Enactment as a concept can serve analytic discourse through its established meaning of an act intended strongly to influence, persuade, or force another to react. We might agree to use the term in two complementary ways: Broadly, enactment can designate all behaviors of both parties in the analytic relationship, even verbal, in consequence of the intensification of the action intent of our words created by the constraints and regressive push induced by the analytic rules and frame. Patient and analyst are vulnerable to falling back on behaviors that actualize their intentions, doing so in ways motivated by and reflecting transference hopes, fears, and compromises shaped in their developmental past. Specifically, enactment can then be defined as those regressive (defensive) interactions between the pair experienced by either as a consequence of the behavior of the other. While nominally an interpersonal perspective, this concept of enactment facilitates more balanced attention to the involvement of both parties and to the intrapsychic dynamics in both that specifically shape their interactions. A clinical vignette illustrates the analyst's contributions to enactment, especially those reflecting his reactivated conflicts and their relation to his theoretical and technical preferences.

The nicotinic acetylcholine receptor (AChR) is a pentameric transmembrane protein (alpha 2 beta gamma delta) that binds the neurotransmitter acetylcholine (ACh) and transduces this binding into the opening of a cation selective channel. The agonist, competitive antagonist, and snake toxin binding functions of the AChR are associated with the alpha subunit (Kao et al., 1984; Tzartos and Changeux, 1984; Wilson et al., 1985; Kao and Karlin, 1986; Pederson et al., 1986). We used site-directed mutagenesis and expression of AChR in Xenopus oocytes to identify amino acid residues critical for ligand binding and channel activation. Several mutations in the alpha subunit sequence were constructed based on information from sequence homology and from previous biochemical (Barkas et al., 1987; Dennis et al., 1988; Middleton and Cohen, 1990) and spectroscopic (Pearce and Hawrot, 1990; Pearce et al., 1990) studies. We have identified one mutation, Tyr190 to Phe (Y190F), that had a dramatic effect on ligand binding and channel activation. These mutant channels required more than 50-fold higher concentrations of ACh for channel activation than did wild type channels. This functional change is largely accounted for by a comparable shift in the agonist binding affinity, as assessed by the ability of ACh to compete with alpha-bungarotoxin binding. Other mutations at nearby conserved positions of the alpha subunit (H186F, P194S, Y198F) produce less dramatic changes in channel properties. Our results demonstrate that ligand binding and channel gating are separable properties of the receptor protein, and that Tyr190 appears to play a specific role in the receptor site for acetylcholine.

A return to Freud's broader concepts of transference and psychic reality as general psychological principles would include applying both concepts to the analyst as well as to the patient and thus obviate the need for the concept of countertransference.

We have investigated the molecular determinants responsible for ␣-bungarotoxin (␣Bgtx) binding to nicotinic acetylcholine receptors through chimeric analysis of two homologous ␣ subunits, one highly sensitive to ␣Bgtx block (␣1) and the other, ␣Bgtx-insensitive (␣3). By replacing rat ␣3 residues 184 -191 with the corresponding region from the Torpedo ␣1 subunit, we introduced a cluster of five ␣1 residues (Trp-184, Trp-187, Val-188, Tyr-189, and Thr-191) into the ␣3 subunit. Functional activity and ␣Bgtx sensitivity were assessed following co-expression in Nicotinic acetylcholine receptors (nAChRs)1 are multimeric ligand-gated ion channels expressed on skeletal muscle cells and on select groups of nerve cells in the peripheral and central nervous systems (1-3). Muscle nAChRs have pentameric structures made up of two ␣1 subunits and one each ␤1, ␥, and ␦(or ⑀) subunits; they are among the best characterized ion channels and serve as a model for understanding the structure and function of related ligand-gated channels responding to glycine, ␥-aminobutyric acid, and 5-hydroxytryptamine (4). Advances in the characterization of muscle nAChRs have been significantly aided by the discovery of a high affinity competitive antagonist, ␣-bungarotoxin (␣Bgtx). ␣Bgtx is used extensively in experiments on the molecular properties of nAChRs and for following expression, targeting, and clustering of these receptors on muscle during synapse formation (1-4). Much less is known about nAChRs on neurons, in part because comparable antagonists are in limited quantity or are nonexistent. The purpose of this paper is to define the amino acid residues that are essential for high affinity ␣Bgtx binding through a chimeric subunit approach, by conferring ␣Bgtx sensitivity to a neuronal ␣ subunit that is normally insensitive to ␣Bgtx. Previous work indicates that the main ␣Bgtx binding site is between residues 173 and 204 on the ␣1 subunit of the muscle nAChR. Specifically, studies of peptides derived from the Torpedo ␣1 sequence capable of binding ␣Bgtx with sub-micromolar affinity suggest that the major determinants of toxin binding are located in a region adjacent to the vicinal cysteines 192 and 193 (e.g. see Ref. 5). Recent studies of heterologously expressed muscle nAChRs have identified residues in this region of the native receptor that appear to interact with the short ␣-neurotoxin I from Naja mossambica mossambica (NmmI (6, 7)) and with ␣Bgtx (8, 9). Residues in this region are also involved in forming the binding sites for agonists and non-␣-neurotoxin antagonists (6, 7, 10 -12). In such studies, singlesite mutations in the muscle type ␣1 subunit have not been very helpful in fully defining the ␣-neurotoxin binding site in the native nAChR, as most mutations studied fail to produce large changes in ␣Bgtx affinity (6 -9). Therefore, in this study, rather than eliminate ␣Bgtx binding, we have used site-directed mutagenesis of a neuronal nAChR to introduce a toxin binding site. As a consequence, we identified the molecular determin...

The molecular mechanisms that couple agonist binding to the gating of Cys-loop ionotropic receptors are not well understood. The crystal structure of the acetylcholine (ACh) binding protein has provided insights into the structure of the extracellular domain of nicotinic receptors and a framework for testing mechanisms of activation. Key ligand binding residues are located at the C-terminal end of the ␤9 strand. At the N-terminal end of this strand (loop 9) is a conserved glutamate [E 172 in chick ␣7 nicotinic acetylcholine receptors (nAChRs)] that is important for modulating activation. We hypothesize that agonist binding induces the movement of loop 9. To test this, we used the substituted-cysteine accessibility method to examine agonist-dependent changes in the modification of cysteines introduced in loop 9 of L 247 T ␣7 nAChRs. In the absence of agonist, ACh-evoked responses of E 172 C/L 247 T ␣7 nAChRs were inhibited by 2-trimethylammonioethylmethane thiosulfonate (MTSET). Agonist coapplication with MTSET reduced the extent and rate of modification. The dose-dependence of ACh activation was nearly identical with that of ACh-dependent protection from modification. ACh increased the inhibition by methanethiosulfonate reagents of N 170 C and did not change inhibition of G 171 C receptors. The antagonist dihydro-␤-erythroidine did not mimic the effects of ACh. Combined with a structural model, the data suggest that receptor activation includes subunit rotation and/or intrasubunit conformational changes that move N 170 to a more accessible position and E 172 to a more protected position away from the vestibule. Thus, loop 9, located near the junction between the extracellular and transmembrane domains, participates in conformational changes triggered by ligand binding.