Ultrastructural heterogeneity of enkephalin-containing terminals in the rat hippocampal formation

Sex differences in the rodent hippocampal opioid system following stress and oxycodone associated learning processes

Over the past two decades, opioid abuse has risen especially among women. In both sexes hippocampal neural circuits involved in associative memory formation and encoding of motivational incentives are critically important in the transition from initial drug use to drug abuse/dependence. Opioid circuits, particularly the mossy fiber pathway, are crucial for associative memory processes important for addiction. Our anatomical studies, especially those utilizing electron microscopic immunocytochemistry, have provided unique insight into sex differences in the distribution of opioid peptides and receptors in specific hippocampal circuits and how these distributions are altered following stress and oxycodone-associative learning processes. Here we review the hippocampal opioid system in rodents with respect to ovarian hormones effects and baseline sex differences then sex differences following acute and chronic stress. Next, we review sex differences in the hippocampal opioid system in unstressed and chronically stressed rats following oxycodone conditioned place preference. We show that opioid peptides and receptors are distributed within hippocampal circuits in females with elevated estrogen states in a manner that would enhance sensitivity to endogenous and exogenous opioids. Moreover, chronic stress primes the opioid system in females in a manner that would promote opioid-associative learning processes. In contrast, chronic stress has limited effects on the opioid system in males and reduces its capacity to support opioid-mediated learning processes. Interestingly, acute stress appears to prime males for opioid associative learning. On a broader scale the findings highlighted in this review have important implications in understanding sex differences in opioid drug use and abuse.

Sex and chronic stress alter delta opioid receptor distribution within rat hippocampal CA1 pyramidal cells following behavioral challenges

Following oxycodone (Oxy) conditioned place preference (CPP), delta opioid receptors (DORs) differentially redistribute in hippocampal CA3 pyramidal cells in female and male rats in a manner that would promote plasticity and opioid-associative learning processes. However, following chronic immobilization stress (CIS), males do not acquire Oxy-CPP and the trafficking of DORs in CA3 pyramidal neurons is attenuated. Here, we examined the subcellular distribution of DORs in CA1 pyramidal cells using electron microscopy in these same cohorts.

CPP

Saline (Sal)-females compared to Sal-males have more cytoplasmic and total DORs in dendrites and more DOR-labeled spines. Following Oxy-CPP, DORs redistribute from near-plasmalemma pools in dendrites to spines in males.

CIS

Control females compared to control males have more near-plasmalemmal dendritic DORs. Following CIS, dendritic DORs are elevated in the cytoplasm in females and near-plasmalemma in males.

CIS plus CPP

CIS Sal-females compared to CIS Sal-males have more DORs on the plasmalemma of dendrites and in spines. After Oxy, the distribution of DORs does not change in either females or males.

Conclusion

Following Oxy-CPP, DORs within CA1 pyramidal cells remain positioned in naïve female rats to enhance sensitivity to DOR agonists and traffic to dendritic spines in naïve males where they can promote plasticity processes. Following CIS plus behavioral enrichment, DORs are redistributed within CA1 pyramidal cells in females in a manner that could enhance sensitivity to DOR agonists. Conversely, CIS plus behavioral enrichment does not alter DORs in CA1 pyramidal cells in males, which may contribute to their diminished capacity to acquire Oxy-CPP.

Oxycodone injections not paired with conditioned place preference have little effect on the hippocampal opioid system in female and male rats

Oxycodone (Oxy) conditioned place preference (CPP) in Sprague Dawley rats results in sex-specific alterations in hippocampal opioid circuits in a manner that facilitates opioid-associative learning processes, particularly in females. Here, we examined if Oxy (3 mg/kg, I.P.) or saline (Sal) injections not paired with behavioral testing similarly affect the hippocampal opioid system. Sal-injected females compared to Sal-injected males had: (1) higher densities of cytoplasmic delta opioid receptors (DOR) in GABAergic hilar dendrites suggesting higher baseline reserve DOR pools and (2) elevated phosphorylated DOR levels, but lower phosphorylated mu opioid receptor (MOR) levels in CA3a suggesting that the baseline pools of activated opioid receptors vary in females and males. In contrast to CPP studies, Oxy-injections in the absence of behavioral tests resulted in few changes in the hippocampal opioid system in either females or males. Specifically, Oxy-injected males compared to Sal-injected males had fewer DORs near the plasma membrane of CA3 pyramidal cell dendrites and in CA3 dendritic spines contacted by mossy fibers, and lower pMOR levels in CA3a. Oxy-injected females compared to Sal-injected females had higher total DORs in GABAergic dendrites and lower total MORs in parvalbumin-containing dendrites. Thus, unlike Oxy CPP, Oxy-injections redistributed opioid receptors in hippocampal neurons in a manner that would either decrease (males) or not alter (females) excitability and plasticity processes. These results indicate that the majority of changes within hippocampal opioid circuits that would promote opioid-associative learning processes in both females and males do not occur with Oxy administration alone, and instead must be paired with CPP.

Sex Differences in the Rat Hippocampal Opioid System After Oxycodone Conditioned Place Preference

Although opioid addiction has risen dramatically, the role of gender in addiction has been difficult to elucidate. We previously found sex-dependent differences in the hippocampal opioid system of Sprague-Dawley rats that may promote associative learning relevant to drug abuse. The present studies show that although female and male rats acquired conditioned place preference (CPP) to the mu-opioid receptor (MOR) agonist oxycodone (3 mg/kg, I.P.), hippocampal opioid circuits were differentially altered. In CA3, Leu-Enkephalin-containing mossy fibers had elevated levels in oxycodone CPP (Oxy) males comparable to those in females and sprouted in Oxy-females, suggesting different mechanisms for enhancing opioid sensitivity. Electron microscopy revealed that in Oxy-males delta opioid receptors (DORs) redistributed to mossy fiber-CA3 synapses in a manner resembling females that we previously showed is important for opioid-mediated long-term potentiation. Moreover, in Oxy-females DORs redistributed to CA3 pyramidal cell spines, suggesting the potential for enhanced plasticity processes. In Saline-injected (Sal) females, dentate hilar parvalbumin-containing basket interneuron dendrites had fewer MORs, however plasmalemmal and total MORs increased in Oxy-females. In dentate hilar GABAergic dendrites that contain neuropeptide Y, Sal-females compared to Sal-males had higher plasmalemmal DORs, and near-plasmalemmal DORs increased in Oxy-females. This redistribution of MORs and DORs within hilar interneurons in Oxy-females would potentially enhance disinhibition of granule cells via two different circuits. Together, these results indicate that oxycodone CPP induces sex-dependent redistributions of opioid receptors in hippocampal circuits in a manner facilitating opioid-associative learning processes and may help explain the increased susceptibility of females to opioid addiction acquisition and relapse.

Endogenous opioids regulate moment-to-moment neuronal communication and excitability

Fear and emotional learning are modulated by endogenous opioids but the cellular basis for this is unknown. The intercalated cells (ITCs) gate amygdala output and thus regulate the fear response. Here we find endogenous opioids are released by synaptic stimulation to act via two distinct mechanisms within the main ITC cluster. Endogenously released opioids inhibit glutamate release through the δ-opioid receptor (DOR), an effect potentiated by a DOR-positive allosteric modulator. Postsynaptically, the opioids activate a potassium conductance through the μ-opioid receptor (MOR), suggesting for the first time that endogenously released opioids directly regulate neuronal excitability. Ultrastructural localization of endogenous ligands support these functional findings. This study demonstrates a new role for endogenously released opioids as neuromodulators engaged by synaptic activity to regulate moment-to-moment neuronal communication and excitability. These distinct actions through MOR and DOR may underlie the opposing effect of these receptor systems on anxiety and fear.

The endogenous opioid system regulates fear and anxiety, but the underlying cellular mechanism is unclear. Winters et al. shows that in the intercalated cells (ITC) of the amygdala, endogenous opioids suppress glutamatergic inputs via the δ-opioid receptor presynaptically, and reduce the excitability of ITCs via the μ-opioid receptor postsynaptically.

Understanding the broad influence of sex hormones and sex differences in the brain

Sex hormones act throughout the entire brain of both males and females via both genomic and nongenomic receptors. Sex hormones can act through many cellular and molecular processes that alter structure and function of neural systems and influence behavior as well as providing neuroprotection. Within neurons, sex hormone receptors are found in nuclei and are also located near membranes, where they are associated with presynaptic terminals, mitochondria, spine apparatus, and postsynaptic densities. Sex hormone receptors also are found in glial cells. Hormonal regulation of a variety of signaling pathways as well as direct and indirect effects on gene expression induce spine synapses, up- or downregulate and alter the distribution of neurotransmitter receptors, and regulate neuropeptide expression and cholinergic and GABAergic activity as well as calcium sequestration and oxidative stress. Many neural and behavioral functions are affected, including mood, cognitive function, blood pressure regulation, motor coordination, pain, and opioid sensitivity. Subtle sex differences exist for many of these functions that are developmentally programmed by hormones and by not yet precisely defined genetic factors, including the mitochondrial genome. These sex differences and responses to sex hormones in brain regions, which influence functions not previously regarded as subject to such differences, indicate that we are entering a new era of our ability to understand and appreciate the diversity of gender-related behaviors and brain functions. © 2016 Wiley Periodicals, Inc.

