In the brain, estrogen receptor β (ERβ) plays important roles in autonomic functions, stress reactivity and learning and memory processes. However, understanding the function of ERβ has been restricted by the limited availability of specific antisera, by difficulties discriminating the discrete localization of ERβ-immunoreactivity (ir) at the light microscopic level in many brain regions and the identification of ERβ-containing neurons in neurophysiological and molecular studies. Here, we demonstrate that a Esr2 bacterial artificial chromosome (BAC) transgenic mouse line that expresses ERβ identified by enhanced green fluorescent protein (EGFP) overcomes these shortcomings. Throughout the brain, ERβ-EGFP was detected in the nuclei and cytoplasm of cells, the majority of which resembled neurons. EGFP often extended into dendritic processes and could be identified either natively or following intensification of EGFP using immunolabeling. The distribution of ERβ-EGFP cells in brain closely corresponded to that reported for ERβ protein and mRNA. In particular, ERβ-EGFP cells were found in autonomic brain regions (i.e., hypothalamic paraventricular nucleus, rostral ventrolateral medulla and nucleus of the solitary tract), in regions associated with anxiety and stress behaviors (i.e., bed nucleus of the stria terminalis, amygdala, periaqueductal gray, raphe and parabrachial nuclei) and in regions involved in learning and memory processes (i.e., basal forebrain, cerebral cortex and hippocampus). Additionally, dual label light and electron microscopic studies in select brain areas demonstrate that cell containing ERβ-EGFP colocalize with both nuclear and extranuclear ERβ-immunoreactivity. These findings support the utility of Esr2 BAC transgenic reporter mice for future studies understanding the role of ERβ in CNS function.

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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.

The connectivity of large neurons of the nucleus reticularis gigantocellularis (NRGc) in the medullary reticular formation potentially allows both for the integration of stimuli, in several modalities, that would demand immediate action, and for coordinated activation of cortical and motoric activity. We have simultaneously recorded cortical local field potentials, neck muscle electromyograph (EMG), and the neural activity of medullary NRGc neurons in unrestrained, unanesthetized rats to determine whether the activity of the NRGc is consistent with the modulation of general arousal. We observed excitatory responses of individual NRGc neurons to all modalities tested: tactile, visual, auditory, vestibular, and olfactory. Excitation was directly linked to increases in neck muscle EMG amplitude and corresponded with increases in the power of fast oscillations (30 to 80 Hz) of cortical activity and decreases in the power of slow oscillations (2 to 8 Hz). Because these reticular formation neurons can respond to broad ranges of stimuli with increased firing rates associated with the initiation of behavioral responses, we infer that they are part of an elementary "first responder" CNS arousal mechanism.

In the centennial year of the birth of Hans Selye, this review compares his classical concepts of stress with a modern approach to mechanisms of CNS arousal. Relations between the two concepts are described. Neuroanatomical, neurophysiological, and functional genomic mechanisms underlying CNS arousal are briefly reviewed. Controls over stress responses and arousal are compared to particular concepts of control system engineering. Understanding these two systems is of crucial importance because their dysregulation is associated with large numbers of disease states.