252 counting frame 15 ×15, which resulted in 300–500 markers per animal. Because of the relatively sparse occurrence and frequent clustering of Ki-67-positive and BrdU-positive cells, these cells were counted manually by means of a modified stereological procedure using a Zeiss microscope (×200 magnification) and multiplied by 10 to estimate the total number of Ki-67-positive and BrdU-positive cells in the DG (Oomen et al. 2007; van Praag et al. 1999). Dentate gyrus granule cell layer (GCL) and molecular layer surface area and volume measurements were performed according to Cavalieri’s principle using the appropriate tools for this in the StereoInvestigator system. Effects of maternal deprivation on the individual granule cell morphology were determined by analysing the dendritic tree of Golgi-stained neurons using 3D reconstruction software. For this purpose, 8- to 13-week- old CON (n = 7) and MD (n = 7) animals were decapitated in the morning between 9 and 10 a.m., and trunk blood was collected in EDTA-covered tubes (experiment I). Immediately after decapitation, brains were rapidly removed and cut into two hemispheres. One hemisphere was used for Golgi–Cox impregnation and the other for electrophysiological recordings (experiment IV). For Golgi–Cox impregnation, a similar procedure was used as described previously (Bagot et al. 2009; Boekhoorn et al. 2006; Champagne et al. 2008). Immediately after decapitation, brains were incubated in a Golgi–Cox solution (5% K2CrO4, 5% HgCl and 5% K2Cr2O7) for 28 days, after which they were imbedded in celloidin and cut into 200- μm-thick sections. From each animal, Z-stacks (step size 1 μm) from five to seven dentate granule cells were generated using a confocal microscope (LSM510, Zeiss, Germany) in bright field mode (×20 objective) and reconstructed in ImagePro in combination with the Neuro- draw toolbox (kindly provided by G. Ramakers, J. van Heerikhuize and C. Pool, Netherlands Institute for Neuro- science, Amsterdam). Criteria for inclusion were as follows: (1) only neurons from the supra-pyramidal blade of the rostral dentate gyrus, from bregma −2.5 to −4.0 (Paxinos and Watson 1986), were selected; (2) cells had to be evenly filled, without any severed dendrites exiting the section; and (3) only cells from the middle third of the granular cell layer were chosen to avoid the inclusion of relatively young or newborn granule cells residing in the inner part of the cell layer, or cells formed early in ontogeny (E21–22) which reside in the outer third part of the cell layer (Altman and Bayer 1990a, b). Total dendritic length, number of branch points and the number of primary dendrites were analysed for every neuron. In addition, spine density was determined in two segments of dendrites at a distance of 90–110 μm (proximal) and 190– 210 μm (distal) from the soma. Psychopharmacology (2011) 214:249–260 Experiment III. Electrophysiological properties of the dentate gyrus To determine the effects of maternal deprivation on electrophysiological properties of the DG network, 59 8- to 13-week-old female rats (n= 7 or eight per group) were decapitated in the morning between 9 and 10 a.m. For these experiments, stress from individual housing was avoided by decapitating the last two animals from the same cage simultaneously. Immediately after decapitation, the brain was rapidly removed and cut into two hemispheres. The left hemisphere was collected for Golgi–Cox impregnation (experiment III) and the right hemisphere was collected for electrophysiological recordings in ice-cold artificial cerebrospinal fluid (aCSF) containing 120 mM NaCl, 3.5 mM KCl, 1.3 mM MgSO4·7H2O, 1.25 mM NaH2PO4, 2.5 mM CaCl2·2H2O, 10 mM glucose and 25 mM NaHCO3, oxygenated with 95% O2 and 5% CO2. Coronal slices (400 μm) were cut using a vibratome (Leica VT1000S) at 5°C and then kept in oxygenated aCSF at room temperature for at least 2 h prior to recording. Sections containing the rostral part (bregma −2.5 to −4.0; Paxinos and Watson 1986) of the hippocampal dentate gyrus were placed in a recording chamber maintained at 30–32°C with a constant flow of oxygenated aCSF. Field excitatory postsynaptic potentials (fEPSPs) were recorded as previously described (Bagot et al. 2009; Pu et al. 2007) in the absence or presence of the GABAergic antagonist bicuculline methiodide (Tebu-bio, 10 μM). fEPSPs were evoked using a stainless steel bipolar stimulation electrode (60 μm diameter, insulated except for the tip) positioned in the medial perforant pathway and recorded through a glass electrode (2–5 MΩ impedance, filled with aCSF) positioned in the middle third of the molecular layer of the upper blade. A stimulus–response curve was generated by gradually increasing the stimulus intensity to define a level that generated the half-maximal response which was used for the remainder of the experiment. Once the input–output curve for each recording was established, baseline synaptic transmission was monitored (0.017 Hz) during 20 min. When recordings were stable, theta burst stimulation (TBS; four pulses of 100 Hz followed by a 200-ms interval, followed by another four pulses) was applied. This sequence was repeated five times with a 30-s interval. After TBS, the degree of potentiation was determined by recording the fEPSP every minute during 1 h (0.017 Hz). The magnitude of the fEPSP was assessed by analysing the slope of the signal. To determine whether the presence of the stress hormone corticosterone during stimulation modulates the degree of long-term potentiation (LTP), corticosterone (Sigma- Aldrich, The Netherlands; 100 nM, dissolved in 0.01%