Sex differences in subcellular distribution of delta opioid receptors in the rat hippocampus in response to acute and chronic stress

Drug addiction requires associative learning processes that critically involve hippocampal circuits, including the opioid system. We recently found that acute and chronic stress, important regulators of addictive processes, affect hippocampal opioid levels and mu opioid receptor trafficking in a sexually dimorphic manner. Here, we examined whether acute and chronic stress similarly alters the levels and trafficking of hippocampal delta opioid receptors (DORs). Immediately after acute immobilization stress (AIS) or one-day after chronic immobilization stress (CIS), the brains of adult female and male rats were perfusion-fixed with aldehydes. The CA3b region and the dentate hilus of the dorsal hippocampus were quantitatively analyzed by light microscopy using DOR immunoperoxidase or dual label electron microscopy for DOR using silver intensified immunogold particles (SIG) and GABA using immunoperoxidase. At baseline, females compared to males had more DORs near the plasmalemma of pyramidal cell dendrites and about 3 times more DOR-labeled CA3 dendritic spines contacted by mossy fibers. In AIS females, near-plasmalemmal DOR-SIGs decreased in GABAergic hilar dendrites. However, in AIS males, near-plasmalemmal DOR-SIGs increased in CA3 pyramidal cell and hilar GABAergic dendrites and the percentage of CA3 dendritic spines contacted by mossy fibers increased to about half that seen in unstressed females. Conversely, after CIS, near-plasmalemmal DOR-SIGs increased in hilar GABA-labeled dendrites of females whereas in males plasmalemmal DOR-SIGs decreased in CA3 pyramidal cell dendrites and near-plasmalemmal DOR-SIGs decreased hilar GABA-labeled dendrites. As CIS in females, but not males, redistributed DOR-SIGs near the plasmalemmal of hilar GABAergic dendrites, a subsequent experiment examined the acute affect of oxycodone on the redistribution of DOR-SIGs in a separate cohort of CIS females. Plasmalemmal DOR-SIGs were significantly elevated on hilar interneuron dendrites one-hour after oxycodone (3 mg/kg, I.P.) administration compared to saline administration in CIS females. These data indicate that DORs redistribute within CA3 pyramidal cells and dentate hilar GABAergic interneurons in a sexually dimorphic manner that would promote activation and drug related learning in males after AIS and in females after CIS.

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Females have more near-plasmalemmal DORs in pyramidal CA3 dendrites than males.

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Acute stress in males relocates DORs in CA3 & GABA dendrites to promote activation.

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Chronic stress in females relocates DORs in GABA dendrites in females to promote activation.

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Chronic stress in males relocates DORs in GABA dendrites opposite of females.

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DOR-stress relocation may contribute to sexually dimorphic effects on drug related learning.

Hippocampal mossy fiber leu-enkephalin immunoreactivity in female rats is significantly altered following both acute and chronic stress

Research indicates that responses to stress are sexually dimorphic, particularly in regard to learning and memory processes: while males display impaired cognitive performance and hippocampal CA3 pyramidal cell dendritic remodeling following chronic stress, females exhibit enhanced performance and no remodeling. Leu-enkephalin, an endogenous opioid peptide found in the hippocampal mossy fiber pathway, plays a critical role in mediating synaptic plasticity at the mossy fiber-CA3 pyramidal cell synapse. Estrogen is known to influence the expression of leu-enkephalin in the mossy fibers of females, with leu-enkephalin levels being highest at proestrus and estrus, when estrogen levels are elevated. Since stress is also known to alter the expression of leu-enkephalin in various brain regions, this study was designed to determine whether acute or chronic stress had an effect on mossy fiber leu-enkephalin levels in females or males, through the application of correlated quantitative light and electron microscopic immunocytochemistry. Both acute and chronic stress eliminated the estrogen-dependence of leu-enkephalin levels across the estrous cycle in females, but had no effect on male levels. However, following acute stress leu-enkephalin levels in females were consistently lowered to values comparable to the lowest control values, while following chronic stress they were consistently elevated to values comparable to the highest control values. Ultrastructural changes in leu-enkephalin labeled dense core vesicles paralleled light microscopic observations, with acute stress inducing a decrease in leu-enkephalin labeled dense core vesicles, and chronic stress inducing an increase in leu-enkephalin labeled dense-core vesicles in females. These findings suggest that alterations in leu-enkephalin levels following stress could play an important role in the sex-specific responses that females display in learning processes, including those important in addiction.

Sex and estrogen receptor expression influence opioid peptide levels in the mouse hippocampal mossy fiber pathway

The opioid peptides, dynorphin (DYN) and enkephalin (L-ENK) are contained in the hippocampal mossy fiber pathway where they modulate synaptic plasticity. In rats, the levels of DYN and L-ENK immunoreactivity (-ir) are increased when estrogen levels are elevated (Torres-Reveron et al., 2008, 2009). Here, we used quantitative immunocytochemistry to examine whether opioid levels are similarly regulated in wildtype (WT) mice over the estrous cycle, and how these compared to males. Moreover, using estrogen receptor (ER) alpha and beta knock-out mice (AERKO and BERKO, respectively), the present study examined the role of ERs in rapid, membrane-initiated (6 h), or slower, nucleus-initiated (48 h) estradiol effects on mossy fiber opioid levels. Unlike rats, the levels of DYN and L-ENK-ir did not change over the estrous cycle. However, compared to males, females had higher levels of DYN-ir in CA3a and L-ENK-ir in CA3b. In WT and BERKO ovariectomized (OVX) mice, neither DYN- nor L-ENK-ir changed following 6 or 48 h estradiol benzoate (EB) administration. However, DYN-ir significantly increased 48 h after EB in the dentate gyrus (DG) and CA3b of AERKO mice only. These findings suggest that cyclic hormone levels regulate neither DYN nor L-ENK levels in the mouse mossy fiber pathway as they do in the rat. This may be due to species-specific differences in the mossy fiber pathway. However, in the mouse, DYN levels are regulated by exogenous EB in the absence of ERα possibly via an ERβ-mediated pathway requiring new gene transcription.

The influences of reproductive status and acute stress on the levels of phosphorylated delta opioid receptor immunoreactivity in rat hippocampus

In the hippocampus, ovarian hormones and sex can alter the trafficking of delta opioid receptors (DORs) and the proportion of DORs that colocalize with the stress hormone, corticotropin releasing factor. Here, we assessed the effects of acute immobilization stress (AIS) and sex on the phosphorylation of DORs in the rat hippocampus. We first localized an antibody to phosphorylated DOR (pDOR) at the SER363 carboxy-terminal residue, and demonstrated its response to an opioid agonist. By light microscopy, pDOR-immunoreactivity (ir) was located predominantly in CA2/CA3a pyramidal cell apical dendrites and in interneurons in CA1-3 stratum oriens and the dentate hilus. By electron microscopy, pDOR-ir primarily was located in somata and dendrites, associated with endomembranes, or in dendritic spines. pDOR-ir was less frequently found in mossy fibers terminals. Quantitative light microscopy revealed a significant increase in pDOR-ir in the CA2/CA3a region of male rats 1h following an injection of the opioid agonist morphine (20mg/kg, I.P). To look at the effects of stress on pDOR, we compared pDOR-ir in males and cycling females after AIS. The level of pDOR-ir in stratum radiatum of CA2/CA3a was increased in control estrus (elevated estrogen and progesterone) females compared to proestrus and diestrus females and males. However, immediately following 30min of AIS, no significant differences in pDOR levels were seen across estrous cycle phase or sex. These findings suggest that hippocampal levels of phosphorylated DORs vary with estrous cycle phase and that acute stress may dampen the differential effects of hormones on DOR activation in females.

Hormonal regulation of delta opioid receptor immunoreactivity in interneurons and pyramidal cells in the rat hippocampus

Clinical and preclinical studies indicate that women and men differ in relapse vulnerability to drug-seeking behavior during abstinence periods. As relapse is frequently triggered by exposure of the recovered addict to objects previously associated with drug use and the formation of these associations requires memory systems engaged by the hippocampal formation (HF), studies exploring ovarian hormone modulation of hippocampal function are warranted. Previous studies revealed that ovarian steroids alter endogenous opioid peptide levels and trafficking of mu opioid receptors in the HF, suggesting cooperative interaction between opioids and estrogens in modulating hippocampal excitability. However, whether ovarian steroids affect the levels or trafficking of delta opioid receptors (DORs) in the HF is unknown. Here, hippocampal sections of adult male and normal cycling female Sprague-Dawley rats were processed for quantitative immunoperoxidase light microscopy and dual label fluorescence or immunoelectron microscopy using antisera directed against the DOR and neuropeptide Y (NPY). Consistent with previous studies in males, DOR-immunoreactivity (-ir) localized to select interneurons and principal cells in the female HF. In comparison to males, females, regardless of estrous cycle phase, show reduced DOR-ir in the granule cell layer of the dentate gyrus and proestrus (high estrogen) females, in particular, display reduced DOR-ir in the CA1 pyramidal cell layer. Ultrastructural analysis of DOR-labeled profiles in CA1 revealed that while females generally show fewer DORs in the distal apical dendrites of pyramidal cells, proestrus females, in particular, exhibit DOR internalization and trafficking towards the soma. Dual label studies revealed that DORs are found in NPY-labeled interneurons in the hilus, CA3, and CA1. While DOR colocalization frequency in NPY-labeled neuron somata was similar between animals in the hilus, proestrus females had fewer NPY-labeled neurons that co-labeled with DOR in stratum oriens of CA1 and CA3 when compared to males. Ultrastructural analysis of NPY-labeled axon terminals within stratum radiatum of CA1 revealed that NPY-labeled axon terminals contain DORs that are frequently found at or near the plasma membrane. As no differences were noted by sex or estrous cycle phase, DOR activation on NPY-labeled axon terminals would inhibit GABA release probability equally in males and females. Taken together, these findings suggest that ovarian steroids can impact hippocampal function through direct effects on DOR levels and trafficking in principal cells and broad indirect effects through reductions in DOR-ir in NPY-labeled interneurons, particularly in CA1.

