Inhibition of the rat brain sodium channel Nav1.2 after prolonged exposure to gabapentin

Prolonged exposure of neurons to gabapentin inhibits repetitive firing of Na⁺-dependent action potentials. Here, we studied the effect of such prolonged exposure to gabapentin on a rat sodium channel, Nav1.2. After 3 days of continuous incubation with gabapentin (10–1000 μM), Nav1.2 current density was decreased dose-dependently relative to untreated cells. The reduction was 57% at 30 μM gabapentin, while higher concentrations (100–1000 μM) did not result in greater effects. Prolonged treatment with gabapentin also caused the channel to inactivate at more hyperpolarized potentials. These effects provide a mechanistic basis for the inhibition of Na⁺-dependent repetitive firing upon prolonged exposure to gabapentin and may contribute to its anticonvulsant activity.     

Effects of lead exposure during development on neocortical dendritic and synaptic structure

The effect of lead (Pb) on neocortical dendritic and synaptic development was examined in rats. Newborn pups were indirectly exposed to Pb by placing 4% Pb carbonate in their mother's diet from postnatal days 1 to 25. The mean brain weight of the Pb-treated animals was reduced 13.2%; neocortical thickness was reduced 13.9%. For analysis of dendritic development, layer V pyramidal cells from area 3 of the sensorimotor cerebral cortex were examined using the Scholl method to determine the number of dendritic branches at 20-μm intervals from the cell body. Although there were no differences in the number of processes leaving the cell body, reductions in dendritic branches were observed at distances greater than 40 μm, reaching significance at 80 and 100 μm. In addition the length of the primary apical dendrite was reduced 5.6% in Pb-treated animals. Synaptic parameters were examined in the molecular layer of the occipital cortex in ethanol phosphotungstic acid-stained tissue. There was a 22.7% reduction in the number of synapses per 15,000× field. No significant differences were observed in the following synaptic parameters measured at 300,000×: presynaptic length and thickness, postsynaptic length and thickness, and cleft width.     

A dual moveable stimulating electrode and its application to the behavioral version of the collision test

The construction and testing of a dual moveable electrode for chronic stimulation in small mammals are described. The assembly is lightweight (0.9 g), its components are completely re-usable, and the two electrodes may be independently advanced in steps as small as 80 μm over a maximum vertical excursion of 6 mm. The unit performed as well as fixed electrodes do in daily behavioral tests conducted over a 3 month period. Ipsiversive rotation was elicited by stimulation of 15 pontine-mesencephalic sites in a single rat. Behavioral evidence for the collision of heterodromic action potentials in circling-relevant fibers was obtained in 4 position sets.     

Synaptic organization of the mammalian olfactory glomerulus: new findings including an intraspecific variation

The olfactory glomeruli of several mammalian species have been analyzed in serial thin sections. The types and frequencies of glomerular synaptic connections were determined in fields at least 8 μm × 8 μm × 7 μm in size.

In all animals examined, axons of olfactory receptors synapsed onto mitral and tufted cell dendrites which were both pre- and postsynaptic to dendrites of periglomerular cells. Receptor cell axons synapsed onto periglomerular cell dendrites of the Mus musculus molossinus, dwarf Netherlands rabbit, and Sprague-Dawley rat, but receptor cell axons did not synapse onto periglomerular cell dendrites of the Balb/c mouse. Synapses between two periglomerular cell dendritic segments were noted in the rabbit, rat, and Balb/c mouse.

We conclude that the synaptic arrangements found in the olfactory glomeruli of the molossinus, rabbit and rat are characteristic of the mammalian olfactory glomerulus, and that the synaptic connections of the Balb/c mouse represent a modification of this basic pattern. The finding that the olfactory glomeruli of two strains of mice possess different synaptic connections implies that data derived from even closely related experimental animals should be compared with caution.     

