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.

alpha2-Adrenergic receptors (alpha2ARs) are essential components of the neural circuitry regulating cardiovascular function. The role of specific alpha2AR subtypes (alpha2a, alpha2b, and alpha2c) was characterized with hemodynamic measurements obtained from strains of genetically engineered mice deficient in either alpha2b or alpha2c receptors. Stimulation of alpha2b receptors in vascular smooth muscle produced hypertension and counteracted the clinically beneficial hypotensive effect of stimulating alpha2a receptors in the central nervous system. There were no hemodynamic effects produced by disruption of the alpha2c subtype. These results provide evidence for the clinical efficacy of more subtype-selective alpha2AR drugs.

␤-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 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.