Age- and hormone-regulation of opioid peptides and synaptic proteins in the rat dorsal hippocampal formation

Circulating estrogen levels and hippocampal-dependent cognitive functions decline with aging. Moreover, the responses of hippocampal synaptic structure to estrogens differ between aged and young rats. We recently reported that estrogens increase levels of post-synaptic proteins, including PSD-95, and opioid peptides leu-enkephalin and dynorphin in the hippocampus of young animals. However, the influence of ovarian hormones on synaptic protein and opioid peptide levels in the aging hippocampus is understudied. Here, young (3- to 5-month-old), middle-aged (9- to 12-month-old), and aged (about 22-month-old) female rats were ovariectomized and then, 4 weeks later, subcutaneously implanted with a silastic capsule containing vehicle or 17β-estradiol. After 48 h, rats were subcutaneously injected with progesterone or vehicle and sacrificed 1 day later. Coronal sections through the dorsal hippocampus were processed for quantitative peroxidase immunohistochemistry of leu-enkephalin, dynorphin, synaptophysin, and PSD-95. With age, females showed opposing changes in leu-enkephalin and dynorphin levels in the mossy fiber pathway, particularly within the hilus, and regionally specific changes in synaptic protein levels. 17β-estradiol, with or without progesterone, altered leu-enkephalin levels in the dentate gyrus and synaptophysin levels in the CA1 of young but not middle-aged or aged females. Additionally, 17β-estradiol decreased synaptophysin levels in the CA3 of middle-aged females. Our results support and extend previous findings indicating 17β-estradiol modulation of hippocampal opioid peptides and synaptic proteins while demonstrating regional and age-specific effects. Moreover, they lend credence to the "window of opportunity" hypothesis during which hormone replacement can modulate hippocampal structure and circuitry to improve cognitive outcomes.

Hippocampal dynorphin immunoreactivity increases in response to gonadal steroids and is positioned for direct modulation by ovarian steroid receptors

The hippocampal formation (HF) is involved in modulating learning related to drug abuse. While HF-dependent learning is regulated by both endogenous opioids and estrogen, the interaction between these two systems is not well understood. The mossy fiber (MF) pathway formed by dentate gyrus (DG) granule cell axons is involved in some aspects of learning and contains abundant amounts of the endogenous opioid peptide dynorphin (DYN). To examine the influence of ovarian steroids on DYN expression, we used quantitative light microscopic immunocytochemistry to measure DYN levels in normal cycling rats as well as in two established models of hormone-treated ovariectomized (OVX) rats. Rats in estrus had increased levels of DYN-immunoreactivity (ir) in the DG and certain CA3 lamina compared with rats in proestrus or diestrus. OVX rats exposed to estradiol for 24 h showed increased DYN-ir in the DG and CA3, while those with 72 h estradiol exposure showed increases only in the DG. Six hours of estradiol exposure produced no change in DYN-ir. OVX rats chronically implanted with medroxyprogesterone also showed increased DYN-ir in the DG and CA3. Next, dual-labeling electron microscopy (EM) was used to evaluate the subcellular relationships of estrogen receptor (ER) alpha-, ERbeta and progestin receptor (PR) with DYN-labeled MFs. ERbeta-ir was in some DYN-labeled MF terminals and smaller terminals, and had a subcellular association with the plasmalemma and small synaptic vesicles. In contrast, ERalpha-ir was not in DYN-labeled terminals, although some DYN-labeled small terminals synapsed on ERalpha-labeled dendritic spines. PR labeling was mostly in CA3 axons, some of which were continuous with DYN-labeled terminals. These studies indicate that ovarian hormones can modulate DYN in the MF pathway in a time-dependent manner, and suggest that hormonal effects on the DYN-containing MF pathway may be directly mediated by ERbeta and/or PR activation.

Ovarian steroids modulate leu-enkephalin levels and target leu-enkephalinergic profiles in the female hippocampal mossy fiber pathway

In the hippocampal formation (HF), the enkephalin opioids and estrogen are each known to modulate learning and cognitive performance relevant to drug abuse. Within the HF, leu-enkephalin (LENK) is most prominent in the mossy fiber (MF) pathway formed by the axons of dentate gyrus (DG) granule cells. To examine the influence of ovarian steroids on MF pathway LENK levels, we used quantitative light microscopic immunocytochemistry to evaluate LENK levels in normal cycling rats and in estrogen-treated ovariectomized rats. Rats in estrus had increased levels of LENK-immunoreactivity (ir) in the DG hilus compared to rats in diestrus or proestrus. Rats in estrus and proestrus had higher levels of LENK-ir in CA3a-c compared to rats in diestrus. Ovariectomized (OVX) rats 24 h (but not 6 or 72 h) after estradiol benzoate (EB; 10 microg) administration had increased LENK-ir in the DG hilus and CA3c. Electron microscopy showed a larger proportion of LENK-labeled small terminals and axons in the DG hilus compared to CA3 which may have contributed to region-specific changes in LENK-ir densities. Next we evaluated the subcellular relationships of estrogen receptor (ER) alpha, ERbeta and progestin receptor (PR) with LENK-labeled MF pathway profiles using dual-labeling electron microscopy. ERbeta-ir colocalized in some LENK-labeled MF terminals and smaller terminals while PR-ir was mostly in CA3 axons, some of which also showed colocalization with LENK. ERalpha-ir was in dendritic spines, but no colocalization with LENK-labeled profiles was observed. The present studies indicate that estrogen can modulate LENK in subregions of the MF pathway in a dose-and time-dependent manner. These effects might be triggered by direct activation of ERbeta or PR in LENK-containing terminals.

Dynorphin and stress-related peptides in rat locus coeruleus: contribution of amygdalar efferents

The interaction between the stress axis and endogenous opioid systems has gained substantial attention, because it is increasingly recognized that stress alters individual sensitivity to opiates. One site at which opiates and stress substrates may interact to have global effects on behavior is within the locus coeruleus (LC). We have previously described interactions of several opioid peptides [e.g., proopiomelanocortin, enkephalin (ENK)] with the stress-related peptide corticotropin-releasing factor (CRF) in the LC. To examine further the interactions among dynorphin (DYN), ENK, and CRF in the LC, sections were processed for detection of DYN and CRF or DYN and ENK in rat brain. DYN- and CRF-containing axon terminals overlapped noradrenergic dendrites in this region. Dual immunoelectron microscopy showed coexistence of DYN and CRF; 35% of axon terminals containing DYN were also immunoreactive for CRF. In contrast, few axon terminals contained both DYN and ENK. A potential DYN/CRF afferent is the central nucleus of the amygdala (CeA). Dual in situ hybridization showed that, in CeA neurons, 31% of DYN mRNA-positive cells colocalized with CRF mRNA, whereas 53% of CRF mRNA-containing cells colocalized with DYN mRNA. Finally, to determine whether limbic DYN afferents target the LC, the CeA was electrolytically lesioned. Light-level densitometry of DYN labeling in the LC showed a significant decrease in immunoreactivity on the side of the lesion. Taken together, these data indicate that DYN- and CRF-labeled axon terminals, most likely arising from amygdalar sources, are positioned dually to affect LC function, whereas DYN and ENK function in parallel.

Ultrastructural evidence for pre- and postsynaptic localization of Cav1.2 L-type Ca2+ channels in the rat hippocampus

In the hippocampal formation, Ca(v)1.2 (L-type) voltage-gated Ca(2+) channels mediate Ca(2+) signals that can trigger long-term alterations in synaptic efficacy underlying learning and memory. Immunocytochemical studies indicate that Ca(v)1.2 channels are localized mainly in the soma and proximal dendrites of hippocampal pyramidal neurons, but electrophysiological data suggest a broader distribution of these channels. To define the subcellular substrates underlying Ca(v)1.2 Ca(2+) signals, we analyzed the localization of Ca(v)1.2 in the hippocampal formation by using antibodies against the pore-forming alpha(1)-subunit of Ca(v)1.2 (alpha(1)1.2). By light microscopy, alpha(1)1.2-like immunoreactivity (alpha(1)1.2-IR) was detected in pyramidal cell soma and dendritic fields of areas CA1-CA3 and in granule cell soma and fibers in the dentate gyrus. At the electron microscopic level, alpha(1)1.2-IR was localized in dendrites, but also in axons, axon terminals, and glial processes in all hippocampal subfields. Plasmalemmal immunogold particles representing alpha(1)1.2-IR were more significant for small- than large-caliber dendrites and were largely associated with extrasynaptic regions in dendritic spines and axon terminals. These findings provide the first detailed ultrastructural analysis of Ca(v)1.2 localization in the brain and support functionally diverse roles of these channels in the hippocampal formation.

