The nature of conditioned anti-analgesia: spinal cord opiate and anti-opiate neurochemistry

The central nervous system contains circuitry that inhibits pain sensitivity (analgesia), as well as circuitry that opposes pain inhibition (anti-analgesia). Activation of analgesia systems and anti-analgesia systems can each be brought under environmental control using classical conditioning procedures. Analgesia can be produced by cues present before and during aversive events such as electric shock, while active inhibition of analgesia comes to be produced by cues never present immediately before or during shock and therefore signal safety. We have recently reported that these analgesia and anti-analgesia systems interact at the level of the spinal cord. A series of 3 experiments were performed to examine how such interactions occur. First, potential opioid mediation of conditioned analgesia was investigated using systemic and intrathecal (i.t.) delivery of opiate antagonists. Conditioned analgesia was found to be mediated by activation of spinal μ and δ opiate receptors. Second, analgesia produced by each of these receptor subtypes was challenged by environmental signals for safety. Analgesias produced by μ and δ opiate agonists were each abolished by safety signals. Third, antagonists/antisera directed against several putative anti-opiate neurotransmitters were tested i.t. to identify which mediate conditioned anti-analgesia at the level of the spinal cord. A cholecystokinin antagonist abolished conditioned anti-analgesia. In contrast, neuropeptide FF antiserum and a κ opiate antagonist were without effect.     

Central Effects of Neuropeptide K on Water and Food Intake in the Rat

The present study investigated the effect on water and food intake in the rat of the intracerebroventricular (ICV) injection of neuropeptide K (NPK), the N-terminally extended form of neurokinin A. NPK inhibited water deprivation-induced water intake even at 31.2 ng/rat. At higher doses, it inhibited also water intake induced by ICV angiotensin II or by subcutaneous hypertonic NaCl, and food-associated drinking, the threshold dose being 125 ng/rat. In response to 125 ng/rat, food intake following 16 h food deprivation was not reduced. NPK inhibited food intake only at 500 ng/rat, a dose that evoked excessive grooming in treated animals. Thus NPK is a potent inhibitor of water deprivation-induced drinking and at higher doses it exerts a general antidipsogenic effect towards several dipsogenic determinants, without affecting food intake. On the other hand, it inhibits food intake only at high doses, 500 ng/rat or more, but this inhibition might be just related to the intense grooming evoked. The effects of NPK on ingestive behavior are markedly different from those of neurokinin A, which selectively inhibits osmotic drinking and food-associated drinking. These differences suggest that NPK itself may exert its effects on the central nervous system, not necessarily through the conversion to neurokinin A.     

Effects of peripheral axotomy of the inferior alveolar nerve on the levels of neuropeptide Y in rat trigeminal primary afferent neurons

The effects of peripheral axotomy of the inferior alveolar nerve (IAN) on the presence and distribution of neuropeptide Y (NPY)-like immunoreactivity (IR) in the trigeminal sensory nuclear complex (TSNC) and trigeminal ganglion were investigated in the rat by immunohistochemistry. In the normal trigeminal ganglion, there were no NPY-IR cells, and some perivascular nerve fibres exhibited NPY-IR. In normal TSNC, many NPY-IR axons and nerve terminals were observed in the superficial layers of the subnucleus caudalis (SpVc) and paratrigeminal nucleus (paraV), but were sparse in the other subnuclei of the TSNC. Fourteen days following peripheral axotomy of the IAN, many large- and medium-sized cells in the trigeminal ganglion displayed NPY-IR, and marked increases in the numbers and staining densities of NPY-IR were observed in deeper laminae (laminae III-V) of the dorso-medial region of the SpVc and other nuclei, in addition to the dorso-medial region of the spinal trigeminal tract. Degrees of alterations of the levels of NPY were most marked in the SpVc. The present results indicate that peripheral axotomy of the IAN evokes the appearance of NPY-IR in the trigeminal ganglion and alterations of NPY-IR in the entire IAN projection areas of the TSNC.     

