No abstract

Low doses of naloxone have been shown to affect the motivational aspects of opiate withdrawal in morphinedependent rats. Conditioned place aversion to opiate withdrawal is one of the most sensitive of motivational indices of opiate withdrawal and is thought to be mediated by the basal forebrain. Expression of the transcription factor Fos is known to increase during opiate withdrawal, but its presence during low-dose antagonist-precipitated withdrawal has not previously been established. In order to determine if there is a relationship between withdrawal-induced neuronal activity and conditioned place aversion, immunocytochemical localization of Fos was examined in the basal forebrain of opiate-dependent animals receiving one of several doses of naloxone (0, 3.25, 7.5, 15, 30, or 1000 g/kgChronic opiate use is partially characterized by the emergence of withdrawal symptoms upon cessation of drug administration. These symptoms include a wellcharacterized group of both physical and affective changes. In humans, affective symptoms often involve feelings of anxiety, restlessness, tension, irritability, and dysphoria (Haertzen and Hooks 1969;Henningfield et al. 1987;Jaffe 1990). Animal models of withdrawal include a number of characteristic somatic signs (Blasig et al. 1973;Gellert and Sparber 1977;Martin et al. 1963;Wei et al. 1975), as well as behavioral changes which are thought to reflect the affective symptoms associated with opiate withdrawal (Koob et al. 1993;Koob et al. 1989).In opiate-dependent rats, affective or motivational signs of withdrawal are apparent at low doses of opiate antagonists at which physical signs of withdrawal are not seen (Higgins and Sellers 1994). Animals experiencing low-dose antagonist-precipitated opiate withdrawal show suppressed locomotor activity (Brady and Holtzman 1981) and operant responding for food (Gellert and Sparber 1977;Higgins and Sellers 1994;Koob et al. 1989), increased intracranial self-stimulation (ICSS) thresholds (Schaefer and Michael 1986), and aversion to the environment associated with withdrawal (Hand et al. 1988;Mucha 1987;Schulteis et al. 1994;Stinus et al. 1990). Conditioned place aversion is one of the most sensitive of these indices and can be shown in dependent animals administered doses of opiate antagonists well below those able to induce physical signs of withdrawal (Schulteis et al. 1994).Many of the behaviors associated with the motivational aspects of opiate withdrawal have been shown to be modulated by certain structures of the basal forebrain known to be a part of the extended amygdala (Heinrichs et al. 1995;Koob et al. 1989;Stinus et al. 1990), notably the nucleus accumbens (Acb) and the central nucleus (ACe) of the amygdala (Alheid and Heimer 1988). Studies by Stinus et al. (1990) showed that both regions are involved in opiate withdrawalinduced conditioned place aversion.The transcription factor c-fos is commonly used as a marker of neuronal activity. Several studies have shown that opiate withdrawal increases the expression of the c-fos protei...

The effectiveness of NMDA antagonists in modulating the motor and motivational effects of opiates is attributed, in part, to functional associations involving NMDA receptors and -opioid receptors (MORs) in the shell of the nucleus accumbens (Acb). To determine the subcellular sites for potential functional interactions between opiate ligands and NMDA receptors in this region, we examined the ultrastructural localization of antipeptide antisera against MOR and the R1 subunit of the NMDA receptor in the Acb shell of the adult rat brain. MOR-like immunoreactivity (MOR-LI) was seen primarily in dendrites, whereas NMDAR1-like immunoreactivity (NMDAR1-LI) was detected more often in axon terminals forming asymmetric synapses. In these profiles, MOR labeling was localized mainly to extrasynaptic plasma membranes, whereas NMDAR1-LI was associated with both synaptic and extrasynaptic sites. Of 307 MOR-labeled processes, 17.9% of the dendrites and 9.4% of the axon terminals also contained NMDAR1-LI. In addition, 24.7% of the dendrites containing only MOR-LI were apposed to NMDAR1-labeled axons or terminals. We conclude that in the shell of the Acb, the output of single neurons can be dually modulated by (1) activation of MOR and NMDA receptors in the same dendrites or (2) combined activation of presynaptic NMDA receptors in afferents contacting dendrites containing MOR. In addition, the colocalization of MOR and NMDAR1 in certain axon terminals in the Acb suggests their dual involvement in the presynaptic release of neurotransmitters in this region.

The N-methyl-D-aspartate (NMDA)-type glutamate receptor in the basolateral amygdala (BLA) has been implicated in activity-dependent plasticity important for cortically evoked acquisition of fear-potentiated startle response. We examined the ultrastructural immunoperoxidase labeling of the R1 subunit of the NMDA receptor in the BLA of adult rats to determine the potential cellular and subcellular sites mediating the effects generated by NMDA activation. The localization was compared with that seen in the bed nucleus of the stria terminalis (BNST), the major efferent pathway from the central nucleus of the amygdala, which has a more pronounced involvement in autonomic function. Electron microscopy established that in the BLA, 68.4% (n = 177) of the profiles showing NMDAR1-like immunoreactivity (NMDAR1-LI) were dendrites, and 19.8% were distal tips of astrocytic processes. In contrast, profiles containing NMDAR1-LI (n = 262) in the BNST were more equally distributed between dendrites (37.4%) and axons (38.2%). The subcellular localization of NMDAR1 immunoreactivity was, however, similar in both regions. Our findings provide the first ultrastructural evidence that glutamate may prominently act through NMDAR1 receptors to elicit postsynaptic actions on intrinsic neurons in the BLA and BNST. The results also indicate that, in the BLA, the NMDAR1 receptor plays an important role in astrocytic function, whereas the receptor is more preferentially a presynaptic modulator in axons which terminate in or pass through the BNST.

The patch compartments of the caudate‐putamen nucleus (CPN) are enriched in μ‐opioid receptors (MORs) and have been recently implicated in reward‐related behaviors. This function has been established more clearly in the nucleus accumbens, where physiological and anatomical studies show reward‐associated interactions involving MORs and N‐methyl‐D‐aspartate‐type glutamate receptors (NMDARs). We examined the immunolabeling for MOR and NMDAR subunit NR1 in patches of the rat CPN to determine the potential relevance of dual activation of the respective receptors. Electron microscopy showed the presence of MOR and/or NR1 immunoreactivity (IR) in many perikarya, dendrites, and spines and in morphologically heterogeneous axon terminals. In each 1,000‐μm2 area, the dually labeled dendrites and spines constituted 65% (37/57) and 37% (9/25) of the total NR1‐labeled and 34% (37/109) and 13% (9/71) of the total MOR‐labeled dendritic profiles. Dually labeled spines received asymmetric excitatory‐type synapses from terminals, which were generally unlabeled, but also occasionally contained MOR and/or NR1. The asymmetric synapses comprised the majority (81%) of the total 263 synaptic contacts between MOR‐ and NR1‐labeled neuronal profiles. In dendrites and spines, MOR‐IR was localized mainly along nonsynaptic plasma membranes, whereas NR1‐IR was more often associated with asymmetric postsynaptic densities and cytoplasmic organelles. In contrast to dendrites, 6% (1.3/22) of NR1‐IR and 4% (1.3/33) of MOR‐IR axon terminals were dually labeled in each 1,000‐μm2 area. Most singly or dually labeled terminals formed asymmetric synapses with MOR‐ or NR1‐labeled spines. Our results suggest that opioids acting through MOR and excitatory neurotransmitters through NMDAR dually regulate the output of single spiny neurons and some of their excitatory afferents in the CPN. J. Comp. Neurol. 412:132–146, 1999. © 1999 Wiley‐Liss, Inc.