Cerebral blood flow increases evoked by electrical stimulation of rat cerebellar cortex: relation to excitatory synaptic activity and nitric oxide synthesis

The purpose of this study was to examine mechanisms involved in the coupling of neuronal activity to cerebral blood flow (CBF). CBF was measured in rat cerebellum using laser-Doppler flowmetry during stimulus-evoked neuronal activity and related to the distribution of the extracellular field potential. Local electrical stimulation of the cerebellar cortex activated a narrow beam of parallel fibers (PFs) 100 μm across and evoked increases of CBF along (On-B) and perpendicular (Off-B) to the beam. Increases of CBF and field potentials were recorded for a distance of up to 1500 μm along the activated beam, and perpendicular to the beam, in a zone approximately 1000 μm wide, i.e. about 10 times wider than the zone in which synaptic excitation took place. CBF increased as a function of stimulus frequency up to 75 Hz, the response being larger On-B than Off-B. TTX abolished both the field potentials and the CBF responses at all frequencies, suggesting that action potentials were mechanistically related to the evoked CBF increases. CBF changes were unchanged by picrotoxin, a blocker of GABA_A receptors, consistent with the idea that inhibitory synaptic activity does not contribute to CBF increases. The latency to the CBF rise was much shorter On-B than Off-B for the same distance from the stimulating electrode. This may suggest that the CBF response Off-B is dependent on diffusion of vasoactive substances from neuronal structures activated by the parallel fibers On-B. Nitric oxide (NO) synthase inhibition withN^G-nitro-l-Arginine increased the time latency to onset of CBF rise by 2–4 times and attenuated the evoked CBF increase by approximately 50%. Sodium nitroprusside, a NO donor, increased baseline CBF, but did not reverse the effects ofl-NNA. Thus the initial part of the evoked CBF rise is probably mediated by NO, which also contributes to the later part of the response. This study provides insight into the distribution and mechanism of neurally evoked increases of CBF, of putative importance for the interpretation of activation studies in animals and humans.     

Protein tyrosine kinase inhibitors reduce high-voltage activating calcium currents in CA1 pyramidal neurones from rat hippocampal slices

We have investigated the effects of protein tyrosine kinases (PTKs) inhibitors on high-threshold voltage activating (HVA) calcium currents in CA1 pyramidal neurones, whole-cell patch-clamp recorded from rat hippocampal slices. Genistein (100 μM) and tyrphostin B42 (100 μM), two PTKs inhibitors, reduced the steady-state barium current (I_Ba). On the other hand, daidzein and genistin (100 μM), two inactive analogues of genistein, had no effect on I_Ba amplitude. The inhibition induced by genistein was more pronounced at negative potentials. In order to characterize the calcium channels subtypes inhibited by PTKs inhibitors, we examined the effect of genistein in the presence of different calcium channel blockers. When L-type calcium channels were blocked by nifedipine, genistein induced a strong inhibition of the nifedipine-resistant I_Ba, suggesting an effect on non-L-type channels. Genistein did not antagonize the depressant effect of ω-Conotoxin-GVIA, a selective N-type calcium channel blocker, suggesting that N-type channels were not blocked by genistein. ω-Conotoxin-MVIIC (3–10 μM), a selective P/Q-type calcium channel blocker, greatly antagonized the depressant effect of genistein. Our results suggest that PTKs inhibitors reduce P-/Q-type, but not L- or N-types calcium currents in neurones of the CNS. The possible modulation of calcium channels by endogenous PTKs is discussed.     

Cerebral hemodynamics during cortical spreading depression in rabbits

Effects of a sibgle cortical spreading depression (CSD), elicited by KCl microinjection, on diameter of pial arterioles and venules in the parieto-occipital cortex were examined in urethane-anesthetized adult rabbits using a closed cranial window. The velocity of CSD propagation was2.7±0.1mm/min (mean±S.E.M.). All arterioles (n=39) except for those in the retrosplenial region (n=6) increased their diameter significantly during CSD. The arteriolar dilation lasted for1.5±0.1min. Location of dilating arteriole and propagating CSD showed that they were always closely associated temporally. As a percentage change, diameters of smaller arterioles significantly increased (from60 ± 1to103 ± μm, 71%, n = 12) more than those of larger ones (from82 ± 2to129 ± 3 μm, 57%, n = 27). While venules with initial diameter of85 ± 4 μm(n = 5) did not dilate, those with initial diameter of49 ± 3 μm increased to57 ± 3 μm(16%, n = 8) for1.4 ± 0.2 min during CSD. The majority of the dilated venules started to increase their diameter after nearby arterioles had dilated maximally. Pial arterioles, which dilated during ipsilateral CSD, decreased their diameter significantly from78 ± 2to72 ± 3 μm(8%, n = 11) during contralateral CSD for13.8 ± 3.6min with similar onset latencies as those observed for the dilation. Indomethacin pretreatment significantly enhanced arteriolar dilation during CSD (from73 ± 4to138 ± 6 μm, 89%, n =4). The results indicate that pial arteriolar dilation observed during CSD is an active response, and probably caused by an excitatory rather than inhibitory effect accompanying CSD, and that prostanoids may play an important modulatory role.     