Selective protection of neuropeptide containing dentate hilar interneurons by non-NMDA receptor blockade in an animal model of status epilepticus

We used a 24 h perforant path stimulation model of status epilepticus to study the role of non-NMDA receptors in the loss of hilar interneurons and paired pulse inhibition associated with the model. In one experiment, NBQX administered i.v. at 1.0 mg/kg/h significantly reduced the loss of hematoxylin and eosin-stained hilar neurons from 360.2 to 125.3 but failed to protect against the loss of paired pulse inhibition. In a second experiment, i.v. NBQX at 1.5 mg/kg/h significantly protected against loss of SS- and NPY-positive hilar interneurons but also failed to protect against loss of paired pulse nhibition. These results demonstrate that the neuronal loss associated with sustained stimulation of this excitatory pathway is mediated in part through non-NMDA receptors. The lack of protection against loss of paired pulse inhibition suggests that SS- and NPY-immunoreactive interneurons may not be responsible for frequency-dependent paired-pulse inhibition of dentate granule cells.     

CGRP-like immunoreactivity in A11 dopamine neurons projecting to the spinal cord and a note on CGRP-CCK cross-reactivity

Using the indirect immunofluorescence technique and double labelling procedures combined awith retrograde tracing it could be demonstrated that the A11 dopamine cell group, located at the border between the diencephalon and mesencephalon of the rat brain and some of which project to the spinal cord, contains calcitonin gene-related peptide (CGRP)-like immunoreactivity. Thus, another catecholamine group in the rat brain has been shown to have a coexisting peptide. One of the CGRP antisera used in the present study also stained cholecystokinin (CCK) containing neurons in various brain areas. Absorption and displacement experiments using immunohistochemistry and radioimmunoassay showed that this cross-reactivity was confined to the C-terminal portion of the peptide molecule. Therefore, the present results suggest that CGRP antisera used for immunohistochemistry and radioimmunoassay should be tested for possible cross-reactivity with CCK.     

EEG effect of basal forebrain neuropeptide Y administration in urethane anaesthetized rats

Neuropeptide Y (NPY) is present both in local neurons as well as in fibers in the basal forebrain (BF), an area that plays an important role in the regulation of cortical activation. In previous studies, NPY axons were found to innervate corticopetal cholinergic cells in this area. In addition, identified NPY positive neurons have been shown to be silent during cortical activation, but active during slow EEG waves. However, no in vivo studies have shown the effect of local NPY release in the BF on the EEG. In the present experiments, the EEG was examined following NPY injection (0.5 microl, 300-500 pmol) into the BF of urethane-anaesthetized rats. Fronto-parietal EEG was recorded on both sides and relative EEG power was calculated in the delta (0-3 Hz), theta (3-9 Hz), alpha (9-16 Hz) and beta (16-48 Hz) frequency bands. We found a significant increase in relative delta power and a decrease in the power of all higher frequency bands (theta, alpha, beta) after NPY injection. These results suggest that NPY can inhibit cortical activation via the BF.     

Distribution of methionine and leucine enkephalin neurons within the social behavior circuitry of the male Syrian hamster brain

Enkephalin plays a role in the social behaviors of many species, but no corresponding role for this peptide has been investigated in the male Syrian hamster, a species in which brain nuclei controlling social behaviors have been identified. Previous studies have shown the distribution of dynorphin and beta-endorphin throughout social behavior circuits within the male hamster brain. To date, the only studies of enkephalin in the hamster brain address the distribution of this peptide in the olfactory bulb and hippocampus. The present study provides a complete map of enkephalinergic neurons within the forebrain and midbrain of the male Syrian hamster and addresses the question of whether enkephalin immunoreactive (Enk-ir) cells are found within brain regions relevant to male hamster social behaviors. Following immunocytochemistry for either methionine enkephalin (met-enkephalin) or leucine enkephalin (leu-enkephalin), we observed enkephalin localization consistent with data that have previously been reported in the rat, with notable exceptions including lateral septum, ventromedial nucleus of the hypothalamus and cingulate gyrus. Additionally, met- and leu-enkephalin localization patterns largely overlap. Consistent with the post-translational processing of preproenkephalin, met-enkephalin was more abundant than leu-enkephalin both within individual cells (darker staining), and within given brain nuclei (more met-enkephalin immunoreactive cells). Two exceptions were the posterointermediate bed nucleus of the stria terminalis, containing more neurons heavily labeled for leu-enkephalin, and the main olfactory bulb, where only met-enkephalin was observed. Of most interest for this study was the observation of Enk-ir cells and terminals in areas implicated in both sexual and agonistic behaviors in this species.     

