Morphine and ACTH1-24: correlative behavioral excitations following micro-injections in rat periaqueductal gray

Chronic infusion of a melanocortin receptor agonist modulates dopamine receptor binding in the rat brain

Previous studies have shown that melanocortin peptides have facilitatory effects on dopaminergic neurotransmission. In the present study we tested the hypothesis that chronic exposure to melanocortin receptor agonists causes a prolonged release of dopamine resulting in changes in the expression of dopamine receptor subtypes. Using an autoradiographic approach we found that a 2 week intracerebroventricular infusion of the melanocortin receptor agonist melanotan-II induced changes in dopamine D(1)-like and D(2)-like receptor binding in several regions of the rat brain. D(1)-like receptor binding was increased in the nucleus accumbens and the caudate putamen, but reduced in the substantia nigra (reticular part), whereas D(2)-like receptor binding was reduced in the caudate putamen, but increased in the periaqueductal grey, substantia nigra (compact part) and the ventral tegmental area. These data suggest that chronic infusion of a melanocortin receptor agonist alters the activity of dopaminergic neurons in the ventral tegmental area and substantia nigra, and support the hypothesis that melanocortin peptides may regulate the activity of central dopamine neurons.

Excitatory amino acids: role in morphine excitation in rat periaqueductal gray

Morphine was previously found to elicit an explosive excitatory behavior following its injection at a high dose in the rat periaqueductal gray (PAG). This non-naloxone reversible excitatory action of morphine was mimicked by the GABAA receptor antagonist, bicuculline, suggesting that morphine excitation was due in part to GABAA receptor blockade. In this paper, we report that injections of the excitatory amino acid (EAA) analogues, N-methyl-D-aspartate (NMDA), quisqualate (Q) or kainate (K) in the rat PAG resulted in similar (but not identical) behaviors. The excitatory actions of morphine or of NMDA (but not Q or K) were blocked or attenuated by the NMDA receptor antagonist, 2-amino-7-phosphonoheptanoate. These results show that both GABAA receptors as well as receptors for the EAAs may contribute to the excitatory actions of morphine in the PAG, and suggest that GABA may normally function to counterbalance a tonic excitatory influence of the EAAs.

Modulation by morphine of aversive-like behavior induced by GABAergic blockade in periaqueductal gray or medial hypothalamus

Pretreatment with "analgesic" doses (15 nmoles) of morphine injected either into the periaqueductal gray (PAG) or into the medial hypothalamus (MH) were found to modulate flight behavior elicited by bicuculline injected into the same brain sites. When injected into the MH, morphine always suppressed bicuculline-induced flight, while PAG injections paradoxically either suppressed or facilitated the behavioral effects produced by bicuculline. Whenever a facilitation of the bicuculline-induced effects had been observed following pretreatment with 15 nmoles of morphine into the PAG, the infusion of lower doses (6 nmoles) did no longer induce facilitation but clear suppression. In those animals that had shown suppression of the aversive-like effects of bicuculline following the same 15 nmoles pretreatment, infusion of higher doses (24 nmoles) of morphine into the PAG still produced the same kind of suppression. And yet, when injected into the PAG, very high doses of morphine (50 nmoles) were found to induce, by themselves, flight behavior known as explosive motor behavior. In contrast, such high doses of morphine never induced comparable explosive motor behavior when injected into the MH. These data can be explained by the involvement of different types of receptors in the neural mechanisms subserving and controlling the generation of aversion in periventricular brain regions (PAG and MH).

Non-stereospecific excitatory actions of morphine may be due to GABA-A receptor blockade

An explosive motor behavior (EMB) similar to that seen following morphine injection into the rat periaqueductal gray (PAG) was observed following an injection of GABA-A receptor antagonists into the rat PAG. In general, the potencies of certain opiates and known GABA-A antagonists in producing EMB following their injections into the PAG paralleled their potencies as GABA antagonists in a radioreceptor assay. We suggest that one of the dual actions of morphine in the CNS may be GABA blockade.

A neuropharmacological study of the periventricular neural substrate involved in flight

This paper reviews results obtained in experiments concerning the neurochemical characteristics of the substrate involved in the control of flight reactions and the induction of aversive effects in the rat. These experiments investigated the behavioural effects produced by microinjecting into the periaqueductal grey matter (PAG) or the medial hypothalamus (MH) compounds known to interfere with the functioning of some neurotransmitter systems known to exist in these structures. The data obtained show that: the activity of the substrate involved in the production of flight reactions is tonically inhibited by the release of GABA (gamma-aminobutyric acid); the behavioural reactions produced by microinjecting GABA antagonists can be clearly distinguished, depending on whether such drugs were injected into the PAG or the MH, despite the fact that jumps were produced from either level; behavioural effects, comparable to some extent to those produced by microinjections of GABA antagonists, can be obtained by injecting drugs which act on non-GABAergic neurochemical substrates, namely opioidergic or cholinergic systems; and behavioural effects, comparable to those produced by injecting GABA antagonists into the PAG, can be obtained by injecting such drugs into various sites located in other parts of the tectum such as the inferior colliculus or adjacent structures.