The distribution and morphological characteristics of the intracortical VIP-positive cell: An immunohistochemical analysis

Using a sensitive immunohistochemical procedure, we have undertaken a detailed morphological characterization of the vasoactive intestinal peptide-positive (VIP-positive) neuron in the cerebral cortex of the rat. VIP-positive neurons are present in all regions of cortex, and are usually strongly bipolar, possessing long, radially directed processes with very limited branching in the tangential plane. The most extensive dendritic branching occurs in layers I and deep IV-superficial V, and the density of axonal varicosities is highest in layers II–IV.In the visual cortex, approximately 50% of the labeled cell bodies are in layer II and III and 80% of the labeled cell bodies are contained in layers I–IV (superficial 600 μm of cortex). In order to determine the density and 3-dimensional distribution pattern of these cells, we prepared serial tangential sections through the rat visual cortex, mapped the distribution of all labeled cells in each section on transparent acetate sheets, and compressed these superimposed maps. This analysis demonstrated that: (1) approximately 1% of the cortical neurons are VIP-positive, (2) their distribution is fairly uniform and statistically random, (3) there are no large areas (i.e. with a diameter greater than 100 μm) that lack a VIP-positive cell, (4) on the average, there is one VIP-positive cell per column of 30 μm diameter, and (5) the average nearest neighbor distance on the compressed display is 15 μm.Given the morphological characteristics of VIP-positive cells, these data indicate that each VIP-containing cell is identified with a unique radial volume, which is generally between 15 and 60 μm in diameter, and overlaps with the contiguous domains of neighboring VIP-positive cells. These morphological data support the notion that VIP-containing neurons play an important functional role within radially oriented columns of cerebral cortex.     

Latex beads phagocytosis capacity and ecto-NAD+ glycohydrolase activity of rat brain microglia cells in vitro

Conditions are described which allow the preparation in vitro of pure (> 95%) microglial cell cultures isolated from newborn rat brain. Such ameboid cells cultivated in vitro can efficiently phagocytize opsonized latex beads and are capable of ingesting more (100–200 beads of 1.1 μm diameter per cell) and larger (6.4 μm) particles than other nerve cells, such as oligodendrocytes and astroglia. The microglial cells also show an important ecto-NAD⁺ glycohydrolase activity which is characteristic of phagocytic cells. We noted that the phagocytic capacity and ecto-NAD⁺ glycohydrolase of these cells were correlated and increased notably during the in vitro culture.

Microglia cultivated in vitro appear to be a good model to study the activation of phagocytic properties in the central nervous system and corresponding modulation by natural or pharmacological immunomodulators.     

Prostacyclin release from the coronary vascular wall by vasoactive substances

Which vasoactive substances that are synthesized in vivo could induce the release of a sufficient amount of prostacyclin (PGI₂) to inhibit platelet aggregation from the vascular wall was investigated in the isolated dog heart perfused by a modified method of Langendorff. Infusion of 5 μM bradykinin or 25 u/ml crude thrombin into the heart for 30 sec resulted in the transient appearance of inhibitory activity of platelet aggregation. The inhibitory activity was stable at alkaline pH but unstable at acidic pH and thermolabile. The appearance of the inhibitory activity was prevented by treatment of the coronary vessel with 30 μM indomethacin or 1 mM tranylcypromine. These results indicated that the inhibitory activity was caused by PGI2. When 25 μM acetylcholine, 25 μM noradrenaline, 25 μM isoproterenol, 10 μM adenosine triphosphate (ATP 5 μM adenosine, 1 μM angiotensin II, 25 μM histamine or 1 μM serotonin was infused for 30 sec, no inhibitory activity of platele aggregation was observed. Bradykinin (5 × 10⁻⁹ 5 × 10⁻⁶ M) and purified thrombin (1 × 10⁻⁹ 1 × 10⁻⁷ M) induced a dose-dependent release of PGI₂ which was assayed using a radioimmunoassay for 6-keto-prostaglandin F₁ (6-keto-PGF₁).     

