MAP kinase (ERK) translates cell surface signals into alterations in transcription. We have found that ERK also regulates hippocampal neuronal excitability during 5 Hz stimulation and thereby regulates forms of long-term potentiation (LTP) that do not require macromolecular synthesis. Moreover, ERK-mediated changes in excitability are selectively required for some forms of LTP but not others. ERK is required for the early phase of LTP elicited by brief 5 Hz stimulation, as well as for LTP elicited by more prolonged 5 Hz stimulation when paired with beta1-adrenergic receptor activation. By contrast, ERK plays no role in LTP elicited by a single 1 s 100 Hz train. Consistent with these results, we find that ERK is activated by beta-adrenergic receptors in CA1 pyramidal cell somas and dendrites.

␤-Adrenergic receptors (␤-ARs) are members of the superfamily of G-protein-coupled receptors that mediate the effects of catecholamines in the sympathetic nervous system. Three distinct ␤-AR subtypes have been identified (␤1-AR, ␤2-AR, and ␤3-AR). In order to define further the role of the different ␤-AR subtypes, we have used gene targeting to inactivate selectively the ␤2-AR gene in mice. Based on intercrosses of heterozygous knockout (␤2-AR ؉/؊) mice, there is no prenatal lethality associated with this mutation. Adult knockout mice (␤2-AR ؊/؊) appear grossly normal and are fertile. Their resting heart rate and blood pressure are normal, and they have a normal chronotropic response to the ␤-AR agonist isoproterenol. The hypotensive response to isoproterenol, however, is significantly blunted compared with wild type mice. Despite this defect in vasodilation, ␤2-AR ؊/؊ mice can still exercise normally and actually have a greater total exercise capacity than wild type mice. At comparable workloads, ␤2-AR ؊/؊ mice had a lower respiratory exchange ratio than wild type mice suggesting a difference in energy metabolism. ␤2-AR ؊/؊ mice become hypertensive during exercise and exhibit a greater hypertensive response to epinephrine compared with wild type mice. In summary, the primary physiologic consequences of the ␤2-AR gene disruption are observed only during the stress of exercise and are the result of alterations in both vascular tone and energy metabolism.

The molecular basis of mechanosensory transduction by primary sensory neurones remains poorly understood. Amongst candidate transducer molecules are members of the acid-sensing ion channel (ASIC) family; nerve fibre recordings have shown ASIC2 and ASIC3 null mutants have aberrant responses to suprathreshold mechanical stimuli. Using the neuronal cell body as a model of the sensory terminal we investigated if ASIC2 or 3 contributed to mechanically activated currents in dorsal root ganglion (DRG) neurones. We cultured neurones from ASIC2 and ASIC3 null mutants and compared response properties with those of wild-type controls. Neuronal subpopulations [categorized by cell size, action potential duration and isolectin B4 (IB4) binding] generated distinct responses to mechanical stimulation consistent with their predicted in vivo phenotypes. In particular, there was a striking relationship between action potential duration and mechanosensitivity as has been observed in vivo. Putative low threshold mechanoreceptors exhibited rapidly adapting mechanically activated currents. Conversely, when nociceptors responded they displayed slowly or intermediately adapting currents that were smaller in amplitude than responses of low threshold mechanoreceptor neurones. No differences in current amplitude or kinetics were found between ASIC2 and/or ASIC3 null mutants and controls. Ruthenium red (5 microm) blocked mechanically activated currents in a voltage-dependent manner, with equal efficacy in wild-type and knockout animals. Analysis of proton-gated currents revealed that in wild-type and ASIC2/3 double knockout mice the majority of putative low threshold mechanoreceptors did not exhibit ASIC-like currents but exhibited a persistent current in response to low pH. Our findings are consistent with another ion channel type being important in DRG mechanotransduction.

The activation state of ␤-adrenergic receptors (␤-ARs) in vivo is an important determinant of hemodynamic status, cardiac performance, and metabolic rate. In order to achieve homeostasis in vivo, the cellular signals generated by ␤-AR activation are integrated with signals from a number of other distinct receptors and signaling pathways. We have utilized genetic knockout models to test directly the role of ␤1-and/or ␤2-AR expression on these homeostatic control mechanisms. Despite total absence of ␤1-and ␤2-ARs, the predominant cardiovascular ␤-adrenergic subtypes, basal heart rate, blood pressure, and metabolic rate do not differ from wild type controls. However, stimulation of ␤-AR function by ␤-AR agonists or exercise reveals significant impairments in chronotropic range, vascular reactivity, and metabolic rate. Surprisingly, the blunted chronotropic and metabolic response to exercise seen in ␤1/ ␤2-AR double knockouts fails to impact maximal exercise capacity. Integrating the results from single ␤1-and ␤2-AR knockouts as well as the ␤1-/␤2-AR double knockout suggest that in the mouse, ␤-AR stimulation of cardiac inotropy and chronotropy is mediated almost exclusively by the ␤1-AR, whereas vascular relaxation and metabolic rate are controlled by all three ␤-ARs (␤1-, ␤2-, and ␤3-AR). Compensatory alterations in cardiac muscarinic receptor density and vascular ␤3-AR responsiveness are also observed in ␤1-/␤2-AR double knockouts. In addition to its ability to define ␤-AR subtype-specific functions, this genetic approach is also useful in identifying adaptive alterations that serve to maintain critical physiological setpoints such as heart rate, blood pressure, and metabolic rate when cellular signaling mechanisms are perturbed.