The midbrain periaqueductal gray in the rat. I. Nuclear volume, cell number, density, orientation, and regional subdivisions

The midbrain periaqueductal gray is a functionally heterogeneous region which plays an important role in pain modulation. Despite the heterogeneity considerable controversy exists regarding the presence or absence of morphological subdivisions within the region. The present study was designed to evaluate the possibility of morphological subdivisions within the rat periaqueductal gray by using a statistical cluster analysis system. In addition both qualitative and quantitative data concerning neuronal size, shape, and density were obtained. On the basis of measurements of over 12,000 neurons in two planes of section, the mean neuronal length of cell bodies in this region was 14.82 microns and the mean neuronal area was 95.59 microns squared . The mean neuronal density was found to be 16,284 cells per mm3. Neuronal density decreased from rostral to caudal in the periaqueductal gray. The data obtained from cluster maps suggest the presence of four subdivisions within this midbrain region. The medial subdivision contains the smallest neurons and exhibits the lowest cell density. The dorsolateral and ventrolateral divisions contain the largest neurons while the dorsal division displays the highest packing density. These results are discussed in light of recent receptor binding and immunohistochemical studies of this region.

Identification of midbrain neurones mediating defensive behaviour in the rat by microinjections of excitatory amino acids

Unilateral microinjections (0.20 microliter) of excitatory amino acids were made into the midbrain of freely moving rats. Injections made within the midbrain periaqueductal grey matter (PAG) consistently elicited reactions characteristic of defensive behaviour (i.e. explosive jumps, freezing, upright postures), whereas injections made in the tegmentum bordering the PAG did not as reliably elicit such behaviour. As injections of excitatory amino acids depolarize cell bodies but not axons, the results suggest that a population of neurones whose excitation elicits these reactions is found primarily within the midbrain PAG of the rat. Furthermore, the data suggested that such neurones may be localized preferentially within the caudal half of the midbrain PAG. Injections of the GABA antagonist, bicuculline methiodide, at many of the same midbrain sites produced behaviour similar to that elicited by excitatory amino acids indicating a possible GABAergic modulation of these same PAG-mediated reactions. It also was observed following unilateral injection into the PAG, of either the excitatory amino acid, L-aspartic acid or bicuculline, that defensive behaviour was elicited by touching the rat on the body or snout contralateral but not ipsilateral to the injection site. This suggests that the induction of defensive behaviour by unilateral PAG stimulation is due, at least in part, to lateralized alterations in sensorimotor responsiveness.

Behavioral support for an ACTH receptor in the CNS

In this report we present a series of experiments which have led us to support the notion of the presence of an ACTH receptor in the CNS. A short intense heat-stress (hot-plate) applied to the paws of rats will temporarily reduce activity. During the course of experimentation we were able to eliminate a number of logical mediators. Neither adrenalectomy, adrenal-medullectomy, naloxone administration, nor alpha-MSH-(1-12) were able to affect the observed akinesia. Hypophysectomy, however, was able to abolish or mask the behavior and the reduction in activity could be reinstated by the administration of ACTH-(4-10) to hypophysectomised rats. These data support the notion that a short intense stressor can release ACTH and that this ACTH can be responsible for mediating the short term reduction in activity. In addition, the fact that ACTH-(4-10) has only minimal steroidogenic properties and was able to reinstate the behavior led us to speculate that these effects were of central origin. Furthermore, since naloxone was not capable of altering the behavior, the suggestion is that ACTH in this paradigm acted at a receptor site apart from the naloxone sensitive receptor. This site may in fact be an ACTH specific receptor.

Behavioral effects of morphine, levorphanol, dextrorphan and naloxone in the frog Rana pipiens

Systemic morphine induces explosive motor behavior and generalized muscular rigidity in frogs. Naloxone does not reverse either of these effects of morphine but at high doses causes muscular flaccidity and unresponsiveness to stimulation. Intraspinal morphine induces rigidity, but not explosive motor behavior, and this action is blocked by naloxone. Behavioral effects are seen rarely after intraspinal levorphanol (rigidity) and never after intraspinal dextrorphan or naloxone. In contrast to systemic morphine and naloxone, systemic levorphanol and dextrorphan are lethal to frogs at high doses.

Chlordiazepoxide-induced potentiation of hexobarbitone sleeping time is reduced by ACTH(1-24)

The effects of ACTH(1-24) on the potentiation of hexobarbitone sleeping time by chlordiazepoxide (CDP) were investigated in the rat. CDP (10 mg/kg, IP) significantly (p less than 0.001) potentiated hexobarbitone (100 mg/kg, IP)-induced sleeping time. This effect was significantly (p less than 0.01) reduced by ACTH(1-24) (10 micrograms/100 g, IP), although ACTH did not influence the response to hexobarbitone in the absence of CDP. The possible implications of these findings are discussed in relation to recent observations concerning the possible interactions between ACTH and benzodiazepines.

