1997;764:1C8. neurons in the hippocampus. Working out rats receiving an infusion of nonblocking serum showed normal increases in the number of new hippocampal neurons after exercise. Thus, increased uptake of blood-borne IGF-I is necessary for the stimulatory effects of exercise on the number of new granule cells in the adult hippocampus. Taken together with previous results, we conclude that circulating IGF-I is an important determinant of exercise-induced changes in the adult brain. Male Wistar rats (300 gm) from our in-house colony were used. Because the aim of the study was to determine the role of IGF-I on exercise-induced changes in the number of new neurons in the hippocampus, we followed a protocol comparable to that originally reported by van Praag et al. (1999) to determine the effects of exercise on the number of new neurons in the adult hippocampus. In a first series of experiments we administered an IGF-I infusion through a subcutaneous osmotic minipump (Alzet 2001; 50 g??kg?1??d?1, rate of infusion 1 l/hr) for 7 d to normal rats, as described (Fernandez et al., 1998). Control animals received a saline infusion. Simultaneously, both IGF-I-infused (3-Carboxypropyl)trimethylammonium chloride and control animals received daily intraperitoneal injections of BrdU (Sigma, St. Louis, MO) dissolved in 0.9% NaCl at a dose of 50 mg/kg for 7 d. Animals were (3-Carboxypropyl)trimethylammonium chloride anesthetized and killed either 24 hr (short-term) or 3 weeks (long-term) after the last injection of BrdU (days 8 and 30, respectively). BrdU+ cells found after short-term and long-term survival will include newly formed cells that have survived up to 1 1 week and 1 month, respectively. In a second series of experiments aimed to determine the role of endogenous blood-borne IGF-I in exercise-induced changes in the (3-Carboxypropyl)trimethylammonium chloride number of new neurons in the adult hippocampus, we administered an infusion of a blocking anti-IGF-I antiserum (20% in saline; Alzet osmotic minipump 1002, subcutaneously implanted, with an infusion rate of 0.25 l/hr) to a group of rats running in a treadmill for 15 d (see below). The control group of exercising animals received an infusion of nonimmune normal rabbit serum (NRS; 20% in saline) because pilot experiments indicated that exercising rats receiving either a saline infusion or NRS show identical (3-Carboxypropyl)trimethylammonium chloride increases in the number of labeled cells in the hippocampus. All animals received BrdU injections during the 15 d of the study. Thereafter, one group was killed 24 hr after the last injection of BrdU (day 16) to determine short-term survival of BrdU+ cells, whereas a second group was killed 2 weeks later for long-term survival analysis (day 30). In all cases, a decrease in the number of BrdU+nuclei in long-term cell survival experiments was found as compared with short-term studies. This indicates a natural cell death process of the newly given birth to cells along time (Cameron et al., 1993). (3-Carboxypropyl)trimethylammonium chloride The anti-IGF-I antiserum used for blocking experiments has 1% cross-reactivity with either insulin or IGF-II, as determined by competition with125I-IGF-I. IGF-I levels in the CSF were determined by radioimmunoassay as described (Carro et al., 2000). CSF samples (150C200 Mmp23 l) were obtained from the cisterna magna. Animals were familiarized with the treadmill apparatus (Letica, Germany) to minimize novelty stress and then divided in two groups: exercised and nonexercised. The procedures followed are.