Currents evoked by GABA and glycine in acutely dissociated neurons from the rat medial preoptic nucleus

The responses of acutely dissociated medial preoptic neurons to application of GABA and glycine were studied using the perforated-patch whole-cell recording technique under voltage-clamp conditions. GABA, at a concentration of 1 mM, evoked outward currents in all cells (n=33) when studied at potentials positive to −80 mV. The I–V relation was roughly linear. The currents evoked by GABA were partially blocked by 25–75 μM picrotoxin and were also partially or completely blocked by 100–200 μM bicuculline. Glycine, at a concentration of 1 mM, did also evoke outward currents in all cells (n=12) when studied at potentials positive to −75 mV. The I–V relation was roughly linear. The currents evoked by glycine were largely blocked by 1 μM strychnine. In conclusion, the present work demonstrates that neurons from the medial preoptic nucleus of rat directly respond to the inhibitory transmitters GABA and glycine with currents that can be attributed to GABA_A receptors and glycine receptors respectively.     

Block of the endplate acetylcholine receptor channel by the sympathomimetic agents ephedrine, pseudoephedrine, and albuterol

Recent observations suggest that some patients with congenital myasthenic syndromes respond favorably to ephedrine, pseudoephedrine, or albuterol. Conventional microelectrode studies, however, provide no clear explanation for a beneficial effect of ephedrine in endplate diseases. To gain further insight into how these drugs affect neuromuscular transmission, we investigated their effects on the kinetic properties of the acetylcholine (ACh) receptor. Single channel currents were recorded from rat lumbrical muscles endplates using low concentrations of ACh and 2.5–100 μM of drugs. Between 10–100 μM, each drug progressively increased the rate of channel closure in a concentration dependent manner, consistent with an open-channel block. Albuterol acted as a sequential fast-acting channel blocker, increasing the mean burst duration in a concentration dependent manner without altering the total open time per burst or the duration of intraburst blockages. Increasing concentrations of ephedrine and pseudoephedrine also increased the number of intraburst closures but decreased the total open time per burst. None of the drugs altered single channel conductance. The channel blocking effects of ephedrine and pseudoephedrine might reduce the synaptic overactivity that occurs in the slow-channel myasthenic syndromes or in endplate ACh esterase deficiency, but these effects occur at concentrations not attainable in clinical practice.     

Different spatial distributions of sodium channels in the slowly and rapidly adapting stretch receptor neuron of the crayfish

Inward Na⁺ currents were studied, using a two-microelectrode intracellular voltage-clamp technique, in the slowly adapting (SA) and rapidly adapting (RA) stretch receptor neurons of the crayfish after the axons were cut at different distances from the soma. In the SA neuron, inward Na⁺ currents were recorded in the soma even when the axon was cut as close as 100 μm from the center of the soma, indicating the presence of Na⁺ channels in these parts. Also, two populations of Na⁺ channels seem to exist in the SA neuron. In the RA neuron, only minute Na⁺ currents were observed if the axon was shorter than 250 μm. The results strongly indicate that the voltage-gated Na⁺ channels in the SA and RA neurons have different distributions and that the difference in the spatial distribution of Na⁺ channel types may be important for the difference in firing properties in the two types of neurons.     

TRPC channels underlie cholinergic plateau potentials and persistent activity in entorhinal cortex

Persistent neuronal activity lasting seconds to minutes has been proposed to allow for the transient storage of memory traces in entorhinal cortex and thus could play a major role in working memory. Nonsynaptic plateau potentials induced by acetylcholine account for persistent firing in many cortical and subcortical structures. The expression of these intrinsic properties in cortical neurons involves the recruitment of a non‐selective cation conductance. Despite its functional importance, the identity of the cation channels remains unknown. Here we show that, in layer V of rat medial entorhinal cortex, muscarinic receptor‐evoked plateau potentials and persistent firing induced by carbachol require phospholipase C activation, decrease of PIP₂ levels, and permissive intracellular Ca²⁺ concentrations. Plateau potentials and persistent activity were suppressed by the generic nonselective cation channel blockers FFA (100 μM) and 2‐APB (100 μM), as well as by the TRPC channel blocker SKF‐96365 (50 μM). However, plateau potentials were not affected by the TRPV channel blocker ruthenium red (40 μM). The TRPC3/6/7 activator OAG did not induce or enhance persistent firing evoked by carbachol. Voltage clamp recordings revealed a carbachol‐activated, nonselective cationic current with a heteromeric TRPC‐like phenotype. Moreover, plateau potentials and persistent firing were inhibited by intracellular application of the peptide EQVTTRL that disrupts interactions between the C‐terminal domain of TRPC4/5 subunits and associated PDZ proteins. Altogether, our data suggest that TRPC cation channels mediating persistent muscarinic currents significantly contribute to the firing and mnemonic properties of projection neurons in the entorhinal cortex. © 2010 Wiley‐ Liss, Inc.     