The relative significance of spinal and supraspinal actions in the antinociceptive effect of morphine in the dorsal horn: an evaluation of the microinjection technique

Large quantities of morphine injected directly into the brainstem of spinal anaesthetized cats inhibited the noxious heat-evoked excitation of dorsal horn neurones. The amounts required were similar to those that were required intravenously in cats with the spinal cord intact or transected. When the spinal cord was intact the amount of morphine microinjected into the brainstem required to inhibit the excitation of dorsal horn neurones was about ten fold less than it was in spinal animals. It is concluded that large, but not small doses of morphine microinjected into the brainstem can exert effects on the spinal cord after first entering the circulation. The effects of small doses are attributed to a local action in the brainstem which causes inhibition of spinal neurones either by activating descending inhibitory neuronal systems or by liberating endogenous substances which reach the spinal cord via the cerebro-spinal fluid. The concentrations of morphine achieved at various distances from the site of injection by the microinjection of microgram quantities and the time courses of the concentration changes were calculated from diffusion equations, assuming diffusion coefficients of 3 or 5 X 10(6) cm2 s-1. The curves obtained closely approximated those obtained experimentally. The concentrations achieved at distances up to 2 mm from the site of injection of 10 micrograms of morphine were calculated to exceed 10(-4)M and the time-courses of these concentration changes were compatible with the time course of inhibition of spinal neurones, or the production of analgesia after microinjection. Such concentrations are vastly in excess of those achieved in the brain after the systemic administration of morphine in analgesic doses. It is concluded that the local effects in the brainstem produced by the microinjection of microgram quantities of morphine have no relevance to the mechanism of analgesia produced by systemic administration.

Dual actions of morphine on the central nervous system: parallel actions of beta-endorphin and ACTH

In the studies described above, the intracerebral microinjection technique was used to study the actions of morphine at morphine-sensitive sites, the periaqueductal gray (PAG) and the midbrain reticular formation (MRF). In the PAG, morphine exerted dual actions: inhibitory and excitatory. In the MRF, morphine exerted an excitatory action only, indicating that the dual actions of morphine are dissociable and site specific. Following microinjection into the PAG, beta-endorphin exerted an inhibitory, and ACTH an excitatory, action, i.e., each duplicated one of morphine's dual actions. These results indicated that receptors for the endogenously occurring peptides, beta-endorphin and ACTH, may play a role in morphine's potent pharmacological actions. Although these studies do not shed direct light on the physiological role of these neuropeptides and their receptors, nor on their potential roles in the functional regulation of brain (especially in diseased mental states), it may be permissible to offer some speculations. We previously proposed that beta-endorphin may be an endogenous antipsychotogen, and that a deficiency in brain beta-endorphin may underlie some forms of psychopathology. In view of beta-endorphin's biosynthetic link to ACTH, and the behavioral effects of beta-endorphin (sedated immobility) that was found to be opposite in kind to those of ACTH (fearful hyperreactivity) following administration into brain, it is possible that these two neuropeptides may have regulatory roles in maintaining a functional balance in brain. (It may be speculated that these biosynthetically linked neuropeptides served survival functions of "flight" and "freeze" in our evolutionary ancestors.) An imbalance in the bioavailability of either of the two neuropeptides, e.g., a deficiency of beta-endorphin or an excess of ACTH (perhaps due to the lack of the specific enzymes that cleave these peptides from their parent prohormone) may be an etiological factor in some forms of chronic functional disorders of the brain.

Postural asymmetry and movement disorder after unilateral microinjection of adrenocorticotropin 1-24 in rat brainstem

A unilateral microinjection of adrenocorticotropin 1-24 in the rat brainstem in the region of the locus ceruleus resulted in postural asymmetry and movement disorder that resembled human dystonia, the severity and duration (2 to 3 days) being dose-dependent. These results show for the first time that neuropeptides in the brainstem may modulate posture and movement, and they suggest that some forms of movement disorder such as dystonia may be due to a disordered regulation of postural and locomotor mechanisms by adrenocorticotropin 1-24.

Effects of opiate antagonists on social and aggressive behavior of isolated mice

Opiate antagonists naloxone (1 and 1.5 mg/kg IP) and naltrexone (2.5 and 5 mg/kg IP) inhibit aggressive responses of DBA/2 isolated mice, while increasing the duration of some social activities such as sniff-body, sniff-nose and following. At the doses employed naloxone and naltrexone did not affect motor activity and self-grooming of paired mice. These findings are discussed in terms of the endogenous opioids system involvement in arousability, in the response of the organism to stressful events, in the motivational mechanisms which control social behavior and in the functioning of some neurotransmitter systems which are known to play an important role in the control of isolation-induced aggressive behavior.