Table 2.

| Kinetic constants for σ₁ receptor ligand binding. All data were obtained by scintillation proximity assay (SPA) at 23 °C. Errors represent standard deviation.

Constant	[3H](+)-pentazocine	[3H]haloperidol
Kd (nM):	22.5 ± 10.8	N.D.
kslow (min−1)*:	(3.2 ± 0.6) × 10−3	(4.4 ± 0.2) × 10−3
kfast (min−1) †
at 1 nM:	(1.6 ± 0.6) × 10−2	(2.8 ± 1.6) × 10−2
3 nM:	(2.3 ± 0.3) × 10−2	(3.3 ± 0.2) × 10−2
10 nM:	(2.6 ± 0.4) × 10−2	(4.3 ± 0.8) × 10−2
30 nM:	(3.0 ± 0.5) × 10−2	N.D.
100 nM:	(3.4 ± 0.4) × 10−2	N.D.
300 nM:	(3.4 ± 0.4) × 10−2	N.D.
koff (min−1) ‡
at 1 nM:	(9.4 ± 6.1) × 10−4	(4.2 ± 1.6) × 10−4
3 nM:	(10.0 ± 7.0) × 10−4	(5.1 ± 0.4) × 10−4
10 nM:	(8.0 ± 5.8) × 10−4	(4.9 ± 0.6) × 10−4
30 nM:	(6.7 ± 4.8) × 10−4	N.D.
100 nM:	(5.1 ± 3.6) × 10−4	N.D.
300 nM:	(4.3 ± 2.9) × 10−4	N.D.

^*k_slow is the observed rate constant for the slow step for ligand association to the σ₁ receptor in the biphasic fit.

^†k_fast is the observed rate constant for the fast step in ligand association to the σ₁ receptor in the biphasic fit.

^‡k_off is the observed off rate for a given concentration in the SPA measurements, based on a monophasic fit.