The islands of Calleja complex of rat basal forebrain II: Connections of medium and large sized cells

The connections of the medium (10–20 μm) and large (20–35 μm) cells of the islands of Calleja Complex (ICC) were studied in the albino rat with anterograde and retrograde transport of horseradish peroxidase (HRP) and fluorescent tracers. The medium and large size cells were found to project to the ipsilateral olfactory tubercle, ventral pallidum, septum, piriform cortex, periamygdaloid cortex, cortical nuclei of the amygdala, ventral endopiriform nucleus, lateral hypothalamic area, Forel's field H, ventral tegmental area, supramammillary complex, and nuclei gemini of the hypothalamus, midline, intralaminar and medial thalamic nuclei, and lateral habenula. Afferents of the ICC appear to include the same nuclei with the exception of the lateral habenula. In addition, the dorsal raphe projects to the ICC. These connections are consistent with the concept that the ICC is a striato-pallidal structure.     

Chronic depolarization induced by veratridine increases the survival of rat retinal ganglion cells ‘in vitro’

During the last decades it has been shown that trophic molecules released by target, afferent and glial cells play a pivotal role controlling neuronal cell death. Trophic molecules are able to inhibit this regressive event during development as well as during degenerative diseases. One of the mechanisms involved in the control of neuronal survival by afferent cells requires the release of trophic molecules stimulated by electrical activity. It has been demonstrated that veratridine (a depolarizing agent that keeps the Na⁺ channels opened) induces an increase in neuronal survival. In the present work we show that 3 μM veratridine induced a two-fold increase on the survival of retinal ganglion cells after 48 h in culture. The veratridine effect was inhibited by 50 μM amiloride (an inhibitor of Ca²⁺ channels), 25 μM benzamil (an inhibitor of Na⁺ channels), 30 μM dantrolene and 7.5 μM caffeine (both inhibitors of Ca²⁺ release from the endoplasmatic reticulum) and 10 μM BAPTA-AM (an intracellular Ca²⁺ chelator). However, 5 μM nifedipine (a selective inhibitor of voltage-dependent -type Ca²⁺ channels) and 100 μM MK 801 (an inhibitor of NMDA receptors) did not block the veratridine effect. On the other hand, treatment with 10 μM genistein (an inhibitor of tyrosine kinase enzymes), 20 μM fluorodeoxyuridine (an inhibitor of cell proliferation) or 10 μM atropine (an antagonist of muscarinic receptors) completely abolished the effect of veratridine. Taken together, our results indicate that veratridine increases the survival of rat retinal ganglion cells through mechanisms involving Na⁺ influx, intracellular Ca²⁺ release, activation of tyrosine kinase enzymes and cellular proliferation. They also indicate that cholinergic activity plays an important role in the veratridine effect.     

Differential modulation of auditory thalamocortical and intracortical synaptic transmission by cholinergic agonist

To investigate synaptic mechanisms underlying information processing in auditory cortex, we examined cholinergic modulation of synaptic transmission in a novel slice preparation containing thalamocortical and intracortical inputs to mouse auditory cortex. Extracellular and intracellular recordings were made in cortical layer IV while alternately stimulating thalamocortical afferents (via medial geniculate or downstream subcortical stimulation) and intracortical afferents. Either subcortical or intracortical stimulation elicited a fast, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX)-sensitive, monosynaptic EPSP followed by long-duration, polysynaptic activity. The cholinergic agonist carbachol suppressed each of the synaptic potentials to different degrees. At low concentrations (5 μM) carbachol strongly reduced (>60%) the polysynaptic slow potentials for both pathways but did not affect the monosynaptic fast potentials. At higher doses (10–50 μM), carbachol also reduced the fast potentials, but reduced the intracortically-elicited fast potential significantly more than the thalamocortically-elicited fast potential, which at times was actually enhanced. Atropine (0.5 μM) blocked the effects of carbachol, indicating muscarinic receptor involvement. We conclude that muscarinic modulation can strongly suppress intracortical synaptic activity while exerting less suppression, or actually enhancing, thalamocortical inputs. Such differential actions imply that auditory information processing may favor sensory information relayed through the thalamus over ongoing cortical activity during periods of increased acetylcholine (ACh) release.     

