Periaqueductal gray neurons response to microiontophoretically injected morphine in naive and morphine-dependent rats

Periaqueductal gray afferents synapse onto dopamine and GABA neurons in the rat ventral tegmental area

The midbrain central gray (periaqueductal gray; PAG) mediates defensive behaviors and is implicated in the rewarding effects of opiate drugs. Projections from the PAG to the ventral tegmental area (VTA) suggest that this region might also regulate behaviors involving motivation and cognition. However, studies have not yet examined the morphological features of PAG axons in the VTA or whether they synapse onto dopamine (DA) or GABA neurons. In this study, we injected anterograde tracers into the rat PAG and used immunoperoxidase to visualize the projections to the VTA. Immunogold-silver labeling for tyrosine hydroxylase (TH) or GABA was then used to identify the phenotype of innervated cells. Electron microscopic examination of the VTA revealed axons labeled anterogradely from the PAG, including myelinated and unmyelinated fibers and axon varicosities, some of which formed identifiable synapses. Approximately 55% of these synaptic contacts were of the symmetric (presumably inhibitory) type; the rest were asymmetric (presumably excitatory). These findings are consistent with the presence of both GABA and glutamate projection neurons in the PAG. Some PAG axons contained dense-cored vesicles indicating the presence of neuropeptides in addition to classical neurotransmitters. PAG projections synapsed onto both DA and GABA cells with no obvious selectivity, providing the first anatomical evidence for these direct connections. The results suggest a diverse nature of PAG physiological actions on midbrain neurons. Moreover, as both the VTA and PAG are implicated in the reinforcing actions of opiates, our findings provide a potential substrate for some of the rewarding effects of these drugs.

Functional characteristics of the midbrain periaqueductal gray

The major functions of the midbrain periaqueductal gray (PAG), including pain and analgesia, fear and anxiety, vocalization, lordosis and cardiovascular control are considered in this review article. The PAG is an important site in ascending pain transmission. It receives afferents from nociceptive neurons in the spinal cord and sends projections to thalamic nuclei that process nociception. The PAG is also a major component of a descending pain inhibitory system. Activation of this system inhibits nociceptive neurons in the dorsal horn of the sinal cord. The dorsal PAG is a major site for processing of fear and anxiety. It interacts with the amygdala and its lesion alters fear and anxiety produced by stimulation of amygdala. Stimulation of PAG produces vocalization and its lesion produces mutism. The firing of many cells within the PAG correlates with vocalization. The PAG is a major site for lordosis and this role of PAG is mediated by a pathway connecting the medial preoptic with the PAG. The cardiovascular controlling network within the PAG are organized in columns. The dorsal column is involved in pressor and the ventrolateral column mediates depressor responses. The major intrinsic circuit within the PAG is a tonically-active GABAergic network and inhibition of this network is an important mechanism for activation of outputs of the PAG. The various functions of the PAG are interrelated and there is a significant interaction between different functional components of the PAG. Using the current information about the anatomy, physiology, and pharmacology of the PAG, a model is proposed to account for the interactions between these different functional components.

Reduction of morphine dependence and potentiation of analgesia by chronic co-administration of nifedipine

Nifedipine, 5 mg/kg IP, potentiated the morphine-induced analgesia measured in the hot-plate, but not in the tail-flick test. Further experiments were carried out using the hot-plate test only. Pretreatment with nifedipine partially restores the analgesic action of morphine in morphine-tolerant rats. Co-administration of nifedipine with morphine in a chronic experiment did not prevent the loss of morphine efficiency (an increase in latency of 44% was not significant) and did not prevent the debilitating effect of chronic morphine administration reflected by an inhibition of the body weight gain, but prevented naloxone-induced withdrawal syndrome (quantified by counting head shakes) in the test carried out 24 h after the injection of nifedipine, when the drug did not affect morphine analgesia. Chronic treatment with either morphine or nifedipine did not produce a significant increase in the density of [3H] naloxone or [3H]prazosin binding sites in the cortex and in the rest of the brain (measured 24 h after the last dose), but the combined treatment resulted in a significant increase in the cortical [3H]prazosin binding site density. The present results suggest that opiate tolerance and physical dependence may be separated by co-administration of nifedipine and suggest that the combined chronic treatment with morphine and nifedipine may increase the efficacy of morphine during chronic treatment and prevent development of abstinence.

