enhanced HVA currents. Preincubating slices with the N-type channel blocker omega-conotoxin GVIA completely blocked the phenylephrine response, suggesting that the N-type channel is essential. Phenylephrine also stimulated LVA currents in approximately two-thirds of neurones in slices from both vehicle- and oestradiol-treated rats. Our data show that oestradiol increases LVA currents in the VMN. Oestradiol also amplifies alpha(1)-adrenergic signalling by increasing the proportion of neurones showing phenylephrine-stimulated HVA currents mediated by N- and L-type calcium channels. In this way, oestradiol may increase excitatory responses to arousing adrenergic inputs to VMN neurones governing oestradiol-dependent reproductive behaviour.

Oestradiol up-regulates glutamine synthetase mRNA and protein expression in the hypothalamus and hippocampus: implications for a role of hormonally responsive glia in amino acid neurotransmission

Rapidly emerging evidence suggests that glial cells in the central nervous system are sensitive to oestrogen actions. However, the functional consequences of the cellular mechanisms of these cells have proven difficult to study in vivo because of the intimate relationships between neurones and glia. Microarray technology offers the potential to uncover steroid hormone regulation of glial-specific genes that may play a role in hormone-dependent neuronal-glial interactions. Analysis of transcriptomes from the medial basal hypothalamus (MBH) of oestradiol and vehicle-treated adult ovariectomised mice revealed an up-regulation of several glial specific genes by oestradiol, including glutamine synthetase (GS), which facilitates the conversion of glutamate to glutamine and plays an integral role in amino acid neurotransmission. In situ hybridisation confirmed that oestradiol treatment resulted in an up-regulation of GS gene expression in the arcuate and ventromedial nuclei of the MBH, as well as the medial amygdala and hippocampus. Moreover, oestradiol increased protein expression of GS in both the MBH and hippocampus. Neurones are incapable of de novo net synthesis of glutamate from glucose and are dependent on glial-provided precursors such as glutamine to renew their amino acid transmitter pools. Thus, oestradiol induced expression of GS suggests a significant role for glial cells in hormonal modulation of glutamatergic neurotransmission important to female reproductive behaviours, neuroendocrine physiology and cognitive functions.

Acute estradiol application increases inward and decreases outward whole-cell currents of neurons in rat hypothalamic ventromedial nucleus

Acute estradiol (E2) can potentiate the excitatory responses of hypothalamic ventromedial nucleus (VMN) neurons to neurotransmitters. To investigate the mechanism(s) underlying the potentiation, the whole-cell patch voltage clamp technique was used to study VMN neurons in hypothalamic slices prepared from female juvenile (3-5 weeks) rats. A voltage step and/or ramp was applied every 5 min to evoke whole-cell currents before, during and after a treatment with E2 (10 nM), corticosterone (10 nM) or vehicle for up to 20 min. Acute E2 increased inward currents in 38% of neurons tested. Their average peak inward current amplitudes started to increase within 5 min and reached the maximum of 163% of pretreatment level (Pre) at 20 min of treatment before recovering toward Pre. These increases are significantly greater than the Pre and corresponding vehicle controls and non-responsive neurons. Outward currents were decreased significantly by E2 in 27% of E2-treated cells, down to 60% of Pre levels. E2 also appeared to affect the kinetics of the inward and outward currents of estrogen-responsive neurons. Whenever observed, the effects of acute E2 were reversible after a 5- to 10-min washing. Probability analysis indicates that E2 affected the inward and the outward currents independently. The E2 effects are specific in that they were not produced by similar treatment with vehicle or corticosterone. Pharmacological characterizations using ion replacement and channel blockers showed that the inward currents were mediated practically all by Na(+) and the outward currents mainly by K(+). Thus, acute E2 can enhance inward Na(+) and attenuate outward K(+) currents. Since both effects will lead to an increase in neuronal excitability, they may explain our previous observation that E2 potentiates the excitation of VMN neurons.

Sex and estrogenic effects on coexpression of mRNAs in single ventromedial hypothalamic neurons

Regulated gene expression in single neurons can be linked to biophysical events and behavior in the case of estrogen-regulated gene expression in neurons in the ventrolateral portion of the ventromedial nucleus (VMN) of the hypothalamus. These cells are essential for lordosis behavior. What genes are coexpressed in neurons that have high levels of mRNAs for estrogen receptors (ERs)? We have been able to isolate and measure certain mRNAs from individual VMN neurons collected from rat hypothalamus. Large numbers of neurons express mRNA for ERalpha, but these neurons are not identical with the population of VMN neurons expressing the likely gene duplication product, ERbeta. An extremely high proportion of neurons expressing either ER also coexpress mRNA for the oxytocin receptor (OTR). This fact matches the known participation of oxytocin binding and signaling in sexual and affiliative behaviors. In view of data that ER and OTR can signal through PKCs, we looked at coexpression of selected PKCs in the same individual neurons. The most discriminating analysis was for triple coexpression of ERs, OTR, and each selected PKC isoform. These patterns of triple coexpression were significantly different for male vs. female VMN neurons. Further, individual neurons expressing ERalpha could distribute their signaling across the various PKC isoforms differently in different cells, whereas the reverse was not true. These findings and this methodology establish the basis for systematic linkage of the brain's hormone-sensitive signaling pathways to biophysical and behavioral mechanisms in a well studied mammalian system.

Female oxytocin gene-knockout mice, in a semi-natural environment, display exaggerated aggressive behavior

Compared to results from a generation of neuropharmacological work, the phenotype of mice lacking the oxytocin (OT) peptide gene was remarkably normal. An important component of the current experiments was to assay OT-knockout (OTKO) and wild-type (WT) littermate control mice living under controlled stressful conditions designed to mimic more closely the environment for which the mouse genome evolved. Furthermore, our experimental group was comprised of an all-female population, in contrast to previous studies which have focused on all-male populations. Our data indicated that aggressive behaviors initiated by OTKO during a food deprivation feeding challenge were considerably more intense and diverse than aggressive behaviors initiated by WT. From the measures of continuous social interaction in the intruder paradigm, it emerged that OTKO mice were more offensively aggressive (attacking rumps and tails) than WT. In a test of parental behaviors, OTKO mice were 100% infanticidal while WT were 16% infanticidal and 50% maternal. Finally, 'alpha females' (always OTKO) were identified in each experiment. They were the most aggressive, the first to feed and the most dominant at nesting behaviors. Semi-natural environments are excellent testing environments for elucidating behavioral differences between transgenic mice and their WT littermates which may not be ordinarily discernible. Future studies of mouse group behavior should include examining female groupings in addition to the more usual all-male groups.