The transient potassium current, the A-current, is involved in spike frequency adaptation in rat amygdala neurons

The possible functional roles of the transient K⁺ current, I_A, in basolateral amygdala (BLA) neurons were studied using a rat brain slice preparation and conventional intracellular recording techniques. Conditioning depolarization, which inactivates I_A slowed the action potential repolarization while conditioning hyperpolarization accelarated the action potential repolarization. 4-Aminopyridine (4-AP, 100 μM), a specific I_A antagonist, also caused a clear delay in spike repolarization similar to the effect of conditioning depolarization suggesting that I_A is involved in the action potential repolarization.

When BLA neurons were excited by injecting long depolarizing current pulses (500 ms), they responded with an initial rapid discharge of action potentials which slowed or accommodated; an afterhyperpolarization (AHP) followed the depolarizing current pulses. Superfusion of 4-AP (100 μM) blocked accommodation resulting in an increase in action potential discharge in 74% (32 out of 43) neurons tested. The remaining 11 cells responded with an increased frequency of discharge of the first few action potentials. Unlike the effect of cadmium (Cd²⁺, 100 μM), a calcium channel blocker, 4-AP did not reduce the AHP. In the presence of norepinephrine (NE, 10 μM), a neurotransmitter which has been shown to block calcium-activated potassuim conductance, 4-AP caused a further increase in the number and frequency of action potential discharge. In addition, in BLA neurons, spontaneous interictal and ictal-like events were observed at low and high concentrations of 4-AP, respectively. We conclude that I_A is involved in the action potential repolarization as well as spike frequency adaptation in BLA neurons and that these actionsmay contribute to the convulsant effect of 4-AP     

Cell size-specific appearance of neuropeptide Y in the trigeminal ganglion following peripheral axotomy of different branches of the mandibular nerve of the rat

The effect of peripheral axotomy of the mental nerve (MN) and the cutaneous branch of the mylohyoid nerve (MhN) on the appearance of neuropeptide Y-like immunoreactivity (NPY-IR) in cells in the trigeminal ganglion of the rat was examined with combined retrograde-tracing and immunohistochemistry. Retrograde-tracing with True Blue (TB) revealed that the cell-size spectrum of the trigeminal cells sending peripheral processes to the MN (TB MN cells) ranged from 75.9 to 1560.5 μm² (or from 9.8 to 44.6 μm in diameter); 53% of TB MN cells were 300–600 μm². TB MhN cells ranged from 47.7 to 1261.5 μm² (or from 7.8 to 40.1 μm in diameter); 56% of TB MhN cells were <300 μm². In the normal trigeminal ganglion, there were no NPY-IR cells. 14 days after MN transection, 35% of TB MN cells displayed NPY-IR. The distribution of the cross-sectional areas of NPY-IR cells after MN transection was very similar to that of TB MN cells. Transection of MhN also induced the appearance of NPY-IR in the trigeminal ganglion but to a lesser extent (17% of TB MhN cells). The distribution of the cross-sectional areas of NPY-IR cells after MhN transection was similar to that of NPY-IR cells after MN transection. These results indicate that injury-evoked NPY-IR is specific for the medium- and large-sized ganglion cells.     

Regional sex differences in spine density along the apical shaft of visual cortex pyramids during postnatal development

Dendritic spines from the apical shaft of layer V pyramids were counted on Golgi-stained sections of the monocular subfield of the primary visual cortex of 10-, 20-, 40- and 60-day-old male and female rats. Dendritic segments located in layer IV and at 100–300 μm from the soma had a significantly higher spine content in 10-day-old females when compared to males. This sex difference was extended to outer dendritic segments with increasing age, and became restricted to dendritic segments of outer layers (II–III) located at 400–550 μm from the perikaryon in 40-day-old rats. Sex differences in spine content finally disappear by day 60. These results show the existence of specific laminar and temporal sex differences in the development of dendritic spines in the apical shaft of visual cortex pyramids.