The response of individual nucleus raphe magnus neurons to microinjections of met-enkephalin at midbrain and at bulbar loci: evidence for midbrain-bulbar convergence on individual raphe neurons

Local microinjection of met-enkephalin into both the nucleus reticularis paragigantocellularis (Pgc) and into the midbrain periaqueductal gray (PAG) altered the spontaneous discharge of many nociresponsive units in the nucleus raphe magnus (NRM) of the urethane-anesthetized rat. The activity of the majority of units (11 of 14) was suppressed following microinjections into both the PAG and Pgc. Two units showed an increase in spontaneous activity after PAG microinjections and a decrease after Pgc microinjections. One cell showed a decrease in activity after microinjection into Pgc and was unaffected by the PAG microinjection. These findings demonstrate that many nociresponsive units in NRM are affected by microinjections of met-enkephalin into both Pgc and the PAG, indicating a convergent influence of these two regions on single NRM neurons.

Microiontophoretic application of morphine and naloxone to neurons in hypothalamus of rat

The present experiments used urethane-anesthetized rats and single cell recording to study the electrophysiological properties of ventromedial hypothalamic (VMH) cells following different doses of morphine and naloxone, applied microiontophoretically. More than 45% of ventromedial hypothalamic units reacted in a dose-response fashion to local application of morphine. In the majority of the ventromedial hypothalamic neurons, naloxone failed to reverse the effects of morphine. Naloxone alone had effects on 37% of the ventromedial hypothalamic units. The ventromedial hypothalamic units exhibited different response patterns from those observed from other CNS sites in response to the microiontophoretic application of morphine and naloxone; this difference is discussed. The present neurophysiological findings support the existence of opiate target sites with multiple opiate receptors within the ventromedial hypothalamus.

EEG effects of microinjection of naloxone in the region of the nucleus tractus solitarius of the rat

In recent years, we have carried out several studies on brainstem mechanisms associated with pain modulation. Of particular interest has been the region of the nucleus tractus solitarius (NTS) at the level of the area postrema (AP) in the dorsomedial medulla. This region has been found to be rich in opiate receptors and to be involved in pain modulation. Having previously demonstrated the EEG and behavioral effects of systemic administration of morphine, we initiated the present study to determine whether microinjection of naloxone directly into the region of the NTS will modify any or all of the EEG effects induced by subsequent systemic administration of morphine. Our results have shown that microinjections of naloxone into the region of the NTS block the cortical EEG synchronizing effects of systemically administered morphine. These results indicate that morphine-induced cortical EEG synchronization is mediated through the region of the NTS. This region may, therefore, comprise part of an opiate sensitive system which can influence cortical EEG activity.

Caudate neuronal response to microiontophoretically injected morphine in naive and morphine-dependent rats

1. The response of caudate nucleus neurons to morphine was found to be dose-dependent and could be divided into two classes: neurons which responded monophasically either by increase or decrease in their firing rate, and neurons whose response can be described as biphasic, exhibiting increase followed by decrease in their firing rate or vice versa, with the increase in morphine concentration. These responses were found in both naive and morphine-dependent rats. 2. Naloxone antagonized the effects of morphine in 74 out of 102 neurons tested. 3. Caudate neurons of morphine-dependent rats showed super-sensitivity to morphine compared to naive rats. 4. Differences were found in the distribution of the spontaneously active neurons between naive and morphine-dependent rats, indicating the existence of two different opiate receptor populations within the caudate nucleus.