Prenatal morphine exposure attenuates the maintenance of late LTP in lateral perforant path projections to the dentate gyrus and the CA3 region in vivo

Previously we reported that prenatal exposure to morphine twice daily during gestation decreases proenkephalin levels in adult progeny within the brain, including the dentate gyrus, and alters mu and delta opioid receptors in the hippocampal CA3 region. The lateral aspect of the perforant path contains and releases enkephalin-derived opioid peptides, and induction of long-term potentiation (LTP) in lateral perforant path projections to both the dentate gyrus and the hippocampal CA3 region is blocked by antagonists of opioid receptors. Thus LTP induction at these synapses involves opioid receptor activation mediated by the release of proenkephalin-derived opioid peptides with lateral perforant path activation. Here we show in adult behaving animals, neither LTP induction nor the early phase of LTP (E-LTP) maintenance is altered by prenatal morphine exposure in the lateral perforant path projections to the dentate gyrus and the CA3 region. However, maintenance and longevity of late LTP (L-LTP), as reflected in the magnitude of LTP over days, was attenuated in animals prenatally exposed to morphine. In contrast, in medial perforant path projections to the dentate gyrus and CA3 region, both LTP induction and the maintenance of E- and L-LTP were unaffected by prenatal morphine treatment. Thus a brief prenatal exposure to the opiate morphine produces sustained, and possibly permanent, alterations in L-LTP in the opioidergic lateral perforant path projection. This suggests that prenatal morphine exposure disrupts LTP via disruption of opioid mechanisms involved in LTP maintenance or via disruption of opioid receptor activation during LTP induction, which can subsequently alter LTP maintenance.

Methylphenidate administration to juvenile rats alters brain areas involved in cognition, motivated behaviors, appetite, and stress

Thousands of children receive methylphenidate (MPH; Ritalin) for attention deficit/hyperactivity disorder (ADHD), yet the long-term neurochemical consequences of MPH treatment are unknown. To mimic clinical Ritalin treatment in children, male rats were injected with MPH (5 mg/kg) or vehicle twice daily from postnatal day 7 (PND7)-PND35. At the end of administration (PND35) or in adulthood (PND135), brain sections from littermate pairs were immunocytochemically labeled for neurotransmitters and cytological markers in 16 regions implicated in MPH effects and/or ADHD etiology. At PND35, the medial prefrontal cortex (mPFC) of rats given MPH showed 55% greater immunoreactivity (-ir) for the catecholamine marker tyrosine hydroxylase (TH), 60% more Nissl-stained cells, and 40% less norepinephrine transporter (NET)-ir density. In hippocampal dentate gyrus, MPH-receiving rats showed a 51% decrease in NET-ir density and a 61% expanded distribution of the new-cell marker PSA-NCAM (polysialylated form of neural cell adhesion molecule). In medial striatum, TH-ir decreased by 21%, and in hypothalamus neuropeptide Y-ir increased by 10% in MPH-exposed rats. At PND135, MPH-exposed rats exhibited decreased anxiety in the elevated plus-maze and a trend for decreased TH-ir in the mPFC. Neither PND35 nor PND135 rats showed major structural differences with MPH exposure. These findings suggest that developmental exposure to high therapeutic doses of MPH has short-term effects on select neurotransmitters in brain regions involved in motivated behaviors, cognition, appetite, and stress. Although the observed neuroanatomical changes largely resolve with time, chronic modulation of young brains with MPH may exert effects on brain neurochemistry that modify some behaviors even in adulthood.

Opioid systems in the dentate gyrus

Opiate drugs alter cognitive performance and influence hippocampal excitability, including long-term potentiation (LTP) and seizure activity. The dentate gyrus (DG) contains two major opioid peptides, enkephalins and dynorphins, which have opposing effects on excitability. Enkephalins preferentially bind to delta- and mu-opioid receptors (DORs and MORs) while dynorphins preferentially bind to kappa-opioid receptors (KORs). Opioid receptors can also be activated by exogenous opiate drugs such as the MOR agonist morphine. Enkephalins are contained in the mossy fiber pathway, in the lateral perforant path (PP) and in scattered GABAergic interneurons. MORs and DORs are predominantly in distinct subpopulations of GABAergic interneurons known to inhibit granule cells, and are present at low levels within granule cells. MOR and DOR agonists increase excitability and facilitate LTP in the molecular layer. Anatomical and physiological evidence is consistent with somatodendritic and axon terminal targeting of both MORs and DORs. Dynorphins are in the granule cells, most abundantly in mossy fibers but also in dendrites. KORs have been localized to granule cell mossy fibers, supramammillary afferents to granule cells, and PP terminals. KOR agonists, including endogenous dynorphins, diminish the induction of LTP. Recent evidence indicates that opiates and opioids also modulate other processes in the hippocampal formation, including adult neurogenesis, the actions of gonadal hormones, and development of neonatal transmitter systems.

Extranuclear estrogen receptor beta immunoreactivity is on doublecortin-containing cells in the adult and neonatal rat dentate gyrus

In adult female rats, estrogen receptor (ER) activation, particularly of ERbeta, promotes hippocampal neurogenesis. We previously reported that extranuclear ERbeta immunoreactivity (ir) in adult rats is on cellular profiles in or near the granule cell layer, which is the location of newly generated cells. During development, cells in or near the granule cell layer transiently express high levels of estrogen binding and nuclear ERs. Thus, we sought to determine if extranuclear ERbeta is in newly generated cells in adult and neonatal rat dentate gyrus. Sections from the dentate gyrus of adult proestrus or postnatal day 7 and 14 female rats were dual-labeled for ERbeta and the new-cell marker doublecortin (DCX) and examined by electron microscopy. DCX-containing neurons were found in the subgranular hilus in adult rats and were more widespread throughout the granule cell layer and hilus of neonatal rats. In both adults and neonatal rats, ERbeta immunoreactivity was found in a subset of DCX-labeled neurons. Electron microscopic examination of the adult dentate gyrus revealed that most perikarya with DCX-ir had the morphological characteristics of granule cells, although a few resembled interneurons. Dendrites with DCX-ir also were observed. In both adults and neonates, DCX-labeled neuronal perikarya and dendrites contained ERbeta-ir; ERbeta-ir usually was aggregated near the plasma membrane, mitochondria or endoplasmic reticula. ERbeta-ir was in glial profiles that apposed DCX-labeled perikarya and dendrites. These findings are consistent with data showing that estrogens can exert non-genomic effects directly and indirectly on newly generated cells in neonatal and adult rat dentate gyrus.

Ultrastructural localization of estrogen receptor beta immunoreactivity in the rat hippocampal formation

Several lines of evidence indicate that estrogen affects hippocampal synaptic plasticity through rapid nongenomic mechanisms, possibly by binding to plasma membrane estrogen receptors (ERs). We have previously shown that ERalpha immunoreactivity (ir) is in select interneuron nuclei and in several extranuclear locations, including dendritic spines and axon terminals, within the rat hippocampal formation (Milner et al., [2001] J Comp Neurol 429:355). The present study sought to determine the cellular and subcellular locations of ERbeta-ir. Coronal hippocampal sections from diestrus rats were immunolabeled with antibodies to ERbeta and examined by light and electron microscopy. By light microscopy, ERbeta-ir was primarily in the perikarya and proximal dendrites of pyramidal and granule cells. ERbeta-ir was also in a few nonprincipal cells and scattered nuclei in the ventral subiculum and CA3 region. Ultrastructural analysis revealed ERbeta-ir at several extranuclear sites in all hippocampal subregions. ERbeta-ir was affiliated with cytoplasmic organelles, especially endomembranes and mitochondria, and with plasma membranes primarily of principal cell perikarya and proximal dendrites. ERbeta-ir was in dendritic spines, many arising from pyramidal and granule cell dendrites. In both dendritic shafts and spines, ERbeta-ir was near the perisynaptic zone adjacent to synapses formed by unlabeled terminals. ERbeta-ir was in preterminal axons and axon terminals, associated with clusters of small, synaptic vesicles. ERbeta-labeled terminals formed both asymmetric and symmetric synapses with dendrites. ERbeta-ir also was detected in glial profiles. The cellular and subcellular localization of ERbeta-ir was generally similar to that of ERalpha, except that ERbeta was more extensively found at extranuclear sites. These results suggest that ERbeta may serve primarily as a nongenomic transducer of estrogen actions in the hippocampal formation.

Intracerebroventricular d-Pen2, d-Pen5-enkephalin administration soon after stressor imposition influences behavioral responsivity to a subsequent stressor encounter in CD-1 mice

Endogenous opioid peptide systems diminish stress-induced autonomic nervous system, neuroendocrine (hypothalamic-pituitary-adrenal axis) and behavioral responses, attenuating a collection of physiological symptoms basic to emotional and affective states. Neurogenic stressors may incite specific central changes in opioid peptide availability as well as changes in mu and delta-opioid receptor function. The present investigation evaluated the proactive influence of an intracerebroventricular injection of the opioid receptor agonist D-Pen2, D-Pen5-enkephalin (DPDPE) (0 microg, 0.005 microg, 1.0 microg or 2.5 microg) on locomotor behavior of mice following uncontrollable footshock (Shock) or novel shock chamber exposure (No Shock). It was expected that DPDPE administration following Shock on Day 1 would restore locomotor activity up to 1 week and prevent shock-associated behavior of mice encountering a brief session of footshock 18 days later. Exposure to Shock reduced horizontal locomotor and vertical locomotor (rearing) activity of mice while 2.5 microg DPDPE restored behavior. Eighteen days following Shock and DPDPE challenge, mice were exposed to either an abbreviated session of footshock (Mild Stress) or the shock chamber (Cues). Mice in the No Shock and Shock groups administered 2.5 microg DPDPE on Day 1 did not exhibit any locomotor deficits in response to Mild Stress on Day 18. Mice in the Shock group administered 0.005 microg DPDPE on Day 1, did not exhibit exaggerated rearing deficits following ensuing Mild Stressor encounter relative to mice reexposed to Cues on Day 18. Taken together, these data show that (a) footshock differentially affects rearing and locomotor activity, (b) DPDPE administration increases locomotor activity for up to 1 week following footshock and DPDPE administration, (c) reexposure to Mild Stress affects rearing and locomotor performance differently depending on previous stressor history and DPDPE dose, (d) DPDPE affords long-lasting protection to previously non-stressed mice against the deleterious effects of subsequent mild stress on locomotor activity, while a low dose of DPDE is sufficient to prevent shock-induced sensitization of rearing deficits, 18 days following original stressor and drug presentation. Finally, our investigation demonstrates that DPDPE administration alters the behavioral impact of future stressful encounters and emphasizes the importance of investigating opioid mechanisms in chronic stress disorders.