Functional interaction between neuropeptide Y receptors and modulation of calcium channels in the rat hippocampus

We investigated the functional interaction between neuropeptide Y (NPY) receptors using nerve terminals and cultured rat hippocampal neurons, and we evaluated the involvement of voltage-gated Ca(2+) channels (VGCCs) in NPY receptors-induced inhibition of Ca(2+) influx and glutamate release. The KCl-evoked release of glutamate from hippocampal synaptosomes was inhibited by 1 microM NPY and this effect was insensitive to either BIBP3226 (Y1 receptor antagonist) or L-152,804 (Y5 receptor antagonist), but was sensitive to BIIE0246 (Y2 receptor antagonist). We could also pharmacologically dissect the NPY receptors activity by using Y1, Y2 and Y5 receptor agonists ([Leu(31),Pro(34)]NPY, NPY13-36, NPY (19-23)-(Gly(1),Ser(3),Gln(4),Thr(6),Ala(31),Aib(32),Gln(34))-pancreatic polypeptide (PP), respectively), and in all the cases we observed that these agonists could inhibited the KCl-induced release of glutamate. However, the selective and specific co-activation of both Y1 and Y2 or Y2 and Y5 receptors resulted in non-additive inhibition, and this effect was prevented in the presence of the Y2 antagonist, but was insensitive to the Y1 or Y5 receptor antagonist. Moreover, as we previously showed for Y1 receptors, we also observed that the activation of Y5 receptors inhibited the glutamate release in the dentate gyrus and CA3 subregion, without significant effect in the CA1 subregion of the hippocampus. The same qualitative results were obtained when we investigated the role of NPY Y1 and Y2 receptors in modulating the changes in [Ca(2+)](i) due to KCl depolarisation in cultured hippocampal neurons. The inhibitory effect of nitrendipine (L-type VGCC blocker) or omega-conotoxin GVIA (omega-CgTx; N-type VGCC blocker) was not potentiated by the simultaneous activation of Y1 or Y2 receptors. Moreover, the exocytotic release of glutamate was inhibited by omega-agatoxin IVA (omega-Aga; P-/Q-type VGCC blocker), and this VGCC blocker did not potentiate Y1, Y2 or Y5 receptor-mediated inhibition of glutamate release. Also, the effect of ionomycin in inducing the exocytotic release of glutamate from hippocampal synaptosomes was insensitive to the activation of NPY receptors. In the present paper, we identified a role for NPY Y1, Y2 and Y5 receptors in modulating the exocytotic release of glutamate and the [Ca(2+)](i) changes in the rat hippocampus. In conditions of co-activation, there appears to exist a physiological cross-talk between Y1 and Y2 and also between Y2 and Y5 receptors, in which Y2 receptors play a predominant role. Moreover, we also show that Y1 and Y2 receptors exert their inhibitory action by directly modulating L-, N-, and P-/Q-type VGCCs, whereas the inhibition of glutamate release mediated by the Y5 receptors seems to involve P-/Q-type VGCCs.     

Integration of hypothalamic feeding and metabolic signals: focus on neuropeptide Y

Research is reviewed on effects of neuropeptide Y (NPY) on energy substrate utilization and central interactions among NPY, serotonin and urocortin, particularly in neurons of the paraventricular nucleus of the hypothalamus.