Effects of apomorphine, clonidine or 5-methoxy-NN-dimethyltryptamine on approach and escape components of lateral hypothalamic and mesencephalic central gray stimulation in two inbred strains of mice

Lateral hypothalamic glutamatergic inputs to VTA glutamatergic neurons mediate prioritization of innate defensive behavior over feeding

The lateral hypothalamus (LH) is involved in feeding behavior and defense responses by interacting with different brain structures, including the Ventral Tegmental Area (VTA). Emerging evidence indicates that LH-glutamatergic neurons infrequently synapse on VTA-dopamine neurons but preferentially establish multiple synapses on VTA-glutamatergic neurons. Here, we demonstrated that LH-glutamatergic inputs to VTA promoted active avoidance, long-term aversion, and escape attempts. By testing feeding in the presence of a predator, we observed that ongoing feeding was decreased, and that this predator-induced decrease in feeding was abolished by photoinhibition of the LH-glutamatergic inputs to VTA. By VTA specific neuronal ablation, we established that predator-induced decreases in feeding were mediated by VTA-glutamatergic neurons but not by dopamine or GABA neurons. Thus, we provided evidence for an unanticipated neuronal circuitry between LH-glutamatergic inputs to VTA-glutamatergic neurons that plays a role in prioritizing escape, and in the switch from feeding to escape in mice.

A narrative synthesis of research with 5-MeO-DMT

BACKGROUND

5-Methoxy-N,N-dimethyltryptamine (5-MeO-DMT) is a naturally occurring, short-acting psychedelic tryptamine, produced by a variety of plant and animal species. Plants containing 5-MeO-DMT have been used throughout history for ritual and spiritual purposes. The aim of this article is to review the available literature about 5-MeO-DMT and inform subsequent clinical development.

METHODS

We searched PubMed database for articles about 5-MeO-DMT. Search results were cross-checked against earlier reviews and reference lists were hand searched. Findings were synthesised using a narrative synthesis approach. This review covers the pharmacology, chemistry and metabolism of 5-MeO-DMT, as well epidemiological studies, and reported adverse and beneficial effects.

RESULTS

5-MeO-DMT is serotonergic agonist, with highest affinity for 5-HT1A receptors. It was studied in a variety of animal models, but clinical studies with humans are lacking. Epidemiological studies indicate that, like other psychedelics, 5-MeO-DMT induces profound alterations in consciousness (including mystical experiences), with potential beneficial long-term effects on mental health and well-being.

CONCLUSION

5-MeO-DMT is a potentially useful addition to the psychedelic pharmacopoeia because of its short duration of action, relative lack of visual effects and putatively higher rates of ego-dissolution and mystical experiences. We conclude that further clinical exploration is warranted, using similar precautions as with other classic psychedelics.

Measurement issues in curve-shift analysis of apomorphine effects on rewarding brain stimulation

The direct dopamine agonist apomorphine has been reported to reduce the rewarding efficacy of lateral hypothalamic (LH) self-stimulation. This effect has been claimed to support the notion that dopamine mediates the rewarding effects of LH self-stimulation. Using a standard rate-frequency curve-shift paradigm with ascending order of frequency presentation, we also found that apomorphine (0.1-0.8 mg/kg, SC) appeared to decrease LH self-stimulation reward. These apparent rightward curve shifts were exacerbated by shortening the test duration, which also produced a number of sessions in which the subjects did not respond at all. When the presentation order of stimulation frequencies was reversed, apomorphine did not produce large reward decreases. These results suggest that the previously reported effects of apomorphine on LH self-stimulation were the result of artifact, perhaps related to apomorphine-induced stereotypical behavior combined with rapid pharmacological recovery.

Apomorphine and electrical self-stimulation of rat brain

The participation of dopamine neurons in reward produced by electrical stimulation of the brain was examined by measuring self-stimulation thresholds after injections of apomorphine, a direct agonist of dopamine receptors. Rats were trained to press a lever to obtain 0.3-s trains of electrical stimulation applied to lateral hypothalamic electrodes in a paradigm where the pulse frequency was decreased every eight stimulations by approximately 20%. The pulse frequency interpolated at 50% of maximum rate was taken as threshold. In a completely within-subject design, five doses of apomorphine from 0.01 to 1.00 mg/kg and the ascorbic acid vehicle were injected in a random order and thresholds were tracked at intervals of 5 min for 2 h postinjection. Low doses from 0.01 to 0.10 mg/kg caused thresholds to increase while the two higher doses, 0.30 and 1.00 mg/kg, caused thresholds to drop; the switch in the direction of the behavioural effect is thought to parallel the shift in apomorphine's action from presynaptic to predominantly postsynaptic activation of dopamine receptors as the concentration of apomorphine increases.

Stressor induced variations of intracranial self-stimulation from the mesocortex in several strains of mice

Intracranial self-stimulation (ICSS) from the mesocortex was assessed in BALB/cByJ, C57BL/6J and DBA/2J mice immediately, 24 h and again 168 h following stressor application. Stressor exposure failed to influence ICSS performance in C57BL/6J mice, while self-stimulation performance was reduced among BALB/cByJ mice only in the immediate post-stressor interval. In contrast, DBA/2J mice exhibited reduced rates of responding for brain stimulation at each of the post-stressor intervals. The potential contribution of DA alterations to the strain-dependent variations of ICSS performance induced by uncontrollable footshock are discussed.

The midbrain periaqueductal gray in the rat. II. A Golgi analysis

This study consists of a detailed analysis of neurons in the midbrain periaqueductal gray of the rat utilizing four variants of the Golgi technique. Neurons were classified into three major categories based on soma shape, number of primary dendrites, number of dendritic bifurcations, interspinous distance, axonal origin, and axon trajectory. Neurons in each category were further subdivided into large and small varieties based predominantly on soma size and dendritic patterns. Both quantitative and qualitative data concerning each neuronal type is provided as well as data relating to its relative distribution among the four periaqueductal gray subdivisions. The small bipolar neuron, characterized by its small size and spindle-shaped soma, was the most prominent cell type observed, composing 37% of the impregnated neurons in our material. This cell type was most numerous in the medial subdivision and least prominent in the dorsolateral subdivision. The small triangular neuron composed 23% of the neuronal population and was relatively evenly distributed through the periaqueductal gray. The remaining four cell types include the large and small multipolar neurons, the large fusiform neurons, and the large triangular neurons. Axons originated from either the perikaryon or a proximal dendrite, with a dendritic origin being most common for large and small triangular neurons and large fusiform neurons. The trajectory of axons in single thick coronal sections originating from periaqueductal gray neurons is typically away from the mesencephalic aqueduct. The exact trajectory is dependent on the location of the neuron. Axons arising from cells in the dorsal subdivision usually project in a dorsal or dorsolateral direction while axons of ventrolateral neurons may project dorsally, laterally, or ventrally. In sum, these data indicate a complex level of internal organization of the periaqueductal gray. The results are discussed in terms of previous immunohistochemical studies of neurons in this region.