Ultrastructural evidence for mu-opioid modulation of cholinergic pathways in rat dentate gyrus

Within the rat hippocampal formation, cholinergic afferents and mu-opioid receptors (MORs) are involved in many crucial learning processes, including those associated with drug reward. Pharmacological data, and the overlapping distributions of cholinergic and mu-opioid systems, particularly in the dentate gyrus, suggest that MOR activation is a potential mechanism for endogenous opioid modulation of cholinergic activity. To date, anatomical evidence supporting this has not been reported. To delineate the relationship between cholinergic afferents and MOR-containing processes in the dentate gyrus, hippocampal sections were dually immunolabeled for vesicular acetylcholine transporter (VAChT) and MOR-1 and examined by electron microscopy. VAChT immunoreactivity was in unmyelinated axons and axon terminals, and was most often associated with small synaptic vesicles. MOR immunoreactivity was found in axons, axon terminals and, to a lesser extent, perikarya, which resembled GABAergic basket cells. Semi-quantitative ultrastructural analysis revealed that from 5% to 13% (depending on laminar location) of VAChT-immunoreactive (ir) presynaptic profiles contained MOR immunoreactivity. Additionally, 7% of VAChT-ir presynaptic profiles directly apposed MOR-ir axons and terminals, and there were almost no appositions to MOR-ir dendrites. These data suggest that opioids may directly and indirectly modulate acetylcholine release and/or reuptake. In the hilus and molecular layer, 4% of VAChT-ir terminals contacted dendritic shafts that were also contacted by MOR-ir terminals. This suggests that cholinergic afferents and MOR-containing afferents can converge on granule cell dendrites (which are restricted to the molecular layer) and on interneuron dendrites in the hilus. The results of this study provide ultrastructural evidence for direct and indirect modulation of cholinergic systems by mu-opioids in the hippocampal formation.

Field-specific changes in hippocampal opioid mRNA, peptides, and receptors due to prenatal morphine exposure in adult male rats

Alterations in the opioid system in the hippocampal formation and some of the possible functional consequences were investigated in adult male rats that were prenatally exposed to either saline or morphine (10 mg/kg twice daily on gestational days 11-18). In situ hybridization and Northern blots were used to measure proenkephalin and prodynorphin mRNA, and radioimmunoassays quantified proenkephalin- and prodynorphin-derived peptide levels in the dentate gyrus, CA3, and CA1 subfields of the hippocampal formation. Prenatal morphine exposure in male rats decreases proenkephalin and increases prodynorphin mRNA selectively in the granule cell layer of the dentate gyrus. Similarly, met-enkephalin peptide levels are decreased and dynorphin B peptide levels are increased in the dentate gyrus but not CA3 or CA1 of prenatally morphine-exposed males. In addition, there are decreases in dynorphin-derived peptides in the CA3 subfield. Receptor autoradiography revealed increases in the density of micro but not delta receptor labeling in discrete strata of specific hippocampal subfields in morphine-exposed males. Because alterations in the hippocampal opioid system suggest possible alterations in the excitability of the hippocampal formation, changes in opioid regulation of seizures were examined. Morphine exposure, however, does not alter the latency to onset or number of episodes of wet dog shakes or clonic seizures induced by infusion of 10 nmol [D-Ala2, MePhe4, Gly-ol5]enkephalin into the ventral hippocampal formation. Interestingly, a naloxone (5 mg/kg) injection 30 min before bicuculline administration reverses the increased latency to onset of clonic and tonic-clonic seizures in morphine-exposed males. Thus, the present study suggests that exposure of rats to morphine during early development alters the hippocampal opioid system, suggesting possible consequences for hippocampal-mediated functions.

Neuronal types expressing μ- and δ-opioid receptor mRNA in the rat hippocampal formation

Opioids are thought to control the excitability of hippocampal principal neurons indirectly by inhibiting GABAergic interneurons. However, direct inhibition of hippocampal principal neurons by opioids has also been reported. To understand better the neuromodulatory role of opioids in rat hippocampal circuits, we analyzed types of micro- and delta-opioid receptor (MOR, DOR)-expressing hippocampal neurons. Most MOR-immunoreactive neurons in the granular and pyramidal cell layers exhibited multipolar morphologies characteristic of GABAergic neurons. Virtually all neurons in the hippocampal formation expressing high MOR mRNA levels cocontained the mRNA for glutamic acid decarboxylase (GAD). Most parvalbumin-, several calretinin-, and several pre-proenkephalin-containing neurons expressed the MOR gene in the hippocampal formation. Expression of high DOR mRNA levels was restricted to GAD-positive neurons in the principal cell layers, oriens layer and hilus. More than 90% of the parvalbumin-positive neurons in the hippocampal formation strongly expressed the DOR gene. Granule cells expressing vesicular glutamate transporter 1 (VGLUT1) mRNA contained very low MOR and DOR transcript levels. In VGLUT1-positive pyramidal cells, weak DOR but no MOR gene expression was detected. Whereas most somatostatinergic hilar neurons were negative for MOR and DOR mRNA, somatostatinergic oriens layer neurons frequently expressed these receptors. Taken together, weak expression of MOR and DOR genes in hippocampal principal cells is in concordance with direct opioid-mediated inhibition of principal cells. However, strong expression of the MOR and DOR genes in the hippocampus is restricted to gamma-aminobutyric acid (GABA)ergic neurons, with DORs being selectively expressed in the parvalbumin- and somatostatin-containing subpopulations. Activation of MOR and/or DOR in parvalbumin- and somatostatin-containing neurons, which provide GABAergic inhibition to the perisomatic and distal dendritic regions of principal cells, respectively, is likely to facilitate principal cell excitation.

Hypothalamic orexin (hypocretin) neurons express vesicular glutamate transporters VGLUT1 or VGLUT2

Initially recognized for their importance in control of appetite, orexins (also called hypocretins) are neuropeptides that are also involved in regulating sleep, arousal, and cardiovascular function. Loss of orexin appears to be the primary cause of narcolepsy. Cells expressing the orexins are restricted to a discrete region of the hypothalamus, but their terminal projections are widely distributed throughout the brain. With the diversity of function and broad distribution of orexin terminals, it is not known whether the orexin cells constitute a homogeneous population. Because orexins produce neuroexcitatory effects, we hypothesized that orexin-containing neurons are glutamatergic. In the present study we used digoxigenin-labeled cRNA probes for the vesicular glutamate transporters, VGLUT1 and VGLUT2, for in situ hybridization studies in combination with immunohistochemical detection of orexin cell bodies in the hypothalamus. In general, cells in the hypothalamus expressed low levels of the vesicular glutamate transporters relative to other areas of the forebrain, such as the cortex and thalamus. Light labeling for VGLUT2 mRNA was detected in about 50% of the orexin-immunoreactive neurons, and a much smaller percentage (approximately 13%) of orexin-immunoreactive cells was found to express VGLUT1. Despite the fact that intense labeling for GAD67 mRNA was found in a large number of cells throughout the hypothalamus, none of the orexin-immunoreactive cells was found to be GABAergic. These findings, showing that many of the orexin neurons are glutamatergic, are consistent with the neuroexcitatory effects of orexin but suggest that another neurochemical phenotype may define the remaining subset of orexin neurons.

Increased mu-opioid receptor labeling is found on inner molecular layer terminals of the dentate gyrus following seizures

The hippocampal formation is a brain region sensitive to seizure development, a phenomenon thought to be mediated in part by mu-opioid receptor (MOR) activation. Previous studies have found a delayed increase in MOR immunoreactivity (IR) in the inner molecular layer (IML) of the dentate gyrus after experimentally induced seizures. However, whether these increases in MOR-IR are restricted to certain cell types or cellular compartments (i.e., presynaptic, postsynaptic, or glial profiles) has not been determined. Thus, the present study examined which subcellular profiles demonstrate changes in MOR-IR after kainic acid (KA)-induced seizures. Light microscopic (LM) analysis demonstrated seizure-induced increases in MOR-IR at three points of the IML (dorsal blade, ventral blade, and crest) at three levels of section (septal, mid-septotemporal, and temporal). Electron microscopic analysis of the IML revealed that MOR-IR was present in the same types of cellular profiles in both control and KA-treated rats. However, a significant increase in the number of MOR-labeled terminal profiles was revealed in KA-treated rats compared to controls. Additionally, some MOR-labeled terminals in KA-treated rats possessed excitatory-type morphology and contained enkephalin or dynorphin, peptides found in mossy fiber terminals. These data suggest that most of the seizure-induced increases in MOR expression in the IML are associated with terminals originating from several different neuronal populations, including granule cells, and possibly, surviving GABAergic interneurons, septal cholinergic, and/or supramamillary projection neurons.

Inspiratory augmenting bulbospinal neurons express both glutamatergic and enkephalinergic phenotypes

Many of the inspiratory augmenting (I-AUG) neurons of the rostral ventral respiratory group (rVRG) are premotor neurons that excite phrenic motor neurons during inspiration, probably by releasing glutamate. In the present study, we demonstrate that these neurons are indeed glutamatergic, in that their cell bodies contain vesicular glutamate transporter-2 (VGLUT2) mRNA and spinal terminals from neurons in the region of the rVRG contain VGLUT2 protein. We also demonstrate by using parallel in situ hybridization and immunocytochemical evidence that most rVRG inspiratory premotor neurons are enkephalinergic. After iontophoretic deposits of biotinylated dextran amine (BDA) in the area of the rVRG, many BDA-labeled terminals in the ventral horn of cervical spinal cord (C4-C5) were immunoreactive for enkephalin and VGLUT2. Injections of Fluoro-Gold amidst phrenic motor neurons in C4-C5 labeled neurons in the area of the rVRG that contained both VGLUT2 mRNA and preproenkephalin (PPE) mRNA as revealed by double in situ hybridization. Thirty-eight bulbospinal I-AUG neurons were recorded in the rVRG and filled with biotinamide by using the juxtacellular labeling technique. Every biotinamide-filled cell tested was positively labeled for VGLUT2 mRNA (n = 14), and most of the cells tested in a separate population exhibited PPE mRNA (16/18). We conclude that most of the phrenic inspiratory premotor neurons of the rVRG are glutamatergic neurons that may also release enkephalins.

Mu opioid receptors are in discrete hippocampal interneuron subpopulations

In the rat hippocampal formation, application of mu opioid receptor (MOR) agonists disinhibits principal cells, promoting excitation-dependent processes such as epileptogenesis and long-term potentiation. However, the precise location of MORs in particular inhibitory circuits, has not been determined, and the roles of MORs in endogenous functioning are unclear. To address these issues, the distribution of MOR-like immunoreactivity (-li) was examined in several populations of inhibitory hippocampal neurons in the CA1 region using light and electron microscopy. We found that MOR-li was present in many parvalbumin-containing basket cells, but absent from cholecystokinin-labeled basket cells. MOR-li was also commonly in interneurons containing somatostatin-li or neuropeptide Y-li that resembled the "oriens-lacunosum-moleculare" (O-LM) interneurons innervating pyramidal cell distal dendrites. Finally, MOR-li was in some vasoactive intestinal peptide- or calretinin-containing profiles resembling interneurons that primarily innervate other interneurons. These findings indicate that MOR-containing neurons form a neurochemically and functionally heterogeneous subset of hippocampal GABAergic neurons. MORs are most frequently on interneurons that are specialized to inhibit pyramidal cells, and are on a limited number of interneurons that target other interneurons. Moreover, the distribution of MORs to different neuronal types in several laminae, some relatively far from endogenous opioids, suggests normal functional roles that are different from the actions seen with exogenous agonists such as morphine.

Neurons with mu opioid receptors interact indirectly with enkephalin-containing neurons in the rat dentate gyrus

In the dentate gyrus, mu opioid receptors (MORs) and their enkephalin agonists have overlapping distributions and influence excitability and plasticity. Released endogenous enkephalins can activate at least some of these MORs; however, whether these interactions involve synaptically associated profiles or more distant associations and whether some subcellular compartments (e.g., terminals or dendrites) are more likely to be targeted than others are not known. To elucidate the relationships between potential sites of enkephalin release and MORs, MOR1 and leucine-enkephalin (LE) immunoreactivities were localized in the hilus by electron microscopy, using immunoperoxidase and immunogold markers. Of the 573 MOR-immunoreactive (ir) profiles analyzed, most were axons and terminals (51 and 30%, respectively), and fewer were dendrites (12%), glia (3%), or unclassifiable (4%). Most MOR-ir profiles resembled interneuron processes, while most LE-ir terminals resembled mossy fibers. One third of MOR-ir profiles were within 3 microm and approximately half were within 4 microm of the nearest LE-ir profile. In contrast, few (3%) MOR-ir profiles contacted LE-ir profiles; only 16% of these contacts included observable synapses, and very few profiles (0.5%) colocalized MOR and LE immunoreactivity. MOR-ir axons, terminals, and dendrites were not distributed differently relative to LE-ir profiles. These results suggest that activation of hilar MORs by LE usually involves short-range volume transmission and that dendritic MORs are as likely as axonal and terminal MORs to be activated by released LE. However, the greater abundance of MOR-ir axons and terminals compared to dendrites indicates that presynaptic profiles are a more prominent target for enkephalins and exogenous MOR agonists such as morphine.

Dentate hilar mossy cells and somatostatin-containing neurons are immunoreactive for the alpha8 integrin subunit: characterization in normal and kainic acid-treated rats

Integrins are heterodimeric cell surface receptors composed of different alpha and beta subunits that mediate cell-cell and cell-extracellular matrix interactions. They have been implicated in the regulation of neuronal migration, differentiation, process outgrowth, and plasticity. The alpha8 integrin subunit associates exclusively with the beta1 subunit to form a receptor (alpha8beta1) for fibronectin, vitronectin, tenascin, and osteopontin. In a previous study, we demonstrated that hippocampal dentate hilar neurons are immunoreactive for alpha8. The present study identifies the major types of alpha8-immunoreactive hilar neurons and characterizes the effects of kainic acid-induced seizures on alpha8-immunoreactivity in these cells. Examination of the hilus in normal rats revealed alpha8-immunoreactivity in the somatodendritic compartments of large hilar neurons identified as mossy cells, including a subset of dendritic thorny excrescences that were contacted by large mossy fiber terminals. alpha8-immunoreactivity also was found in approximately 71% of somatostatin-containing hilar cells. Kainic acid-induced seizures dramatically and rapidly altered the levels and distribution of alpha8-immunoreactivity in hilar neurons. After 1.5 h of seizures, alpha8-immunoreactivity in their dendrites was reduced greatly. One day after kainic acid treatment, labeling was diminished throughout the somatodendritic compartments of most hilar cells. This decrease appeared to be transient, since alpha8 labeling returned to normal levels in surviving hilar neurons within 2 weeks of treatment. In addition, many alpha8-immunoreactive hilar neurons, particularly in caudal dentate regions, were lost 3-5 weeks after kainic acid treatment. Our findings suggest that alpha8beta1 may mediate adhesive interactions of the dendritic processes of mossy cells and somatostatin-containing hilar neurons with other cellular elements or with extracellular matrix components. They also suggest that alpha8 may be susceptible to activity-dependent proteolysis that could modulate its function in the somatodendritic compartment of these cells.

Subunit heterogeneity of cytoplasmic dynein: Differential expression of 14 kDa dynein light chains in rat hippocampus

Cytoplasmic dynein is a multi-subunit protein complex in which each subunit is encoded by a few genes. How these subunit isoforms are assembled and regulated to mediate the diverse functions of cytoplasmic dynein is unknown. We previously have shown that two highly conserved 14 kDa dynein light chains, Tctex-1 and RP3, have different cargo-binding abilities. In this report, coimmunoprecipitation revealed that Tctex-1 and RP3 were present in mutually exclusive dynein complexes of brain. Two specific antibodies were used to examine the localization of these two dynein light chains in adult rat hippocampal formation and cerebral cortex. By light microscopy, Tctex-1 and RP3 immunoreactivities exhibited distinct and almost complementary distribution patterns in both brain regions. In hippocampal formation, Tctex-1 immunoreactivity was most enriched in somata of newly generated granule cells and scant in the mature granule and pyramidal cell somata. In contrast, RP3 immunoreactivity was abundant in pyramidal and granule cell somata. Ultrastructural analysis of the dentate gyrus revealed both dynein light chains were associated with various membranous organelles that often were affiliated with microtubules. In addition, Tctex-1 and RP3 immunoreactivities were preferentially and highly enriched on membranous organelles and/or vesicles of axon terminals and dendritic spines, respectively. These results suggest that dynein complexes with different subunit composition, and possibly function, are expressed differentially in a spatially and temporally regulated manner. Furthermore, Tctex-1 and RP3 may play important roles in synaptic functions.

Ultrastructural evidence for presynaptic mu opioid receptor modulation of synaptic plasticity in NMDA-receptor-containing dendrites in the dentate gyrus

Physiological studies have demonstrated that long-term potentiation (LTP) induction in N-methyl-D-aspartate (NMDA) receptor containing dentate granule cells following lateral perforant path stimulation is opioid dependent, involving mu-opioid receptors (MORs) on gamma-aminobutyric acid (GABA)-ergic neurons. To determine the cellular relationships of MORs to postsynaptic NMDA receptor-containing dendrites, immunoreactivity (-I) against MOR and the NMDA receptor subunit 1 (NMDAR1) was examined in the outer molecular layer of the dentate gyrus using electron microscopy. MOR-I was predominantly in axons and axon terminals. NMDAR1-I was almost exclusively in spiny dendrites, but was also in a few terminals. Using immunogold particles to localize precisely NMDAR1, one-third of the NMDAR1-I was detected on the dendritic plasmalemma; in dendritic spines plasmalemmal immunogold particles were near synaptic densities. Many MOR-labeled axons and terminals contacted NMDAR1-labeled dendrites. MOR-labeled terminals formed symmetric (inhibitory-type) synapses on NMDAR1-labeled dendritic shafts or nonsynaptically contacted NMDAR1-labeled shafts and spines. MOR-labeled axons often abutted NMDAR1-containing dendritic spines which received asymmetric (excitatory-type) synapses from unlabeled terminals. Occasionally, MOR-labeled terminals and dendrites were apposed to NMDAR1-containing terminals. These results provide anatomical evidence that endogenous enkephalins or exogenous opioid agonists could inhibit GABAergic terminals that modulate granule cell dendrites, thus boosting depolarizing events in granule cells and facilitating the activation of NMDA receptors located on their dendrites.

Ultrastructural evidence for enkephalin mediated disinhibition in the ventromedial nucleus of the hypothalamus

The ventromedial nucleus of the hypothalamus (VMN) regulates the estrogen-dependent appearance of female mating behavior, lordosis. Accumulating evidence suggests that estrogen might exert its control over lordosis by acting, in part, on neurons that contain enkephalin in the VMN. The expression of the enkephalin precursor gene is robustly stimulated by estrogen and is correlated with the later appearance of lordosis. GABA has also been implicated as an important neurotransmitter for the appearance of lordosis. Because enkephalin is thought to act in several brain areas to modulate the activity of GABAergic neurons, we studied the ultrastructural morphology and relationship between neurons containing these neurochemicals using dual-labeling immunocytochemistry in ovariectornized rats, half of which received estrogen replacement. Immunolabeling for enkephalin was almost always detected within axon terminals (695 axonal profiles sampled), while GABA immunoreactivity was more often localized to cell bodies and dendrites (191 profiles), than to axons (63 profiles). Axon terminals containing enkephalin immunolabeling provided a major innervation to soma or dendrites containing GABA. That is, over one third (94/245) of the axon terminals in contact with GABA-immunoreactive dendrites contained enkephalin. Furthermore, these GABA-immunoreactive dendrites accounted for a fifth of the somatodendritic processes associated with enkephalin-containing axon terminals. These findings support the hypothesis that enkephalin may act in the VMN by inhibiting GABAergic neurons, which could result in the disinhibition of neural circuits relevant for lordosis.

The multifarious hippocampal mossy fiber pathway: a review

The hippocampal mossy fiber pathway between the granule cells of the dentate gyrus and the pyramidal cells of area CA3 has been the target of numerous scientific studies. Initially, attention was focused on the mossy fiber to CA3 pyramidal cell synapse because it was suggested to be a model synapse for studying the basic properties of synaptic transmission in the CNS. However, the accumulated body of research suggests that the mossy fiber synapse is rather unique in that it has many distinct features not usually observed in cortical synapses. In this review, we have attempted to summarize the many unique features of this hippocampal pathway. We also have attempted to reconcile some discrepancies that exist in the literature concerning the pharmacology, physiology and plasticity of this pathway. In addition we also point out some of the experimental challenges that make electrophysiological study of this pathway so difficult.Finally, we suggest that understanding the functional role of the hippocampal mossy fiber pathway may lie in an appreciation of its variety of unique properties that make it a strong yet broadly modulated synaptic input to postsynaptic targets in the hilus of the dentate gyrus and area CA3 of the hippocampal formation.

Prenatal morphine exposure enhances seizure susceptibility but suppresses long-term potentiation in the limbic system of adult male rats

The present study examined the effects of prenatal morphine exposure on NMDA-dependent seizure susceptibility in the entorhinal cortex (EC), and on activity-dependent synaptic plasticity at Schaffer collateral and perforant path synapses in the hippocampus. During perfusion with Mg(2+)-free ACSF, an enhancement of epileptiform discharges was found in the EC of slices from prenatally morphine-exposed male rats. A submaximal tetanic stimulation (2x50 Hz/1 s) in control slices elicited LTP at the Schaffer collateral-CA1 synapses, but neither LTP nor LTD was evoked at the perforant path-DG synapses. In slices from prenatally morphine-exposed adult male rats, long-term potentiation of synaptic transmission was not observed at Schaffer collateral-CA1 synapses, while the submaximal tetanus now elicited frank LTD of synaptic EPSPs at perforant path synapses. These data suggest that prenatal morphine exposure enhances the susceptibility of entorhinal cortex to the induction of epileptiform activity, but shifts long-term plasticity of hippocampal synapses in favor of LTD.

beta-adrenergic receptors primarily are located on the dendrites of granule cells and interneurons but also are found on astrocytes and a few presynaptic profiles in the rat dentate gyrus

In the rat dentate gyrus, beta-adrenergic receptor (beta-AR) activation is thought to be important in mediating the effects of norepinephrine (NE). beta-AR-immunoreactivity (beta-AR-I) was localized in this study by light and electron microscopy in the rat dentate gyrus by using two previously characterized antibodies to the beta-AR. By light microscopy, dense beta-AR-I was observed in the somata of granule cells and a few hilar interneurons. Diffuse and slightly granular beta-AR-I was found in all laminae, although it was most noticeable in the molecular layer. Ultrastructurally, the cytoplasm of granule cell and interneuronal perikarya (some of which contained parvalbumin immunoreactivity) contained beta-AR-I. beta-AR-I was associated primarily with the endoplasmic reticula; however, a few patches were observed near the plasmalemma. Quantitative analysis revealed that the greatest proportion of beta-AR-labeled profiles was found in the molecular layer. The majority of beta-AR-labeled profiles were either dendritic or astrocytic. In dendritic profiles, beta-AR-I was prominent near postsynaptic densities in large dendrites, many of which originated from granule cell somata. Moreover, some beta-AR-I was found in dendritic spines, sometimes affiliated with the spine apparati. Astrocytic profiles with beta-AR-I were commonly found next to unlabeled terminals which formed asymmetric (excitatory-type) synapses with dendritic spines. Additionally, beta-AR-I was observed in a few unmyelinated axons and axon terminals, many of which formed synapses with dendritic spines. Dual-labeling studies revealed that axons and axon terminals containing tyrosine hydroxylase (TH), the catecholamine synthesizing enzyme, often were near both neuronal and glial profiles containing beta-AR-I. These studies demonstrate that hippocampal beta-AR-I is localized: 1) principally in postsynaptic sites on granule cells and a few interneurons (some of which were basket cells); and 2) in glial processes. These observations add further support to the contention that beta-AR-activation modulates synaptic function through disparate pathways: directly, at either postsynaptic densities or presynaptic processes, or indirectly, through adjacent glial processes.

Lack of expression of long-term potentiation in the dentate gyrus but not in the CA1 region of the hippocampus of mu-opioid receptor-deficient mice

The possible involvement of the mu-opioid receptor subtype in mechanisms of long-term potentiation (LTP) of the lateral perforant pathway to the dentate gyrus neurons, as well as of the Schaffer collateral-commissural input of CA1 neurons, was investigated using mu-opioid receptor-deficient mutant mice. In transversal hippocampal slices from mice lacking the mu-opioid receptor (MOR) only a short potentiation in the dentate gyrus after tetanization of the lateral perforant pathway was found. In contrast, the loss of the mu-opioid receptor in the CA1 region did not affect the potentiation of the field potentials induced by tetanization of the Schaffer collaterals. In parallel experiments, the application of 10 microM of the selective MOR-antagonist, funaltrexamine, decreased LTP in the dentate gyrus of wild-type mice but again did not alter the potentiation of the field potentials in the CA1. The loss of MOR-binding in the hippocampus was accompanied by a reduction in D2-binding sites indicating a possible compensatory role of the dopaminergic system. The D1- and glutamate binding was not affected. These observations confirm earlier results with pharmacological blockade of opioid receptors in the dentate gyrus and demonstrate an essential role of MOR activation for the generation of LTP in the dentate gyrus of the mouse but not necessarily in the CA1 region.

Mu opioid receptors are in somatodendritic and axonal compartments of GABAergic neurons in rat hippocampal formation

Activation of mu opioid receptors (MORs) has a net excitatory effect in the hippocampal formation through inhibition of gamma-amino butyric acid (GABA)-containing interneurons. To determine the precise subcellular targets of MOR agonists, immunoreactivity against MOR1 and GABA was examined in single sections of the hippocampal formation prepared for dual-labeling electron microscopy. In both the CA1 region of hippocampus and the dentate gyrus, MOR-like immunoreactivity (-li) was present in neuronal somata, dendrites, axons, and axon terminals, as well as a very few glial processes. Axon terminals with MOR-li formed symmetric synapses with principal cell dendrites and somata. Many MOR-labeled profiles of all types also contained GABA-li, and the vast majority possessed the ultrastructural characteristics of interneurons. Additionally, in the dentate gyrus a very small proportion of granule cell dendrites contained MOR-li. MOR-li, identified using immunogold-silver particles, was often affiliated with the extrasynaptic regions of neuronal plasma membranes, consistent with responsiveness to diffusing endogenous neuropeptide ligands. Semiquantitative analysis of the distribution of MOR-li revealed significantly more "presynaptic" (axons and terminals) than "postsynaptic" (somata and dendrites) labeled profiles in most laminae. We conclude that in addition to previously described somatodendritic MOR-li, a substantial amount of MOR-li in hippocampal formation is presynaptic. Furthermore, MORs are almost exclusively in GABAergic interneurons.

In the ventromedial nucleus of the rat hypothalamus, GABA-immunolabeled neurons are abundant and are innervated by both enkephalin- and GABA-immunolabeled axon terminals

Immunohistochemical-labeling for the neurochemicals gamma-aminobutyric acid (GABA) and enkephalin are abundant in the ventromedial nucleus of the hypothalamus (VMN). In VMN, both GABA and enkephalin may function to regulate feeding behavior, as well as other hormone-controlled behaviors. Importantly, in several brain areas, enkephalin is often thought to modulate GABAergic neurotransmission. Therefore, we used dual-labeling immunohistochemistry with electron microscopic analysis to study the circuitry of neurons containing GABA- and/or enkephalin-labeling within the VMN. Somato-dendritic profiles containing GABA-labeling were three fold more abundant than GABA-labeled axon terminals (117 soma or dendrites vs. 34 axons). In addition, axon terminals containing GABA-labeling sometimes synapsed onto GABA-labeled somata or dendrites (25% or 9/34). In contrast, under these conditions labeling for enkephalin was primarily restricted to axon terminals, which were very abundant throughout VMN. Enkephalin-containing terminals accounted for a large fraction (25% 23/92) of the axons in contact with GABA-labeled dendrites, although they also contacted unlabeled dendrites. These observations suggest that a population of VMN neurons are GABAergic. These may be either local circuit 'interneurons' or projection neurons. In addition, GABA-labeled VMN neurons may be regulated by either enkephalin or GABA. These morphologic observations provide the basis for disinhibitory mechanisms to function within the VMN.

Morphological substrates underlying opioid, epinephrine and gamma-aminobutyric acid inhibitory actions in the rat locus coeruleus

The locus coeruleus (LC) has been implicated in attentional processes related to orienting behaviors, learning and memory, anxiety, stress, the sleep-wake cycle, and autonomic control, as well as to contributing to the affective state. Direct activation of LC neurons causes desynchronization of the electroencephalogram, suggesting that the LC is an important modulator of the behavioral state. The LC has been an intensely studied neuronal system, as the physiology and pharmacology of this nucleus is well understood. This is mainly because of the similarity in neurochemical composition of LC cells which all contain norepinephrine in the rat. However, the homogeneity in neurotransmitter content in LC neurons is sharply contrasted by the heterogeneity of neurochemicals found in its afferent processes. Among these are axon terminals that contain inhibitory and excitatory amino acids, monoamines, and neuropeptides, many of which have been shown to exert differential physiological effects on LC discharge activity. Although much attention has focused on physiological activation of LC neurons, substantial evidence indicates that diverse afferents prominently inhibit noradrenergic cellular activity. Such inhibitory neurochemicals, which arise from local and extrinsic sources, include gamma-aminobutyric acid (GABA) and epinephrine as well as the neuropeptides methionine5-enkephalin and leucine5-enkephalin. Inhibitory transmission in the LC has widespread implications for norepinephrine release at diverse postsynaptic targets, and clinically useful pharmacological agents such as clonidine, an alpha2 adrenergic receptor agonist that potently inhibits the firing of LC neurons, alleviate some negative physical symptoms observed following withdrawal from opiates. In the present review, the synaptic and functional organization of selected inhibitory-type neurotransmitters in the LC obtained from immunoelectron microscopic data will be discussed.

Endogenous activation of mu and delta-1 opioid receptors is required for long-term potentiation induction in the lateral perforant path: dependence on GABAergic inhibition

Opioid peptides costored with glutamate have emerged as powerful regulators of long-term potentiation (LTP) induction in several hippocampal pathways. The objectives of the present study were twofold: (1) to identify which opioid receptor types (mu, delta, or kappa) regulate LTP induction at lateral perforant path-granule cell synapses and (2) to test the hypothesis that endogenous opioids regulate LTP induction via modulation of GABAergic inhibition. LTP of lateral perforant path-evoked field EPSPs was induced selectively by high-frequency stimulation applied to the outer third of the molecular layer of the dentate gyrus of rat hippocampal slices. No changes in medial perforant path responses occurred. LTP was blocked when high-frequency stimulation was applied in the presence of the mu receptor antagonist CTAP, the selective delta-1 receptor antagonist BNTX, or the delta-1 and delta-2 receptor antagonist naltrindole. By contrast, the kappa-1 opioid receptor antagonist NBNI had no effect on LTP induction. The role of GABAergic inhibition was investigated by comparing the effect of naloxone on LTP induction in slices maintained in standard buffer and picrotoxin-containing buffer. Naloxone blocked LTP in standard buffer, whereas normal LTP was induced in picrotoxin-treated, disinhibited slices. Finally, NMDA receptor blockade completely inhibited LTP in both standard and disinhibited slices. The results show that mu and delta-1 opioid receptors regulate LTP induction and that this mechanism critically depends on GABAergic inhibition. A key issue then becomes how endogenous opioids fine-tune the activity of intact inhibitory networks in the dentate gyrus, effectively gating synaptic plasticity in specific dendritic strata.

Electron microscopic evidence for coexistence of leucine5-enkephalin and gamma-aminobutyric acid in a subpopulation of axon terminals in the rat locus coeruleus region

We recently described ultrastructural evidence for morphologically heterogeneous axon terminals containing the endogenous opioid peptide, methionine5-enkephalin (ENK), that formed synapses with neurons containing the catecholamine synthesizing enzyme, tyrosine hydroxylase, in the locus coeruleus (LC) of the rat brain. The morphological characteristics of these terminals suggested that ENK may be co-localized with either an excitatory or inhibitory amino acid. To further test this hypothesis, we combined immunogold-silver localization of gamma-aminobutyric acid (GABA) and immunoperoxidase labeling for ENK in single sections through the LC, in the present study, to determine whether ENK and GABA were contained within single axon terminals. Light microscopic analysis of ENK and GABA immunoreactivities in the LC indicated that both transmitters were enriched in the dorsal pons. Although electron microscopy revealed that ENK and GABA were located primarily in axon terminals, some dendrites also contained immunolabeling for GABA. The dense core vesicles were consistently the most immunoreactive in ENK containing axon terminals and were identified toward the periphery of the axon terminal distal to the synaptic specialization. Axon terminals containing either ENK or GABA immunoreactivities contained pleomorphic vesicles as well as large dense core vesicles, varied in size and formed heterogeneous types of synaptic specializations (i.e. asymmetric vs. symmetric). Approximately 38% (n = 76) of the axon terminals containing ENK immunoreactivity (n = 200) also contained GABA. Some axon terminals containing peroxidase labeling for ENK (22%; n = 44) converged on common targets with GABA-labeled axon terminals. Finally, a few ENK-labeled axon terminals (14%; n = 28) formed asymmetric (excitatory-type) synapses with dendrites containing gold-silver labeling for GABA. The results, therefore, indicate that the opioid peptide, ENK, and the inhibitory amino acid, GABA, may influence LC neurons by concerted actions via (1) release from a common axon terminal, and (2) via separate sets of afferents converging on similar portions of the plasmalemma of target neurons. Furthermore, these studies also suggest a cellular substrate for opioid inhibition of LC neurons via activation (i.e. asymmetric synapses) of inhibitory GABAergic neurons. Future studies are required to determine whether the receptive sites for ENK and GABA are located at similar sites on the plasma membranes of LC neurons pre- or postsynaptically and whether there is differential release of either transmitter from single terminals in the LC.

Cellular and subcellular localization of delta opioid receptor immunoreactivity in the rat dentate gyrus

To study a potential locus of action of opioids in the rat dentate gyrus, we examined the localization of the delta opioid receptor (DOR) by immunocytochemistry. Two antisera raised to unique, non-overlapping peptide sequences located within the extracellular N-terminal sequence of DOR were tested. By light microscopy, numerous neurons in the central hilar region were intensely labeled for DOR, while the granule cell layer contained light DOR immunoreactivity. To further characterize hilar neuron cell types which contained DOR, sections through the dentate gyrus were double labeled using immunofluorescence with antisera to DOR and either gamma-aminobutyric acid (GABA), neuropeptide Y (NPY), or somatostatin-28 antisera. Most DOR-labeled perikarya also contained GABA and NPY, while a subpopulation contained somatostatin. Electron microscopic examination of sections labeled for DOR revealed that the immunoreactivity was common in profiles which exhibited the morphological characteristics of granule cells, as well as those of non-granule cells. DOR immunoreactivity was located at postsynaptic sites within neuronal perikarya (2%), dendrites (27%), and dendritic spines (22%); as well as in presynaptic axon terminals (25%) and glia (23%) (n = 279). In dendrites and dendritic spines, DOR immunoreactivity was most often associated with the plasmalemmal surface near asymmetric synapses. In axon terminals, DOR immunoreactivity primarily surrounded small, clear vesicles, and was less consistently found on the plasmalemmal surface. The distribution of DOR-labeled profiles overlapped with, but was not restricted to regions known to contain enkephalin. These data suggest that opiates acting at the DOR can modulate both hilar neurons and granule cells both pre- and postsynaptically.

Ultrastructural relationships between leu-enkephalin- and GABA-containing neurons differ within the hippocampal formation

Electrophysiological studies have suggested that the excitatory actions of opioids in the hippocampal formation are mediated by inhibition of interneurons containing GABA; however, an anatomical basis for this interaction has never been established. Thus, we sought to determine the relationship between leu-enkephalin (LE)-containing axon terminals and GABAergic neurons using dual labeling immunohistochemistry and electron microscopy. In the CA1 region of the hippocampus, LE-labeled terminals (n = 99) were in direct contact with GABA-labeled perikarya and dendrites (18%), and directly apposed to GABA-labeled axon terminals (14%). In the molecular layer of the dentate gyrus, LE-containing terminals (n = 125) occasionally apposed GABA-containing terminals (8%). In the hilus of the dentate gyrus, LE-containing terminals (n = 165) often contacted GABA-containing perikarya and dendrites (39%), but rarely apposed GABA-containing terminals (3%). In the CA3 region of the hippocampus, only a few LE-labeled mossy fiber boutons (n = 102) contacted the shafts of GABA-labeled dendrites (4%). The results demonstrate that leu-enkephalin-containing terminals have a different anatomical relationship with GABA-containing profiles in each subregion of the hippocampal formation. In the CA1 region of the hippocampus, the data support the numerous electrophysiological studies indicating that LE functions in modulating inhibitory GABAergic neurons by both pre- and postsynaptic mechanisms. In the outer molecular layer of the dentate gyrus the localization suggests some presynaptic regulation of GABAergic terminals. In the hilus of the dentate gyrus, the study also supports the contention that LE may have an important role in regulating inhibition of GABA-containing neurons. In comparison, in the CA3 region of the hippocampus, LE may have a more limited role in regulating GABAergic inhibition by direct association.